Hydrosilanes. From laboratory curiosity to semiconductors

Abstract

Hydrosilanes represent a versatile class of silicon compounds that bridge fundamental main-group chemistry with technologically relevant materials science. Since the early discovery of monosilane in the nineteenth century, the chemistry of silicon hydrides has evolved from a laboratory curiosity into a cornerstone of modern semiconductor processing and silicon materials synthesis. This review provides a comprehensive overview of hydrosilane chemistry, focusing on three central aspects: (i) synthetic approaches to mono- and oligohydrosilanes, (ii) their functionalization through silanide intermediates and related transformations, and (iii) their application as molecular precursors for silicon-based materials and deposition technologies.

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Daniel Griess

Daniel Griess MSc obtained his Master's degree in Chemistry in 2024 under the supervision of Prof. Rolf Breinbauer. He is currently a Ph.D. student in Haas’ group at the Institute of Inorganic Chemistry at Graz University of Technology (Austria). His research interests cover mainly the synthesis of aminosilanes.

Madeleine Heurix

DI Madeleine Heurix obtained her Master's degree in Chemistry in 2024 under the supervision of Dr Michael Haas. She is currently a Ph.D. student in Haas’ group at the Institute of Inorganic Chemistry at Graz University of Technology (Austria). Her research interests cover mainly the synthesis of oligohydrogermanes and silylgermanes.

Matthias Paris

DI Matthias Paris obtained his Master's degree in Chemistry in 2024 under the supervision of Dr Michael Haas. He is currently a Ph.D. student in Haas’ group at the Institute of Inorganic Chemistry at Graz University of Technology (Austria). His research interests cover mainly the synthesis of silanides.

Philomena Vatter

Philomena Vatter BSc obtained her Bachelor's degree in Chemistry in 2024 from Karl Franzens University Graz with thesis supervison from Prof. Tremel at JGU Mainz. She is currently a Master's student in Haas’ group at the Institute of Inorganic Chemistry at Graz University of Technology (Austria). Her research interests cover mainly the liquid phase deposition of functionalized hydrosilanes.

Michael Haas

Prof. Michael Haas received his Ph.D. under the supervision of Prof. H. Stueger. He subsequently carried out postdoctoral research at Monash University (Australia) in the group of Prof. C. Jones. He is currently a group leader at Graz University of Technology (Austria). His research interests include the enolate chemistry of heavier carbon homologues, silane chemistry, liquid-phase deposition of silicon heterostructures, and the design of new group 14 photoinitiators.

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Introduction

Hydrosilanes are a class of silicon-based compounds that have gained significant attention due to their unique chemical reactivity and broad applications in material science and nanotechnology. Although silicon chemistry shares some conceptual similarities with the more extensively explored realm of carbon chemistry, it boasts its own distinct characteristics and complexities. Unlike their carbon-based counterparts, hydrosilanes exhibit distinctive properties owing to the weaker Si–Si bonds and the polar nature of Si–H bonds. These properties make hydrosilanes highly reactive and suitable for various applications, including silicon deposition, semiconductor processing, hydrogen storage, and the development of next-generation optoelectronic materials. Their role in forming advanced silicon-based nanostructures has also sparked interest in fields such as catalysis and biomedical research.

Recent advances in hydrosilane chemistry have focused on improving their synthetic accessibility, stability, and processability. Methods such as reductive coupling, dehydrogenative coupling, plasma synthesis, and chloride-induced disproportionation have been explored to optimize their production. Additionally, applications in printable electronics, thin-film solar cells, and silicon-based nanostructures have further driven research in this field. Several reviews and reference works have summarized aspects of hydrosilane chemistry over the past decades. Early comprehensive treatments can be found in classical sources such as the Gmelin Handbook of Inorganic Chemistry, as well as survey articles and encyclopedia entries published during the 1980s and 1990s.^1,2 These works primarily focused on the fundamental properties, preparation, and industrial relevance of simple silanes. Later contributions,³ including chapters in Comprehensive Inorganic Chemistry II,⁴ expanded on these topics but remained largely centered on established synthetic methodologies and basic silicon hydride chemistry.

More recently, a review by Gerwig, Böhme, and Friebel has revisited the field with emphasis on hydrosilanes; however, this work mainly addresses hydrosilanes themselves and does not cover the broader chemistry of higher hydrosilanes, their functionalization, and their emerging roles as precursors for modern deposition technologies.⁵ Therefore, despite the availability of these valuable contributions, a comprehensive and updated overview connecting classical hydrosilane chemistry with recent developments in functionalization and materials applications remains highly desirable.

Consequently, this review summarizes the broad field of hydrosilanes in terms of their synthesis (i), their functionalization (ii) and, if investigated, their applicability with respect to several deposition techniques (iii).

This review does not deal with hydrosilanes, which have only one hydride R₃SiH (e.g.: (Me₃Si)₃SiH, Ph₃SiH, or HPh₂SiSiPh₂H etc.), as in these cases the organic substituents largely dominate the chemical reactivity. Furthermore, molecular species of the type R₂SiH₂ (e.g.: (Me₃Si)₂SiH₂, Ph₂SiH₂, or Et₂SiH₂ etc.) are likewise excluded for the same reason. However, R₂SiH₂ units within higher silicon hydrides and Si–Si bonded systems are implicitly included in this review. For reviews about hydrosilylation the reader is referred to two very recent publications.⁶ For a summary of inorganic tetrylenes the reader is referred to the excellent review of Rivard.⁷

Historical development

The classical approach is the reaction of silicides with diluted acids. The pioneers in this field were F. Wöhler and H. Buff, who synthesized SiH₄ by the reaction of diluted acids with aluminium silicides.⁸ In 1902, H. Moissan and S. Smiles identified Si₂H₆ as another product formed by this reaction.⁹ Between 1919 and 1921 A. Stock and K. Somieski pioneered the field of organosilanes and aminosilanes with the synthesis of methylsilane¹⁰ and aminosilanes with trisilylamine.¹¹ In the following years they showed that also higher homologs of the series Si_nH_2n+2 were formed.¹⁰ G. Fritz was able to obtain the first silylphosphane in 1953 by pyrolysis of PH₃ and SiH₄ ¹² and in 1955 B. J. Aylett, H. J. Emeléus and A. G. Maddock were able to obtain evidence of the existence of trisilylphosphane.¹³ The next milestone in the synthesis of hydrosilanes was achieved by F. Fehér and co-workers, who used the same methodology and implemented procedures for a targeted isolation up to Si₈H₁₈ on a preparative scale. His scientific life is excellently portrayed in M. Baudler's reminiscence.¹⁴ M. A. Ring and D. M. Ritter were the first to report the synthesis of silyl potassium KSiH₃.¹⁵ E. Amberger and H. Boeters were the first to isolate and report the properties of trisilylphosphane in 1962.¹⁶

A change in the paradigm was the targeted synthesis of the first cyclic hydrosilane derivatives by E. Hengge and co-workers in 1973.¹⁷

In 1993 W. Sundermeyer reported the synthesis of branched silylgermanes, demonstrating the structural diversity accessible in mixed silicon–germanium hydride systems.¹⁸

13 years later T. Shimoda and co-workers demonstrated that these higher silanes have an application as precursors for liquid phase deposition (LPD).¹⁹ This landmark development resulted in a boom of this chemistry with many groups in academia and industry working on the implementation of solution-based deposition in several industrial processes. In 2015, M. Wagner and co-workers reported the synthesis of silafullerens, expanding the structural landscape of silicon-based cluster chemistry.²⁰

In 2017 H. Stueger and co-workers published the synthesis of 2,2,3,3,4,4-hexasilylpentasilane, which is to date the highest isolated hydrosilane on preparative scale.²¹ Recently our group pioneered the use of functionalized hydrosilanes as precursors for LPD.²² In summary this can be seen as a showcase example of an ideal workflow in science. Starting from the very fundamental studies to the several applications. In Fig. 1 the historical development of this research topic is summarized.

Fig. 1 Historical development.

Silane synthesis

Hydrolysis of silicides

The hydrolytic decomposition of silicides with aqueous acids was first observed by Wöhler and Buff in 1857, marking the first documented formation of a silicon- and hydrogen-containing compound.⁸ Subsequently, in 1902, Moissan and Smiles were the first to characterize SiH₄ along with higher hydrosilanes following the decomposition of magnesium silicide with aqueous HCl.⁹ This discovery laid the foundation for the use of silicide hydrolysis as a commonly applied method for silane synthesis. The reaction produces monosilane (SiH₄) alongside disilane (Si₂H₆) and minor amounts of higher hydrosilanes. The yield and product distribution are highly dependent on factors like purity of Mg₂Si, the solvent and acid used, and overall reaction conditions. Stock implemented extensive studies on this topic and successfully isolated and characterized higher silanes, including tetrasilane (Si₄H₁₀), pentasilane (Si₅H₁₂) and hexasilane (Si₆H₁₄) (see Scheme 1). The application of preparative gas chromatography further enabled the separation and identification of both linear and branches silanes up to octasilane (Si₈H₁₈).^23–25 Fehér expanded upon these studies by investigating the hydrolysis of Mg₂Si using sulfuric and phosphoric acid, leading to the formation and identification of silanes up to pentadecasilane (Si₁₅H₃₂). Moreover, distinct linear silanes up to heptasilane (Si₇H₁₆), as well as branched species such as 2-silyltrisilane, 2-silyltetrasilane and 2-silylpentasilane, were isolated and characterized.²⁶

Scheme 1 Synthesis of monosilane and oligohydrosilanes with product distribution according to the work of Stock et al.

Johnson reported that using ammonium bromide (NH₄Br) in liquid ammonia instead of aqueous HCl increased silane yields to 80–90%.²⁷

Hydrogenation of chlorosilanes

The hydrogenation of chlorosilanes with mild reducing agents such as lithium aluminium hydride (LiAlH₄) or diisobutylaluminium hydride (i-Bu₂AlH) is a well-established method for synthesizing hydrosilanes on a laboratory scale (see Scheme 2). However, this approach is limited by the availability of chlorosilane precursors and side reactions involving Si–Si bond cleavage (especially LiAlH₄ results in the cleavage of silyl groups).

Scheme 2 Formation of oligohydrosilanes via the hydrogenation of chlorosilanes.

Finholt et al. first reported this reaction in 1947, demonstrating the formation of disilane (Si₂H₆) via the reduction of hexachlorosilane (Si₂Cl₆) with LiAlH₄ (see Scheme 2). The yield of hydrosilanes varies significantly depending to reaction conditions and chlorosilane employed.²⁸ In 1973, Höfler reported the formation of neopentasilane, short NPS, (neo-Si₅H₁₂) via the hydrogenation of dodecachloroneopentasilane (neo-Si₅Cl₁₂) with LiAlH₄, though low yields were observed due to Si–Si bond cleavage. The use of i-Bu₂AlH instead of LiAlH₄ improved selectivity and increased product yields.²⁹

The Litz–Ring process (see Scheme 3), while primarily employed for the reduction of tetrachlorosilane (SiCl₄) to monosilane using alkali metal hydrides regenerated in situ by molten-salt electrolysis, has also been investigated for related hydride-mediated silicon transformations. The process enables continuous, high-yield production of ultra-pure monosilane and was demonstrated at pilot scale with strong industrial relevance for low-cost, high-purity silicon production for semiconductors.³⁰

Scheme 3 Simplified Litz–Ring process.

Hengge et al. reported the synthesis of the cyclic hydrosilanes, cyclopentasilane (cy-Si₅H₁₀) and cyclohexasilane (cy-Si₆H₁₂) via the hydrogenation of their respective cyclic chlorosilanes using LiAlH₄, achieving excellent yields (see Scheme 4).³¹

Scheme 4 Hydrogenation of cyclic chlorosilanes.

In 2013, Kroke and co-workers reported the successful crystallization of cyclopentasilane, enabling its structural characterization by X-ray diffraction (see Fig. 2).³²

Fig. 2 Crystal structure of Si₅H₁₀ adapted from original literature by Kroke and co-workers.

An alternative approach involves the reduction of [Si₆Cl₁₄]²⁻ complexes, which are generated from HSiCl₃ through a redistribution reaction, facilitated by alkylated amines such as tetraethylenediamine (TEEDA) or pentaethyldiethylenetriamine (PEDETA) and the presence of a base like ethyldiisopropylamine (EDIPA). The resulting hexacoordinate species can then be hydrogenated with LiAlH₄, yielding cy-Si₆H₁₂ (see Scheme 5).³³ Wagner and co-workers have further explored the rich amine-mediated chemistry of chlorosilanes and related silicon cluster species; a comprehensive overview of this chemistry can be found in their review.³⁴

Scheme 5 Synthesis of cyclohexasilane via the reduction of the [Si₆Cl₁₄]²⁻ complexes.

The first synthesis of an unsubstituted bicyclic halosilane, bi(cylclopentasilanyl), was reported by Stueger, Lassacher and Hengge in 1995.³⁵ The key intermediate bis(nonachlorocyclopentasilanyl) is obtained either by Hg-mediated coupling of nonachlorocyclopentasilane using (t-Bu)₂Hg in n-heptane or by catalytic phenyl abstraction from the perphenylated bicyclic compound with HCl/AlCl₃, both routes affording Si₁₀Cl₁₈ in good yields. Subsequent hydride reduction with LiAlH₄ at −40 °C selectively converts the Si–Cl functionalities into Si–H bonds, yielding the unsubstituted bicylic Si₁₀H₁₈ (see Scheme 6). This reduction is accompanied by the partial cleavage of the central Si–Si bond, leading to approximately 15% cyclopentasilane (cy-Si₅H₁₀) as a side product.

Scheme 6 Synthesis of bicyclic hydrosilane. Yields of the final product not reported since it could not be separated from the side-product (cy-Si₅H₁₀).

Disproportionation of chlorosilanes

The disproportionation is the main large-scale method used nowadays for the production of monosilane (SiH₄). Trichlorosilane (HSiCl₃) undergoes disproportionation in the presence of a catalyst (e.g. AlCl₃, ZnCl₂, BCl₃, FeCl₃). The reaction leads to the formation of SiH₄ in high purity alongside SiCl₄. The process is continuous and operates in a closed-loop system, allowing recycling of SiCl₄ (see Scheme 7).³⁶

Scheme 7 Large-scale production of SiH₄.

Direct synthesis of Si and H₂

It is also possible to synthesize monosilane and also higher hydrosilanes from silicon powder and hydrogen in the presence of catalytic amounts of H₂S or various sulfides (see Scheme 8). The product distribution is controlled by the H₂ pressure. However, this method requires high temperatures and pressures, making it less practical compared to other methodologies.³⁷

Scheme 8 Reaction of Si-powder and hydrogen at high temperatures. *Product distribution and total yield when CuS is used as the catalyst.

Catalytic dehydrogenative coupling

The formation of Si–Si bonds from monosilane (SiH₄) can be achieved through dehydrogenative coupling reactions, in which primary and secondary organomonosilanes undergo hydrogen abstraction in the presence of a catalyst (see Scheme 9). Such reactions typically require transition metal complexes from group 4 to 10 which work as catalysts.³⁸

Scheme 9 Schematic equation for dehydrogenative coupling of silanes.

Harrod was the first to report the catalytic formation of higher hydrosilanes from SiH₄ in 1988.³⁹ Building on this work, Okumura et al. demonstrated the synthesis of disilane (Si₂H₆) and trisilane (Si₃H₈) via catalytic condensation of SiH₄ using platinum, rhodium and ruthenium-based complexes.⁴⁰ Moreover, n-tetrasilane was afforded via catalytic conversion of SiH₄, Si₂H₆ and Si₃H₈.⁴¹ However, a significant challenge associated with these approaches is the concurrent formation of insoluble polymeric byproducts, which can negatively impact selectivity and complicate product isolation.

Wurtz-type coupling

A Wurtz-type approach to hydrosilane synthesis was introduced by Craig et al. in 1962, involving the reaction of iodosilane (H₃SiI) with a liquid sodium/mercury amalgam at room temperature (see Scheme 10).⁴²

Scheme 10 Synthesis of disilane (Si₂H₆) via Wurtz-type coupling.

This method resulted in the formation of disilane with a reported yield of 67%.⁴² However, the broader application of this method remains limited, as alkali metals tend to also react with Si–H bonds.

An alternative route to higher hydrosilanes involves the nucleophilic substitution of halosilanes with silanide anions at low temperatures (see Scheme 11).^43,44 A detailed discussion on the synthesis of silanides follows in Functionalization with group 1 (synthesis of silanides).

Scheme 11 Formation of mono-, di-, and trisilane via Wurtz-type coupling.

This method not only enables the formation of linear oligomers but also the synthesis of branched silanes, such as neopentasilane (neo-Si₅H₁₂) and neohexasilane (Si(SiH₃)₃(Si₂H₅)) (see Scheme 12). These branched species can be obtained through the reaction of phenylchlorosilane with a mixture of different potassiumsilanides (KSiH(SiH₃)₂, KSi(SiH₃)₃ and KSi(SiH₃)₂(Si₂H₅)) followed by sequential bromination and reduction of the resulting phenyl-substituted oligosilanes using HBr and LiAlH₄, respectively. Nevertheless, the separation and isolation of the individual products was not reported.⁴⁵

Scheme 12 Formation of branched silanes in a 3-step synthesis.

Building on Fehér's work on silanide synthesis (see Functionalization with group 1 (synthesis of silanides)), Sundermeyer and co-workers investigated the protonation of higher silanides (KSi_nH_2n+1) with phenylsulfonic acid (PhSO₃H) resulting in a mixture of various branched hydrosilanes with the general formula (SiH_4−n(SiH₃)_n).^46,47

However, they noted that a selective synthesis of higher silanides could not be achieved under the investigated conditions, consequently the reactions were leading to mixtures or less well-defined products.

Stueger et al. demonstrated that LiSi(SiH₃)₃ (prepared from isolated neopentasilane) reacts selectively with SiCl₂Ph₂ to form the branched nonasilane (SiH₃)₃SiSiPh₂Si(SiH₃)₃. Subsequent dephenylation using triflic acid, followed by hydrogenation with i-Bu₂AlH, resulted in the formation of 2,2,4,4-tetrasilylpentasilane in good overall yield (see Scheme 13).^48,49

Scheme 13 Synthesis of (SiH₃)₃SiSiPh₂Si(SiH₃)₃ and subsequent treatment with TfOH and i-BuAlH.

In addition, an alternative synthesis of 2,2,4,4-tetrasilylpentasilane was achieved independently by two groups. While Dow Corning employed borosilicate glass surfaces to promote the conversion of neopentasilane,⁵⁰ the groups led by Stueger and Haas, as well as Evonik, demonstrated that 2,2,4,4-tetrasilylpentasilane also forms in the presence of a Lewis acid (see Scheme 14).⁵¹

Scheme 14 Alternative synthesis of 2,2,4,4-tetrasilylpentasilane.

In addition, Haas et al. turned to an alternative method inspired by earlier work of Gilman and Harrell.⁵² In this approach, 0.5 equiv. of dibromoethane (BrCH₂CH₂Br) were added to LiSi(SiH₃)₃ at −80 °C, leading the formation of 2,2,3,3-tetrasilyltetrasilane (a branched octasilane) shown in Scheme 15 accompanied by minor amounts of 2,2,3,3,4,4-hexasilylpentasilane (a branched undecasilane). This reaction is driven by an initial halogen–metal exchange, forming BrSi(SiH₃)₃ as key intermediate.²¹

Scheme 15 Synthesis of branched octasilane with minor amounts of branched undecasilane.

Moreover, Stueger and co-workers successfully obtained crystals of the branched undecasilane appropriate for X-ray diffractometry (see Fig. 3).²¹

Fig. 3 Crystal structure from 2,2,3,3,4,4-hexasilylpentasilane (a branched undecasilane). Reproduced from Haas et al.²¹ with permission from Angewandte Chemie International Edition, © 2017.

Impact of energy on SiH₄

An alternative route to higher hydrosilanes involves energy-induced transformation of monosilane, such as by pyrolysis, photolysis, or electrical discharge (see Scheme 16). These approaches rely on the homolytic cleavage of Si–H and Si–Si bonds, promoting the stepwise build-up of longer-chained silanes.

Scheme 16 Impact of energy on SiH₄. Total yield and product distribution depends on the energy source and reaction conditions.

The most straightforward method employs thermal energy. As early as 1950, Fritz demonstrated that disilane could be synthesized through the pyrolysis of monosilane. Continued pyrolysis of disilane leads to the formation of trisilane, while further thermal treatment of trisilane results in the generation of tetrasilane, indicating that this process enables chain elongation through successive decomposition and recombination steps.^53,54

In addition to thermal activation, monosilane can also be exposed to silent electrical discharge within an ozonizer. This method yields a mixture of disilane, trisilane, and smaller amounts of higher homologues.^2,55 A further photochemical approach involves the irradiation of SiH₄ with ultraviolet light. When mercury is employed as a photosensitizer, a distribution of di-, tri-, and higher silanes can be generated.⁵⁶

Silafullerenes

A major conceptual advance in silicon hydride chemistry was achieved with the realization of silafullerenes, more precisely endohedral silafullerenes (see Fig. 4), by Wagner and co-workers in 2015–2021.^20,57 Building on earlier theoretical predictions that halide ions could template silicon cages, they reported the synthesis of the dodecahedral silicon cluster [Cl@Si₂₀H₂₀]⁻, the silicon analogue of the smallest fullerene C₂₀H₂₀ (see Scheme 17).

Fig. 4 Crystal structures from silafullerenes by Wagner et al. Reproduced from ref. 57 with permission from Journal of American Chemical Society, © 2021.

Scheme 17 Formation of silafullerenes by Wagner et al.

The synthesis proceeds via controlled stepwise reduction of perchlorinated precursor, ultimately converting all exohedral Si–Cl bonds into Si–H functionalities while preserving an encapsulated chloride ion at the centre of the cage. Although formally ionic compounds and therefore isolated as a salt with a counter-cation, the cluster is structurally and chemically dominated by twenty-terminal Si–H bonds. As such, these silafullerenes represent a unique class of molecular hydrosilanes, bridging classical neutral hydrosilanes and anionic silicon clusters, and they offer a hydrogen-terminated silicon surface that is amenable to further functionalization while being stabilized by the endohedral Cl⁻ template.

Silane derivatization

Functionalization with group 1 (synthesis of silanides). In recent years, alkali silanides MSiH₃ (M = Li, Na, K, Rb, Cs) have emerged as promising complex hydrides for solid-state hydrogen storage, showing reversible hydrogenation/dehydrogenation with their parent MSi phases and practical capacities near ambient conditions.⁵⁸ Recent studies have demonstrated favourable thermodynamics that enable hydrogen uptake and release under moderate operating windows, and catalytic approaches have further improved kinetics while maintaining reversibility.⁵⁹ Collectively, these advances position MSiH₃ as viable candidates that have been actively tested and refined for practical hydrogen storage applications.⁶⁰

Furthermore, alkali metal silanides with the general formula MSi_xH_y (M = Li, Na, K, Rb, Cs) serve as versatile precursors for subsequent functionalization, enabling the incorporation of diverse heteroatoms while preserving the Si–H functionality. The scope and methodology of these transformations are discussed in detail in this review.

In 1961, Ring and Ritter reported the formation of potassium silanide (KSiH₃) by reducing monosilane with potassium metal in DME. Reaching 75–100% consumption of silane required about two months (see Scheme 18, path a). A Na/K alloy greatly accelerated the process, achieving 92% SiH₄ consumption in 14 days (see Scheme 18, path b). They also showed that KH reacts with disilane to give quantitative conversion within one day, whereas lithium hydride (LiH) affords only 12% conversion after 48 hours (see Scheme 18, path c). Although isolated yields were not determined directly, subsequent reactions of KSiH₃ proceeded in essentially quantitative yield.¹⁵

Scheme 18 Synthesis of MSiH₃ via reduction (path a–c). No yields were reported.

Hagenmuller and Pouchard prepared sodium silanide (NaSiH₃) by introducing SiH₄ into a finely dispersed suspension of Na in DME. Under these conditions, both reactants partially (approx. 20%) dissolve within 6 hours in equimolar proportions, affording a yellowish solution. However, the resulting NaSiH₃ solution is unstable and undergoes decomposition under the synthesis conditions.⁶¹

In 1966 Kennedy et al. reported a significant improvement in the preparation of potassium silanide by reacting disilane with potassium sand at −78 °C, achieving complete conversion to KSiH₃ within 24–48 h.⁴³

Subsequent studies by Amberger and Römer demonstrated that using a K/Na alloy (5 : 1 w/w) in DME or diglyme affords full conversion to KSiH₃ at room temperature within 3 days. In contrast, THF and 2,2-dimethyl-1,3-dioxolane give slower reactions. While triglyme accelerates the conversion further, the resulting KSiH₃ solutions are less stable than those obtained in DME or diglyme. Notably, the addition of diethyl ether (Et₂O) to solutions of KSiH₃ induces precipitation of a white, crystalline solid that retains its reactivity under N₂ or vacuum and can be fully redissolved thereafter.⁶²

Cradock, Gibbon, and van Dyke later showed that hexamethylphosphoramide (HMPA) enables complete conversion to NaSiH₃ and KSiH₃ at 10 °C within just 15 minutes. However, isolation of the free silanides from HMPA proved unsuccessful due to the high boiling point of the solvent and the high solubility of the products, which prevented precipitation upon addition of nonpolar solvents (see Scheme 19).⁶³

Scheme 19 Synthesis of MSiH₃ in HMPA. Isolation of the silanide failed.

Amberger, Römer, and Layer undertook a detailed study of alkali-metal reactivity in DME and diglyme, establishing a clear increase in rate down group 1 (Na < K < Rb < Cs). Relative to sodium, caesium reduced the time required to reach full conversion by up to 92%. Under otherwise comparable conditions, diglyme generally afforded faster conversions than DME (see Scheme 20, path a and b). Mixed-metal systems offered additional benefits: an Rb/Na alloy shortened reaction times relative to rubidium alone (see Scheme 20, path c). By contrast, an Na/Hg amalgam was distinctly sluggish, reaching only 17% conversion after 23 days (see Scheme 20, path d). Isolation of discrete MSiH₃ (M = Na, K, Rb, Cs) was not attempted; instead, the reactive solutions were directly subjected to subsequent functionalization.⁶⁴

Scheme 20 Synthesis of MSiH₃ by Amberger, Römer, and Layer (path a–d). No yields were reported.

Bürger and Eujen were the first in 1974 to study the generation of higher silanides in more detail. They used K or KSiH₃ and reacted it with varying equivalents of silanes with the formula Si_nH_2n+1 (n = 1–3) and were able to prove via nuclear magnetic resonance (NMR) spectroscopy the existence of KSiH_n(SiH₃)_3−n (n = 1–3). When reacting KSiH₃ with an excess of SiH₄ they observe next to KSiH₃ the formation of KSi₂H₅ (see Scheme 21, path a). On the other hand, upon the reaction of K with di- or trisilane, or KSiH₃ with trisilane they report the formation of the higher silanides KSiH_n(SiH₃)_3−n (n = 1–3) (see Scheme 21, path b). Furthermore, they note that when an excess of trisilane is used the main product is KSi(SiH₃)₃ (see Scheme 21, path c), and an excess of K leads to cleavage of Si–Si-bonds. Finally, when they reacted the formed silanides with HCl, they always observe SiH₄ and Si₂H₆ next to small amounts of the expected silanes Si_nH_2n+2.⁶⁵

Scheme 21 Synthesis of higher silanides, isolated yields no reported (path a–c).

In the same year Fehér and Freund extended the generation of higher alkali-metal silanides using DME as solvent. Reaction of KSiH₃ with disilane furnished KSi₂H₅ cleanly. With equimolar trisilane, NMR spectroscopy revealed a product ensemble comprising KSiH₃, KSi₂H₅, KSiH(SiH₃)₂ and KSi(SiH₃)₃, with KSiH(SiH₃)₂ as the major species. In line with the observations of Bürger and Eujen, employing 3–4 equivalents of trisilane yielded exclusively KSi(SiH₃)₃ (see Scheme 22).^65,66 Isolation of the individual silanides for yield determination was not attempted; instead, the in situ solutions were directly subjected to electrophilic functionalization (see Scheme 51).⁶⁶

Scheme 22 Synthesis of higher silanides by Fehér and Freund, yields were not determined.

Fehér, Betzen, and Krancher (1981) achieved the first isolation of solvent-free potassium silanide KSiH₃. Concentration of freshly prepared KSiH₃ solutions (generated using a Na/K alloy, 1 : 3 w/w) led to crystallization; subsequent high-vacuum drying afforded analytically pure KSiH₃ in 63.5% isolated yield. They further quantified the solvent dependence of KSiH₃ solubility, finding a clear increase with donor atom count (triglyme > diglyme > DME), consistent with enhanced cation coordination. In benzene, addition of 18-crown-6 (18-c-6) increased solubility linearly with crown concentration. A temperature-dependent study in DME (10–51 °C) revealed an inverse temperature dependence, with the highest solubility at 10 °C.⁶⁷

In a subsequent report, Fehér and Krancher showed that replacing the Na/K with an ultrasonically prepared K dispersion reduced the reaction time by up to 9 h. Moreover, adding 17% (v/v) benzene to 0.8–0.9 M DME solutions of KSiH₃ accelerated recrystallization to the solvent-free KSiH₃. The isolated yields were essentially unchanged: 64.4% using the Na/K dispersion and 64.9% with the K dispersion.⁶⁸

Fehér and Krancher (1983)⁶⁹ then examined the formation of higher silanides from various hydrosilane precursors, targeting the homologous series KSiH_n(SiH₃)_3−n (n = 1–3) over a range of silane stoichiometries (see Tables 1–3). “Reaction time” was defined as the point at which the analyzed composition became invariant; longer times were generally required at higher silane loadings. Transient higher silanides – namely KSi₅H₁₁ and KSi₆H₁₃ – were detected at early stages but disappeared upon extended reaction, while increasing the silane-to-metal ratio favored formation of higher silanides overall. Product ratios were established by quenching with a fivefold excess of benzyl chloride followed by GC analysis of the resulting products. No H₂ evolution during the build-up reaction was observed; rather, SiH₄ was the only gaseous byproduct. In addition to the targeted silanides and SiH₄, the reactions produced KH and a potassium-containing silicon hydride polymer, (K_0.09SiH_1.19)_n. On the basis of these observations, a mechanism (see Scheme 23) was proposed in which stepwise nucleophilic substitutions at silicon generate KH and higher silanes as intermediates; these silanes undergo competing silylation and metalation. Because the intermediate silanes (other than SiH₄) re-enter the reaction network, the sequence converges to mixtures of KSiH_n(SiH₃)_3−n, SiH₄, and the potassium containing polymeric silicon hydride.⁶⁹

Scheme 23 Proposed mechanism by Fehér and Krancher towards the formation of higher silanides.

Table 1 Results of the reactions between KSiH₃ + xSi₂H₆ with the composition of the reaction solution at the respective time in mol%

Molar ratio x	1	2	3	4	5
Reaction time [h]	0.25	0.5	6	24	72
KH	Trace	Trace	4.8	3.9	1.5
KSiH3	33.3	Trace	Trace	—	—
KSi2H5	63.9	6.5	Trace	—	—
KSiH(SiH3)2	2.8	93.5	68.8	22.2	5.0
KSi(SiH3)3	—	Trace	26.4	73.9	93.5

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Table 2 Results of the reactions between KSiH₃ + xSi₃H₈ with the composition of the reaction solution at the respective time in mol%

Molar ratio x	0.5	1.0	1.5	2.0	2.5	3.0
Reaction time [h]	0.25	0.33	24	48	72	72
KH	—	—	2.7	3.9	2.0	—
KSiH3	42.6	2.5	—	—	—	—
KSi2H5	50.8	61.5	Trace	Trace	—	—
KSiH(SiH3)2	6.6	34.5	91.0	22.3	2.5	2.2
KSi(SiH3)3	—	1.5	6.3	73.8	95.5	97.8

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Table 3 Results of the reactions between KSiH₃ and the respective silane (A = iso-Si₄H₁₀, B = n-Si₄H₁₀, C = Si₅H₁₂) with their compositions at the respective time in mol%

Reaction	KSiH3 + 2A		KSiH3 + 2B		KSiH3 + 2C
Reaction time [h]	1	72	1	72	24	48
KH	2.2	3.6	1.8	4.2	7.1	1.8
KSiH3	Trace	—	Trace	—	—	—
KSi2H5	Trace	—	Trace	—	—	—
KSiH(SiH3)2	82.6	1.5	85.9	1.7	66.9	3.4
KSi(SiH3)3	15.2	94.9	12.3	94.1	26.0	94.8

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Fieselmann and Dickson showed that adding 18-c-6 to the reaction of SiH₄ with potassium in glyme markedly accelerates formation of [K(18-c-6)SiH₃], reducing the reaction time to hours (see Scheme 24). Under these conditions the silanide precipitates as a white solid; after 3 h, monitoring of hydrogen evolution indicated that 87% of the SiH₄ had been consumed. The resulting silanide was subsequently subjected to electrophilic functionalization (see Scheme 48).⁷⁰

Scheme 24 Formation of KSiH₃ complexed with 18-c-6, no yields reported.

F. Fehér, Krancher, and M. Fehér (1991) revisited the Na/SiH₄ system and established that reactions of SiH₄ with sodium in ether solvents invariably yield mixtures of sodium silanides NaSiH_3−n(SiH₃)_n (n = 0–3), rather than exclusively NaSiH₃ (see Scheme 25, path a). Product distributions determined by benzyl chloride trapping followed by GC analysis, together with direct NMR characterization of the silanide solutions, contradict earlier claims by Hagenmuller and Pouchard of sole NaSiH₃ formation.^61,71 The sodium silanides proved markedly more stable in solution than previously suggested, showing no detectable degradation over weeks at room temperature. In DME, the steady-state composition contained approximately twice as much NaSiH₃ as NaSi₂H₅; in diglyme, the ratio increased to about 4.5 : 1. Minor NaSiH(SiH₃)₂ was consistently observed, with only trace amounts of NaSi(SiH₃)₃. Substantial NaH formation was detected only at −30 °C, and the overall conversion of SiH₄ to sodium silanides at room temperature was 54–60%. Further investigations examined the effects of used sodium, reaction vessel size, solvent volume, reaction time, and temperature; full details are provided in the original publication. Attempts to isolate discrete sodium silanides by concentrating the solutions promoted redistribution toward higher silanides (see Table 4), whereas evaporation to dryness gave only NaH and polysilanes, indicating complete silanide decomposition (see Scheme 25, path b). The related reaction of KSiH₃ with excess of SiH₄ in DME exhibited a pronounced concentration dependence: dilute solutions favored the distribution KSi₂H₅ > KSiH₃ > KSiH(SiH₃)₂, whereas more concentrated mixtures reacted approximately twice as fast but afforded KSiH₃ > KSi₂H₅ > KSiH(SiH₃)₂ > KSi(SiH₃)₃ (see Scheme 25, path c).⁷¹

Scheme 25 a) Monosilane reacting with Na or K forming different silanides MSiH_n(SiH₃)_3−n (n = 0–3). (b) Attempted isolation of formed silanides leading to degradation. (c) Concentration dependency of formed silanides.

Table 4 Results of the silanide solution composition before and after concentrating the solution

Silanide	Composition of the starting solution [mol%]	Composition of the solution after concentrating it [mol%]
NaSiH3	65	25.7
NaSi2H5	30.4	16.1
NaH(SiH3)2	4.6	30.3
NaSi(SiH3)3	Trace	27.9

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Lobreyer, Oeler, and Sundermeyer (1991) designed a purpose-built reactor (design details in Fig. 5) that enabled complete consumption of 15 g of potassium to form KSiH₃ at 65 °C in only 105 minutes in diglyme and 150 minutes in DME. The yield of KSiH₃ was quantitative with respect to potassium.^46,72 This represented a substantial acceleration over the fastest previously reported procedure, which required 11–12 hours to consume 15 g of potassium.⁶⁸

Fig. 5 Reactor design developed by Lobreyer, Oeler, and Sundermeyer. (a) Stirrer bearing, (b) gas-sparging (hollow) stirrer, (c) vortex breaker, (d) G4 frit, (e) gas inlet, (f) temperature probe. Reproduced from ref. 46 with permission from European Chemical Society. © 1991.

In a follow-up study, Lobreyer et al. attempted to prepare NaSiH₃ in diglyme using the same reactor. Instead, they consistently obtained mixtures of sodium silanides NaSiH_n(SiH₃)_3−n (n = 0–3). Spectroscopic analysis after functionalization with p-toluenesulfonic acid (PTSA) indicated approximately 50% conversion to NaSi(SiH₃)₃. Analogously, direct reaction of potassium metal with SiH₄ in the specialized reactor furnished higher potassium silanides KSiH_3−n(SiH₃)_n (n = 0–3) in approx. 90% yield relative to the potassium input, as determined by titration. For both K and Na, extended reaction times at 100 °C afford greater quantities of K/NaSiH_n(SiH₃)_3−n (n = 0, 1); concentrating the solution by solvent removal likewise promotes formation of higher silanides. Using K at 70 °C yields exclusively KSiH₃ with no evidence of the build-up reaction. The resulting solutions were subsequently engaged in follow-up transformations with C-, P-, and Sn-based electrophiles (see Schemes 53, 129 and 73).⁴⁷

Stueger et al. accessed iso-tetrasilanides by the reaction of methyllithium (MeLi) with neopentasilane.⁷³ Later Lainer et al. succeeded in the isolation of iso-tetrasilanide through the addition of tetramethylethylenediamine (TMEDA) (see Scheme 26, path a). Full removal of solvent leads to decomposition, but the isotetrasilanide precipitates from the reaction solution, enabling isolation.

Scheme 26 Synthesis of isotetrasilanides via different routes (path a–c). Yields according to NMR-spectroscopy are quantitative.

Analogously, neopentasilane with MOtBu (M = Li, Na, K, Rb, Cs) in THF, and with LiN(iPr)₂ in Et₂O (see Scheme 26, path b and c) also led to the isotetrasilanides.^48,74 For M = K, identical outcomes were obtained in Et₂O and DME as in THF.⁴⁸ Attempts at isolation or recrystallization led to yellow, insoluble polymers; however, ethereal solutions were stable for several hours and could be employed for downstream functionalization (see Scheme 55). Formation of iso-tetrasilanides was quantitative as determined via NMR-spectroscopy.⁴⁸

In 2015, Leich, Spaniol, and Okuda introduced a concise, high-yield route to the α-polymorphs of potassium silanide, α-KSiH₃ and α-KSiD₃, via hydrogenolysis/deuterolysis of the triphenylsilyl complex [K(Me₆TREN)SiPh₃] (Me₆TREN = tris[2-(dimethylamino)ethyl]amine), affording isolated yields of up to 98% (see Scheme 27).⁷⁵

Scheme 27 Generation of silanide through hydrogenolysis/deuterolysis.

Schuhknecht et al. broadened the hydrogenolysis strategy by using the chelating ligand 1,4,7,10-tetramethyl-1,4,7,10-tetraaminocyclododecane (Me₄TACD). The triphenylsilyl precursors [(L)MSiPh₃]_n (M = Li, Na, K, Rb, Cs; L = Me₄TACD) underwent hydrogenolysis with H₂ (Scheme 28, path a) to give the corresponding hydridosilanide complexes [(L)MSiH₃]_m in yields up to 93% for M = Na, K, and Rb. In the lithium system, only small amounts (<10%) of [(L)LiSiH₃]_m could be isolated; extensive ligand decomposition was observed alongside neutral silanes H_n+1SiPh_3−n (n = 0–3). For cesium, hydrogenolysis afforded the ligand-free, polymeric trihydridosilanide [CsSiH₃]_∞.

Scheme 28 Generation of silanides through a hydrogenolysis reaction (path a) and a redistribution route (path b).

Alternatively, [(L)MSiH₃]_m (M = Li, Na, K, Rb) can be prepared via a redistribution route (Scheme 28, path b): catalytic amounts of [(L)MSiPh₃]_n promote scrambling of HSiPh₃ in solution to generate H₂SiPh₂ and SiH₄ in situ; subsequent reaction of SiH₄ furnishes the hydridosilanide complexes. This indirect route is particularly advantageous for M = Li, substantially improving the final yield relative to direct hydrogenolysis.⁷⁶

Functionalization with group 2 (synthesis of silanides). The field of silanide chemistry has been dominated by studies of alkali metal derivatives, whereas alkaline earth metal analogues have been almost entirely overlooked. It was only in very recent work that these compounds were shown to be stable and isolable.

Lainer et al. prepared the magnesium silanide Mg[Si(SiH₃)₃]₂ in low yield by salt metathesis of the isotetrasilanide LiSi(SiH₃)₃ with MgBr₂ (see Scheme 29, path a); however, the product is unstable and decomposes over several days at room temperature. In contrast, reaction of LiSi(SiH₃)₃ with [(^ArylNacNac)MgI(OEt₂)] afforded the corresponding ligand-supported magnesium silanide [(^ArylNacNac)MgSi(SiH₃)₃] (Aryl = 2,6-diisopropylphenyl (Dipp), 2,4,6-trimethylphenyl (Mes)) in good yields (see Scheme 29, path b). This complex remains stable after solvent removal, when stored at low temperature. Single-crystal X-ray diffraction of [(^DippNacNac)MgSi(SiH₃)₃]·Et₂O (see Fig. 6) provided the first crystallographic characterization of a higher silanides.⁷⁴

Scheme 29 Synthesis of magnesium isotetrasilanides by Lainer et al. (path a and b).

Fig. 6 Crystal structure of the magnesium isotetrasilanide [(^DippNacNac)MgSi(SiH₃)₃]·Et₂O from ref. 74 published by wiley under CC-BY 4.0. © 2022.

Functionalization with group 3 (synthesis of silyltetrels)

Silylboranes. In silicon device manufacturing, p-type films are typically deposited by co-flowing diborane with silane in chemical vapor deposition (CVD), with the B₂H₆/SiH₄ ratio, temperature, and plasma conditions governing boron incorporation into polycrystalline, amorphous, or epitaxial silicon.^77,78 In contrast, molecules that already contain Si–B bonds while retaining Si–H provide cleaner, single-source feedstocks that co-deliver silicon and boron and, ideally, evolve only hydrogen as a byproduct, thereby minimizing carbon/halogen contamination and facilitating precise control of dopant concentration.⁷⁹

Amberger and Römer employed KSiH₃ to access a range of silyl boranes via salt metathesis with chloroboranes. While several new SiH₃–BR₂ derivatives were obtained (see Scheme 30, path a–c), reactions with ClB(n-Bu)₂, Cl₂BNMe₂, BCl₃, and HBCl₂ did not yield isolable target compounds (see Scheme 30, path d). From these results they concluded that isolable silyl boranes require compensation of the electron deficiency at boron by neighbouring donor substituents capable of pi donation (e.g., amino groups), whereas purely inductive electron release from alkyl substituents is insufficient; consistent with this, H₃SiB(n-Bu)₂ could not be isolated.⁶²

Scheme 30 Synthesis of silylboranes using SiH₃K (path a–d).

Gaines and Iorns prepared the silylborane µ-H₃SiB₅H₈ in 80% isolated yield by reacting LiB₅H₈ with H₃SiCl, representing the first fully hydrogenated silicon–borane compound (see Scheme 31, path a). Stirring µ-H₃SiB₅H₈ with diethyl ether at ambient temperature for 8 h afforded 2-H₃SiB₅H₈ (see Scheme 31, path b).⁸⁰ In a subsequent study, heating of 2-H₃SiB₅H₈ to 150 °C for 13 h gave 1-H₃SiB₅H₈, identified as the most stable isomer (see Scheme 31, path c).^80,81 Geisler and Norman selectively halogenated the silyl substituent in silyl-substituted pentaboranes using BCl₃, BBr₃, and HBr (see Scheme 32, path a–c). Under otherwise comparable conditions, BCl₃-mediated chlorination of µ-H₃SiB₅H₈ required longer reaction times than chlorination of 2-H₃SiB₅H₈ (Scheme 32, path a and c).⁸²

Scheme 31 Generation of µ-silylpentaborane and its isomerization reactions (path a–c).

Scheme 32 Halogenation reactions of 2- and µ-silylpentaborane (path a–c).

Geisler, Soice, and Norman investigated BCl₃-mediated halogenation of the silyl substituent in silylpentaboranes (see Scheme 33, path a).⁸³ For 1-H₃SiB₅H₈, the halogenation rate was indistinguishable from that of 2-H₃SiB₅H₈, whereas the µ isomer required higher temperatures and longer reaction times to reach full conversion (see Scheme 33, path c and d).^82,83 They also synthesized µ-disilylpentaborane by treating LiB₅H₈ with Si₂H₅Cl (see Scheme 33, path b). The µ isomer rearranged to the 2 isomer upon stirring in Et₂O at room temperature for 4 h, and to the 1 isomer only under more forcing conditions (120 °C, 14 h) (see Scheme 33, path c and d). This behaviour parallels that of the corresponding monosilylpentaboranes; in both series, the thermodynamic stability follows µ ≪ 2 < 1.^80,81,83

Scheme 33 Halogenation of 1-silylpentaborane and generation of disilylpentaborane and its isomers (path a–d).

Lainer et al. synthesized the lithium hypersilyl borate [(H₃Si)₃SiBH₃]Li in a single step by treating the isotetrasilanide LiSi(SiH₃)₃ with Me₂S·BH₃. The obtained salt is notably robust, remaining unchanged for extended periods as a solid or in solution at ambient temperature (see Scheme 34). Attempts to access cationic or neutral derivatives by hydride abstraction with B(C₆F₅)₃ or with Me₃N·HCl were unsuccessful; in both cases, uncharacterized polysilanes formed. The structure of the new hypersilyl borate was confirmed by single-crystal X-ray diffraction (see Fig. 7).⁸⁴

Scheme 34 Synthesis of lithium hypersilyl borate and attempted hydride abstraction reactions.

Fig. 7 Crystal structure of hypersilyl borate [(H₃Si)₃SiBH₃]Li from ref. 84 published by Wiley under CC-BY 4.0. © 2024.

Silylalane. Hagenmuller and Pouchard (1964) reported the synthesis of Al(SiH₃)₃ by salt metathesis of NaSiH₃ with AlCl₃·DME; however, the product is highly labile and decomposes at 0 °C or upon exposure to a stream of H₂ (see Scheme 35, path a).⁶¹ In related work, Semenenko and Taisumov obtained H₂AlSiH₃ by reacting KSiH₃ with AlH₂Cl·Et₂O (see Scheme 35, path b).⁸⁵ Amberger and Römer synthesized the silylalane H₃SiAl(n-Bu)₂ by salt metathesis of KSiH₃ with ClAl(n-Bu)₂ (see Scheme 36). After solvent removal, the material was obtained as the contact adduct H₃SiAl(n-Bu)₂·KCl. Separation of KCl required heating to 220 °C, affording pure H₃SiAl(n-Bu)₂.⁸⁶ Notably, H₃SiAl(n-Bu)₂ exhibits high thermal stability; in contrast, related silylboranes already undergo slow decomposition at ambient temperature.^62,86

Scheme 35 Generation of trisilylalane by Hagenmuller and Pouchard (path a) and monosilylalane by Semenenko and Taisumov (path b). For path (a) no yields were reported.

Scheme 36 Synthesis of silylalane by Amberger and Römer.

Functionalization with group 4 (synthesis of silyltetres)

Organosilanes. Organosilanes and related derivatives, such as silicon carbide coatings are increasingly important across semiconductor, aerospace, aircraft engines, and mechanical engineering. In semiconductor fabrication, alkylsilanes act as precursors for surface passivation, enable area-selective deposition, and improve device stability,⁸⁷ they also strengthen dielectric layers through siloxane networks⁸⁸ and promote adhesion between photoresists and SiO₂ or other substrates.⁸⁹ Furthermore high-purity alkylsilanes are critical for high-quality coatings and can function as anti-reflective layers to suppress reflections and enhance photolithography efficiency.⁹⁰

For the synthesis of organosilanes two prevalent methods are used in the literature, either the reduction of silyl halides or via transformations of inorganic silanes.

Reduction of silyl halides is the most commonly used approach for the synthesis of alkylsilylhydrides. Most frequently LiAlH₄ is used for the reduction, though other common reductants like lithium hydride, sodium hydride, triethyl aluminium (AlEt₃) and diisobutylaluminium hydride can be employed as well (see Scheme 37).^28,91

Scheme 37 Synthesis of organosilanes via reduction with LiAlH₄. The yields for R = Me, vinyl, n-Bu and i-Bu were only given as range of 80 to 90% by Tannenbaum et al. For R = Pr, no detailed yield was given.

Consequently using di- to tetrabromosilylmethane with LiAlH₄ allows for the synthesis of the respective silylmethane.^92–94 When bromosilylmethanes were treated in standard solvents such, as n-Bu₂O, the yields were very poor, however, using tetraline in combination with benzyltriethylammoniumchloride (TEBAC), excellent yields were achieved (see Scheme 38, path b).⁹³

Scheme 38 Synthesis of C(SiH₃)₄ starting from C(SiH₂Ph)₄ (path a) and synthesis of CH_4–n(SiH₃)_n (path b).

Schmidbaur and co-workers were the first to report tetrasilylmethane (C(SiH₃)₄). The usual choice of a polar solvent was replaced with a two-phase system with phase transfer catalyst, in order to supress the cleavage of Si–C bonds, but the sideproduct trisilylmethane (HC(SiH₃)₃) was still observed (see Scheme 38, path a).^94,95 Furthermore they were able to obtain 2,2-disilylpropane from 2,2-dibromosilylpropane Me₂C(SiH₂Br)₂.⁹⁶

While commonly Et₂O is used as the solvent of choice for the synthesis of lower boiling silanes, Kozhevnikov and co-workers were able to obtain methylsilane (MeSiH₃) in excellent yield, using n-Bu₂O as solvent (see Scheme 39, path a).⁹⁷ Bellama and co-workers prepared disilylmethane (H₂C(SiH₃)₂), via reduction of bistrichlorosilylmethane (H₂C(SiCl₃)₂), in 50% yield (see Scheme 39, path b).⁹⁸

Scheme 39 Synthesis of MeSiH₃ (path a) and H₂C(SiH₃)₂ (path b) via reduction with LiAlH₄.

A representative example of metal-hydride reduction of alkylsilyl halides is the conversion of 1,1,3,3-tetrachloro-1,3-disilabutane to 1,3-disilabutane. The tetrachloro precursor is prepared in 41% yield by reacting HCl with elemental Si and MeSiCl₂CH₂Cl using Cu based catalysts,⁹⁹ followed by reduction with LiAlH₄ to afford 1,3-disilabutane (see Scheme 40).¹⁰⁰

Scheme 40 Synthesis of 1,3-disilabutane by reduction with LiAlH₄. No yield and experimental details were reported for the LiAlH₄ reduction.

Fritz and co-workers showed that treatment of chlorinated C-spiro-linked 2,4-disilacyclobutanes with LiAlH₄ results in the cleavage of the four membered rings (see Schemes 41 and 42, path b). In contrast, when they employed i-Bu₂AlH no cleavage was observed, and the hydrogenated C-spiro-linked 2,4-disilacyclobutanes were obtained (see Schemes 41 and 42, path a).¹⁰¹

Scheme 41 Example of the hydration of a C-spiro linked 2,4-disilacyclobutane with LiAlH₄ (path b) and i-Bu₂AlH (path a). No yield for the LiAlH₄ reaction was reported.

Scheme 42 Hydration of a C-spiro linked 2,4-disilacyclobutane with LiAlH₄ (path b) and i-Bu₂AlH (path a). No yield for the LiAlH₄ reaction was reported.

Pyrolysis of the methylchlorosilanes methyltrichlorosilane (MeSiCl₃), dichlorodimethylsilane (Me₂SiCl₂) and chlorotrimethylsilane (Me₃SiCl) at 700 °C afford mixtures of linear and cyclic alkylchlorosilanes, for example, 1,3-disilapropane. The chlorine containing pyrolysis products were then reduced with LiAlH₄, though isolation of pure compounds is problematic.¹⁰² Furthermore, pyrolysis of a mixture of SiH₄ and ethylene leads to formation of mixtures of the alkylsilanes ethylsilane (EtSiH₃), diethylsilane (Et₂SiH₂), triethylsilane (Et₃SiH), though no yields were reported.¹⁰³ The copyrolysis of disilane with methylsilane (MeSiH₃) afforded methyldisilane (MeH₂SiSiH₃), with dimethylsilane (Me₂SiH₂) it led to formation of 1,1-dimethyldisilane (Me₂SiHSiH₃) and with trimethylsilane (Me₃SiH) it resulted in the formation of 1,1,1-trimethyldisilane (Me₃SiSiH₃), though again no isolation was performed.¹⁰⁴

Furthermore, freshly prepared bis- and tristrifluoromethanesulfonatosilylmethanes are effective precursors for di- and trisilylmethanes. Although the formation of these intermediates proceeds in essentially quantitative yield, they undergo decomposition and isomerization at ambient temperature; therefore, immediate reduction is required to achieve optimal yields (see Scheme 43).¹⁰⁵

Scheme 43 Synthesis of di- and trisilylmethane via reduction.¹⁰⁵

Westermark et al. employed LiAlH₄ in combination with aluminium chloride (AlCl₃) to reduce alkyltriethoxysilanes to the respective alkylsilanes in good yields (see Scheme 44).¹⁰⁶

Scheme 44 Reduction of alkyltriethoxysilanes to afford alkylsilanes.

Trialkylstannanes can be used together with Lewis-base catalysts for the partial and full reduction of alkylsilylhalides to alkylhydrosilanes. Usually a distribution of products is obtained, depending on the catalyst used, the stannane used and the molar ratio of stannane to silane.^107–109 When using tributyltin hydride (Bu₃SnH) and catalytic amounts of triphenylphosphine (PPh₃), methylchlorodisilanes show significant Si–Si bond cleavage; in contrast, no cleavage is observed when the analogous methylbromodisilanes are used (see Scheme 45).¹⁰⁷

Scheme 45 Reduction of methylbromodisilanes with Bu₃SnH and PPh_3. No specific yields were reported for the individual compounds, but yields were given as 80% to quantitative yields for the reductions.

One of the first reported synthesis of monoalkylsilanes is the synthesis of MeSiH₃ from H₃SiCl, by Stock and Somieski in 1919. But no information about the purity of the obtained MeSiH₃ is given (see Scheme 46).¹⁰

Scheme 46 Synthesis of MeSiH₃ via reaction of H₃SiCl with ZnMe₂.

Another approach is the direct hydrosilylation of unsaturated substrates, though early work demonstrated that direct addition of SiH₄ to alkenes or alkynes generally affords product mixtures with low yields under both thermal and photochemical conditions.¹¹⁰ The use of catalysts, for example alkali metal aluminates¹¹¹ and LiAlH₄ ¹¹² promotes more selective addition of SiH₄ to alkenes, significantly improving the selectivity. The best results were achieved by employing Pt(PPh₃)₄ as catalyst (see Scheme 47).¹¹³

Scheme 47 Synthesis of alkylsilanes via addition of SiH₄ to an alkene.

SiH₄ was also employed as feedstock for nucleophilic substitution via silyl anions. Conversion of SiH₄ into KSiH₃ allows for subsequent reaction with organic halides, for example MeSiH₃ was obtained by reaction of methyliodide (MeI) with KSiH₃ (see Scheme 48, path a).¹¹⁴ Ritter and Ring obtained MeSiH₃ in excellent yields by treating KSiH₃ with excess methyl chloride (MeCl) (see Scheme 48, path b).¹⁵ Fieselmann and Dickson improved the yields by using potassium(18-crown-6) silanide ([K(18-c-6)]SiH₃) and demonstrated the synthesis of H₂C(SiH₃)₂ (see Scheme 48, path c).⁷⁰ Similar to Na- and KSiH₃, RbSiH₃ and CsSiH₃ are suitable for the reaction with MeI as well.⁶⁴

Scheme 48 Synthesis of alkylsilanes by reaction of KSiH₃ with MeX (X = Cl, I) (path a–c).

Nucleophilic substitution between KSiH₃ and bromomethylsilane (MeSiH₂Br) only leads to formation of methyldisilane in poor yields of 5%, a more suitable approach is the reaction between a monohalodisilane with methyllithium (MeLi) (see Scheme 49, path a and b).⁹²

Scheme 49 Synthesis of methyldisilane (path a and b).

Alkali metal silanides derived from SiH₄ can be converted to alkylsilanes in hexamethylphosphoramide upon treatment with the corresponding alkyl halides, typically in good yields (see Scheme 50).⁶³

Scheme 50 Synthesis of methyl- and ethylsilane from K- or NaSiH₃. No detailed procedures with exact amounts of reagents were given for R = Me, Et.

Treatment of mixtures of potassium silanides with ethyl chloride (EtCl) afforded the corresponding ethylsilanes in approximately 25% overall yield, notably individual product yields were not reported (see Scheme 51).⁶⁶

Scheme 51 Synthesis of ethyl substituted silanes from potassium silanides with ethylchloride. No specific yield for the individual compounds was given.

Furthermore, treatment of mixtures of higher potassium silanides with phenyl chlorosilane (PhH₂SiCl) afforded the corresponding phenylsilanes in approximately 20% overall yield, notably no specific yield was given for the individual products (see Scheme 52).⁴⁵

Scheme 52 Synthesis of phenylisotetra-, phenylneopenta-, and phenylneohexasilane from the respective potassium silanide and phenylchlorosilane. No specific yield for the individual compounds was assigned.

Sundermeyer and co-workers accessed the methylsubstituted silanes MeSiH₃, methyldisilane (MeSiH₂SiH₃), 2-methyltrisilane (SiH₃SiHMeSiH₃) and MeSi(SiH₃)₃ via the buildup reaction between SiH₄ and K in diglyme, and subsequent alkylation with methyl p-toluenesulfonate (p-TosMe) (see Scheme 53).^47,115

Scheme 53 Synthesis of methylsilanes from silanide mixtures. While products are formed with n = 0–3, the isolated yield was only given for n = 2 and n = 3.

Stueger and co-workers employed lithium and potassium isotetrasilanides to access methylisotetrasilane (MeSi(SiH₃)₃) and a range of alkyl- and phenyl-substituted neopentasilanes. Treatment of solutions of lithium isotetrasilanide (LiSi(SiH₃)₃) with methyl p-toluenesulfonate afforded methylisotetrasilane in 36% yield, while using triphenylchlorosilane (ClSiPh₃) as the electrophile furnished triphenylsilylneopentasilane (SiPh₃Si(SiH₃)₃) in 50% yield (see Scheme 54, path a and b). When dimethylphenylchlorosilane (ClSiMe₂Ph) or phenylchlorosilane (ClSiH₂Ph) were employed, both the corresponding monosubstituted neopentasilanes and the disubstituted derivatives were obtained (see Scheme 55, path a and b). To suppress disubstitution, the authors avoided an excess of potassium silanide by adding the silanide slowly to the electrophile.⁴⁸

Scheme 54 Synthesis of methylisotetrasilane (path a) and triphenylsilylneopentasilane (path b) from lithiumisotetrasilanide.

Scheme 55 Synthesis of dimethylphenyl- and phenylneopentasilane from potassiumisotetrasilanide, with concomitant formation of the respective disubstituted product (path a and b). The yield for disubstituted bisdimethylphenylneopentasilane was not reported.

Stueger, Haas and co-workers obtained several phenyl-substituted organosilanes in good yields by treating lithium isotetrasilanide with phenylchlorosilanes as electrophiles. When dichlorophenylsilane (Cl₂HSiPh) and 1,2-dichloro-1,2-tetrakisphenyl-disilane were employed as electrophiles, the corresponding branched nonasilane and branched decasilane were isolated as pure compounds, however, the silane byproducts were not identified (see Scheme 56, path a and b). A crystal X-ray structure was determined for (SiH₃)₃Si(SiPh₂)₂Si(SiH₃)₃ (see Fig. 8).⁴⁹

Scheme 56 Synthesis of phenylsubstituted nona- and decasilanes from lithium isotetrasilanide (path a and b). Both compounds were isolated as pure compounds, but during the reaction undefined silanes emerged as byproducts.

Fig. 8 Crystal structure of (SiH₃)₃Si(SiPh₂)₂Si(SiH₃)₃, adapted from Stueger and Haas et al.⁴⁹ with permission from American Chemical Society © 2019.

In contrast, when dichlorodiphenylsilane (Cl₂SiPh₂) was used as electrophile the corresponding diphenylnonasilane, and two cyclic phenylsilane byproducts were obtained in low yields (see Scheme 57). A crystal structure from (SiH₃)₃SiPh₂(SiH₃)₃ was obtained (see Fig. 9).⁴⁹

Scheme 57 Synthesis of phenylsubstituted organosilanes with dichlorodiphenylsilane.

Fig. 9 Crystal structure of (SiH₃)₃SiPh₂(SiH₃)₃, adapted from Stueger and Haas et al.⁴⁹ with permission from American Chemical Society © 2019.

Moreover, treatment of (H₃Si)₃Si(SiPh₂)₂Si(SiH₃)₃ with triflic acid gave the partially hydrogenated derivative (H₃Si)₃Si(SiPhH)₂Si(SiH₃)₃, this compound was not isolated but was used in situ to prepare the fully hydrogenated product (H₃Si)₃Si(SiH₂)₂Si(SiH₃)₃ (see Scheme 58).⁴⁹

Scheme 58 Synthesis of (H₃Si)₃Si(SiPhH)₂Si(SiH₃)₃. The compound was not isolated and reacted further to (H₃Si)₃Si(SiH₂)₂Si(SiH₃)₃.

Haas and co-workers converted dodecamethoxyneopentasilane to the corresponding potassium silanide using potassium t-butoxide (KOt-Bu) (see Scheme 59, path a). The subsequent treatment of the silanide with electrophiles, such as MeI resulted in the formation of the corresponding nonamethoxyisotetrasilanes (see Scheme 59, path b). Reduction of these derivatives with a slight excess of i-Bu₂AlH afforded the corresponding organosilanes in good yields (see Scheme 59, path c).^22,116 Applying the same strategy to a potassium disilanide delivered the cyclic 1,1,4,4-tetrakis(silyl)octamethyl-cyclohexasilane (see Scheme 60). A crystal structure of this compound was also obtained (see Fig. 10).²²

Scheme 59 Synthesis of organosilanes from potassium silanides via reaction with electrophiles and subsequent reduction with i-Bu₂AlH (path a–c).

Scheme 60 Synthesis of organohexasilane compounds from a potassiumdisilanide.

Fig. 10 Crystal structure of 1,1,4,4-tetrakis(silyl)octamethylcyclohexasilane. Adapted from Haas et al.²² published by American Chemical Society under CC-BY 4.0.

Silyl halides are viable substrates for Grignard based synthesis of alkylsilanes, though only in moderate yields. As an example, H₃SiBr reacts with various Grignard reagents to afford the respective alkylsilane (see Scheme 61).¹¹⁷

Scheme 61 Synthesis of alkylsilanes from silyl halides and Grignard reagents. No specific molar ratios or temperatures were given for the synthesis.

Moreover, halomethylsilanes, such as, bromo- and chlorosilylmethane are suitable precursors to silylmethyl Grignard reagents (XMgCH₂SiH₃), which subsequently react with halosilanes to furnish silyl-substituted methanes, exemplified by CH₂(SiH₃)₂ (see Scheme 62). However, neither bromo- nor chloromethylsilane reacts with magnesium under standard conditions, the corresponding Grignard reagent is obtained only when activated magnesium is generated by reducing magnesium chloride (MgCl₂) with K in THF under reflux. They stated that they obtained 1,2-disilylethane (H₃SiEtSiH₃) in 26% yield from the preparation of H₃SiCH₂MgBr due to Wurtz-coupling. Further examples of this synthesis approach are given in Table 5.¹¹⁸

Scheme 62 Example synthesis of alkylsilanes from silylmethylgrignard compounds.

Table 5 Synthesis of silyl substituted methanes from halosilanes and grignard reagents

Grignard reagent	Halosilane	Product	Yield [%]
Yields marked with asterisk are crude yields.
Me3SiCH2MgCl	H3SiBr	Me3SiCH2SiH3	86
Me2SiHCH2MgCl	H3SiBr	Me2SiHCH2SiH3	20–30*
MeSiH2CH2MgCl	H3SiBr	MeSiH2CH2SiH3	92
H3SiCH2MgBr	H3SiBr	CH2(SiH3)2	91
H3SiCH2MgBr	Me2SiHCl	H3SiCH2SiHMe2	30*
H3SiCH2MgBr	Me3SiCl	H3SiCH2SiMe3	85

---

Dimethyltitanocene (Me₂TiCp₂) is able to catalyze redistribution reactions of diethoxymethylsilane ((EtO)₂MeSiH) affording triethoxymethylsilane ((EtO)₃MeSi) and methylsilane in excellent yields.¹¹⁹ Other catalysts like sodium ethoxide are used for the redistribution as well (see Scheme 63).¹²⁰

Scheme 63 Catalytic redistribution of (EtO)₂MeSiH.

Ozonizer-type silent electrical discharge tubes were also employed in the synthesis of methyldisilane from a mixture of Me₂O and SiH₄, though this approach is not synthetically useful due to the low yield of 4% and the poor selectivity.¹²¹

Treatment of silanes with organolithium reagents provides an alternative route to organosilanes. Bolduc and Ring reported that condensing disilane into a solution of ethyllithium (LiEt) in benzene (C₆H₆), furnishes ethyldisilane (EtH₂Si–SiH₃) in 37% yield by the following reaction at room temperature (see Scheme 64).¹²²

Scheme 64 Synthesis of ethyldisilane from disilane and ethyllithium.

This approach can be expanded to larger silanes such as tri-, n-tetra- and n-pentasilane as well. When solutions of the respective silanes in n-hexane are treated with n-butyllithium (n-BuLi) solutions, the respective n-butylsilane is afforded (see Scheme 65, path a and b). Disubstituted dibutylsilanes are observed as well and are the main product of the reaction when 2 equivalents of n-BuLi are used. Additionally, a solid polysilane is formed as well. While it was mentioned that SiH₄ and disilane are formed in the reaction with trisilane as well, it was not indicated if these silanes are observed with tetra- and pentasilane as well.¹²³

Scheme 65 Synthesis of butylsilanes from tri-, tetra- and pentasilane (path a and b). No specific yields for the disubstituted n-butylsilanes with 1 equivalent n-BuLi and no individual yields for the monosubstituted yields were given.

Silylgermanes. Hydrosilanes with a direct silicon–germanium bond, commonly referred to as silylgermanes, have recently attracted increasing attention as single source precursors for the deposition of mixed Si–Ge materials. In conventional processes, SiGe thin films are typically obtained from mixtures of monogermane (GeH₄) and monosilane. However, the significantly different activation energies and surface reaction kinetics of these two hydrides often lead to non-uniform incorporation during growth. As a result, classical co-flow deposition can suffer from compositional inhomogeneities, gas-phase depletion effects, and segregation phenomena such as the formation of germanium-rich cluster or islands. Silylgermanes offer a promising strategy to circumvent these intrinsic limitations. The presence of a covalent Si–Ge bond within a single molecular precursor ensures that silicon and germanium are delivered to the substrate in a fixed stoichiometric relationship, potentially enabling more uniform incorporation at the surface. Moreover, many silylgermanes decompose readily under mild conditions, allowing high deposition rates even at comparatively low substrate temperatures. This combination of single-source stoichiometric control and enhanced low-temperature reactivity makes silylgermanes an appealing class of feedstocks for chemical vapor deposition, liquid phase deposition, and related techniques aimed at producing SiGe:H amorphous alloys, mixed semiconductors, and advanced thin-film architectures. Their potential enables both: fundamental material research and technologically relevant applications.^46,124

The most basic member of the Si–Ge hydride family is monosilylgermane (H₃SiGeH₃). Its first targeted preparation was reported by Cox and Varma in 1964, who achieved the formation of H₃SiGeH₃ in approximately 20% yield via the nucleophilic substitution of chlorogermane with potassium silanide (see Scheme 66). This study represents the earliest attempt to generate a Si–Ge single-bonded hydride under controlled conditions.¹²⁵

Scheme 66 Synthesis of silylgermane (H₃GeSiH₃).

In the same year, Phillips and co-workers identified H₃SiGeH₃ as a minor product during silent electrical discharge experiments, although the hydride was obtained only in trace amounts (∼2%).¹²⁶

An alternative approach to the silylation of germyl anions employs silyltriflate (H₃Si-OTf) or silylnonaflate (H₃Si-ONf) as highly electrophilic silylating agents (see Scheme 67). This approach was demonstrated by Sundermeyer and co-workers (1994) and later expanded by Kouvetakis and co-workers (2005). Using these reagents, H₃SiGeH₃ can be obtained in moderate yields ranging from 22–35%, representing a significant improvement over earlier methods based on halide replacement.^46,127

Scheme 67 Synthesis of silylgermane (H₃GeSiH₃) after treatment of KGeH₃ with SiH₃-OTf and SiH_3-ONf.

Sundermeyer and co-workers also demonstrated the synthesis of branched silylgermane with the general formula (SiH₃)_nGeH_4−n (n = 2–4). In their studies, a mixture of branched silylsilanides (SiH₃)_3−nSiNa (n = 0–2) was first generated by reacting sodium with monosilane in diglyme. Subsequent treatment of this anion mixture with monogermane induced a rearrangement in which the germanium atom migrated into the central position of the silyl framework, forming branched silylgermanides. Finally, silylation with H₃SiONf afforded the corresponding silylgermanes (see Scheme 68). However, the individual products could not be separated, and yields were therefore only estimated by gas chromatography, limiting the characterization of the mixture.¹⁸

Scheme 68 3-Step-synthesis of silylgermanes. *No isolated yields; only determined via gaschromatography.

With a similar procedure, Ritter et al. synthesized the different silylgermanes with the general formular (H₃Ge)_nSiH_4−n. They also employed silyltriflates (H_nSi(OTf)_4−n) and silylnonaflates (H_nSi(ONf)_4−n), enabling the selective synthesis and isolation of the individual products (Scheme 69).¹²⁷

Scheme 69 Synthesis of germylsilanes.

More recently, Stueger et al. introduced a complementary and highly selective approach to silyl-germanes based on nucleophilic substitution reactions of well-defined silanide reagents.⁷³ Using lithium tris(silyl)silanide, LiSi(SiH₃)₃, the previously unknown germaisotetrasilane Ph₃GeSi(SiH₃)₃ could be synthesized in high yield on a multigram scale by reaction with Ph₃GeCl (see Scheme 70). The resulting Ph₃Ge-substituted silyl-germanes serve as versatile intermediates for further derivatization. Selective treatment with methyllithium induces cleavage of a single Si–Si bond while preserving the Si–Ge linkage, generating lithium silanide species that can be reacted with other chlorosilanes or chlorogermanes. This strategy enables the efficient synthesis of higher substituted silyl-germanes containing one or two Ph₃Ge groups in excellent yields. In contrast, attempts to access hydrogen rich silylgermanes via stepwise dephenylation at the germanium center using triflic acid followed by hydride reduction were significantly less selective.

Scheme 70 Synthesis of different branched germasilicon hydrides.

It was also possible to obtain single crystals suitable for X-ray diffraction analysis (Fig. 11 and 12).

Fig. 11 Crystal structure of Si(GePh₃)(SiH₃)₃. Reproduced from ref. 73 with permission from American Chemical Society, © 2016.

Fig. 12 Crystal structure of Si(GePh₃)₂(SiH₃)₂. Reproduced from Stueger et al.⁷³ with permission from American Chemical Society, ©2016.

Mixed silylgermanes with preformed Si–Ge bonds were reported by Wagner and co-workers as part of a two-step synthesis strategy toward single-source precursors for Si–Ge materials.¹²⁸ In a first step, dichlorogermanes (R₂GeCl₂, R = Ph, n-Bu) were reacted with hexachlorodisilane in the presence of catalytic [n-Bu₄N]Cl, generating bis(trichlorosilyl)-germanes Cl₃Si–GeR₂–SiCl₃ in high yields. The reaction proceeds via in situ formation of the nucleophilic [SiCl₃]⁻ species, enabling efficient Si–Ge bond formation under mild conditions. Subsequent hydride reduction of the SiCl₃ groups with LiAlH₄ afforded the corresponding hydrosilanes H₃Si–GeR₃–SiH₃ in good to excellent yields (see Scheme 71).

Scheme 71 Formation of germylsilanes with the general formula H₃Si–(GR)_m–SiH₃.

Moreover, Wagner and co-workers were successful in gaining crystals for X-ray diffractometry of intermediates and products (see Fig. 13). This modular approach provides convenient access to well-defined silylgermanes while avoiding preformed Si–Cl bonds and suppressing scrambling reactions typically encountered for fully hydride-substituted Si–Ge compounds.

Fig. 13 Crystal structures of Cl₃Si–(GePh₂)₂–SiCl₃ and H₃Si–(GePh₂)₂–SiH₃. Reproduced from Wagner and co-workers¹²⁸ with permission from American Chemical Society, ©2022.

Silylstannanes. Hydridic silylstannanes, defined by a direct Si–Sn bond, represent the heaviest mixed group-14 hydrides accessible to experimental investigation. In contrast to silylgermanes, whose synthesis, stability and application as single-source precursors have been explored in considerable detail, purely hydridic Si–Sn compounds remain rarely studied. The difference is mainly due to the high instability of Sn–H and Si–Sn bonds, which strongly limits the stability, isolation and handling of silylstannanes.

The simplest hydridic silylstannane, H₃Si–SnH₃, has been experimentally synthesized only under highly controlled conditions. The first targeted synthesis was reported by Wiberg and co-workers, who obtained the compound via low-temperature hydride reduction of prefunctionalized Si–Sn acetates or chlorides using LiAlH₄, generating ethereal solutions of the mixed hydride (see Scheme 72).¹²⁹

Scheme 72 Formation of silylstannane (H₃SnSiH₃) via the treatment of chloro-, or acetatesilylstannane with LiAlH₄. No yields reported.

However, even under these conditions, H₃Si–SnH₃ was found to be stable only in highly diluted solutions below −80 °C, while enrichment or warming led to rapid decomposition into silane, metallic tin and hydrogen. Complementary matrix-isolation IR studies later provided spectroscopic evidence for hydridic Si–Sn species formed by insertion of atomic tin into silane, yielding HSnSiH₃ complexes stabilized in inert argon matrices at cryogenic temperatures.¹³⁰ Together, these studies demonstrate while hydridic silylstannanes can be generated and spectroscopically characterized, their extreme thermal lability fundamentally limits their isolation and practical application. Sundermeyer and co-workers were successful in generating silyl stannides with the general formular NaSn(SiH₃)_nH_3−n via the reaction of NaSi(SiH₃)_nH_3−n and SnH₄ at −40 °C (see Scheme 73).⁴⁷ In contrast to the corresponding silylgermanides systems (see Scheme 68), no further follow-up chemistry or reactivity studies were reported.

Scheme 73 Generation of silylstannides.

Functionalization with group 5 (synthesis of silylpnictogens)

Silylamines. Silicon nitride SiN_x is a workhorse in metal–oxide–semiconductors and memory as spacers, liners, hard masks, barriers, and passivation layers, but continued scaling demands conformal, low-temperature films with minimal C/O impurities that can degrade fixed charge, trap density, and leakage.⁷⁷ This has sharpened interest in aminosilanes composed exclusively of H, Si, and N, most notably silylamine (H₃SiNH₂), disilylamine (HN(SiH₃)₂), trisilylamine (N(SiH₃)₃), and the preceramic polymer perhydropolysilazane (PHPS); [SiH₂NH]_n. These molecules deliver direct Si–N bonds and high hydrogen content, enabling clean decomposition to volatile byproducts for example H₂ and SiH₄ and facilitating low-impurity SiN_x via CVD and plasma-enhanced atomic layer deposition ALD at lower temperatures.^131–134 In particular, N(SiH₃)₃ and related small H–Si–N aminosilanes can function as single-source Si–N precursors or as nitrogen-rich co-reactants with silane, improving nucleation and conformality in high-aspect-ratio features while avoiding carbon bearing ligands.^131–133 At the polymeric scale, PHPS offers a purely H–Si–N spin-on route that converts at comparatively low temperatures to dense SiN_x, supporting gap-fill, barriers, and passivation without introducing heteroatom contaminants.¹³⁴ Collectively, these H–Si–N-only chemistries target key integration needs at ≤400 °C, with ongoing work focused on tuning precursor volatility and reactivity, controlling incorporated hydrogen, and optimizing plasma densification for device reliability.^131–133

A widely used route to silylamines involves the reduction of silylamino halides. Although several hydride reagents are effective, LiAlH₄ is most commonly employed and usually Cl is the halide of choice. The employed silylamino halides are typically prepared by reacting silyl halides with the corresponding lithium amides,^135,136 or via reaction of the respective amine with the silylhalide,¹³⁷ followed by hydride reduction to afford the target silylamines. A general reaction is given in Scheme 74, path a, several examples can be found in Scheme 74, path b to g.^135–139

Scheme 74 General synthesis of silylamines from chlorosilanes and example synthesis (path a–g).

Another widely used reductant for aminohalosilanes is diisobutylaluminium hydride. For example, reduction of diisopropylaminopentachlorodisilane with i-Bu₂AlH affords the partially halogenated diisopropylaminodichlorodisilane in 83% yield (see Scheme 75).¹⁴⁰

Scheme 75 Example synthesis of partially halogenated aminochlorosilanes using i-Bu₂AlH.

An alternative, synthetically straightforward route exploits nucleophilic substitution at halogenated silanes by the corresponding amines, typically generating the ammonium halide of the amine as a byproduct.^141–143 Stock and Somieski were able to obtain trisilylamine from the reaction between ammonia (NH₃) and H₃SiCl, notably if an excess of NH₃ was used the reaction was unselective, while excess H₃SiCl leads to the selective formation of trisilylamine.^11,144,145 They reasoned that the reaction proceeds stepwise with silylamine and disilylamine (HN(SiH₃)₂) as intermediates, but could not isolate them and they observed the formation of a polymeric solid and SiH₄ (see Scheme 76).¹⁴⁴ Later Aylett and Hakim showed that disilylamine decomposes to trisilylamine and NH₃ and in the presence of NH₃ disilylamine decomposes to SiH₄ and a solid polymer.¹⁴⁶

Scheme 76 Synthesis of trisilylamine from H₃SiCl and NH₃, according to Stock. No yield was reported.

Burg and Kuljian adapted Stocks and Somieskis synthesis by slowly mixing gaseous ammonia, from below, into H₃SiCl, resulting in the formation of trisilylamine in 80% yield (see Scheme 77). Notably they mentioned that slow addition of ammonia is necessary to obtain high yields and good selectivity.¹⁴⁷ When ammonia and H₃SiCl were condensed into a flask the reaction afforded trisilylamine in low yields.¹⁴⁸

Scheme 77 Synthesis of trisilylamine with gaseous ammonia and H₃SiCl, according to Burg.

Emeléus and Miller already employed this method in 1939 for the synthesis of dimethylsilylamine (Me₂NSiH₃), ethyldislylamine (EtN(SiH₃)₂) and methyldisilylamine (MeNSiH₃) (see Scheme 78, path a and b), though they only identified them and gave no yields.¹⁴¹ For instance, monochlorodisilane (H₃SiSiH₂Cl) undergoes substitution with diisopropylamine (i-Pr₂NH) to furnish diisopropylaminodisilane (H₃Si–SiH₂Ni–Pr₂), concomitant with formation of diisopropylammonium chloride (i-Pr₂NH₂Cl) (see Scheme 78, path c).¹⁴³ Both isopropylamine (i-PrNH₂) and t-butylamine (t-BuNH₂) react with H₃SiCl to afford the corresponding disilylamines in good yields (see Scheme 78, path b),¹⁴² while monochlorodisilane reacts with diisopropylamine to give diisopropylaminodisilane.¹⁴⁹ Burg and Kuljian prepared methyldisilylamine (MeN(SiH₃)₂) from methylamine (MeNH₂) and ammonia in 85% yield (see Scheme 78, path b).¹⁴⁷ Bromodisilane (H₃Si–SiH₂Br) reacts with dimethylamine (Me₂NH) to afford dimethylaminodisilane (H₃Si–SiH₂NMe₂) in 8% yield (see Scheme 78, path d).¹⁵⁰

Scheme 78 Example synthesis of silylamines from halosilanes and amines (path a–d). When no yields are given for the product, no yield was reported.

Furthermore, disilylamine reacts with H₃SiI to afford trisilylamine in 67% yield, when a slight excess of H₃SiI is used to afford trisilylamine the yield increases to 87%, though no experimental details were given (see Scheme 79, path a).^146,151 Similarly, anhydrous hydrazine reacts with H₃SiI to give tetrasilylhydrazine in 40% yield, which is a strong reducing agent, but explodes in contact with air (see Scheme 79, path b).¹⁵²

Scheme 79 Synthesis of trisilylamine (path a) and tetrasilylhydrazine (path b) using H₃SiI.

Ward and MacDiarmid treated iododisilane with NH₃ to afford trisdisilylamine in 64% yield (see Scheme 80).¹⁵³

Scheme 80 Synthesis of tridislanylamine from monoiododisilane and NH₃.

Diaminosilylamines, such as t-BuN(SiH₂NMe₂)₂, can be obtained from dichlorosilane (H₂SiCl₂) in a two-step synthesis in good yields. H₂SiCl₂ reacts with the respective primary amine, either in excess¹⁵⁴ or under use of a aiding base like trimethylamine (NMe₃),¹⁵⁵ for example t-BuNH₂ to obtain a dichlorosilylamine, which subsequently reacts with an amine to afford the respective diaminosilylamine (see Scheme 81).^154,155

Scheme 81 Synthesis of diaminosilylamines from H₂SiCl₂ and the respective amines.

Wells and Schaeffer investigated the reaction of trisilylamine with ammonia by condensing both reagents into a flask and allowing them to react in the liquid phase. The reaction afforded N,N′,N″-trisilylcyclotrisilazane only in trace amounts, however, multiple runs provided sufficient material for characterization. They reasoned that ammonia catalyses the elimination of SiH₄ from trisilylamine (see Scheme 82).¹⁴⁸

Scheme 82 Reaction between trisilylamine and ammonia.

Various azapolysilanes can be prepared by reacting diisopropylaminodisilane with primary amines, such as t-BuNH₂. Alternatively by the reaction of chlorodisilane with primary amines in the presence of triethylamine as a base, or by the treatment of 1,2-dichlorodisilane (ClH₂Si–SiH₂Cl) with primary amines and triethylamine (NEt₃) (see Scheme 83).¹⁵⁶

Scheme 83 Example synthesis of silylamines from diisopropylaminodisilane, monochlorodisilane and 1,2-chlorodisilane. No isolated yields were given.

Furthermore, alkalimetal amides, such as sodium amide, are suitable reagents to form silylamines via reaction with silylhalides. After formation of the respective sodium amide a silylhalide is added affording the formation of silylamines, this approach allows for the synthesis of diisopropylaminosilane (i-Pr₂NSiH₃) (see Scheme 84).¹⁵⁷

Scheme 84 Synthesis of silylamines from alkalimetal amides and silylhalides. No isolated yields were given.

Schmidbaur and co-workers accessed 1,1- and 1,2-diaminodisilanes from solutions of 1,2-disilanediylditriflate. The solution of 1,2-disilanediylditriflate is afforded from di-p-tolyldisilane and triflic acid and reacts with solutions of the respective amine and triethylamine to afford the 1,2-diaminodisilanes (see Scheme 85, path a and b). Notably, in the case of diethylamine (HNEt₂) the 1,1-substituted diethylaminodisilane is also formed (see Scheme 85, path a), whereas the 1,1-diisopropylaminodisilane is not observed (see Scheme 85, path b).¹⁵⁸

Scheme 85 Synthesis of diaminodisilanes from disilanediylditriflate using secondary amines (path a and b).

When primary amines are employed, the reaction furnishes six-membered cyclic aminosilanes containing two disilyl units, rather than the 1,1- and 1,2-substituted linear diaminodisilanes obtained with secondary amines (see Scheme 86, path a). When 1,4-difunctional diamines are used instead, the reaction yields five-membered cyclic compounds bearing an exocyclic silyl substituent (see Scheme 86, path b).¹⁵⁸

Scheme 86 Synthesis of cyclic silylamines from disilanediylditriflate. (path a and b).

Dehydrogenative coupling offers an attractive, atom-economical route to silylamines. A variety of catalysts promote the coupling of amines with hydrosilanes, including the Lewis acid trispentafluorophenylborane (B(C₆F₅)₃),¹⁵⁹ transition metal systems such as triruthenium dodecacarbonyl (Ru₃(CO)₁₂),¹⁶⁰ main-group reagents such as di-n-butylmagnesium (n-Bu₂Mg),¹⁶¹ Mn based catalysts¹⁶² and Zn based catalysts.¹⁶³ Sanchez et al. reported the dehydrogenative coupling of amino- and diaminosilanes derived from disilane and trisilane using ruthenium on carbon (Ru/C) as the catalyst, although only non-isolated yields were provided (see Scheme 87, path a and b).¹⁶⁴

Scheme 87 Synthesis of aminodisilane (path a) and aminotrisilane (path b) using Ru/C as catalyst. Only non-isolated yields were reported. Several R groups can be employed.

SiH₄ can be dehydrogenatively coupled with diisopropylamine using di-n-butylmagnesium n-Bu₂Mg as catalyst, as shown by Maddock et al. (see Scheme 88).^159,161

Scheme 88 Dehydrogenative coupling of SiH₄ and diisopropylamine, using n-Bu₂Mg.

Moreover, Rekken employed Zn-based catalysis to achieve dehydrogenative coupling, affording diisopropylaminodisilane and bisdiisopropylaminodisilane. They further demonstrated that Zn-based catalysis is applicable to SiH₄, albeit with low conversions. No isolated yields were reported, the product compositions were determined by GC-FID, and diisopropylamine was used in large excess (see Scheme 89).¹⁶³

Scheme 89 Dehydrogenative coupling of disilane with diisopropylamine, using Zn based catalysts. Only conversions were reported.

Using Zn(NTf₂)₂ as a catalyst enabled the dehydrogenative coupling to afford diisopropylaminoneopentasilane, however, only crude product conversions determined by GC-FID were reported (see Scheme 90).¹⁶³

Scheme 90 Dehydrogenative coupling of neopentasilane with Zn(NTf₂)₂ and diisopropylamine. Only conversions were reported.

In 2022, Trovitch and co-workers demonstrated that the dehydrogenative coupling of SiH₄ with primary and secondary amines proceeds using the manganese-based catalyst [(^2,6-iPr₂PhBDI)Mn(μ-H)]₂.¹⁶² With primary or secondary amines, di- and trisubstituted aminosilanes are obtained, and the steric demand of the amine strongly influences the reaction time. Diamines and triamines furnish polycarbosilazanes, whereas ammonia affords perhydropolysilazane (see Scheme 91).¹⁶²

Scheme 91 Synthesis of aminosilanes from SiH₄ via Mn based catalysis.

A different approach to obtain aminosilanes is the amine exchange pathway. Various aminosilanes are accessible via transamination with amines at room temperature or elevated temperature. The educts of choice for transaminations in the literature are diisopropylaminosilanes. For example, diisopropylaminosilane forms disilylamine with NH₃ and subsequently forms trisilylamine (see Scheme 92).¹⁵⁹

Scheme 92 Synthesis of di- and trisilylamine, via transamination of diisopropylamine. No detailed ratios of the reagents were given; therefore, the overall equation is given.

Aylett and Hakim reported the synthesis of disilylamine in 75% yield by treating diphenylaminosilane (Ph₂NSiH₃) in toluene at low temperature with ammonia (see Scheme 93).¹⁵¹

Scheme 93 Synthesis of disilylamine via transamination of Ph₂NSiH₃ with NH₃.

Transamination of diisopropylaminosilane proceeds with a broad range of secondary amines; with primary amines, the corresponding disilylamines are obtained, while polyfunctional amines undergo silylation at multiple amino groups (see Scheme 94, path a to c).¹⁴³

Scheme 94 Example transaminations of diisopropylaminosilane (path a–c).

This methodology extends to larger aminosilane frameworks, for example, Zhou et al. employed diisopropylaminodisilane in transamination reactions to access a range of monosubstituted as well as bissubstituted (disilanyl)amines, although only GC-MS conversions were reported and no isolated yields (see Scheme 95 and Table 6).¹⁶⁵

Scheme 95 Transamination of diisopropylaminosilane. For conversions see Table 6. R = organic rest, R′ = H or organic rest, see Table 6.

Table 6 Conversions of transamination reactions

Amine (2 eq.)	GC conversion [%]
	A	B
Diethyl	38	—
Methyl	69	22
Ethyl	73	23
Propyl	62	21
Butyl	71	25
2-Aminobutane	96	1
Pentylamine	51	13
2-Aminopentane	95	1
1,2-Dimethylpropyl	98	—
t-Pentyl	83	—
Cyclopentyl	84	12
Cyclohexyl	92	5
Aniline	74	—
o-Toluidine	38	—

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Using an NH₃ : diisopropylaminodisilane ratio of 1 : 3 gives 99% conversion to bisdisilanylamine; with an additional equivalent of diisopropylaminodisilane, trisdisilanylamine is obtained.¹⁶⁵ Furthermore, Rekken et al. obtained trisdisilanylamine by thermal degradation of bisdisilanylamine at 110 °C (see Scheme 96, path a to c).¹⁶⁶

Scheme 96 Transamination of diisopropylaminosilane affording bis- and trisdisilanylamine and thermal degradation of bisdisilanylamine (path a–c). No isolated yields were reported. For the thermal redistribution of bisdisilanylamine only the general equation is given.

Silyl iso-cyanide (H₃SiNC) was previously prepared but wrongfully assigned as silyl cyanide (H₃SiCN) and no detailed analysis of its properties was carried out.¹⁶⁷ MacDiarmid was able to obtain and analyze both silyl iso-cyanide and silyl iso-thiocyanate (H₃SiNCS). The treatment of silver cyanide (AgNC) with H₃SiI at room temperature afforded silyl iso-cyanide in 90% yield and by reaction with mercuric cyanide at elevated temperatures, though no yield was reported (see Scheme 97, path a). Silyl iso-thiocyanate was afforded via the reaction between H₃SiI and silver thiocyanate (AgNCS) in 66% yield (see Scheme 97, path b).¹⁶⁸ Furthermore, the reaction between trisilylamine and cyanic acid (HOCN) affords silyl iso-cyanate (H₃SiNCO) in 95% yield, and with thiocyanic acid (HSCN), it leads to the formation of silyl iso-thiocyanate in 58% yield (see Scheme 97, path c and d).¹⁶⁹

Scheme 97 Synthesis of silyl iso-cyanide, silyl iso-cyanate and silyl iso-thiocyanate from H₃SiI (path a and b) and trisilylamine (path c and d).

Gaseous silylphosphane (H₃SiPH₂) reacts readily with cyanic acid as well as thiocyanic acid affording silyl iso-cyanate or silyl iso-thiocyanate in yields greater than 80% (see Scheme 98, path a and b).¹⁷⁰

Scheme 98 Synthesis of silyl iso-cyanate (path a) and silyl iso-thiocyanate (path b). No amounts of the used reagents were given.

Disilylsulfide (S(SiH₃)₂) is a viable feedstock for the synthesis of silyl i-thiocyanate. In combination with silver thiocyanate (AgNCS), it afforded H₃SiNCS in 86% yield (see Scheme 99). When instead cyanogen chloride was employed, the reaction led to a mixture of disilylsulfide, silyl cyanide, sulfur, and silyl i-thiocyanate, but the mixture proved to be inseparable.¹⁷¹

Scheme 99 Synthesis of silyl i-thiocyanate from disilylsulfide. The amount of AgNCS used was not reported.

Ebsworth and Mays reported the synthesis of silyl i-cyanate via treatment of silvercyanate (AgOCN) and powdered glass with iodosilane vapor with an overall yield of 20% (see Scheme 100).¹⁷²

Scheme 100 Synthesis of silyl i-cyanate from iodosilane and silvercyanate.

Silylamines such as N(SiH₃)₃ and N(H₂Si–SiH₃)₃ serve as precursors to silylaminoboranes. Trisilylamine reacts quantitatively at low temperature with boron trihalides BX₃ (X = Cl, F) to afford the corresponding disilylaminohaloboranes.^147,173 Witz and Sujishi showed that boron trifluoride (BF₃) forms adducts with trisilylamine, methyldisilylamine, and dimethylsilylamine, these adducts decompose at room temperature to give silyl fluoride and the corresponding silylaminoborane (see Scheme 101, path a and b).¹⁷³

Scheme 101 Synthesis of bissilylaminodichloro- and bissilylaminodifluoroborane (path a and b).

Furthermore, trisilylamine reacts readily with bromodiborane (B₂H₅Br) at low temperature to afford disilylaminoborane in quantitative yield; subsequent treatment with diborane furnishes bissilylaminodiborane in 80% yield (see Scheme 102, path a and b).¹⁴⁷ The same methodology extends to methyldisilylamine, but instead of the desired methylsilylaminoborane it yields methylsilylaminodiborane in 20% yield, accompanied by formation of H₃SiBr, SiH₄ and B₂H₆ (see Scheme 102, path c). The yield was improved to 44% by adding 1.3 equivalents of diborane, an effect attributed to reduced disproportionation of the intermediate methylsilylaminoborane (see Scheme 102, path c).¹⁴⁷

Scheme 102 Reactions between trisilylamine (path a and b) and methyldisilylamine (path c and d) with bromodiborane and subsequent reaction of the formed silylamnioboranes with diborane.

MacDiarmid and Abedini formed the corresponding adduct of trisdisilanylamine with BF₃ at low temperature; upon heating, the adduct decomposed to afford bisdisilanylamino-fluoroborane and fluorodisilane, although only the isolated yield of the fluorodisilane was reported (see Scheme 103).¹⁵⁰

Scheme 103 Synthesis of F₂BN(SiH₂SiH₃)₂. In the second reaction only the yield for fluorodisilane was reported.

Norman and Scantlin reported that boranes catalyze the condensation of trisilylamine and methyldisilylamine to give the corresponding silazanes with simultaneous liberation of SiH₄, in good yields (see Scheme 104, path a and b). When B₂H₆ or B₅H₉ is used, the reaction affords H₂Si(N(SiH₃)₂)₂ with only minor formation of higher condensation products Si₇N₃H₁₆. In contrast 1- and 2-BrB₅H₈ catalyze much faster reactions that are more difficult to control, leading to increased formation of Si₇N₃H₁₆.¹⁷⁴

Scheme 104 Borane catalyzed condensation of trisilylamine (path a) and methyldisilylamine (path b). Path (a) no isolation was performed under catalysis by 1- and 2-BrB₅H₈.

Silyl azide (H₃SiN₃) can be prepared by reacting trisilylamine with hydrazoic acid (HN₃) in n-Bu₂O, however, it is unstable at room temperature and slowly decomposes with evolution of SiH₄ (see Scheme 105, path a).¹⁷⁵ Alternatively, treatment of H₃SiBr with tributyltinazide (Bu₃SnN₃) affords silylazide in 71% yield (see Scheme 105, path b).¹⁷⁶ A further route involves treating H₃SiPH₂ with a dilute solution of hydrazoic acid, though no yield was reported (see Scheme 105, path c).¹⁷⁰

Scheme 105 Synthesis of silylazide (path a–c). No yield or experimental details were reported for the reaction of H₃SiPH₂ with hydrazoic acid.

Fritz and Berkenhoff investigated the reaction between silylphosphane and NH₃ and concluded that at −80 °C silylamine forms and subsequently converts to polymeric solids of the type [SiH₂NH]_x. To support this interpretation, they also examined the reaction of H₃SiCl and NH₃, which led to the same solids, although the proposed intermediates could not be directly observed (see Scheme 106).¹⁷⁷

Scheme 106 Ammonolysis of H₃SiPH₂ according to Fritz. No details about the amounts of H₃SiPH₂ and NH₃ as well as reaction time were given. For the following reaction of the formed H₃SiNH₂ only the general formula is given.

Norman and Jolly further investigated the H₃SiPH₂ and NH₃ system. They found that depending on reaction time, temperature, and ratio of H₃SiPH₂ to NH₃, a complex product mixture is formed. At −78 °C to −63 °C and a ratio above 6 : 1 H₃SiPH₂ : NH₃ mainly PH₃ and HN(SiH₃)₂ are formed in a 2 : 1 ratio, with small amounts of N(SiH₃)₃ and SiH₄ from subsequent reactions.¹⁷⁸ When the reaction was carried out with a ratio of H₃SiPH₂ : NH₃ below 6 : 1 or at higher temperatures, only a complex product mixture was observed, notably small amounts of the silazanes SiH₂(NHSiH₃)₂ and (SiH₃)₂NSiH₂NHSiH₃ were formed. They also could not find direct evidence for the formation of H₃SiNH₂.¹⁷⁸

Glidewell further examined the reactions between H₃SiPH₂ and amines and imines. In the gas phase, methylamine and diethylamine react with PH₃ to give the corresponding silylamines in quantitative yield. In contrast, methylamine does not react with gaseous H₃SiPH₂ and with condensed H₃SiPH₂ only a white polymeric solid is obtained (see Scheme 107).¹⁷⁰

Scheme 107 Synthesis of silylamines from H₃SiPH₂ with amines. The amounts of reagents used were not given.

Treatment of cyanamide (CH₂N₂) with H₃SiPH₂ or with disilylsulfide affords disilylcarbodiimide (C(NSiH₃)₂) in excellent yields (see Scheme 108, path a and b). However, the yield of the S(SiH₃)₂ reaction was not explicitly reported. In contrast, trisilylphosphane (P(SiH₃)₃) gives the carbodiimide only in poor yield with concomitant decomposition of P(SiH₃)₃, and SiH₃PMe₂ undergoes complete decomposition under the reaction conditions.¹⁷⁰

Scheme 108 Synthesis of disilylcarbodiimide (path a and b). No experimental details were given, therefore only the general formula is given.

Passing H₃SiI vapor over powdered glass wool and silver cyanamide (Ag₂CN₂) led to the formation of disilylcarbodiimide in 20% yield, with SiH₄, and HCN as side products. Neither using PbCN₂, or H₃SiBr with Ag₂CN₂ improves the yields and results in 10–20% yield of disilylcarbodiimide. Notably, H₃SiI and undiluted Ag₂CN₂ led to an explosion.¹⁷⁹ When H₃SiI vapor is diluted with N₂, the yield of the reaction is increased to up to 60%.¹⁸⁰ While it was first unclear if disilylcyanamide NCN(SiH₃)₂ or disilylcarbodiimide was formed during the reaction, later IR spectra studies¹⁸⁰ and gas phase electron diffraction¹⁸¹ showed that disilylcarbodiimide is formed (see Scheme 109).

Scheme 109 Synthesis of disilylcarbodiimide from halosilanes and Ag₂CN₂.

Schmidbaur and Schuh employed NaH and NaNH₂ in the presence of 18-c-6 as a phase-transfer catalyst to convert 1,4-disilabutane and n-tetrasilane into their corresponding aminosilanes (see Scheme 110, path a and b). While 1,4-disilabutane reacts with diethylamine using the NaNH₂/18-c-6 system in pentane to afford the aminosilanes with evolution of H₂, n-tetrasilane undergoes cleavage, giving mixtures of smaller aminosilanes and higher silanes. They found that NaH alone does not catalyze these reactions. When pyrrole was used with n-tetrasilane instead of diethylamine, the main product was Si(NC₄H₄)₄, accompanied by formation of trisilane.¹⁸²

Scheme 110 Reactions of 1,4-disilabutane (path a) and n-tetrasilane (path b) with amines under NaNH₂ catalysis.

Perhydropolysilazanes such as bisdisilylaminosilane are obtainable from trisilylamine with a suitable catalyst, such as B(C₆F₅)₃ or PdCl₂ (for more examples see Scheme 111 and Table 7). Though it is not stated if the product obtained is pure and no number average molecular weight M_n for the products was given. When the reaction is carried out with increased catalyst loading, M_n increases and the ratio of SiH₂ : SiH₃ of the products increases as well.¹⁸³

Scheme 111 Synthesis of bis(disilylamino)silane and perhydropolysilazanes. Yields for path a are given in Table 7. No yields or molecular weight distributions were given for path b.

Table 7 Synthesis of bis(disilylamino)silane

Catalyst	Catalyst loading [mol%]	Cocatalyst [mol%]	T [°C]	t [h]	Yield [%]
Pt/C	5	NEt3 31%	100	18	48
Pt/C	5	NMe2Et 25%	100	4	36
Pt/C	5	NMe2Et 25%	100	22	46
Pt/C	0.4	NMe2Et 25%	100	4	31
Pt/C	0.4	NMe2Et 25%	105	25	39
[RuCl2((AMPY) (DPPB))]	0.4	—	100	18	10
[RuCl2((AMPY) (DPPB))]	0.4	NEt3 30%	100	18	83
[RhCl(PPh3)3]	0.4	NMe2Et 25%	100	1	86
[(R,R)-teth-TsDpenRuCl]	0.6	NMe2Et 25%	100	1.5	83
Ru3(CO)12	1.1	NMe2Et 25%	80	1	80

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Silylphosphanes. For further information about silylphosphanes containing partially hydrated and fully non hydrated silyl groups Fritz and Scheer wrote an excellent review, which is beyond the scope of fully hydrated silylphosphanes of this review.¹⁸⁴

The composition of silylphosphane precursors is especially relevant for semiconductor applications. Silylphosphanes consisting only of P, Si, and H are attractive for semiconductor processing because they are carbon- and halogen-free and decompose to benign H₂, reducing impurity incorporation that degrades carrier lifetimes and interface quality.^185,186 Their preformed Si–P bonds make them single-source reagents that deliver Si and P together, enabling in situ n-type doping of Si/Ge and the growth of Si–P materials with improved dopant incorporation efficiency and spatial uniformity compared with separate feeds.^186,187

The first reported silylphosphane H₃SiPH₂ was prepared by Fritz, in 1952, via thermal decomposition of equimolar amounts of SiH₄ and PH₃, proceeding through a radical mechanism.^12,53,188 Furthermore, under the reaction conditions, less volatile species such as H₂Si(PH₂)₂ and SiP₂ are generated.^189,190 This pathway enabled the first investigations into the properties of silylphosphanes but was problematic for the synthesis of H₃SiPH₂ in a preparative scale, due to the further decomposition of H₃SiPH₂ towards SiP₂ during the reaction (see Scheme 112).¹⁹⁰

Scheme 112 Synthesis of H₃SiPH₂ via pyrolysis of SiH₄/PH₃ mixtures. No yields were reported.

Sabherwal and Burg adapted the thermal decomposition conditions to a decreased temperature of 300 °C in the presence of trace amounts of I₂, leading to a significantly increased yield of 61% for H₃SiPH₂.¹⁹¹ The enhanced yield was attributed to a lower degree of product degradation, due to the lower temperature and to the formation of H₃SiI, which reacts with PH₃ to give H₃SiPH₂ and HI. The formed HI then reacts with SiH₄ to regenerate H₃SI.¹⁹¹ Another early report of silylphosphanes was made by Aylett et al. in 1955, who demonstrated that H₃SiI reacts with white phosphorous to afford PI₂SiH₃ with various side products, most notably P(SiH₃)₃, which they could not characterize fully. They further showed that H₃SiPH₂ can be formed via reaction of NMe₃SiH₃I with PH₃.¹³

In addition to the thermal route photochemistry was also investigated to yield silylphosphanes. Blazejowski obtained small quantities of H₃SiPH₂ by IR irradiation of a PH₃/SiH₄ mixture, photosensitized by SiF₄,¹⁹² later by irradiation of a PH₃/SiH₄ mixture,¹⁹³ and by photolysis of PH₃/SiH₄ mixtures at 147 nm.¹⁹⁴ The formation of H₃SiPH₂ and hydrosilanes such as disilane and trisilane is rationalized by the generation of silylene groups that insert into Si–H and P–H bonds.^192,193 Another alternative approach towards silylphosphanes such as H₃SiPH₂ employed electrical discharge methods. Mixtures of SiH₄ and PH₃ form silylphosphanes in an ozonizer-type silent electrical discharge tube.^195,196 Using this setup Jolly and Gokhale obtained HP(SiH₃)₂ from a mixture of H₃SiPH₂ and SiH_4.^196,197 Furthermore ozonizer-type silent electrical discharge tubes have also facilitated the preparation of H₃SiSiH₂PH₂ from disilane and PH_3. However the silent electrical discharge approach has proven to be unsuitable for larger scales, due to the low yields and poor selectivity (see Scheme 113, path a and b).^196,197

Scheme 113 Synthesis of HP(SiH₃)₂ (path a) and Si₂H₅PH₂ (path b) via silent electrical discharge. No yields for P₂H₄ and H₃SiPH₂ were given in the reaction of disilane.

Halide mediated exchange reactions between silylphosphane and halodisilanes were also investigated in order to obtain silylphosphanes. Drake et al. obtained disilanylphosphane (H₃SiSiH₂PH₂) via exchange reaction between H₃SiPH₂ and disilylhalides (see Scheme 114).^198,199

Scheme 114 Synthesis of H₃SiSiH₂PH₂ from H₃SiPH₂ and H₃SiSiH₂X. They noted that all compounds are present in equimolar amounts at the equilibrium.

They further employed SiH₂Cl₂ and SiHCl₂SiH₂Cl with H₃SiPH₂ to synthesize the chlorinated silylphosphanes SiH₂ClPH₂ and SiHCl₂SiH₂PH₂ (see Scheme 115, path a and b). Whereas 1,1-dichlorodisilane does not react with H₃SiPH₂, 1,2-dichlordisilane undergoes facile conversion to 1,2-diphosphinodisilane via the intermediate SiH₂ClSiH₂PH₂ (see Scheme 115, path c). Furthermore 1,1,2-trichlorodisilane affords 1,1-dichloro-2-phosphinodisilane.¹⁹⁹

Scheme 115 Synthesis of silylphosphanes and chlorosilylphosphanes from H₃SiPH₂ with chlorosilanes (path a–d). No yields were given for the reaction of H₃SiPH₂ with 1,1,2-trichlorodisilane.

Silylphosphane synthesis can also be achieved through nucleophilic substitution of silyl halides by alkali metal phosphides. Amberger and Boeters demonstrated that the direct reaction of bromosilane with potassium dihydrophosphide (KPH₂) does not afford H₂PSiH₃, but instead leads to the synthesis of P(SiH₃)₃ (see Scheme 116).²⁰⁰

Scheme 116 Synthesis of P(SiH₃)₃ from KPH₂ and H₃SiBr.

A sequence of transmetalation steps results in the formation of P(SiH₃)₃ steps involving mono- and disilylated intermediates (see Scheme 117).^16,200–202

Scheme 117 Formation of P(SiH₃)₃ from H₃SiBr through the intermediates H₂PSiH₃ and HP(SiH₃)₃.

Glidewell and Sheldrick demonstrated that the intermediate mono- and disilylsubstituted silylphosphanes can be obtained as well, when using an excess of potassium dihydrophosphide with H₃SiBr, followed by acidification of the non-volatile products with hydrogen sulfide.²⁰²

Lewis acid driven redistribution reactions also lead to the formation silylphosphanes. MacDiarmid and Russ demonstrated that H₃SiPH₂ forms a Lewis adduct with BF₃, which subsequently decomposes at low temperatures to a mixture of compounds containing P(SiH₃)₃. Both pathways (a and b) proceed to comparable extents (see Scheme 118).²⁰³

Scheme 118 Decomposition pathways of H₃SiPH₂·BF₃. Both reactions a and b occur equally.

Further investigations showed that boranes promote the redistribution of silylphosphanes and their mixtures into trisilylphosphanes.^203–206 In the first step of these redistribution reactions the silylphosphanes form Lewis acid–base complexes (see Scheme 119, path a) and, depending on the used borane, then undergo redistribution into trisilylphosphanes (see Scheme 119, path b).²⁰⁴ The best yields are obtained when using an excess of BF₃ with 83% formation of P(SiH₃)₃ and small amounts of HP(SiH₃)₂ and 90% for P(SiH₂SiH₃)₃ with small amounts of HP(SiH₃)₂ as side product.^204,206 BCl₃ induces only limited redistribution, while BBr₃ predominantly led to the formation of H₃SiBr and BH₃ complexes only redistributed in trace amounts.²⁰⁴ Mixed trisilylphosphanes are obtained when mixtures of H₃SiPH₂ and H₃SiH₂SiPH₂ are subjected to these conditions.²⁰⁴

Scheme 119 Formation of Lewis acid–base complexes of boranes and silylphosphanes and subsequent BF₃ promoted redistribution reaction, forming trisilylphosphanes (path a and b). No yields were given for the BX₃ and diborane adducts.

When disilanylphosphane is converted to borane adducts and subsequently heated, redistribution to trisilylphosphanes is not observed, instead SiH₄ and a polymeric substance is formed (see Scheme 120, path a). Only when 9 equivalents of disilanylphosphane were employed could trisdisilanylphosphane be detected, though no yields were given (see Scheme 120, path b).²⁰⁵

Scheme 120 Formation of the borane adduct of H₂PSiH₂SiH₃ (path a) and thermal degradation into trisdisilanylphosphane (path b). No isolated yields were given.

In 2010 Tice et al. reported a solvent free approach in which P(SnMe₃)₃ reacts with H₃SiBr to afford P(SiH₃)₃ in 98% yield (see Scheme 121).²⁰⁷

Scheme 121 Synthesis of P(SiH₃)₃ from P(SnMe₃)₃.

Recently Li et al. employed exchange reactions between P(SiMe₃)₃ and an excess of monochlorosilanes to generate the corresponding trisilylphosphanes (see Scheme 122).²⁰⁸

Scheme 122 Synthesis of trisilylphosphanes via exchange reaction of P(SiMe₃)₃ with monochlorosilanes.

The direct reaction between metalated phosphanes and silylhalides was also thoroughly investigated. Lithium tetrakisdihydrogenphosphidoaluminate was prepared by Finholt et al. in 1963,²⁰⁹ which proved as a useful reagent for phosphanylation. In contrast to reactions of silylhalides with KPH₂ ^16,200–202 the transmetalation sequence leading to the formation of P(SiH₃)₃ is not observed when metal tetrakisdihydrogenphosphidoaluminates are employed, giving access to H₃SiPH₂ (see Scheme 123, path a and b).^210,211

Scheme 123 Synthesis of silylphosphanes from metal tetrakis(dihydrogenphosphido)aluminates (path a and b).

Use of LiAl(PH₂)₄ as phosphanylation agent enables the synthesis of diverse silylphosphanes from silylhalides, notably they were able to use SiH₂Br₂ to obtain SiH₂(PH₂)₂ and SiHBr₃ for SiH(PH₂)₃ respectively.^210,212,213 Norman and co-workers showed that analogous methodology employing sodium tetrakisdi-hydrogenphosphidoaluminate (NaAl(PH₂)₄) enables access to several silylphosphanes, as well as germylphosphanes.^210,212 The reaction of NaAl(PH₂)₄ with H₃SiBr has proven to be a viable synthetic approach for the phosphanylation of silicon halides, allowing the synthesis of silylphosphanes such as H₂PSiH₃.²¹¹ In 1994 Becker et al. successfully employed silyltriflate (CF₃SO₂OSiH₃) as silylation agent for NaAl(PH₂)₄ to obtain H₃SiPH₂ in 60% yield (see Scheme 124).²¹⁴

Scheme 124 Synthesis of H₃SiPH₂ from CF₃SO₂OSiH₃. No yield for silyltriflate was given, as it is freshly prepared and used immediately without isolation.

Lithium dialkylphosphides are useful for preparing metalated silylphosphanes via reaction with H₃SiPH₂ (see Scheme 125, path a and b). Solutions of the monometalated silylphosphanes with concentrations above 0.1 mol l⁻¹ undergo disproportionation at room temperature in apolar solvents, establishing an equilibrium between H₃SiPHLi and LiP(SiH₃)₂ (see Scheme 125, path b).²¹⁵ This behavior prevents the direct synthesis of disilylphosphanes from monometalated H₃SiPHLi by subsequent reaction with silylhalides, as trisilylphosphanes will be formed as well.²¹⁵ Precipitation of LiPH₂ after removing most of the solvent and adding benzene drives the equilibrium towards LiP(SiH₃)₂ and allows for recrystallization in Et₂O.²¹⁵

Scheme 125 Synthesis of metalated silylphosphanes from H₃SiPH₂ (path a and b) and disproportionation equilibrium of H₃SiPHLi. No isolated yield was given for the metalated silylphosphanes but for the associated byproducts.

The monometalated H₃SiPHLi can be treated with AlCl₃ in diglyme to form LiAl(PHSiH₃)₄, which reacts with H₃SiBr to afford HP(SiH₃)₂. Owing to the disproportionation of H₃SiPHLi into LiP(SiH₃)₂ and LiPH₂, the initially formed LiAl(PHSiH₃)₄ also contains LiAlP(SiH₃)₂ and LiAlPH₂, which in turn yield H₃SiPH₂ and P(SiH₃)₃ upon reaction with H₃SiBr. Additionally, HP(SiH₃)₂ undergoes dismutation to monosilyl- and trisilylphosphane, leading to the approximately equimolar formation of mono-, di-, and trisilylphosphanes. By quickly separating the disilylphosphanes the dismutation can be circumvented. The dismutation is only observed with PH containing silylphosphides (see Scheme 126).²¹⁵

Scheme 126 Synthesis HP(SiH₃)₂ from LiAl(PHSiH₃)₄ and subsequent disproportionation into H₃SiPH₂ and P(SiH₃)₃.

Drake and Anderson converted P(SiH₃)₃ with LiAlH₄ to the disilylphosphinoaluminate ion LiAlH[P(SiH₃)₂]₃, which further reacts with halides, such as, monobromodisilane H₃SiSiH₂Br to give P(SiH₃)₂SiH₂SiH₃ (see Scheme 127, path a).²¹⁶ H₃SiPEt₂ derived from H₃SiBr undergoes H/Li exchange with LiPEt₂ to form H₂Si(PEt₂)₂ and LiH. Subsequent reactions of H₂Si(PEt₂)₂ with 2 equivalents of LiPEt₂ provides HSi(PEt₂)₃ (see Scheme 127, path b).²¹⁷

Scheme 127 Path (a) synthesis of disilylphosphinoaluminate and subsequent reactions with halides. Path (b) synthesis of H₃SiP(Et)₂, H₂Si(P(Et)₂)₂ and HSi(P(Et)₂)₃ from H₃SiBr. No isolated yields were given.

HSi(PEt₂)₃ is interesting in the context of silylphosphanes, as it reacts with LiPEt₂ to give LiSi(PEt₂)₃, which reacts with H₃SiBr, resulting in the formation of H₃SiSiH(PEt₂)₂, but was only isolated with sideproducts (see Scheme 128).²¹⁷ Metalation of silylphosphanes using n-BuLi leads to cleavage of the Si–P bond.^215,217

Scheme 128 Synthesis of H₃SiSiH(PEt₂)₂ from LiSi(PEt₂)₃. The formation of H₃SiSiH(PEt₂)₃ was confirmed but isolation as not possible due to degradation during distillation.

Silylphosphides as interesting reagents for further functionalization were obtained either via metalation of silylphosphanes,^218,219 or by employing build up reactions of silanes.¹¹⁵ In 1973, Cradock et al. obtained LiP(SiH₃)₂ from P(SiH₃)₃ in 90% yield by metalation with MeLi (see Scheme 129, path a).^218,219 In 1994 Sundermeyer and co-workers accessed KPHSiH₃ and KP(SiH₃)₂ via the buildup reaction between SiH₄ and K in diglyme and subsequent reaction with PH₃ (see Scheme 129, path b).⁴⁷ LiP(SiH₃)₂ can be synthesized through the dismutation of H₃SiPH₂ and LiPHCH₃ (see Scheme 125, path b), and isolated as [(TMEDA)₂Li][P(SiH₃)₂] (see Scheme 129, path c). A crystal structure is shown in Fig. 14.²¹⁴

Scheme 129 Synthesis of LiP(SiH₃)₂ (path a), KPHSiH₃, KP(SiH₃)₂ (path b) and [Tmeda₂Li][P(SiH₃)₂] (path c).

Fig. 14 Crystal structure of the anion (a) and cation (b) of [(TMEDA)₂Li][P(SiH₃)₂]. For better visibility the anion and cation are not shown in the same scale. Adapted from G. Becker, B. Eschbach, D. Käshammer, et al.²¹⁴ with permission from John Wiley and Sons © 1994.

Silylarsanes. Silylarsanes containing only As, Si, and H, are promising precursors for semiconductor fabrication because they are carbon- and halogen-free, which helps minimize contamination during CVD/PECVD/ALD and supports cleaner, low-temperature decomposition to volatile byproducts, mainly H₂.^186,220 Arsenic is a shallow donor in group-IV semiconductors, so silylarsanes can serve as single-source, in situ n-type dopant feeds for Si, Ge, and Si-rich alloys, leveraging the high activation of As and notably low diffusivity in Si to form ultra-shallow, abrupt junctions that are advantageous for modern complementary metal–oxide–semiconductors source/drain engineering.¹⁸⁵

The smallest member of the silylarsane family is silylarsane (H₃SiAsH₂) and was prepared by Jolly and Drake in 1962. Similar to the synthesis of H₃SiPH₂, using an equimolar mixture of SiH₄ and arsine (AsH₃) in ozonizer-type silent electrical discharge tube yields H₃SiAsH₂, as well as disilanylarsane (H₃SiH₂SiAsH₂) (see Scheme 130).¹⁹⁵ Notably disilane as well as trisilane are formed as sideproducts.²²¹

Scheme 130 Synthesis of H₃SiAsH₂ and Si₂AsH₇ from a SiH₄/AsH₃ mixture. No experimental details were given.

When H₃SiAsH₂ and B₂H₆ are condensed into a vessel they react upon warming to 0 °C by forming a solid as well as disilylarsane (HAs(SiH₃)₂) and trisilylarsane (As(SiH₃)₃) (see Scheme 131).²²¹ Reactions between H₃SiAsH₂ and bromodiborane (B₂H₅Br) only afford the formation of bromosilane and a arsine containing polymer.²²¹ H₃SiAsH₂ forms the 1 : 1 adduct with BX₃ (X = Cl, Br), but H₃SiAsH₂BX₃ decomposes at low temperatures forming H₃SiX.²²¹

Scheme 131 Synthesis of HAs(SiH₃)₂ and As(SiH₃)₃ from H₃SiAsH₂ and B₂H₆.

Aylett et al. showed in 1955, that monoiodosilane reacts with arsenic to afford diiodosilylarsane (AsI₂SiH₃) with various side products, most notably trisilylarsane, additionally they showed that trimethylarsane (AsMe₃) undergoes reaction with iodosilane, giving access to trisilylarsane.¹³

Arsenides show similar reactivity towards silyl halides as phosphides. Trisilylarsane can be accessed through nucleophilic substitution of silyl halides by alkali metal arsenides. Amberger and Boeters used bromosilane with potassium dihydroarsenide (KAsH₂) to afford As(SiH₃)₃ (see Scheme 132).²⁰⁰ The same reported method of Glidewell and Sheldrick, in which H₃SiPH₂ and HP(SiH₃)₂ can be obtained from the reaction of KPH₂ with H₃SiBr via acidification with H₂S also works for potassium arsenide KAsH₂, but leads mostly to silylarsine in 54% yield.²⁰² The formation of As(SiH₃)₃ from KAsH₂ proceeds via a series of transmetalation steps, similar to the respective synthesis of trisilylphosphane P(SiH₃)₃ (see Scheme 117).²⁰²

Scheme 132 Synthesis of As(SiH₃)₃ from KAsH₂ and H₃SiBr.²⁰⁰

Metalated silylarsanes can react with silyl halides to form the respective silylarsanes. Lithium tetrakisdihydrogenarsido-aluminate (LiAl(AsH₂)₄) reacts with silylhalides H₃SiX (X = Br, I) to afford silylarsine, though monochlorosilane only led to formation of H₃SiAsH₂ in trace amounts (see Scheme 133).^222,223

Scheme 133 Synthesis of H₃SiAsH₂ from LiAl(AsH₂)₄ and silylhalides.

Anderson and Drake were able to obtain disilylarsane HAs(SiH₃)₂ by treating LiAl(AsH₂)₄ with bromosilane. After removal of the volatiles the remaining solid was treated with dihydrogen selenide (H₂Se), resulting in the formation of HAs(SiH₃)₂.²⁰² Furthermore the treatment of lithium tetrasilylarsenide (LiAs(SiH₃)₄) with bromodisilane led to the formation of disilanylarsane (H₃SiH₂SiAsH₂) in 59% yield (see Scheme 134).²²³

Scheme 134 Synthesis of H₃SiH₂SiAsH₂ from LiAl(AsH₂)₄ and H₃SiSiH₂Br.

Disilanylarsane can also be obtained from the exchange reaction with monohalodisilanes H₃SiH₂SiX (X = Cl, Br) at low temperatures. The yields for H₃SiH₂SiAsH₂ can be brought to 30%, by stepwise removal of the formed silyl halide H₃SiX (see Scheme 135).¹⁹⁸

Scheme 135 Synthesis of H₃SiSiH₂AsH₂ via exchange reaction from H₃SiSiH₂X (X = Cl, Br) and H₃SiAsH₂.

Lithium disilylarsenide (LiAs(SiH₃)₂) can be obtained from trisilylarsane in 90% yield by metalation with MeLi, as shown by Cradock et al. in 1973.^218,219

Similar to trisilylphosphanes, trisilylarsanes are also available through an exchange reaction between tristrimethylsilylarsane (As(SiMe₃)₃) and monohalosilanes, though only the reaction with chlorotrisilane is reported (see Scheme 136).²⁰⁸

Scheme 136 Synthesis of As(SiH₂SiH₂SiH₃)₃ via exchange reaction from H₃SiH₂SiH₂SiCl and As(SiMe₃)₃.

Silylstibanes. Antimony can act similar to other group V elements as n-type dopant.¹⁸⁵ Antimony has a significantly larger atomic size compared to phosphorous and arsenic, leading to slower diffusion and therefore more precise control of dopant profiles, which is especially useful for specialized applications, such as, power electronics and high-temperature devices.²²⁴ Compared to phosphorous and arsenic there are only few investigations into the synthesis of silylstibanes.

The nucleophilic substitution of phosphides and arsenides with silyl halides gave access to their respective trisilyl compound, but similar reactions with antimonides and stibanes did not lead to the desired trisilylstibane (Sb(SiH₃)₃).^200,225 Amberger and Boeters instead accessed Sb(SiH₃)₃ via the salt methathesis reaction of lithium antimonide (Li₃Sb) with H₃SiBr (see Scheme 137).^200,225

Scheme 137 Synthesis of Sb(SiH₃)₃ via salt metathesis reaction of Li₃Sb and H₃SiBr.

Functionalization with group 6 (synthesis of silylchalcogenes)

Siloxanes. Single-source Si–O feedstocks deliver silicon and oxygen in a fixed stoichiometric ratio, simplifying process control compared with separate silane and oxidant flows, and reducing parasitic gas-phase reactions.²²⁶ When designed to be carbon- and halogen-free, they can evolve only hydrogen or water as byproducts, lowering contamination.²²⁷

In 1917, Stock, Somieski, and Wintgen reported the first isolation of O(SiH₃)₂. Their method involved shaking H₃SiBr with degassed H₂O, allowing the mixture to stand, and then purifying the product by fractional distillation (see Scheme 138). They proposed that hydrolysis generates H₃SiOH as intermediate, which rapidly self-condenses to give O(SiH₃)₂ with loss of water.²²⁸

Scheme 138 Synthesis of O(SiH₃)₂ via hydrolysis of SiH₃Br.

In a follow-up study Stock and Somieski isolated O(Si₂H₅)₂ by reacting Si₂H₅Br (which was contaminated with Si₂H_nBr_6−n, n = 0–6) with degassed H₂O, and then extracting it with benzene (see Scheme 139). They failed in isolating the pure compound and always had some H₂O next to O(Si₂H₅)₂. The higher silyl bromides present in the educt as contaminations did not interfere with the reaction, since their resulting siloxanes polymerized and then precipitated as a white solid.¹⁴⁵

Scheme 139 Hydrolysis of Si₂H₅Br, contaminated with higher bromosilanes, leading to O(Si₂H₅)₂ and polymeric siloxanes. Yields of the siloxane were not determined.

Emeléus, MacDiarmid, and Maddock attempted the reduction of O(SiCl₃)₂ with LiAlH₄, but were unable to prepare O(SiH₃)₂ via this route. Instead, they obtained O(SiH₃)₂ in 37% yield by passing H₂ through boiling O(SiCl₃)₂, then directing the saturated gas stream over aluminium foil at 500 °C (see Scheme 140, path a). They also generated O(SiH₃)₂ via hydrolysis of SiH₃I, S(SiH₃)₂, and Se(SiH₃)₂ (see Scheme 140, path b and c). For the Se(SiH₃)₂ route, the yield could not be determined because H₂Se could not be separated from the silylether.²²⁹

Scheme 140 Synthesis of O(SiH₃)₂ from O(SiCl₃)₂ (path a), SiH₃I (path b), S(SiH₃)₂ and Se(SiH₃)₂ (path c). Yields of Se(SiH₃)₂ hydrolysis reaction not determined.

Kriner, MacDiarmid, and Evers investigated reactions of O(SiH₃)₂ with aluminum halides. Treatment with Me₄Al₂Br₂ afforded Me₄Al₂(OSiH₃)₂ (see Scheme 141), which is stable at −78 °C but decomposes at room temperature to give SiH₄ and a non-volatile viscous residue. In contrast, reactions of O(SiH₃)₂ with Al₂X₆ (X = Cl, Br, I) yielded only the corresponding halosilanes; no aluminum-containing products could be isolated.²³⁰

Scheme 141 Synthesis of Me₄Al₂(OSiH₃)₂ from Me₄Al₂Br₂ and O(SiH₃)₂.

Ward and MacDiarmid isolated pure O(Si₂H₅)₂ by hydrolysing purified Si₂H₅I. Purification of O(Si₂H₅)₂ required an extensive protocol: multiple fractional distillations, drying over P₂O₅ (noting that prolonged contact causes degradation of the silylether and formation of PH₃), followed by repeated distillations. The final product was obtained in 96% yield (see Scheme 142).²³¹

Scheme 142 Hydrolysis of O(Si₂H₅)₂ from Si₂H₅I.

Sternbach and MacDiarmid (1961) investigated routes to H₃SiOCH₃. Reacting SiH₄ with methanol (MeOH) at room temperature gave only unreacted SiH₄, H₂, and a complex mixture of methoxysilanes. With copper powder as a promoter, low yields of H₂Si(OCH₃)₂ and HSi(OCH₃)₃ were isolated (see Scheme 143, path a). Direct reaction of SiH₃I with methanol was highly vigorous, leading mainly to polymeric material; only traces of H₃SiOCH₃ were detected in the volatile fraction (see Scheme 143, path b). In contrast, treating the amine adduct SiH₃I·N(Me)₃ with methanol furnished MeOSiH₃ in 28% yield (see Scheme 143, path c). They further showed that MeOSiH₃ reacts with BF₃ to give SiH₃F and MeOBF₂ (see Scheme 143, path d).²³²

Scheme 143 Synthesis of alkoxysilanes (path a–c) and reaction of H₃SiOMe with BF₃ (path d).

Onyszchuk found that O(SiH₃)₂ reacts with BF₃ and BCl₃ to give H₃SiOBX₂ (X = F, Cl), but both products are highly unstable. The fluorinated derivative decomposed during isolation, even at low temperature, and could not be isolated. The chloro analogue decomposes more slowly and was successfully isolated at −64 °C; several subsequent distillations at −112 °C were used to remove H₃SiCl formed in the process (see Scheme 144).²³³

Scheme 144 Reactions of O(SiH₃)₂ with BF₃ (path a) and BCl₃ (path b).

Van Dyke and MacDiarmid prepared the unsymmetrical silylether H₃SiOSi₂H₅ by two routes: path (a) hydrolysis of a mixture containing SiH₃I and Si₂H₅Br; path (b) equilibration between O(SiH₃)₂ and O(Si₂H₅)₂ (see Scheme 145).²³⁴

Scheme 145 Synthesis of the unsymmetrical silylether H₃SiOSi₂H₅ via hydrolysis (path a) and equilibration reactivity of silylethers (path b). *Yield of product mixture.

Van Dyke examined the reactions of O(SiH₃)₂ with phosphorus halides PX₃ (X = F, Cl, Br). No reaction was observed with PF₃ or PCl₃. In contrast, PBr₃ caused H₂ evolution and formation of a yellow solid; the product mixture contained, in addition to unreacted starting materials, SiH₃Br and (H₃SiO)_nSiH_4−n (n = 2, 3) (see Scheme 146, path a). These products were inseparable, so no yields were reported. The absence of SiH₄ formation and the appearance of a yellow, presumably polymeric phosphorus hydride (PH)_x led Van Dyke to propose the transformation shown in (Scheme 146, path b).²³⁵

Scheme 146 a) Reaction of O(SiH₃)₂ with PBr₃ and (b) the proposed reaction mechanism, no yields were reported.

Glidewell and Rankin prepared the aryl silylether PhOSiH₃ in nearly quantitative yield by reacting S(SiH₃)₂ with phenol (PhOH) (see Scheme 147).²³⁶

Scheme 147 Synthesis of PhOSiH₃ using S(SiH₃)₂.

Ebsworth and Thompson prepared silyl esters RCO₂SiH₃ (R = CF₃, H, Me) by protonolysis of trisilylamine (N(SiH₃)₃) with the corresponding carboxylic acids (see Scheme 148, path a). Side products depended on R: with R = CF₃ they observed traces of monosilane; with R = H they detected SiH₄, H₃SiOSiH₃, and traces of CO₂. They next examined the reaction of silver trifluoroacetate (F₃CC(O)OAg) with silyl halides H₃SiX (X = Cl, Br, I) (see Scheme 148, path b). SiH₃Cl gave the trifluoroacetate silyl ester in low yield, SiH₃Br improved the yield to about 60%, and with SiH₃I isolation of the ester failed. Finally, they attempted the analogous synthesis with silver formate (HC(O)OAg). With SiH₃I, substantial iodine was liberated along with non-condensable gases; the mixture contained disiloxane, formic acid, and SiH₃I bearing only small amounts of silyl formate (HC(O)OSiH₃). With H₃SiBr only traces of silyl formate were obtained, and warming the reaction tube to 0 °C caused it to explode (see Scheme 148, path c).¹⁶⁹

Scheme 148 Synthesis towards silyl esters using N(SiH₃)₃ (path a) and SiH₃X (X = Cl, Br, I) (path b and c). Yield of reaction between H₃SiI and silver trifluoroacetate not reported.

Cradock et al. reported that treatment of O(SiH₃)₂ with MeLi liberates MeSiH₃ in approximately 85% of the theoretical amount, forming Li(OSiH₃) (see Scheme 149). They then attempted to react solutions containing Li(OSiH₃) with different electrophiles, but never observed the expected product.²¹⁹

Scheme 149 Synthesis of Li(OSiH₃), no reaction details reported and yields were not determined.

Drake, Henderson, and Hemmings synthesized the silyl esters H₃SiOC(O)CX₃ (X = F, Cl) by adapting the Ebsworth–Thompson protocol, replacing N(SiH₃)₃ with a bis(silyl)carbodiimide [C(NSiH₃)₂] as the silylating agent (see Scheme 150).^169,237 This provided a complementary route to the trihaloacetate esters.

Scheme 150 Synthesis of silyl esters using C(NSiH₃)₂.

Glidewell prepared mixed silyl ethers ROSiH₃ (R = Me, Et, i-Pr, t-Bu) by reacting the corresponding lithium tetraaluminates with SiH₃Br (see Scheme 151, path a). The phenyl silyl ether PhOSiH₃ was obtained from PhONa using either SiH₃Br or silyl trifluoroacetate (Scheme 151, path b). He further showed that silylphosphine can serve as an SiH₃-transfer reagent: 2,2,2-trifluoroethanol was smoothly converted to CF₃CH₂OSiH₃ (a transformation that failed under the tetraaluminate/SiH₃Br conditions), and benzyl alcohol gave BnOSiH₃ selectively (Scheme 151, path b and c).²³⁸

Scheme 151 Synthesis towards mixed silyl ethers using SiH₃Br and tetraaluminates, SiH₃R (R = Br, OC(O)CF₃) and PhONa, and H₂PSiH₃ with ROH (R = F₃CCH₂, Bn). Treatment of mixed silylethers ROSiH₃ (R = Et, i-Pr, t-Bu, F₃CCH₂) with HI (path a–c). Yield of the reactions between lithium aluminate and H₃SiBr, and silylphosphine and BnOH were not determined.

Fehér, Fischer, and Skrodzki found that UV irradiation (medium-pressure Hg lamp) of trisilane in the presence of acetone affords a mixture of alkoxysilanes (silylethers) (see Scheme 152, path a). They note that the substitution pattern can be tuned by conditions: short irradiation times with an excess of silane favour predominantly monosubstituted alkoxysilanes, whereas excess ketone and prolonged irradiation shift the distribution toward highly alkoxylated silanes. They further showed that the formed isopropoxysilanes undergo acid-promoted hydrolysis with dilute, non-oxidizing acids to give the corresponding bis-silyl ethers (disiloxanes), in high yields (see Scheme 152, path b).²³⁹

Scheme 152 UV irradiation of trisilane in the presence of acetone leading to different mixed silylethers (path a) and the hydrolysis of formed silylethers with aqueous HCl (path b).

Lobreyer, Oeler, and Sundermeyer (1991) reported two routes towards silyl nonaflate. Reaction of silver nonaflate with SiH₃Cl afforded low yields (see Scheme 153, path a), whereas an alternative approach employing phenylsilane with perfluorobutanesulfonic acid gave significantly improved isolated yields (see Scheme 153, path b). The resulting silyl nonaflate was then used in further functionalization reactions (Scheme 67).⁴⁶

Scheme 153 Formation of silyl nonaflate using silver nonaflate with SiH₃Cl (path a) and perbutanesulfonic acid with phenylsilane (PhSiH₃) (path b).

Cradock et al. generated (TfOSiH₂)₂ in situ by protonolysis of bis(p-tolyl)disilane with triflic acid (TfOH) (see Scheme 154). The disilyl di(triflate) was not isolated; instead, it was used directly for subsequent functionalization (Scheme 86).¹⁵⁸

Scheme 154 Synthesis towards disilyl di(triflate) using (p-tolyl-SiH₂)₂ and triflic acid. Isolated yields not reported.

Silylthiones. Single-source Si–S feedstocks deliver silicon and sulfur simultaneously in a fixed stoichiometric ratio, simplifying process control. They also suppress gas-phase parasitic reactions and reactor memory effects compared with separate silane/H₂S or organosulfide co-flows.¹³⁴ Because Si and S are pre-bonded, these precursors can enable lower-temperature deposition; used examples like bis(trimethylsilyl)sulfide demonstrate processability.²⁴⁰ By contrast, a carbon-free analogue such as disilyl sulfide (S(SiH₃)₂) would, in principle, evolve only hydrogen or hydrogen sulfide as volatiles and minimize residual carbon.

Emeléus, MacDiarmid, and Maddock prepared disilyl sulfide S(SiH₃)₂ by passing SiH₃I vapour three times through a tube packed with HgS. Subsequent reaction of S(SiH₃)₂ with H₂S at room temperature for 31 days furnished SiH₃SH (see Scheme 155).²²⁹

Scheme 155 Synthesis of S(SiH₃)₂ using SiH₃I and HgS, reaction of S(SiH₃)₂ with H₂S forming SiH₃SH.

Downs and Ebsworth prepared F₃CSSiH₃ by passing H₃SiI vapor over Hg(SCF₃)₂ deposited on glass wool. The initially obtained material partially decomposed to give SiH₃F and CSF₂; repeated fractional distillation removed these volatile by products and furnished pure F₃CSSiH₃ (see Scheme 156).²⁴¹

Scheme 156 Reaction between SiH₃I and Hg(SCF₃)₂ forming F₃CSSiH₃.

Sternbach and MacDiarmid synthesized MeSSiH₃ by reacting SiH₃I·NMe₃ with MeSH at 0 °C (see Scheme 157).²⁴²

Scheme 157 Formation of MeSSiH₃ by reacting SiH₃I·NMe₃ with MeSH.

Ward and MacDiarmid prepared S(Si₂H₅)₂ by passing Si₂H₅I vapor over excess HgS at room temperature (see Scheme 158).¹⁵³

Scheme 158 Synthesis of S(Si₂H₅)₂ by the reaction of Si₂H₅I with HgS.

Schmeißer and Frouzanfar attempted to prepare monosubstituted thiosilanes (RS)SiH₃ by LiAlH₄ reduction of RS-SiCl₃, but instead obtained di-, tri-, and tetrasubstituted products. They attributed this outcome to AlCl₃ formed in situ, which promotes disproportionation of thiolato groups. The product distribution depends on the substituent R: with n-alkyl groups, (RS)₃SiH formed together with SiH₄ (see Scheme 159, path a); with a branched alkyl group (i-Bu), (RS)₂SiH₂ and SiH₄ were obtained (see Scheme 159, path b); and with phenyl, a mixture of tri- and tetrasubstituted thiosilanes accompanied by SiH₄ (Scheme 159, path c). Monosubstituted thiosilanes were obtained only when employing lead mercaptides in aliphatic or aromatic solvents, which furnished isolable (RS)SiH₃ (Scheme 159, path d).²⁴³

Scheme 159 Synthesis of mono-, di-, tri- and tetrasubstituted thiosilanes starting either from trichlorosilanes and LiAlH₄ (path a–c), or from SiH₃I and lead mercaptides (path d).

Glidewell and Rankin prepared PhSSiH₃ by salt metathesis of PhSK with SiH₃Br. They also examined the reactivity of PhSK toward boron halides: no reaction was observed with BF₃, whereas BCl₃ afforded a white 1 : 1 adduct at low temperature that dissociated on warming to ambient temperature (see Scheme 160).²³⁶

Scheme 160 Formation of PhSSiH₃ and reaction of PhSSiH₃ with BCl₃.

Glidewell later obtained disilyl sulfide S(SiH₃)₂ in excellent yields by salt metathesis of Li₂S with SiH₃Br (see Scheme 161). Alternatively, treatment of SiH₃Br with Me₃N·H₂S also furnished S(SiH₃)₂.¹⁷¹

Scheme 161 Synthesis of disilylsulfide by Glidewell.

Ebsworth, Glidewell, and Sheldrick monitored the reaction of trisilylphosphine P(SiH₃)₃ with elemental sulfur in CS₂ in an NMR tube over an extended period. After 3 weeks, NMR spectroscopy indicated 16% formation of S(SiH₃)₂; after 2 months, conversion to S(SiH₃)₂ was essentially complete. In contrast, P(SiH₃)₃ showed no detectable reaction with H₂S under analogous conditions (see Scheme 162).²⁴⁴

Scheme 162 Synthesis of disilylsulfide by reacting trissilylphosphane with sulfur.

Drake and Riddle subjected gas mixtures to silent electrical discharge and observed broad product formation. For an equimolar SiH₄/H₂S mixture, in addition to higher silanes, S(SiH₃)₂ and H₃SiSH were detected. Using SiH₄/MeSH, the products included S(SiH₃)₂, H₃SiSMe, Me₂S and Me₂S₂, along with higher silanes. Finally, a SiH₄/GeH₄/H₂S mixture afforded H₃SiSGeH₃, H₃SiSH, H₃GeSH, S(SiH₃)₂, S(GeH₃)₂, higher silanes and germanes, as well as mixed silylgermanes (see Scheme 163).²⁴⁵

Scheme 163 Silent electrical discharge reactions of different SiH₄ mixtures. Yields of individual products were not reported.

Glidewell obtained MeSSiH₃ by reacting LiAl(SMe)₄ with H₃SiBr (see Scheme 164). Treatment of Li₂S with H₃SiBr furnished the expected S(SiH₃)₂, but it could not be isolated, contrary to his previous work were isolation was successful (Scheme 161).^171,238

Scheme 164 Synthesis of silylsulfides starting from alkali sulfides. No yields for S(SiH₃)₂ reported.

Anderson and Drake prepared a range of thiosilanes by using the thiolato-aluminate LiAl(SMe)₄ as nucleophilic RS-source. Treatment of LiAl(SMe)₄ with H₃SiBr or H₅Si₂Br furnished the corresponding MeSSiH₃ and MeSSi₂H₅, respectively (see Scheme 165), providing a convenient route to both silyl- and disilanyl-substituted thiosilanes.²⁴⁶

Scheme 165 Synthesis of silylsulfides using the thiolato-aluminate LiAl(SMe)₄.

Cradock, Ebsworth, and Jessep obtained S(SiH₃)₂ in quantitative yield by treating N(SiH₃)₃ with H₂S, also forming the ammonium salt NH₃·HSSiH₃. The same ammonium salt was also obtained quantitatively by combining HSSiH₃ with NH₃ (see Scheme 166, path a). Using partially substituted amines derived from N(SiH₃)₃, they similarly prepared NMe₂H·HSSiH₃ and NMeH₂·HSSiH₃ (see Scheme 166, path b). They then explored the electrophile scope of NH₃·HSSiH₃. MeI afforded MeSSiH₃, and Me₃SiCl gave Me₃SiSSiH₃, as expected (see Scheme 166, path c). Reaction with H₃GeBr produced H₃GeSSiH₃ together with the follow-up products S(GeH₃)₂ and S(SiH₃)₂ (see Scheme 166, path d). With acetyl chloride, using an excess led predominantly to SiH₃Cl, whereas sub-equimolar MeC(O)Cl furnished MeC(O)SSiH₃, which isomerized in solution to MeC(S)OSiH₃ with an isomer ratio of at least 7 : 1 in favor of MeC(S)OSiH₃ (see Scheme 166, path e).²⁴⁷

Scheme 166 Generation of disilylsulfide and the ammonium salt HSSiH₃·NH₃ and reactivity studies of the ammonium salt (path a–e). Yields of the reactions between the ammonium salt and H₃GeBr and acetylchloride not reported.

Cradock et al. reported that treatment of S(SiH₃)₂ with MeLi liberates MeSiH₃ in approximately 90% of the theoretical amount, forming Li(SSiH₃) (see Scheme 167). A solution of Li(SSiH₃) can then be quenched with Me₃SiCl to give the expected (Me₃Si)S(SiH₃).²⁴⁸

Scheme 167 Generation of Li(SSiH₃) and its reactivity with Me₃SiCl. No yields were reported.

Finch and Van Dyke generated H₃GeSSiH₃ by treating S(SiH₃)₂ with H₃GeCl, but the product disproportionates to S(GeH₃)₂ and S(SiH₃)₂, preventing isolation of pure H₃GeSSiH₃ (see Scheme 168). S(GeH₃)₂ can be removed readily, but H₃GeSSiH₃ and S(SiH₃)₂ have very similar volatilities and could not be separated by distillation.²⁴⁹ This behaviour mirrors the disproportionation reaction observed by Cradock, Ebsworth, and Jessep when NH₃·HSSiH₃ was reacted with GeH₃Br, which likewise furnished H₃GeSSiH₃ together with S(GeH₃)₂ and S(SiH₃)₂ (see Scheme 166, path d).²⁴⁷

Scheme 168 Synthesis of H₃GeSSiH₃ and its disproportionation towards digermyl- and disilylsulfide, no yields were reported.

Cradock et al. published follow-up chemistry on Li(SSiH₃), reacting it with Me₃SiCl yields Me₃SiSSiH₃ (see Scheme 169, path a). Reaction with MeI results in the expected MeSSiH₃ formation; acetylchloride giving the thioester MeC(O)SSiH₃ and with Me₂SiCl₂ the corresponding Me₂Si(SSiH₃)₂ (see Scheme 169, path b and c). They also reported several unsuccessful attempts that did not deliver the targeted S-substituted products (see Scheme 169, path d). Reactions with MeSiCl₃, SiCl₄, PBrF₂, PF₃, HgCl₂, SnCl₂, and BCl₃ mainly produced S(SiH₃)₂, SiH₃F, or SiH₄. Finally, chalcogen exchange experiments showed that reacting Li(SSiH₃) with Se(SiH₃)₂, or alternatively Li(SeSiH₃) with S(SiH₃)₂, converged in both cases to Li(SeSiH₃) and S(SiH₃)₂ (see Scheme 169, path e).²¹⁹

Scheme 169 Reactivity of Li(SSiH₃) with different electrophiles (path a–e). Yields of MeC(O)SSiH₃ and Me₂Si(SSiH₃)₂ were not reported.

Haas and Vongher isolated the cyclic trimer (SSiH₂)₃ in 1978. Their route began with diiodosilane (SiH₂I₂) and HgS at elevated temperature, followed by filtration and solvent removal to give an oligomeric mixture. After leaving the oligomeric mixture exposed to light for about one week, this mixture converted into a gummy-like, polymeric solid. Vacuum depolymerization at 210 °C for 2 h furnished (SSiH₂)₃ (see Scheme 170), which is stable at −80 °C but slowly polymerizes at 20 °C over several days. Thermolysis in the presence of Al₂S₃ showed no change up to 170 °C; above this temperature the trimer decomposes to give mainly H₃SiSH and S(SiH₃)₂, with traces of H₂S.²⁵⁰

Scheme 170 Synthesis of the cyclic (SSiH₂)₃ by reacting HgS with SiH₂I₂.

Haas and Hitze reported new routes to cyclotrisilathiane (SSiH₂)₃ in 1984. When Li(SSiH₃) was combined with SiH₂Cl₂, the expected H₂Si(SSiH₃)₂ was not obtained; instead, its decomposition products S(SiH₃)₂ and (SSiH₂)₃ were formed (see Scheme 171, path a). The analogous reaction with SiH₂I₂ gave similar outcomes. Notably, (SSiH₂)₃ prepared from SiH₂Cl₂ was more stable than material obtained from SiH₂I₂, which the authors attributed to iodide catalyzing polymerization. Alternatively, reacting S(SiH₃)₂ directly with SiH₂Cl₂ furnished cyclotrisilathiane and, in parallel, H₂Si(SSiH₃)₂, which they succeeded in isolating (see Scheme 171, path b). Using the purified H₂Si(SSiH₃)₂, they conducted a thermal stability study and determined its degradation pathway leading to cyclotrisilathiane (see Scheme 171, path c).²⁵¹

Scheme 171 Synthesis of the cyclotrisilathiane via different routes (path a–c). Yields for H₂ClSiSSiH₃ were not reported.

Haas, Süllentrup, and Krüger generated the new six-membered cyclic disilathiane (SSi₂H₄)₂ by two routes. By combining (H₂SiCl)₂ with S(SiH₃)₂ (see Scheme 172, path a), or by reacting Li(SSiH₃) with one equivalent of (ClSiH₂)₂, wherein two molecules of the intermediate (ClH₅Si₂)S(SiH₃) undergo self-condensation with elimination of SiH₃Cl to close the ring (see Scheme 172, path b).²⁵²

Scheme 172 Two pathways towards the cyclic disilathiane (SSi₂H₄)₂ by Haas, Süllentrup and Krüger (path a and b).

A thermolysis study at 90 °C for 1 h showed that, alongside the recovered ring, S(Si₂H₅)₂ is the principal degradation product (Scheme 173, path a). They also accessed linear disilathianes of the type (H₃Si)_nS(Si₂H₅)_2−n (n = 0, 1) by treating S(SiH₃)₂ with two or one equivalent(s) of Si₂H₅Cl, respectively (Scheme 173, path b and c).²⁵²

Scheme 173 Thermolysis of disilathiane (SSi₂H₄)₂ (path a) and generation of the unsymmetrical silylsulfide (path b) and disilylsulfide (path c). No yields reported for H₃SiS(Si₂H₅).

Silylselenides. Relative to oxygen and sulfur, selenium has a larger covalent radius, higher electronic polarizability, and greater metallic character, yielding softer Si–Se bonds, greater structural compliance, and stronger dielectric screening in Si-based semiconductors. Consequently, Se enables tunable band gaps and defect energetics, higher carrier mobilities with reduced interface-state densities in passivation layers, and superior electronic conductivity for conductive interlayers. Its higher atomic mass also enhances spin–orbit coupling, providing an additional lever for band-structure engineering.²⁵³

In 1955, Emeléus, MacDiarmid, and Maddock attempted to synthesize Se(SiH₃)₂ by reacting SiH₃I with either Se or HgSe over extended periods, but both approaches failed. By contrast, using Ag₂Se led to rapid formation of Se(SiH₃)₂ within about 30 minutes (see Scheme 174).²²⁹

Scheme 174 Synthesis of disilylselenide using Ag₂Se and SiH₃I.

Ebsworth, Emeléus, and Welcman prepared perfluoroalkyl silyl selenides RSeSiH₃ (R = CF₃, C₃F₇) by reacting H₃SiI with Hg(SeR)₂ (see Scheme 175).²⁵⁴

Scheme 175 Synthesis of mixed silylselenides.

Contrary to the H₂S case, Ebsworth, Glidewell, and Sheldrick reported that P(SiH₃)₃ reacts with H₂Se to furnish the expected Se(SiH₃)₂ (see Scheme 176).²⁴⁴

Scheme 176 Generation of disilylselenide through the reaction of trisilylphosphide and H₂Se.

Cradock and Ebsworth obtained Se(SiH₃)₂ in quantitative yield by reacting Li₂Se with H₃SiBr (see Scheme 177), providing a straightforward halide/selenide metathesis.²⁵⁵

Scheme 177 Synthesis of disilylselenide using lithiumselenide and SiH₃Br.

Drake and Riddle applied silent electrical discharge to equimolar SiH₄/H₂Se and observed, in addition to higher silanes, the formation of Se(SiH₃)₂ and H₃SiSeH (see Scheme 178). With ternary SiH₄/GeH₄/H₂Se mixtures, the product spectrum expanded to include H₃SiSeGeH₃, H₃ESeH and Se(EH₃)₂ (E = Si, Ge), along with higher silanes, higher germanes, and silylgermanes.²⁴⁵

Scheme 178 Silent electrical of SiH₄/H₂Se and SiH₄/GeH₄/H₂Se mixtures. Yields of individual products were not reported.

Anderson and Drake synthesized MeSeSiH₃ by combining H₃SiBr with LiAl(SeMe)₄ (see Scheme 179), directly analogous to the sulfur congener prepared from LiAl(SMe)₄ (Scheme 165).²⁴⁶

Scheme 179 Synthesis of the mixed MeSeSiH₃ selenide using LiAl(SeMe)₄ and SiH₃Br.

Cradock, Ebsworth, and Jessep showed that N(SiH₃)₃ reacts with H₂Se to give Se(SiH₃)₂ together with the ammonium selenosilane NH₃·HSeSiH₃ (see Scheme 180, path a). Replacing NH₃ with partially substituted amines such as HNMeSiH₃ or Me₂NSiH₃ resulted in the formation of the corresponding salts HNRR′·HSeSiH₃ (R, R′ = H, Me) (see Scheme 180, path b). The NH₃·HSeSiH₃ adduct was examined in detail: methylation with MeI delivered MeSeSiH₃; treatment with Me₃SiCl afforded Me₃SiSeSiH₃, which slowly disproportionated at room temperature to (Me₃Si)₂Se and Se(SiH₃)₂ (see Scheme 180, path c); and exposure to PF₂Br produced a complex mixture containing PF₃, SiH₃F, unreacted PF₂Br, H₂Se, Se(SiH₃)₂, and SiH₃·NH·PF₂. Finally, reaction with acetyl chloride yielded a mixture of Se(SiH₃)₂, diacetyl selenide (Se[(O)CMe]₂), and the desired silylselenoacetate MeC(O)SeSiH₃, which could not be isolated; nonetheless, the isomer ratio at room temperature was determined to be approximately 2.5 : 1, with MeC(O)SeSiH₃ as the major isomer (see Scheme 180, path d).²⁴⁷

Scheme 180 Generation of disilylselenide and the ammonium salt HSeSiH₃·NH₃ and reactivity studies of the ammonium salt (path a–d). Yields of the reaction between the ammonium salt and Me₃SiCl and acetylchloride not reported.

Cradock et al. reported that treating Se(SiH₃)₂ with MeLi evolves methylsilane (MeSiH₃) in approximately 90% of the theoretical amount and forms Li(SeSiH₃). The resulting solution of Li(SeSiH₃) can then be quenched with Me₃SiCl to afford the expected Me₃SiSeSiH₃ (see Scheme 181).²⁴⁸

Scheme 181 Generation of Li(SeSiH₃) by the reaction of disilylselenide and MeLi and its reaction with Me₃SiCl. Yields of products were not determined.

Barker, Drake, and Hemmings showed that LiAl(SeMe)₄ reacts with H₃SiBr to give H₃SiSeMe and with H₂SiBr₂ to give H₂Si(SeMe)₂; both products were successfully isolated, and in each case only traces of SiH₄ were detected as a byproduct (see Scheme 182).²⁵⁶

Scheme 182 Generation of mixed silylselenides using LiAl(SeMe)₄.

Cradock et al. showed that Li(SeSiH₃) reacts cleanly with Me₃SiCl to furnish Me₃SiSeSiH₃ (see Scheme 183, path a). If NMe₃ is present prior to quenching, the Lewis adduct Me₃SiSeSiH₃·NMe₃ is formed, subsequent treatment with BF₃ releases the free selenide (with formation of NMe₃·BF₃) (see Scheme 183, path b). Acylation with acetyl chloride affords MeC(O)SeSiH₃, which isomerizes in solution and exists in equilibrium with MeC(Se)OSiH₃ (see Scheme 183, path c). Extending this electrophile scope gave divergent outcomes. With MeC(O)₂O or MeCO₂H, the main product is again the acetyl silyl selenide MeC(O)SeSiH₃. In contrast, CF₃CO₂H reacts mainly to the silyl trifluoroacetate CF₃C(O)OSiH₃, while the acyl halides CF₃C(O)Br and CClH₂C(O)Cl lead predominantly to Se(SiH₃)₂. No reaction is observed with NMe₃ alone under the same conditions (see Scheme 183, path d).²¹⁹

Scheme 183 Reactivity scope of Li(SeSiH₃) (path a–d).

Drake and Hemmings synthesized phenylselenosilanes by two complementary routes: reaction of Li(SePh) with H₃SiBr to give PhSeSiH₃, and reaction of the selenoaluminate LiAl(SePh)₄ with H₂SiBr₂ to furnish H₂Si(SePh)₂ (see Scheme 184).²⁵⁷

Scheme 184 Generation of phenylselenosilanes.

Drake, Glavinčevski, and Hemmings prepared Se(SiH₃)₂ by reacting H₃SiI with the selenoaluminate LiAl(SeH)₄, which serves as an efficient selenide-transfer reagent (see Scheme 185).²⁵⁸

Scheme 185 Synthesis of disilylselenide using selenoaluminate and SiH₃I.

Haas and Hitze were the first to obtain the cyclic selenosilane (SeSiH₂)₃ by heating HgSe with H₂SiI₂ to 120 °C. The initial reaction produced oligomeric mixtures, which upon depolymerization under vacuum at 140 °C, followed by recrystallization in benzene furnished crystalline (SeSiH₂)₃ (see Scheme 186, path a). The crystals are unstable at 20 °C and revert to an oligomeric mixture within 1–2 hours. Attempts to reach the same product via Na₂Se and H₂SiI₂ in benzene at 20 °C led to decomposition into iodine and selenium; performing the reaction in ether gave the same outcome, with decomposition occurring above −40 °C. Reactions between SiH₂I₂ and K₆[HgSe₄] behaved similarly (see Scheme 186, path b). When Ag₂Se was combined with H₂SiI₂, no reaction was observed at 20 °C, and upon refluxing the mixture, SiH₃I and Se(SiH₃)₂ were obtained instead of (SeSiH₂)₃ (see Scheme 186, path c).²⁵⁹

Scheme 186 Attempted reactions towards the cyclic selenosilane (SeSiH₂)₃ and its successful generation using HgSe and SiH₂I₂ (path a–c).

Haas, Süllentrup, and Krüger generated the new cyclic compound (SeSi₂H₄)₂ by reacting (SiH₂Cl)₂ with Se(SiH₃)₂, with elimination of H₃SiCl (see Scheme 187, path a). Thermolysis of this ring at 90 °C for 1 h gave Se(Si₂H₅)₂ as the principal degradation product together with Si₂H₆ (see Scheme 187, path b). They also obtained the new linear disilaselenides (H₃Si)_nSe(Si₂H₅)_2−n (n = 0, 1) by reacting Se(SiH₃)₂ with one or two equivalents of Si₂H₅Cl (see Scheme 187, path c and d).²⁵²

Scheme 187 Synthesis of the cyclic disilylselenide (SeSi₂H₄)₂ via different pathways (path a) and generation of mixed and symmetrical disilylselenides (path b–d).

Silyltelluride. Relative to O, S, and Se, tellurium has the largest covalent radius, highest electronic polarizability, and greatest metallic character in group 16, yielding the softest Si–Te bonds and enhanced lattice compliance in Si-based semiconductors. These attributes enable deeper band-structure modulation, tuning band gaps or sub-bandgap states into the near-IR, and can produce higher electronic conductivity in Te-rich chalcogenide passivation or interlayer stacks.²⁶⁰

Bürger and Goetze were the first in 1967 to synthesize Te(SiH₃)₂ by reacting Li₂Te with SiH₃I in tetralin and heating the mixture at 70 °C for 3 days (see Scheme 188).²⁶¹

Scheme 188 Synthesis of Te(SiH₃)₂ using TeLi₂ and SiH₃I in tetralin.

Two years later, Cradock, Ebsworth, and Rankin obtained Te(SiH₃)₂ by reacting SiH₃Br with Li₂Te in Me₂O at low temperature (see Scheme 189).²⁵⁵

Scheme 189 Synthesis of Te(SiH₃)₂ using TeLi₂ and SiH₃Br.

Anderson and Drake prepared MeTeSiH₃ by reacting the telluroaluminate LiAl(TeMe)₄ with SiH₃Br, but the product was obtained only in low yield (see Scheme 190).²⁴⁶

Scheme 190 Synthesis of the mixed MeTeSiH₃ using LiAl(TeMe)₄ and SiH₃Br.

Drake and Hemmings attempted to prepare RTeSiH₃ (R = Me, Ph) by reacting LiTeR with SiH₃Br, but they were unable to isolate the desired products; instead, they observed Te(SiH₃)₂ and traces of silane among the volatile species alongside intractable polymeric material. In contrast, an exchange route employing RTeMe₃Si (R = Me, Ph) with SiH₃Br or SiH₃I, eliminating Me₃SiBr or Me₃SiI, results in a successful RTeSiH₃ formation (see Scheme 191, path a). They also found that MeTeSiH₃ decomposes after prolonged storage or upon laser excitation to give H₂Si(TeMe)₂ and SiH₄ (see Scheme 191, path b).²⁶²

Scheme 191 Attempted and successful reaction towards RTeSiH₃ (path a) and decomposition products of MeTeSiH₃ (path b).

Drake, Glavinčevski, and Hemmings later synthesized Te(SiH₃)₂ by reacting SiH₃I with an excess of Li₂Te at low temperatures in Me₂O (see Scheme 192).²⁵⁸

Scheme 192 Generation of disilyltelluride using SiH₃I and TeLi₂ in ether.

Functionalization with group 7 (synthesis of silylhalogenes)

Halohydrosilanes represent a central class of silicon compounds in which Si–X bonds (X = F, Cl, Br, I) replace one or more Si–H or Si–Si functionalities. The strong polarity and thermodynamic stability of Si–halogen bonds significantly influence their chemical behaviour: halohydrosilanes are highly sensitive to hydrolysis, readily undergo redistribution reactions, and frequently serve as activated intermediates in silicon bond construction. From a synthetic perspective, they occupy a strategic position between elemental silicon and hydrosilanes, acting both as precursors to reduce silicon species and electrophilic building blocks for tailored silicon frameworks.

Industrial and technological interest in halohydrosilanes is driven largely by thin-film deposition processes. In chemical vapor deposition and liquid phase deposition or solution-based routes, halohydrosilanes provide a controlled silicon delivery, variable reactivity, and favourable volatility. Chlorinated and fluorinated silanes, in particular, are widely exploited for semiconductor fabrication, surface passivation, and the formation of silicon-containing coatings. Their decomposition pathways allow precise regulation of film growth kinetics, impurity profiles, and microstructure – critical parameters for semiconductor devices, photovoltaics, and protective layers. Furthermore, halohydrosilanes serve as key intermediates in the preparation of higher hydrosilanes and functional silicon oligomers, linking fundamental synthesis with applied materials chemistry.²⁶³

Synthetic access to halohydrosilanes generally follows several recurring strategies (see Scheme 193).

Scheme 193 Method (i)–(v) for the formation of different halohydrosilanes.

Structurally, halohydrosilanes can be devided into acyclic perhalogenated, cyclic compounds and mixed derivatives. The following sections summarize the principal compound classes and their synthetic approaches, organized according to the halogen substituent – beginning with fluorohydrosilanes and proceeding through chlorosilanes and bromohydrosilanes to iodohydrosilanes – reflecting their sequence in the periodic table.

Fluorohydrosilanes. Intensive investigations towards the synthesis, properties and reactivity of fluorohydrosilanes began in the early 1970s. Particular attention has been devoted to disilane derivatives because of their relevance to semiconductor processing, whereas comparatively fewer studies have addressed the preparation of higher fluorinated silanes.²⁶⁴

Monofluorosilane (H₃SiF) has been prepared primarily through electrophilic fluorination or Lewis-acid-induced Si–X bond cleavage reactions. Early work by Emeléus and Maddock demonstrated that H₃SiF can be obtained in approximately 79% yield by fluorination of chlorosilane with antimony trifluoride at room temperature (see Scheme 194, path a), although competing oxidative fluorination and disproportionation limited product stability and purity.²⁶⁵ A significant cleaner and higher-yielding approach was later introduced by Onyszchuk, who showed that boron trifluoride induces efficient Si–O bond cleavage in disiloxane, affording H₃SiF in up to 85% isolated yield under solvent-free conditions (see Scheme 194, path b).²³³ Related studies by Sternbach and MacDiarmid further established that boron trifluoride readily converts alkoxysilanes into H₃SiF, confirming the general applicability of BF₃-mediated fluorination pathways, although this route was not optimized for preparative yields (see Scheme 194, path c).²³² In addition, Ebsworth and Mays reported the formation of H₃SiF from the reaction of silyl isocyanate with boron trifluoride at −78 °C, an observation made in the course of probing the reactivity of silylisocyanate rather than as dedicated synthetic approach to H₃SiF (see Scheme 194, path d).¹⁷² Treatment of PhOSiH₃ with HF affords H₃SiF in excellent yields (see Scheme 194, path e).²³⁶ The most common method for the synthesis of fluorohydrooligosilanes is the co-condensation of SiF₂ with B₂H₆ at a cool copper surface (method iv). The reactive SiF₂ intermediate is generated in situ by passing SiF₄ over elemental silicon at high temperatures. Under these conditions, a mixture of F₃Si–SiH₃, F₃Si–SiF₂H and H₂Si–(SiF₃)₂ is formed (see Scheme 195). Although isolated yields were not determined, spectroscopic analyses indicated that H₂Si–(SiF₃)₂ is formed in relatively higher amounts, while F₃Si–SiH₃ and F₃Si–SiF₂H are produced in comparable portions. This species were separated by high vacuum, low temperature fractional condensation.²⁶⁶

Scheme 194 Methods for the synthesis of monofluorosilane (path a–e). For the reactions of H₃SiOMe + BF₃ (path c) and SiH₃·NCO + BF₃ (path d) no yields reported.

Scheme 195 Formation of different fluorohydrosilanes from Si₂F with B₂H₆; no yields reported.

F₃Si–SiH₃ is also formed as a byproduct during the synthesis of SiF₂HPH₂ and SiF₃PH₂ via the condensation of SiF₂ with PH₃ (method iv) (see Scheme 196).²⁶⁷

Scheme 196 Formation of F₃Si–SiH₃ as a byproduct during the condensation of SiF₂ and PH₃; no yields reported.

Reactions of SiF₂ with protic reagents (such as HBr) typically generate initially formed fluorinated bromodisilanes that are thermally unstable and undergo rapid secondary transformation. Scheme 197 shows that the reaction with HBr first produces 1-bromo-1,1,2,2-tetrafluorodisilane, which decomposes quickly to a mixture of different mixed halohydrosilanes. Subsequent treatment of the resulting mixture with excess SbF₃ converts it efficiently into F₃Si–SiF₂H.²⁶⁸

Scheme 197 Treatment of SiF₂ with HBr yielding in the unstable BrF₂Si–SiF₂H. Subsequent treatment of decomposition products with SbF₃ affording F₃Si–SiF₂H. No yields reported.

A similar pattern is observed with H₂S: the initially formed silanethiol F₂HSi–SiF₂SH decomposes within minutes, again yielding F₃Si–SiF₂H (see Scheme 198).²⁶⁹

Scheme 198 Reaction of SiF₂ with H₂S. Decomposition at room temperature affords F₃Si–SiF₂H. No yields reported.

Direct fluorination routes of hydrosilanes (method i) have not been reported, but indirect access is possible through selective hydrogenation of mixed halodisilanes (method v). In compounds containing both, fluorine and chlorine or bromine substituents, the heavier halides can be selectively reduced with Me₃SnH without cleaving Si–F bonds. Using this strategy, F₃Si–SiH₃ can be obtained in high yields from the corresponding mixed halo precursors (see Scheme 199).²⁷⁰

Scheme 199 Hydrogenation of mixed halodisilane yielding in F₃Si–SiH₃.

In contrast, partially fluorinated bromodisilanes do not undergo clean hydrogenation. Instead, F/Br redistribution occurs prior to reduction, producing mixtures of fluorinated disilanes. Stronger hydride reagent led to even less selective outcomes, promoting Si–Si bond cleavage and partial reduction of Si–F bonds. Together, these observations highlight the delicate balance between halogen redistribution, bond stability and reductive pathways in fluorinated disilane chemistry.²⁷⁰

FH₂Si–SiH₃ was obtained by fluorination of dichlorodisilanes or dichlorotrisilane with SbF₃ (method iii) (see Scheme 200).^271,272

Scheme 200 Fluorination of oligohydrosilanes with SbF₃.

ZnF₂ is another well-established fluorinating agent and has been used to prepare F₂HSi–SiHF₂ and FH₂Si–(SiH₂)₂–SiH₂F from their corresponding chlorinated or brominated precursors (method iii) (see Scheme 201).^273,274

Scheme 201 Fluorination of chloro-, and bromooligohydrosilanes with ZnF₂.

In addition, cleavage of Si–N bonds in N(Si₂H₅)₃ by BF₃ provides another pathway to FH₂Si–SiH₃ (see Scheme 202).¹⁵⁰

Scheme 202 Formation of FH₂Si–SiH₃ via the reaction of N(H₅Si₂)₃ and BF₃.

Moreover, it is possible to fluorinate different methoxyoligosilanes to the corresponding branched fluorohydrosilanes with BF₃ (see Scheme 203).²⁷⁵

Scheme 203 Fluorination of branched methoxysilanes with BF₃.

Finally, fluorohydrosilanes can be accessed via an electrical discharge approach. In this method, various monofluorosilanes were converted into F₂HSi–SiH₃, FH₂Si–SiH₂F, F₂HSi–SiHF₂, F₃Si–SiF₂H, F₂HSi–Si₂H₅ and F₃Si–Si₂H₅. However, the resulting product mixtures proved difficult to impossible to separate.^271,276

Chlorohydrosilanes. Chlorosilanes, particularly chlorohydrosilanes, serve as key precursors in the production of high-purity polycrystalline silicon. Driven by the rapid expansion of photovoltaic technologies, global polysilicon production capacity reached approximately 1.6 million metric tons in 2023.²⁷⁷ The overwhelming majority of this material is manufactured via chemical vapour deposition (CVD) using trichlorosilane as primary feedstock.²⁷⁸

Already in 1919 Stock and Somieski reported the synthesis of monochlorosilane (H₃SiCl).¹⁰ They treated monosilane with hydrogen chloride and catalytic amounts of AlCl₃ (method i) (see Scheme 204). Heating this mixture to 100 °C for 30 hours afforded ClSiH₃ in 50–55% yield, with dichlorosilane as byproduct (ratio ClSiH₃ : Cl₂SiH₂ = 4 : 1). More recent studies showed, that the yields of ClSiH₃ can be increased to 76% when a zeolite catalysator (Na-ZSM-5) is used.²⁷⁹

Scheme 204 Methods for the synthesis of monochlorosilane (path a–d).

Hollandsworth et al. showed, that monosilane can also be chlorinated with silver chloride to afford monochlorosilane (see Scheme 204).²⁸⁰ When phenylsilane (PhSiH₃) is used as starting material, H₃SiCl could be obtained in yields of 78% (method ii) (see Scheme 204, path c).²⁸¹ In addition, Glidewell and Rankin showed, that treatment of PhOSiH₃ with HCl affords H₃SiCl (see Scheme 204, path d).²³⁶

Starting in the 1960s, synthetic efforts towards chlorooligohydrosilanes increasingly focused on the chlorination (method i) of smaller hydrosilanes such as disilane, trisilane and tetrasilane.

Early studies by Drake et al. and van Dyke et al. established boron trichloride as an efficient chlorination reagent for these substrates (see Scheme 205, path a).^{271,276,282,283} In analogy to observations made of monosilane, hydrogen chloride²⁸⁴ and silverchloride^285,286 were subsequently shown to be suitable chlorinations agents for disilane as well (see Scheme 205, path b and c). Beyond these systems, tetrachlorostannane was demonstrated to chlorinate di-, tri-, and tetrasilane efficiently, thereby broadening the scope of applicable chlorination reagents (see Scheme 205, path d).²⁸⁷ Finally, direct halogenation using molecular chlorine was reported by Fehér and co-workers, providing a more straightforward, albeit harsher, approach to the chlorination of disilane (see Scheme 205, path e).²⁸⁸

Scheme 205 Chlorination of oligohydrosilanes using different chlorination agents (path a–e).

However, all of the mentioned methods for direct halogenation of oligohydrosilanes yield complex product mixtures that often require time consuming separation or further derivatization before reliable analysis is possible.^{271,276,284,286–289} Another challenge is the tendency of higher silanes to undergo isomerization.²⁸³

Van Dyke, and MacDiarmid prepared the silylethers H₃SiOSi₂H₅ and H₃SiOSi₂H₅ (see Scheme 145). In subsequent reactions, both O(Si₂H₅)₂ and H₃SiOSi₂H₅ reacted with BCl₃ to give Si₂H₅Cl as the main product (see Scheme 206).²³⁴

Scheme 206 Reaction of silylethers with BCl₃.

In 2012 Stueger et al. demonstrated the selective chlorination of neopentasilane with 3.5 equivalents SnCl₄ to afford tetra(chlorosilyl)silane (see Scheme 207 and Fig. 15).²⁹⁰ However, a significant drawback of employing SnCl₄ as halogenating reagent is the formation of large quantities of SnCl₂, which are difficult to remove completely, particularly on a preparative scale. To address limitation, the authors also examined an alternative protocol based on gaseous HCl in presence of catalytic AlCl₃ and obtained the selective formation of tetra(chlorosilyl)silane (85%), accompanied only by minor amounts of 1,2,3-trichloroneopentasilane as byproduct (15%) in a total yield of more than 60%.²⁹¹

Scheme 207 Synthesis of branched chlorooligohydrosilanes.

Fig. 15 Crystal structure of Si(SiH₂Cl)₄ reproduced from Stueger et al.⁴⁸ with permission from American Chemical Society, © 2012.

An additional example for the synthesis of branched chlorohydrosilanes, is the preparation of HSi(SiCl₃)₃. Höfler et al. reported two efficient synthetic approaches to this compound. The first involves the reaction of dodecachloroneopentasilane (Si(SiCl₃)₄) with HCl, eliminating SiCl₄, affording the target product in 70% yield (see Scheme 208). The second approach is based on the chlorination of HSi[Si(OMe)₃]₃ with nine equivalents of BCl₃, providing HSi(SiCl₃)₃.²⁹²

Scheme 208 Two methods for the synthesis of hexachlorotrisilane.

The synthesis of an even higher branched chlorooligohydrosilane was demonstrated by Christopoulos et al. First, they generated phenylnonasilane via the reaction of the trissilyllithium anion and 0.5 equivalents dichlorophenylsilane. Subsequent treatment with triflic acid affords the triflate substited nonasilane, which can be chlorinated with an excess of lithiumchloride and the chlorononasilane can be obtained in yields of 81% (see Scheme 209).⁴⁹

Scheme 209 Synthesis of chlorononasilane starting from neopentasilane.

An alternative method for the synthesis of chlorohydrosilanes is the partial hydration of perchlorooligosilanes using hydrating reagents, such as LiAlH₄ or Bu₃SnH (method v). But also these approach were found to yield in unselective product mixtures.^107,109

A more attractive strategy involves the controlled partial hydrogenation (method v) with substoichiometric amounts of i-Bu₂AlH (see Scheme 210). This methodology enables access to broader range of chlorohydrooligosilanes under comparatively mild conditions. i-Bu₂AlH promotes selective Si–Cl hydrogenation in both linear and branched chlorooligosilanes without inducing Si–Si bond cleavage.²⁹¹

Scheme 210 Synthesis of chlorooligosilanes via partial hydrogenation.

An additional method to prevent the complete hydrogenation of oligochlorosilanes with LiAlH₄ is the implementation of N(SiMe₃)₂ or N(SiMe₂Ph)₂ substituents as protecting groups into the molecule (see Scheme 211). After the hydrogenation, these groups can easily be cleaved off with HCl.¹³⁹

Scheme 211 Synthesis of chlorodisilanes using N(SiMe₃)₂ (path a) and N(SiMe₂Ph)₂ (path b) as protecting groups. No reported yields of ClH₂Si–SiH₂Cl.

In contrast to hydrosilanes, aryl-substituted silanes (Ar_mSi_nH_2n+2−m) are excellent starting materials for the selective synthesis of chlorohydrosilanes (Cl_mSi_nH_2n+2−m, n = 2–7, m = 1–9) (Scheme 212). The aryl substituents can be cleanly replaced by chlorine upon treatment with liquefied hydrogen chloride or with HCl solutions in benzene, enabling controlled halogen incorporation with good selectivity (method ii).^{252,273,274,293,294}

Scheme 212 Synthesis of chlorooligosilanes via dearylation.

To suppress equilibrium of the chlorinated products,²⁹⁵ Uhlig et al. proposed a stepwise dephenylation strategy in which triflic acid is first used to remove the organic substituents, followed by either hydride reduction with LiAlH₄ or chlorination with Et₃NHCl (see Scheme 213). Using Ph₃Si–SiH₃ as precursor, this route enables the selective formation of HCl₂Si–SiH₃ and Cl₃Si–SiH₃, respectively.²⁹⁶

Scheme 213 Multi-step synthesis of chlorodisilanes by Uhlig et al.

An analogous electrical discharge strategy for the formation of fluorohydrosilanes, has also been applied for chlorohydrosilanes. Under comparable conditions, monochlorosilane undergoes coupling reactions to give mixtures of chloro-substituted di- and oligosilanes. Similar to the fluorinated systems, these discharge processes produce complex product distributions, and separation of the individual components is challenging.²⁷⁶

Bromohydrosilanes. Also bromohydrosilanes have attracted high interest as alternative silicon precursors for vapor-phase deposition processes, particularly in efforts to move beyond the well-established chlorosilane chemistry used in large-scale silicon production.²⁹⁷

Monobromosilane can be synthesized by reacting monosilane with various bromination agents, including HBr¹¹⁷ (see Scheme 214, path a), SnBr₄ ²⁹⁸ (see Scheme 214, path b), and AgBr²⁸⁰ (see Scheme 214, path c) (method i). A more efficient approach involves the conversion of phenylsilane with hydrogen bromide and catalytic amounts of AlBr₃ (see Scheme 214, path d) (method ii).^281,299

Scheme 214 Different methods for the synthesis of BrSiH₃ (path a–d). No reported yields for the bromination of SiH₄ with HBr (path a).

As already shown in the section Siloxanes, H₃SiBr is also formed at the reaction of O(SiH₃)₂ with PBr₃ (see Scheme 146, path a).

Similar to the preparation of chlorooligohydrogermanes, different bromooligohydrosilanes can be generated through the bromination of oligohydrosilanes (method i) using reagents like BBr₃ ^271,272 (see Scheme 215, path a) HBr^117,286 (see Scheme 215, path b), SnBr₄ ²⁹⁸ (see Scheme 215, path c), AgBr²⁸⁰ and Br₂ ^288,300 (see Scheme 215, path d). These transformations typically proceed through non-selective halogen exchange leading to complex mixtures of mono- and oligobrominated silanes rather than single, well-defined products.

Scheme 215 Methods for the synthesis of bromooligosilanes (path a–d). *Yield was not isolated. **Yield of the product mixture.

Also similar to the synthesis of chlorohydrosilanes, is the possibility of electrophilic cleavage of silicon–aryl bonds with HBr (method ii) (see Scheme 216).^274,294,301

Scheme 216 Synthesis of bromooligosilanes via dearylation.

Iodohydrosilanes. Iodohydrosilanes have attracted increasing interest as highly reactive silicon precursors for CVD and LPD processes, owing to the weak Si–I bond, which enables low-temperature decomposition and efficient surface activation. Early synthetic and reactivity studies demonstrated that iodosilanes exhibit significantly higher reactivity than their chloro- and bromo-analogues, making them particularly attractive for low-energy silicon deposition.¹⁶⁷ More recent work has highlighted their potential as single-source precursors, although their practical use remains limited by thermal instability and challenging handling properties.³⁰²

Maddock and co-workers first reported the preparation of monoiodosilane in 1939, describing its formation via the reaction of monosilane and hydrogen iodide in the presence of catalytic amounts of AlI₃ (method i) (Scheme 217, path a).^167,303 An alternative efficient method towards monoiodosilane employs phenylsilane as the starting material (method ii) (Scheme 217, path b).^281,304 Ward et al. further demonstrated that H₃SiI can be generated via the chlorination of phenylchlorosilane and subsequent hydration to form the chlorophenylsilane, which upon treatment with hydrogen iodide afforded monoiodosilane in yields of up to 65% (Scheme 217, path c).²⁹⁹ Across a range of silyl ethers, treatment with HI afforded H₃SiI in quantitative yield and the corresponding alcohol (ROH) as byproduct (Scheme 217, path d).²³⁸ It is also possible to convert PhOSiH₃ to H₃SiI by treatment with HI (Scheme 217, path e).²³⁶

Scheme 217 Methods for the synthesis of ISiH₃ (path a–e).

In 1960, Ward and MacDiamid extended these studies to iodohydrooligosilanes.²³¹ In their work, disilane was reacted with hydrogen iodide in the presence of catalytic amounts aluminium iodide, leading to the formation of iododisilane IH₂Si–SiH₃ (method i) (see Scheme 218).

Scheme 218 Iodation of disilane with HI yielding in IH₂Si–SiH₃.

Approximately a decade later Fehér et al. carried out intense studies to the synthesis of different iodooligohydrosilanes following an alternative approach applied to di-, tri- and tetrasilane, in which iodide serves as iodation reagent (see Scheme 219).^305,306 These reactions consistently produced mixtures of mono- and diiodinated silanes. Nevertheless, individual isomers of iodinated di-, tri-, and tetrasilanes could be separated and identified by gas chromatographic methods.

Scheme 219 Reaction of di-, tri- and n-tetrasilane with iodide. *Yield of the monoiodotetrasilane was not reported.

Nearly twenty years afterward, Hassler and co-workers reported significant progress in the synthesis of iodooligohydrosilanes. Analogous to strategies established for chloro- and bromohydrosilanes, their work demonstrated that electrophilic cleavage of silicon-aryl bonds with hydrogen iodine enables the selective introduction of iodine substituents (see Scheme 220). Using this approach, a broad range of iodohydrosilans was prepared in high efficiency.^294,307

Scheme 220 Synthesis of iodooligosilanes via dearylation.

Cyclic halohydrosilanes

Nonahalocylcopentasilanes (HSi₅X₉) and octahalocyclo-pentasilanes (1,3-H₂Si₅X₈) (X = Cl, Br, I) can be prepared via dephenylation of the corresponding phenyl-substituted precursors HSi₅Ph₉ and H₂SiPh₈ (see Scheme 221).^35,308

Scheme 221 Synthesis of cyclic halohydrosilanes.

Partially chlorinated cyclopentasilanes were reported by Roewer and co-workers through the stepwise hydride reduction of decachlorocyclopentasilane using Me₃SnH (see Scheme 222). The reaction proceeds without detectable formation of SiHCl moieties. The major product observed was assigned to 1,1,3,3,-tetrachlorocyclopentasilane, while 1,1-dihydrooctachlorocyclopentasilane was not detected. Structural assignments were based on ²⁹Si NMR spectroscopic data; the intermediates were not isolated.¹⁰⁷

Scheme 222 Synthesis of cyclic chlorohydrosilanes.

Stueger et al. reported the partial hydrogenation of perchlorinated cyclopentasilane in 2012 in a three-step synthesis (see Scheme 223).²⁹⁰ The final yield of the product (Si₅HCl₉) could not be determined since it could not be separated from cyclopentasilane which was formed as byproduct.

Scheme 223 Synthesis of nonachlorocyclopentasilane by Stueger et al. No yield of the final product reported.

Mixed halohydrosilanes

The bromination of Si–H bonds using BBr₃ was employed to generate chlorobromohydrosilanes. Thus, ClH₂Si–SiH₃ and Cl₂HSi–SiH₃ were converted into HBrClSi–SiH₃ and BrCl₂Si–SiH₃, respectively (see Scheme 224). These products were detected by GC; however, no isolation was reported.²⁷¹

Scheme 224 Synthesis of mixed halodisilanes; no yields reported.

Mixed halohydrosilanes of the general formula Si₂X_mY_nH_6−m−n (X = F, Y = Br) are formed via co-condensation of SiF₂ with HBr. Upon warming the reaction mixture to room temperature, its composition changes, although the mixed halohydrosilanes BrF₂Si–SiF₂H, F₃Si–SiFHBr and F₃Si–SiHBr₂ could be isolated but no yields were reported (see Scheme 225). The compounds HBrFSi–SiF₂Br and HBr₂Si–SiF₂Br were identified in the final reaction mixture by ¹⁹F NMR spectroscopy, but were not isolated. Subsequent chlorination of BrF₂Si–SiF₂H and F₃Si–SiHBr₂ with SnCl₄ afforded F₃Si–SiHCl₂ and ClF₂Si–SiHF₂. Yields of the isolated products have not been reported.^270,309

Scheme 225 Synthesis of mixed halodisilanes; no yields reported.

Key physical and chemical properties for the central hydrosilanes

In Table 8 the key physical and chemical properties for the central hydrosilanes are depicted.

Table 8 Summary of key physical and chemical properties of central hydrosilanes

Silane	m.p. [°C]	b.p. [°C]	Physical/state	Solubility	Safety
SiH4	−185	−112	Gas	Slightly in all organic and inorganic solvents	Pyrophoric
Si2H6	−133	−15	Gas	All organic and inorganic solvents	Pyrophoric
Si3H8	−115	53	Liquid	All organic and inorganic solvents	Pyrophoric
n-Si4H10	−89.9	108	Liquid	All organic and inorganic solvents	Pyrophoric
iso-Si4H10	−99.4	102	Liquid	All organic and inorganic solvents	Pyrophoric
n-Si5H12	−72.2	153	Liquid	All organic and inorganic solvents	Pyrophoric
neo-Si5H12	−57.8	134	Liquid	All organic and inorganic solvents	Pyrophoric
cyclo-Si5H10	−10.5	194	Liquid	All organic and inorganic solvents	Pyrophoric
n-Si6H14	−44.7	194	Liquid	All organic and inorganic solvents	Pyrophoric
cyclo-Si6H12	16.5	226	Liquid	All organic and inorganic solvents	Pyrophoric
n-Si7H16	−30.1	227	Liquid	All organic and inorganic solvents	Pyrophoric
Si8H18	28	x	Oil	All organic and inorganic solvents	Pyrophoric
N(SiH3)3	−106	52	Liquid	All organic and inorganic solvents	Pyrophoric
N(SiH2SiH3)3	−97	176	Liquid	All organic and inorganic solvents	Pyrophoric
Et2NSiH2SiH2NEt2	x	23/0.05 Torr	Liquid	All organic and inorganic solvents	Pyrophoric
SiH3SiH(NEt2)2	x	23/0.2 Torr	Liquid	All organic and inorganic solvents	Pyrophoric
iPr2NSiH2SiH2NiPr2	x	68/0.05 Torr	Liquid	All organic and inorganic solvents	Pyrophoric
H3SiPH2	<135	12.7	Gas	All organic and inorganic solvents	Pyrophori, toxic
P(SiH3)3	−73	114	Liquid	All organic and inorganic solvents	Pyrophori, toxic
O(SiH3)2	−144	−15.2	Gas	All organic and inorganic solvents	Not pyrophoric
S(SiH3)2	−70.0	58.8	Liquid	All organic and inorganic solvents	Pyrophori, toxic
Se(SiH3)2	−68.0	85.2	Liquid	All organic and inorganic solvents	Toxic
Te(SiH3)2	x	49/50 Torr	Liquid	All organic and inorganic solvents	Toxic

---

Deposition methods

Silicon (Si) is the second most abundant element in the Earth's crust constituting approximately 28%.³¹⁰ As a semiconductor, silicon plays a crucial part in the electronics industry. Depending on deposition method and process conditions silicon thin films can exhibit monocrystalline, polycrystalline or amorphous structures, which significantly influence the electronic properties and application potential. Monocrystalline silicon (c-Si) possesses a long-range ordered crystal lattice and high charge carrier mobility, making it the preferred material for microelectronic devices. Polycrystalline silicon (poly-Si) consists of crystalline grains separated by grain boundaries, which reduce carrier mobility but allow deposition at lower temperatures, enabling its use in thin film transistors (TFTs) and photovoltaic devices. Amorphous silicon (a-Si) lacks long-range order and exhibits a high density of dangling bonds.^311–313 The structural motifs are summarized in Fig. 16.

Fig. 16 Structural motifs of (a) monocrystalline silicon, (b) polycrystalline silicon and (c) amorphous silicon. Dangling bonds marked as dotted line.³¹⁴

Early on, a-Si attracted scientific interest as a semiconductor, however, it exhibited significant drawbacks with respect to electrical conductivity. At the beginning of the 1970s, new deposition methods were developed, that enabled the preparation of amorphous silicon thin-films with improved electronic properties through the incorporating of hydrogen, resulting in hydrogenated amorphous silicon (a-Si:H).³¹⁵ As shown in Fig. 17, passivation of dangling bonds with hydrogen leads to a drastic reduction in defect density, thereby improving conductivity and carrier mobility. Furthermore, effective p- and n-type doping of a-Si:H can be achieved by incorporating appropriate dopant species, such as diborane or phosphine, during deposition, as dopant atoms are no longer compensated by dangling bond defects.³¹⁶

Fig. 17 (a) Schematic passivation of dangling bonds by hydrogen and (b) schematic illustration of density of states in a-Si and a-Si:H. Graphic adapted from Vora.³¹⁷

Since then, several different deposition techniques have been employed to produce silicon thin films. These silicon thin films have a wide range of applications, including thin film transistors, flexible large area displays, and photovoltaics.^318,319

Chemical vapor deposition

Chemical vapor deposition describes the deposition of a solid component on the surface of a substrate from the vapor phase by means of a chemical reaction. The term was first introduced by J. M. Blocher in 1960 to distinguish the process from other deposition methods, that do not involve chemical reactions such as sputtering.³²⁰ During the CVD process, gaseous reactant species are supplied and transported into a reaction chamber, as schematically illustrated in Fig. 18. Upon entering the reactor, the reactant gases are conveyed toward the substrate by mass transport and may either diffuse directly through the boundary layer to adsorb onto the heated substrate surface or participate in gas-phase reactions, forming intermediate species and gaseous byproducts. These intermediates can subsequently reach the substrate by diffusion and adsorption. On the substrate surface, adsorbed species undergo surface diffusion and heterogeneous chemical reactions, ultimately leading to thin-film growth. Any unreacted gaseous precursors or byproducts are subsequently desorbed from the surface and removed from the chamber in the gas phase.^{312,320–323}

Fig. 18 Schematic illustration of the CVD process. Graphic adapted from Sun et al.³²³ and Katsui et al.³²²

A variety of CVD processes and techniques exist, including thermal CVD (TCVD), plasma-enhanced CVD (PECVD), photo-assisted CVD (PACVD), and hot-wire CVD (HWCVD). Depending on the deposition conditions such as temperature, pressure, and plasma environment, CVD can yield silicon films with different microstructures, including amorphous, microcrystalline, polycrystalline, or epitaxial silicon. The various CVD techniques therefore differ not only in deposition rate and defect concentration but also in the structural properties of the resulting films. A special form of CVD is atomic layer epitaxy (ALE), which enables the growth of monoatomic layers on a substrate surface. In the following section, these different methods are described in more detail.³¹⁵

Thermal chemical vapor deposition

The most common sub-technique of CVD is the thermal CVD. In this process, thermal energy is used to activate the chemical reactions. The required energy can be supplied by several different methods, such as radio-frequency heating, thermal radiation or resistance heating. A typical reactor configuration for TCVD, including gas delivery, substrate heating, and exhaust, is schematically illustrated in Fig. 19.

Fig. 19 Scheme of a typical reactor configuration for TCVD. Graphic adapted from Nnadozie et al.³²⁴

The industrial implementation of silicon chemical vapor deposition emerged in the early 1960s at Bell Telephone Laboratories, where Theuerer demonstrated controlled epitaxial growth of single-crystalline silicon via hydrogen reduction of SiCl₄.³²⁵ This chlorosilane-based approach enabled precise control over film thickness, doping, and conductivity type, establishing CVD as a technologically viable route for device-grade epitaxial layers. However, these processes required elevated substrate temperatures typically exceeding 1100 °C and involved kinetically demanding Si–Cl bond cleavage with concomitant HCl elimination. These limitations motivated the exploration of hydrosilanes as thermodynamically more labile alternatives for thermal silicon deposition. Lewis and co-workers demonstrated that monosilane undergoes efficient thermal decomposition to silicon and hydrogen, enabling high-purity silicon deposition at temperatures substantially lower than those required for chlorosilane reduction.³²⁶ Subsequent kinetic studies by Joyce and Bradley revealed two distinct growth regimes during silane-based epitaxy: a transport-limited regime above ∼1100 °C and a reaction-controlled regime at lower temperatures. Together, these studies established silane as a kinetically tuneable and technologically viable precursor for thermal silicon CVD. Further experimental and kinetic studies have demonstrated that the initial step of silane pyrolysis is most likely the formation of highly reactive silylene species (:SiH₂), accompanied by the release of molecular hydrogen as illustrated in Scheme 226. These intermediates readily insert into Si–H bonds of silanes, leading to the formation of higher polysilanes and initiating silicon hydride cluster growth. Subsequent dehydrogenation and rearrangement reactions generate unsaturated intermediates, including silenes and additional silylenes, which further propagate the reaction network. With increasing cluster size, intramolecular reactions can promote ring formation, yielding cyclic silicon hydrides that represent important intermediates in the early stages of silicon cluster and particle formation during silane pyrolysis. Depending on the reaction conditions, particularly temperature, pressure, and residence time, these species either contribute to heterogeneous film growth on heated surfaces or undergo homogeneous reactions leading to particle and powder formation in the gas phase. While the general mechanistic framework of silane pyrolysis is well established, the detailed reaction network remains highly complex and continues to be investigated through experimental and theoretical studies.^327–329

Scheme 226 Simplified mechanism of silane pyrolysis illustrating the formation of higher silanes and cyclic silicon hydrides.^327,328

Based on the operating pressure, TCVD can be further subdivided into atmospheric pressure CVD (APCVD), low-pressure CVD (LPCVD) and ultrahigh vacuum CVD (UHVCVD). In 1997, Sturm et al. demonstrated low-temperature LPCVD growth of β-SiC on Si using methylsilane as a single-source precursor, yielding polycrystalline, device-compatible SiC films.³³⁰ Two years later, Madaura et al. reported for the first time the APCVD of SiC films on a Si substrate using trimethylsilane as a single source precursor. Although trimethylsilane contains only a single Si–H bond and therefore falls outside the scope of classical hydrosilanes discussed in this review, this study is noteworthy as an early example of silicon deposition chemistry employing molecular silane precursors. The SEM microphotographs of the SiC/Si interface are shown in Fig. 20.³³¹

Fig. 20 Microphotographs of the SiC/Si interface as seen by cross-sectional SEM adapted from Madapura et al.³³¹ With permission from ECS – The Electrochemical Society, © 1999.

Hazbun et al. investigated the use of tetrasilane (Si₄H₁₀) as a precursor for ultra-high-vacuum CVD of silicon epitaxial layers. They demonstrated that tetrasilane enables significantly higher growth rates than monosilane and allows crystalline silicon deposition at temperatures as low as 400 °C, making it suitable for low-temperature epitaxy.³³² Furthermore, Byeon et al. investigated the epitaxial growth of Si and SiGe using high-order silanes such as disilane, trisilane, and tetrasilane under UHVCVD and LPCVD conditions without a carrier gas. They showed that higher-order silanes enable silicon epitaxy at reduced temperatures, with trisilane and tetrasilane providing significantly higher growth rates, while disilane yielded the highest crystal quality of the deposited films.³³³ Halogenated hydrosilanes were likewise employed as silicon precursors for both oxide and carbide thin films. In high-temperature thermal CVD, silicon tetrachloride in combination with propane enabled the growth of epitaxial SiC layers, where chlorine-containing species suppress gas-phase nucleation and improve stoichiometric control of Si : C ratios.³³⁴ In a related context, Sneh et al. reported atomic-layer controlled growth of SiO₂ using a binary reaction sequence with SiCl₄ and H₂O.³³⁵ Nitrogen-functionalized hydrosilanes subsequently emerged as important precursors for the deposition of silicon nitride (Si₃N₄) thin films. Gumpher et al. demonstrated LPCVD of Si₃N₄ from di(t-butylamino)silane (BTBAS) and NH₃ at 550–600 °C, producing high-quality nitride films suitable for microelectronic applications.³³⁶ The incorporation of preformed Si–N bonds in these precursors enhances surface reactivity while avoiding corrosive halide byproducts, thereby improving film purity and conformality in advanced deposition schemes.³³⁷

Silylphosphanes and related phosphinosilanes were explored primarily in patent literature as single-source Si–P precursors for in situ phosphorus doping during silicon CVD. Although conceptually attractive as potential alternatives to PH₃, limited thermal robustness and the maturity of established PH₃-based processes curtailed their broader implementation.³³⁸ Silylgermanes containing preformed Si–Ge bonds were systematically developed as molecular single-source precursors for SiGe thin-film growth. Kouvetakis and co-workers first established the complete hydride series (H₃Ge)_xSiH_4−x (x = 1–4), including silylgermane H₃GeSiH₃ and tetrasilylgermane Ge(SiH₃)₄, and demonstrated their application in low-pressure deposition of epitaxial Si_1−xGe_x layers at 300–450 °C. Thermal activation proceeds via Si–H and Ge–H bond cleavage with H₂ elimination while preserving the intrinsic Si : Ge stoichiometry encoded in the precursor, enabling compositionally controlled alloy growth without separate silane and germane feeds. This molecular design strategy facilitates the preparation of a new class of Si-based semiconductors with Ge-rich stoichiometry. Fig. 21 shows a X-ray transmission electron microscopy (XTEM) image of a SiGe₂ layer on a Si substrate.¹²⁷

Fig. 21 XTEM micrograph of a SiGe₂ layer grown on Si(100) adapted from Kouvetaksi et al.¹²⁷ With permission from American Chemical Society, © 2005.

More recently, Wagner and co-workers introduced mixed-substituted Si–Ge hydrides such as H₃SiR₂GeSiH₃ (R = Ph, n-Bu), synthesized via Si₂Cl₆-mediated coupling and subsequent hydride formation, and demonstrated their conversion by low-pressure CVD into predominantly amorphous Si_1−xGe_x coatings with predefined Si : Ge ratios and reduced pyrophoricity compared to purely hydridic systems.¹²⁸

While thermal processes are widely used in industry, primarily due to the technical ease of implementation, they also exhibit clear disadvantages. In particular, temperature-sensitive substrates may be damaged by the high process temperature. For this reason, alternative energy sources have been developed over the decades of research.^321,339,340

Plasma-enhanced chemical vapor deposition

PECVD, also known as glow-discharge chemical vapor deposition is a variation of CVD in which electrical energy is used to activate the chemical reaction. By applying a high frequency voltage between two electrodes at reduced pressure a glow discharge is developed in which a non-thermal plasma consisting of electrons, ions and electronically excited species is formed. Through electron impact within the plasma, the precursor gases are ionized and dissociated, thereby generating chemically active ions and radicals. As a result, the electron temperature can reach values of 2000 K or higher, while the gas temperature, depending on process conditions, remains near room temperature. The activated species can take part in heterogeneous reactions on or near the substrate surface, leading to the deposition of a thin film. Since the reactive species are predominantly formed in the gas phase, only minimal thermal activation is required. This enables PECVD processes to be carried out at significantly lower temperatures than TCVD, in many cases at temperatures below 200 °C. Consequently, depositions on temperature-sensitive substrates such as glass and polymeric materials becomes possible.^321,339,341 The plasma-induced dissociation pathway of silane are summarized in the following Fig. 22.

Fig. 22 Schematic dissociation pathway of SiH₄ and H₂ in the plasma adapted from Kasap et al.³¹³ With permission from Springer-Verlag US, © 2007.

In 1965, Sterling and Swann³⁴² were the first to report the deposition of an amorphous silicon layer from gaseous SiH₄ using plasma-assisted CVD. In the early 1970s, a research group at Havard University demonstrated the incorporation of hydrogen into a-Si leading to the development of hydrogenated amorphous silicon (a-Si:H) with a significantly reduced defect density and improved electronical properties.³⁴³ Building on these findings, Spear and LeComber³⁴⁴ first demonstrated the controlled n- and p-doping of a-Si:H by adding phosphine and diborane to SiH₄ in a reactor with a plasma at radio frequency voltages. This enabled precise control of the electrical conductivity over several orders of magnitude and paved the way for the realization of a-SiH-based devices, in particular thin-film solar cells and TFTs.³⁴⁵ As a result of these advances, PECVD became the standard procedure for the deposition of silicon thin films in the 1980s. In 1983, Matsuda et al. deposited amorphous a-SiC:H films from SiH₄/CH₄ mixtures at substrate temperatures of 300–400 °C, producing films suitable for microelectronic and optical applications.³⁴⁶ Pokhodnya et al. studied the intrinsic stress in a-Si:H films using cyclohexasilane (CHS) as a precursor.³⁴⁷ Lee et al. reported the deposition of trisilylamine using PECVD forming highly conformal SiCN films. In Fig. 23 a transmission electron microscopy (TEM) cross-sectional view of the substrate with the deposited layer is shown.³⁴⁸

Fig. 23 TEM cross-sectional view of the substrate with the deposited SiCN layer adapted from Lee et al.³⁴⁸ With permission from Elsevier B.V., © 2018.

In the following decades, research efforts focused on optimizing the PECVD process with respect to material quality, increased deposition rates and long-term stability. New approaches such as very-high-frequency PECVD (VHF-PECVD),³⁴⁹ microwave-assisted CVD (MW-CVD)³⁵⁰ and “high power high pressure” regime RF-PECVD³⁵¹ were developed. Today RF-PECVD at a excitation frequency of 13.56 MHz remains the dominating deposition technique, notwithstanding its relatively low deposition rate of 1–2 Å s⁻¹.³⁵²

The following Fig. 24 shows a schematic illustration of a PECVD reactor.

Fig. 24 Schematic illustration of a plasma-enhanced chemical vapor deposition (PECVD) reactor. Graphic adapted from Verma et al.³⁵³ and altered with the help of AI. With permission from Springer Nature Singapore Pte Ltd., © 2023.

Photo-assisted chemical vapor deposition

Photo-assisted chemical vapor deposition (PACVD), also known as photo-initiated CVD, is a variant of CVD in which photon energy, typically in the ultraviolet (UV) or vacuum-ultraviolet (VUV) range, is used to activate chemical reactions. Reaction initiation occurs via photolytic dissociation of the precursor molecules, leading to the formation of reactive radicals and electronically excited species, which subsequently participate in heterogeneous surface reactions resulting in thin film growth.^220,354 In 1983, Migitaka et al. and Takahashi et al. reported the preparation of a-Si:H films from monosilane³⁵⁵ and disilane.³⁵⁶ A schematic illustration of a PACVD reactor can be found in the article of Dorval Dion et al.³⁵⁴

A key advantage of PACVD is the absence of plasma, which eliminates ion-induced damage to both the substrate and the growing film. Furthermore, in contrast to TCVD, which relies on pyrolytic mechanisms and requires high substrate temperatures, PACVD enables deposition at significantly lower temperatures, in some cases at room temperature. As a result, the choice of substrate material is not restricted by thermal stability. PACVD also has limitations, including the requirement that precursor molecules exhibit sufficient absorption within the emission spectrum of the light source, as well as relatively low deposition rates.^339,354

PACVD is often discussed together with laser-assisted CVD (LACVD), also known as laser chemical CVD, and the two processes are not always strictly distinguished terminologically in literature, as both rely on photonic activation. The crucial difference, however, lies in the method of energy input. In contrast to PACVD, LACVD uses coherent, high-intensity laser radiation. LACVD can generate very high local energy densities and induce both photolytic and locally pyrolytic processes, for example, through targeted heating of the substrate surface. Laser-CVD is therefore particularly suitable for locally confined deposition or structuring processes.^220,339,357 A typical schematic of the LACVD process is shown in Fig. 25.

Fig. 25 Schematic illustration of a LACVD reactor. With permission from the Institute of Optics and Electronics, Chinese Academy of Science³⁵⁸ © 2022.

Hot-wire chemical vapor deposition

Hot-wire chemical vapor deposition (HWCVD), also known as catalytic CVD (Cat-CVD), is a plasma-free CVD process in which the decomposition of precursor gases occurs on a highly heated metal filament. Tungsten or tantalum are typically used as the filament material, which can be heated to temperatures of approximately 1400 to 2100 °C. At the surface of the hot wire, gaseous silane can be efficiently dissociated, with an almost complete separation into its atomic components. The reactive species generated subsequently undergo further reactions with SiH₄ in the gas phase, forming various secondary precursors that diffuse to the substrate surface and contribute to thin-film deposition.^359,360 The following Fig. 26 gives a schematic view of a HWCVD reactor.

Fig. 26 Illustration of a schematic HWCVD reactor adapted from Schropp.³⁶⁰ With permission from ECS – The Electrochemical Society, © 2009.

A characteristic feature of HWCVD is the spatial separation of precursor decomposition and layer growth. Since the generation of reactive species occurs exclusively on the filament, the process is often referred to as a remote decomposition method. The temperatures of the filament and substrate can be adjusted independently, enabling deposition at comparatively low substrate temperatures while simultaneously achieving high deposition rates.

The basic principle of HWCVD was first published and patented in 1979 by Wiesmann et al. under the name thermal vapor deposition.³⁶¹ In the mid-1980s, Matsumura et al. first demonstrated the deposition of amorphous silicon using this method and introduced the term catalytic CVD.³⁶² Independently, Doyle et al. described a comparable concept under the name evaporative surface decomposition (ESD).³⁶³

A significant milestone was achieved in 1991 with the work of Mahan et al., who demonstrated that amorphous silicon layers deposited using HWCVD can exhibit higher material quality than comparable films produced by PECVD.³⁶⁴ The high quality of the deposited layers is attributed to the efficient decomposition of the precursor gases at filament temperatures above approximately 1500 °C. Typical deposition rates for amorphous silicon range from 10 to 50 Å s⁻¹, significantly exceeding those of conventional RF-PECVD processes.

Due to the absence of a plasma, ion-induced structural damage to the growing layer, that can be caused by ion bombardment or high-energy radiation, is largely avoided. The term Cat-CVD refers to the catalytic effect of the hot wire in the dissociation of the precursor gases, although the filament is not an ideal catalyst in the strict chemical sense, as it undergoes aging and erosion during the process.³⁶⁵ In the literature, the term HWCVD is predominantly used in physical and photovoltaic contexts, while Cat-CVD is particularly common in chemical research and Japanese literature. Current research is investigating, among other things, the application of HWCVD for the production of TFTs and thin-film solar cells.³⁶⁰

Atomic layer deposition

Atomic layer deposition is a thin film deposition technique, that can be classified as a special modification of CVD. In the literature, it is also referred to as atomic layer epitaxy or Atomic Layer Chemical Vapor Deposition (ALCVD). A key characteristic of ALD is the deposition of inorganic thin films with thicknesses down to a range of single monolayers, which is enabled by sequential, self-limiting surface reactions. As a result, highly homogeneous and conformal layers with excellent thickness control can be deposited.^131,366

In ALD, gaseous precursors are introduced into the reactor in a strictly sequential manner. Each precursor reacts exclusively with the available reactive sites on the substrate surface until a saturated adsorption level is reached. The reactor is purged with inert gas between the individual precursor pulses to remove excess precursor molecules and volatile byproducts. This procedure effectively suppresses gas-phase reactions, such that the film growth is governed almost entirely by surface reactions.^133,367 The scheme of a sequential ALD cycle is shown in Fig. 27. Hirose et al. demonstrated the use of monosilane for room temperature ALD.³⁶⁸ Matsunami et al. established epitaxial growth of β-SiC (3C-SiC) on Si substrates by reacting SiH₄ with hydrocarbons such as propane at temperatures above 1000 °C, enabling crystalline SiC layers for electronic applications and demonstrating in situ doping with nitrogen donors and aluminium or boron acceptors.³⁶⁹ Halogenated hydrosilanes were also employed for SiC epitaxy: Pedersen et al. employed methyltrichlorosilane in a hot-wall CVD reactor to obtain high-quality 4H-SiC layers.³⁷⁰ Additional chlorinated silanes, including trichlorosilane, dichlorosilane, and hexachlorodisilane (Si₂Cl₆), have likewise been utilized for high-quality SiC epitaxial growth.³⁷¹

Fig. 27 Scheme of a sequential ALD cycle adapted from George.¹³¹ With permission from American Chemical Society, © 2009.

Nitrogen-functionalized hydrosilanes became important precursors for the deposition of silicon dioxide (SiO₂) thin films. Bis(dimethylamino)silane (BDMAS) was introduced for atomic layer deposition of SiO₂ with H₂O³³⁷ or O₃ ³⁷² as oxidants, where ligand-exchange reactions with surface OH groups followed by ozone or oxygen plasma oxidation yield SiO₂.³⁷² Furthermore Faraz et al. reported the deposition of silicon nitride layer through plasma-enhanced atomic layer deposition of di(sec-butylamino)silane (DSBAS) on planar and on high aspect ratio 3D trench nanostructures. Fig. 28 shows a TEM image of the layer as-deposited and post wet-etch with dilute hydrofluoric acid.³⁷³

Fig. 28 TEM image of the silicon nitride layer as-deposited (left) and post wet-etch with dilute hydrofluoric acid (right) adapted from Faraz et al.³⁷³ With permission from American Chemical Society, © 2017.

In contrast to hydrosilanes, oxygen-functionalized silicon precursors such as tetraethyl orthosilicate (TEOS, Si(OEt)₄) are widely used for the deposition of SiO₂ thin films in LPCVD, PECVD, and sub-atmospheric CVD processes.³⁷⁴

Liquid phase deposition

A viable alternative to established vapour-based deposition techniques is solution-based processing of silicon, commonly referred to as liquid phase deposition. In contrast to CVD methods, which require pyrophoric precursor gases, elaborate vacuum or plasma infrastructure, and often elevated deposition temperatures, LPD can generally be carried out at lower temperatures and under atmospheric pressure. This reduces equipment complexity as well as operating costs. Furthermore, LPD is compatible with flexible, large-area, and temperature-sensitive substrates and can enable higher throughput, particularly when combined with coating or printing techniques. Such approaches also facilitate direct patterning of materials and may reduce the number of required processing steps. Consequently, LPD is considered a promising route for applications in printable electronics, including large-area flexible displays, thin-film solar cells, TFTs and silicon-based anode materials in lithium-ion batteries. The following section discusses the underlying chemical mechanisms of hydrosilane-based LPD and summarizes the key contributions in this field.^19,375,376

The conceptual foundations of hydrosilane-based liquid processing date back to the late 1980s. West and Wolff³⁷⁷ as well as Miller and Michl³⁷⁸ demonstrated that organopolysilanes can be deposited as thin films and subsequently converted into silicon-rich amorphous networks by thermal or photochemical treatment, thereby establishing the processing and photochemistry toolkit for later hydrosilanes routes. However, true hydrosilanes themselves were only mentioned in passing in these studies, and a dedicated liquid-phase deposition strategy based explicitly on hydrosilane precursors had not been established. A decisive step toward practical implementation was achieved by Shimoda et al. in 2006,¹⁹ who established cy-Si₅H₁₀ as a molecular precursor for solution-processed silicon films. Cy-Si₅H₁₀ was selected due to its favourable properties: it exhibits comparatively high stability and good availability making it suitable for handling and processing under inert conditions, and also exhibiting a high photoreactivity upon irradiation with ultraviolet light. In Shimodas LPD approach cy-Si₅H₁₀ is first irradiated with UV light with a wavelength of 365 nm leading to a ring-opening polymerization and the formation of polydihydrosilane. The resulting polyhydrosilane can be dissolved in organic solvents to yield a processable liquid silicon material, often referred to as “silicon ink”. The silicon ink is deposited by spin-coating on a substrate forming a uniform precursor. Subsequently thermal treatment induces the conversion of the polysilane network into a a-Si film. During initial heating, volatile components such as the solvent and cy-Si₅H₁₀ evaporate. At a temperature below approximately 280 °C cleavage of Si–Si bonds begins, leading to a release of SiH₂ and SiH₃ species. Above a temperature of around 300 °C the Si–H bonds break resulting in the formation of the amorphous silicon film.^{19,376,379–381} Fig. 29 illustrates the colour changes associated with variations in the chemical composition of the thin film at different annealing temperatures.

Fig. 29 Photographic image of the solution-processed polysilane films coated on quartz glass adapted from Trifunovic et al.³⁸¹ With permission from American Institute of Physics, © 2015.

The material can subsequently be further processed by additional thermal treatment or laser annealing to obtain polycrystalline silicon. Fig. 30 depicts a scheme for the solution-based formation of amorphous and polycrystalline silicon films.

Fig. 30 Scheme for the solution-based formation of amorphous and polycrystalline silicon films adapted from Gerwig et al.⁵ With permission from Chemistry – A European Journal published by Wiley-VCH GmbH, © 2024.

In comparison to a-Si thin films formed through PECVD, the LPD produced films show a significantly lower mobility which is connected to a low concentration of hydrogen atoms. Films processed at temperatures below 300 °C exhibit charge carrier mobilities comparable to those obtained by PECVD. However, under these conditions the material can no longer be unambiguously classified as amorphous silicon and shows increased susceptibility to oxidation. Shimoda et al. therefore emphasizes the necessity of balancing low processing temperatures with sufficient structural stabilization and oxidation resistance in order to achieve device-relevant film quality.³⁸⁰

Beyond the preparation of intrinsic silicon inks, Shimoda and co-workers also developed n-type and p-type silicon inks by incorporating suitable dopant precursors, such as white phosphorus and decaborane, into the cy-Si₅H₁₀-derived system prior to film deposition. During subsequent thermal conversion, the dopant species are incorporated into the forming silicon network. These developments were later summarized under the term “liquid silicon family materials” (LSFMs), describing a materials platform in which cy-Si₅H₁₀ acts as a common precursor for different silicon-based functional layers. In this concept, intrinsic, n-type, and p-type silicon films as well as device structures such as TFTs and solar cells can be fabricated using entirely liquid-based processing routes.

Subsequent studies expanded the precursor chemistry, the available activation strategies as well as the deposition methods for hydrosilane-based LPD. Following Shimoda's initial work, cy-Si₅H₁₀ remained the dominant hydrosilane precursor, and numerous studies focused on refining its processing and activation. In contrast to the spin-coated polysilane ink approach described by Shimoda, Frey et al. employed an aerosol-assisted spray deposition technique using neat cy-Si₅H₁₀ as precursor. The cy-Si₅H₁₀ was atomized and deposited onto heated substrates under inert atmosphere, while ultraviolet irradiation was applied within the deposition chamber to promote precursor activation. Film formation proceeded primarily via thermally assisted decomposition of the liquid precursor during and after deposition, followed by high-temperature annealing (700–950 °C) to obtain polycrystalline silicon.³⁸² Trifunovic et al. employed doctor-blade coating of cy-Si₅H₁₀, followed by ultraviolet curing at 365 nm and subsequent excimer laser at crystallization at 308 nm to obtain poly-Si at a maximum process temperature of 150 °C. In this approach, the solution-deposited precursor was directly transformed into crystalline silicon without requiring intermediate high-temperature annealing above 350 °C or a separate amorphous silicon conversion step. Therefore, this process is suitable for deposition on temperature-sensitive substrates like paper or polymer and facilitating low-cost electronics. Fig. 31 shows a poly Si film fabricated on paper.³⁸¹

Fig. 31 Poly Si film fabricated on paper adapted from Trifunovic et al.³⁸¹ With permission from American Institute of Physics, © 2015.

In parallel, alternative activation methods for cy-Si₅H₁₀ were investigated. Cádiz Bedini et al. reported on ring-opening polymerisation induced by ultrasonic irradiation at a frequency of 26 kHz. The temperature was maintained below 75 °C, and it was confirmed, that no thermal activation of the precursor occurred under these conditions.

Masuda et al. demonstrated that cy-Si₅H₁₀-derived liquid silicon can be employed for solution-based silicon patterning using both inkjet printing and nanoimprinting strategies. In both approaches, cy-Si₅H₁₀ was first photochemically polymerised at 365 nm to yield a processable polysilane precursor. In the inkjet printing approach, the polysilane solution was deposited in a digitally defined, maskless manner onto the substrate. Subsequent thermal conversion at approximately 380–400 °C yielded hydrogenated amorphous silicon, while additional annealing at 1000 °C enabled crystallisation to polycrystalline silicon. The method further allowed the incorporation of p- and n-type dopants directly into the printed structures. Fig. 32 shows a printed silicon pattern on a glass substrate.³⁸³

Fig. 32 Inkjet printed silicon pattern on a glass substrate adapted from Masuda et al.³⁸³ With permission from Wiley-VCH GmbH, © 2020.

In contrast, the nanoimprinting approach consisted of spin-coating the cy-Si₅H₁₀-derived Liq-Si precursor onto the substrate. The deposited film was subsequently pre-cured at 140–220 °C, which initiated partial crosslinking of the polysilane network. Successful pattern replication was achieved only within this temperature window under an applied pressure of 10 MPa. At lower temperatures, insufficient crosslinking prevented stable feature transfer, whereas at higher temperatures the film lost its deformability. Crosslinking proceeds via thermally activated 1,2-hydrogen shift reactions, accompanied by the release of SiH₄ and H₂, leading to progressive solidification of the film. After imprinting and controlled cooling, post-annealing at approximately 380 °C completed the conversion to amorphous silicon. Despite significant volumetric shrinkage during polymer-to-silicon transformation, nanoscale patterns were retained. Fig. 33 shows a nanoimprinted pattern of a-Si.³⁸⁴

Fig. 33 Nanoimprinted pattern of a-Si adapted from Masuda et al.³⁸⁴ With permission from American Chemical Society, © 2016.

These studies demonstrate that cy-Si₅H₁₀-derived polysilanes are suitable precursors for lithography-free micro- and nanostructured silicon fabrication.

Gerwig et al. investigated the oligomerisation of cy-Si₅H₁₀ using electron spin resonance (ESR), NMR, and Raman spectroscopy supported by DFT and molecular dynamics simulations. Their results showed that cy-Si₅H₁₀ does not undergo a classical ring-opening polymerisation. Instead, short-lived silyl radicals are formed, which recombine to generate highly branched hydrooligosilanes. Silylene intermediates were found not to play a significant role. Competing decomposition and disproportionation reactions produce volatile fragments, reducing hydrogen content and overall yield. ESR measurements confirmed the presence of tertiary silyl radicals as reactive intermediates.⁵

The influence of solvent and processing parameters on film formation was further examined. Optimised spin-coating and UV irradiation conditions yielded homogeneous silicon films, which were subsequently passivated by hydrogen plasma. The resulting thin films and TFT devices exhibited performance comparable to PECVD-processed materials.

Shen et al. introduced a liquid-source vapor deposition (LVD), where cy-Si₅H₁₀ thermally decomposes between two parallel hot substrates at atmospheric pressure. In this configuration, a fraction of the precursor is directly converted into amorphous silicon on the substrate surface, while volatile silicon hydride radicals are simultaneously generated. These reactive intermediates diffuse within the confined deposition space and act as secondary growth species, thereby sustaining further film formation. This allows for the formation of denser amorphous silicon layers with reduced oxygen incorporation compared to conventional spin-coated films. The schematic set-up for LVD is shown in Fig. 34.³⁸⁵

Fig. 34 Schematic set-up of the LVD process with cy-Si₅H₁₀ as the precursor adapted from Shen et al.³⁸⁵ Published by Royal Society of Chemistry under CC-BY 4.0, © 2015.

However, the demanding synthesis and purification of cy-Si₅H₁₀ highlight the need to consider alternative precursors such as cyclohexasilane (CHS) and NPS.^5,376 Iyer et al. investigated CHS as an alternative cyclic hydrosilane precursor. The CHS solution was spin-coated onto the substrate under simultaneous UV irradiation to induce ring-opening polymerisation and network formation. Subsequent thermal annealing and excimer laser treatment enabled the transformation of the amorphous intermediate into crystalline silicon. Furthermore, hydrogen plasma exposure was applied to passivate residual defect states and improve the structural and electronic quality of the films.³⁸⁶ The following Fig. 35 shows SEM images of the surface of the thin film after different points in the formation process.

Fig. 35 SEM images of the thin film surface of (a) the amorphous silicon film, (b) the thermally treated silicon film, (c) the laser treated silicon film and (d) the laser treated silicon film at higher magnification adapted from Iyer et al.³⁸⁶ With permission from American Chemical Society, © 2012.

Cadiz Bedini et al. demonstrated the preparation of silicon ink from trisilane using ultrasonic irradiation. After subsequent UV irradiation, they used the resulting oligomeric mixture deposit a a-Si–H thin film via spin coating. Furthermore, the authors also reported on the deposition of Si nanoparticles from the sonicated silicon ink.³⁸⁷

Bronger et al. were the first to introduce neopentasilane in solution-based silicon processing. Using spin-coating and slot-die coating as deposition techniques, they showed that the branched molecular structure and increased Si–Si bond content of NPS significantly influence the thin film formation. Compared to cy-Si₅H₁₀, NPS exhibits improved solubility, which results in enhanced film homogeneity and uniformity. In addition, they reported an improved p-type doping efficiency for NPS-derived films relative to cy-Si₅H₁₀. The electronic performance approached that of comparable PECVD-deposited materials.³¹⁹ The working group of Haase executed further research to determine the usability of liquid silicon ink based on NPS for local n-and p-doping of solar cells.³⁸⁸

Further development explored hydrosilanes beyond simple cyclic or small branched molecules. Haas et al. introduced highly branched perhydrosilanes, namely 2,2,3,3-tetrasilyltetrasilane and 2,2,3,3,4,4-hexasilylpentasilane, which show a reduced pyrophoric character and a bathochromically shifted UV-absorption. After photolysis the obtained oligomeric polyhydrosilane mixture was spin-coated on glass substrates and thermally treated at 500 °C resulting in a homogeneous a-Si:H layer. Fig. 36 shows a photographic image and an optical micrograph of the solution processed a-Si:H film.²¹

Fig. 36 Photographic image (B) and optical micrograph (C) of the solution processed a-Si: film adapted from Haas et al.²¹ With permission from Wiley-VCH, © 2017.

In this context, Christopoulos et al. reported the synthesis and structural characterisation of highly branched hydrogenated nonasilanes and decasilanes. Although no thin-film deposition experiments were performed, the study provided detailed information on molecular structure, bonding motifs, and stability of higher nuclearity hydrosilanes. Compared to smaller cyclic precursors, these compounds exhibit modified volatility and decomposition behaviour due to their increased silicon content and branching degree. The work therefore contributes to the understanding of structure–property relationships in hydrosilanes and identifies potential candidates for future liquid-phase silicon deposition studies.⁴⁹

The increasing focus on precursor structure is also reflected in patent literature. Several patents disclose synthetic routes toward branched and doped hydridosilanes intended for thin-film deposition applications. These developments address precursor stability, controlled oligomerisation, and reduced pyrophoric character, highlighting the technological relevance of structure-tailored hydrosilanes for liquid-phase silicon processing.³⁸⁹ More recently, precursor-engineered strategies have focused on single-source hydrosilane-based precursors that contain one or more heteroatoms covalently linked to silicon to enable controlled Si–C network formation. Matsuda and co-workers developed a polymeric polydihydrosilane precursor with pendant hexyl groups (PSH), which was deposited by solution-based methods and subsequently pyrolyzed to form an a-SiC coating. Fig. 37 shows a photographic image of PSH films coated on glass substrates and pyrolyzed at different temperatures.³⁹⁰

Fig. 37 Photographic image of PSH films coated on glass substrates and pyrolyzed at different temperatures adapted from Masuda et al.³⁹⁰ With permission from Royal Society of Chemistry, © 2015.

In a related molecular design approach, Haas and co-workers reported the systematic functionalization of higher silicon hydrides with carbon-containing substituents and demonstrated their application in LPD, where spin-coated precursor films were thermally converted into homogeneous Si/C thin films. Fig. 38 exemplary shows a picture and a light microscopy image of a thin layer formed from the precursor 1,1,1-trimethyl-2,2-disilyltrisilane.²²

Fig. 38 A picture and a light microscopy image of a thin layer formed from the precursor 1,1,1-trimethyl-2,2-disilyltrisilane on glass adapted from Haas et al.²² Published by American Chemical Society under CC-BY 4.0, © 2023.

Structure–property–application relationships

Across CVD and LPD, recent work clarifies how structure–property–application relationships are essential to fully exploit the potential of hydrosilanes in modern materials science. The molecular structure of hydrosilanes governs their activation routes, processing windows, and ultimately film properties. In CVD, high volatility and clean gas-phase decomposition are structure-dependent parameters. Small, volatile hydrides such as SiH₄ or Si₂H₆ dissociate cleanly under plasma, hot-wire, or photon activation and remain advantageous for transport-limited growth formation of high-purity films. This includes hydrosilanes, where the incorporation of a Si–heteroatom bond can enhance surface reactivity, thereby improving the purity and conformality of a film. These attributes are particularly critical, as reduced impurity incorporation minimizes defect-mediated recombination, while improved conformality ensures uniform coverage and consistent electronic properties across complex device architectures. Consequently, both factors directly contribute to enhanced device performance and reliability in both microelectronics and thin-film solar cells.

In LPD, the balance shifts toward solubility, manageable reactivity, and photochemical responsiveness: higher and especially branched hydrosilanes exhibit superior solubility, reduced pyrophoricity, and bathochromically shifted UV absorption, enabling efficient photochemical activation and yielding homogeneous a-Si:H films. These features promote homogeneous film formation and improved control over hydrogen incorporation and network structure in a-Si:H materials. For example, branched hydrosilanes support more uniform coating behavior and facilitate dopant incorporation, while cyclic precursors follow distinct radical-mediated pathways that yield highly branched intermediates, influencing film densification and defect evolution. Thus, precursor structure directly governs the resulting microstructure and optoelectronic properties of the deposited films.

Across both deposition regimes, molecular functionalization (e.g., preformed Si–N, Si–C, or Si–Ge bonds) provides a direct handle to encode composition and doping at the molecular level, linking precursor design directly to device-relevant properties. In the context of thin-film solar cells, such tailored precursors can influence key properties such as crystallinity, electronic structure, and impurity incorporation, thereby directly affecting device performance. These examples clearly demonstrate that the rational design of hydrosilane precursors-linking molecular structure to decomposition behavior and ultimately to material properties-represents a central strategy for advancing silicon-based technologies encoding film stoichiometry and network chemistry at the molecular level.

Conclusions

Over more than a century of research, the chemistry of hydrosilane has evolved from the early isolation of simple silanes to a sophisticated field spanning synthetic inorganic chemistry, materials science, and semiconductor technology. The development of reliable synthetic methodologies-from classical silicide hydrolysis to chlorosilane reduction, catalytic coupling reactions, and silanide-based strategies-has enabled access to a broad spectrum of linear, branched, cyclic, and cluster-type hydrosilanes. These advances have revealed the remarkable structural diversity of silicon hydrides and highlighted the different reactivity of Si–H and Si–Si bonds compared to their carbon analogues.

A central theme emerging from recent research is the pivotal role of silanide intermediates and Si–H bond activation processes in hydrosilane functionalization. Alkali-metal silanides and related species provide powerful synthetic entry points to a wide range of silicon-element bonds, enabling the preparation of silyl derivatives with elements across the periodic table. In parallel, dehydrogenative coupling reactions have emerged as a particularly powerful methodology for the controlled construction of Si–Si bonds, allowing the transformation of simple monosilanes into higher oligohydrosilanes and polysilane frameworks through catalytic hydrogen elimination. Together with advances in silicon cluster chemistry, including the synthesis of silafullerenes and higher branched hydrosilanes, these developments have significantly expanded the conceptual and structural landscape of molecular silicon hydride chemistry.

Beyond their fundamental chemical interest, hydrosilanes have become indispensable precursors for the preparation of silicon-containing materials and thin films. Chemical vapour deposition represents the most widely employed industrial technique for silicon film formation, where simple hydrosilanes such as monosilane and disilane serve as key feedstocks for the growth of polycrystalline, amorphous, or epitaxial silicon layers used in semiconductor devices and photovoltaic technologies. The controlled thermal or plasma-assisted decomposition of these molecules enables the deposition of high-purity silicon films with precisely tuneable properties. Building on this established technology, recent research has increasingly explored molecularly defined higher hydrosilanes and functionalized derivatives as advanced precursors. In particular, solution-based approaches such as liquid-phase deposition has emerged as attractive complementary strategy that offers advantages in terms of processability, scalability, and compatibility with emerging device architectures.

Outlook and challenges

The chemistry of hydrosilanes has evolved from a largely academic field into a key enabling technology for semiconductor processing and silicon-based materials. As highlighted throughout this review, hydrosilanes represent highly versatile molecular precursors that bridge fundamental silicon chemistry with industrially relevant applications such as chemical vapor deposition (CVD) and liquid phase deposition (LPD).

Building on this progress, future developments are expected to focus on improving synthetic accessibility, scalability, and the targeted functionalization of hydrosilane derivatives.

From an application perspective, hydrosilanes and especially higher oligomeric species offer significant potential as tailored single-source precursors for advanced materials. Their tunable reactivity and decomposition behavior enable precise control over film growth, composition, and doping in semiconductor fabrication. In particular, the development of functionalized hydrosilanes that incorporate heteroatoms directly into the molecular backbone represents a promising strategy toward cleaner and more efficient deposition processes, avoiding the need for co-feeding multiple gaseous precursors.

However, several key challenges currently limit the broader industrial implementation of these compounds. A major bottleneck is the economic feasibility, as the high cost of polysilanes has so far prevented their widespread use in large-scale applications such as solar cell manufacturing. Despite their attractive properties as solution-processable silicon precursors, their synthesis often involves multi-step procedures, low overall yields, and demanding purification protocols, all of which significantly increase production costs.

In addition, synthetic limitations remain a critical issue. For many technologically relevant doping elements, no scalable and robust synthetic routes to suitable hydrosilane-based precursors are currently available. While initial approaches toward Si–B, Si–P, or other heteroatom-functionalized systems have been reported, these methods often suffer from limited selectivity, stability issues, or poor scalability.

This lack of general and efficient synthetic strategies restricts the systematic exploration of doped silicon materials derived from hydrosilanes.

Another challenge lies in the control of reactivity and stability. The intrinsic reactivity of Si–H and Si–Si bonds, while advantageous for materials synthesis, can lead to undesired side reactions such as polymerization or decomposition, complicating handling and processing. This is particularly relevant for higher hydrosilanes, where competing reaction pathways often result in complex product mixtures and reduced selectivity.

Looking forward, addressing these challenges will require integrated efforts in synthesis, mechanistic understanding, and process development. Advances in catalytic methodologies, improved precursor design, and the development of scalable production routes will be essential to unlock the full potential of hydrosilanes in industrial applications. In this context, the continued interplay between fundamental research and applied process engineering will be crucial for translating laboratory-scale discoveries into viable technologies.

Author contributions

D. G., M. Hx., M. P. and P. V.: were responsible for visualization, data presentation and writing original draft (lead). M. H. was in charge for methodology and conceptualization, review and editing of the manuscript (lead), project administration and funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Data availability

This article is a review and does not report any original data. Therefore, no datasets were generated or analyzed during the current study.

Acknowledgements

The financial support by the Austrian Federal Ministry of Labour and Economy, the National Foundation for Research, Technology and Development and the Christian Doppler Research Association is gratefully acknowledged. The authors acknowledge the use of artificial intelligence tools (OpenAi (GPT 5)) for language polishing of the manuscript.

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