s-Block amidoboranes: syntheses, structures, reactivity and applications

Abstract

Metal amidoborane compounds of the alkali- and alkaline earth metals have in recent years found applications in diverse disciplines, notably as hydrogen storage materials, as reagents for the reduction of organic functional groups and as catalysts and intermediates in dehydrocoupling reactions. These functions are connected by the organometallic chemistry of the MNR₂BH₃ group.† This review focusses on central aspects of the s-block amidoborane compounds – their syntheses, structures and reactivity. Well-defined amidoborane complexes of group 2 metals are now available by a variety of solution-phase routes, which has allowed a more detailed analysis of this functional group, which was previously largely confined to solid-state materials chemistry. Structures obtained from X-ray crystallography have begun to provide increased understanding of the fundamental steps of key processes, including amine–borane dehydrocoupling and hydrogen release from primary and secondary amidoboranes. We review structural parameters and reactivity to rationalise the effects of the metal, nitrogen substituents and supporting ligands on catalytic performance and dehydrogenative decomposition routes. Mechanistic features of key processes involving amidoborane compounds as starting materials or intermediates are discussed, alongside emerging applications such as the use of group 1 metal amidoboranes in synthesis. Finally, the future prospects of this vibrant branch of main group chemistry are evaluated.

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Tom E. Stennett

Tom Stennett was born in Preston, England in 1986. He received his master's degree from the University of Durham in 2008 after a project in the lab of Prof. Jörg Sundermeyer at the Phillips-Universität Marburg. He then moved to the University of Bristol to undertake his PhD studies under the supervision of Prof. Duncan Wass on chromium-based catalysts for selective ethylene oligomerisation. After the award of his doctorate in 2013, he had a brief stay with Prof. Paul Pringle in Bristol before moving to the University of Erlangen-Nürnberg, where he is working as a postdoctoral researcher with Prof. Sjoerd Harder on early main-group organometallic chemistry.

Sjoerd Harder

Sjoerd Harder, born in The Netherlands, obtained his PhD with Lambert Brandsma (1990, Utrecht) and was awarded the H. J. Backer prize in organic chemistry for his thesis. After post-doctoral stays with Paul Rague von Schleyer (1991, Erlangen), Andrew Streitwieser (1992, Berkeley) and Hans Brintzinger (1993, Konstanz) he received his habilitation in 1998. In 2004 he became a professor of Inorganic Chemistry at the University of Duisburg-Essen and in 2010 he accepted the chair of Molecular Inorganic Chemistry at the University of Groningen. Currently he holds the chair of Organometallic and Inorganic Chemistry at the University of Erlangen-Nürnberg.

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1 Introduction

Although the current interest in hydrogen storage materials has propelled amidoborane compounds to the forefront of research, the story of metal amidoboranes has humble beginnings. In 1938, Schlesinger and Burg were researching the then elusive structure of the “diammoniate of diborane”, B₂H₆·2NH₃.¹ As part of their extensive investigations, they found that the reaction of sodium metal with BH₃·OMe₂ in liquid ammonia produced hydrogen and a compound with an elemental analysis consistent with NaNH₂BH₃. This was taken as evidence for the structure [NH₄⁺][BH₃NH₂BH₃⁻] for the diammoniate, on the basis that the presence of sodium inhibits the formation of ammonium ions, and thus the diammoniate does not form under these conditions. This turned out to be an erroneous conclusion: after the discovery of the borohydride anion by the same group,² Shore and Parry later confirmed the formulation of the diammoniate as [NH₃BH₂NH₃⁺][BH₄⁻]³ and a crystal structure was reported recently.⁴ In spite of this, the novel NH₂BH₃⁻ anion represented an interesting development, and the existence of a BH₃NH₂BH₃⁻ anion was eventually confirmed by the recent report of the thermal rearrangement of NaNH₂BH₃ to Na(BH₃NH₂BH₃) in refluxing THF.⁵ Nevertheless, it was over two decades after Schlesinger and Burg's initial discovery before the first amidoborane compounds were truly isolated and characterised. First Aftandilian et al., then Gilje and Ronan, reported syntheses of NaNMe₂BH₃ by reaction of dimethylamine borane with NaH and Na metal, respectively.^6,7 This compound was investigated sporadically over the ensuing years in the broader context of fundamental boron–nitrogen chemistry.^8,9

The first significant practical application of metal amidoborane compounds came in 1984, when Hutchins et al. reported the use of the sodium amidoborane compounds, NaN(H)^tBuBH₃ and NaNMe₂BH₃, as reducing agents for a variety of functional groups, including aldehydes, esters, ketones and certain amides.¹⁰ Although the sodium derivatives never found widespread use, the extension of this work to lithium amidoboranes by the group of Singaram^11–14 and others^15,16 established this class of compound as a useful alternative to classical reducing agents. These so-called ‘LAB reagents’ are more powerful reducing agents than borohydrides, safer and easier to handle than the ubiquitous LiAlH₄, and essentially infinitely variable by modification of the nitrogen substituents.^17,18 In some cases, unique chemistry can also be accessed by transfer of the amino group of the amidoborane to the substrate, for example the amination of aryl halides.^19,20

The resurgent interest in metal amidoboranes of the last decade is largely due to the recognition of the potential of the parent compounds, M(NH₂BH₃)_x, in hydrogen storage. The finite nature of the fossil fuels that are currently the source of the vast majority of the world's energy supplies, and the pollution and associated global warming that result from their combustion, implore us to discover alternatives. A vast amount of research effort is therefore being directed towards the possibility of transition to a hydrogen-based economy.²¹ Although in terms of production and consumption hydrogen could, at least in principle, be a renewable and pollution-free energy source (the only combustion product being water), there are currently significant obstacles to its storage and transportation.²² The U.S. Department of Energy currently has targets set for 2020 for on-board hydrogen storage/release systems for automobiles that require a gravimetric H₂ density of 55 g kg⁻¹ and a volumetric density of 40 g L⁻¹ for the whole system.²³ Compressed hydrogen gas requires a pressure of 700 bar to attain such parameters at ambient temperature, before one has even considered the dimensions of the special containers and pressure valves required for its safe handling. Liquid H₂ (boiling point: −252.9 °C) has a somewhat higher volumetric density (70 g L⁻¹), but comes with similar problems associated with storage at such low temperature. In addition, both of these solutions come with a large energy penalty for pressurizing or liquefaction, respectively.

Storing hydrogen is especially a volumetric problem and therefore the only realistic approach is chemical storage in a much denser solid or fluid that can release H₂ ‘on demand’. Probably the most studied compound in this regard is ammonia-borane (AB, NH₃BH₃). AB contains 19.6% hydrogen by weight, is a stable, non-flammable solid under standard conditions that does not react with water, and releases hydrogen either thermally or in the presence of a catalyst.^22,24,25 While unparalleled in terms of hydrogen content, AB suffers from numerous problems as a storage material. These include slow kinetics, resulting in a higher-than-ideal H₂-release temperature (>100 °C), concurrent release of the rather stable borazine as by-product, which acts as a fuel-cell poison, and foaming during hydrogen release.²⁶ Furthermore, the dehydrogenation reaction is highly exothermic, making the prospect of reversibility unlikely.

In 2007, Burrell and co-workers discovered that some of these problems could be overcome, simply by replacing one of the N–H protons with a metal cation.²⁷ They reported that Ca(NH₂BH₃)₂ releases 3.6 equivalents of H₂ (7.2 wt%) within the range of 120 °C and 170 °C, with no induction period and no foaming. Nevertheless, traces of ammonia and borazine were still detected. Shortly afterwards, the group of Chen reported further improved properties by using alkali metals. LiNH₂BH₃ and NaNH₂BH₃ were both found to release two equivalents of H₂, representing 10.9 and 7.5 wt% of the materials, at the considerably lower temperatures of 92 °C and 89 °C, respectively.²⁸ Moreover, there was no evidence of borazine release, and no foaming was observed during the decomposition. Differential scanning calorimetry experiments on the materials also indicated that hydrogen release from these compounds is considerably less exothermic than for AB itself, potentially making fuel regeneration more feasible. This decomposition chemistry was rapidly extended to amidoborane compounds of other s-block metals.^29,30 Unfortunately, in all cases, the products of amidoborane thermal decomposition are amorphous solids, and characterisation has generally been limited to elemental analysis and solid-state NMR spectroscopy. Since this breakthrough, efforts within the materials chemistry community have focussed on developing these compounds to improve the available hydrogen content, release temperature and regeneration properties.^31–39 From an organometallic chemistry perspective, an interest has also developed in amidoborane moieties as ligands. Solution-phase NMR spectroscopy and single crystal X-ray diffraction techniques have allowed further insight into their unique reactivity and provided mechanistic clues about solid-state processes. The advent of catalytic amine borane dehydrocoupling as a route to new B–N oligo- and polymers has only increased the need for improved understanding of the fundamental steps involved.^40–45 The extension of this transition metal dominated chemistry to cheaper, less toxic metals is beginning to be realised.

2 Scope

The aim of this review is to summarise the chemistry of s-block metal amidoborane compounds from the point of view of the main-group organometallic chemist. This enables us to discuss this old but highly relevant class of compound from a perspective in which synthetic methods, detailed structures, reactivity and mechanisms are important. We have chosen to focus on well-defined, soluble species, and predominantly those that have been crystallographically characterised. Due to the wide applicability of metal amidoborane complexes, aspects of their chemistry have previously been covered in review articles from the perspectives of hydrogen storage materials,^31,46,47 the catalytic dehydrogenation of amine–boranes^48–50 and the reduction of unsaturated organic functional groups.^17,18 However, these existing articles either touch only briefly on the s-block amidoboranes in the context of a broader topic, or focus on the applications of the compounds in materials chemistry or organic synthesis. Our emphasis here is on the synthesis, structures and reactivity of s-block metal amidoborane complexes, although applications of the compounds in H₂ storage, catalysis and reduction chemistry will be discussed for context where appropriate.

3 Bonding in early main-group metal amidoboranes

The bonding situation in amidoborane compounds is interesting, due to the plethora of intra- and intermolecular interactions that can occur. The bonding between the metal cation and the amidoborane anion takes place primarily via interaction of the amide nitrogen with the metal centre. A further contact is typically present between the borohydride moiety and the metal. In the parent amidoborane compounds, Mⁿ⁺(NH₂BH₃)_n, these two interactions lead to extended polymeric networks in the solid state, while further N–H^δ+⋯H^δ−–B, and even homopolar B–H^δ−⋯H^δ−–B and N–H^δ+⋯H^δ+–N dihydrogen bonding interactions contribute to the stability of the structure, as well as influencing the propensity for H₂ release.^51–53 In compounds containing larger supporting ligands, intermolecular interactions between amidoborane units are inhibited, and both terminal and bridging amidoborane ligands are possible. Terminal amidoborane ligands display M⋯H–B interactions, which display notable similarities to β-agostic M⋯H–C interactions in metal ethyl complexes, and are believed to play a similar role in β-hydride elimination processes.^54,55

The bonding of borohydride compounds to transition metals is predominantly a donation from the σ_(B–H) orbital to the metal centre, forming a three-centre, two-electron bond.^56–58 For the s-block metals, metal–ligand bonding is primarily electrostatic, and any covalent contribution is likely to be minor.⁵⁹ Mulliken population analysis performed by Wu et al. on LiNH₂BH₃ and Ca(NH₂BH₃)₂ supported the assignment as largely ionic compounds;⁶⁰ charges of +0.98 for Li and +1.67 for Ca are close to the formula valences, while in each case the nitrogen atom bears a charge of roughly −1. The slightly negative charges on boron (−0.33 and −0.28, respectively) and the B-bound hydrogens (between −0.09 and −0.17) suggest a weaker, more diffuse interaction between the metal and this group. The labile nature of the M⋯H–B bonding is supported by the observation of a single resonance in the solution phase ¹H{¹¹B} NMR spectrum for the BH₃ group, for compounds where this data is available. The ¹¹B chemical shifts of an amine–borane and its s-block metal salts are also typically remarkably similar. The ¹¹B–¹H J-coupling constant can, however, provide spectroscopic evidence for amidoborane formation, as it typically becomes smaller upon deprotonation at nitrogen (e.g.¹J_BH(HNMe₂BH₃) = 96 Hz,⁶¹¹J_BH(LiNMe₂BH₃) = 86 Hz).⁵³ Although the BH₃⋯M interaction has yet to be comprehensively investigated for s-block amidoborane compounds, computational studies have shown the related borohydrides, LiBH₄ and Ca(BH₄)₂, to display exclusively ionic interactions between the cations and anions.^62,63 The subsequently discussed compounds display a range of BH₃⋯M bonding motifs, including the metal being ligated by a single hydride of a BH₃ group, by two geminal hydrides (bifurcating), and also the bridging of a hydride between two metal ions. Rather than being determined by orbital interactions, the choice of bonding mode is thus more heavily dependent on steric and conformational factors.

4 Group 1 metals

4.1 Group 1 amidoboranes in hydrogen storage

Despite the prominence of the group 1 metal amidoboranes in materials chemistry and synthesis, there are only a relatively small number of structurally characterised examples. The parent amidoborane complexes, MNH₂BH₃ (M = Li (1), Na (2), K (3)), are well defined, with structures determined by single crystal and/or powder X-ray diffraction. All of the structures consist of polymeric networks held together by intermolecular interactions. The lithium and sodium compounds are structurally very similar,^28,64 with the alkali metal cation in a pseudo-tetrahedral environment, coordinated by one nitrogen atom and three borohydride moieties (Fig. 1). This similarity is presumably partly responsible for the compounds' practically identical decomposition patterns. Conversely, the larger cation in KNH₂BH₃ is octahedrally coordinated by three nitrogen atoms and three borohydride moieties.³⁰ Despite the obvious size differences between the cations, the structure of the amidoborane fragment remains relatively constant, with all of the B–N distances falling between 1.53 Å and 1.56 Å. As well as conventional B–H^δ−⋯H^δ+–N heteropolar dihydrogen bonds,⁶⁵ the Li and Na compounds display counterintuitive, homopolar B–H^δ−⋯H^δ−–B and N–H^δ+⋯H^δ+–N dihydrogen bonding interactions.^66,67 Although the observation of these latter homopolar interactions may be quite unexpected, there is a notable similarity to the C–H⋯H–C interactions that are becoming increasingly recognised.^68–70 Homopolar B–H^δ−⋯H^δ−–B interactions are dominated by van der Waals attraction, but AIM analysis shows that they can be quite significant. For example, calculations on LiNH₂BH₃ show a bond critical point between hydride atoms. The accumulation of electron density at this point is comparable to that found for Li⁺⋯H^δ−–B bonding, and nearly twice as high as that found for more conventional N–H^δ+⋯H^δ−–B interactions. Also noteworthy is the observation that LiND₂BH₃ eliminates some H₂ as well as HD upon heating, indicating that such homopolar interactions may play a role in hydrogen release.⁵² Potassium amidoborane is unique in that it melts before eliminating hydrogen, but the decomposition temperature is nevertheless remarkably similar to that of the Li and Na salts (80 °C).

Fig. 1 Portion of the crystal structure of LiNH₂BH₃ (1), displaying the pseudo-tetrahedral coordination environment at Li.

Treatment of compound 1 with ammonia results in formation of the monoammoniate, LiNH₂BH₃·NH₃, which releases up to 11 wt% hydrogen in the remarkably low temperature range of 40–55 °C.^33,71 The dehydrogenated material can also be partially regenerated by treatment with hydrazine.⁷² Unfortunately, facile absorption/desorption of ammonia rules out the use of this compound in fuel cells, and as yet no structure has been reported. The AB adduct of 1, LiNH₂BH₃·NH₃BH₃, was found by Chen and co-workers to release 14 wt% of H₂, with peak release temperatures of 80 °C and 140 °C – remarkably, lower than both LiNH₂BH₃ and AB themselves – and with no detectable ammonia or borazine formation.⁷³ The crystal structure of this compound shows alternating LiNH₂BH₃ and AB layers, with lithium pseudo-tetrahedrally coordinated by one nitrogen atom and three borohydride groups, as in the parent compound.

A small number of mixed-metal amidoboranes has been prepared, with the objective of improving the dehydrogenation characteristics of the materials. The first of these, Na[Li(NH₂BH₃)₂], was synthesised by Grochala and co-workers.⁷⁴ The crystal structure of the compound consists of pseudo-tetrahedral lithium ions coordinated by three N atoms, alongside one Li–borohydride interaction, while the sodium atoms are solely coordinated by borohydride moieties. The compound releases hydrogen, beginning at 75 °C, albeit contaminated with ammonia.³⁷ Shortly afterwards, Wu prepared Na₂[Mg(NH₂BH₃)₄], which also produces a significant quantity of H₂ (8.4%) starting at 65 °C; in this case, contamination with ammonia and borazine was minimal.³⁸ The structure of this compound contains [Mg(NH₂BH₃)₄]²⁻ dianions held together by sodium ions. A similar structure is also observed for the potassium derivative, K₂[Mg(NH₂BH₃)₄].⁷⁵ The formation of NaMg(NH₂BH₃)₃ has been invoked for the extremely low H₂-release temperature (from 45 °C) of ball-milled mixtures of AB and NaMgH₃.⁷⁶ Some of these mixed-metal species display lower hydrogen-release temperatures, and with less contamination from fuel-cell poisons, than their monometallic counterparts. Additionally, it is thought that the presence of a secondary, ‘softer’ cation within the material can improve the thermodynamic stability – a fact that bodes well for the development of a reversible process.⁷⁷

Nöth and co-workers performed a structural study on the known reducing agent LiNMe₂BH₃ (4) in combination with various solvents.⁷⁸ While the compound was found to be poorly soluble in aromatic solvents, addition of coordinating solvents led to a variety of different adducts. 4·TMEDA (TMEDA = N,N,N′,N′-tetramethylethylenediamine) has a dimeric structure containing an 8-membered ring formed by Li⁺⋯H^δ−–B interactions (Fig. 2). Adducts with 1,4-dioxane, trioxane, and 1,3-dioxolane all form layered polymeric structures, whereas crystallisation from the polyether [12]crown-4 allowed retention of the monomeric structure. More recently, McGrady and coworkers found that both 4 and KNMe₂BH₃ (5) could be crystallised as solvent-free compounds from THF. Both possess polymeric structures, and the authors found further evidence for B–H⋯H–B interactions in both compounds in the absence of conventional protic hydrogen bond donors.⁵³ The diisopropyl derivative, LiNⁱPr₂BH₃, crystallises with [12]crown-4 to produce the salt [Li([12]crown-4)₂⁺][Li(NⁱPr₂BH₃)₂⁻].⁷⁹ This species is unique among lithium amidoboranes for possessing the only structurally characterised example of a ‘side-on’ bound amidoborane ligand, with the nitrogen and borohydride moieties bound to the same metal atom (Fig. 2). Comparison with the structure of 4·[12]crown-4 illustrates the profound impact of the nitrogen substituents on the aggregation of these compounds.

Fig. 2 Crystal structures of TMEDA and [12]crown-4 adducts of LiNMe₂BH₃ (left) and [Li([12]crown-4)₂⁺][Li(NⁱPr₂BH₃)₂⁻] (right).

4.2 Group 1 metals in amine–borane dehydrocoupling

Keller reported in 1975 that the reaction of dimethylamine borane with LiH or KH produces B–N coupled complexes, M(NMe₂BH₂NMe₂BH₃), that react further with metal hydride to afford MNMe₂BH₃ compounds.⁸⁰ The group of Hill recently went on to report the crystal structure of the potassium complex, K(NMe₂BH₂NMe₂BH₃), as its dimeric THF adduct (6, Fig. 3), in this case prepared from KN(SiMe₃)₂ and Me₂NHBH₃ (Scheme 1).⁸¹ The dimeric structure of 6 is held together by K⋯H–B interactions. They also found that the group 1 amides, MN(SiMe₃)₂ (M = Li, Na, K), can perform the catalytic dehydrocoupling of dimethylamine borane, producing predominantly the cyclic diborazane (Me₂NBH₂)₂. The rate of reaction was considerably slower than for the equivalent Mg-catalysed process (vide infra), which may be due to the formation of insoluble metal hydride aggregates (MH)_∞. Details of the dehydrocoupling process are discussed in more detail in Section 5.2.

Fig. 3 Crystal structure of [K(NMe₂BH₂NMe₂BH₃)]₂·(THF)₃.

Scheme 1 Dehydrogenative on-metal coupling of two amidoborane units.

4.3 Group 1 MNR₂BH₃ compounds as reagents in salt metathesis reactions

There are a number of examples of group 1 metal amidoboranes being used in salt metathesis reactions with other organometallics (Scheme 2). The first characterised transition metal amidoborane complexes, reported by Roesler and co-workers, were prepared via a salt metathesis route.⁸² The zirconocene complexes (η⁵-C₅H₅)₂Zr(H)NH₂BH₃ and (η⁵-C₅Me₅)₂Zr(H)NH₂BH₃ were prepared by the reaction of the corresponding zirconocene dichloride with NH₃BH₃ and ⁿBuLi (2 equivalents). Use of a single equivalent of AB and base led to the complex (η⁵-C₅H₅)₂Zr(Cl)NH₂BH₃. Although lithium amidoborane was not specifically identified, its intermediacy in the reaction is to be assumed. McGrady and co-workers produced crystals of the Ti(III) compound (η⁵-C₅H₅)₂TiNH₂BH₃ from the reaction between (η⁵-C₅H₅)₂TiCl₂ and LiNH₂BH₃ (Scheme 2).⁵⁴ Similarly, Helten et al. later reported that titanocene dichloride reacts with two equivalents of LiNMe₂BH₃ to produce the species (η⁵-C₅H₅)₂TiNMe₂BH₃ in good yield, with concurrent release of H₂.⁸³ The corresponding reaction with (η⁵-C₅H₅)₂ZrCl₂ produces (η⁵-C₅H₅)₂Zr(H)NMe₂BH₃.

Scheme 2 Salt metathesis reactions of group 1 amidoboranes.

In several cases the amidoborane complex is not the final product. The groups of Harder and Schulz exploited this chemistry to gain access to zinc hydride compounds by reaction of mono- and bimetallic diketiminate zinc iodide complexes with KN(H)ⁱPrBH₃ (Scheme 2).^84,85 Likewise, treatment of the aluminium complex (DIPP-nacnac)AlCl₂ (DIPP = 2,6-ⁱPr₂C₆H₃; DIPP-nacnac⁻ = (2,6-ⁱPr₂C₆H₃)NC(Me)C(H)C(Me)N(2,6-ⁱPr₂C₆H₃)⁻) with KN(H)RBH₃ (R = ⁱPr, DIPP) produces (DIPP-nacnac)AlH₂ in good yield by presumed double β-hydride elimination.⁸⁶ NaNMe₂BH₃ has also been used to convert ReBr₂(NO)(η²-H₂)(PR₃)₂ species into tetrahydrides Re(H)₄(NO)(PR₃)₂.⁸⁷ Quantitative formation of (Me₂NBH₂)₂ suggests an insertion/β-hydride elimination process (vide infra). In a similar reaction, RuHCl(η²-H₂)(PⁱPr₃)₂ reacts with LiNMe₂BH₃, in this case forming the ruthenium aminoborane complex RuH₂(η²:η²-H₂BNMe₂)(PⁱPr₃)₂.⁸⁸

Lancaster and co-workers have studied the effect of electron-withdrawing substituents at boron on the reactivity of lithium amidoboranes.^89–91 To this end, the compounds LiNH₂B(C₆F₅)_n(H)_3−n (n = 1–3) were prepared as THF solvates, by reaction of the appropriate ammonia-borane precursor with n-butyllithium or LiN(SiMe₃)₂.⁹¹ Addition of [12]crown-4 to LiNH₂B(C₆F₅)₂H·(THF)₂ (7·THF₂) produced the crown ether adduct ([12]crown-4)LiNH₂B(C₆F₅)₂H, which could be structurally characterised by X-ray diffraction. At 1.544(2) Å, the B–N bond is markedly shorter than in the parent amine–borane, NH₃B(C₆F₅)₂H (1.6034(19) Å), but unremarkable compared to other amidoborane species. Reactions of these species with group 4 metal complexes produced a variety of interesting products. Combination of 7 with Cp₂ZrCl₂ led to either Cp₂Zr(NH₂B(C₆F₅)₂H)₂ or Cp₂Zr(Cl)(NH₂B(C₆F₅)₂H), depending on the stoichiometry, the former of which exists in equilibrium with Cp₂Zr(H)(NH₂B(C₆F₅)₂H) and aminoborane H₂NB(C₆F₅)₂, presumably via a reversible β-hydride elimination/insertion.⁸⁹ The corresponding reaction with Cp₂HfCl₂ predominantly yields the N–H activation product Cp₂Hf(NHB(C₆F₅)₂H).⁹⁰ It is proposed that this occurs by the second equivalent of 7 acting as a base to deprotonate the hafnium amidoborane group. Increasing the number of pentafluorophenyl substituents at boron has the expected effect of reducing the nucleophilicity of the amide group. This is demonstrated by the failure of LiNH₂B(C₆F₅)₃ (8) to react with Cp₂MCl₂ (M = Ti, Zr).⁸⁹ This amidoborane ligand could, however, be transferred to Zr and Hf by reaction of 8 with the zwitterionic compounds Cp₂M(Me)(MeB(C₆F₅)₃), with precipitation of the salt Li[MeB(C₆F₅)₃] as by-product. In a similar fashion, the amidoborane group could also be transferred to other Lewis acids, such as E(C₆F₅)₃ (E = B, Al), forming ‘ate’ complexes with a [Li(THF)₄]⁺ counterion.⁸⁹

5 Group 2 metals

5.1 Amidoborane complexes as models for hydrogen storage compounds

Calcium amidoboranes. The first example of a group 2 amidoborane compound was the calcium species [Ca(NH₂BH₃)₂(THF)₂]_∞ (9), reported by Burrell and co-workers in 2007, formed by the reaction of NH₃BH₃ with CaH₂ in THF (Scheme 3).²⁷ Compound 9 crystallises as a coordination polymer with short intermolecular contacts between the B–H hydrogen atoms and calcium (Fig. 4). The remaining THF could be removed under vacuum to afford the salt-like [Ca(NH₂BH₃)₂]_∞, a material containing 10.1 wt% hydrogen, which could be characterised as a 3-dimensional network by powder X-ray diffraction.⁶⁰ Thermal gravimetric analysis of the compound showed that it released H₂, with no induction period, over a temperature range of 120–170 °C. A total of 3.6 equivalents were generated, corresponding to 7.2% of the initial mass, and only minimal contamination of the released hydrogen with ammonia and borazine was observed. As with the alkali metal amidoboranes, the residue formed after heating was amorphous and could not be characterised beyond an elemental composition of CaN₂B₂H₂C_0.2. The difficulty in the characterisation of solid-state amidoborane compounds, and especially of unknown products of thermal decomposition, sparked an interest in the solution-phase study of amidoborane complexes of the group 2 metals, which have since begun to reveal a rich and varied chemistry.

Scheme 3 Synthesis of calcium amidoborane coordination polymers.

Fig. 4 Crystal structure of [Ca(NH₂BH₃)₂(THF)₂]_∞ (9).

Divalent metals present a unique opportunity for mechanistic study when compared to the alkali metal amidoboranes, in that it is possible to introduce a monoanionic spectator ligand to create a well-defined active site and solubilise the resulting complexes. The strongly coordinating NN-bidentate β-diketiminate ligand, (2,6-ⁱPr₂C₆H₃)NC(Me)C(H)C(Me)N(2,6-ⁱPr₂C₆H₃)⁻ (DIPP-nacnac⁻) has considerable precedent in alkaline earth metal chemistry. This is due to its ability to stabilise a variety of unusual structures and to disfavour the formation of homoleptic species via Schlenk equilibria.^92–96 Harder and co-workers turned to this system to investigate the thermal decomposition of calcium amidoborane complexes under homogeneous conditions. The previously reported, stable, hydrocarbon-soluble calcium hydride complex [(DIPP-nacnac)CaH·(THF)]₂ (10)⁹⁷ was treated with NH₃BH₃ to produce the monomeric amidoborane (DIPP-nacnac)CaNH₂BH₃·(THF)₂ (11-H) in high yield.⁹⁸ In sharp contrast to 9, the crystal structure of compound 11-H revealed a ‘side-on’ coordination mode of the amidoborane ligand, in which a B–H hydride atom interacts with the calcium atom of the same molecule rather than containing intermolecular M⋯H interactions or N–H⋯H–B dihydrogen bonds.⁹⁹

While thermally stable up to 64 °C in the presence of THF, compound 11-H was found to decompose at temperatures as low as 20 °C in benzene solution, with concurrent evolution of H₂. The product of the reaction was identified as [(DIPP-nacnac)Ca(THF)]₂(HN-BH-NH-BH₃) (12-H, Scheme 5). The formation of this unprecedented [HN-BH-NH-BH₃]²⁻ dianion implied the doubly-dehydrogenative coupling of two [NH₂BH₃]⁻ units, and it was interpreted as an intermediate species on the way to fully dehydrogenated [NBH]⁻. The presence of excess THF presumably inhibits the reaction by preventing the aggregation of two molecules of 11-H.

A selection of other derivatives of 11-H with varied substituents at the nitrogen of the amidoborane was then prepared in order to investigate the influence on the dehydrogenation process. The complexes (DIPP-nacnac)CaN(R)HBH₃·(THF) (R = Me (11-Me),⁹⁸ⁱPr (11-ⁱPr),¹⁰⁰ DIPP (11-DIPP)¹⁰⁰) were prepared by reaction of the corresponding amine–borane either with [(DIPP-nacnac)CaH·(THF)]₂ (10) or the related (DIPP-nacnac)Ca(N(SiMe₃)₂)·(THF) (13). More recently, Hill et al. also used the latter compound to prepare 11-^tBu in the same fashion.¹⁰¹Table 1 shows a selection of structural data and decomposition temperatures for these compounds, from which the following trends can be deduced: (1) increasing steric bulk generally increases both the length of the Ca–N bond and the linearity of the Ca–N–C moiety, (2) the B–N bond distance is essentially independent of the R substituent, and (3) increasing the steric bulk increases the decomposition temperature relating to H₂ release.

Table 1 Relevant bond lengths and angles and decomposition temperatures of amidoborane compounds

Compound	M–NAB/Å	N–B^a/Å	M–NAB–C/°	T dec /°C	Ref.
a Bond length between the amido nitrogen and adjacent boron atom. b Temperature of initial hydrogen release.
NH3BH3	0.96(3), 1.07(4); 0.80(6), 0.96(4)	1.58(2); 1.599(8)	—	120	26, 65 and 102
LiNH2BH3	1.973	1.560	—	92	28
LiNH2BH3·NH3BH3	2.1(1)	1.55(9)	—	80	73
Li([12]crown-4)NH2B(C6F5)2H	2.015(3)	1.544(2)	—	—	90
Li([12]crown-4)NH2B(C6F5)3	2.052(3), 2.046(4)	1.552(3), 1.549(2)	—	—	89
Li([12]crown-4)NMe2BH3	1.999(8), 2.005(8)	1.553(7), 1.561(6)	108.1(3), 112.3(3), 112.9(3), 116.3(3)	—	78
[Li([12]crown-4)][Li(N^iPr2BH3)2]	2.052(9), 2.05(1)	1.546(7), 1.558(8)	108.5(3), 112.1(4), 120.2(3), 120.7(4)	—	79
NaNH2BH3	2.35	1.56	—	89	28
KNH2BH3	2.907(5)–3.364(5)	1.532(8)	—	80	30
Mg(NH2BH3)2	—	—	—	104	103
Mg(NH2BH3)2·NH3	2.181(6), 2.135(6)	1.539(8), 1.548(8)	—	50	104
Ca(NH2BH3)2	—	—	—	120	27
Ca(NH2BH3)2·(THF)2	2.069(7), 2.362(7)	1.52(2)	—	—	27
Sr(NH2BH3)2	2.68	1.53	—	60	29
(DIPP-nacnac)CaNH2BH3·(THF)2	2.399(2)	1.581(4)	—	20	98
(DIPP-nacnac)CaNH(Me)BH3·(THF)	2.382(4)	1.581(8)	125.5(3)	40	98
(DIPP-nacnac)CaNH(^iPr)BH3·(THF)	2.406(4)	1.582(7)	133.7(3)	100	100
(DIPP-nacnac)CaNH(DIPP)BH3·(THF)	2.460(2)	1.587(4)	147.6(2)	120	100
(DIPP-nacnac)CaNH(^tBu)BH3·(THF)	2.414(3)	1.554(6)	123.8(2)	—	101
(DIPP-nacnac)CaNH2BH3·(NH3)3	2.491(2), 2.528(2)	1.555(3), 1.566(3)	—	50	105
[(DIPP-nacnac)CaNH(^iPr)BH3]2	2.394(3)	1.516(4)	131.3(2)	—	105
(DIPP-nacnac)CaNMe2BH3·(THF)	2.375(3)	1.497(6)	119.4(2)	—	106
(DIPP-nacnac)CaN(CH2)4BH3·(THF)	2.405(3)	1.533(6)	118.2(5), 126.5(5)	—	107
(DIPP-nacnac)MgNH2BH3·(THF)	2.056(3)	1.544(6)	—	80	108
[(DIPP-nacnac)MgNH(^iPr)BH3]2	2.122(1)	1.556(2)	122.57(9)	110	108
(DIPP-nacnac)MgNH(DIPP)BH3	2.083(4)	1.626(9)	145.2(4)	120	109
PYR-[MgNH(^iPr)BH3]2	2.123(4)	1.571(6)	113.4(2), 113.8(2)	90	108
PYR-[MgNH(DIPP)BH3]2	2.121(1), 2.130(1)	1.591(2), 1.595(2)	129.64(9), 130.15(9)	90	108
NN-[MgNH(^iPr)BH3]2	2.093(2)	1.590(3)	125.3(2), 131.2(1)	60	108
[K(NMe2BH2NMe2BH3)]2·(THF)3	2.8703(17)	1.503(3)	98.0(1), 98.5(1), 100.9(1), 102.9(1)	—	81
Mg(NMe2BH2NMe2BH3)2·(THF)	2.1667(11)	1.5647(11)	111.39(9), 111.57(8), 111.79(8), 112.32(9)	—	106
(DIPP-nacnac)MgNMe2BH2NMe2BH3	2.0929(14)	1.586(3)	111.5(1), 112.9(1)	—	106
(DIPP-nacnac)MgN(CH2)4BH2N(CH2)4BH3	2.1146(19)	1.562(3)	113.1(2), 113.4(2)	—	107

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11-Me and 11-ⁱPr thermally release H₂ at 40 and 100 °C, respectively, to afford compounds (12, Scheme 5) containing [RN-BH-NR-BH₃]²⁻ dianions. 12-Me displays a similar structure to 12-H, albeit with only one Ca atom bearing a THF ligand (Fig. 5), while in 12-ⁱPr, the steric bulk prevents the terminal nitrogen from bridging both Ca atoms and it only ligates that without a THF ligand. In contrast, the highly bulky 11-DIPP decomposes to form borylamide complex 14. This is likely due to a steric barrier to dimerization, but could also relate to the higher acidity of the remaining N–H proton in comparison to that of alkyl derivatives. Compound 11-^tBu was not tested for thermal decomposition, instead having been prepared for catalytic dehydrocoupling investigations (vide infra).

Fig. 5 Crystal structure of compound 11-Me (l) and its thermal decomposition product 12-Me (r), displaying the [NMe-BH-NMe-BH₃]²⁻ dianion. ⁱPr groups and most H atoms are omitted for clarity.

Calcium amidoborane ammine complexes. The imbalance of protic and hydridic hydrogen atoms within metal NH₂BH₃ complexes represents one of their drawbacks for use in hydrogen storage; this makes it problematic to release the entire hydrogen content of the molecule. Chua et al. serendipitously discovered that the reaction of Ca(NH₂)₂ and NH₃BH₃ yields the amidoborane–ammine compound Ca(NH₂BH₃)₂·(NH₃)₂ (15).³⁶ The presence of ammonia in the crystal lattice of 15 increases protic hydrogen content, facilitates intermolecular dihydrogen bonding and thus favours hydrogen release.¹¹⁰ This compound releases up to 5.5 equivalents of H₂ over the temperature range of 70–150 °C in a closed system, confirming that hydrogen from NH₃ is also released, and suggesting that ammoniate complexes could display improved storage characteristics. In order to probe this behaviour in solution, the Harder group therefore went on to prepare DIPP-nacnac-supported calcium amidoborane complexes with NH₃ as an auxiliary ligand.¹⁰⁵

Complexes of the form (DIPP-nacnac)Ca[NH(R)BH₃]·(NH₃)_x (R = H (16-H), Me (16-Me), ⁱPr (16-ⁱPr), DIPP (16-DIPP); x = 2 or 3) were prepared by the reaction of the precursor [(DIPP-nacnac)Ca(μ-NH₂)·(NH₃)₂]₂ with the respective amine boranes. Solid-state structures of the isolated complexes revealed varying connectivity of the ammonia molecules; not only were N–H⋯H–B dihydrogen bonds observed, but in one case there was also an intermolecular interaction between the protic N–H hydrogen atoms and the central carbon of the DIPP-nacnac ligand.

The thermal decomposition of these complexes occurred at the relatively low temperature of 50 °C, but the dehydrogenation was complicated by side-reactions. Loss of NH₃ was generally observed upon heating, allowing crystallisation of monoammine and ammine-free products. Nevertheless, hydrogen release was observed in all cases, and evidence for the involvement of NH₃ was provided by the formation of [(DIPP-nacnac)Ca(μ-NH₂)]₂ (17). The protonation of the DIPP-nacnac anion was also found to be a significant side-reaction, and the formation of DIPP-nacnacH was accompanied by the precipitation of an insoluble residue, proposed to be the coordination polymer {H₂NCa[NH(R)BH₃]}_∞. No soluble boron-containing species were formed, supporting the likely formation of polymeric residues after dehydrogenation. Interestingly, and in contrast to the ammine-free derivatives, the decomposition temperature of compounds 16 was independent of the nitrogen substituent, R, of the amidoborane. This was a clear indication that the protic hydrogen originates from ammonia, rather than from the amidoborane unit. This is in agreement with theoretical calculations by Chen and co-workers, which conclude that the lowest-energy hydrogen release process from Ca(NH₂BH₃)₂·(NH₃)₂ involves combination of ammonia N–H and amidoborane B–H moieties.¹¹¹ It was proposed that the amidoborane ligand undergoes hydride elimination to a hydride–ammine complex, which then rapidly releases H₂ to produce the calcium amide. This, in combination with results from solid-state experiments, suggests that metal amide formation is a feasible decomposition pathway for solid-state amidoborane–ammine materials.^33,104

Magnesium amidoboranes. The parent magnesium amidoborane, Mg(NH₂BH₃)₂ (18), was recently reported,¹⁰³ despite earlier unsuccessful attempts to prepare it^110,112 and suggestions of its instability.¹⁰⁴ It was prepared by the aging of a ball-milled mixture of MgH₂ (or Mg powder) and NH₃BH₃ at 70 °C, and it released roughly 10 wt% of pure H₂ in several stages at temperatures between 104 °C and 300 °C. Although strong evidence was presented for the compound's formation, no crystal structure could be obtained. The identity of the decomposition product(s) remains unclear, although solid-state ¹¹B NMR and FTIR spectroscopic studies indicate the presence of 3-coordinate boron atoms and B–H bonds, respectively.¹⁰³ Two ammoniates of magnesium amidoborane have, however, been prepared in crystalline form. Chen and co-workers produced the compound Mg(NH₂BH₃)₂·(NH₃) (19) from the reaction of MgNH with NH₃BH₃.¹⁰⁴ Compound 19 releases up to 11.4 wt% of H₂ up to 300 °C, and in contrast to the calcium derivative, shows quantitative conversion of NH₃, probably due to the smaller amount of coordinated NH₃. Also, the higher charge density of Mg²⁺ makes it more Lewis-acidic than Ca²⁺, thus resulting in significantly stronger metal–NH₃ bonding. The compound Mg(NH₂BH₃)₂·(NH₃)₃ (20) has also been prepared by ball-milling AB with MgH₂ under NH₃.¹¹³ This species is interesting in that its crystal structure consists of alternating layers of [Mg(NH₂BH₃)₄]²⁻ and [Mg(NH₃)₆]²⁺ units. Compound 20 releases seven equivalents of H₂ at 300 °C, or 10.6 wt%.

In order to study the influence of the metal on the properties of amidoborane complexes, Harder and co-workers went on to perform experiments with magnesium β-diketiminate complexes. Reaction of (DIPP-nacnac)MgN(SiMe₃)₂ (21) with primary amine boranes RNH₂BH₃ produced the corresponding magnesium amidoboranes, (DIPP-nacnac)MgNH(R)BH₃ (R = H (22-H), Me (22-Me), ⁱPr (22-ⁱPr),¹⁰⁸ DIPP (22-DIPP)¹⁰⁹). With the exception of 22-DIPP, the compounds exist as dimers or aggregates, which can be fragmented and solubilised by addition of THF. The increased tendency of the amidoborane to bridge between metal centres is presumably a result of the smaller magnesium dication leading to a more strained four-membered M–N–B–H ring in terminal amidoborane complexes. Structural comparisons are hampered by the differing geometries and coordination numbers of the crystallised products as well as disorder in the amidoborane fragments. Nevertheless, it is evident that the M–N bonds are roughly 0.3 Å shorter with the smaller magnesium dication compared to calcium (see Table 1). An increase in the amidoborane C–N bond length with increasing steric hindrance could also be deduced, as is the case for the calcium analogues. At 1.626(9) Å, 22-DIPP contains the longest B–N bond of any amidoborane compound discussed here. Jones, Stasch and co-workers went on to show that the known amidoborane complexes (22-H)₂ and 22-H·THF and the equivalent new compounds based on a bulkier nacnac ligand, (2,6-ⁱPr₂C₆H₃)NC(^tBu)C(H)C(^tBu)N(2,6-ⁱPr₂C₆H₃)⁻, could be obtained via a variety of different routes (Scheme 4).¹¹⁴ These included reaction of NH₃BH₃ with [(DIPP-nacnac)Mg(μ-H)]₂ (H₂ elimination), [(DIPP-nacnac)Mg(μ-Bu)]₂ (butane elimination) and the Mg(I) dimer [{(DIPP-nacnac)Mg}₂] (reductive dehydrogenation). Salt metathesis of [(DIPP-nacnac)MgI·(OEt₂)] with LiNH₂BH₃ also afforded the same compounds.

Scheme 4 Summary of synthetic routes to β-diketiminate group 2 amidoborane complexes. Coordinated solvent molecules (THF, NH₃, Et₂O) are present in some cases but are omitted for generality.

Scheme 5 Predominant decomposition pathways of β-diketiminate calcium amidoborane complexes.

Thermal decomposition was again performed by Spielmann et al. for the Mg complexes 22 (Scheme 6).¹⁰⁸ The complexes were found to be considerably more stable than their Ca counterparts, requiring temperatures of at least 80 °C to effect decomposition. A clear dependence on the nature of the nitrogen substituent was again observed. The complexes 22-H and 22-Me both decomposed unselectively at 80 °C in benzene. The only defined product identified, albeit in small quantities, was [(DIPP-nacnac)Mg]₂(H₃B-NMe-BH-NMe)·(THF), in which, unlike its calcium analogue, the BNBN dianion bridges unsymmetrically between the metal centres. 22-ⁱPr decomposed at the higher temperature of 110 °C, but instead of yielding a new amidoborane complex, the known magnesium hydride dimer, [(DIPP-nacnac)Mg(μ-H)]₂ ([23]₂) was formed in high yield alongside H₂ and the borazine (ⁱPrNBH)₃. This was proposed to occur by release of ⁱPr(H)NBH₂ (via β-H elimination), followed by a magnesium hydride-catalysed oligomerisation to the trimer. The high yield (89%) of the magnesium hydride suggests that amidoborane complexes could be promising precursors to other main-group metal hydride complexes. The bulky 22-DIPP decomposed at 120 °C in the same fashion as its calcium analogue, yielding the borylamide complex (DIPP-nacnac)MgN(DIPP)BH₂·(THF) (24).

Scheme 6 Decomposition pathways of monometallic Mg amidoborane complexes.

The mechanistic conclusions drawn from these results are summarised in Scheme 7. Mg and Ca amidoborane complexes initially decompose via β-hydride elimination to form metal hydride species and free aminoboranes, R(H)N=BH₂, which may initially coordinate to the metal centre. The subsequent course is dictated by the reactivity of the metal hydride species (Ca > Mg) and the nitrogen substituent. In case of metal hydrides of low reactivity, as is notably the case for zinc hydrides, the aminoborane R(H)N=BH₂ is simply released and forms oligomers, thus giving the hydride complex as a product.¹¹⁵ In case of more reactive metal hydride complexes (Mg, Ca) the coordinated aminoborane R(H)N=BH₂ may react. Species with small N-substituents readily dimerise, likely via nucleophilic attack of a bound amide group at the boron of a coordinated aminoborane, leading eventually to the stable [RN-BH-NR-BH₃]²⁻ dianion. Larger N-substituents such as DIPP produce aminoboranes that do not dimerise and are then deprotonated by the metal hydride to form borylamide complexes.

Scheme 7 Alternative bimetallic decomposition routes of metal amidoborane complexes.

Related computational studies by Kim and co-workers on the decomposition of LiNH₂BH₃ also advocated a dehydrogenation process involving a coupling of two BN units.³² These authors found that the “BNBN” decomposition route had a considerably lower activation barrier than borylamide formation, which correlates to the decomposition temperatures of group 2 amidoborane model complexes; decomposition via B–N coupling is a more facile process, and thus proceeds at lower temperatures, than formation of borylamide complexes. On the basis of DFT investigations, direct H₂ elimination from amidoborane complexes is regarded to be unlikely for both monometallic³² and bimetallic¹⁰⁸ decomposition pathways.

Bimetallic magnesium amidoborane complexes. The Harder group prepared dinuclear amidoborane complexes based on bis-β-diketiminate ligands.¹⁰⁸ Due to the necessarily bimolecular nature of some of the aforementioned dehydrogenation reactions, enforcing the proximity of two magnesium amidoborane units in the same molecule was expected to affect the decomposition. Furthermore, this system provides a better model for the decomposition of solid-state amidoborane hydrogen-storage materials, Mⁿ⁺(NH₂BH₃)_n. Magnesium amidoborane complexes of ligands consisting of external DIPP-nacnac units bridged by pyridylene (PYR) and meta-phenylene (META) moieties, and one containing a direct N–N linkage (NN, Scheme 8), were prepared via the reaction of their MgⁿBu analogues with ⁱPrNH₂BH₃. The C₂-symmetric PYR complex, PYR-[MgNH(ⁱPr)BH₃]₂ (25-PYR) exhibits two amidoborane ligands that bridge the two magnesium centres via a M–N bond and B–H⋯M interactions. The N–N bound complex, 25-NN, could only be crystallised in the presence of THF, producing an asymmetric structure with a THF molecule coordinated to one magnesium centre, with one bridging amidoborane and one side-on bound amidoborane present.

Scheme 8 Synthesis and thermal decomposition of bimetallic magnesium amidoborane complexes.

Both 25-NN and 25-PYR were found to decompose (T_dec = 60 °C (25-NN), 90 °C (25-PYR)) with release of H₂, producing complexes LMg₂[(ⁱPr)NBHN(ⁱPr)BH₃] (26) rather than metal hydride species as observed for the monometallic analogue. Clearly, the enforced proximity of two amidoborane ligands favours the formation of the BNBN unit. The complex META-[MgNH(ⁱPr)BH₃]₂ (25-META) decomposed in a similar fashion at 90 °C, although in this case ligand exchange reactions allowed isolation of the products META-Mg₃[(ⁱPr)N-BH-N(ⁱPr)-BH₃]₂ (27) and (META-Mg)₂ (28, Scheme 8). The crystal structure of 27 revealed one Mg atom to be solely bound to two BNBN dianion moieties through both N atoms and one short borohydride contact each. It is distinctly possible that this could be illustrative of the bonding situation in certain decomposition products of solid-state alkaline earth metal amidoboranes. Interestingly, the complex PYR-[Mg(NH(DIPP)BH₃)]₂ (29) undergoes a single β-hydride elimination upon exposure to THF, yielding the magnesium hydride species PYR-Mg[NH(DIPP)BH₃](μ-H)Mg·(THF) (30), in contrast to the borylamide formation from the monometallic analogue 22-DIPP.¹¹⁶

5.2 Group 2 amidoboranes in amine–borane dehydrocoupling

Following extensive research using transition metals, several group 2 metal compounds have recently been applied in the catalytic dehydrocoupling of amine–boranes. This process has attracted considerable attention from the perspectives of both hydrogen storage and materials chemistry, but has traditionally been dominated by metals such as Rh,^117–119 Ir,^120,121 Ru,^122,123 and Ti.^83,124–126 While NH₃BH₃ itself is naturally the most attractive substrate with regard to hydrogen storage, due to its low molecular weight and equal numbers of protic and hydridic hydrogen atoms, study of its dehydrocoupling is somewhat hampered by complicated and/or insoluble product mixtures, caused by polymerisation and multiple dehydrocoupling. Primary and secondary amine–boranes, RNH₂BH₃ and R₂NHBH₃, on the other hand, show improved solubility and can typically be selectively dehydrogenated to form monomeric aminoboranes, cyclic dimers or trimers, depending on the nature of the substituent(s).¹¹⁸

The first catalytic amine–borane dehydrocoupling reaction by a group 2 metal complex was reported by Harder and co-workers.¹⁰⁹ Here, the bulky derivative, DIPPNH₂BH₃ was found to undergo a catalytic reaction with 5 mol% of (DIPP-nacnac)MgN(SiMe₃)₂ (21) giving quantitative conversion to the diaminoborane HB[N(H)DIPP]₂, since shown to be a competent ligand in f-block chemistry in its doubly deprotonated form,^127,128 along with H₂ and diborane. The active catalyst was proposed to be (DIPP-nacnac)MgBH₄.

The group of Hill has made great strides in the dehydrocoupling of secondary amine–boranes using alkaline earth metals, and they have reported a number of secondary amidoboranes as likely catalytic intermediates. In their initial publication, the simple organometallic magnesium reagent, Mg[CH(SiMe₃)₂]₂·(THF)₂ (31), was reported to catalyse the dehydrocoupling of Me₂NHBH₃. Although slow compared to some transition metal systems (5 mol% cat., 60 °C, 3 days), the catalysis selectively produced the cyclic aminoborane dimer, (Me₂NBH₂)₂. In order to obtain mechanistic insight, stoichiometric reactions were also performed. The starting compound 31 was found to react with four equivalents of Me₂NHBH₃ to produce a species containing new B–N bonds, Mg(NMe₂BH₂NMe₂BH₃)₂·(THF) (32, Scheme 9).¹⁰⁶ X-ray diffraction revealed the presence of two monoanionic BNBN ligands, each bound to the magnesium centre through a Mg–N bond and two B–H⋯Mg interactions from the δ-BH₃ moiety (Fig. 6). Crystallographically determined Mg–N and N–B bond lengths were not significantly different from other magnesium amidoborane species (see Table 1).

Scheme 9 Isolated Intermediates in the Mg-mediated dehydrocoupling of dialkylamine boranes.

Fig. 6 Crystal structure of compound 32, displaying the NMe₂-BH₂-NMe₂-BH₃⁻ anion.

Heating compound 32 to 60 °C yielded the cyclic diborazane, (Me₂NBH₂)₂, over the course of several days. This process was proposed to occur via concerted δ-hydride elimination and B–N bond formation, although no direct evidence of magnesium hydride species was observed. In order to test this hypothesis, the complex (DIPP-nacnac)Mg(NMe₂-BH₂-NMe₂-BH₃) (33-Me₂) was prepared from Me₂NHBH₃ and (DIPP-nacnac)MgⁿBu. The NBNB ligand displays similar geometry to that in the homoleptic derivative, and the complex was indeed found to eliminate (Me₂NBH₂)₂, with concomitant production of the previously reported¹²⁹ hydride species [(DIPP-nacnac)Mg(μ-H)·(THF)]₂ (23·THF). A catalytic cycle was therefore proposed (Scheme 10) involving σ-bond metathesis of a magnesium hydride compound with the amine–borane to form an amidoborane complex and H₂, followed by β-hydride elimination. The liberated aminoborane, Me₂N=BH₂, was then proposed to insert into an amidoborane M–N bond, before undergoing δ-hydride elimination and releasing the cyclic diborazane.

Scheme 10 Mechanism of s-block-catalysed amine–borane dehydrocoupling.

The analogous reactions with calcium demonstrated the strong influence of the metal in this chemistry. The reaction of (DIPP-nacnac)Ca[N(SiMe₃)₂]·(THF) (5) with Me₂NHBH₃ afforded the amidoborane complex (DIPP-nacnac)Ca(NMe₂BH₃)·(THF) (34-Me₂). In contrast to the magnesium case, no significant B–N coupling could be observed in the presence of excess amine–borane. Heating complex 34-Me₂ to 80 °C resulted in slow decomposition via β-hydride elimination, forming a calcium hydride complex, traces of HB(NMe₂)₂ and the monomeric Me₂N=BH₂. In contrast to the case where the amidoborane has a single alkyl substituent (compounds 3-R), the lack of protic hydrogens allows the highly reactive calcium hydride to be observed.

Very similar reactivity was subsequently reported with pyrrolidine borane, (CH₂)₄NHBH₃, with B–N coupling again observed for Mg but not for Ca.¹⁰⁶ The structures of the complexes (DIPP-nacnac)Mg[N(CH₂)₄BH₂N(CH₂)₄BH₃] (33-(CH₂)₄) and (DIPP-nacnac)Ca[N(CH₂)₄BH₃]·(THF) (34-(CH₂)₄) displayed similar parameters to their dimethyl-substituted analogues, and the Mg complex decomposed at elevated temperatures to produce the cyclic diborazane [(CH₂)₄NBH₂]₂. In the same publication, NMR spectroscopic evidence was also obtained for the formation of a strontium amidoborane complex, (DIPP-nacnac)Sr[N(CH₂)₄BH₃]·(THF) (35); however, this compound could not be crystallised, and was found to be unstable towards Schlenk-type ligand redistribution, complicating its chemistry even further.¹⁰⁷

The release of cyclic diborazane from complexes 33-Me₂ and 34-Me₂ can also be effected by the room-temperature addition of B(C₆F₅)₃ as a hydride abstraction reagent, with the resulting hydrido-borate complexes (36 and 37, Scheme 11) providing a convenient route into CO₂ reduction chemistry.¹³⁰ The mechanism of B–N coupling for Ca amidoboranes is unclear.

Scheme 11 Release of cyclic diborazane from amidoborane complexes with B(C₆F₅)₃.

Subsequent experiments by Hill et al. looked into the reactivity of heteroleptic group 2 amides, M[N(SiMe₃)₂]₂, with respect to secondary amine–boranes.¹⁰⁷ Although the absence of a β-diketiminate ligand apparently thwarted attempts to obtain crystalline products for structural analysis, NMR spectroscopy allowed interesting deductions to be made regarding the dimerization process and, in particular, the role of the metal. All of the group 2 metal amides tested (Mg, Ca, Sr, Ba) formed amidoborane complexes upon reaction with Me₂NHBH₃, as judged by ¹¹B NMR spectroscopy. Rather than mediating dehydrocoupling, the Mg complex underwent β-hydride elimination, eliminating the monomeric aminoborane, Me₂N=BH₂. The analogous reaction with Ca also produced some Me₂N=BH₂, albeit much more slowly. For Sr and Ba, NMR experiments indicated the formation of amidoborane compounds in both cases, but no evidence for Me₂N=BH₂ production was found. Very similar behaviour was observed when using the bulkier substrate, ⁱPr₂NHBH₃, with the added advantage of rapid catalytic dehydrogenation in the case of Mg[N(SiMe₃)₂]₂. These results demonstrate the stability of amidoborane complexes of the heavier alkaline earth metals, and illustrate the decreasing propensity of amidoborane complexes to undergo β-hydride elimination as the charge density of the metal decreases.

In this context, it is of interest to note that the primary calcium amidoborane complexes (11), containing additional acidic N–H units, react much faster to form BNBN or borylamide products than their magnesium analogues (22). Since these reactions are also proposed to proceed through a metal hydride intermediate (Scheme 7), it may be possible that β-hydride elimination in metal amidoborane complexes to produce metal hydride species is reversible (as proposed for Zr–amidoborane complexes).⁸⁹ The much more reactive calcium hydride intermediates easily react further, whereas less reactive magnesium hydride intermediates may be present to a larger extent (and can even be isolated) but need much more forcing conditions for further reaction. Only the presence of excess acidic amine-borane (or another proton source), like during catalytic dehydrogenation, may cause fast conversion of magnesium hydride intermediates. Decomposition of the heaviest metal amidoborane complexes (Sr, Ba) by β-hydride elimination would generate the hitherto unknown strontium and barium hydride complexes that would be highly reactive and immediately insert R₂N=BH₂ to reform the metal amidoborane complexes, i.e. the equilibrium lies strongly or completely to the metal amidoborane side. The latter may explain their inactivity in catalytic dehydrogenation.

Hill and co-workers went on to report the dehydrocoupling of the primary amine–borane, ^tBuNH₂BH₃, with (DIPP-nacnac)Ca[N(SiMe₃)₂]·(THF) (13).¹⁰¹ The catalysis was found to proceed through the intermediate amidoborane complex, 11-^tBu (Scheme 5). In contrast to catalysis with the aforementioned dialkylamine–boranes, the reaction was rather unselective, affording a complicated mixture of products including the cyclic diborazane [^tBu(H)N-BH₂]₂, the borazine (^tBuNBH)₃ and the diaminoborane HB[N(H)^tBu]₂. One likely explanation for this lack of selectivity is the availability of a second protic N–H moiety, which could be deprotonated by the proposed intermediate calcium hydride complex (vide supra). Nevertheless, the operation of different reaction pathways, most notably the proposed dehydrogenation/ring expansion from [^tBu(H)N-BH₂]₂ to (^tBuN-BH)₃, perhaps reveals a greater depth to this calcium catalysis that warrants further investigation.

5.3 Insertion of aminoborane units into M–R bonds

The isoelectronic nature of amidoborane compounds and aminoboranes to alkyl compounds and alkenes, respectively, has led to interest in the extent to which this analogy can be applied to the organometallic chemistry of B–N compounds.^56,131–134 The amine–borane dehydrocoupling mechanism (Scheme 10) describes the alkene-like re-insertion of a dehydrogenated aminoborane unit into a metal–nitrogen bond; Hill and co-workers provided support for this mechanism with their report of the insertion of a BN unit of an amidoborane complex into a metal–carbon bond.¹³⁵ The strontium complexes, [(Me₃Si)₂HC]Sr(NR₂BH₃)·(THF)₂ (R = Me (38-Me₂), (CH₂)₂ (38-(CH₂)₄)) were prepared by σ-bond metathesis from homoleptic Sr[CH(SiMe₃)₂]₂·(THF)₂. Heating to 80 °C resulted in β-hydride elimination and insertion of the aminoborane unit into the remaining strontium–carbon bond, after which further β-hydride elimination afforded the aminoalkylborane product, [(Me₃Si)₂HC]B(H)NR₂, 39, in both cases (Scheme 12).

Scheme 12 Aminoborane insertion into M–C bonds.

In this case, the reduced tendency of strontium amidoboranes to undergo β-hydride elimination proved advantageous. While some conversion to the aminoalkylborane was observed with Mg[CH(SiMe₃)₂]₂·(THF)₂ and Ca[CH(SiMe₃)₂]₂·(THF)₂, the reactions were less selective and the low decomposition temperature prevented the isolation of the amidoborane species.¹³⁶ This publication also contained details of a tetrametallic strontium amidoborane complex, Sr₄[CH(SiMe₃)₂]₂[NH(^tBu)BH₃]₆·(THF)₄ (40), formed from the equimolar combination of ^tBuNH₂BH₃ and Sr[CH(SiMe₃)₂]₂·(THF)₂. The mismatch between the number of alkyl and amidoborane ligands in the crystallised product was proposed to be the result of a Schlenk equilibrium. Similar insertion chemistry was also observed with this derivative, affording the aminoalkylborane [(Me₃Si)₂HC]B(H)NH^tBu.

6 Conclusions and outlook

Over the last two decades, the chemistry of metal amidoboranes has progressed from relative obscurity to an active multidisciplinary field. The use of lithium amidoboranes as reducing agents in organic synthesis has reportedly begun to be implemented in undergraduate teaching laboratories,¹⁸ and solution-phase amidoborane chemistry has shed some light on the mechanism of potentially lucrative hydrogen storage and release processes.

The realisation that early main group MNH₂BH₃ complexes can release H₂ at significantly more convenient temperatures than the hydrogen-rich NH₃BH₃, with many other merits, such as the absence of foaming and induction periods, and thermoneutrality, clearly puts a spotlight on amidoboranes. The next breakthrough for the practical use of metal amidoboranes in hydrogen storage, however, will only be achieved by finding reversible systems. Efficient fuel regeneration (i.e. rehydrogenation of decomposition products) is an absolute prerequisite for future applications.¹³⁷

The extension of amine–borane dehydrocoupling to the abundant, cheap and non-toxic metals of the s-block is an interesting development and is likely to be a target for further research, although for group 1 any success likely depends upon the ability to solubilise the metal hydride intermediates. Solution-phase reactivity studies have revealed other possible applications of amidoboranes, in particular for the generation of reactive metal hydride species for both main group and transition metals. There are, nonetheless, significant strides to be made before practical applications of these compounds can be realised. The amidoborane chemistry of the heavier alkaline earth metals also remains in its infancy, while expanding the scope of supporting ligands for solution-phase Mg and Ca amidoborane chemistry beyond the ubiquitous DIPP-nacnac is likely to reveal new reactivity. One thing that can be stated with confidence is that s-block amidoborane compounds will continue to play a role in challenging the notion of transition metal dominance in organometallic chemistry and homogeneous catalysis.

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Footnote

† Although the strict definition of the term ‘organometallic‘ refers only to species containing a metal carbon bond, we feel that this term best describes the coordination chemistry and on-metal reactivity reviewed in this article. We refer the reader to the scope of the journal ‘Organometallics’: “…manuscripts dealing with metal-containing compounds that do not contain metal–carbon bonds will be considered if there is a close relationship between the subject matter and the principles and practice of organometallic chemistry…”.

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