Facile access to mid-valent Group 5 and 6 metal synthons

Abstract

Group 5 and 6 metal chlorides, MCl_x (M = Nb, Ta, Mo, W) are easily and controllably reduced, in a stepwise fashion, by stoichiometric PhMe₂SiH, yielding only PhMe₂SiCl, a useful reagent, and H₂ as the byproducts. Addition of n moles of PhMe₂SiH to toluene solutions of MCl_x yields stepwise uncoordinated reduction products of the form MCl_x−n (M = Nb, Ta, Mo, W; x = 5 for Nb, Ta, Mo, x = 6 for W; n = 1 for Nb and Ta, n = 1, 2 for Mo and W). The reactions proceed cleanly furnishing quantitative, analytically pure yields of the desired mid-valent binary chlorides. The obtained products are very reactive and can be further derivatized with coordinating ethers or phosphines for the on-demand preparation of desired inorganic synthons.

---

Introduction

When a specific oxidation state is targeted for the synthesis of a transition metal complex, it is typically preferred to begin with a precursor of that oxidation state and follow simple ligation or metathesis type routes to install the desired ligand framework. This necessitates either the availability of stable but reactive commercial precursors of variable oxidation states, typically halide salts, or facile routes to access them synthetically. An area of particular interest are the group 5 and 6 metals which can exist in a wide range of oxidation states, e.g., Mo displays states covering 0, +1, +2, +3, +4, +5, and +6. Therein, and germane to the content of this report, Nb, Ta, Mo, and W have drawn decades worth of interest for the synthesis of coordination compounds in service of fundamental structural chemistry,¹ catalysis,² and biomimetics.³ More recently with a growing interest in vapor-phase growth processes (chemical vapor deposition and atomic layer deposition)⁴ for the fabrication of metal oxides and 2-D semiconductor transition metal dichalcogenides,⁵ the demand for group 5⁶ and 6⁷ molecular precursors with diverse oxidation states and ligand topologies has increased. The synthesis of these molecular precursors, and the aforementioned coordination compounds is reliant on the availability of their MCl_x (M = Nb, Ta, Mo, W) synthons. Unfortunately, for the group 5 and 6 metals, commercial availability of mid-valent halides is comparatively sparse compared to high- and low-valent analogues. Correspondingly, those which are available are often significantly more costly, and can often be less reactive due to a combination of their polymeric nature, and/or high degree of crystallinity stemming from their preparative routes.

Over the past seven decades, a variety of routes targeting mid-valent early transition metal chlorides have been reported; encompassing both high temperature and in situ reduction protocols (Scheme 1).⁸ The high temperature routes allow for the straightforward preparation of the desired oxidation state. However, resultant of the high temperature process, these materials are typically highly crystalline and often display lower reactivity. Conversely, as the behaviour of high-valent transition metal chlorides is generally well-understood, in situ reductions to the desired oxidation state become more appealing. They can be carried out via both heterogeneous or homogeneous reducing agents, and in the presence of a coordinating solvent or ligand to capture the desired product as a discreet molecular complex. A perfect example of this is the diethyl ether adduct of molybdenum(IV) chloride, MoCl₄(Et₂O)₂. Heterogenous tin metal^8c and homogenous allyltrimethylsilane⁹ are both capable of reducing MoCl₅ in diethyl ether, leading to the formation of MoCl₄(Et₂O)₂. A downside to these in situ reactions is that strict control of reaction conditions is necessary to prevent unwanted side/by-products, e.g., the formation of MoOCl₃(Et₂O)₂.^9,10

Scheme 1 Select examples of common group 5 and 6 MCl_x reduction processes.

Alternatively, amorphous, uncoordinated MCl_x can be prepared via reductions of high-valent starting materials, in non-coordinating solvents, with heterogenous reducing agents, e.g., the preparation of amorphous WCl₄via reduction of WCl₆ with tin metal.^8d Although reliable, such methods utilize tin metal as the reductant which generates chlorinated tin waste. In general, most in situ reductions necessitate reagents that are either difficult to handle (pyrophoric, toxic) and/or generate unwanted by-products (metal/salt wastes). An elegant potential solution was first reported by Hampden-Smith in 1992, whereby MoCl₅ was reacted with Ph₂SiH₂ under mild conditions in thf to generate MoCl₄(thf)₂ and Ph₂SiHCl.

This process utilizes a common organo-reductant that is easy to handle and yields the desired product with a readily separated chlorosilane as by-product; the latter is also synthetically useful. Unfortunately, this methodology has not been expounded on adequately in the decades since its first report. We surmise that this is likely due to the lower importance given to green chemistry at the time of publication, and looser restrictions placed on use of hazardous reagents in both industrial, academic, and national laboratory settings.

Leveraging this finding, our group recently reported the use of commercially available PhMe₂SiH, as a stoichiometric reducing agent for the preparation of dimeric Mo(III) synthons.¹¹ Herein, we report an expansion of this process as a general methodology for the reduction of high-valent Group 5 and 6 transition metal chlorides (Scheme 1). Our approach requires mild reaction conditions and generates highly reactive, uncoordinated reduced metal chlorides as a precipitate for facile separation, and only PhMe₂SiCl as by-product; itself a very useful reagent for a variety of processes.¹² Critically, the degree of reduction is readily controlled by reductant stoichiometry: 1e⁻ for 1 equiv. of PhMe₂SiH. To the best of our knowledge there are no examples of uncoordinated transition metal halides being formed and isolated in a similar manner. Inspired by the reduction of MoCl₄(dme) to ‘MoCl₃(dme)’ (dme = dimethoxyethane) using PhMe₂SiH, we sought to explore the generality of this reduction process with other Mo, W, Nb, and Ta chlorides, and target the generation of their uncoordinated mid-valent products.

Experimental

Materials

All chemical manipulations were performed using standard glovebox and Schlenk techniques in ultra-high purity (UHP) nitrogen environments. Basic alumina (50–200 μm) was purchased from Acros Organics and activated by heating to 220 °C under vacuum (10⁻² Torr) for 12 h. Molecular sieves were purchased from Fisher Scientific and dried at 150 °C for 24 h prior to use. Diethyl ether, hexanes, and toluene were collected from an mBraun solvent purification system (SPS) and stored over basic alumina for 12 h in the glovebox prior to use. Dimethoxyethane (dme) was purchased from Sigma-Aldrich and dried over activated basic alumina for 12 h prior to use. Molybdenum pentachloride (99.6%), tungsten hexachloride (99.9%-W), niobium pentachloride (99+%-Nb), and tantalum pentachloride (99.9%-Ta) were purchased and used as received from STREM Chemicals, Inc. Dimethylphenylsilane (≥98%) was purchased from Sigma-Aldrich and stored over activated 3 Å molecular sieves for 12 h prior to use. Benzene-d₆ was purchased from Sigma-Aldrich and was dried over activated basic alumina for 12 h prior to use. Triethylphosphine (99%) was purchased and used as received from STREM Chemicals, Inc.

Characterization

The purity of organic reagents was checked via¹H NMR. Elemental analyses (C, H, Cl) were performed by Galbraith Laboratories, Inc. (Knoxville, TN). ¹H NMR spectra were recorded on a Bruker Avance III HD 600 instrument equipped with a cryoprobe and were processed using Mestrelab's MestReNova software. Spectra were referenced to residual C₆D₅H (¹H δ 7.16).

General preparation of MCl₄

When starting from an MCl₅ precursor, a typical experiment proceeded as follows: inside the glovebox, an Erlenmeyer flask was charged with 1 g of the commercial grade MCl₅, a stir bar, and approximately 25 ml of toluene. Whilst stirring, 1 molar equivalent of dimethylphenylsilane was introduced by syringe. The flask was then capped with a rubber septum, which was then punctured with an 18-gauge needle to function as a ventilation port for the H₂ gas by-product. In the case of preparing WCl₄ from WCl₆, the only experimental modification is the addition of 2 molar equivalents of dimethylphenylsilane. The reactions were then left to stir overnight. The preparation of MoCl₄ is an exception, where complete conversion occurs in approximately 1 h. The next day, the resulting suspensions were filtered through fine porosity sintered glass frits and washed with approximately 25 mL of fresh toluene. Then, the collected solid was washed with approximately 25 mL of hexanes and dried in vacuo for 0.5 hours. The resulting yields were quantitative, with slight deviations due to mechanical loss. Elemental Analyses of each MCl₄ product are as follows: Anal. Calcd for NbCl₄: Cl, 60.42. Found: Cl, 60.38; Anal. Calcd for TaCl₄: Cl, 43.94. Found: Cl, 43.48; Anal. Calcd for MoCl₄: Cl, 59.64. Found: Cl, 59.69; Anal. Calcd for WCl₄: Cl, 43.55. Found: Cl, 43.45. Species-specific observations can be found in the ESI.†

Preparation of MoCl₃

Addition of two equivalents of dimethylphenylsilane to the yellow-brown suspension of MoCl₅ in toluene results in the immediate, visual release of H₂ and the precipitation of MoCl₄. If left to sit with no stirring, the reduction of MoCl₄ to MoCl₃ does not proceed. However, with vigorous stirring, MoCl₄ is fully reduced to MoCl₃ which is recovered as a dark, red-brown solid. This reduction is also achievable via the addition of one equivalent of dimethylphenylsilane to MoCl₄ prepared via the silane reduction route described above. Anal. Calcd for MoCl₃: Cl, 52.57. Found: Cl, 52.66.

Preparation of WCl₅

Addition of one equivalent of dimethylphenylsilane to a stirred, royal blue suspension of WCl₆ in toluene results in the immediate evolution of H₂. This reaction is rapid, and amorphous, black WCl₅ settles to the bottom of the reaction vessel and a clear supernatant is observed. Anal. Calcd for WCl₅: Cl, 49.09. Found: Cl, 48.73.

Preparation of MCl₄(PEt₃)₂ (M = Nb, Ta)

0.5 g of starting MCl₄ powder was suspended in approximately 50 mL of dry toluene inside of a Schlenk flask equipped with a stir bar. This suspension was then cycled onto the Schlenk line and kept under a UHP N₂ environment. While under direct nitrogen flow, 2 equivalents (0.63 mL for NbCl₄, 0.46 mL for TaCl₄) of triethylphosphine were syringed into the reaction vessel at room temperature and the reaction was allowed to stir overnight. The next day, the reaction was filtered on the Schlenk line through a freshly dried, fine porosity swivel frit to remove any insoluble materials. The obtained dark, red-yellow (Nb) and clear, yellow-green (Ta) solutions were then stripped under reduced pressure, which afforded 0.65 g of dark red NbCl₄(PEt₃)₂ (65%) and 0.43 g of beige TaCl₄(PEt₃)₂ (50%). Anal. Calcd for NbCl₄P₂C₁₂H₃₀: Cl, 30.10; C, 30.60; H, 6.42. Found: Cl, 30.03; C, 30.22; H, 6.12. Anal. Calcd for TaCl₄P₂C₁₂H₃₀: Cl, 25.36; C, 25.78; H, 5.41. Found: Cl, 25.64; C, 24.63; H, 4.53. C, H analyses were conducted in the presence of a combustion agent (V₂O₅) to obtain reliable data.

Preparation of MoCl₄(dme)

0.4 g of MoCl₄ powder were added to a 22 mL scintillation vial, along with a freshly dried stir bar. Then, approximately 10 mL of dry dme were added and the mixture was stirred for 1 h. At the end of the period, a homogenous solution was obtained. This solution was then treated with an equal volume of dry hexanes which resulted in the precipitation of a dull orange powder. The powder was then isolated via vacuum filtration and washed with fresh hexanes and dried in vacuo, yielding 0.52 g (95%) of MoCl₄(dme). Purity was assessed via¹H NMR in d₈-toluene: δ 16.32 (s, 6H), 1.17 (s, 4H).

Preparation of WCl₄(dme)

2 g of WCl₄ powder were charged to a freshly dried 125 mL Erlenmeyer flask equipped with a stir bar. Approximately 45 mL of dry dme was then added to the flask, which was then capped with a glass stopper and stirred overnight. The next day, the green reaction mixture was passed through a 30 mL fine porosity sintered glass frit charged with a quarter volume of dried Celite, via vacuum filtration, to remove any unreacted WCl₄. The Celite filter cake was then washed with 2 × 10 mL of fresh dme to recover any adduct left behind. The obtained emerald-green filtrate was then transferred to a freshly dried 100 mL Schlenk flask and cycled onto the Schlenk line. The solution is then stripped under reduced pressure with gentle heating to afford 2.17 g of a brown solid. Yield: 85%. The isolated material is then ready for employment in subsequent syntheses. Purity was assessed via¹H NMR in d₈-toluene: δ 9.93 (s, 6H), 0.77 (s, 4H).

Single crystal X-ray diffraction

Detailed tables of crystallographic data, structural refinement information, and bond lengths and angles can be found in the ESI.† Single crystals of NbCl₄(dme) were obtained by creating a concentrated solution of NbCl₄ in dry dme and setting up a vapor diffusion crystallization with hexanes as the counter solvent. Over a period of two days, well-formed, orange needles precipitated from the solution. Single crystals of NbCl₄(Et₂O)₂ were obtained by saturating a solution of diethyl ether with NbCl₄. The resulting orange solution was then passed through a filter plug made of glass wool and Celite to remove any insoluble material. The homogenous solution was then chilled in a −35 °C freezer over a period of two days and orange, block-like crystals formed. Single crystals of TaCl₄(Et₂O)₂ and Ta₂Cl₈(Et₂O)₂ were obtained via the dissolution of TaCl₄ in minimal, dry diethyl ether. The resulting teal-green solution was then placed in the freezer and two crystalline morphologies precipitated over a period of 30 minutes. Lime green needles corresponding to TaCl₄(Et₂O)₂ and teal needles corresponding to Ta₂Cl₈(Et₂O)₂ were obtained. Single crystals of [Ta₂Cl₆(dme)₂][TaCl₆] were obtained by the addition of minimal dme to TaCl₄. The resulting suspension was filtered through a filter plug consisting of glass wool and Celite to obtain a homogenous, dark blue solution. The solution was then placed in a −35 °C freezer and left to sit overnight, during which time yellow blocks and thin, blue plates corresponding to [Ta₂Cl₆(dme)₂][TaCl₆] precipitated. X-ray diffraction studies of NbCl₄(dme) were conducted on a XtaLAB Synergy, single source at offset/far, HyPix diffractometer with Mo Kα (λ = 0.7103 Å) micro-focus sealed X-ray tube PhotonJet source and a mirror detector at 100(1) K. X-ray diffraction studies of NbCl₄(Et₂O)₂, TaCl₄(Et₂O)₂, Ta₂Cl₈(Et₂O)₂, and [Ta₂Cl₆(dme)₂][TaCl₆] were conducted on a Rigaku XTA-Lab Mini II diffractometer using a Mo Kα (λ = 0.7103 Å) source and a CCD plate detector at 100(1) K. Data collection, cell parameter determinations, data reduction, and absorption corrections were performed via CrysAlis Pro.¹³ Structure solution, refinement, and publication materials were generated via SHELXL, SHELXLT, and Olex2.^14–16 All hydrogen atoms were attached via the riding model at calculated positions.

Results and discussion

Synthesis

The addition of PhMe₂SiH to small quantities of crystalline MoCl₅, partially dissolved in toluene to form a brown-yellow suspension, resulted in the vigorous evolution of gas and the precipitation of a black powder with a clear supernatant. A droplet of the supernatant was then suspended in d₈-toluene and analysed by ¹H NMR. The septet at 4.42 ppm, corresponding to the hydridosilane hydride, had disappeared and the doublet at 0.21 ppm, corresponding to the methyl groups collapsed to a singlet and shifted to 0.43 ppm, indicating the formation of the chlorinated by-product, PhMe₂SiCl (Scheme 2i). The formation of the chlorinated by-product and the vigorous evolution of gas lead us to believe that this process proceeds via the reductive elimination of H₂ (vide infra). Elemental analysis of the isolated and dried product confirmed that we had made analytically pure MoCl₄. This process was then scaled to multigram quantities, with 5 grams of starting MoCl₅, and left to react overnight which ultimately yielded 4.14 g (95%) of MoCl₄. Any unreacted MoCl₅ can be removed by washing with anhydrous dichloromethane or anhydrous toluene.

Scheme 2 List of demonstrated 1e⁻ and 2e⁻ MCl_x (M = Mo, W, Nb, Ta) reduction processes with stoichiometric PhMe₂SiH.

Next, we sought to understand the extent of reduction, and the degree of stoichiometric control available with PhMe₂SiH. Reaction of MoCl₅ with 2 equiv. of PhMe₂SiH, or a suspension of isolated MoCl₄ (vide supra) with 1 equiv. of PhMe₂SiH accompanied by vigorous stirring overnight yielded dark red MoCl₃ (Scheme 2ii and iii). Notably, the MoCl₄ → MoCl₃ sequence is not accompanied by vigorous gas evolution, indicating a slower reaction rate.

Both reaction sequences were conducted on the gram scale affording near quantitative yields with high purity product as confirmed by elemental analysis (see Experimental section and ESI†).

The isolated MoCl₄ can be used to obtain solvated complexes. As an example, MoCl₄ was suspended in dme and stirred for one hour, furnishing MoCl₄(dme) in 95% yield (Scheme 3i). The silane can also be employed as a reducing agent for previously reported Mo-based systems: the syntheses of MoCl₄(Et₂O)₂ and MoCl₃(thf)₃ in lieu of Me₃Si(C₃H₅) and Ph₂SiH₂, respectively. For MoCl₄(Et₂O)₂, a comparable yield of 86% is obtained in 30 min vs. the reported 2 h.⁹ A 70% yield of MoCl₃(thf)₃ is obtained in 1 h, vs. the reported 85% yield in 4 h.^8e

Scheme 3 Derivatization reactions for amorphous MCl₄ (M = Nb, Ta, Mo, W) to form MCl₄(PEt₃)₂ (M = Nb, Ta) and MCl₄(dme) (M = Mo, W).

Next, we targeted the synthesis of WCl₄. Starting with WCl₆ partially dissolved in toluene, stepwise reductions were performed with stoichiometric amounts of PhMe₂SiH to obtain analytically pure WCl₅ and WCl₄, respectively, in essentially quantitative yields (Scheme 2iv and v). The reduction of WCl₆ to WCl₅ is complete in 2 h while WCl₆ to WCl₄ reduction was left to react overnight. We note here that WCl₆ is not reduced to WCl₃ with 3 equiv. of PhMe₂SiH. As with Mo, we utilized the as synthesized WCl₄ to obtain WCl₄(dme) in 86% yield (2 g scale, Scheme 3ii).

We then shifted our attention to Nb and Ta chlorides. The only commercially available chlorides are MCl₅ and, to the best of our knowledge, preparative routes to uncoordinated lower oxidation states are limited to high-temperature processes (Scheme 1). These are energy intensive, lengthy, and require further purification. Employing our reduction strategy, small quantities of NbCl₅ and TaCl₅ were added to toluene which resulted in dark orange-red and golden-yellow solutions, respectively. We then added excess PhMe₂SiH; in this instance vigorous gas evolution was not observed. We left the reactions to sit overnight and returned to brown NbCl₄ and olive-green TaCl₄, suspended in colourless supernatants, which were isolated and determined to be analytically pure. The reactions were repeated on the 5 g scale, resulting in a 96% yield of both NbCl₄ and TaCl₄. We note that our obtained olive-green TaCl₄ differs from the reported orange TaCl₄ intermediate obtained by Hayton et al.¹⁷via the addition of triethylsilane to TaCl₅ but matches the reported olive-green TaCl₄ obtained by Manzer via the high-temperature reduction of TaCl₅ with Al powder.^8a Efforts to reduce the Nb and Ta chlorides below the (IV) oxidation state were unsuccessful, even with mild heating or vigorous stirring.

To probe the utility of our amorphous NbCl₄ and TaCl₄ powders, we sought to form the established bis(triethyl phosphine) adducts. Both binary halides were suspended in toluene and two equivalents of triethylphosphine (PEt₃) were introduced at room temperature (Scheme 3iii and iv). Both trans-bis(triethylphosphine) complexes were formed, with 65% yield for Nb and 50% yield for Ta, comparable with reported yields of 76% and 48%, respectively.¹⁸ Next, we examined the reactivity of our NbCl₄ and TaCl₄ with ethereal solvents, with the goal of developing new synthons. While there are examples of mid-valent Nb and Ta complexes with N, S, P, and Se donor ligands, the libraries of neutral, ether-coordinated, non-oxo Nb and Ta synthons are limited.¹⁹

The addition of NbCl₄ to cold (−35 °C) dme produced an orange solution, which readily precipitated bright orange NbCl₄(dme) in near quantitative yields upon addition of hexanes. Addition of NbCl₄ to thf and diethyl ether also produce the desired NbCl₄(thf)₂ and NbCl₄(Et₂O)₂ adducts, respectively. Single crystal X-ray structures were readily obtained (Fig. 1A and B). These routes now provide one-step access to ethereal NbCl₄L₂ synthons directly from NbCl_4. The reaction of as prepared TaCl₄ with room temperature diethyl ether resulted in the immediate formation of a teal solution, from which two crystalline morphologies precipitated in ∼30 min at −35 °C. The first were lime green needles that were determined to be the monomeric trans-TaCl₄(Et₂O)₂ (Fig. 1C). The second were teal needles determined to be dimeric Ta₂Cl₈(Et₂O)₂ (Fig. 1D). The addition of TaCl₄ to cold dme results in the immediate formation of a blue solution. Filtration and storage of the solution at −35 °C precipitated blue crystals found to be [Ta₂Cl₆(dme)₂][TaCl₆] (Fig. 1E) and yellow crystals yet to be structurally characterized. The Ta–Cl distances of the [TaCl₆]⁻ anion closely align with those of previously reported sodium hexachlorotantalate(V),²⁰ pointing towards the formation of a mixed valent, Ta(III)–Ta(IV) dimer stabilized by a Ta(V) counter ion. At present, we are calling this a dme-induced disproportionation; further studies will be required to fully elucidate this reaction.

Fig. 1 Single crystal X-ray structures of (A) NbCl₄(dme), (B) NbCl₄(Et₂O)₂, (C) TaCl₄(Et₂O)₂, (D) [TaCl₄(Et₂O)]₂, and (E) [TaCl₃(dme)]₂[TaCl₆]. Thermal ellipsoids drawn at 50%; for bond lengths and angles, see ESI.†

The results highlighted herein provide a validation of the high reactivity of the in situ reduced uncoordinated metal salts and offer a proof-of-concept for potential applicability.

Conclusions

In summary, we have developed a benign, facile, solution-based route for the on-demand preparation of Group 5 and 6 mid-valent binary chlorides. This process employs the use of commercially available PhMe₂SiH and affords analytically pure, quantitative yields of the desired mid-valent chloride on a short time scale. The work-up procedure for each reaction is simple and requires no further separation of heterogenous by-products. These mid-valent chlorides are highly reactive and can be leveraged to reproduce classic inorganic synthons or build towards new species as exemplified by our preliminary findings on etherates. Ongoing work is focused on elucidating the latter and expanding the scope of this process beyond Groups 5 and 6.

Author contributions

T. E. Shaw: investigation, methodology, formal analysis, writing – original draft, writing – reviewing & editing. C. L. Stern: investigation, formal analysis. A. P. Sattelberger: supervision, methodology, conceptualization, formal analysis, project administration, funding acquisition, writing – original draft, writing – reviewing & editing. T. Jurca: supervision, methodology, conceptualization, formal analysis, project administration, funding acquisition, writing – original draft, writing – reviewing & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Department of Chemistry, College of Sciences, and the Faculty Cluster Initiative at the University of Central Florida. The use of the IMSERC Crystallography facility at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-2025633), and Northwestern University, is gratefully acknowledged. The authors acknowledge insightful input from Drs. Brian L. Scott and Samuel M. Greer (Los Alamos National Lab) and Professor Thomas M. Gilbert (Northern Illinois University).

References

1. D. Singh, W. R. Buratto, J. F. Torres and L. J. Murray, Activation of Dinitrogen by Polynuclear Metal Complexes, Chem. Rev., 2020, 120, 5517–5581.
2. C. Coperet, A. Comas-Vives, M. P. Conley, D. P. Estes, A. Fedorov, V. Mougel, H. Nagae, F. Nunez-Zarur and P. A. Zhizhko, Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities, Chem. Rev., 2016, 116, 323–421.
3. S. Groysman and R. H. Holm, Biomimetic Chemistry of Iron, Nickel, Molybdenum, and Tungsten in Sulfur-Ligated Protein Sites, Biochemistry, 2009, 48, 2310–2320.
4. (a) J. Lu, J. W. Elam and P. C. Stair, Atomic layer deposition-Sequential self-limiting surface reactions for advanced catalyst “bottom-up” synthesis, Surf. Sci. Rep., 2016, 410–472; (b) T. J. Kunene, L. K. Tartibu, K. Ukoba and T.-C. Jen, Review of atomic layer deposition process, application and modelling tools, Mater. Today: Proc., 2022, 62, S95–S109.
5. J. Cai, X. Han, X. Wang and X. Meng, Atomic Layer Deposition of Two-Dimensional Layered Materials: Processes, Growth Mechanisms, and Characteristics, Matter, 2020, 2, 587–630.
6. (a) K. Khumaini, H. Roh, H. Han, H.-L. Kim, H.-S. Kim, J.-H. Seok, J. W. Park and W.-J. Lee, Surface reaction mechanism of atomic layer deposition of niobium oxide: In situ characterization and first-principle study, Appl. Surf. Sci., 2023, 615, 156340; (b) S. J. Song, T. Park, K. J. Yoon, J. H. Yoon, D. E. Kwon, W. Noh, C. Lansalot-Matras, S. Gatineau, H.-K. Lee, S. Gautam, D.-Y. Cho, S. W. Lee and C. S. Hwang, Comparison of the Atomic Layer Deposition of Tantalum Oxide Thin Films Using Ta(N^tBu)(NEt₂)₃, Ta(NtBu)(NEt2)2Cp, and H₂O, ACS Appl. Mater. Interfaces, 2017, 9, 537–547.
7. (a) T. Jurca, M. J. Moody, A. Henning, J. D. Emery, B. Wang, J. M. Tan, T. L. Lohr, L. J. Lauhon and T. J. Marks, Low-Temperature Atomic Layer Deposition of MoS₂ Films, Angew. Chem., Int. Ed., 2017, 56, 4991–4995; (b) M. Mattinen, J.-L. Wree, N. Stegmann, E. Ciftyurek, M. El Achhab, P. J. King, K. Mizohata, J. Raisanen, K. D. Schierbaum, A. Devi, M. Ritala and M. Leskela, Atomic Layer Deposition of Molybdenum and Tungsten Oxide Thin Films Using Heteroleptic Imido-Amidinato Precursors: Process Development, Film Characterization, and Gas Sensing Properties, Chem. Mater., 2018, 30, 8690–8701.
8. (a) L. E. Manzer, Preparation of the paramagnetic alkyls bis(cyclopentadienyl)dimethylniobium and bis(methylcyclopentadienyl)dimethyltantalum and some six- and eight-coordinate phosphine derivatives of niobium(IV), Inorg. Chem., 1977, 16, 525–528; (b) B. Bajan and H.-J. Meyer, Crystal structure of tantalum tetrachloride, TaCl₄, Z. Kristallogr. – Cryst. Mater., 1996, 211, 818; (c) F. Stoffelbach, D. Saurenz and R. Poli, Improved Preparations of Molybdenum Coordination Compounds from Tetrachlorobis(diethyl ether)molybdenum(IV), Eur. J. Inorg. Chem., 2001, 2699–2703; (d) V. Kolesnichenko, D. C. Swenson and L. Messerle, Facile Reduction of Tungsten Halides with Nonconventional, Mild Reductants. I. Tungsten Tetrachloride: Several Convenient Solid-State Syntheses, a Solution Synthesis of Highly Reactive (WCl₄)_x, and the Molecular Structure of Polymeric Tungsten Tetrachloride, Inorg. Chem., 1998, 37, 3257–3262; (e) D. Zeng and M. J. Hampden-Smith, High yield routes to molybdenum(III) compounds by diphenylsilane and tin(II) chloride reduction of molybdenum(V) and molybdenum(IV) chlorides, Polyhedron, 1992, 11, 2585–2589.
9. C. Persson and C. Andersson, Reduction of tungsten(VI) and molybdenum(V) by allyltrimethylsilane and cyclopentene. Simple high yield syntheses of MoCl₄(OEt₂)₂, MoCl₄(dme), WCl₄(thf)₂, WCl₄(dme), and WOCl₃(thf)₂, Inorg. Chim. Acta, 1993, 203, 235–238.
10. (a) L. Castellani and M. C. Gallazzi, Facile preparation of diethyl ether complexes of molybdenum and tungsten tetrachloride, Transition Met. Chem., 1985, 10, 194–195; (b) T. E. Shaw, T. J. Diethrich, C. L. Stern, B. L. Scott, T. Jurca, T. M. Gilbert and A. P. Sattelberger, Synthesis, characterization, X-ray and electronic structures of diethyl ether and 1,2-dimethoxyethane adducts of molybdenum(IV) chloride and tungsten(IV) chloride, Dalton Trans., 2022, 51, 7856–7863; (c) T. E. Shaw, P. LeMagueres, A. P. Sattelberger and T. Jurca, Crystal structure and Hirshfeld surface analysis of the elusive trichlorobis(diethyl ether)oxomolybdenum(V), Acta Crystallogr., Sect. C: Struct. Chem., 2020, 76, 947–951.
11. T. E. Shaw, T. J. Diethrich, B. L. Scott, T. M. Gilbert, A. P. Sattelberger and T. Jurca, “MoCl₃(dme)” Revisited: Improved Synthesis, Characterization, and X-ray and Electronic Structures, Inorg. Chem., 2021, 60, 12218–12225.
12. (a) P. Deglmann, E. Ember, P. Hofmann, S. Pitter and O. Walter, Experimental and Theoretical Investigations on the Catalytic Hydrosilylation of Carbon Dioxide with Ruthenium Nitrile Complexes, Chem. – Eur. J., 2007, 13, 2864–2879; (b) K. Izod, C. Wills, W. Clegg and R. W. Harrington, Acyclic Dialkylstannylene and -Plumbylene Compounds That Are Monomeric in the Solid State, Organometallics, 2009, 28, 5661–5668; (c) X. Zhang and Z. Cheng, Performance of combined use of chlorosilanes and AlCl₃ in the carboxylation of toluene with CO₂, AIChE J., 2016, 63, 185–191.
13. Rigaku Oxford Diffraction, CrysAlis PRO, Rigaku Oxford Diffraction, Yarnton, Oxforshire, England, 2020.
14. SHELXTL, ver. 6.12, Bruker AXS, Inc., Madison, WI, 2005.
15. Olex 2 1.2 (compiled 2014-06-27 svn.r2953 for OlexSys, GUI svn. r4855).
16. O. Dolomanov, L. Bourhis, R. Gildea, J. Howard and H. Puschmann, OLEX2: a complete structure solution, refinement and analysis program, J. Appl. Crystallogr., 2009, 42, 339–341.
17. P. L. Damon, C. J. Liss, R. A. Lewis, S. Morochnik, D. E. Szpunar, J. Telser and T. W. Hayton, Quantifying the Electron Donor and Acceptor Abilities of the Ketimide Ligands in M(N=C^tBu₂)₄ (M = V, Nb, Ta), Inorg. Chem., 2015, 54, 10081–10095.
18. (a) F. A. Cotton, S. A. Duraj and W. J. Roth, Further studies of the phosphine complexes of niobium(IV) chloride, Inorg. Chem., 1984, 23, 3592–3596; (b) F. A. Cotton, S. A. Duraj and W. J. Roth, Preparation and Structural Characterization of Phosphine Adducts of Tantalum(IV) Chloride, Inorg. Chem., 1984, 23, 4046–4050.
19. (a) E. J. Roskamp and S. F. Pedersen, The first practical niobium(III) reagent in organic synthesis. A convenient route to 2-amino alcohols via the coupling of imines with aldehydes or ketones promoted by NbCl₃(DME), J. Am. Chem. Soc., 1987, 109, 6551–6553; (b) L. G. Hubert-Pfalzgraf, M. Tsunoda and J. G. Riess, Synthesis and characterization of niobium(III) trichloride ether and sulfide derivatives, Inorg. Chim. Acta, 1980, 41, 283–286; (c) G. Bresciani, S. Zacchini, F. Marchetti and G. Pampaloni, Non-precious metal carbamates as catalysts for the aziridine/CO₂ coupling reaction under mild conditions, Dalton Trans., 2021, 50, 5351–5359; (d) R. Bini, F. Marchetti, G. Pampaloni and S. Zacchini, Further insights into the chemistry of niobium and tantalum pentahalides with 1,2-dialkoxyalkanes: Synthesis of bromo-and iodoalkoxides, spectroscopic and computational studies, Polyhedron, 2011, 30, 1412–1419; (e) R. Bini, C. Chiappe, F. Marchetti, G. Pampaloni and S. Zacchini, Structures and Unusual Rearrangements of Coordination Adducts of MX₅ (M = Nb, Ta; X = F, Cl) with Simple Diethers. A Crystallographic, Spectroscopic, and Computational Study, Inorg. Chem., 2010, 49, 339–351.
20. H. Henke, Zur kristallchemischen Einordnung von NaSbCl₆, NaNbCl₆, und NaTaCl₆, Z. Kristallogr., 1992, 198, 1–16.

---

Footnote

† Electronic supplementary information (ESI) available. CCDC 2254035–2254039. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi00910f

---