Tuning Philicity of Dichlorosilylene: Nucleophilic Behavior of the

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Tuning Philicity of Dichlorosilylene: Nucleophilic Behavior of the Dichlorosilylene−NHC Complex Cl2Si−IPr Vladimir Ya. Lee,*,† Satoru Horiguchi,†,∥ Akira Sekiguchi,*,†,⊥ Olga A. Gapurenko,‡ Tatyana N. Gribanova,‡ Vladimir I. Minkin,‡ and Heinz Gornitzka§ †

Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan Institute of Physical and Organic Chemistry, Southern Federal University, 194/2 Stachki Avenue, Rostov on Don 344090, Russian Federation § CNRS, LCC, Université de Toulouse, UPS, INPT, 205 Route de Narbonne, BP 44099, Toulouse Cedex 4 F-31077, France ‡

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S Supporting Information *

ABSTRACT: Novel unsaturated four-membered ring disilene, 3,3-dichloro-1,2,4,4tetrakis[di-tert-butyl(methyl)silyl]-1Δ-1,2,3,4-trisilagermetene, was synthesized by the ring expansion reaction of the three-membered ring 1-disilagermirene with the silylene− NHC complex Cl2Si−IPr. The mechanism of the unexpected formation of this compound was verified by high-level density functional theory computations, which revealed nSi:(HOMO)−π*SiSi(LUMO) as the dominant orbital interaction.

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INTRODUCTION Carbenes, possessing both a lone pair and vacant p-orbital at the divalent carbon center, are intrinsically ambiphilic.1 Likewise, silylenes, as silicon analogues of carbenes, are also ambiphilic, manifesting dual-type reactivity: (1) electrophilic, as electron pair acceptors, the acceptor orbital is a silicon 3p-orbital [lowest unoccupied molecular orbital (LUMO)] (Lewis acid reactivity) and (2) nucleophilic, as electron pair donors, the donor orbital is a silicon lone pair n-orbital [highest occupied molecular orbital (HOMO)] (Lewis base reactivity) (Chart 1).2

donors. However, our computations on the free Cl2Si: do not support notable importance of such nCl → 3pSi interaction: (1) electron population of the dichlorosilylene 3p-orbital is still low (0.12 e−) and (2) Si−Cl natural resonance theory bond order of 1.06 is indicative of a small extent of the double bonding. Therefore, it might be expected that the free dihalosilylenes, including dichlorosilylene Cl2Si:, are primarily electrophilic (like dichlorocarbene Cl2C:1b), thus undergoing reactions involving attack of the vacant p-orbital of the silicon center by the Lewis basic reagent as the initial step of the process. However, like dichlorocarbene, dichlorosilylene does not exist as a stable compound, as it participates in a vast number of chemical transformations as a reactive intermediate.3 The major breakthrough came in 2009, when Roesky and co-workers reported the synthesis and crystal structure of the isolable complex of dichlorosilylene with the N-heterocyclic carbene (NHC), Cl2Si−IPr (IPr = 1,3-bis(2,6-diisopropylphenyl)-2H-imidazol2-ylidene).4 In the same year, the synthesis of dibromosilylene− NHC complex Br2Si−IPr was reported by Filippou and coworkers,5 and it is also relevant to mention here the remarkable synthesis of metastable SiF2 and SiCl2 solutions by Schnepf and co-workers.6 The germanium analogue of dichlorosilylene, namely, dichlorogermylene Cl2Ge:, also could not be isolated as a

Chart 1. Dual Reactivity of Silylenes

The reactivity of silylenes in each particular case is totally governed by the substituents at the divalent silicon center: σelectron-withdrawing substituents (such as halogens) promote preferentially electrophilic behavior of silylenes, whereas πelectron-donating groups (such as amino-substituents) favor nucleophilic reactivity of silylenes. Given their lone pairs, halogen substituents could alternatively be considered as π© 2019 American Chemical Society

Received: December 6, 2018 Accepted: January 30, 2019 Published: February 8, 2019 2902

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room temperature-stable derivative because of its exceptionally high reactivity including a polymerization process.7 However, ndonor ligands can and do readily and effectively stabilize the lowcoordinate germanium center, and accordingly, a range of dichlorogermylene complexes with such n-donor ligands was successfully prepared long ago.8 Among them, by far, the most common and readily available is the complex of dichlorogermylene with 1,4-dioxane (diox), [Cl2Ge−diox].9 Previously, we reported a rather unusual reaction between the [Cl2Ge−diox] complex with the heavier analogue of cyclopropene, namely, the unsaturated three-membered ring 1disilagermirene 1,10 which unexpectedly forms the fourmembered ring trans-1,2-dichloro-1,2,3,4-tetrakis[di-tert-butyl(methyl)silyl]-3Δ-1,2,3,4-disiladigermetene 2 (Scheme 1).11 Overall, this ring-expansion reaction involved a tricky transformation of a SiSi bond in the starting 1-disilagermirene 1 into a GeGe bond in the final disiladigermetene 2.

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RESULTS AND DISCUSSION

In contrast to the fast reaction of disilagermirene 1 with Cl2Ge− diox, which quantitatively forms the disiladigermetene 2 even at low temperature,11 reaction of 1 with Cl2Si−IPr required remarkably harsher conditions, namely, heating in tetrahydrofuran (THF) at 70 °C for 34 h. The only product isolated from the reaction mixture was 3,3-dichloro-1,2,4,4-tetrakis[di-tertbutyl(methyl)silyl]-1Δ-1,2,3,4-trisilagermetene 4, a structural isomer of the expected 3 (Scheme 3). Scheme 3. Synthesis of 4

Scheme 1. Proposed Reaction Pathway for the Formation of 2 (Adapted from ref 9)

The product 4, isolated by glove-box column chromatography as a yellow-orange solid, was formed along with free IPr12 and t Bu2MeSiCl as the only identifiable side products. The side product tBu2MeSiCl was formed in tiny amounts and identified by the comparison of its NMR and GC−MS spectroscopic characteristics with those of the authentic sample (tBu2MeSiCl: colorless liquid, bp 60−61 °C/10 mm. 1H NMR (C6D6, δ, ppm): 0.19 (s, 3 H, CH3), 0.98 (s, 18 H, 2 tBu); 13C NMR (C6D6, δ, ppm): −4.7 (CH3), 21.6 (2 C(CH3)3), 27.6 (2 C(CH3)3); 29Si NMR (C6D6, δ, ppm): 37.9; GC−MS (m/z): 192 [M+], 157 [M+ − Cl], 135 [M+ − tBu]). Its formation can be realized in terms of the elimination of tBu2MeSi-substituent from the starting 1, taking place upon thermal reaction of the latter with Cl2Si−IPr playing a role of the chlorinating agent (the ability of the latter complex to function as a chlorinating agent was previously acknowledged: see ref 13). This side reaction unavoidably proceeded upon heating, simultaneously with the main reaction giving 4, resulting in the complex mixture containing unidentifiable compounds along with tBu2MeSiCl. The constitution of 4 was established on the basis of its spectroscopic (NMR and HRMS) data. Thus, in the 29Si NMR spectrum of 4, there were two low-field resonances observed at +168.0 and +176.9 ppm, both in the region diagnostic for the four-membered ring cyclic disilenes.14 This contradicts our initial expectations of obtaining compound 3 with a SiGe double bond, which requires observation of only one signal for the sp2-Si atom, implying that rather than forming the cyclic silagermene, the unsymmetrically substituted cyclic disilene with a SiSi bond was actually produced. This was finally confirmed by X-ray crystallography of 4 (Figure 1). The fourmembered ring is composed of three silicon atoms and one germanium atom, featuring an endocyclic Si2−Si3 double bond and two chlorine atoms attached to the cyclic sp3-Si1 atom. The Ge1Si1Si2Si3-four-membered ring is folded (folding angle is 24.2°, calculated value 18.3°), and the length of the Si2−Si3 bond of 2.199(9) Å is typical for the double bond in the cyclotetrasilenes (2.16−2.36 Å,14 calculated value 2.180 Å). As the structure of the isolated product 4 is distinctly different from that of the expected product 3, the mechanisms of the formation of these two compounds are most likely different. We probed this hypothesis with the high-level computations at the M06/Def2TZVP15 level of theory.16 In the first step of the previously reported disiladigermetene 2, we proposed initial addition of the Cl2Ge: across the SiSi

Expanding the applicability range of such an attractive reaction, we were prompted to explore the possibility of a similar reaction of 1 with the source of dichlorosilylene, namely, Cl2Si−IPr. As the final product, we expected formation of the compound similar to disiladigermetene 2, that is, trisilagermetene 3 with a SiGe double bond and two chlorine atoms at the skeletal sp3-Si atoms (Scheme 2). In this paper, we report on the results of these studies, discussing the actual structure of the product and proposing the mechanisms of its formation, in comparison with the formation pathways for the previously reported disiladigermetene 2. Scheme 2. Proposed Synthesis of 3

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Scheme 4. Proposed Reaction Pathway for the Formation of 4

Overall, the exceptional electron-donating properties of the NHC ligand strongly affects and alters the reactivity pattern of dichlorosilylene in the Cl2Si−IPr complex: if free dichlorosilylene Cl2Si: is primarily electrophilic, its NHC-complex Cl2Si− IPr is no longer electrophilic but rather nucleophilic instead.

Figure 1. Crystal structure of 4 (ORTEP view, thermal ellipsoids are given for Ge, Si, and Cl atoms at the 50% probability level, C atoms are given as spheres, and H atoms are not shown. Part of the molecule is positionally disordered, and only the major contribution (79%) is shown).

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CONCLUSIONS The unsaturated four-membered ring cyclic disilene, namely, 3,3-dichloro-1,2,4,4-tetrakis[di-tert-butyl(methyl)silyl]- 1 Δ1,2,3,4-trisilagermetene, was unexpectedly formed upon the ring expansion reaction of the three-membered ring 1-disilagermirene and the Cl2Si−IPr complex. According to high-level density functional theory computations, the most likely mechanistic scenario for the formation of the product may involve nSi:(HOMO)−πSiSi(LUMO) * as the dominant orbital interaction.

bond forming the intermediate chlorogermylene 5 (Scheme 1). Given the primarily electrophilic nature of dichlorogermylene, one would expect initial interaction between the occupied πorbital of the SiSi of disilagermirene 1 (HOMO) and the vacant 4p-orbital of the Ge center of Cl2Ge−diox (LUMO), finally resulting in the formation of 2 (Scheme 1). This was supported by our computations, which showed that such a πSiSi(HOMO) → 4pGe:(LUMO) orbital interaction is indeed facilitated compared with the alternative nGe:(HOMO) → πSiSi(LUMO) * interaction mode: ΔE(πSiSi → 4pGe:) and ΔE(nGe: → π*SiSi) are 43 and 151 kcal/mol, respectively. Likewise, when the hypothetical free dichlorosilylene Cl2Si: was calculated to react with disilagermirene 1, the first step of the reaction primarily involved a πSiSi(HOMO) → 3pSi:(LUMO) orbital interaction: ΔE(πSiSi → 3pSi:) and ΔE(nSi: → π*SiSi) are 52 and 143 kcal/mol, respectively. Such electrophilic behavior is expected for the dichlorosilylene with its strongly electronwithdrawing chlorine substituents. By contrast, when the NHCcomplex of dichlorosilylene Cl2Si−IPr was reacted with disilagermirene 1, the nSi:(HOMO)−π*SiSi(LUMO) orbital interaction became substantially more favorable because of the remarkable population of the 3pSi:-orbital caused by the coordination of the strongly σ-donating IPr-ligand [actually, the MO involving interaction between the 3p-orbital of silicon and the lone pair orbital of NHC ligand (HOMO − 22) is much lower in energy than the vacant p-orbital (LUMO) in the free Cl2Si:]. Because of such unavailability of the silicon 3p-orbital in the SiCl2−IPr complex for πSiSi → 3pSi: bonding (which was the case of the free Cl2Si:, vide supra), the primary nSi: → πSiSi * interaction involves the lone pair n-orbital on silicon, thus manifesting nucleophilic behavior of the dichlorosilylene (Scheme 4). It should be noted here that although predominantly nucleophilic behavior of dichlorosilylene in the Cl2Si−IPr complex was not previously unambiguously acknowledged, the Lewis basic reactivity of the latter complex was documented in its reactions with Lewis acids, such as boranes and transition metal complexes.13,17 The initial nucleophilic attack results in the generation of the intermediate zwitter-ion 6, followed by the intramolecular ring-expansion of the latter accompanied with the recovery of the SiSi double bond to form finally unsaturated four-membered ring product 4 (Scheme 4).

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EXPERIMENTAL SECTION General Methods. All experimental manipulations were performed using high-vacuum line techniques or in an argon atmosphere of MBRAUN MB 150B-G glove box. All solvents were predried by conventional methods and finally dried and degassed over a potassium mirror in vacuum immediately prior to use. NMR spectra were recorded on Bruker AV-400FT NMR spectrometer (1H NMR at 400.1 MHz; 13C NMR at 100.6 MHz; 29 Si NMR at 79.5 MHz). High-resolution mass spectra were measured on a Bruker Daltonics micrOTOF-TU mass spectrometer with the atmospheric pressure chemical ionization (APCI) method. Starting 1-disilagermirene 1 was prepared according to a previously published procedure.10 Experimental Procedure and Spectroscopic and Crystallographic Data for 3,3-Dichloro-1,2,4,4-tetrakis[di-tert-butyl(methyl)silyl]-1Δ-1,2,3,4-trisilagermetene 4. A mixture of 1-disilagermirene 1 (387 mg, 0.51 mmol) and Cl2Si−IPr (375 mg, 0.77 mmol) was placed in a reaction tube with a magnetic stirring bar. Then, dry and degassed dioxygenfree THF (4 mL) was introduced into the reaction tube by vacuum transfer, and the reaction mixture was stirred at 70 °C for 34 h. THF was removed under vacuum, and dry hexane was introduced. In the next step, the free IPr was separated by glovebox column chromatography on silica gel with hexane as eluent. Finally, the solvent and tBu2MeSiCl were removed under vacuum to give 4 as an orange-yellow solid (81 mg, 19%). mp 147−149 °C. 1H NMR (C6D6, δ, ppm): 0.32 (s, 3 H, CH3), 0.43 (s, 3 H, CH3), 0.72 (s, 6 H, 2 CH3), 1.20 (s, 18 H, 2 tBu), 1.21 (s, 18 H, 2 tBu), 1.25 (s, 18 H, 2 tBu), 1.30 (s, 18 H, 2 tBu); 13C NMR (C6D6, δ, ppm): −0.8 (CH3), 1.49 (CH3), 1.52 (2 CH3), 22.4 (2 C(CH3)3), 22.7 (2 C(CH3)3), 23.1 (2 C(CH3)3), 23.4 (2 C(CH3)3), 29.9 (2 C(CH3)3), 30.6 (2 C(CH3)3), 31.1 (2 C(CH3)3), 31.8 (2 C(CH3)3); 29Si NMR (C6D6, δ, ppm): 22.7, 24.7, 25.6, 29.0, 168.0 (sp2-Si), 176.9 (sp2-Si); HRMS (APCI): 2904

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m/z calcd for C36H85Cl2GeSi7, 857.3620 [M + H]+; found, 857.3886 [M + H]+. Details of the crystallographic data for 4 are given in the Supporting Information. Although the final refinement of 4 was not ideal, its crystal structure and structural parameters can be reliably discussed in this preliminary crystallographic report. In the paper, we briefly discuss only the most essential structural characteristics (SiSi double bond length and four-membered ring folding angle), whereas all other metric parameters of 4 are given in the Supporting Information. Computational Details. Calculations were performed using the Gaussian 16 program16 based on the M06/Def2TZVP level of theory. This calculation level was selected as the one, best reproducing the X-ray crystallographic data, after testing several other levels (M06/6-311+G**, TPSSh/6-311+G**, TPSSh/Def2TZVP, B3LYP/6-311+G**, and B3LYP/ Def2TZVP).

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Complexes. In Organosilicon Compounds: Theory and Experiment (Synthesis); Lee, V. Ya., Ed.; Elsevier, 2017; Volume 1, Chapter 8. (3) Teichmann, J.; Wagner, M. Silicon Chemistry in Zero to Three Dimensions: From Dichlorosilylene to Silafullerane. Chem. Commun. 2018, 54, 1397−1412. (4) (a) Ghadwal, R. S.; Roesky, H. W.; Merkel, S.; Henn, J.; Stalke, D. Lewis Base Stabilized Dichlorosilylene. Angew. Chem., Int. Ed. 2009, 48, 5683−5686. (5) Filippou, A. C.; Chernov, O.; Schnakenburg, G. SiBr2(Idipp): A Stable N-Heterocyclic Carbene Adduct of Dibromosilylene. Angew. Chem., Int. Ed. 2009, 48, 5687−5690. (6) Uhlemann, F.; Köppe, R.; Schnepf, A. Synthesis of Metastable SiIIX2 Solutions (X = F, Cl). A Novel Binary Halide for Synthesis. Z. Anorg. Allg. Chem. 2014, 640, 1658−1664. (7) Neumann, W. P. Germylenes and Stannylenes. Chem. Rev. 1991, 91, 311−334. (8) (a) Nefedov, O. M.; Kolesnikov, S. P.; Rogozhin, I. S. New Germanium Dichloride Complexes. Bull. Acad. Sci. USSR, Div. Chem. Sci. 1973, 22, 2762. (b) Nefedov, O. M.; Kolesnikov, S. P.; Rogozhin, I. S. Molecular Complexes of Germylenes with n-Donor Ligands. Izv. Akad. Nauk SSSR, Ser. Khim. 1980, 170−173. (9) (a) Kolesnikov, S. P.; Shiryaev, V. I.; Nefedov, O. M. Complex of Germanium Dichloride. Bull. Acad. Sci. USSR, Div. Chem. Sci. 1966, 15, 562. (b) Kolesnikov, S. P.; Rogozhin, I. S.; Nefedov, O. M. Preparation of Complex of Germanium Dichloride with 1,4-Dioxane. Bull. Acad. Sci. USSR, Div. Chem. Sci. 1974, 23, 2297. (10) Lee, V. Ya.; Ichinohe, M.; Sekiguchi, A.; Takagi, N.; Nagase, S. The First Three-Membered Unsaturated Rings Consisting of Different Heavier Group 14 Elements: 1-Disilagermirene with a Si=Si Double Bond and Its Isomerization to a 2-Disilagermirene with a Si=Ge Double Bond. J. Am. Chem. Soc. 2000, 122, 9034−9035. (11) (a) Lee, V. Ya.; Takanashi, K.; Ichinohe, M.; Sekiguchi, A. A Chemical Trick: How to Make a Digermene from a Disilene, Formation of 3Δ-1,2,3,4-Disiladigermetene. J. Am. Chem. Soc. 2003, 125, 6012− 6013. (b) Lee, V. Y.; Ito, Y.; Sekiguchi, A. 1,2-Dibromo-3Δ-1,2,3,4disiladigermetene. Phosphorus, Sulfur Silicon Relat. Elem. 2011, 186, 1351−1355. (12) Bantreil, X.; Nolan, S. P. Synthesis of N-Heterocyclic Carbene Ligands and Derived Ruthenium Olefin Metathesis Catalysts. Nat. Protoc. 2010, 6, 69−77. (13) Li, J.; Merkel, S.; Henn, J.; Meindl, K.; Döring, A.; Roesky, H. W.; Ghadwal, R. S.; Stalke, D. Lewis-Base Stabilized Dichlorosilylene: A Two-Electron σ-Donor Ligand. Inorg. Chem. 2010, 49, 775−777. (14) Lee, V. Ya.; Sekiguchi, A. Organometallic Compounds of LowCoordinate Si, Ge, Sn and Pb (From Phantom Species to Stable Compounds); Wiley, 2010; Chapter 5. (15) Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Functionals. Theor. Chem. Acc. 2008, 120, 215− 241. (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A. V.; Bloino, J.; Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J. V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams-Young, D.; Ding, F.; Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson, T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.; Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Throssell, K.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M. J.; Heyd, J. J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Keith, T. A.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M.; Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J. Gaussian 16, Revision A.03; Gaussian, Inc.: Wallingford CT, 2016.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b03429. Details of the X-ray crystallography for the trisilagermetene 4 (tables of the crystallographic data including atomic positional and thermal parameters) (PDF) Cartesian coordinates of the calculated molecules are given in XYZ format (XYZ) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (V.Ya.L.). *E-mail: [email protected] (A.S.). ORCID

Vladimir Ya. Lee: 0000-0002-6527-5342 Present Addresses ∥

Asia Industry Co., Ltd., 22 Kawarai-cho, Kuki-shi, Saitama, 346-0028, Japan. ⊥ Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology, Central-5, 1-1-1 Higashi, Tsukuba, Ibaraki 3058565, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the JSPS KAKENHI Grants program (nos. JP18K05137, JP16K05682) from the Ministry of Education, Science, Sports, and Culture of Japan. V.I.M. acknowledges financial support by the Ministry of Science and Education of the Russian Federation (grant no. 4.9792017/4.6).

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REFERENCES

(1) (a) Bourissou, D.; Guerret, O.; Gabbaï, F. P.; Bertrand, G. Stable Carbenes. Chem. Rev. 2000, 100, 39−92. (b) Fleming, I. Molecular Orbitals and Organic Chemical Reaction; Wiley, 2009. (2) (a) Becerra, R.; Walsh, R. What Have We Learnt About Heavy Carbenes Through Laser Flash Photolysis Studies? Phys. Chem. Chem. Phys. 2007, 9, 2817−2835. (c) Mizuhata, Y.; Sasamori, T.; Tokitoh, N. Stable Heavier Carbene Analogues. Chem. Rev. 2009, 109, 3479. (d) Iwamoto, T.; Ishida, S. Stable Silylenes and Their Transition Metal 2905

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(17) (a) Ghadwal, R. S.; Roesky, H. W.; Merkel, S.; Stalke, D. Ambiphilicity of Dichlorosilylene in a Single Molecule. Chem.Eur. J. 2010, 16, 85−88. (b) Azhakar, R.; Tavčar, G.; Roesky, H. W.; Hey, J.; Stalke, D. Facile Synthesis of a Rare Chlorosilylene−BH3 Adduct. Eur. J. Inorg. Chem. 2011, 475−477. (c) Tavčar, G.; Sen, S. S.; Azhakar, R.; Thorn, A.; Roesky, H. W. Facile Syntheses of Silylene Nickel Carbonyl Complexes from Lewis Base Stabilized Chlorosilylenes. Inorg. Chem. 2010, 49, 10199−202. (d) Ghadwal, R. S.; Azhakar, R.; Pröpper, K.; Holstein, J. J.; Dittrich, B.; Roesky, H. W. N-Heterocyclic Carbene Stabilized Dichlorosilylene Transition-Metal Complexes of V(I), Co(I), and Fe(0). Inorg. Chem. 2011, 50, 8502−8508.

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