Si–H bond activation at {(NHC)2Ni0} leading to hydrido silyl and bis(silyl) complexes: a versatile tool for catalytic Si–H/D exchange, acceptorless dehydrogenative coupling of hydrosilanes, and hydrogenation of disilanes to hydrosilanes

Abstract

The unique reactivity of the nickel(0) complex [Ni₂(iPr₂Im)₄(COD)] (1) (iPr₂Im = 1,3-di-isopropyl-imidazolin-2-ylidene) towards hydrosilanes in stoichiometric and catalytic reactions is reported. A series of nickel hydrido silyl complexes cis-[Ni(iPr₂Im)₂(H)(SiH_n−1R_4−n)] (n = 1, 2) and nickel bis(silyl) complexes cis-[Ni(iPr₂Im)₂(SiH_n−1R_4−n)₂] (n = 1, 2, 3) were synthesized by stoichiometric reactions of 1 with hydrosilanes H_nSiR_4−n, and fully characterized by X-ray diffraction and spectroscopic methods. These hydrido silyl complexes are examples where the full oxidative addition step is hindered. They have, as a result of the remaining Si–H interactions, remarkably short Si–H distances and feature a unique dynamic behavior in solution. Cis-[Ni(iPr₂Im)₂(H)(SiMePh₂)] (cis-5) shows in solution at room temperature a dynamic site exchange of the NHC ligands, H–D exchange with C₆D₆ to give the deuteride complex cis-[Ni(iPr₂Im)₂(D)(SiMePh₂)] (cis-5-D), and at elevated temperatures an irreversible isomerization to trans-[Ni(iPr₂Im)₂(D)(SiMePh₂)] (trans-5-D). Reactions with sterically less demanding silanes give cis-configured bis(silyl) complexes accompanied by the release of dihydrogen. These complexes display, similarly to the hydrido silyl complexes, interestingly short Si–Si distances. Complex 1 reacts with 4 eq. HSi(OEt)₃, in contrast to all the other silanes used in this study, to give the trans-configured bis(silyl) complex trans-[Ni(iPr₂Im)₂Ni(Si(OEt)₃)₂] (trans-12). The addition of two equivalents of Ph₂SiH₂ to 1 results, at elevated temperatures, in the formation of the dinuclear complex [{(iPr₂Im)Ni-μ²-(HSiPh₂)}₂] (6). This diamagnetic, formal Ni(I) complex exhibits a long Ni–Ni bond in the solid state, as established by X-ray diffraction. The capability of the electron rich {Ni(iPr₂Im)₂} complex fragment to activate Si–H bonds was applied catalytically in the deuteration of Et₃Si–H to Et₃Si–D employing C₆D₆ as a convenient deuterium source. Furthermore, we show that 1 serves as a catalyst for the acceptorless dehydrogenative coupling of Ph₂SiH₂ to the corresponding disilane Ph₂HSi–SiHPh₂ and trisilane Ph₂HSi–Si(Ph)₂–SiHPh₂, and the coupling of PhSiH₃ to give a mixture of cyclic and linear polysilanes with high polydispersity (M_w = 1119; M_n = 924; M_w/M_n = 1.2). The capability of 1 to catalyze the formal reverse reaction as well is demonstrated by the hydrogenation of disilanes. The hydrogenation of the disilanes Ph₂MeSi–SiMePh₂ and PhMe₂Si–SiMe₂Ph to the corresponding hydrosilanes Ph₂MeSi–H and PhMe₂Si–H, respectively, proceeds effectively in the presence of 1 under very mild conditions (room temperature, 1.8 bar H₂ pressure).