Si-H bond activation at {(NHC)_2Ni~0} 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

摘要

The unique reactivity of the nickel(0) complex [Ni_2(iPr_2Im)_4(COD)] (1) (iPr_2Im = 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(iPr_2Im)_2(H)(SiH_(n-1)R_(4-n))] (n = 1, 2) and nickel bis(silyl) complexes cis-[Ni- (iPr_2Im)_2(SiH_(n-1_R_(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(iPr_2Im)_2(H)(SiMePh_2)] (cis-5) shows in solution at room temperature a dynamic site exchange of the NHC ligands, H-D exchangewith C_6D_6 to give the deuteride complex cis-[Ni(iPr_2Im)_2(D)- (SiMePh_2)] (cis-5-D), and at elevated temperatures an irreversible isomerization to trans-[Ni(iPr_2Im)_2(D)- (SiMePh_2)] (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(iPr_2Im)_2Ni- (Si(OEt)_3)_2] (trans-12). The addition of two equivalents of Ph_2SiH_2 to 1 results, at elevated temperatures, in the formation of the dinuclear complex [{(iPr_2Im)Ni-μ~2-(HSiPh_2)}_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(iPr_2Im)_2} complex fragment to activate Si-H bonds was applied catalytically in the deuteration of Et_3Si-H to Et_3Si-D employing C_6D_6 as a convenient deuterium source. Furthermore, we show that 1 serves as a catalyst for the acceptorless dehydrogenative coupling of Ph_2SiH_2 to the corresponding disilane Ph_2HSi-SiHPh_2 and trisilane Ph_2HSi-Si(Ph)_2-SiHPh_2, and the coupling of PhSiH_3 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 Ph_2MeSi-SiMePh_2 and PhMe_2Si-SiMe_2Ph to the corresponding hydrosilanes Ph_2MeSi-H and PhMe_2Si-H, respectively, proceeds effectively in the presence of 1 under very mild conditions (room temperature, 1.8 bar H_2 pressure).