[(NHC)CuI–ER3] Complexes (ER3 = SiMe2Ph, SiPh3, SnMe3): From Linear, Mononuclear Complexes to Polynuclear Complexes with Ultrashort CuI···CuI Distances

A series of complexes of the type [(NHC)­Cu–ER3] (NHC = IDipp, IMes, ItBu, Me2IMe, and ER3 = SiMe2Ph, SiPh3, SnMe3) and [(NHC)­Cu–R′] (NHC = IDipp, Me2IMe and R′ = Ph, CCPh) was synthesized in good yields by the reaction of the corresponding [(NHC)­Cu–OtBu] complex with the respective silylborane pinB–ER3 (pin = OCMe2CMe2O; ER3 = SiMe2Ph, SiPh3), the stannylborane ((C2H4)­(iPrN)2)­B–SnMe3, or a boronic acid ester pinB–R′ (R′ = Ph, CCPh). Solid structures of all complexes were systematically studied by X-ray diffraction analysis. The solid state structures of the complexes [(NHC)­Cu–ER3] show a dependence of the structural motif from the steric properties of the NHC ligand. The sterically demanding NHC ligands (IDipp, IMes, ItBu) lead to monomeric, linear complexes [(NHC)­Cu–ER3], while with the less demanding Me2IMe ligand, polynuclear, μ-ER3-bridged complexes with ultrashort Cu···Cu distances are observed. For the related complexes [(NHC)­Cu–R′] no analogous complexes with bridging anionic ligands are realized. Instead, irrespective of the NHC ligand, linear coordinated copper complexes of different types are formed. 29Si heteronuclear solution NMR spectroscopic data on [(NHC)­CuI–SiR3] exhibit distinctly different chemical shifts for the (in the solid state) monomeric and dimeric complexes suggesting different structure types also in solution. This agrees well with the observation of a trinuclear complex [(Me2IMe)­Cu–SnMe3]3 both in the solid state and in solution. Initial catalytic studies suggest that [(NHC)­Cu–OtBu] complexes (NHC = ItBu, Me2IMe) are, in addition to the established [(IDipp)­Cu–OtBu] complex, efficient precatalysts for the silylation of aldehydes and α,β-unsaturated ketones with pinB–SiMe2Ph.