Half-Sandwich Silane σ-Complexes of Ruthenium Supported by NHC Carbene

Abstract

Reactions of carbene complex [Cp(IPr)Ru(pyr)2][BF4] (6, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with excess acetonitrile and LiCl in THF afford complexes [Cp(IPr)Ru(NCCH3)2][PF6] (7) and Cp(IPr)RuCl (8), respectively. Complex 8 was characterized by NMR and X-ray diffraction analysis. Addition of hydrosilanes to 8 results in silane σ-complexes Cp(IPr)Ru(η2-HSiR3)Cl (4), which were characterized by NMR and X-ray studies of Cp(IPr)Ru(η2-HSiMeCl2)Cl (4b) and Cp(IPr)Ru(η2-H3SiPh)Cl (4d). The hydrogen-silicon coupling constants of complexes 4 show an unusual trend in that the J(H-Si) values increase from the less-chlorinated complex Cp(IPr)Ru(η2-HSiMe2Cl)Cl (4c) to the trichloro derivative Cp(IPr)Ru(η2-HSiCl3)Cl (4a). Reaction of Cp(IPr)RuCl (8) with two equivalents of HSiCl3 gave the ruthenate complex [IPrH]+[CpRuCl(H) (SiCl3)2]-, characterized by NMR and X-ray study. Addition of hydrosilanes to the cationic complex [Cp(IPr)Ru(NCCH3)2][BArF4] (9) furnished very unstable cationic silane σ-complexes [Cp(IPr)Ru(η2-HSiR3)(NCCH3)]+ (5), characterized by low-temperature NMR. Reaction of complex 9 with two equivalents of HSiCl3 gives the neutral bis(silyl) complex CpRu(NCCH3)(H)(SiCl3)2 and [IPrH][BArF4]. Catalytic studies showed that 9 is a poorer catalyst for hydrosilylation of benzaldehyde, benzonitrile, and pyridine than its phosphine analogue [Cp(iPr3P)Ru(NCCH3)2][BArF4]. The reason for this reduced activity was assigned to the easy dissociation of carbene from the former catalyst.