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

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.