Cyclometalated N-heterocyclic carbene complexes of ruthenium for access to electron-rich silylene complexes that bind the Lewis acids CuOTf and AgOTf

The synthesis of the cyclometalated complexes CpRu(IXy-H) (2) [IXy = 1,3-bis(2,6-dimethylphenyl)imidazol-2-ylidene; IXy-H = 1-(2-CH₂C ₆H₃-6-methyl)-3-(2,6-dimethylphenyl)imidazol-2-ylidene-1- yl (the deprotonated form of IXy); Cp* = η⁵-C ₅Me₅] and CpRu(IXy-H)(N₂) (3) was achieved by dehydrochlorination of CpRu(IXy)Cl (1) with KCH₂Ph. Complexes 2 and 3 activate primary silanes (RSiH₃) to afford the silyl complexes Cp(IXy-H)(H)RuSiH₂R [R = p-Tol (4), Mes (5), Trip (6)]. Density functional theory studies indicated that these complexes are close in energy to the corresponding isomeric silylene species Cp(IXy)(H)Ru=SiHR. Indeed, reactivity studies indicated that various reagents trap the silylene isomer of 6, Cp(IXy)(H)Ru=SiHTrip (6a). Thus, benzaldehyde reacts with 6 to give the [2 + 2] cycloaddition product 7, while 4-bromoacetophenone reacts via C-H bond cleavage and formation of the enolate Cp(IXy)(H)₂RuSiH[OC(=CH ₂)C₆H₄Br]Trip (8). Addition of the O-H bond of 2,6-dimethylphenol across the Ru=Si bond of 6a gives Cp(IXy)(H) ₂RuSiH(2,6-Me₂C₆H₃O)Trip (9). Interestingly, CuOTf and AgOTf also react with 6 to provide unusual Lewis acid-stabilized silylene complexes in which MOTf bridges the Ru-Si bond. The AgOTf complex, which was crystallographically characterized, exhibits a structure similar to that of [Cp(ⁱPr₃P)Ru(μ-H) ₂SiHMes]⁺, with a three-center, two-electron Ru-Ag-Si interaction. Natural bond orbital analysis of the MOTf complexes supported this type of bonding and characterized the donor interaction with Ag (or Cu) as involving a delocalized interaction with contributions from the carbene, silylene, and hydride ligands of Ru.