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 Cp*Ru(IXy-H) (2) [IXy = 1,3-bis(2,6-dimethylphenyl)imidazol-2-ylidene; IXy-H = 1-(2-CH2C6H3-6-methyl)-3-(2,6-dimethylphenyl)imidazol-2-ylidene-1-yl (the deprotonated form of IXy); Cp* = η(5)-C5Me5] and Cp*Ru(IXy-H)(N2) (3) was achieved by dehydrochlorination of Cp*Ru(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(═CH2)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*((i)Pr3P)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.