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* = η⁵-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*­(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.