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.