Synthesis, structures, and reactivity studies of cyclometalated N-heterocyclic carbene complexes of ruthenium

Abstract

An unusual cyclometalation reaction results from a C–C bond activation in Cp*(IPr)RuCl to give Cp*(IPr′)Ru(L) featuring a NHC-C(sp²) chelating ligand (5-L; L = propene, N₂; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; IPr′ = 1-(6-isopropylphenyl)-3-(2,6-diisopropylphenyl)imidazol-2-ylidene). DFT calculations were employed to elucidate the C–C bond activation pathway. Reactions of cyclometalated ruthenium complexes bearing NHC-C(sp²) and NHC-C(sp³) ligands (5-L and Cp*(IXy-H)Ru(N₂), 1a, respectively where IXy = 1,3-bis(2,6-dimethylphenyl)-imidazol-2-ylidene; IXy-H is the deprotonated form of (IXy)) are reported. Deprotonation of 1a by an equimolar mixture of benzyl potassium and 18-crown-6 afforded a doubly-cyclometalated complex [Cp*(IXy-2H)Ru][K(18-crown-6)] (7). A lower CO stretching frequency in Cp*(IXy-H)Ru(CO) (8) vs. Cp*(IPr′)Ru(CO) (9) suggests that the NHC-C(sp³) ligand is more electron donating. Complexes 5-L reacted with H₂ to give the dihydride Cp*(IPr′)RuH₂ (11). In comparison, after an initial oxidative addition of H₂, complex 1a with its more reactive Ru–C(sp³) bond underwent C–H bond reductive elimination, and a second oxidative addition of H₂ afforded the trihydride Cp*(IXy)RuH₃ (10). Reaction of 1a with B(C₆F₅)₃ resulted in a zwitterionic complex Cp*Ru(IXy′′) (12; IXy′′ = 1-[2-((C₆F₅)₃BCH₂)C₆H₃-6-methyl]-3-(2,6-dimethylphenyl)imidazol-2-ylidene-1-yl) by the formation of a new C–B bond. In contrast, B(C₆F₅)₃ abstracted a hydride from 5-L and promoted a very unusual C–C bond formation involving insertion of an allyl ligand into a Ru–C bond to form [Cp*Ru(IPr′′)][HB(C₆F₅)₃] (IPr′′ = 1-[2-(CH₂CHCH₂)C₆H₃-6-isopropyl]-3-(2,6-diisopropyl)imidazol-2-ylidene-1-yl) (13).