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(sp(2)) chelating ligand (5-L; L = propene, N-2; 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(sp(2)) and NHC-C(sp(3)) ligands (5-L and Cp*(IXy-H)Ru(N-2), 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(sp(3)) ligand is more electron donating. Complexes 5-L reacted with H-2 to give the dihydride Cp*(IPr)RuH2 (11). In comparison, after an initial oxidative addition of H-2, complex 1a with its more reactive Ru-C(sp(3)) bond underwent C-H bond reductive elimination, and a second oxidative addition of H-2 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)(3)BCH2)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-(CH2?CHCH2)C6H3-6-isopropyl]-3-(2,6-diisopropyl)imidazol-2-ylidene-1-yl) (13).