Molybdenum Imido Alkylidene Complexes Containing N- and C-Chelating N-Heterocyclic Carbenes

Six molybdenum imido alkylidene complexes bearing an Nchelating N-heterocyclic carbene (NHC), i.e., Mo(N-2,6-Me-2-C6H3)(CHCMe2Ph)(1-(2-pyridyl)methylene-3-mesitylimidazol-2-ylidene)(OTf)(2) (1), Mo(N-2,6-Me-2-C6H3)(CHCMe2Ph)(1-(2-pyridyl)methylene-3-mesitylimidazol-2-ylidene)(OTf)(C6F5) (3), Mo(N-2,6-Me-2-C6H3)(CHCMe2Ph)(1-(2-pyridyl)methylene-3-mesitylimidazol-2-ylidene) (OTf) (OCMe3) (4), and their cationic counterparts [Mo(N-2,6-Me-2-C6H3)(CHCMe2Ph)(1-(2pyridyl)methylene-3-mesitylimidazol-2-ylidene)(0TO]B(Ar94 (2), [Mo(N2,6-Me-2-C6H3)(CHCMe2Ph)(1-(2-pyridyl)methylene-3-mesitylimidazol-2- ylidene)(OC6P5)B(Ar-F)(4) (5), and [Mo(N-2,6-Me-2-C6H3)(CHCMe2Ph)(1-(2-pyridyl)methylene-3-mesitylimidazol-2-ylidene) (OCMe3)]B (Ar-F)(4) (6), have been prepared. The influence of the alkoxide ligand on the loss of triflate in the hexacoordinated complexes 1, 3, and 4 resulting in the corresponding cationic complexes was studied. The 2-pyridyl-donor-functionalized NHC ligand renders the high oxidation state cationic molybdenum imido alkylidene complexes highly stable and in ultimate consequence mostly inactive in olefin metathesis even at elevated temperatures. Notably, the molybdenum imido alkylidene complex Mo(N-2,6-MerC(6)H(3))(CHCMe2Ph)(1mesity1-3-(C9H11)-imidazol-2-ylidene)(OTO (9) with a C-chelating NHC formed via C H activation and was isolated in 76% yield.