Understanding Synthetic Peculiarities of Cationic Molybdenum(VI) Imido Alkylidene N-Heterocyclic Carbene Complexes

We report on novel synthetic approaches to molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes including the first NHC alkylidene complexes featuring pentafluorophenyl-substituted imido ligands, highly basic 6-Mes (6-Mes = 1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene) NHC ligands and the preparation of the first representatives of cationic 14-electron Mo imido alkylidene NHC catalysts bearing sterically demanding anionic terphenoxide ligands. Also, the Mo imido alkylidene and alkylidyne NHC bisalkoxide complexes [Mo(NR)(CHCMe2Ph)(NHC)(OR ')(2)], [Mo(NH-C6F5)(CCMe2Ph)(aIMes){OCMe(CF3)(2)}(2)] (aIMes = 1,3-dimesitylimidazol-5-ylidene, an abnormally C-5-bound imidazolylidene) and [Mo(NH-2-CF3-C6H4)(CCMe2Ph)(6-Mes){OCMe(CF3)(2)}(2)] are presented. Anionic Mo amido alkylidyne complexes have been found to form preferentially in the presence of sterically demanding NHC and alkoxide ligands. The first Mo imido alkylidene NHC complexes containing the strongly basic 6-Mes ligand, namely [Mo(N-C6F5)(CHCMe2Ph)(6-Mes)(Br)(2)], [Mo(N-2-CF3-C6H4)(CHCMe2Ph)(6-Mes)(Br)(2)] and [Mo(N-2,6-Me-2-C6H3)(CHCMe2Ph)(6-Mes)(Br)(2)], were synthesized from the corresponding Mo imido dibromo alkylidene-DME complexes (DME = 1,2-dimethoxyethane) and transformed into their cationic monobromo or monoalkoxide derivatives. Finally, we developed the high yield synthesis of Mo imido alkylidene NHC dichloro complexes via direct protonation of [Mo(N-Adamantyl)(2)(CH2CMe2Ph)(2)] with HCl followed by stabilization with NHCs.