Neutral and Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes: Reactivity in Selected Olefin Metathesis Reactions and Immobilization on Silica

The synthesis and single-crystal X-ray structures of the novel molybdenum imido alkylidene N-heterocyclic carbene complexes [Mo(N-2,6-Me2C6H3)(IMesH(2))(CHCMe2Ph)(OTf)(2)] (3), [Mo(N-2,6-Me2C6H3)(IMes)(CHCMe2Ph)(OTf)(2)] (4), [Mo(N-2,6-Me2C6H3)(IMesH(2))(CHCMe2Ph)(OTf){OCH(CF3)(2)}] (5), [Mo(N-2,6-Me2C6H3)(CH3CN)(IMesH(2))(CHCMe2Ph)(OTf)]+BArF- (6), [Mo(N-2,6-Cl2C6H3)(IMesH(2))(CHCMe3)(OTf)(2)] (7) and [Mo(N-2,6-Cl2C6H3)(IMes)(CHCMe3)(OTf)(2)] (8) are reported (IMesH(2)=1,3-dimesitylimidazolidin-2-ylidene, IMes=1,3-dimesitylimidazolin-2-ylidene, BArF-=tetrakis-[3,5-bis(trifluoromethyl)phenyl] borate, OTf=CF3SO3-). Also, silica-immobilized versions I1 and I2 were prepared. Catalysts 3-8, I1 and I2 were used in homo-, cross-, and ring-closing metathesis (RCM) reactions and in the cyclopolymerization of ,-diynes. In the RCM of ,-dienes, in the homometathesis of 1-alkenes, and in the ethenolysis of cyclooctene, turnover numbers (TONs) up to 100000, 210000 and 30000, respectively, were achieved. With I1 and I2, virtually Mo-free products were obtained (<3ppm Mo). With 1,6-hepta- and 1,7-octadiynes, catalysts 3, 4, and 5 allowed for the regioselective cyclopolymerization of 4,4-bis(ethoxycarbonyl)-1,6-heptadiyne, 4,4-bis(hydroxymethyl)-1,6-heptadiyne, 4,4-bis[(3,5-diethoxybenzoyloxy)methyl]-1,6-heptadiyne, 4,4,5,5-tetrakis(ethoxycarbonyl)-1,7-octadiyne, and 1,6-heptadiyne-4-carboxylic acid, underlining the high functional-group tolerance of these novel Group6 metal alkylidenes.