Cationic Group VI Metal Imido Alkylidene N-Heterocyclic Carbene Nitrile Complexes: Bench-Stable, Functional-Group-Tolerant Olefin Metathesis Catalysts.

Despite their excellent selectivities and activities, Mo-and W-based catalysts for olefin metathesis have not gained the same widespread use as Ru-based systems, mainly due to their inherent air-sensitivity. Here we describe the synthesis of cationic-at-metal molybdenum and tungsten imido alkylidene NHC nitrile complexes that are long-time stable in air. They catalyze olefin metathesis reactions of substrates containing functional groups such as (thio-) esters, (thio-) ethers and alcohols without the need for prior activation, e.g. by a Lewis acid. The presence of a nitrile ligand was found to be essential for their stability towards air, while no decrease in activity and productivity could be observed upon coordination of a nitrile. Variations of the imido and anionic ligand revealed that alkoxide complexes with electron-withdrawing imido ligands offer the highest reactivities and excellent stability compared to analogous triflate and halide complexes. The molybdenum imido alkylidene NHC nitrile complexes maintain their selectivity for the formation of trans -isotactic poly(norbornene)s.