Cationic Tungsten-Oxo-Alkylidene-N-Heterocyclic Carbene Complexes: Highly Active Olefin Metathesis Catalysts

The synthesis, structure, and olefin metathesis activity of the first neutral and cationic W-oxo-alkylidene-N-heterocyclic carbene (NHC) catalysts are reported. Neutral W-oxo-alkylidene-NHC catalysts can be prepared in up to 90% isolated yield. Depending on the ligands used, they possess either an octahedral (<i>O</i>_<i>h</i>) or trigonal bipyramidal ligand sphere. They can be activated with excess AlCl₃ to form cationic olefin metathesis-active W-complexes; however, these readily convert into neutral chloro-complexes. Well-defined, stable cationic species can be prepared by stoichiometric substitution of one chloro ligand in the parent, neutral W-oxo-alkylidene-NHC complexes with Ag­(MeCN)₂B­(Ar^F)₄ or NaB­(Ar^F)₄; B­(Ar^F)₄ = B­(3,5-(CF₃)₂-C₆H₃)₄. They are highly active olefin metathesis catalysts, allowing for turnover numbers up to 10,000 in various olefin metathesis reactions including alkenes bearing nitrile, sec-amine, and thioether groups