Highly Reactive Cationic Molybdenum Alkylidyne N‑Heterocyclic Carbene Catalysts for Alkyne Metathesis

The tetracoordinated cationic molybdenum alkylidyne N-heterocyclic carbene (NHC) complexes [Mo­(CC6H4-p-OMe)­(IMes)­(OCMe­(CF3)2)2]­[BPh4] (Mo5) and [Mo­(CC6H4-p-OMe)­(IMes)­(OCMe­(CF3)2)2]­[B­(ArF)4] (Mo6, IMes = 1,3-dimesitylimidazol-2-ylidene)) were synthesized from the pentacoordinated progenitor Mo­(CC6H4-p-OMe)­(IMes)­(OCMe­(CF3)2)2(OTf) (Mo4). Complexes Mo4–Mo6 were evaluated for their ability to catalyze the self-metathesis of several internal alkynes. The presence of a triflate group facilitates formation of a cationic species while preformation of the cationic molybdenum center in molybdenum alkylidyne NHC complexes indeed results in a strong increase in catalyst productivity and activity, also in the presence of functional groups, compared to previously reported neutral congeners. The most striking feature of this class of tetracoordinate cationic complexes is the excellent catalytic activity in the alkyne metathesis of non-protic substrates, thereby supporting our previously published proposal of a tetracoordinate cationic active species in alkyne metathesis formed from the neutral, pentacoordinate molybdenum alkylidyne NHC progenitors. Catalyst productivity, expressed as turnover number, reached 20 000 in the self-metathesis of 1-phenyl-1-propyne (S1) using Mo­(CC6H4-p-OMe)­(1,3-dimesitylimidazol-2-ylidene)­(OCMe­(CF3)2)2[B­(ArF)4] (Mo6) and 5-(benzyloxy)-2-pentyne (S2) at catalyst loadings as low as 0.005 mol %.