Tungsten alkylidynes [CF3-ONO]W≡CC(CH3)3(THF)2 (1) and [(t)BuOCO]W≡CC(CH3)3(THF)2 (3) react with ethylene. Complex 1 reacts reversibly with ethylene to give the metallacyclobutene (2). Complex 3 reacts with ethylene to form the tethered alkylidene (4) featuring a tetraanionic pincer ligand. Complexes 1 and 3 initiate the polymerization of norbornene at room temperature. The polymerization of norbornene by 1 is not stereoselective, whereas 3 generates a highly cis and syndiotactic cyclic polynorbornene. Comparison of the intrinsic viscosity, radius of gyration, and elution time of the synthesized cyclic polynorbornene with those of linear analogues provides conclusive evidence for a cyclic topology.

This report details the preparation of the trianionic pincer ligand supported Mo–alkylidyne complex [ t BuOCO]­MoCMes­(THF)2 (5). The catalytic activity of [ t BuOCO]­MoCMes­(THF)2 (5) for alkyne polymerization is limited in contrast to its W-congener. Solution and crystallographic evidence for the conversion of the trianionic pincer complex [ t BuOCO]­MoCMes­(THF)2 (5) to a tetraanionic pincer complex [O2C­(MesC)­Mo­(η2-CHC t Bu)­(THF)] (6) upon exposure to tert-butylacetylene is provided.

A tungsten metallacyclopentadiene complex is generated upon treating a trianionic pincer tungsten alkylidyne with dipropargyl fluorene.