Catalytic and stoichiometric reactions by half-metallocene diene complexes of niobium and tantalum

We have systematically prepared ethylene, benzyne, and alkylidene complexes of niobium and tantalum bearing the fragments of 'MCp(eta(4)-1, 3-diene)' (M = Nb and Ta), and we found that these complexes have the similar stereochemistry and reactivity to that observed for the corresponding metallocene complexes of Group 4 metals, indicating clearly that the fragments of half-metallocene diene were isoelectronic and isolobal to those Of 'MCp2' (M = Zr and Hf). According to these results, we found three kinds of polymerization of ethylene, norbornene, and methyl methacrylate by using half-metallocene complexes of niobium and tantalum. Mono(diene) complexes 2 and 14 and bis(diene) complexes 16 and 17 upon being activated by MAO catalyzed the living polymerization of ethylene. Di(benzyl) tantalum complexes 18 and 23 served as a unique stereoselective polymerization of norbornene. The half-metallocene tantalum complexes 42 bearing MMA and DAD ligands reacted with one equiv of AlMe3 at -20degreesC to give a zwitter inonic hetero-bimetallic enolate species 43, which catalyzed the living polymerization of MMA.