Development of novel highly active ROMP catalysts based on Group 5 transition metals

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Pentavalent tantalum and niobium complexes with aryloxy ligands, (2,6-R_2C_6H_3O)MCl_4 (M=Ta, Nb; R=H, Me, ^iPr, Ph), were prepared, and their catalytic behaviors for the ring-opening metathesis polymerization (ROMP) of norbornene (NBE) were systematically studied in the presence of alkylaluminum cocatalysts, R'_3Al (R'=Me, Et, ^iBu). Tantalum complexes were found to show very high activity for the ROMP of NBE in combination with ^iBu_3Al to give high-molecular-weight polymers (M_n>10^5) and relatively narrow molecular weight distributions (M_w/M_n〜2), implying that these are single-site catalysts. With increasing the bulkiness of the ligand, the tantalum catalyst systems became less active. In sharp contrast, the aryloxyniobium complexes were revealed to exhibit very high activity upon activation with Me_3Al to give high-molecular weight polymers. With increasing the bulkiness of the ligand, the activity of the niobium catalyst systems increased. We also studied synthesis of pentavalent tantalum and niobium complexes having catecholato, tropolonato, hinokitiolato, biphenolato, and binaphtolato ligands, and their catalyses for the ROMP of NBE in the presence of trialkylaluminum as a cocatalyst. Among those complexes, the tantalum complexes also showed high activity upon activation with ^iBu_3Al for ROMP of NBE. In contrast, the biphenolato and binaphtholato complexes of niobium were effectively activated with Me_3Al rather than with ^iBu_3Al. The polymers obtained with those complexes had high molecular weight (M_n>10^5) and relatively narrow molecular weight distributions (M_w/M_n〜2) implying that these are single-site catalysts. These findings will contribute to future development of ROMP catalysts.