Synthesis of novel oxo complexes of tungsten and molybdenum with various chalcogen-bridged chelating bis(aryloxo) ligands and their catalytic behavior for ring-opening metathesis polymerization

Oxo tungsten complexes coordinated by OEO (E=S, Te, CH2) chelating bis(aryloxo) ligands, WO[E(4-Me-6-tBuC6H2O)2]Cl2, (1: E=S, 2: E=Te, 3: E=CH2), were synthesized by the reaction of WOCl4 with 2,2′-E(4-Me-6-tBuC6H2OH)2 [E=S (tBu2tbpH2), E=Te (TebpH2), E=CH2 (mbpH2)]. Similarly a molybdenum complex, MoO[S(4-Me-6-tBuC6H2O)2]Cl2 (4), was also prepared. The structures of the oxodichloro tungsten complexes 1 and 3 were determined by single-crystal X-ray analysis to have pseudo Cs symmetry. A μ-oxo binuclear tungsten complex with two tBu2tbp ligands, {WO[S(4-Me-6-tBuC6H2O)2]Cl}2(μ-O) (5), was synthesized by refluxing a mixture of W2O3Cl6 and two equivalents of tBu2tbpH2 in THF, whose structure was determined by single-crystal X-ray analysis. We study here the catalytic behavior of these complexes for the ring-opening metathesis polymerization (ROMP) of norbornene. In the catalytic systems, 1–AlEt3 and 2–AlEt3, cis–trans selectivity varies depending on the catalyst concentration and on the polymerization temperature. The resulting poly(norbornene) tend to have higher cis-content with increasing catalyst concentration and with lowering polymerization temperature. Oppositely, the polymerization under low catalyst concentration and at high polymerization temperature gave the trans-rich poly(norbornene). On the other hand, the complex 3 with the methylene-bridged bis(aryloxo) ligand did not show such a dependence of cis–trans selectivity. We proposed that these observations might come from oxotungsten–aluminum interaction.