Synthesis of neutral and cationic monocyclopentadienyl tantalum alkoxo complexes and polymerization of methyl methacrylate

The alkoxo compounds [TaCp*Me-3(OR)] (R = SiiPr(3), 1a; 2,6-Me2C6H3, 1b; 2,6-iPr(2)C(6)H(3), 1c) were obtained by reaction of [TaCp*Me-4] with the corresponding alcohol ROH, at room temperature for R = SiiPr(3) and upon heating at 100 degrees C over 2 and 3 days for R = 2,6-Me2C6H3 and 2,6-iPr(2)C(6)H(3), respectively. The alkoxo cations [TaCp*Me-2(OR)](+) (R = SiiPr(3), 2a; 2,6-Me2C6H3, 2b; 2,6-iPr(2)C(6)H(3), 2c) were formed when one of the methyl groups of the neutral trimethyl complexes 1 was dissociated by reaction with 1 equiv of B(C6F5)(3) to generate the borate anion [MeB(C6F5)(3)](-) (B) and with 1 and 2 equiv of Al(C6F5)(3) to give the mononuclear [MeAl(C6F5)(3)](-) (Al) and dinuclear [Me{Al(C6F5)(3)}(2)](-) (Al2) aluminate anions, respectively. A similar reaction of the neutral compound [TaCp*Me-4] with 1 and 2 equiv of Al(C6F5)(3) gave salts of the [MeAl(C6F5)(3)](-) (Al) and [Me{Al(C6F5)(3)}(2)](-) (Al2) anions containing the [TaCp*Me-3](+) (3) cation. Compounds 1 and [TaCp*Me-4] did not show catalytic activity for MMA polymerization when B(C6F5)(3) was used as cocatalyst but were active in the presence of Al(C6F5)(3), yielding high molecular weight syndiotactic PMMA.