Synthesis of Neutral and Cationic Monocyclopentadienyl Tantalum Alkoxo Complexes and Polymerization of Methyl Methacrylate

The alkoxo compounds [TaCp*Me3(OR)] (R = SiiPr3, 1a; 2,6-Me2C6H3, 1b; 2,6-iPr2C6H3, 1c) were obtained by reaction of [TaCp*Me4] with the corresponding alcohol ROH, at room temperature for R = SiiPr3 and upon heating at 100 °C over 2 and 3 days for R = 2,6-Me2C6H3 and 2,6-iPr2C6H3, respectively. The alkoxo cations [TaCp*Me2(OR)]+ (R = SiiPr3, 2a; 2,6-Me2C6H3, 2b; 2,6-iPr2C6H3, 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*Me4] 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*Me3]+ (3) cation. Compounds 1 and [TaCp*Me4] 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.