Synthesis of neutral and cationic monocyclopentadienyl alkyl niobium and tantalum complexes

Compound [NbCp′Me4] (Cp′ = η5-C5H4SiMe3, 1) reacted with several ROH compounds (R = tBu, SiiPr3, 2,6-Me2C6H3) to give the derivatives [NbCp′Me3(OR)] (R = tBu 2a, SiiPr32b, 2,6-Me2C6H32c). The diaryloxo tantalum compound [TaCp∗Me2(OR)2] (Cp∗ = η5-C5Me5, R = 2,6-Me2C6H33) was obtained by reaction of [TaCp∗Cl2Me2] with 2 equiv of LiOR (R = 2,6-Me2C6H3). Abstraction of one methyl group from these neutral compounds 1–3 with the Lewis acids E(C6F5)3 (E = B, Al) gave the ionic derivatives [NbCp′Me2X][MeE(C6F5)3] (X = Me 4-E. X = OR; R = SiiPr35b-E, 2,6-Me2C6H35c-E. E = B, Al) and [TaCp∗Me(OR)2][MeE(C6F5)3] (R = 2,6-Me2C6H36-E; E = B, Al). Polymerization of MMA with the aryloxoniobium compound 2c and Al(C6F5)3 gave syndiotactic PMMA in a low yield, whereas the tetramethylniobium compound 1 and the diaryloxotantalum derivative 3 were inactive.