Synthesis of neutral and cationic monocyclopentadienyl alkyl niobium and tantalum complexes

Abstract

Compound [NbCp′Me 4] (Cp′ = η 5-C 5H 4SiMe 3, 1) reacted with several ROH compounds (R = tBu, Si iPr 3, 2,6-Me 2C 6H 3) to give the derivatives [NbCp′Me 3(OR)] (R = tBu 2a, Si iPr 3 2b, 2,6-Me 2C 6H 3 2c). The diaryloxo tantalum compound [TaCp ∗Me 2(OR) 2] (Cp ∗ = η 5-C 5Me 5, R = 2,6-Me 2C 6H 3 3) was obtained by reaction of [TaCp ∗Cl 2Me 2] with 2 equiv of LiOR (R = 2,6-Me 2C 6H 3). Abstraction of one methyl group from these neutral compounds 1–3 with the Lewis acids E(C 6F 5) 3 (E = B, Al) gave the ionic derivatives [NbCp′Me 2X][MeE(C 6F 5) 3] (X = Me 4-E. X = OR; R = Si iPr 3 5b-E, 2,6-Me 2C 6H 3 5c-E. E = B, Al) and [TaCp ∗Me(OR) 2][MeE(C 6F 5) 3] (R = 2,6-Me 2C 6H 3 6-E; E = B, Al). Polymerization of MMA with the aryloxoniobium compound 2c and Al(C 6F 5) 3 gave syndiotactic PMMA in a low yield, whereas the tetramethylniobium compound 1 and the diaryloxotantalum derivative 3 were inactive.