Synthesis of neutral and cationic monocyclopentadienyl alkyl niobium and tantalum complexes

摘要

Compound [NbCp′Me_4] (Cp′ = η~5-C _5H_4SiMe_3, 1) reacted with several ROH compounds (R = tBu, SiiPr_3, 2,6-Me_2C_6H_3) to give the derivatives [NbCp′Me_3(OR)] (R = tBu 2a, SiiPr _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 = SiiPr _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.