(Silox)3Nb(η2-N,C-4-picoline) as a source of (silox)3Nb: structure of [(silox)3Nb]2(μ-η2,η2-C6H6) (silox= Bu3SiO)

Abstract

Abstract Electrochemical and chemical reductions of (silox)3NbCl2 (1, −1.42 V versus Ag/Ag+) and (silox)3TaCl2 (2, −2.18 V) generated [(silox)3NbCl2]− (1−) and [(silox)3TaCl2]− (2−). The anions were subject to Cl− loss, leading to (silox)3NbCl (3) and probable disproportionation (Ta). Complex 3 formed reversible solvent adducts (silox)3ClNbL (3-L, L=THF, py). Na/Hg reduction of (silox)3NbCl2 (1) in the presence of 4-picoline afforded (silox)3Nb(η2-N,C-4-NC5H4CH3) (6), which can be considered a source of ‘(silox)3Nb’, analagous to (silox)3Ta (4). Adducts such as (silox)3Nb(η2-H2CCHR) (R=H, 7; Ph, 8) may be formed upon addition of olefins, and 6 abstracted an oxygen from N2O, NO or ethylene oxide to give (silox)3NbO (9) and N2 or C2H4, respectively, which are reactions also observed for 4. 4-Picoline may be abstracted from 6 by 4 to give {(silox)3Nb}2(μ:η2,η2-C6H6) (11) and (silox)3Ta(η2-N,C-4-NC5H4CH3) (12). An X-ray structure determination of 11 revealed a disordered bridge that was modeled as a μ:η2,η2-benzene. Extended Huckel molecular orbital (EHMO) calculations showed that the μ:η2,η2-configuration was energetically better than a μ:η6,η6-alternative, but about the same as a plausible μ:η3,η3-arrangement.