Studies on bis(η-indenyl)niobium complexes

Abstract

The reaction between [NbCl₄(thf)₂](thf = tetrahydrofuran) and Li[C₉H₇] or Na[C₉H₇]·thf gives [Nb(η-C₉H₇)₂Cl₂]1, which reacts with NaBH₄ to give [Nb(η-C₉H₇)₂(µ-H₂BH₂)]2. The bridging and terminal tetrahydroborate protons in 2 exchange on the NMR time-scale (ΔG^‡= 55.2 ± 1.6 kJ mol^–1). Compound 2 reacts with PMe₂Ph or pyridine via BH₃ extraction to give [NbH(η-C₉H₇)₂(PMe₂Ph)]3 and [NbH(η-C₉H₇)₂(NC₅H₅)]4 respectively. The pyridine ligand in 4 undergoes restricted rotation about the Nb–N bond (ΔG^‡= 52 ± 1 kJ mol^–1). The reaction of 2 with NMe₃ in the presence of CO, H₂CCCH₂, trans,trans-hexa-2,4-diene or styrene gives the new complexes [Nb(η-C₉H₇)(CO)₄]5, [Nb(η-C₉H₇)₂(η-CH₂CHCH₂)]6, [Nb(η-C₉H₇)₂(η-MeCHCHCHCH₂Me)]7 and endo-[NbH(η-C₉H₇)₂(η-H₂CCHPh)]9 respectively. The styrene hydride (complex 9) is fluxional and this process has been studied using both one- and two-dimensional NMR techniques. The activation barrier to olefin hydride exchange has been determined by coalescene methods to be ΔG^‡= 70.0 ± 2 kJ mol^–1 at 50 °C, this is significantly lower than that for the analogous η-C₅H₅ and η-C₅Me₅ complexes. A mechanism that involves a ring slipped intermediate is proposed.