Niobium Pentamethoxide: a Multi-facet Study of an Early Transition Metal Alkoxide

Abstract

Niobium pentamethoxide, Nb₂(OCH₃)₁₀, was chosen as a typical early transition metal alkoxide on which to undertake a multifacet study, including such aspects as crystal and molecular structure, exchange of methoxo groups among coordination sites, exchange mechanisms, solvation and coordination properties, for the purpose of gaining some deeper insight into the structure and behaviour of such alkoxides. The X-ray structure determination showed the presence of two different conformers in the unit cell, a moderately strained bridge, evidence for π-bonding in the Nb-O bonds, and no detectable trans effect. The exchange of methoxo groups among the three non-equivalent sites of structure II is rapid in solution on the NMR time scale; the terminalbridge and terminal-terminal exchanges occur via independent processes. It is shown that, contrary to earlier conclusions, the mechanism of exchange of the bridging groups is very likely to be intermolecular. The exchange of the terminal substituents is also discussed. A mixed transition metal alkoxide, NbTa(OCH₃)₁₀ (V), was isolated. Evidence for the formation of a solvation cage by polar solvents is presented. The coordination chemistry of the alkoxide has also been investigated, and the ability of the most common ligands to compete with auto-association of Nb(OCH₃)₅ into a dimer is discussed. The exchange of methoxo groups between the non-equivalent sites of adduct VI, and between dimer and adduct, are also dynamic on the NMR timescale.