Nuclear magnetic resonance studies on niobium and tantalum penta-alkoxides

Abstract

Variable-temperature n.m.r. studies have proved the double alkoxide-bridged bi-octahedral (edge-shared) structure of the dimeric penta-alkoxides M₂(OR)₁₀, where M = Nb or Ta and R = Me, Et, or Buⁱ. Rapid intramolecular double exchange of alkoxide groups occurs between (i) the two types of terminal group and (ii) the terminal and bridging groups and the activation energies E_tt and E_tb were evaluated for Ta₂(OMe)₁₀. The presence of a monomer–dimer equilibrium was proved for the pentaisopropoxides and from equilibrium constants at various temperatures the enthalpy of dissociation of the dimer was evaluated. The monomeric t-butoxides showed a rapid intramolecular exchange down to –90°. Alcohol interchange with dimeric species was slower than with monomers.