Nuclear magnetic resonance studies on niobium and tantalum penta-alkoxides
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 Ett and Etb 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.