M(IV)- and M(V)-Substituted {MW5} Lindqvist-Type Polyoxometalates from Plenary [W6O19]2– or WO3·H2O: A Gateway to Wider Systematic Reactivity Studies

The reactivity of polyoxometalates (POMs) may be tuned by heterometal substitution into the POM framework, and systematic reactivity studies of these nanoscale molecular metal oxide structures require reliable, efficient synthetic methods. We have now shown that nonaqueous base degradation of either WO3·H2O or plenary (TBA)2[W6O19] (TBA)21 followed by reaction with heterometal precursors provides convenient, high-yield access to MIV- and MV-substituted {MW5} Lindqvist-type hexametalates [(iPrO)TiW5O18]3–2, [(MeO)MW5O18]3– (M = Ti 3, Sn 4), [{(MeO)MW5O18}2]6– (M = Zr 5, Hf 6) and [MW5O19]3– (M = V 7, Nb 8, Ta 9). Crystal structures of (TBA)65 and (TBA)66 showed the anions to be isostructural, and kinetic studies of CD3OD exchange revealed slightly slower exchange at Hf than Zr. For 5, DFT analysis of MeOH exchange revealed a transition state involving a six-membered ring via insertion of MeOH into a Zr(μ-OMe)Zr bridge. Alkoxide protonolysis reactions provided monomeric {LMW5} derivatives with 7-coordinate Zr or Hf, and X-ray crystal structures of (TBA)3[(PhCO2)ZrW5O18] (TBA)312, (TBA)3[(PhCO2)HfW5O18] (TBA)313 and (TBA)3[(acac)HfW5O18] (TBA)314 were obtained. Cyclic voltammetry showed one-electron couples for 3, 4, 7, 8, and 9 but not for dimeric 5 and 6, consistent with calculated reduction potentials for these {MW5} anions.