Stabilization of Tetravalent Cerium upon Coordination of Unsaturated Heteropoly tungstate Anions

Monovacant Keggin derivative anions Xⁿ⁺W₁₁O₃₉^(12-n)- (X = P, Si, Ge, B) and a monovacant Dawson derivative anion P₂W₁₇O₆₁^10- were adopted as the ligand for cerium(III). The formation of the complexes [CeL]^(m-3)- and [CeL2]^(2m-3)- was confirmed by means of absorption spectroscopy, where ^Lm- denotes an unsaturated heteropolytungstate anion of P, Si, or Ge, while clear evidence for the complexation of BW₁₁O₃₉^9- was not obtained. Electrochemical behaviors of heteropolytungstocerates(III) were studied by cyclic and differential pulse voltammetry in aqueous solution. The redox potential of the cerium(IV/III) couple was considerably lowered (below +1 V vs Ag/AgCl) upon coordination of heteropolytungstate in aqueous solution at pH 4.5. The magnitude of the potential shift for [CeL]^(m-3)- increased in the order PW₁₁O₃₉^7-, P₂W₁₇O₆₁^10-, GeW₁₁O₃₉^8-, and SiW₁₁O₃₉^8-. A similar trend was observed for [CeL₂]^(2m-3)- with larger potential shift than those of [CeL]^(m-3)-. The shift of the redox potential for cerium(IV/III) is caused primarily by the large negative charge of the unsaturated heteropolytungstate.