STABILIZATION OF TETRAVALENT CERIUM UPON COORDINATION OF UNSATURATED HETEROPOLYTUNGSTATE ANIONS

Monovacant Keggin derivative anions Xn+W11O39(12-n)- (X = P, Si, Ge, B) and a monovacant Dawson derivative anion P2W17O6110- 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 L- denotes an unsaturated heteropolytungstate anion of P, Si, or Ge, while clear evidence for the complexation of BW11O399- 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 PW11O397-, P2W17O6110-, GeW11O398-, and SiW11O398-. 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.