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, o r Ge, while clear evidence for the complexation of BW11O399- was not o btained. Electrochemical behaviors of heteropolytungstocerates(III) we re studied by cyclic and differential pulse voltammetry in aqueous sol ution. The redox potential of the cerium(IV/III) couple was considerab ly lowered (below +1 V vs Ag/AgCl) upon coordination of heteropolytung state in aqueous solution at pH 4.5. The magnitude of the potential sh ift for [CeL](m-3)- increased in the order PW11O397-, P2W17O6110-, GeW 11O398-, 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 larg e negative charge of the unsaturated heteropolytungstate.