Stabilization of uranium(IV) in heteropoly anions

The heteropoly anions (UIVMO12O42)8-(UMo12), (UIVW10O36)8-(UW10), (UIV(PW11O39)2)10-(U(PW11)2) and (UIV(SiW11O39)2)12-(U(SiW11)2) were examined by cyclic voltammetry on a wax-impregnated carbon electrode. Reversible one-electron oxidations were observed for UMO12 and U(SiW11)2. No oxidation of UW10 was detected at potentials prior to oxygen discharge (ca. +0.9 V at pH 7). Controlled potential oxidation of a aqueous solutions of UMo12 gave unstable solutions of (UVMo12O42)7-. Oxidation of U(PW11)2 was achieved in aqueous and nonaqueous (acetonitrile, propylene carbonate) solution. The electronic spectra of UVMo12 and UV(PW11)2 are reported and are discussed in terms of UO12(Ieta) and UO8 (D4d) chromophores respectively. Possibilities for geometrical and optical isomers of U(XW11)2 anions are considered. Solutions of brucinium salts of U(PW11)2 and UW10 in dimethyl formamide show induced cotton effects at wavelengths corresponding to the f-f transitions of UIV. The voltammograms of UMo12, ThMo12 and CeMo12 show an irreversible twelve-electron reduction at -0.25 V. The pale brown reduced solutions cannot be reoxidized to the original heteropoly anions. (orig./RB)