Stabilization of uranium(IV) in heteropoly anions*

Abstract

The heteropoly anions [ uIVMo 1204218-(UMo 12 ), [uIVw1003618-(WW10), [uIV(PWI IO39)2]l°-[U(PWII )2] and [uIV(siwI IO39)2112-[U(SiWI 1)2] were examined by cyclic voltammetry on a wax-impregnated carbon electrode. Reversible one-electron oxidations were observed for UMoI2 (E = +0.91 V vs sce at pH = ca. 0), U(PW11)2 (E = +0.60 V at pH 4.4) and U(SiW11)2 (E = +0.19 V at pH 4.4). No oxidation of UWI 0 was detected at potentials prior to oxygen discharge (ca. +0.9 V at pH 7). Controlled potential oxidation of aqueous solutions of UMo12 gave unstable solutions of [UVMo12042] 7-. Oxidation of U(PWI 1)2 was achieved in aqueous and nonaqueous (acetonitrile, propylene carbonate) solution. The electronic spectra of UVMo12 and uV(pwI 1)2 are reported and are discussed in terms of UO12 (lh) and UO8 (D4a) chromophores respectively. Possibilities for geometrical and optical isomers of U(XW11)2 anions are considered. Solutions of brucinium salts of U(PWI 1)2 and UWI 0 in dimethyl formamide show induced Cotton effects at wavelengths corresponding to the f-f transitions of U Iv. The voltammograms of UMolz, 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.

Redox Chemistry of Actinide Ions in Wells−Dawson Heteropolyoxoanion Complexes

“…The one-electron oxidation of U 4ϩ has been previously reported in both Keggin . [26] The oxidation was achieved through electrolytic means, with electrode potentials of ϩ0.23 Յ E°Ј Յ ϩ0.64 V, and through potentiometric titrations with dichromate and permanganate. [27] Our E 1/2 value agrees with the formal potential of ϩ0.56Ϯ0.…”

Chirality in Polyoxometalate Chemistry

“…The resultant dimeric structures are chiral because of the distorted square antiprism geometry of the 8‐coordinated cation sandwiched between the two lacunary anions. This can be evidenced by 183 W NMR spectroscopy100,101 or the induced Cotton effect 102. Crystallization usually occurred in a centrosymmetric space group, such as, for example, Cs 11 [Eu(α‐PW 11 O 39 ) 2 ]103 or K 17 [Lu(α 2 ‐P 2 W 17 O 61 ) 2 ] 104.…”

On the Aqueous Chemistry of the UIV–DOTA Complex

“…The equivalent U V to U IV oxidation process of the chelated cation is not reported and its overall feasibility is questionable because it ought to withhold some serious energetic constraints; the U V cation is a very hard Lewis acid and is usually present as a short‐lived linear UO 2 + cation (pentavalent uranyl) in aqueous solutions . Interesting exceptions are the aqueous U V complexes with unsaturated heteropoly anions (HPAs), which are accessible by chemical or electrochemical oxidation of the equivalent U IV complex. The U V species in these complexes is reported to exist both in the dioxo form (uranyl‐like) and as the “bare” cation.…”