Redox Chemistry of Actinide Ions in Wells−Dawson Heteropolyoxoanion Complexes

The redox behavior has been characterized for several actinide ( An ) complexes with the monovacant Wells−Dawson anion, of the form [ An n + (α‐2‐P 2 W 17 O 61 ) 2 ] n −20 ( An = Th 4+ , U 4+ , Np 4+ , Pu 4+ , and Am 3+ ). Two complexes, with An = U 4+ and Am 3+ , show redox activity under oxidizing conditions, which is attributed to the actinide oxidation. Am 3+ is oxidized to Am 4+ with an E 1/2 = +1.21±0.01 V, and U 4+ oxidizes to U 5+ with a measured E 1/2 = +0.55±0.01 V vs. Ag/AgCl. Although the cyclic voltammetry (CV) data are consistent with a reversible redox couple, bulk oxidative electrolysis of [U 4+ (α‐2‐P 2 W 17 O 61 ) 2 ] 16− results in the decomposition of this complex to produce uranyl acetate and the free monovacant Wells−Dawson anion. In contrast, all of the CV data from the actinide coordination complexes differ from equivalent data obtained from the [α‐2‐P 2 W 17 O 61 ] 10− ligand itself. There are two complexed An 4+ ions, Np and Pu, that undergo reduction over the same potential range as the ligands themselves. In situ X‐ray spectroelectrochemistry is used to quantify the actinide response. The Np 4+ /Np 3+ redox behavior is a classically single ion process, with a formal potential of −0.84±0.01 V that was determined from a Nernst plot of X‐ray absorption near‐edge structure (XANES) data. The Pu 4+ /Pu 3+ formal reduction potential in the complex [Pu(α‐2‐P 2 W 17 O 61 ) 2 ] n − was determined to be −0.17±0.01 V using the same methodology. However, in this latter case, the slope of the Nernst plot indicates that 0.72±0.03 electrons are involved in the reduction. This is a significant deviation from the 1 electron expected for the Pu couple, and is discussed in terms of the concomitant reduction of the P−W−O framework of the Wells−Dawson anion. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)