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Keywords:

  • Actinides;
  • Cyclic voltammetry;
  • Polyoxometalates;
  • Redox chemistry;
  • Nernst analysis;
  • Intermediate valence;
  • Spectroelectrochemistry;
  • XANES

Abstract

The redox behavior has been characterized for several actinide (An) complexes with the monovacant Wells−Dawson anion, of the form [Ann+(α-2-P2W17O61)2]n−20 (An = Th4+, U4+, Np4+, Pu4+, and Am3+). Two complexes, with An = U4+ and Am3+, show redox activity under oxidizing conditions, which is attributed to the actinide oxidation. Am3+ is oxidized to Am4+ with an E1/2 = +1.21±0.01 V, and U4+ oxidizes to U5+ with a measured E1/2 = +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 [U4+(α-2-P2W17O61)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-P2W17O61]10− ligand itself. There are two complexed An4+ 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 Np4+/Np3+ 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 Pu4+/Pu3+ formal reduction potential in the complex [Pu(α-2-P2W17O61)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)