pubs.acs.org/IC Published on Web 01/28/2010 r 2010 American Chemical Society Inorg. Chem. 2010, 49, 2349–2359 2349 DOI: 10.1021/ic902225f Molecular Structure and Electrochemical Behavior of Uranyl(VI) Complex with Pentadentate Schiff Base Ligand: Prevention of Uranyl(V) Cation-Cation Interaction by Fully Chelating Equatorial Coordination Sites Koichiro Takao, †,‡ Masaru Kato, § Shinobu Takao, ‡ Akira Nagasawa, § Gert Bernhard, ‡ Christoph Hennig, ‡ and Yasuhisa Ikeda* ,† † Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1-N1-34, O-okayama, Meguro-ku, Tokyo 152-8550, Japan, ‡ Institute of Radiochemistry, Forschungszentrum Dresden-Rossendorf, P.O. Box 51 01 19, 01314 Dresden, Germany, and § Department of Chemistry, Graduate School of Science and Engineering, Saitama University, 255, Shimo-Okubo, Sakura-ku, Saitama City, Saitama 338-8570, Japan Received November 13, 2009 The U VI complex with a pentadentate Schiff base ligand (N, N 0 -disalicylidenediethylenetriaminate = saldien 2- ) was prepared as a starting material of a potentially stable U V complex without any possibility of U V O 2 þ 333 U V O 2 þ cation-cation interaction and was found in three different crystal phases. Two of them had the same composition of U VI O 2 - (saldien) 3 DMSO in orthorhombic and monoclinic systems (DMSO = dimethyl sulfoxide, 1a and 1c, respectively). The DMSO molecule in both 1a and 1c does not show any coordination to U VI O 2 (saldien), but it is just present as a solvent in the crystal structures. The other isolated crystals consisted only of U VI O 2 (saldien) without incorporation of solvent molecules (1b, orthorhombic). A different conformation of the coordinated saldien 2- in 1c from those in 1a and 1b was observed. The conformers exchange each other in a solution through a flipping motion of the phenyl rings. The pentagonal equatorial coordination of U VI O 2 (saldien) remains unchanged even in strongly Lewis-basic solvents, DMSO and N,N-dimethylforma- mide. Cyclic voltammetry of U VI O 2 (saldien) in DMSO showed a quasireversible redox reaction without any successive reactions. The electron stoichiometry determined by the UV-vis-NIR spectroelectrochemical technique is close to 1, indicating that the reduction product of U VI O 2 (saldien) is [U V O 2 (saldien)] - , which is stable in DMSO. The standard redox potential of [U V O 2 (saldien)] - /U VI O 2 (saldien) in DMSO is -1.584 V vs Fc/Fc þ . This U V complex shows the characteristic absorption bands due to f-f transitions in its 5f 1 configuration and charge-transfer from the axial oxygen to U 5þ . 1. Introduction Actinide elements at oxidation states þ5 and þ6 form a typical “actinyl” ion (MO 2 nþ ) in many cases. This species has a linear OdMdO structure. Due to this structural character, the coordination of additional ligands occurs only in the equatorial plane of MO 2 nþ . 1,2 The number of the equatorial coordination sites varies between 3 and 6. Uranyl(V) is unusually unstable in solutions due to dispro- portionation. 1 Recently, the chemistry of U V has attracted special in- terest, because this field of actinides is poorly explored. Uranyl(V) carbonate, U V O 2 (CO 3 ) 3 5- , is currently the solely known stable U V species in aqueous solution. 3 In our previous articles, the electrochemical behavior of U VI com- plexes with organic ligands [Lewis-basic solvent molecules (L), β-diketonates, Schiff bases] in nonaqueous solvents was studied. 4-6 As a result, two stable U V complexes in nonaqu- eous systems have been found: [U V O 2 (salophen)DMSO] - in DMSO and [U V O 2 (dbm) 2 DMSO] - in DMSO (salophen 2- = N, N 0 -disalicylidene-o-phenylenediaminate, dbm - =dibenzoyl- methanate, DMSO = dimethyl sulfoxide). In the same period, Berthet et al. incidentally obtained a single crystal of *To whom correspondence should be addressed. Phone: þ81 3-5734-3061. E-mail: yikeda@nr.titech.ac.jp. (1) Katz, J. J.; Seaborg, G. T.; Morss, L. R. The Chemistry of the Actinide Elements, 2nd ed.;, Chapman and Hall: London, 1986. (2) Cotton, S. Lanthanide and Actinide Chemistry; John Wilety & Sons Ltd: West Sussex, England, 2006. 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