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

The UVI complex with a pentadentate Schiff base ligand (N,N’-disalicylidenediethylene-triaminate = saldien2–) was prepared as a starting material of a potentially stable UV complex without any possibility of UVO2+•••UVO2+ cation-cation interaction, and was found in three different crystal phases. Two of them had the same composition of UVIO2(saldien)•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 UVIO2(saldien), but it is just present as a solvent in the crystal structures. The other isolated crystals consisted only of UVIO2(saldien) without incorporation of solvent molecules (1b, orthorhombic). Different conformation of the coordinated saldien2– 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 UVIO2(saldien) remains unchanged even in strongly Lewis-basic solvents, DMSO and N,N-dimethylformamide. Cyclic voltammetry of UVIO2(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 UVIO2(saldien) is [UVO2(saldien)]– which is stable in DMSO. The standard redox potential of [UVO2(saldien)]–/UVIO2(saldien) in DMSO is –1.584 V vs. Fc/Fc+. This UV complex shows the characteristic absorption bands due to f-f transitions in its 5f1 configuration and charge-transfer from the axial oxygen to U5+.