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 U-VI complex with a pentadentate Schiff base ligand (N,N'-disalicylidenediethylenetriaminate = saldien(2-)) was prepared as a starting material of a potentially stable U-V complex without any possibility of (UO2+)-O-V center dot center dot center dot(UO2+)-O-V cation-cation interaction and was found in three different crystal phases. Two of them had the same composition of (UO2)-O-VI(saldien)center dot 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 (UO2)-O-VI(saldien), but it is just present as a solvent in the crystal structures. The other isolated crystals consisted only of (UO2)-O-VI(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 (UO2)-O-VI(saldien) remains unchanged even in strongly Lewis-basic solvents, DMSO and N,N-dimethylformamide. Cyclic voltammetry of (UO2)-O-VI(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 (UO2)-O-VI(saldien) is [(UO2)-O-V(saldien)](-), which is stable in DMSO. The standard redox potential of [(UO2)-O-V(saldien)](-)/(UO2)-O-VI(saldien) in DMSO is -1.584 V vs Fc/Fc(+). 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 U5+.