Effects of coordinating heteroatoms on molecular structure, thermodynamic stability and redox behavior of uranyl(vi) complexes with pentadentate Schiff-base ligands

Uranyl(vi) complexes with pentadentate N3O2-, N2O3- and N2O2S1-donating Schiff base ligands, tBu,MeO-saldien-X2- (X = NH, O and S), were synthesized and thoroughly characterized by H-1 NMR, IR, elemental analysis, and single crystal X-ray diffraction. The crystal structures of UO2(tBu,MeO-saldien-X) showed that the U-X bond strength follows U-O approximate to U-NH > U-S. Conditional stability constants (beta(X)) of UO2(tBu,MeO-saldien-X) in ethanol were investigated to understand the effect of X on thermodynamic stability. The log beta(X) decrease in the order of UO2(tBu,MeO-saldien-NH) (log beta(NH) = 10) > UO2(tBu,MeO-saldien-O) (log beta(O) = 7.24) > UO2(tBu,MeO-saldien-S) (log beta(S) = 5.2). This trend cannot be explained only by Pearson's Hard and Soft Acids and Bases (HSAB) principle, but rather follows the order of basicity of X. Theoretical calculations of UO2(tBu,MeO-saldien-X) suggested that the ionic character of U-X bonds decreases in the order of U-NH > U-O > U-S, while the covalency increases in the order U-O < U-NH < U-S. Redox potentials of all UO2(tBu,MeO-saldien-X) in DMSO were similar to each other regardless of the difference in X. Spectroelectrochemical measurements and DFT calculations revealed that the center U6+ of each UO2(tBu,MeO-saldien-X) undergoes one-electron reduction to afford the corresponding uranyl(v) complex. Consequently, the difference in X of UO2(tBu,MeO-saldien-X) affects the coordination of tBu,MeO-saldien-X2- with UO22+. However, the HSAB principle is not always prominent, but the Lewis basicity and balance between ionic and covalent characters of the U-X interactions are more relevant to determine the bond strengths.