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 1H 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 ≈ U–NH > U–S. Conditional stability constants (βX) of UO2(tBu,MeO–saldien–X) in ethanol were investigated to understand the effect of X on thermodynamic stability. The log βX decrease in the order of UO2(tBu,MeO–saldien–NH) (log βNH = 10) > UO2(tBu,MeO–saldien–O) (log βO = 7.24) > UO2(tBu,MeO–saldien–S) (log βS = 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.