Effects of Substituents on the Molecular Structure and Redox Behavior of Uranyl(V/VI) Complexes with NO-Donating Schiff Base Ligands.

Uranyl(VI) complexes with pentadentate NO-donating Schiff base ligands having various substituents at the (R) and/or (R) positions on phenolate moieties, R,R-saldien, were synthesized and thoroughly characterized by H nuclear magnetic resonance, infrared, elemental analysis, and single-crystal X-ray diffraction. Molecular structures of UO(R,R-saldien) are more or less affected by the electron-donating or -withdrawing nature of the substituents. The redox behavior of all UO(R,R-saldien) complexes was investigated to understand how substituents introduced onto the ligand affect the redox behavior of these uranyl(VI) complexes. As a result, the redox potentials of UO(R,R-saldien) in dimethyl sulfoxide increased from -1.590 to -1.213 V with an increase in the electron-withdrawing nature of the substituents at the R and R positions. The spectroelectrochemical measurements and theoretical calculation [density functional theory (DFT) and time-dependent DFT calculations] revealed that the center U of each UO(R,R-saldien) complex undergoes one-electron reduction to afford the corresponding uranyl(V) complex, [UO(R,R-saldien)], regardless of the difference in the substituents. Consequently, the redox active center of uranyl(VI) complexes seems not to be governed by the redox potentials but to be determined by whether the LUMO is centered on a U 5f orbital or on one π* orbital of a surrounding ligand.