Experimental and Theoretical Approaches to Redox Innocence of Ligands in Uranyl Complexes: What Is Formal Oxidation State of Uranium in Reductant of Uranyl(VI)?

Redox behavior of [UO2(gha)­DMSO]−/UO2(gha)­DMSO couple (gha
= glyoxal bis­(2-hydroxanil)­ate, DMSO = dimethyl sulfoxide) in DMSO
solution was investigated by cyclic voltammetry and UV–vis–NIR
spectroelectrochemical technique, as well as density functional theory
(DFT) calculations. [UO2(gha)­DMSO]− was
found to be formed via one-electron reduction of UO2(gha)­DMSO
without any successive reactions. The observed absorption spectrum
of [UO2(gha)­DMSO]−, however, has clearly
different characteristics from those of uranyl­(V) complexes reported
so far. Detailed analysis of molecular orbitals and spin density of
the redox couple showed that the gha2– ligand in
UO2(gha)­DMSO is reduced to gha•3– to give [UO2(gha)­DMSO]− and the formal
oxidation state of U remains unchanged from +6. In contrast, the additional
DFT calculations confirmed that the redox reaction certainly occurs
at the U center in other uranyl­(V/VI) redox couples we found previously.
The noninnocence of the Schiff base ligand in the
[UO2(gha)­DMSO]−/UO2(gha)­DMSO
redox couple is due to the lower energy level of LUMO in this ligand
relative to those of U 5f orbitals. This is the first example of the noninnocent ligand system in the coordination chemistry
of uranyl­(VI).