Utility of redox-active ligands for reversible multi-electron transfer in uranyl(vi) complexes

Abstract

In most cases, the redox activity of a U^VIO₂²⁺ complex is regarded as metal-centered phenomena, because uranium has small energy gaps amongst the 5f/6d/7s subshells, thereby exhibiting a wide range of oxidation states, commonly from +III to +VI or in some cases even +I or +II. While a wide variety of redox-active ligands are known for use as transition metal complexes including multi-electron reduction that could facilitate inert bond or small molecule activation, only a few such examples are known for U^VIO₂²⁺. In this study, three U^VIO₂²⁺ complexes bearing α-diimine-, o-quinonediimine- and 2,6-diiminopyridine-based ligands were synthesized, which exhibited two redox couples in the range of −0.79 V to −2.02 V vs. Fc^+/0 to give singly- and doubly-reduced complexes by stepwise reduction. Unique electronic transitions of U^VIO₂²⁺ complexes with a variety of low-lying excited states helped us to combine spectroelectrochemistry and time-dependent density functional theory (TD-DFT) calculations which complemented each other to assign the redox-active site in these U^VIO₂²⁺ complexes, i.e., whether or not a ligand of interest becomes redox-active. During all the redox processes observed here, the ligands employed are found to be exclusively redox-active, i.e., non-innocent, whereas the centered U^VIO₂²⁺ is just “spectating” and remains unchanged, i.e., innocent. Whereas the double reduction of the U^VIO₂²⁺ complexes usually involves breaking of strong UO bonds, in the present examples this is not required and therefore a basis for the synthesis of new types of uranium molecular catalysts and magnetic materials may be found.