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

In most cases, the redox activity of a U VI O 2 2+ 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 VI O 2 2+ . In this study, three U VI O 2 2+ 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 VI O 2 2+ 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 VI O 2 2+ 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 VI O 2 2+ is just "spectating" and remains unchanged, i.e. , innocent . Whereas the double reduction of the U VI O 2 2+ complexes usually involves breaking of strong U&z.tbd;O 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. Three uranyl( vi ) complexes with redox-active ligands showed reversible multi-step redox processes. Their redox centers are always the coordinating ligand and the centered U exhibits redox-innocent behavior.