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

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Abstract
In most cases, the redox activity of a UVIO2 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 UVIO2 2+. In this study, three UVIO2 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 UVIO2 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 UVIO2 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 UVIO2 2+ is just “spectating” and remains unchanged, i.e., innocent. Whereas the double reduction of the UVIO2 2+ 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.
Funding Information
  • Japan Society for the Promotion of Science (20H02663)
  • Japan Society for the Promotion of Science (21J11942)
  • Tokyo Institute of Technology
  • Japan Atomic Energy Agency (JPJA19P19209861)

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