Analyzing the photophysics and photochemistry of [UO2(acetylacetonate)2(L)] complexes

Abstract

Photocatalysis using the uranyl cation, UO₂²⁺, is underdeveloped. Under visible light illumination, uranyl photocatalysts generate potent excited state oxidants (>2.6 V vs. SHE), though the impact of the equatorial ligand environment on the photophysics and photocatalysis of the complexes is poorly understood. To better understand the effect of the equatorial ligands, a series of bis(diketonate) uranyl photocatalysts of the form UO₂(acetylacetonate)₂(L) (L = substituted pyridine) were prepared and characterized. The use of electron withdrawing or donating substituents on the pyridine does little to disturb the geometric structure of the complexes, but significant changes in their electronic structure are observed. Specifically, the use of electron-withdrawing cyano- and fluoropyridines increases the ground state reduction potential and weakens the UO bond. Computational data demonstrates that the ligand-to-metal charge transfer in these complexes involves both the uranium oxo bonds and the acetylacetonate ligand, while the choice of pyridine shifts the absolute energies of the HOMO and LUMO within the complex. Finally, dehydrogenation of 1-phenylethanol was used as a model reaction to explore photochemical reactivity against the performance of uranyl nitrate hexahydrate, the standard choice in uranyl photocatalysis.