A Diuranyl(VI) Complex and Its Application in Electrocatalytic and Photocatalytic Hydrogen Evolution from Neutral Aqueous Medium

Abstract

The reaction of equimolar amounts of UO2(OAc)2·2H2O, 2,6-diformyl-4-methylphenol, and N-(hydroxyethyl)ethylenediamine in methanol affords a dinuclear trans-uranyl(VI) complex of the molecular formula [(UO2)2(μ-L)2] (L2- = 2-formyl-4-methyl-6-((2-(2-oxidoethylamino)ethylimino)methyl)phenolate) in 65% yield. Detailed structural elucidation of the complex was performed by using single-crystal X-ray crystallographic and spectroscopic studies. In [(UO2)2(μ-L)2], the metal centers are in edge-shared pentagonal-bipyramidal N2O5 coordination spheres assembled by the two meridional ONNO-donor bridging L2- and two pairs of mutually trans oriented oxo groups. The complex is redox active and displays two successive metal-centered one-electron reductions at Epc = -0.71 and -1.03 V in N,N-dimethylformamide solution. The redox-active complex was used as a heterogeneous catalyst for electrochemical hydrogen evolution from aqueous medium at pH 7 with a turnover frequency (TOF) of 384 h-1 and a Tafel slope of 274 mV dec-1. The Faradaic efficiency of [(UO2)2(μ-L)2] was found to be 84%. Beyond the electrocatalytic response, the [(UO2)2(μ-L)2]-TiO2-N719 composite also exhibited significant heterogeneous photocatalytic hydrogen evolution activity in neutral aqueous medium under visible light and provided a yield of 3439 μmol gcat-1 of H2 in 4 h with a TOF of 172 h-1 and apparent quantum yield (AQY) of 7.6%.