Dinuclear uranium(vi) salen coordination compound: an efficient visible-light-active catalyst for selective reduction of CO2 to methanol

Abstract

A new dinuclear uranyl salen coordination compound, [(UO₂)₂(L)₂]·2MeCN [L = 6,6′-((1E,1′E)-((2,2-dimethylpropane-1,3-diyl)bis(azaneylylidene))-bis(methaneylylidene))bis(2-methoxyphenol)], was synthesized using a multifunctional salen ligand to harvest visible light for the selective photocatalytic reduction of CO₂ to MeOH. The assembling of the two U centers into one coordination moiety via a chelating–bridging doubly deprotonated tetradentate ligand allowed the formation of U centers with distorted pentagonal bipyramid geometry. Such construction of compounds leads to excellent activity for the photocatalytic reduction of CO₂, permitting a production rate of 1.29 mmol g⁻¹ h⁻¹ of MeOH with an apparent quantum yield of 18%. Triethanolamine (TEOA) was used as a sacrificial electron donor to carry out the photocatalytic reduction of CO₂. The selective methanol formation was purely a photocatalytic phenomenon and confirmed using isotopically labeled ¹³CO₂ and product analysis by ¹³C-NMR spectroscopy. The spectroscopic studies also confirmed the interaction of CO₂ with the molecule of the title complex. The results of these efforts made it possible to understand the reaction mechanism using ESI–mass spectrometry.