Electronic Tunability of Ruthenium Formyl and Hydroxymethyl Intermediates Relevant to Sustainable CO-to-Methanol Conversion

Metal formyl and hydroxymethyl complexes are implicated as key intermediates in the (photo)electrochemical reduction of carbon oxides (CO2 or CO) to liquid fuels, such as methanol. Formyl complexes, and to a lesser extent hydroxymethyl complexes, have been previously synthesized and characterized; nevertheless, the influence of electronic modifications to ligands supporting these reactive carbon fragments is not well understood. Herein, we report the synthesis of a family of ruthenium polypyridyl carbonyl complexes of the form [Ru(4,4′-R,R-bpy)(tpy)(CO)]2+ bearing different substituents on the bipyridyl (bpy) ligand (R = OMe, H, CF3). Treatment with NaBH4 as a chemical reductant results in the formation of the formyl and subsequently the hydroxymethyl and methyl complexes; each is characterized by comprehensive NMR spectroscopy, mass spectrometry, and isotopic labeling studies. An electron-donating modification (R = OMe) to the bpy ligand is shown to significantly increase the lifetime of the formyl intermediate and the yield of released methanol. We observe a clear linear dependence of thermodynamic parameters on bpy electronics; however, the stability of the formyl complex and the reactivity of the resulting hydroxymethyl complex do not depend linearly on ligand electronics. We anticipate that these results may be extended to future development of (photo)electrocatalytic systems for CO-to-methanol conversion.