Iron-CatalyzedHydrogenation of Alkyl Formates andCarbon Dioxide

Alkyl formates are crucial components in biomass polyoxygenated derivatives, and their hydrogenation favors methanol and alcohol production, emerging substances as alternative energy sources. In addition, these substrates are intermediates in the hydrogenation of carbon dioxide in the presence of alcohols, pointing to their importance in the field. Despite there being several reports on the catalyzed hydrogenation of aryl esters, the hydrogenation of alkyl formates remains largely unexplored. This study addresses alkyl and aryl formate hydrogenation using iron(II) precursors with nitrogenated ligands based on imino- and aminopyridine-substituted azoles (benzotriazole and pyrazole). The investigation also sheds light on the catalytic activity’s dependence on the ligands’ electronic and steric effects, as well as the influence of amine and imine substituents. A systematic study on additives that provide iron-hydrides like LiBHEt₃ and dihydrogen activation using LiHMDS enhanced the activity and selectivity of iron complexes. The catalytic system of iron complexes and additives shows heightened activity, influenced by the ligand’s azole ring, amine or imine substitution, and coordinating halides and solvents. The catalytic system achieved a remarkable activity and selectivity in the hydrogenation of ethyl formate as well as the hydrogenation of carbon dioxide in the presence of ethanol.