Ruthenium complexes bearing small bite angle PN ligands are effective catalysts for the upgrading of ethanol/methanol to isobutanol via the Guerbet reaction.

Three mixed PN-donor ligand structures based on 2-(diphenylphosphino)-3-methylindole (1H), and diphenylphosphinopyridines (2 and 3) have been used to design simple and stable ruthenium catalysts for the Guerbet reaction (4-7). Two of these complexes (6, 7) have been structurally characterised, showing the hemilability of these PN-donor ligands, depending on the other ligands coordinated to ruthenium. These complexes are effective Guerbet catalysts for the upgrading of ethanol/methanol mixtures to isobutanol, a potential sustainable drop-in fuel replacement, reaching isobutanol yields of up to 62%. Previously unidentified precipitated side products, e.g. sodium alkyl carbonates, have been discovered to be present in post reaction mixtures. Analysis of ruthenium phosphinopyridine complexes under simulated catalytic environments provided further detail into catalytic degradation species and potential active catalyst structures.