Effective electrocatalysts for hydrogen production from acetic acid by screening of monodentate ligands in cationic diiron hetero-carbyne complexes

The development of efficient electrocatalysts for proton reduction (H2 production) based on earth-abundant metals is a scientific challenge with implications for energy storage and generation. Inspired by [FeFe]-hydrogenase enzymes, several compounds based on the {Fe2Cp2(CO)2} framework have been investigated in this respect but, to date, only two charge-neutral derivatives, with a terminal cyanide and a bridging dimethylamino- (CNMe2+) or methylthio- (CSMe+) carbyne ligand, were found to be effective electrocatalysts. Herein, we extended the investigation to related cationic derivatives with the cyanide ligand being replaced with various monodentate, hydrophilic N- P- or S-donor ligands. Aminocarbyne complexes [Fe2Cp2(CO)(L)(mu-CO)(mu-CNMe2)]+ (L = NH3, [2a]+; imidazole, [2b]+; pyrazole, [2c]+; thiourea, [2d]+; 1,3,5-triaza-7-phosphadamantane, PTA, [2e]+) and thiocarbyne complexes [Fe2Cp2(CO)(L)(mu-CO)(mu-CSMe)]+ (L = imidazole, [4b]+; PTA, [4e]+; 4-dimethylaminopyridine, DMAP, [4f]+; MeCN [4g]+), five of which are unprecedented, were prepared, isolated as triflate salts and characterized by IR and NMR spectroscopy and X-ray diffraction in two cases. These nine diiron compounds were screened by cyclic voltammetry for their redox chemistry in acetonitrile and the electrocatalytic activity in the H2 evolution reaction from acetic acid. Two thiocarbyne complexes, featuring imidazole ([4b]+) and DMAP ([4f]+) as monodentate ligands emerged for their remarkable electrocatalytic activity, outperforming the previously-investigated cyanide derivatives. A mechanism for the electrocatalytic cycle is proposed based on combined electrochemical, spectroscopic and literature data.