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)(μ-CO)(μ-CNMe₂)]⁺ (L = NH₃, [2a]⁺; imidazole, [2b]⁺; pyrazole, [2c]⁺; thiourea, [2d]⁺; 1,3,5-triaza-7-phosphadamantane, PTA, [2e]⁺) and thiocarbyne complexes [Fe₂Cp₂(CO)(L)(μ-CO)(μ-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.