New 1,2-bis(diphenylphosphino)ethane-laden Co(iii) dithiolate anions: dithiolate-mediated tuning of electrocatalytic performance for non-aqueous hydrogen evolution reaction

Abstract

Four new heteroleptic 1,2-bis(diphenylphosphino)ethane (dppe) laden cobalt(III) dithiolates having compositions NEt₄ [Co(cdc)₂(dppe)] (Co-cdc), NEt₄ [Co(ecda)₂(dppe)] (Co-ecda), NEt₄ [Co(NED)₂(dppe)] (Co-NED) and NEt₄ [Co(i-mnt)₂(dppe)] (Co-i-mnt) (cdc = cyanodithioimidocarbonate; ecda = 1-ethoxycarbonyl-1-cyanoethylene-2,2-dithiolate; NED = 1-nitroethylene-2,2-dithiolate and i-mnt = 1,1-dicyanoethylene-2,2-dithiolate) have been synthesized and characterized. Single crystal X-ray analysis for Co-i-mnt reveals that the immediate geometry around the Co(III) metal is distorted octahedral, and is satisfied by four sulfur atoms of two i-mnt²⁻ ligands and two phosphorus atoms of dppe. These complexes have been employed as homogeneous electrocatalysts for the hydrogen evolution reaction (HER) using trifluoroacetic acid (TFA) as a proton source. The electrocatalytic proton reduction activity results suggest that amongst all the four complex anions, Co-NED showcases the best HER electrocatalysis with the lowest overpotential of 1.04 V and highest turnover frequency of 493 s⁻¹ following the trend Co-NED > Co-cdc > Co-i-mnt > Co-ecda. Additionally, controlled potential electrolysis (CPE) experiments for these complexes indicated that the amount of charge passed correlates directly to the amount of hydrogen evolved. Furthermore, rinse tests on the most active electrocatalyst, Co-NED, confirm its chemical stability during electrocatalysis, with no detectable conversion to cobalt chalcogenide species. Computational studies reveal that superior HER performance in Co-NED arises from the highest natural charge over cobalt-center charge and minimal steric bulk of dithiolate pendants, thereby highlighting the importance of electronic tuning via strategic ligand substitution in cobalt–dithiolate complexes.