New use of mononuclear Mn/Co/Ni/Cu complexes of symmetric N₂O₂-type Schiff base as hydrogen-evolving catalysts

Development of sustainable hydrogen production methods is crucial for the future hydrogen economy. Hence, designing cheap, stable and efficient catalysts for hydrogen generation is important. Here, we present the use of the family of eight Schiff base ligand-based transition metal complexes [Mn^III(Salen)Cl] 1/[Co^II(Salen)], 2/[Ni^II(Salen)], 3/[Cu^II(Salen)], 4/[Mn^III(CH₃Salen)Cl], 5/[Co^II(CH₃Salen)], 6/[Ni^II(CH₃Salen)], 7/[Cu^II(CH₃Salen)] and 8 (Salen/N,N'-bis(salicylidene)ethylenediamine/H₂ L¹ ; CH₃Salen/N,N'-bis(o-hydroxyacetophenone)ethylenediamine/H₂ L² ) as hydrogen evolution reaction (HER) electrocatalysts. Electrochemical studies for the complexes (in CH₃CN) were carried out in the absence and presence of acids. Catalytic parameters of turnover frequency/overpotential were calculated for the electrocatalytic process. All complexes were found to be efficient HER catalysts in the presence of acids and Co/Ni/Cu complexes were more stable than the Mn complexes. The stability and efficiency order of the catalysts was 2, 4 > 6 > 7 > 3 > 8 (acetic acid); 1 > 5 (trifluoroacetic acid). Density functional theory studies provided detailed insight into the metal- versus ligand-centred redox transitions and the HER mechanism for all complexes. Successive events of electrochemical reduction (E) and protonation (C) along the HER paths were investigated, and thermodynamically feasible routes were assigned. This work provides insights into the effect of a series of earth-abundant transition metal complex structures, the performance of this series of metal centres on electrocatalysis, and the effect of pK _a on reactivity. Hence, it will be of value to develop novel and promising catalysts for HER in the future.