Electrocatalytic HER Performance of [FeFe]-Hydrogenase Mimics Bearing M-salen Moieties (M=Zn, Ni, Fe, Mn)

The synthesis and characterization of novel compounds (5-8) as mimetics of [FeFe]-hydrogenase, combining two distinct systems capable of participating in hydrogen evolution reactions (HER): the [(μ-adt)Fe₂(CO)₆] fragment and M-salen complexes (salen=N,N'-bis(salicylidene)ethylenediamine) (M=Zn, Ni, Fe, Mn), is reported. These complexes were synthesized in high yields via a three-step procedure from N,N'-bis(4-R-salicylidene)ethanediamine) 4 [R=Fe₂(CO)₆(μ-SCH₂)₂NCOCH₂O]. Structural analysis through spectroscopic, spectrometric, and computational (DFT) methods confirmed distorted tetrahedral and square-planar geometries for Zn-salen and Ni-salen complexes (5 and 6) respectively, while complexes Fe-salen 7 and Mn-salen 8 exhibit square-based pyramidal structures typical of Fe(III) and Mn(III) high-spin salen-complexes. Electrochemical studies revealed different reduction events for [(μ-adt)Fe₂(CO)₆] and M-salen moieties, but the electrocatalytic experiments in TFA demonstrated a clear cooperative effect between these components, especially at higher acid concentrations. Notably, the Ni-salen (6), Fe-salen (7), and Mn-salen (8) complexes exhibit significant reductions in overpotential, highlighting their potential for enhanced catalytic performance.