Accelerating Electrochemical Hydrogen Evolution by Introducing Local Proton Source in Non-Heme Iron (III) Complexes

In the present study, we synthesized three non-heme Fe^III complexes (Fe^III[L¹]₂)^- (1), (Fe^III[L²]₂)^- (2), and (Fe^III[L³]₂)^- (3) with 4-hydroxy pyridine-2,6-dicarboxamide (H₂L¹), pyridine-2,6-dicarboxamide (H₂L²), and 4-methoxy pyridine-2,6-dicarboxamide (H₂L³) based ligands and characterized by elemental analysis, single crystal X-ray diffraction (for 1 and 2), EPR, Möβbauer (for 1), UV-visible and infrared spectroscopy. Hydrogen evolution reaction (HER) catalytic activity of 1, 2, and 3 was performed in the presence of acids such as acetic acid (AcOH) and triethylammonium chloride/triethylammonium tetrafluoroborate (HNEt₃Cl/HNEt₃BF₄) in dimethylformamide (DMF). In these compounds, the primary coordination sphere of Fe remained similar, forming bis-chelated hexa-coordinated complexes. However, single-crystal X-ray structures revealed that the Fe-Npyridine bond distances are different in 1 and 2. Noteworthily, complex 1 showed better electrocatalytic activity for HER with a k_cat of 1.4 ± 0.07 × 10⁵ M^-1s^-1 in the presence of HNEt₃Cl, highlighting the significant effect of internal hydroxy functional groups in HER. The presence of additional hydroxy groups in complex 1 plays the role of a local proton source and facilitates an intramolecular proton transfer, which enhances the catalytic rates for HER. The current study emphasizes how tuning the ligand scaffolds in Fe^III complexes could potentially affect their catalytic activity for HER.