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

In the present study, we synthesized three non-heme FeIII complexes (FeIII[L1]2)- (1), (FeIII[L2]2)- (2), and (FeIII[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 kcat 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 FeIII complexes could potentially affect their catalytic activity for HER.