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

Area of Science:

• Inorganic Chemistry
• Catalysis
• Electrochemistry

Background:

• Non-heme iron complexes are crucial in catalysis, but their efficiency in hydrogen evolution reactions (HER) requires optimization.
• Ligand design plays a pivotal role in modulating the electronic and structural properties of metal complexes for enhanced catalytic performance.

Purpose of the Study:

• To synthesize and characterize novel non-heme Fe(III) complexes with pyridine-2,6-dicarboxamide-based ligands.
• To investigate the hydrogen evolution reaction (HER) catalytic activity of these complexes under acidic conditions.
• To elucidate the structure-activity relationship, particularly the influence of ligand functional groups on HER performance.

Main Methods:

• Synthesis and characterization of three Fe(III) complexes using elemental analysis, single crystal X-ray diffraction, EPR, Mössbauer, UV-Vis, and IR spectroscopy.
• Electrocatalytic evaluation of HER activity in dimethylformamide (DMF) using various acidic media (acetic acid, triethylammonium chloride/tetrafluoroborate).
• Kinetic analysis to determine catalytic turnover frequencies (kcat).

Main Results:

• Three bis-chelated hexa-coordinated non-heme Fe(III) complexes, (FeIII[L1]2)− (1), (FeIII[L2]2)− (2), and (FeIII[L3]2)− (3), were successfully synthesized.
• Complex 1, featuring a 4-hydroxy pyridine-2,6-dicarboxamide ligand, demonstrated significantly higher electrocatalytic activity for HER (kcat = 1.4 ± 0.07 × 10^5 M−1s−1) compared to complexes 2 and 3.
• X-ray diffraction revealed differences in Fe-Npyridine bond distances between complexes 1 and 2, suggesting structural influences on activity.

Conclusions:

• The presence of internal hydroxy functional groups in complex 1 enhances HER electrocatalytic activity by acting as a local proton source and facilitating intramolecular proton transfer.
• Tuning ligand scaffolds in Fe(III) complexes is an effective strategy to modulate their catalytic performance for hydrogen evolution.
• This study highlights the potential of specifically designed non-heme iron complexes for efficient hydrogen production.