Facile electrocatalytic proton reduction by a [Fe–Fe]-hydrogenase bio-inspired synthetic model bearing a terminal CN− ligand

Abstract

An azadithiolate bridged CN⁻ bound pentacarbonyl bis-iron complex, mimicking the active site of [Fe–Fe] H₂ase is synthesized. The geometric and electronic structure of this complex is elucidated using a combination of EXAFS analysis, infrared and Mössbauer spectroscopy and DFT calculations. The electrochemical investigations show that complex 1 effectively reduces H⁺ to H₂ between pH 0–3 at diffusion-controlled rates (10¹¹ M⁻¹ s⁻¹) i.e. 10⁸ s⁻¹ at pH 3 with an overpotential of 140 mV. Electrochemical analysis and DFT calculations suggests that a CN⁻ ligand increases the pK_a of the cluster enabling hydrogen production from its Fe(I)–Fe(0) state at pHs much higher and overpotential much lower than its precursor bis-iron hexacarbonyl model which is active in its Fe(0)–Fe(0) state. The formation of a terminal Fe–H species, evidenced by spectroelectrochemistry in organic solvent, via a rate determining proton coupled electron transfer step and protonation of the adjacent azadithiolate, lowers the kinetic barrier leading to diffusion controlled rates of H₂ evolution. The stereo-electronic factors enhance its catalytic rate by 3 order of magnitude relative to a bis-iron hexacarbonyl precursor at the same pH and potential.