Fluorophenyl-substituted Fe-only hydrogenases active site ADT models: different electrocatalytic process for protonreduction in HOAc and HBF4/Et2O

Abstract

A set of fluorophenyl-substituted adt-bridged Fe₂S₂ active site models of Fe-only hydrogenase, [(μ-SCH₂)₂NR]Fe₂(CO)₆ (1, R = C₆F₄CF₃-p; 2, R = C₆H₄CF₃-p) and [(μ-SCH₂)₂NR]Fe₂(CO)₅(PPh₃) (3, R = C₆F₄CF₃-p; 4, R = C₆H₄CF₃-p), have been synthesized and well characterized. Spectroscopic and electrochemical studies demonstrate that the aryl-substituted complexes 1–4 are stable toward a strong acid HBF₄/Et₂O, and electrocatalytic process for the hydrogen production is mostly dependent on the strength of the available proton source. When CH₃COOH is used as the proton source, the electrocatalytic process begins with successively two one-electron reduction processes to produce H₂ at Fe(0)Fe(0) (E²_pc); whereas in the presence of strong acid, HBF₄/Et₂O, the process is initiated by protonation of a N-bridged atom followed by reduction of the protonated N-bridged atom around −1.29 V, and then release of H₂ at Fe(0)Fe(I) (E¹_pc). Varying the strength of acid leads to the initial electron-transfer step from the reduction of a protonated N-bridged atom to the active site of [Fe(I)Fe(I)].