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

A set of fluorophenyl-substituted adt-bridged Fe2S2 active site models of Fe-only hydrogenase, [(m-SCH2)(2)NR]Fe-2(CO)(6) (1, R=C6F4CF3-p; 2, R = C6H4CF3-p) and [(mu-SCH2)(2)NR]Fe-2(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 H-2 at Fe(0) Fe(0) (E-pc(2)); 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 H-2 at Fe(0) Fe(I) (E-pc(1)). 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)].