Tris(N-pyrrolidinyl)phosphine substituted diiron dithiolate related to iron-only hydrogenase active site: Synthesis, characterization and electrochemical properties

A tris(N-pyrrolidinyl)phosphine (P(NC₄H₈)₃) monosubstituted complex, [(μ-pdt)Fe₂(CO)₅P(NC₄H₈)₃] (2) was synthesized as a functional model of the hydrogen-producing capability of the iron hydrogenase active site. The structure was fully characterized by X-ray crystallography. IR and electrochemical studies have indicated that the P(NC₄H₈)₃ ligand has better electron-donating ability than that of those phosphine ligands, such as PMe₃, PTA (1,3,5-triaza-7-phosphaadamantane), PMe₂Ph PPh₃, and P(OEt)₃. The electrocatalytic activity of 2 was recorded in CH₃CN in the absence and presence of weak acid, HOAc. The cathodic shift of potential at −1.98 V and the dependence of current on acid concentration have indicated that complex 2 can catalyze the reduction of protons to hydrogen at its Fe⁰Fe^I level in the presence of HOAc. IR spectroelectrochemical experiments are conducted during the reduction of 2 under nitrogen and carbon monoxide, respectively. The formation of a bridging CO group during the reduction of 2 at −1.98 V has been identified using IR spectroelectrochemical techniques, and an electrocatalytic mechanism of 2 consistent with the spectroscopic and electrochemical results is proposed.