Electrocatalysis of hydrogen production by active site analogues of the iron hydrogenase enzyme: structure/function relationships

Abstract

A series of binuclear Fe^IFe^I complexes, (μ-SEt)₂[Fe(CO)₂L]₂ (L = CO (1), PMe₃ (1-P)), (μ-SRS)[Fe(CO)₂L]₂ (R = CH₂CH₂ (μ-edt): L = CO (2), PMe₃ (2-P); R = CH₂CH₂CH₂(μ-pdt): L = CO (3), PMe₃ (3-P); and R = o-CH₂C₆H₄CH₂ (μ-o-xyldt): L = CO (4), PMe₃ (4-P)), that serve as structural models for the active site of Fe-hydrogenase are shown to be electrocatalysts for H₂ production in the presence of acetic acid in acetonitrile. The redox levels for H₂ production were established by spectroelectrochemistry to be Fe⁰Fe⁰ for the all-CO complexes and Fe^IFe⁰ for the PMe₃-substituted derivatives. As electrocatalysts, the PMe₃ derivatives are more stable and more sensitive to acid concentration than the all-CO complexes. The electrocatalysis is initiated by electrochemical reduction of these diiron complexes, which subsequently, under weak acid conditions, undergo protonation of the reduced iron center to produce H₂. An (η²-H₂)Fe^II–Fe^0/I intermediate is suggested and probable electrochemical mechanisms are discussed.