Tetranuclear Iron Complexes Bearing Benzenetetrathiolate Bridges as Four-Electron Transformation Templates and Their Electrocatalytic Properties for Proton Reduction

Abstract

Two tetranuclear iron-sulfur complexes, (μ,μ-pbtt)[Fe(2)(CO)(6)](2) (pbtt = benzene-1,2,4,5-tetrathiolato, 3) and (μ,μ-obtt)[Fe(2)(CO)(6)](2) (obtt = benzene-1,2,3,4-tetrathiolato, 4), were prepared from reaction of Fe(3)(CO)(12) and the corresponding tetramercaptobenzene in THF, respectively. Complexes 5 and 6, (μ,μ-pbtt)[Fe(2)(CO)(5)L(1)][Fe(2)(CO)(5)L(2)] (L(1) = CO, L(2) = PPyr(3) (Pyr = N-pyrrolyl), 5; L(1) = L(2) = PPyr(3), 6) were obtained by controlling CO displacement of 3 with PPyr(3). Molecular structures of 3-6 were determined by spectroscopic and single-crystal X-ray analyses. All-CO Fe(4)S(4) complexes 3 and 4 each display four-electron reduction processes in consecutive chemically reversible two-electron reduction events with relatively narrow potential spans in the cyclic voltammograms. Phosphine-substituted Fe(4)S(4) complexes 5 and 6 exhibit two consecutive two-electron reduction events, which are not fully reversible. The electrocatalytic properties of 3 and 4 for proton reduction were studied using a series of carboxylic acids of increasing strength (CH(3)COOH, CH(2)ClCOOH, CHCl(2)COOH, CCl(3)COOH, and CF(3)COOH). The mechanisms for electrochemical proton reduction to hydrogen catalyzed by complex 3 as a function of acid strength are discussed.