Electrocatalysis of hydrogen evolution by synthetic diiron units using weak acids as the proton source: Pathways of doubtful relevance to enzymic catalysis by the diiron subsite of [FeFe] hydrogenase

IR spectroelectrochemical studies of bis(thiolate) and dithiolate-bridged diiron carbonyl compounds, [Fe2(μ-SR)2(CO)6], show that the primary reduction process results in rapid chemical reaction, leading to two-electron reduced products. When the reaction is conducted under an inert atmosphere, the major product is [Fe2(μ-SR)(μ-CO)(CO)6]1−, where in the case of dithiolate-bridged neutral compounds the product has one bridging and one non-bound sulfur atom. This product is formed in near-quantitative yield for solutions saturated with CO. Reduction of [Fe2(μ-SR)(μ-CO)(CO)6]1− occurs at potentials near −2.0 V vs. SCE to give a range of products including [Fe(CO)4]2−. Reduction of thiolate-bridged diiron compounds at mild potentials in the presence of CH3COOH leads to formation of [Fe2(μ-SR)(μ-CO)(CO)6]1− and this is accompanied by an acid-base reaction with the dissociated thiolate. The reaction is largely reversible with recovery of ca. 90% of the starting diiron compound and CH3COOH. In the presence of acid, reduction of [Fe2(μ-SR)2(CO)6] proceeds without generation of observable concentrations of the structurally related one-electron reduced compound. Electrocatalytic proton reduction is achieved when the potential is stepped sufficiently negative to reduce [Fe2(μ-SR)(μ-CO)(CO)6]1−, an observation in keeping with the cyclic voltammetry of the system. Since the catalytic species involved in the weak-acid reactions is structurally distinct from the starting material, and the diiron subsite of the hydrogenase H-cluster, these experiments are of dubious relevance to the biological system.