Electron transfer at a dithiolate-bridged diiron assembly: Electrocatalytic hydrogen evolution

Electrochemical reduction of Fe-2(mu-pdt)(CO)(6) 1 (pdt = propane-1,3-dithiolate) leads initially to a short-lived species, 1(-), then subsequently to two-electron reduced products, including a CO-bridged diiron compound, 1B. The assignment of the redox level of 1(-) is based on EPR and UV-vis spectra together with the observation that a CO-saturated solution of 1(-) decays to give 1 and 1B. Hydride reduction of 1 also results in formation of 1B via a relatively long-lived formyl species, 1 formyl. Despite its involvement in hydride transfer reactions, 1B is formulated as [Fe-2(mu-S(CH2)(3)SH)(mu-CO)(CO)(6)](-) based on a range of spectroscopic measurements together with the Fe-Fe separation of 2.527 Angstrom (EXAFS). Electrocatalytic proton reduction in the presence of 1 in moderately strong acids has been examined by electrochemical and spectroelectrochemical techniques. The acid concentration dependence of the voltammetry is modeled by a mechanism with two electron/proton additions leading to 1H(2), where dissociation of dihydrogen leads to recovery of 1. Further reduction processes are evident at higher acid concentrations. Whereas free CO improves the reversibility of the electrochemistry of 1, CO inhibits electrocatalytic proton reduction, and this occurs through side reactions involving a dimeric species formed from 1(-).