Electrochemical proton reduction by thiolate-bridged hexacarbonyldiiron clusters

The reduction of Fe2(μ-SC6H4S)(CO)6 (1) at glassy carbon electrode appears chemically reversible (E1/2=−1.44 V vs. Fc in Bu4NPF6–CH2Cl2, ks,h∼5×10−3 cm s−1) and actually consists of two overlapping one-electron transfers with the second transfer slightly more favorable than the first (E02−E01∼35 mV). Apparently, the reduction of Fe2(μ-SCH2C6H4CH2S)(CO)6 (2) is also a two-electron process (E1/2=−1.68 V vs. Fc in Bu4NPF6–CH2Cl2), but 22− decomposes on the voltammetric time scale. For both 1 and 2, the electrochemical reduction of the neutral cluster generates intermediate species which further catalyze proton reduction in acid-containing electrolyte (i.e., HBF4 in Bu4NPF6–CH2Cl2). Unfortunately voltammetry also reveals a catalytic cycle for electrochemical proton reduction either slow or complicated by side reactions.