Electrochemical insights into the mechanisms of proton reduction by [Fe-2(CO)(6){mu-SCH2N(R)CH2S}] complexes related to the [2Fe](H) subsite of [FeFe]hydrogenase

Electrochemical investigations on a structural analogue of the [2Fe](H) subsite of [FeFe]H-2 ases, namely, [Fe-2(CO)(6){mu-SCH2N(CH2CH2OCH3)CH2S}] (1), were conducted in MeCN/NBu4PF6 in the presence of HBF4/Et2O or HOTS. Two different catalytic proton reduction processes operate, depending on the strength and the concentration of the acid used. The first process, which takes place around -1.2 V for both HBF4/Et2O and HOTS, is limited by the slow release of H-2 from the product of the {2H(+)/2e} pathway, 1-2H. The second catalytic process, which occurs at higher acid concentrations, takes place at different potentials depending on the acid present. We propose that this second mechanism is initiated by protonation of 1-2H when HBF4/Et2O is used, whereas the reduction of 1-2H is the initial step in the presence of the weaker acid HOTS. The potential of the second process, which occurs around -1.4V (reduction potential of 1-3H(+)) or around -1.6V (the reduction potential of 1-2H) is thus dependent on the strength of the available proton source.