Activation of proton by the two-electron reduction of a di-iron organometallic complex

Fe2(bdt)(CO)6 (1) (bdt = benzenedithiolate) exhibits a chemically reversible two-electron reduction at −1.27 V vs. Fc+/0 in CH3CN + Bu4NPF6 electrolyte. The increase in height of the reduction wave of complex 1 upon increasing the concentration of toluenesulfonic acid (pKa ∼ 8 in CH3CN) is consistent with a catalysis of the proton reduction reaction (kcat ∼ 500 M−1 s−1). The reaction scheme, derived from the simulation of the voltammetric responses, involves the initial formation of the dianion 12− followed by two successive reactions with acid. The last protonation step is the rate determining step in the catalytic cycle. Using experiments conducted with acetic acid (pKa ∼ 23 in CH3CN), we further shown that the intermediate species 1H− is a significantly weaker base than the dianion 12−.