Proton-coupled electron transfer in the reduction of diiron hexacarbonyl complexes and its enhancement on electrocatalytic reduction of proton caused by a pendant basic group
Three benzenedithiolate-bridged diiron hexacarbonyl complexes (2, 3 and 4) with different functional groups were designed and synthesized. In addition, a well-defined benzenedithiolate-bridged complex without any functional groups, 5, was employed for comparison. Electrochemical investigations in five various acids showed that the first reduction potential of the complexes shifts positively in acidic media. The potential shift is attributed to PCET (Proton-Coupled Electron Transfer) and depends on the acid strength. When an acid is too weak (pKa 24 in this case), no such potential shift could be observed. On the other hand, increasing the acid strength did not lead always to linear trend in the potential shift for all the complexes due to kinetic resistance in proton transfer for some of the complexes. The presence of a pendant basic group (2) could ease such kinetic resistance and the linear trend holds valid up to pKa at 1.8 whereas for the rest of the complexes (3-5) which do not bear an internal basic group, the correlation between the potential shift and the acid strength would be levelled off after certain pKa value (12). In other word, when the acid is strong enough to provide sufficient proton, further increasing the acid strength cannot improve the potential shift since in this circumstance, the kinetic resistance for a proton to cross the solvated layer becomes dominant.