Proton-coupled electron transfer in the reduction of diiron hexacarbonyl complexes and its enhancement on the electrocatalytic reduction of protons by a pendant basic group

Abstract

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 using five different acids showed that the first reduction potential of the complexes shifted positively in acidic media. This potential shift is attributed to a PCET (Proton-Coupled Electron Transfer) reaction and depends on the acid strength. When an acid is too weak (pK_a ≥ 24 in this case), no potential shift is observed. Moreover, increasing the acid strength does not lead always to a linear trend in the potential shift for all of the complexes due to kinetic resistance during proton transfer for some of the complexes. The presence of a pendant basic group (2) can ease such kinetic resistance, and the linear trend holds valid up to pK_a 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 levels off after a certain pK_a value (12). Alternatively, when the acid is strong enough to provide sufficient proton, further increases in the acid strength do not improve the potential shift and the kinetic resistance for a proton to cross the solvated layer becomes dominant.