Protonation/Reduction of Carbonyl-Rich Diiron Complexes and the Direct Observation of Triprotonated Species: Insights into the Electrocatalytic Mechanism of Hydrogen Formation

Both the reduced and the protonated states of diiron dithiolate complexes, which are key intermediate species for the electrocatalytic production of hydrogen, have been spectroscopically and theoretically investigated in this study. Five important states in the process of H2 evolution have been characterized. In the presence of a superacid, protonation occurs onto the Fe–Fe vector of [(μ-xdt)­Fe2(CO)6] (xdt: pdt, 1,3-propanedithiolate; edt, 1,2-ethanedithiolate; bdt, 1,2-benzenedithiolate) to yield the cationic μ-H species (the C state). A single reduction at 193 K leads to the neutral species (the CE state), with similar structures for the pdt and edt bridgeheads. The CE species of the bdt analogue is unstable under the same conditions. An open structure resulting from the rupture of one Fe–S bond is suggested by DFT calculations. Subsequently, a second reduction induces a dramatic structural rearrangement in which the CEE state possesses an open structure exhibiting a μ-H and a μ-CO group. Protonation onto the terminal sulfur site of the CEE state affords the CEEC state, which readily converts to the parent hexacarbonyl complex accompanied by the liberation of H2 at higher temperatures. In the presence of excess acid, the CEECC state is achieved and the third proton is coordinated to the Fe center. The S-proton and Fe-hydride have been characterized by 1H and 2D NMR spectroscopy. Electrocatalytic hydrogen production involving the CEEC and CEECC states has been investigated by DFT calculations. In combination with the spectroscopic results, this information allows us to construct the possible catalytic routes and study the plausible role of the triply protonated species least explored in biomimetic catalysis.