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.