Spectroscopic and theoretical investigation of the [Fe₂(bdt)(CO)₆] hydrogenase mimic and some catalyst intermediates

In [Fe-Fe] hydrogenase mimic systems the ene-1,2-dithiolene ligands play an important role in the stabilisation of the redox-active metal center. This is demonstrated by the benzenedithiolene (bdt) analogue, featuring six terminal carbonyl ligands connected to a di-iron metal center, i.e. [Fe₂(bdt)(CO)₆]. Here we present a combined experimental and theoretical study that elucidates key intermediates [Fe₂(bdt)(CO)₆]^1- and [Fe₂(bdt)(μ-CO)(CO)₅]^2- in the electrocatalytic production of dihydrogen. A DFT study shows that [Fe₂(bdt)(CO)₆]^1- is the kinetic product after the first one electron reduction, while the previously proposed bridging intermediate species [Fe₂(bdt)(μ-CO)(CO)₅]^1- is kinetically inaccessible. The doubly reduced species [Fe₂(bdt)(μ-CO)(CO)₅]^2- was for the first time structurally characterized using EXAFS. XANES analysis confirms the existence of reduced iron zero species and confirms the distorted geometry that was suggested by the DFT calculations. Combining IR, UV-vis and XAS spectroscopic results with TD-DFT and FEFF calculations enabled us to assign the key-intermediate [Fe₂(bdt)(CO)₆]^2-. This study emphasizes the strengths of combining computational chemistry with advanced spectroscopy techniques.