RESUMEN
Di-iron dithiolate hydrogenase model complexes are promising systems for electrocatalytic production of dihydrogen and have therefore been spectroscopically and theoretically investigated in this study. The direct effect of ligand substitution on the redox activity of the complex is examined. In order to understand and eventually optimize such systems, we characterised both metal and ligand in detail, using element specific X-ray absorption Fe- and S-K edge XAS. The (electronic) structure of three different [Fe2S2] hydrogenase systems in their non-reduced state was investigated. The effect of one- and two-electron reduction on the (electronic) structure was subsequently investigated. The S K-edge XAS spectra proved to be sensitive to delocalization of the electron density into the aromatic ring. The earlier postulated charge and spin localization in these complexes could now be measured directly using XANES. Moreover, the electron density (from S K-edge XANES) could be directly correlated to the Fe-CO bond length (from Fe K-edge EXAFS), which are in turn both related to the reported catalytic activity of these complexes. The delocalization of the electron density into the conjugated π-system of the aromatic moieties lowers the basicity of the diiron core and since protonation occurs at the diiron (as a rate determining step), lowering the basicity decreases the extent of protonation and consequently the catalytic activity.
RESUMEN
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.
