Probing Catalytic Reaction Mechanisms of Biomimetic Diiron Complexes through Time-resolved Absorption Spectroscopy

Abstract: Directed design of improved molecular catalysts for hydrogen evolution reactions relies on rational benchmarking based on a detailed understanding about the mechanism of catalysis. Specifically, investigation of multi-electron redox catalysis, with structural characterization of catalytic intermediates, combined with the kinetics of their transformations, can reveal the rate-limiting step of the overall reaction, possible degradation pathways and the function of structural motives. However, direct spectroscopic observation of catalytic intermediates is in most cases not available due to the rapid turnover of efficient catalysts.In this thesis, time-resolved absorption spectroscopy with UV-Vis and mid-IR detection was used to identify catalytic reaction intermediates and account for kinetics relevant to elementary reactions steps of H2 formation on a nanosecond to second time scale. For a class of FeIFeI (S-R-S)(CO)6-n(PMe3)n complexes (R = propyl, benzyl or azapropyl), inspired by the active site of FeFe-hydrogenase, the key intermediates formed in different catalytic pathways have been characterized. These complexes typically feature very similar coordination geometry, but show different structural rearrangements upon reduction. This could be applied to rationalize their differences in protonation dynamics. Protonation kinetics of singly reduced species, forming a bridging hydride, indicate a direct proton transfer step in the FeIFe0 state, in contrast to that of the neutral complex (FeIFeI state) with phosphine ligands (PMe3) in which the hydride formation is likely mediated by one of the CO-ligands, as had been proposed. In catalysis of FeFe-hydrogenase, the amine function of the bridgehead is known to assist enzymatic H2 formation by proton shuttling. The same role in catalysis by the synthetic diiron complex with the azapropyl bridgehead had been proposed. However, our results show that for the synthetic complex, the aza-group has no role as a proton shuttle in the hydride formation in the FeIFe0 state. Instead, the effect of nitrogen protonation is to lower the catalyst overpotential, without substantially slowing down the hydride formation with external protons. The amine acting as a proton shuttle in the hydride formation could be expected in the Fe0Fe0 level. However, slower second reduction of FeIFeI (S-azapropyl-S)(CO)6 complex impedes observation of the doubly reduced species under the catalytic conditions. For the benzyldithiolate complex, on the other hand, the rigid and unsaturated bridging ligand generally leads to less negative potentials and prevent the reduced forms from rapid degradation. This allows characterization of the later intermediates of the catalytic processes, and to obtain direct kinetic information on the turnover step.