Disputation: Shihuai Wang

2019-04-17

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

Disputationen äger rum den 10 maj klockan 10.15 i Häggsalen på Ångströmlaboratoriet.

Abstract
Directed design of improved molecular catalysts for hydrogen evolution reactions relies on rational benchmarking based on 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 ratelimiting 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 FeI FeI (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 geometry coordination, but show different structural rearrangement upon reduction. This might be applied to elucidate their differences in protonation dynamics. Protonation kinetics of singly reduced species, forming bridging hydride, indicate a direct proton transfer step in the FeI Fe0 state, in contrast to that of the neutral complex (FeI FeI 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 FeFehydrogenase, the amine function of the bridgehead is known to assist enzymatic H2 formation by proton shuttling. The same role in catalysis by 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 proton shuttle in the hydride formation in the FeI Fe0 state. Instead, the effect of aza protonation is to lower the catalyst overpotential, but does not slows down the hydride formation by external proton.

For complex with benzyl bridgehead, the rigid and unsaturated bridging ligand generally lower the reduction potential and stabilize the reduced forms. This allows to characterize the overall catalytic processes, i.e exact structure of hydride intermediate, turnover process, etc.

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