The [Fe-Fe]-hydrogenases are extraordinarily efficient biological catalysts for reducing protons to generate hydrogen gas. The active site of the natural enzyme contains two central iron atoms with a bridging dithiolate ligand, and multiple CO and CN ligands on the irons. Many small-molecule models of the active site have been developed that contain some or all of the structural features of the [FeFe]-hydrogenase active site. Several model compounds also contain a photosensitizer which could permit absorption of light to drive the reduction of hydrogen. Understanding the photo-dynamics of the active site of these and related model compounds is critical for ultimately designing efficient hydrogen-generating, light-driven catalysts since the long-lived photochemistry affects the overall catalytic behaviour at the active site.<br/><br/>In this poster, the time-dependent photochemistry of two ionic CN-substituted hydrogenase models are examined using picosecond time-resolved infrared spectroscopy (TRIR). Substantial differences in the photochemical behaviour are found depending on ligand substitution at the model active site. All previously studied model compounds produce CO-loss products when excited with UV and/or visible light, and these product isomers persist for several microseconds and longer. Model compounds with only CO ligands or CO and PMe<sub>3</sub> ligands also exhibit excitation of long-lived (ca. 150 ps) excited electronic states, either competing with or in addition to CO loss. However, substitution of two CO ligands with bipyridine and 1,10-phenylanthracene exhibit partial short-lived isomerization and only minor amounts of long-lived CO-loss products. This current study of two model derivatives with only CO ligands and one CN ligand on the irons exhibit no evidence of the long-lived excited state and produce CO-loss with multiple isomerization processes. Recently acquired TRIR spectra and Density Functional modelling for these two CN-containing model compounds (see below) will be presented.

Time-resolved infrared spectroscopy of CN-substituted [FeFe]-Hydrogenase model species

ACS Fall 2021

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