Naturally-occurring hydrogenases catalyze the reversible reaction of H+ to H2. To develop a hydrogen source that is not reliant on fossil fuels, synthetic [FeFe]-hydrogenase models are being studied. These compounds have a wing-like structure with a Fe-Fe bond, three pendant ligands on each Fe, two sulfur atoms bridging the irons, and a short organic chain connecting the sulfurs. The turnover rates for these compounds are substantially lower than that of the enzyme, although varying the active site ligands can alter the catalytic properties. One active area of research is to pair hydrogenase models with visible/UV absorbing chromophores, in a mechanism inspired by photosynthesis. To successfully create photo-driven hydrogenase catalysts, their behavior after light exposure must be understood. Our research involved studying Fe2(μ-S2C2H4)(CO)4(PMe3)2 using time-resolved ultraviolet/visible pump, infrared probe spectroscopy. Fe2(μ-S2C2H4)(CO)4(PMe3)2, was dissolved in acetonitrile or n-heptane and excited with 532, 355, and 266 nm light. In the 355 and 532 nm data in both solvents, the bleaches from loss of the original molecule and the photoproduct absorptions both decay to roughly 50% of their original intensities with an average time constant of 360 ± 91 ps. The remaining signals last out to microsecond timescales. These decay behaviors are similar to those observed in the propyl-bridged phosphine model compound, Fe2(μ-S2C3H6)(CO)4(PMe3)2. The assignments of the signals to molecular behaviors are also expected to be similar. The short-lived signals likely correspond to excited state, CO-loss photoproducts. These excited state photoproducts decay back to the ground electronic state, with roughly half recombining with a CO and returning to the original isomer. The long-lasting signals seem to result from a complex mixture of 3-CO and 4-CO molecules in their ground electronic states. The 4-CO molecules isomerize as they relax, resulting in a mixture of isomers significantly different from those found in the original sample.

Time-resolved infrared spectrocopy of [FeFe]-hydrogenase model compounds

249th National Meeting (2015)

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