Time-Resolved Infrared Studies of a Trimethylphosphine Model Derivative of [FeFe]-Hydrogenase

Abstract

Model compounds that structurally mimic the hydrogen-producing active site of [FeFe]-hydrogenases have been studied to explore potential ground-state electronic structure effects on reaction mechanisms compared to hexacarbonyl derivatives. The time-dependent behavior of Fe2(μ-S2C3H6)(CO)4(PMe)2 (A) in room temperature n-heptane and acetonitrile solutions was examined using various ultrafast UV and visible excitation pulses with broadband IR-probe spectroscopy of the carbonyl (CO) stretching region. Ground- and excited-state electronic and CO-stretching mode vibrational properties of the possible isomers of A were also examined using density functional theory (DFT) computations. In n-heptane, 355 and 532 nm excitation resulted in short-lived (135 ± 74 ps) bands assigned to excited-state, CO-loss photoproducts. These bands decay away, forming new long-lived absorptions that are likely a mixture of isomers of both three-CO and four-CO ground-state isomers. These new bands grow in with a time scale of 214 ± 119 ps and persist for more than 100 ns. In acetonitrile, similar results are seen with a 532 nm pump, but the 355 nm data lack evidence of the longer-lived bands. In either solvent, the 266 nm pump data seem to also lack longer-lived bands, but the intensities are significantly lower in this data, making firm conclusions more difficult. We suggest that these wavelength-dependent excitation dynamics significantly alter potential mechanisms and efficiencies for light-driven catalysis.