Ultrafast Photodynamics of Cyano-Functionalized [FeFe] Hydrogenase Model Compounds

Abstract

[FeFe] hydrogenases are efficient enzymes that produce hydrogen gas under mild conditions. Synthetic model compounds containing all CO or mixed CO/PMe ligands were previously studied by us and others with ultrafast ultraviolet or visible pump-infrared probe spectroscopy in an effort to better understand the function and interactions of the active site with light. Studies of anionic species containing cyano groups, which more closely match the biological active site, have been elusive. In this work, two model compounds dissolved in room-temperature acetonitrile solution were examined: [Fe(μ-SCH)(CO)(CN)] (1) and [Fe(μ-SCH)(CO)(CN)] (2). These species exhibit long-lived transient signals consistent with loss of one CO ligand with potential isomerization of newly formed ground electronic state photoproducts, as previously observed with all-CO and CO/PMe-containing models. We find no evidence for fast (ca. 150 ps) relaxation seen in the all-CO and CO/PMe compounds because of the absence of the metal-to-metal charge transfer band in the cyano-functionalized models. These results indicate that incorporation of cyano ligands may significantly alter the electronic properties and photoproducts produced immediately after photoexcitation, which may influence the catalytic activity of model compounds when attached to photosensitizers.

Photodynamics of Asymmetric Di-Iron-Cyano Hydrogenases Examined by Time-Resolved Mid-Infrared Spectroscopy

“…Species 1 exists as four possible ground-state isomers, where 1-1 and 1-2 (as well as 1-3 and 1-4) differ only by the direction of the propyl bridge atoms. For these species, as with others previously studied, 42,44,46 the bridge flip isomeric forms do not significantly affect room-temperature vibrational spectra. 2 can exist as two possible isomers as the species is ethyl-bridged and does not possess a bridge flip (numbered 2-1 and 2-3 for consistency with 1).…”

“…After approximately 20−30 ps, the product species return to their ground vibrational states and the intermediate spectra "relax" to the long-time spectrum, which is narrower and shifted to higher wavenumbers. Evidence of vibrational cooling has been observed in previously examined related molecules 42,43 but not as distinctly as in 1 (and 2, see below). In addition to vibrational relaxation, there is also rotational dynamics potentially contributing to the early-time dynamics, as observed in other similar model compounds.…”

“…43,44,47−49 The exception to this were the di-CN model species, which only showed an ∼20 ps decay followed by a long offset, similar to the mono-CN species studied here. 42 Interestingly, all compounds that exhibit a 150 ps decay component have a weak UV−vis absorption band around 450 nm (see Figure 3). In these spectra, both CN-functionalized species 1 and 5 ([Fe 2 (μ-S 2 C 3 H 6 )(CO) 4 (CN) 2 ] 2− ) do not show a distinct absorption band at 450 nm (although a long tail is present in this area that could obscure a very weak band), and their time-dependences are also missing the 150 ps decay component.…”

“…A 40 fs Ti:sapphire oscillator tuned to 800 nm seeded a home-built linear cavity Ti:sapphire regenerative amplifier. 42 The regenerative amplifier produced 800 nm, 80 fs, 700 μJ pulses at a frequency of 1 kHz. These pulses were used to pump a Spectra-Physics OPA-800F optical parametric amplifier (OPA).…”

Photochemistry and photoinhibition of the H-cluster of FeFe hydrogenases

“…The TRIR investigation of Fe 2 S 2 18-electron models evidenced at least one ultrafast CO dissociation process, with the formation of an 16-electron intermediate species, whose life time depends on the chemical nature of the ligands (e.g. 25 ps in the case of a phen substituted model 152 and much longer for a cyano-functionalized model 151 ). TDDFT calculations 144 suggests that dissociation after excitation in the UV domain results from the population of a dissociative Fe→CO charge transfer (CT) state, which involves an equatorial CO ligand.…”