2 research outputs found
Time-Resolved Infrared Studies of a Trimethylphosphine Model Derivative of [FeFe]-Hydrogenase
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 Fe<sub>2</sub>(μ-S<sub>2</sub>C<sub>3</sub>H<sub>6</sub>)Â(CO)<sub>4</sub>(PMe)<sub>2</sub> (<b>A</b>) in room temperature <i>n</i>-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 <b>A</b> were also examined using
density functional theory (DFT) computations. In <i>n</i>-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
Time-Resolved Vibrational Spectroscopy of [FeFe]-Hydrogenase Model Compounds
Model compounds have been found to structurally mimic
the catalytic hydrogen-producing active site of Fe–Fe hydrogenases
and are being explored as functional models. The time-dependent behavior
of Fe<sub>2</sub>(μ-S<sub>2</sub>C<sub>3</sub>H<sub>6</sub>)Â(CO)<sub>6</sub> and Fe<sub>2</sub>(μ-S<sub>2</sub>C<sub>2</sub>H<sub>4</sub>)Â(CO)<sub>6</sub> is reviewed and new ultrafast UV- and visible-excitation/IR-probe
measurements of the carbonyl stretching region are presented. Ground-state
and excited-state electronic and vibrational properties of Fe<sub>2</sub>(μ-S<sub>2</sub>C<sub>3</sub>H<sub>6</sub>)Â(CO)<sub>6</sub> were studied with density functional theory (DFT) calculations.
For Fe<sub>2</sub>(μ-S<sub>2</sub>C<sub>3</sub>H<sub>6</sub>)Â(CO)<sub>6</sub> excited with 266 nm, long-lived signals (Ï„
= 3.7 ± 0.26 μs) are assigned to loss of a CO ligand. For
355 and 532 nm excitation, short-lived (τ = 150 ± 17 ps)
bands are observed in addition to CO-loss product. Short-lived transient
absorption intensities are smaller for 355 nm and much larger for
532 nm excitation and are assigned to a short-lived photoproduct resulting
from excited electronic state structural reorganization of the Fe–Fe
bond. Because these molecules are tethered by bridging disulfur ligands,
this extended di-iron bond relaxes during the excited state decay.
Interestingly, and perhaps fortuitously, the time-dependent DFT-optimized
exited-state geometry of Fe<sub>2</sub>(μ-S<sub>2</sub>C<sub>3</sub>H<sub>6</sub>)Â(CO)<sub>6</sub> with a semibridging CO is reminiscent
of the geometry of the Fe<sub>2</sub>S<sub>2</sub> subcluster of the
active site observed in Fe–Fe hydrogenase X-ray crystal structures.
We suggest these wavelength-dependent excitation dynamics could significantly
alter potential mechanisms for light-driven catalysis