Structural and Kinetic Studies of Intermediates of a Biomimetic Diiron Proton-Reduction Catalyst

One-electron reduction and subsequent protonation of a biomimetic proton-reduction catalyst [FeFe­(μ-pdt)­(CO)₆] (pdt = propanedithiolate), <b>1</b>, were investigated by UV–vis and IR spectroscopy on a nano- to microsecond time scale. The study aimed to provide further insight into the proton-reduction cycle of this [FeFe]-hydrogenase model complex, which with its prototypical alkyldithiolate-bridged diiron core is widely employed as a molecular, precious metal-free catalyst for sustainable H₂ generation. The one-electron-reduced catalyst was obtained transiently by electron transfer from photogenerated [Ru­(dmb)₃]⁺ in the absence of proton sources or in the presence of acids (dichloro- or trichloroacetic acid or tosylic acid). The reduced catalyst and its protonation product were observed in real time by UV–vis and IR spectroscopy, leading to their structural characterization and providing kinetic data on the electron and proton transfer reactions. <b>1</b> features an intact (μ²,κ²-pdt)­(μ-H)­Fe₂ core in the reduced, <b>1^–</b>, and reduced-protonated states, <b>1H</b>, in contrast to the Fe–S bond cleavage upon the reduction of [FeFe­(bdt)­(CO)₆], <b>2</b>, with a benzenedithiolate bridge. The driving-force dependence of the rate constants for the protonation of <b>1^–</b> (<i>k</i>_pt = 7.0 × 10⁵, 1.3 × 10⁷, and 7.0 × 10⁷ M^–1 s^–1 for the three acids used in this study) suggests a reorganization energy >1 eV and indicates that hydride complex <b>1H</b> is formed by direct protonation of the Fe–Fe bond. The protonation of <b>1^–</b> is sufficiently fast even with the weaker acids, which excludes a rate-limiting role in light-driven H₂ formation under typical conditions

Direct Observation of Key Catalytic Intermediates in a Photoinduced Proton Reduction Cycle with a Diiron Carbonyl Complex

The structure and reactivity of intermediates in the photo­catalytic cycle of a proton reduction catalyst, [Fe₂­(bdt)­(CO)₆] (bdt = benzene­dithiolate), were investigated by time-resolved spectroscopy. The singly reduced catalyst [Fe₂­(bdt)­(CO)₆]⁻, a key intermediate in photo­catalytic H₂ formation, was generated by reaction with one-electron reductants in laser flash-quench experiments and could be observed spectroscopically on the nanoseconds to microseconds time scale. From UV/vis and IR spectroscopy, [Fe₂­(bdt)­(CO)₆]⁻ is readily distinguished from the two-electron reduced catalyst [Fe₂­(bdt)­(CO)₆]^2– that is obtained inevitably in the electro­chemical reduction of [Fe₂­(bdt)­(CO)₆]. For the dispro­portion­ation rate constant of [Fe₂­(bdt)­(CO)₆]⁻, an upper limit on the order of 10⁷ M^–1 s^–1 was estimated, which precludes a major role of [Fe₂­(bdt)­(CO)₆]^2– in photo­induced proton reduction cycles. Structurally [Fe₂­(bdt)­(CO)₆]⁻ is characterized by a rather asymmetrically distorted geometry with one broken Fe–S bond and six terminal CO ligands. Acids with p<i>K</i>_a ≤ 12.7 protonate [Fe₂­(bdt)­(CO)₆]⁻ with bimolecular rate constants of 4 × 10⁶, 7 × 10⁶, and 2 × 10⁸ M^–1 s^–1 (trichloroacetic, trifluoroacetic, and toluenesulfonic acids, respectively). The resulting hydride complex [Fe₂­(bdt)­(CO)₆H] is therefore likely to be an intermediate in photo­catalytic cycles. This intermediate resembles structurally and electronically the parent complex [Fe₂­(bdt)­(CO)₆], with very similar carbonyl stretching frequencies

Ultrafast Electron Transfer Between Dye and Catalyst on a Mesoporous NiO Surface

The combination of molecular dyes and catalysts with semiconductors into dye-sensitized solar fuel devices (DSSFDs) requires control of efficient interfacial and surface charge transfer between the components. The present study reports on the light-induced electron transfer processes of p-type NiO films cosensitized with coumarin C343 and a bioinspired proton reduction catalyst, [FeFe]­(mcbdt)­(CO)₆ (mcbdt = 3-carboxybenzene-1,2-dithiolate). By transient optical spectroscopy we find that ultrafast interfacial electron transfer (τ ≈ 200 fs) from NiO to the excited C343 (“hole injection”) is followed by rapid (<i>t</i>_1/2 ≈ 10 ps) and efficient surface electron transfer from C343^– to the coadsorbed [FeFe]­(mcbdt)­(CO)₆. The reduced catalyst has a clear spectroscopic signature that persists for several tens of microseconds, before charge recombination with NiO holes occurs. The demonstration of rapid surface electron transfer from dye to catalyst on NiO, and the relatively long lifetime of the resulting charge separated state, suggests the possibility to use these systems for photocathodes on DSSFDs

Sensitizer-Catalyst Assemblies for Water Oxidation

Two molecular assemblies with one Ru­(II)-polypyridine photosensitizer covalently linked to one Ru­(II)­(bda)­L₂ catalyst (<b>1</b>) (bda = 2,2′-bipyridine-6,6′-dicarboxylate) and two photosensitizers covalently linked to one catalyst (<b>2</b>) have been prepared using a simple C–C bond as the linkage. In the presence of sodium persulfate as a sacrificial electron acceptor, both of them show high activity for catalytic water oxidation driven by visible light, with a turnover number up to 200 for <b>2</b>. The linked photocatalysts show a lower initial yield for light driven oxygen evolution but a much better photostability compared to the three component system with separate sensitizer, catalyst and acceptor, leading to a much greater turnover number. Photocatalytic experiments and time-resolved spectroscopy were carried out to probe the mechanism of this catalysis. The linked catalyst in its Ru­(II) state rapidly quenches the sensitizer, predominantly by energy transfer. However, a higher stability under photocatalytic condition is shown for the linked sensitizer compared to the three component system, which is attributed to kinetic stabilization by rapid photosensitizer regeneration. Strategies for employment of the sensitizer-catalyst molecules in more efficient photocatalytic systems are discussed