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

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

Tuning the Electronics of Bis(tridentate)ruthenium(II) Complexes with Long-Lived Excited States: Modifications to the Ligand Skeleton beyond Classical Electron Donor or Electron Withdrawing Group Decorations

A series of homoleptic bis­(tridentate) [Ru­(L)₂]²⁺ (<b>1</b>, <b>3</b>) and heteroleptic [Ru­(L)­(dqp)]²⁺ complexes (<b>2</b>, <b>4</b>) [L = dqxp (<b>1</b>, <b>2</b>) or dNinp (<b>3</b>, <b>4</b>); dqxp = 2,6-di­(quinoxalin-5-yl)­pyridine, dNinp = 2,6-di­(<i>N</i>-7-azaindol-1-yl)­pyridine, dqp = 2,6-di­(quinolin-8-yl)­pyridine] was prepared and in the case of <b>2</b> and <b>4</b> structurally characterized. The presence of dqxp and dNinp in <b>1</b>–<b>4</b> result in anodically shifted oxidation potentials of the Ru^3+/2+ couple compared to that of the archetypical [Ru­(dqp)₂]²⁺ (<b>5</b>), most pronounced for [Ru­(dqxp)₂]²⁺ (<b>1</b>) with a shift of +470 mV. These experimental findings are corroborated by DFT calculations, which show contributions to the complexes’ HOMOs by the polypyridine ligands, thereby stabilizing the HOMOs and impeding electron extraction. Complex <b>3</b> exhibits an unusual electronic absorption spectrum with its lowest energy maximum at 382 nm. TD-DFT calculations suggest that this high-energy transition is caused by a localization of the LUMO on the central pyridine fragments of the dNinp ligands in <b>3</b>, leaving the lateral azaindole units merely spectator fragments. The opposite is the case in <b>1</b>, where the LUMO experiences large stabilization by the lateral quinoxalines. Owing to the differences in LUMO energies, the complexes’ reduction potentials differ by about 900 mV [<i>E</i>_1/2(<b>1</b>^2+/1+) = −1.17 V, <i>E</i>_c,p(<b>3</b>^2+/1+) = −2.06 V vs Fc^+/0]. As complexes <b>1</b>–<b>4</b> exhibit similar excited state energies of around 1.80 V, the variations of the lateral heterocycles allow the tuning of the complexes’ excited state oxidation strengths over a range of 900 mV. Complex <b>1</b> is the strongest excited state oxidant of the series, exceeding even [Ru­(bpy)₃]²⁺ by more than 200 mV. At room temperature, complex <b>3</b> is nonemissive, whereas complexes <b>1</b>, <b>2</b>, and <b>4</b> exhibit excited state lifetimes of 255, 120, and 1570 ns, respectively. The excited state lifetimes are thus somewhat shortened compared to that of <b>5</b> (3000 ns) but still acceptable to qualify the complexes as photosensitizers in light-induced charge-transfer schemes, especially for those that require high oxidative power

Fe^II Hexa <i>N</i>‑Heterocyclic Carbene Complex with a 528 ps Metal-to-Ligand Charge-Transfer Excited-State Lifetime

The iron carbene complex [Fe^II(btz)₃]­(PF₆)₂ (where btz = 3,3′-dimethyl-1,1′-bis­(<i>p</i>-tolyl)-4,4′-bis­(1,2,3-triazol-5-ylidene)) has been synthesized, isolated, and characterized as a low-spin ferrous complex. It exhibits strong metal-to-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a ³MLCT state with a 528 ps excited-state lifetime in CH₃CN solution that is more than one order of magnitude longer compared with the MLCT lifetime of any previously reported Fe^II complex. The low potential of the [Fe­(btz)₃]³⁺/[Fe­(btz)₃]²⁺ redox couple makes the ³MLCT state of [Fe^II(btz)₃]²⁺ a potent photoreductant that can be generated by light absorption throughout the visible spectrum. Taken together with our recent results on the [Fe^III(btz)₃]³⁺ form of this complex, these results show that the Fe^II and Fe^III oxidation states of the same Fe­(btz)₃ complex feature long-lived MLCT and LMCT states, respectively, demonstrating the versatility of iron <i>N-</i>heterocyclic carbene complexes as promising light-harvesters for a broad range of oxidizing and reducing conditions

How Rigidity and Conjugation of Bidentate Ligands Affect the Geometry and Photophysics of Iron <i>N</i>‑Heterocyclic Complexes: A Comparative Study

Two iron complexes featuring the bidentate, nonconjugated N-heterocyclic carbene (NHC) 1,1′-methylenebis(3-methylimidazol-2-ylidene) (mbmi) ligand, where the two NHC moieties are separated by a methylene bridge, have been synthesized to exploit the combined influence of geometric and electronic effects on the ground- and excited-state properties of homoleptic FeIII-hexa-NHC [Fe(mbmi)3](PF6)3 and heteroleptic FeII-tetra-NHC [Fe(mbmi)2(bpy)](PF6)2 (bpy = 2,2′-bipyridine) complexes. They are compared to the reported FeIII-hexa-NHC [Fe(btz)3](PF6)3 and FeII-tetra-NHC [Fe(btz)2(bpy)](PF6)2 complexes containing the conjugated, bidentate mesoionic NHC ligand 3,3′-dimethyl-1,1′-bis(p-tolyl)-4,4′-bis(1,2,3-triazol-5-ylidene) (btz). The observed geometries of [Fe(mbmi)3](PF6)3 and [Fe(mbmi)2(bpy)](PF6)2 are evaluated through L–Fe–L bond angles and ligand planarity and compared to those of [Fe(btz)3](PF6)3 and [Fe(btz)2(bpy)](PF6)2. The FeII/FeIII redox couples of [Fe(mbmi)3](PF6)3 (−0.38 V) and [Fe(mbmi)2(bpy)](PF6)2 (−0.057 V, both vs Fc+/0) are less reducing than [Fe(btz)3](PF6)3 and [Fe(btz)2(bpy)](PF6)2. The two complexes show intense absorption bands in the visible region: [Fe(mbmi)3](PF6)3 at 502 nm (ligand-to-metal charge transfer, 2LMCT) and [Fe(mbmi)2(bpy)](PF6)2 at 410 and 616 nm (metal-to-ligand charge transfer, 3MLCT). Lifetimes of 57.3 ps (2LMCT) for [Fe(mbmi)3](PF6)3 and 7.6 ps (3MLCT) for [Fe(mbmi)2(bpy)](PF6)2 were probed and are somewhat shorter than those for [Fe(btz)3](PF6)3 and [Fe(btz)2(bpy)](PF6)2. [Fe(mbmi)3](PF6)3 exhibits photoluminescence at 686 nm (2LMCT) in acetonitrile at room temperature with a quantum yield of (1.2 ± 0.1) × 10–4, compared to (3 ± 0.5) × 10–4 for [Fe(btz)3](PF6)3

Toward Highlighting the Ultrafast Electron Transfer Dynamics at the Optically Dark Sites of Photocatalysts

Building a detailed understanding of the structure–function relationship is a crucial step in the optimization of molecular photocatalysts employed in water splitting schemes. The optically dark nature of their active sites usually prevents a complete mapping of the photoinduced dynamics. In this work, transient X-ray absorption spectroscopy highlights the electronic and geometric changes that affect such a center in a bimetallic model complex. Upon selective excitation of the ruthenium chromophore, the cobalt moiety is reduced through intramolecular electron transfer and undergoes a spin flip accompanied by an average bond elongation of 0.20 ± 0.03 Å. The analysis is supported by simulations based on density functional theory structures (B3LYP*/TZVP) and FEFF 9.0 multiple scattering calculations. More generally, these results exemplify the large potential of the technique for tracking elusive intermediates that impart unique functionalities in photochemical devices