Investigation of the Ultrafast Dynamics Occurring during Unsensitized Photocatalytic H2 Evolution by an [FeFe]-Hydrogenase Subsite Analogue

Understanding the [NiFe] Hydrogenase Active Site Environment through Ultrafast Infrared and 2D-IR Spectroscopy of the Subsite Analogue K[CpFe(CO)(CN)2] in Polar and Protic Solvents

The [CpFe(CO)(CN)2]- unit is an excellent structural model for the Fe(CO)(CN)2 moiety of the active site found in [NiFe] hydrogenases. Ultrafast infrared (IR) pump-probe and 2D-IR spectroscopy have been used to study K[CpFe(CO)(CN)2] (M1) in a range of protic and polar solvents and as a dry film. Measurements of anharmonicity, intermode vibrational coupling strength, vibrational relaxation time, and solvation dynamics of the CO and CN stretching modes of M1 in H2O, D2O, methanol, dimethyl sulfoxide, and acetonitrile reveal that H-bonding to the CN ligands plays an important role in defining the spectroscopic characteristics and relaxation dynamics of the Fe(CO)(CN)2 unit. Comparisons of the spectroscopic and dynamic data obtained for M1 in solution and in a dry film with those obtained for the enzyme led to the conclusion that the protein backbone forms an important part of the bimetallic active site environment via secondary coordination sphere interactions.

Photochemical H2 Evolution from Bis(diphosphine)nickel Hydrides Enables Low-Overpotential Electrocatalysis

Molecules capable of both harvesting light and forming new chemical bonds hold promise for applications in the generation of solar fuels, but such first-row transition metal photoelectrocatalysts are lacking. Here we report nickel photoelectrocatalysts for H₂ evolution, leveraging visible-light-driven photochemical H₂ evolution from bis(diphosphine)nickel hydride complexes. A suite of experimental and theoretical analyses, including time-resolved spectroscopy and continuous irradiation quantum yield measurements, led to a proposed mechanism of H₂ evolution involving a short-lived singlet excited state that undergoes homolysis of the Ni-H bond. Thermodynamic analyses provide a basis for understanding and predicting the observed photoelectrocatalytic H₂ evolution by a 3d transition metal based catalyst. Of particular note is the dramatic change in the electrochemical overpotential: in the dark, the nickel complexes require strong acids and therefore high overpotentials for electrocatalysis; but under illumination, the use of weaker acids at the same applied potential results in a more than 500 mV improvement in electrochemical overpotential. New insight into first-row transition metal hydride photochemistry thus enables photoelectrocatalytic H₂ evolution without electrochemical overpotential (at the thermodynamic potential or 0 mV overpotential). This catalyst system does not require sacrificial chemical reductants or light-harvesting semiconductor materials and produces H₂ at rates similar to molecular catalysts attached to silicon.

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

Two anionic asymmetric Fe-Fe hydrogenase model compounds containing a single cyano (CN) and five carboxyl (CO) ligands, [Et₄N][Fe₂(μ-S₂C₃H₆)(CO)₅(CN)₁] and [Et₄N][Fe₂(μ-S₂C₂H₄)(CO)₅(CN)₁], dissolved in room-temperature acetonitrile, are examined. The molecular asymmetry affects the redox potentials of the central iron atoms, thus changing the photophysics and possible catalytic properties of the compounds. Femtosecond ultraviolet excitation with mid-infrared probe spectroscopy of the model compounds was employed to better understand the ultrafast dynamics of the enzyme-active site. Continuous ultraviolet lamp excitation with Fourier transform infrared (FTIR) spectroscopy was also used to explore stable product formation on the second timescale. For both model compounds, two timescales are observed; a 20-30 ps decay and the formation of a long-lived photoproduct. The picosecond decay is assigned to vibrational cooling and rotational dynamics, while the residual spectra remain for up to 300 ps, suggesting the formation of new photoproducts. Static FTIR spectroscopy yielded a different stable photoproduct than that observed on the ultrafast timescale. Density functional theory calculations simulated photoproducts for CO-loss and CN-loss isomers, and the resulting photoproduct spectra suggest that the picosecond transients arise from a complex mixture of isomerization after CO-loss, while dimerization and formation of a CN-containing Fe-CO-Fe bridged species are also considered.

Protonation and electrochemical properties of a bisphosphide diiron hexacarbonyl complex bearing amino groups on the phosphide bridge

A bisphosphide-bridged diiron hexacarbonyl complex 3 with NEt2 groups on the phosphide bridge was synthesized to examine a new proton relay system from the NEt2 group to the bridging hydride between the two iron centers. As a precursor of the bridging moiety, peri-Et2NP-PNEt2-bridged naphthylene 5 was synthesized by the reaction of 1,8-dilithionaphthylene with two equivalents of Cl2PNEt2 followed by reductive P-P bond formation by magnesium. The reaction of the diphosphine ligand 5 with Fe2(CO)9 gave the diiron hexacarbonyl complex 3, in which the P-P bond of the ligand was cleaved to form the bisphosphide-bridge. The molecular structure of 3 indicated that the trigonal plane of the NEt2 group was forced to face the Fe-Fe bond to avoid steric congestion with the naphthylene group linking the two phosphide groups. The NEt2 group could be protonated by p-toluenesulfonic acid. Density functional theory (DFT) calculations confirmed that the proton of the N(H)Et2 group adopted a position close to the bridging hydride. The DFT results for the ferrocene analogue 1, in which the 1,8-naphthylene group of 3 was replaced with the 1,1'-ferrocenylene group, also revealed that the most stable orientation of the protonated NHEt2 group was that in the protonated 3. As a result, electrochemical proton reduction reactions using complexes 1 and 3 proceeded with similar catalytic efficiencies. Unfortunately, the catalytic efficiencies (CEs) of these complexes were much lower than those of the complexes with a proton relay system of the terminal hydrogen, indicating that the reactive properties of the bridging hydride in the present proton relay system cannot exceed those of the terminal hydride.

Understanding the structure and dynamics of hydrogenases by ultrafast and two-dimensional infrared spectroscopy

Hydrogenases are valuable model enzymes for sustainable energy conversion approaches using H2, but rational utilization of these base-metal biocatalysts requires a detailed understanding of the structure and dynamics of their complex active sites. The intrinsic CO and CN- ligands of these metalloenzymes represent ideal chromophores for infrared (IR) spectroscopy, but structural and dynamic insight from conventional IR absorption experiments is limited. Here, we apply ultrafast and two-dimensional (2D) IR spectroscopic techniques, for the first time, to study hydrogenases in detail. Using an O2-tolerant [NiFe] hydrogenase as a model system, we demonstrate that IR pump-probe spectroscopy can explore catalytically relevant ligand bonding by accessing high-lying vibrational states. This ultrafast technique also shows that the protein matrix is influential in vibrational relaxation, which may be relevant for energy dissipation from the active site during fast reaction steps. Further insights into the relevance of the active site environment are provided by 2D-IR spectroscopy, which reveals equilibrium dynamics and structural constraints imposed on the H2-accepting intermediate of [NiFe] hydrogenases. Both techniques offer new strategies for uniquely identifying redox-structural states in complex catalytic mixtures via vibrational quantum beats and 2D-IR off-diagonal peaks. Together, these findings considerably expand the scope of IR spectroscopy in hydrogenase research, and new perspectives for the characterization of these enzymes and other (bio-)organometallic targets are presented.

Photodynamics of [FeFe]-Hydrogenase Model Compounds with Bidentate Heterocyclic Ligands

Two asymmetrically structured model compounds for the hydrogen-generating [Fe-Fe]-hydrogenase active site were investigated to determine the ultrafast photodynamics, structural intermediates, and photoproducts compared to more common symmetric di-iron species. The bidentate-ligand-containing compounds studied were Fe2(μ-S2C3H6)(CO)4(bipy), 1, and Fe2(μ-S2C3H6)(CO)4(phen), 2, in dilute room temperature acetonitrile solution and low-temperature 2Me-THF matrix isolation using static FTIR difference and time-resolved infrared spectroscopic methods (TRIR). Ultraviolet-visible spectra were also compared to time-dependent density functional theory (TD-DFT) to ascertain the orbital origins of long wavelength electronic absorption features. The spectroscopic evidence supports the conclusions that only a propyl-bridge flip occurs in low-temperature matrix, while early time CO ejection leads to the formation of solvated isomeric species on the 25 ps time scale in room temperature solution.

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

[FeFe] hydrogenases are efficient enzymes that produce hydrogen gas under mild conditions. Synthetic model compounds containing all CO or mixed CO/PMe3 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: [Fe2(μ-S2C3H6)(CO)4(CN)2]2- (1) and [Fe2(μ-S2C2H4)(CO)4(CN)2]2- (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/PMe3-containing models. We find no evidence for fast (ca. 150 ps) relaxation seen in the all-CO and CO/PMe3 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.

Dynamic Flexibility of Hydrogenase Active Site Models Studied with 2D-IR Spectroscopy

Hydrogenase enzymes enable organisms to use H₂ as an energy source, having evolved extremely efficient biological catalysts for the reversible oxidation of molecular hydrogen. Small-molecule mimics of these enzymes provide both simplified models of the catalysis reactions and potential artificial catalysts that might be used to facilitate a hydrogen economy. We have studied two diiron hydrogenase mimics, μ-pdt-[Fe(CO)₃]₂ and μ-edt-[Fe(CO)₃]₂ (pdt = propanedithiolate, edt = ethanedithiolate), in a series of alkane solvents and have observed significant ultrafast spectral dynamics using two-dimensional infrared (2D-IR) spectroscopy. Since solvent fluctuations in nonpolar alkanes do not lead to substantial electrostatic modulations in a solute's vibrational mode frequencies, we attribute the spectral diffusion dynamics to intramolecular flexibility. The intramolecular origin is supported by the absence of any measurable solvent viscosity dependence, indicating that the frequency fluctuations are not coupled to the solvent motional dynamics. Quantum chemical calculations reveal a pronounced coupling between the low-frequency torsional rotation of the carbonyl ligands and the terminal CO stretching vibrations. The flexibility of the CO ligands has been proposed to play a central role in the catalytic reaction mechanism, and our results highlight that the CO ligands are highly flexible on a picosecond time scale.

Synthesis and Photophysical Study of a [NiFe] Hydrogenase Biomimetic Compound Covalently Linked to a Re-diimine Photosensitizer

The synthesis, photophysics, and photochemistry of a linked dyad ([Re]-[NiFe₂]) containing an analogue ([NiFe₂]) of the active site of [NiFe] hydrogenase, covalently bound to a Re-diimine photosensitizer ([Re]), are described. Following excitation, the mechanisms of electron transfer involving the [Re] and [NiFe₂] centers and the resulting decomposition were investigated. Excitation of the [Re] center results in the population of a diimine-based metal-to-ligand charge transfer excited state. Reductive quenching by NEt₃ produces the radically reduced form of [Re], [Re]⁻ (k_q = 1.4 ± 0.1 × 10⁷ M^–1 s^–1). Once formed, [Re]⁻ reduces the [NiFe₂] center to [NiFe₂]⁻, and this reduction was followed using time-resolved infrared spectroscopy. The concentration dependence of the electron transfer rate constants suggests that both inter- and intramolecular electron transfer pathways are involved, and the rate constants for these processes have been estimated (k_inter = 5.9 ± 0.7 × 10⁸ M^–1 s^–1, k_intra = 1.5 ± 0.1 × 10⁵ s^–1). For the analogous bimolecular system, only intermolecular electron transfer could be observed (k_inter = 3.8 ± 0.5 × 10⁹ M^–1 s^–1). Fourier transform infrared spectroscopic studies confirms that decomposition of the dyad occurs upon prolonged photolysis, and this appears to be a major factor for the low activity of the system toward H₂ production in acidic conditions.

Excitation of the [Re] center in the linked-dyad complex ([Re]-[NiFe₂]) populates the ³MLCT excited state, and reductive quenching by NEt₃ produces [Re]⁻. [Re]⁻ reduces the [NiFe₂] center to [NiFe₂]⁻ via inter- and intramolecular electron transfer pathways (k_inter = 5.9 ± 0.7 × 10⁸ M⁻¹ s⁻¹, k_intra = 1.5 ± 0.1 × 10⁵ s⁻¹). For the analogous bimolecular system, where only intermolecular electron transfer could be observed, k_inter = 3.8 ± 0.5 × 10⁹ M⁻¹ s⁻¹.