Deciphering Photoinduced Catalytic Reaction Mechanisms in Natural and Artificial Photosynthetic Systems on Multiple Temporal and Spatial Scales Using X-ray Probes

Utilization of renewable energies for catalytically generating value-added chemicals is highly desirable in this era of rising energy demands and climate change impacts. Artificial photosynthetic systems or photocatalysts utilize light to convert abundant CO2, H2O, and O2 to fuels, such as carbohydrates and hydrogen, thus converting light energy to storable chemical resources. The emergence of intense X-ray pulses from synchrotrons, ultrafast X-ray pulses from X-ray free electron lasers, and table-top laser-driven sources over the past decades opens new frontiers in deciphering photoinduced catalytic reaction mechanisms on the multiple temporal and spatial scales. Operando X-ray spectroscopic methods offer a new set of electronic transitions in probing the oxidation states, coordinating geometry, and spin states of the metal catalytic center and photosensitizers with unprecedented energy and time resolution. Operando X-ray scattering methods enable previously elusive reaction steps to be characterized on different length scales and time scales. The methodological progress and their application examples collected in this review will offer a glimpse into the accomplishments and current state in deciphering reaction mechanisms for both natural and synthetic systems. Looking forward, there are still many challenges and opportunities at the frontier of catalytic research that will require further advancement of the characterization techniques.

Enzymatic and Bioinspired Systems for Hydrogen Production

The extraordinary potential of hydrogen as a clean and sustainable fuel has sparked the interest of the scientific community to find environmentally friendly methods for its production. Biological catalysts are the most attractive solution, as they usually operate under mild conditions and do not produce carbon-containing byproducts. Hydrogenases promote reversible proton reduction to hydrogen in a variety of anoxic bacteria and algae, displaying unparallel catalytic performances. Attempts to use these sophisticated enzymes in scalable hydrogen production have been hampered by limitations associated with their production and stability. Inspired by nature, significant efforts have been made in the development of artificial systems able to promote the hydrogen evolution reaction, via either electrochemical or light-driven catalysis. Starting from small-molecule coordination compounds, peptide- and protein-based architectures have been constructed around the catalytic center with the aim of reproducing hydrogenase function into robust, efficient, and cost-effective catalysts. In this review, we first provide an overview of the structural and functional properties of hydrogenases, along with their integration in devices for hydrogen and energy production. Then, we describe the most recent advances in the development of homogeneous hydrogen evolution catalysts envisioned to mimic hydrogenases.

Five decades of metalloenzymology

Metalloenzymes have been detailed in The Enzymes since its inception over half a century ago. Here, I review selected metal-containing enzyme highlights from early chapters in this series and I describe advances made since those contributions. Three topics are emphasized: nickel-containing enzymes, Fe(II)/2-oxoglutarate-dependent oxygenases, and enzymes containing non-canonical iron-sulfur clusters.

Unravelling the Potential of Ylides in Stabilizing Low-Valent Group 13 Compounds: Theoretical Predictions of Stable, Five-Membered Group 13 (Aluminum and Gallium) Carbenoids Capable of Small-Molecule Activation

Computational investigations provide evidence toward the remarkable ability of strongly electron-donating ylidic functionalities in stabilizing singlet group 13 carbenoids with promising ligand properties. All of the proposed carbenoids are found to be considerably nucleophilic and possess significant singlet-triplet energy separation values. The calculated activation barriers and reaction free energies obtained for the cleavage of different enthalpically strong bonds by these carbenoids are found to be either comparable to or lower than those of the experimentally evaluated aluminum and gallium carbenoids, thereby indicating their potential in small-molecule activation.

Second and Outer Coordination Sphere Effects in Nitrogenase, Hydrogenase, Formate Dehydrogenase, and CO Dehydrogenase

Gases like H2, N2, CO2, and CO are increasingly recognized as critical feedstock in "green" energy conversion and as sources of nitrogen and carbon for the agricultural and chemical sectors. However, the industrial transformation of N2, CO2, and CO and the production of H2 require significant energy input, which renders processes like steam reforming and the Haber-Bosch reaction economically and environmentally unviable. Nature, on the other hand, performs similar tasks efficiently at ambient temperature and pressure, exploiting gas-processing metalloenzymes (GPMs) that bind low-valent metal cofactors based on iron, nickel, molybdenum, tungsten, and sulfur. Such systems are studied to understand the biocatalytic principles of gas conversion including N2 fixation by nitrogenase and H2 production by hydrogenase as well as CO2 and CO conversion by formate dehydrogenase, carbon monoxide dehydrogenase, and nitrogenase. In this review, we emphasize the importance of the cofactor/protein interface, discussing how second and outer coordination sphere effects determine, modulate, and optimize the catalytic activity of GPMs. These may comprise ionic interactions in the second coordination sphere that shape the electron density distribution across the cofactor, hydrogen bonding changes, and allosteric effects. In the outer coordination sphere, proton transfer and electron transfer are discussed, alongside the role of hydrophobic substrate channels and protein structural changes. Combining the information gained from structural biology, enzyme kinetics, and various spectroscopic techniques, we aim toward a comprehensive understanding of catalysis beyond the first coordination sphere.

Bioinorganic Chemistry on Electrodes: Methods to Functional Modeling

One of the major goals of bioinorganic chemistry has been to mimic the function of elegant metalloenzymes. Such functional modeling has been difficult to attain in solution, in particular, for reactions that require multiple protons and multiple electrons (nH⁺/ne^-). Using a combination of heterogeneous electrochemistry, electrode and molecule design one may control both electron transfer (ET) and proton transfer (PT) of these nH⁺/ne^- reactions. Such control can allow functional modeling of hydrogenases (H⁺ + e^- → 1/2 H₂), cytochrome c oxidase (O₂ + 4 e^- + 4 H⁺ → 2 H₂O), monooxygenases (RR'CH₂ + O₂ + 2 e^- + 2 H⁺ → RR'CHOH + H₂O) and dioxygenases (S + O₂ → SO₂; S = organic substrate) in aqueous medium and at room temperatures. In addition, these heterogeneous constructs allow probing unnatural bioinspired reactions and estimation of the inner- and outer-sphere reorganization energy of small molecules and proteins.

Ionization Energies and Redox Potentials of Hydrated Transition Metal Ions: Evaluation of Domain-Based Local Pair Natural Orbital Coupled Cluster Approaches

Hydrated transition metal ions are prototypical systems that can be used to model properties of transition metals in complex chemical environments. These seemingly simple systems present challenges for computational chemistry and are thus crucial in evaluations of quantum chemical methods for spin-state and redox energetics. In this work, we explore the applicability of the domain-based pair natural orbital implementation of coupled cluster (DLPNO-CC) theory to the calculation of ionization energies and redox potentials for hydrated ions of all first transition row (3d) metals in the 2+/3+ oxidation states, in connection with various solvation approaches. In terms of model definition, we investigate the construction of a minimally explicitly hydrated quantum cluster with a first and second hydration layer. We report on the convergence with respect to the coupled cluster expansion and the PNO space, as well as on the role of perturbative triple excitations. A recent implementation of the conductor-like polarizable continuum model (CPCM) for the DLPNO-CC approach is employed to determine self-consistent redox potentials at the coupled cluster level. Our results establish conditions for the convergence of DLPNO-CCSD(T) energetics and stress the absolute necessity to explicitly consider the second solvation sphere even when CPCM is used. The achievable accuracy for redox potentials of a practical DLPNO-based approach is, on average, 0.13 V. Furthermore, multilayer approaches that combine a higher-level DLPNO-CCSD(T) description of the first solvation sphere with a lower-level description of the second solvation layer are investigated. The present work establishes optimal and transferable methodological choices for employing DLPNO-based coupled cluster theory, the associated CPCM implementation, and cost-efficient multilayer derivatives of the approach for open-shell transition metal systems in complex environments.

Research and economic perspectives on an integrated biorefinery approach for the simultaneous production of polyhydroxyalkanoates and biohydrogen

Alarming environmental impacts have been resulted across the globe due to the recovery and consumption of fossil fuels. The elevated global carbon footprint has paved the way to an alternative to combat the prevalent pollution. On the other hand, the fossil-based plastics produced from the byproducts of petroleum remain intact in the environment leading to pollution. Fossil abated bioproducts are in high demand due to the increase in pollution. This call to utilize feedstock for simultaneous production of biologically useful products through carbon capture utilisation where the leftover carbon-rich substrate is converted into usable chemicals like bioplastics, methanol, urea and various other industrially essential components. The present review extensively focuses on the research and economic perspectives of an integrated biorefinery and addresses technical breaches, bottlenecks, and efficient strategies for the simultaneous production of biohydrogen and polyhydroxyalkanoates.

H2 activation by hydrogenase-inspired NiFe catalyst using frustrated Lewis pair: effect of buffer and halide ion in the heterolytic H-H bond cleavage

Hydrogen is a clean fuel alternative to fossil fuels, and it is vital to develop catalysts for its efficient activation and production. We investigate the reaction mechanism of H₂ activation in an aqueous solution by the recently developed NiFe complex (Ogo et al. Sci. Adv. 2020, 6, eaaz8181) using density functional theory (DFT) calculation. Our computational results showed that H₂ is activated using frustrated Lewis pair. That is, H₂ binds to the Fe site of the NiFe complex, acting as a Lewis acid, while the added buffer, acting as Lewis base, abstracts protons to form a hydride complex. Furthermore, the higher basicity in the proton abstraction reaction characterises reaction more exergonic and lowers the reaction barrier. In addition, in the proton abstraction by the water molecule, the reaction barrier was lowered when anion such as Cl⁻ is in the vicinity of the water. Understanding the chemical species that contribute to the catalytic process in cooperation with the metal catalyst at the atomic level should help to maximise the function of the catalyst.

Hydrogen is a clean fuel alternative to fossil fuels, and it is vital to develop catalysts for its efficient activation and production.

Deuteration mechanistic studies of hydrogenase mimics

The role of deuterium in disentangling key steps of the mechanisms of H₂ activation by mimics of hydrogenases is presented. These studies have allowed to a better understanding of the mode of action of the natural enzymes and their mimics.

Biochemical and artificial pathways for the reduction of carbon dioxide, nitrite and the competing proton reduction: effect of 2nd sphere interactions in catalysis

Reduction of oxides and oxoanions of carbon and nitrogen are of great contemporary importance as they are crucial for a sustainable environment.

Activation of small molecules by cyclic alkyl amino silylenes (CAASis) and germylenes (CAAGes): a theoretical study

Quantum chemical calculations have been carried out on a series of skeletally modified cyclic alkyl amino silylenes (CAASis) and germylenes (CAAGes) to understand their ligand properties and reactivity towards the activation of a variety of small molecules. The installation of boron or silicon atoms into the ring framework of these silylenes/germylenes led to a dramatic increase in their σ-basicity while the incorporation of ylidic moieties resulted in a sharp reduction of their π-acidity although it did help in increasing the electron donation ability. The calculated values of energy barriers for the activation of H-H, N-H, C-H and Si-H bonds by many of the cyclic silylenes considered here are found to be comparable to those for experimentally evaluated systems, indicating the potential of these computationally designed molecules in small molecule activation and calling for synthetic efforts towards their isolation. Furthermore, activations employing CAAGes are found to be more demanding than those with CAASis which may be attributed to the significantly lower Lewis basicity of the former than the latter.

Electrocatalytic Reduction of Nitrogen to Hydrazine Using a Trinuclear Nickel Complex

Activation and reduction of N₂ have been a major challenge to chemists and the focus since now has mostly been on the synthesis of NH₃. Alternatively, reduction of N₂ to hydrazine is desirable because hydrazine is an excellent energy vector that can release the stored energy very conveniently without the need for catalysts. To date, only one molecular catalyst has been reported to be able to reduce N₂ to hydrazine chemically. A trinuclear T-shaped nickel thiolate molecular complex has been designed to activate dinitrogen. The electrochemically generated all Ni(I) state of this molecule can reduce N₂ in the presence of PhOH as a proton donor. Hydrazine is detected as the only nitrogen-containing product of the reaction, along with gaseous H₂. The complex reported here is selective for the 4e^-/4H⁺ reduction of nitrogen to hydrazine with a minor overpotential of ∼300 mV.

Biosynthetic Approaches towards the Design of Artificial Hydrogen-Evolution Catalysts

Hydrogen is a clean and sustainable form of fuel that can minimize our heavy dependence on fossil fuels as the primary energy source. The need of finding greener ways to generate H₂ gas has ignited interest in the research community to synthesize catalysts that can produce H₂ by the reduction of H⁺ . The natural H₂ producing enzymes hydrogenases have served as an inspiration to produce catalytic metal centers akin to these native enzymes. In this article we describe recent advances in the design of a unique class of artificial hydrogen evolving catalysts that combine the features of the active site metal(s) surrounded by a polypeptide component. The examples of these biosynthetic catalysts discussed here include i) assemblies of synthetic cofactors with native proteins; ii) peptide-appended synthetic complexes; iii) substitution of native cofactors with non-native cofactors; iv) metal substitution from rubredoxin; and v) a reengineered Cu storage protein into a Ni binding protein. Aspects of key design considerations in the construction of these artificial biocatalysts and insights gained into their chemical reactivity are discussed.

Heterologous Hydrogenase Overproduction Systems for Biotechnology-An Overview

Hydrogenases are complex metalloenzymes, showing tremendous potential as H2-converting redox catalysts for application in light-driven H2 production, enzymatic fuel cells and H2-driven cofactor regeneration. They catalyze the reversible oxidation of hydrogen into protons and electrons. The apo-enzymes are not active unless they are modified by a complicated post-translational maturation process that is responsible for the assembly and incorporation of the complex metal center. The catalytic center is usually easily inactivated by oxidation, and the separation and purification of the active protein is challenging. The understanding of the catalytic mechanisms progresses slowly, since the purification of the enzymes from their native hosts is often difficult, and in some case impossible. Over the past decades, only a limited number of studies report the homologous or heterologous production of high yields of hydrogenase. In this review, we emphasize recent discoveries that have greatly improved our understanding of microbial hydrogenases. We compare various heterologous hydrogenase production systems as well as in vitro hydrogenase maturation systems and discuss their perspectives for enhanced biohydrogen production. Additionally, activities of hydrogenases isolated from either recombinant organisms or in vivo/in vitro maturation approaches were systematically compared, and future perspectives for this research area are discussed.

The oxygen reactivity of an artificial hydrogenase designed in a reengineered copper storage protein

The O₂ reactivity of an artificial biomolecular hydrogenase, the nickel binding protein (NBP) is investigated.

Redesign of a Copper Storage Protein into an Artificial Hydrogenase

We report the construction of an artificial hydrogenase (ArH) by reengineering a Cu storage protein (Cspl) into a Ni-binding protein (NBP) employing rational metalloprotein design. The hypothesis driven design approach involved deleting existing Cu sites of Csp1 and identification of a target tetrathiolate Ni binding site within the protein scaffold followed by repacking the hydrophobic core. Guided by modeling, the NBP was expressed and purified in high purity. NBP is a well-folded and stable construct displaying native-like unfolding behavior. Spectroscopic and computational studies indicated that the NBP bound nickel in a distorted square planar geometry that validated the design. Ni(II)-NBP is active for photo-induced H₂ evolution following a reductive quenching mechanism. Ni(II)-NBP catalyzed H⁺ reduction to H₂ gas electrochemically as well. Analysis of the catalytic voltammograms established a proton-coupled electron transfer (PCET) mechanism. Electrolysis studies confirmed H₂ evolution with quantitative Faradaic yields. Our studies demonstrate an important scope of rational metalloprotein design that allows imparting functions into protein scaffolds that have natively not evolved to possess the same function of the target metalloprotein constructs.

Nickel-Iron Dithiolato Hydrides Derived from H2 Activation by Their μ-Hydroxo Ligand-Containing Analogues

Here, we describe the synthesis and characterization of the two μ-hydrido models of [NiFe]H₂ases prepared from a new type of H₂ activation by the corresponding two μ-hydroxo models. In addition, another μ-hydrido model prepared by the anionic exchange between one of the two μ-hydrido models and KPF₆ is also reported. Interestingly, the synthesis of the two μ-hydrido models from H₂ activation represents the first functional modeling of the H₂ activation catalyzed by the [NiFe]H₂ase (Ni-SIr)_I state to give the Ni-R state.

Hydrogenases

Hydrogenases catalyze the simple yet important interconversion between H₂ and protons and electrons. Found throughout prokaryotes, lower eukaryotes, and archaea, hydrogenases are used for a variety of redox and signaling purposes and are found in many different forms. This diverse group of metalloenzymes is divided into [NiFe], [FeFe], and [Fe] variants, based on the transition metal contents of their active sites. A wide array of biochemical and spectroscopic methods has been used to elucidate hydrogenases, and this along with a general description of the main enzyme types and catalytic mechanisms is discussed in this chapter.

Reactivation of the Ready and Unready Oxidized States of [NiFe]-Hydrogenases: Mechanistic Insights from DFT Calculations

The apparently simple dihydrogen formation from protons and electrons (2H⁺ + 2e^- ⇄ H₂) is one of the most challenging reactions in nature. It is catalyzed by metalloenzymes of amazing complexity, called hydrogenases. A better understanding of the chemistry of these enzymes, especially that of the [NiFe]-hydrogenases subgroup, has important implications for production of H₂ as alternative sustainable fuel. In this work, reactivation mechanism of the oxidized and inactive Ni-B and Ni-A states of the [NiFe]-hydrogenases active site has been investigated using density functional theory. Results obtained from this study show that one-electron reduction and protonation of the active site promote the removal of the bridging hydroxide ligand contained in Ni-B and Ni-A. However, this process is sufficient to activate only the Ni-B state. H₂ binding to the active site is required to convert Ni-A to the active Ni-SI_a state. Here, we also propose a reasonable structure for the spectroscopically well-characterized Ni-SI_r and Ni-SU species, formed respectively from the one-electron reduction of Ni-B and Ni-A. Ni-SI_r, depending on the pH at which the reaction occurs, features a bridging hydroxide ligand or a water molecule terminally coordinated to the Ni atom, whereas in Ni-SU a water molecule is terminally coordinated to the Fe atom, and the Cys64 residue is oxidized to sulfenate. The sulfenate oxygen atom in the Ni-A state affects the stereoelectronic properties of the binuclear cluster by modifying the coordination geometry of Ni, and consequently, by switching the regiochemistry of H₂O and H₂ binding from the Ni to the Fe atom. This effect is predicted to be at the origin of the different reactivation kinetics of the oxidized and inactive Ni-B and Ni-A states.

Advanced surface resonators for electron spin resonance of single microcrystals

Electron spin resonance (ESR) spectroscopy of paramagnetic species in single crystals is a powerful tool for characterizing the latter's magnetic interaction parameters in detail. Conventional ESR systems are optimized for millimeter-size samples and make use of cavities and resonators that accommodate tubes and capillaries in the range 1-5 mm. Unfortunately, in the case of many interesting materials such as enzymes and inorganic catalytic materials (e.g., zeolites), single crystals can only be obtained in micron-scale sizes (1-200 µm). To boost ESR sensitivity and to enable experiments on microcrystals, the ESR resonator needs to be adapted to the size and shape of these specific samples. Here, we present a unique family of miniature surface resonators, known as "ParPar" resonators, whose mode volume and shape are optimized for such micron-scale single crystals. This approach significantly improves upon the samples' filling factor and thus enables the measurement of much smaller crystals than was previously possible. We present here the design of such resonators with a typical mode dimension of 20-50 µm, as well as details about their fabrication and testing methods. The devices' resonant mode(s) are characterized by ESR microimaging and compared to the theoretical calculations. Moreover, experimental ESR spectra of single microcrystals with typical sizes of ∼25-50 µm are presented. The measured spin sensitivity for the 50-µm resonator at cryogenic temperatures of 50 K is found to be ∼1.8 × 10⁶ spins/G √Hz for a Cu-doped single crystal sample that is representative of many biological samples of relevance.

Oxygen uptake in complexes related to [NiFeS]- and [NiFeSe]-hydrogenase active sites

The NiFe hydrogenase biomimetics are protected from oxygen invaders by sulfur and selenium castle guards.

A biomimetic study for S/Se oxygenation in Ni(μ-EPh)(μ-SN₂)Fe, (E = S or Se; SN₂ = Me-diazacycloheptane-CH₂CH₂S); Fe = (η⁵-C₅H₅)Fe^II(CO) complexes related to the oxygen-damaged active sites of [NiFeS]/[NiFeSe]-H₂ases is described. Mono- and di-oxygenates (major and minor species, respectively) of the chalcogens result from exposure of the heterobimetallics to O₂; one was isolated and structurally characterized to have Ni–O–Se_Ph–Fe–S connectivity within a 5-membered ring. A compositionally analogous mono-oxy species was implicated by ν(CO) IR spectroscopy to be the corresponding Ni–O–S_Ph–Fe–S complex; treatment with O-abstraction agents such as P(o-tolyl)₃ or PMe₃ remediated the O damage. Computational studies (DFT) found that the lowest energy isomers of mono-oxygen derivatives of Ni(μ-EPh)(μ-SN₂)Fe complexes were those with O attachment to Ni rather than Fe, a result consonant with experimental findings, but at odds with oxygenates found in oxygen-damaged [NiFeS]/[NiFeSe]-H₂ase structures. A computer-generated model based on substituting ^–SMe for the N-CH₂CH₂S^– sulfur donor of the N₂S suggested that constraint within the chelate hindered O-atom uptake at that sulfur site.

Comprehensive reaction mechanisms at and near the Ni-Fe active sites of [NiFe] hydrogenases

[NiFe] hydrogenase (H2ase) catalyzes the oxidation of dihydrogen to two protons and two electrons and/or its reverse reaction. For this simple reaction, the enzyme has developed a sophisticated but intricate mechanism with heterolytic cleavage of dihydrogen (or a combination of a hydride and a proton), where its Ni-Fe active site exhibits various redox states. Recently, thermodynamic parameters of the acid-base equilibrium for activation-inactivation, a new intermediate in the catalytic reaction, and new crystal structures of [NiFe] H2ases have been reported, providing significant insights into the activation-inactivation and catalytic reaction mechanisms of [NiFe] H2ases. This Perspective provides an overview of the reaction mechanisms of [NiFe] H2ases based on these new findings.

Reaction fronts of the autocatalytic hydrogenase reaction

We have built a model to describe the hydrogenase catalyzed, autocatalytic, reversible hydrogen oxidation reaction where one of the enzyme forms is the autocatalyst. The model not only reproduces the experimentally observed front properties, but also explains the found hydrogen ion dependence. Furthermore, by linear stability analysis, two different front types are found in good agreement with the experiments.

L-edge sum rule analysis on 3d transition metal sites: from d10 to d0 and towards application to extremely dilute metallo-enzymes

According to L-edge sum rules, the number of 3d vacancies at a transition metal site is directly proportional to the integrated intensity of the L-edge X-ray absorption spectrum (XAS) for the corresponding metal complex. In this study, the numbers of 3d holes are characterized quantitatively or semi-quantitatively for a series of manganese (Mn) and nickel (Ni) complexes, including the electron configurations 3d10→ 3d0. In addition, extremely dilute (<0.1% wt/wt) Ni enzymes were examined by two different approaches: (1) by using a high resolution superconducting tunnel junction X-ray detector to obtain XAS spectra with a very high signal-to-noise ratio, especially in the non-variant edge jump region; and (2) by adding an inert tracer to the sample that provides a prominent spectral feature to replace the weak edge jump for intensity normalization. In this publication, we present for the first time: (1) L-edge sum rule analysis for a series of Mn and Ni complexes that include electron configurations from an open shell 3d0 to a closed shell 3d10; (2) a systematic analysis on the uncertainties, especially on that from the edge jump, which was missing in all previous reports; (3) a clearly-resolved edge jump between pre-L3 and post-L2 regions from an extremely dilute sample; (4) an evaluation of an alternative normalization standard for L-edge sum rule analysis. XAS from two copper (Cu) proteins measured using a conventional semiconductor X-ray detector are also repeated as bridges between Ni complexes and dilute Ni enzymes. The differences between measuring 1% Cu enzymes and measuring <0.1% Ni enzymes are compared and discussed. This study extends L-edge sum rule analysis to virtually any 3d metal complex and any dilute biological samples that contain 3d metals.

Heterobimetallic [NiFe] Complexes Containing Mixed CO/CN- Ligands: Analogs of the Active Site of the [NiFe] Hydrogenases

The development of synthetic analogs of the active sites of [NiFe] hydrogenases remains challenging, and, in spite of the number of complexes featuring a [NiFe] center, those featuring CO and CN^- ligands at the Fe center are under-represented. We report herein the synthesis of three bimetallic [NiFe] complexes [Ni( N₂ S₂)Fe(CO)₂(CN)₂], [Ni( S₄)Fe(CO)₂(CN)₂], and [Ni( N₂ S₃)Fe(CO)₂(CN)₂] that each contain a Ni center that bridges through two thiolato S donors to a {Fe(CO)₂(CN)₂} unit. X-ray crystallographic studies on [Ni( N₂ S₃)Fe(CO)₂(CN)₂], supported by DFT calculations, are consistent with a solid-state structure containing distinct molecules in the singlet ( S = 0) and triplet ( S = 1) states. Each cluster exhibits irreversible reduction processes between -1.45 and -1.67 V vs Fc⁺/Fc and [Ni( N₂ S₃)Fe(CO)₂(CN)₂] possesses a reversible oxidation process at 0.17 V vs Fc⁺/Fc. Spectroelectrochemical infrared (IR) and electron paramagnetic resonance (EPR) studies, supported by density functional theory (DFT) calculations, are consistent with a Ni^IIIFe^II formulation for [Ni( N₂ S₃)Fe(CO)₂(CN)₂]⁺. The singly occupied molecular orbital (SOMO) in [Ni( N₂ S₃)Fe(CO)₂(CN)₂]⁺ is based on Ni 3d_z² and 3p S with the S contributions deriving principally from the apical S-donor. The nature of the SOMO corresponds to that proposed for the Ni-C state of the [NiFe] hydrogenases for which a Ni^IIIFe^II formulation has also been proposed. A comparison of the experimental structures, and the electrochemical and spectroscopic properties of [Ni( N₂ S₃)Fe(CO)₂(CN)₂] and its [Ni( N₂ S₃)] precursor, together with calculations on the oxidized [Ni( N₂ S₃)Fe(CO)₂(CN)₂]⁺ and [Ni( N₂ S₃)]⁺ forms suggests that the binding of the {Fe(CO)(CN)₂} unit to the {Ni(CysS)₄} center at the active site of the [NiFe] hydrogenases suppresses thiolate-based oxidative chemistry involving the bridging thiolate S donors. This is in addition to the role of the Fe center in modulating the redox potential and geometry and supporting a bridging hydride species between the Ni and Fe centers in the Ni-C state.

Theoretical investigation of aerobic and anaerobic oxidative inactivation of the [NiFe]-hydrogenase active site

The extraordinary capability of [NiFe]-hydrogenases to catalyse the reversible interconversion of protons and electrons into dihydrogen (H₂) has stimulated numerous experimental and theoretical studies addressing the direct utilization of these enzymes in H₂ production processes.

Quantum chemical approaches to [NiFe] hydrogenase

The mechanism by which [NiFe] hydrogenase catalyses the oxidation of molecular hydrogen is a significant yet challenging topic in bioinorganic chemistry. With far-reaching applications in renewable energy and carbon mitigation, significant effort has been invested in the study of these complexes. In particular, computational approaches offer a unique perspective on how this enzyme functions at an electronic and atomistic level. In this article, we discuss state-of-the art quantum chemical methods and how they have helped deepen our comprehension of [NiFe] hydrogenase. We outline the key strategies that can be used to compute the (i) geometry, (ii) electronic structure, (iii) thermodynamics and (iv) kinetic properties associated with the enzymatic activity of [NiFe] hydrogenase and other bioinorganic complexes.

Synthetic Models for Nickel-Iron Hydrogenase Featuring Redox-Active Ligands

The nickel-iron hydrogenase enzymes efficiently and reversibly interconvert protons, electrons, and dihydrogen. These redox proteins feature iron-sulfur clusters that relay electrons to and from their active sites. Reported here are synthetic models for nickel-iron hydrogenase featuring redox-active auxiliaries that mimic the iron-sulfur cofactors. The complexes prepared are NiII(μ-H)FeIIFeII species of formula [(diphosphine)Ni(dithiolate)(μ-H)Fe(CO)2(ferrocenylphosphine)]+ or NiIIFeIFeII complexes [(diphosphine)Ni(dithiolate)Fe(CO)2(ferrocenylphosphine)]+ (diphosphine = Ph2P(CH2)2PPh2 or Cy2P(CH2)2PCy2; dithiolate = -S(CH2)3S-; ferrocenylphosphine = diphenylphosphinoferrocene, diphenylphosphinomethyl(nonamethylferrocene) or 1,1'-bis(diphenylphosphino)ferrocene). The hydride species is a catalyst for hydrogen evolution, while the latter hydride-free complexes can exist in four redox states - a feature made possible by the incorporation of the ferrocenyl groups. Mixed-valent complexes of 1,1'-bis(diphenylphosphino)ferrocene have one of the phosphine groups unbound, with these species representing advanced structural models with both a redox-active moiety (the ferrocene group) and a potential proton relay (the free phosphine) proximal to a nickel-iron dithiolate.

Electrocatalytic proton reduction by a model for [NiFeSe] hydrogenases

Electrocatalytic proton reduction was studied using [NiFe] complexes as models of [NiFeSe] hydrogenases.

Synthetic [NiFe] models with a fluxional CO ligand

A [NiFe] complex [(dppe)Ni(pdt)FeCp*(CO)]BF₄ was characterized as two isomers, and their interconversions were established by thermal process and electrochemistry.

Nickel–ruthenium-based complexes as biomimetic models of [NiFe] and [NiFeSe] hydrogenases for dihydrogen evolution

Electrocatalytic proton reduction was studied using nickel–ruthenium complexes that were developed as models for [NiFe] and [NiFeSe] hydrogenases.

Heteronuclear assembly of Ni–Cu dithiolato complexes: synthesis, structures, and reactivity studies

A mild route was discovered to synthesize heterometallic [Ni^IICu^I] complexes featuring square-planar Ni(ii) and distorted tetrahedral Cu(i).

Structure and function of [NiFe] hydrogenases

Hydrogenases catalyze the reversible conversion of molecular hydrogen to protons and electrons via a heterolytic splitting mechanism. The active sites of [NiFe] hydrogenases comprise a dinuclear Ni-Fe center carrying CO and CN^- ligands. The catalytic activity of the standard (O₂-sensitive) [NiFe] hydrogenases vanishes under aerobic conditions. The O₂-tolerant [NiFe] hydrogenases can sustain H₂ oxidation activity under atmospheric conditions. These hydrogenases have very similar active site structures that change the ligand sphere during the activation/catalytic process. An important structural difference between these hydrogenases has been found for the proximal iron-sulphur cluster located in the vicinity of the active site. This unprecedented [4Fe-3S]-6Cys cluster can supply two electrons, which lead to rapid recovery of the O₂ inactivation, to the [NiFe] active site.