Molecular Electrocatalysts for the Hydrogen Evolution Reaction: Input from Quantum Chemistry

Peripheral Bromination for Strongly Affecting the Structural, Electronic, and Catalytic Properties of Cobalt Corroles

The feasibility of a hydrogen-based economy critically depends on the development of catalysts for the hydrogen evolution reaction (HER) that do not rely on Pt or other noble metals. Contemporary efforts are focused on developing first-row transition metal complexes that will be operative at low overpotentials and catalyze the reaction with high efficacy and turnover rates. We now report a surprisingly strong effect of bromide substituents on the structure, coordination chemistry, electronic spectrum, reduction potentials, and catalytic activity of an already electron-poor cobalt corrole. The six-coordinate cobalt(III) complex of the brominated corrole displays a very nonplanar macrocycle, its axial pyridines are perpendicular to each other, the maxima in the electronic spectrum are red-shifted by almost 70 nm, and it is reduced by 600 mV less negative potentials. It catalyzes the HER from an organic acid in an organic solvent with a very low onset potential of -0.96 V vs. the Fc+/Fc couple and has been used for the preparation of a catalyst-modified cathode to produce hydrogen gas from acidic water with 97% Faradaic efficacy at potentials as low as -0.4 V vs. RHE.

Pyridyl Aroyl Hydrazone-Based Metal Complexes Showing a Ligand-Centered Metal-Assisted Pathway for Electrocatalytic Hydrogen Evolution

In the past few decades, the concern over the excessive use of fossil fuels and consequent environmental damage has triggered the search for cleaner and renewable energy resources. This has led to the consideration of hydrogen as our future fuel. It can be produced from various sources, and if used as fuel, it generates only water as a byproduct. Electrocatalytic reduction of protons (2H+ + 2e- → H2) is one of the available methods for producing hydrogen gas at large scales. Currently, the most efficient electrocatalysts are Pt-based complexes. In order to make the entire process more cost-effective, it has become necessary to find electrocatalysts that are derived from earth-abundant metals such as Ni, Zn, Fe, etc. Herein, we have demonstrated the electrocatalytic hydrogen evolution reaction (HER) catalyzed by pyridyl aroyl hydrazone ligand (HL)-based metal complexes (M = Fe, Co, Ni, Cu, and Zn). Rate calculations using controlled potential electrolysis revealed the optimal overall catalytic performance of NiL2 among the investigated complexes. For NiL2, a maximum turnover frequency of 7040 s-1 with a 0.42 V overpotential was obtained when triethylamine hydrochloride was used as a proton source. Both experimental and density functional theory (DFT) calculations suggested a ligand-centered metal-assisted catalysis pathway for NiL2 in the presence of triethylammonium chloride.

Ligand Many-Body Expansion as a General Approach for Accelerating Transition Metal Complex Discovery

Methods that accelerate the evaluation of molecular properties are essential for chemical discovery. While some degree of ligand additivity has been established for transition metal complexes, it is underutilized in asymmetric complexes, such as the square pyramidal coordination geometries highly relevant to catalysis. To develop predictive methods beyond simple additivity, we apply a many-body expansion to octahedral and square pyramidal complexes and introduce a correction based on adjacent ligands (i.e., the cis interaction model). We first test the cis interaction model on adiabatic spin-splitting energies of octahedral Fe(II) complexes, predicting DFT-calculated values of unseen binary complexes to within an average error of 1.4 kcal/mol. Uncertainty analysis reveals the optimal basis, comprising the homoleptic and mer symmetric complexes. We next show that the cis model (i.e., the cis interaction model solved for the optimal basis) infers both DFT- and CCSD(T)-calculated model catalytic reaction energies to within 1 kcal/mol on average. The cis model predicts low-symmetry complexes with reaction energies outside the range of binary complex reaction energies. We observe that trans interactions are unnecessary for most monodentate systems but can be important for some combinations of ligands, such as complexes containing a mixture of bidentate and monodentate ligands. Finally, we demonstrate that the cis model may be combined with Δ-learning to predict CCSD(T) reaction energies from exhaustively calculated DFT reaction energies and the same fraction of CCSD(T) reaction energies needed for the cis model, achieving around 30% of the error from using the CCSD(T) reaction energies in the cis model alone.

Mechanisms of hydrogen evolution by six-coordinate cobalt complexes: a density functional study on the role of a redox-active pyridinyl-substituted diaminotriazine benzamidine ligand as a proton relay

The hydrogen evolution reaction is an important process for energy storage. The six-coordinate cobalt complex [CoIII(L1-)(LH)]2+ (LH = N-(4-amino-6-(pyridin-2-yl)-1,3,5-triazin-2-yl)benzamidine) was found to catalyze photocatalytic hydrogen evolution. In this work, we performed density functional calculations to obtain the reduction potentials and the proton-transfer free energy of possible intermediates to determine the preferred pathways for proton reduction. The mechanism involves the metal-based reduction of Co(III) to Co(II) before the protonation at the amidinate N on the pyridinyl-substituted diaminotriazine benzamidinate ligand L1- to form [CoII(LH)(LH)]2+. Essentially, the subsequent electron transfer is not metal-based reduction, but rather ligand-based reduction to form [CoII(LH)(LH˙1-)]1+. Through a proton-coupled electron transfer process, the cobalt hydride [CoIIH(LH)(LH2˙)]1+ is formed as the key intermediate for hydrogen evolution. As the cobalt hydride complex is coordinatively saturated, a structural change is required when the hydride on Co is coupled with the proton on pyridine. Notably, the redox-active nature of the ligand results in the low acidity of the protonated pyridine moiety of LH2˙, which impedes its function as a proton relay. Our findings suggest that separating the proton relay fragment from the electron reservoir fragment of the redox-active ligand is preferred for fully utilizing both features in catalytic H2 evolution.

Small Organic Molecular Electrocatalysts for Fuels Production

In recent years, heterocyclic organic compounds have been explored as molecular electrocatalysts in relevant reactions for energy conversion and storage. Merging mimetics of biological systems that perform hydride transfer with rational synthetic chemical design has opened many opportunities for organic molecules to be tuned at the atomic level conferring them interesting reactivities. These molecular electrocatalysts represent an alternative to traditional metallic materials and metal complexes employed for water oxidation, hydrogen production, and carbon dioxide reduction. This minireview describes recent reports concerning design, catalytic activity and the mechanism of synthetic molecular electrocatalysts towards solar fuels production.

Bioinspired motifs in proton and CO2 reduction with 3d-metal polypyridine complexes

The synthesis of active and efficient catalysts for solar fuel generation is nowadays of high relevance for the scientific community, but at the same time poses great challenges. Critical requirements are mainly associated with the kinetic barriers due to the multi-proton and multi-electron nature of the hydrogen evolution reaction (HER) and the CO2 reduction reaction (CO2RR) as well as to selectivity issues. In this regard, natural enzymes can be a source of inspiration for the design of effective and selective catalysts to target such fundamental reactions. In this Feature Article we review some recent works on molecular catalysts for both the HER and the CO2RR performed in our labs and other research teams which mainly address (i) the role of redox non-innocent ligands, to lower the overpotential for catalysis and control the selectivity, and (ii) the role of internal relays, to assist formation of catalytic intermediates via intramolecular routes. The selected exemplars have been chosen to emphasize that, although the molecular structures and the synthetic motifs are different from those of the active sites of natural enzymes, many affinities in terms of catalytic mechanism and functionality are instead present, which account for the observed remarkable performances under operative conditions. The data discussed herein thus demonstrate the great potential and the privileged role of molecular catalysts towards the design and construction of hybrid photochemical systems for solar energy conversion into fuels.

Two-Coordinate Coinage Metal Complexes as Solar Photosensitizers

Generating sustainable fuel from sunlight plays an important role in meeting the energy demands of the modern age. Herein, we report two-coordinate carbene-metal-amide (cMa, M = Cu(I) and Au(I)) complexes that can be used as sensitizers to promote the light-driven reduction of water to hydrogen. The cMa complexes studied here absorb visible photons (ε_vis > 10³ M^-1 cm^-1), maintain long excited-state lifetimes (τ ∼ 0.2-1 μs), and perform stable photoinduced charge transfer to a target substrate with high photoreducing potential (E^+/* up to -2.33 V vs Fc^+/0 based on a Rehm-Weller analysis). We pair these coinage metal complexes with a cobalt-glyoxime electrocatalyst to photocatalytically generate hydrogen and compare the performance of the copper- and gold-based cMa complexes. We also find that the two-coordinate complexes herein can perform photodriven hydrogen production from water without the addition of the cobalt-glyoxime electrocatalyst. In this "catalyst-free" system, the cMa sensitizer partially decomposes to give metal nanoparticles that catalyze water reduction. This work identifies two-coordinate coinage metal complexes as promising abundant metal, solar fuel photosensitizers that offer exceptional tunability and photoredox properties.

Non-Metallic Phosphorus Corrole as Efficient Electrocatalyst in Hydrogen Evolution Reaction

The economical consideration of using an electrocatalyst in energy-related field, composed of non-precious/sustainable elements is quite noteworthy. In this work, the phosphorus(V) complex of tris-(pentafluorophenyl)corrole [(TPFC)P^V (OH)₂ ] was reported as electrocatalyst for the hydrogen evolution reaction (HER). The electrochemical studies revealed that the HER experienced a ECEC pathway (E: electron transfer step, C: chemical step), and the possible intermediate [P^V ]-H species was suggested. (TPFC)P^V (OH)₂ displayed excellent HER activity in dimethylformamide (DMF) with trifluoroacetic acid (TFA) as the proton source, and the turnover frequency (TOF) reached 31.75 s^-1 at an overpotential of 900 mV. Interestingly, the HER electrocatalytic performance remained extraordinary even applying water as a proton source in acetonitrile/water (v/v=2 : 3), with a TOF of 18.40 mol H 2 ${{_{{\rm H}{_{2}}}}}$  mol_cat ^-1  h^-1 at an overpotential of 900 mV.

Catalytic Mechanism of Competing Proton Transfer Events from Water and Acetic Acid by [CoII(bpbH2)Cl2] for Water Splitting Processes

We performed first principles simulations to explore the water reduction process of the cobalt complex [Co^II(bpbH₂)Cl₂], where bpbH₂ = N,N'-bis(2'-pyridine carboxamide)-1,2-benzene. We considered the sequence steps of electron reduction followed by the proton addition process to observe the hydrogen evolution process. An experimental study of the catalyst showed that the increase in the acetic acid concentration triggers catalytic current and reduction of Co(II) to Co(I), and protonation occurred, yielding a Co(III)-H intermediate. Therefore, we used water and acetic acid as the proton sources. We compare the proton transfer kinetics from both the water and acetic acid. The reduction potentials and proton transfer kinetics from water or acetic acid to the reaction center were studied in a DMF solvent through the implicit solvent model. The first proton transfer from the acetic acid is more favorable, forming a Co^III-H complex and further reducing to Co^II-H. The second proton transfer from water to the Co^II-H moiety requires less free energy than acetic acid and is the rate-limiting step. The nature of the reduction process is also examined through the charge analysis, which reveals that the ligand becomes softer due to the C═O groups.

Priority of Mixed Diamine Ligands in Cobalt Dithiolene Complex-Catalyzed H2 Evolution: A Theoretical Study

Redox non-innocent metal dithiolene or diamine complexes are potential alternative catalysts in hydrogen evolution reaction and have been incorporated into 2D metal-organic frameworks to obtain unexpected electrocatalytic activity. According to an experimental study, Co-bis(dithiolene), Co-bis(diamine), and Co-dithiolene-diamine portions are considered as active sites where the generation of H₂ occurs and a diamine ligand is necessary for high catalytic efficiency. We are interested in the difference between these catalytic active sites, and mechanistic studies on extracted Co-bis(dithiolene), Co-bis(diamine), and Co-dithiolene-diamine complex-catalyzed hydrogen evolution reactions are carried out by using density functional methods. Our calculated results indicate that the priority of ligand mixed complexes resulted from the readily occurring protonation of diamine ligands and large electron affinity of dithiolene ligands as well as the lowest overall barrier for H₂ evolution.

From Enzyme Stability to Enzymatic Bioelectrode Stabilization Processes

Bioelectrocatalysis using redox enzymes appears as a sustainable way for biosensing, electricity production, or biosynthesis of fine products. Despite advances in the knowledge of parameters that drive the efficiency of enzymatic electrocatalysis, the weak stability of bioelectrodes prevents large scale development of bioelectrocatalysis. In this review, starting from the understanding of the parameters that drive protein instability, we will discuss the main strategies available to improve all enzyme stability, including use of chemicals, protein engineering and immobilization. Considering in a second step the additional requirements for use of redox enzymes, we will evaluate how far these general strategies can be applied to bioelectrocatalysis.

Successes, challenges, and opportunities for quantum chemistry in understanding metalloenzymes for solar fuels research

Overview of the rich and diverse contributions of quantum chemistry to understanding the structure and function of the biological archetypes for solar fuel research, photosystem II and hydrogenases.

Borophene and Boron Fullerene Materials in Hydrogen Storage: Opportunities and Challenges

Two-dimensional materials have led to a leap forward in materials science research, especially in the fields of energy conversion and storage. Borophene and its spherical counterpart boron fullerene represent emerging materials that have attracted much attention in the whole area of advanced energy materials and technologies. Owing to their prominent features, such as electronic environment and geometry, borophene and boron fullerene have been used in versatile applications, such as supercapacitors, superconductors, anode materials for photochemical water splitting, and biosensors. Herein, one of the most promising applications/areas of hydrogen storage is discussed. Boron fullerenes have been considered and discussed for hydrogen-storage applications, and recently borophene was also included in the list of materials with promising hydrogen-storage properties. Studies focus mainly on doped borophene systems over pristine borophene due to enhanced features available upon decoration with metal atoms. This Review introduces very recent progress and novel paradigms with respect to both borophene derivatives and boron fullerene based systems reported for hydrogen storage, with a focus on the synthesis, physiochemical properties, hydrogen-storage mechanism, and practical applications.

Electrocatalytic hydrogen production by dinuclear cobalt(ii) compounds containing redox-active diamidate ligands: a combined experimental and theoretical study

The chiral dicobalt(ii) complex [Co^II₂(μ₂-L)₂] (1) (H₂L = N²,N⁶-di(quinolin-8-yl)pyridine-2,6-dicarboxamide) and its tert-butyl analogue [Co^II₂(μ₂-LBu)₂] (2) were structurally characterized and their catalytic evolution of H₂ was investigated.

Ligand-based electronic effects on the electrocatalytic hydrogen production by thiosemicarbazone nickel complexes

This work reports on the synthesis and characterization of a series of mononuclear thiosemicarbazone nickel complexes that display significant catalytic activity for hydrogen production in DMF using trifluoroacetic acid as the proton source.