Identification of an Electrode-Adsorbed Intermediate in the Catalytic Hydrogen Evolution Mechanism of a Cobalt Dithiolene Complex

Bioinspired Diiron Complex with Proton Shuttling and Redox-Active Ligand for Electrocatalytic Hydrogen Evolution

A μ-oxo diiron complex, featuring the pyridine-2,6-dicarboxamide-based thiazoline-derived redox-active ligand, H2L (H2L = N2,N6-bis(4,5-dihydrothiazol-2-yl)pyridine-2,6-dicarboxamide), was synthesized and thoroughly characterized. [FeIII-(μ-O)-FeIII] showed electrocatalytic hydrogen evolution reaction activity in the presence of different organic acids of varying pKa values in dimethylformamide. Through electrochemical analysis, we found that [FeIII-(μ-O)-FeIII] is a precatalyst that undergoes concerted two-electron reduction to generate an active catalyst. Fourier transform infrared spectrum of reduced species and density functional theory (DFT) investigation indicate that the active catalyst contains a bridged hydroxo unit which serves as a local proton source for the Fe(III) hydride intermediate to release H2. We propose that in this active catalyst, the thiazolinium moiety acts as a proton-transferring group. Additionally, under sufficiently strong acidic conditions, bridged oxygen gets protonated before two-electron reduction. In the presence of exogenous acids of varying strengths, it displays electro-assisted catalytic response at a distinct applied potential. Stepwise electron-transfer and protonation reactions on the metal center and the ligand were studied through DFT to understand the thermodynamically favorable pathways. An ECEC or EECC mechanism is proposed depending on the acid strength and applied potential.

Synthesis and coordination behaviour of 1H-1,2,3-triazole-4,5-dithiolates

The preparative access to and first group 10 metal complexes of novel 1H-1,2,3-triazole-4,5-dithiolate ligands (tazdt^2-) are reported. A set of S-protected 1H-1,2,3-triazole-4,5-dithiol derivatives with R¹ = 2,6-dimethylphenyl (Xy) or benzyl (Bn) at N1 and with R² = Bn or trimethylsilylethyl (TMS-ethyl) at both S atoms were synthesized by a 1,3-dipolar cycloaddition catalysed by either Ru(II) or Cu(I). Extensive investigations on the removal of the protective groups resulted the reductive removal of benzyl groups to be superior in isolating the free 4,5-dithiols of R¹N₃C₂(SH)₂ with R¹ = Xy (H₂-8) or Bn (H₂-9). Coordination of these ligands led to the formation of the metal complexes [(η⁵-C₅H₅)₂Ti(8)], [Ni(dppe)(8)], [Ni(dppe)(9)], [Pd(dppe)(9)] {dppe = bis(diphenylphosphanyl)ethane} and homoleptic (NBu₄)_n[Ni(8)₂] (n = 1, 2). All complexes were fully characterized including structure determination by single crystal XRD. The electronic properties of the Ni and Pd complexes were determined by cyclic voltammetry, UV/vis and EPR spectroscopy supported by DFT calculations. According to the spectral and electrochemical data, the tazdt^2- complexes resemble the corresponding benzene-1,2-dithiolate (bdt^2-) type compounds reflecting the restricted influence of the electron-withdrawing N₃ moiety in the backbone. DSC-TGA measurements with [(η⁵-C₅H₅)₂Ti(8)] and [Ni(dppe)(8)] indicate a well-defined thermal process involving simultaneous elimination of both N₂ and CS.

Electrocatalytic syngas generation with a redox non-innocent cobalt 2-phosphinobenzenethiolate complex

A cobalt complex supported by the 2-(diisopropylphosphaneyl)benzenethiol ligand was synthesized and its electronic structure and reactivity were explored. X-ray diffraction studies indicate a square planar geometry around the cobalt center with a trans arrangement of the phosphine ligands. Density functional theory calculations and electronic spectroscopy measurements suggest a mixed metal-ligand orbital character, in analogy to previously studied dithiolene and diselenolene systems. Electrochemical studies in the presence of 1 atm of CO₂ and Brønsted acid additives indicate that the cobalt complex generates syngas, a mixture of H₂ and CO, with faradaic efficiencies up to >99%. The ratios of H₂ : CO generated vary based on the additive. A H₂ : CO ratio of ∼3 : 1 is generated when H₂O is used as the Brønsted acid additive. Chemical reduction of the complex indicates a distortion towards a tetrahedral geometry, which is rationalized with DFT predictions as attributable to the populations of orbitals with σ*(Co-S) character. A mechanistic scheme is proposed whereby competitive binding between a proton and CO₂ dictates selectivity. This study provides insight into the development of a catalytic system incorporating non-innocent ligands with pendant base moieties for electrochemical syngas production.

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.

Highly Conductive Cobalt Perthiolated Coronene Complex for Efficient Hydrogen Evolution

Metal-bis(dithiolene) is one of the most promising structures showing redox activity, excellent electron transport and magnetic properties as well as catalytic activities. Perthiolated coronene (PTC), an emerging highly symmetric ligand containing the smallest graphene nanoplate was employed to manufacture a hybrid material with fused metal-bis(dithiolene) and graphene nanoplate, and it has been demonstrated as an efficient strategy for the construction of multifunctional materials recently. Herein, Co-PTC, a 2D MOF containing Co-bis(dithiolene) and coronene units is prepared via a homogeneous reaction for the first time as powder samples, which are bar-shaped microparticles composed of nanosheets. A neutral formula of [Co₃ (C₂₄ S₁₂ )]_n is verified for Co-PTC. Co-PTC plays an ultrahigh conductivity of approximately 45 S cm^-1 at room temperature as compressed samples, which is among the highest value ever reported for the compressed powder samples of conducting MOFs. Moreover, Co-PTC exhibits good electrocatalytic performance in the hydrogen evolution reaction (HER) with a Tafel slope of 189 mV decade^-1 and an operating overpotential of 227 mV at 10 mA cm^-1 with pH=0, as well as a remarkable stability in the extremely acidic aqueous solutions, which is the best hydrogen evolution properties among metal-organic compounds.

Redox mediators accelerate electrochemically-driven solubility cycling of molecular transition metal complexes

The solubility of molecular transition metal complexes can vary widely across different redox states, leaving these compounds vulnerable to electron transfer-initiated heterogenization processes in which oxidation or reduction of the soluble form of the redox couple generates insoluble molecular deposits. These insoluble species can precipitate as suspended nanoparticles in solution or, under electrochemical conditions, as an electrode-adsorbed material. While this electrochemically-driven solubility cycling is technically reversible, the reverse electron transfer to regenerate the soluble redox couple state is a practical challenge if sluggish electron transfer kinetics result in a loss of electronic communication between the molecular deposits and the electrode. In this work, we present an example of this electrochemically-driven solubility cycling, report a novel strategy for catalytically enhancing the oxidation of the insoluble material using homogeneous redox mediators, and develop the theoretical framework for analysing and digitally simulating the action of a homogeneous catalyst on a heterogeneous substrate via cyclic voltammetry. First, a mix of electrochemical and spectroscopic methods are used to characterize an example of this electrochemically-driven solubility cycling which is based on the two-electron reduction of homogeneous [Ni(PPh 2NPh 2)2(CH3CN)]2+ (PPh 2NPh 2 = 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane). The limited solubility of the doubly-reduced product in acetonitrile leads to precipitation and deposition of molecular [Ni(PPh 2NPh 2)2]. While direct oxidation of this heterogeneous [Ni(PPh 2NPh 2)2] at the electrode surface is possible, this electron transfer is kinetically limited. We demonstrate how a freely diffusing redox mediator (ferrocene) - which shuttles electrons between the electrode and the molecular material - can be used to overcome these slow electron transfer kinetics, enabling catalytic regeneration of soluble [Ni(PPh 2NPh 2)2]2+. Finally, mathematical models are developed that describe the current-potential response for a generic EC' mechanism involving a homogeneous catalyst and surface-adsorbed substrate. This novel strategy has the potential to enable reversible redox chemistry for heterogeneous, molecular deposits that are adsorbed on the electrode or suspended as nanoparticles in solution.

Crystal structure of bis(hydroxydi(pyridin-2-yl)methanolato-κ3 N,N′O)cobalt(III) 7,7,8,8-tetracyanoquinodimethane, C34H22CoN8O4

C34H22CoN8O4, triclinic, P1̄ (no. 2), a = 6.8423(10) Å, b = 8.8919(13) Å, c = 12.6319(18) Å, α = 104.743(2)°, β = 94.606(2)°, γ = 90.934(2)°, V = 740.30(19) Å3, Z = 1, R gt(F) = 0.0369, wRref(F 2) = 0.1179, T = 296(2) K.

Recent advances in the mechanisms of the hydrogen evolution reaction by non-innocent sulfur-coordinating metal complexes

This review comments on the homogeneous HER mechanisms for catalysts carrying S-non-innocent ligands in the light of experimental and computational data.

Design and photophysical studies of iridium(iii)–cobalt(iii) dyads and their application for dihydrogen photo-evolution

We report several new dyads constituted of cationic iridium(iii) photosensitizers and cobalt(iii) catalyst connected via free pendant pyridine on the photosensitizers.

On decomposition, degradation, and voltammetric deviation: the electrochemist's field guide to identifying precatalyst transformation

What is the identity of the true electrocatalytic species?

Reactivity of Two-Electron-Reduced Boron Formazanate Compounds with Electrophiles: Facile N-H/N-C Bond Homolysis Due to the Formation of Stable Ligand Radicals

The reactivity of a boron complex with a redox-active formazanate ligand, LBPh₂ [L = PhNNC(p-tol)NNPh], was studied. Two-electron reduction of this main-group complex generates the stable, nucleophilic dianion [LBPh₂]^2–, which reacts with the electrophiles BnBr and H₂O to form products that derive from ligand benzylation and protonation, respectively. The resulting complexes are anionic boron analogues of leucoverdazyls. N–C and N–H bond homolysis of these compounds was studied by exchange NMR spectroscopy and kinetic experiments. The weak N–C and N–H bonds in these systems derive from the stability of the resulting borataverdazyl radical, in which the unpaired electron is delocalized over the four N atoms in the ligand backbone. We thus demonstrate the ability of this system to take up two electrons and an electrophile (E⁺ = Bn⁺, H⁺) in a process that takes place on the organic ligand. In addition, we show that the [2e^–/E⁺] stored on the ligand can be converted to E^• radicals, reactivity that has implications in energy storage applications such as hydrogen evolution.

A boron complex with a redox-active formazanate ligand in its two-electron-reduced state is shown to react with electrophiles (BnBr and H⁺). The resulting “borataleucoverdazyl” products have weak N−C and N−H bonds; homolytic cleavage reactions lead to stable ligand-based radicals. Thus, the accumulation of [2e⁻/E⁺] on the formazanate ligand and conversion to E^• radicals are demonstrated, and their potential relevance in energy-related electrocatalysis (e.g., proton reduction) is discussed.

An Azoaromatic Ligand as Four Electron Four Proton Reservoir: Catalytic Dehydrogenation of Alcohols by Its Zinc(II) Complex

Electroprotic storage materials, though invaluable in energy-related research, are scanty among non-natural compounds. Herein, we report a zinc(II) complex of the ligand 2,6-bis(phenylazo)pyridine (L), which acts as a multiple electron and proton reservoir during catalytic dehydrogenation of alcohols to aldehydes/ketones. The redox-inactive metal ion Zn(II) serves as an oxophilic Lewis acid, while the ligand behaves as efficient storage of electron and proton. Synthesis, X-ray structure, and spectral characterizations of the catalyst, ZnLCl₂ (1a) along with the two hydrogenated complexes of 1a, ZnH₂LCl₂ (1b), and ZnH₄LCl₂ (1c) are reported. It has been argued that the reversible azo-hydrazo redox couple of 1a controls aerobic dehydrogenation of alcohols. Hydrogenated complexes are hyper-reactive and quantitatively reduce O₂ and para-benzoquinone to H₂O₂ and para-hydroquinone, respectively. Plausible mechanistic pathways for alcohol oxidation are discussed based on controlled experiments, isotope labeling, and spectral analysis of intermediates.

A mechanism study on the hydrogen evolution reaction catalyzed by molybdenum disulfide complexes

Water-mediated intermolecular H⁺/H⁻ coupling between two- or three-electron reduced sulfur hydride complexes with a hydrated proton is preferred to produce H₂ rather than intramolecular couplings between sulfur hydride and metal hydride complexes.

Electro- and Photocatalytic Generation of H2 Using a Distinctive CoII "PN3 P" Pincer Supported Complex with Water or Saturated Saline as a Hydrogen Source

Efficient electrocatalytic production of H₂ from mixed water/acetonitrile solutions was achieved using three new Co^II complexes supported by the neutral pincer ligand bis(diphenylphosphino)-2,6-di(methylamino)pyridine ("PN₃ P"). At -1.9 V vs. Fc/Fc⁺ , these catalysts showed 96 % Faradaic efficiency with added water or saturated aqueous saline at rates of up to 316 L(mol cat)^-1 (cm² )^-1  h^-1 using a glassy carbon working electrode. The complex [Co(κ³ -2,6-{Ph₂ PNMe}₂ (NC₅ H₃ )Br₂ ] (1) was also able to photocatalytically reduce water to hydrogen in the presence of a Ru(bpy)₃²⁺ photosensitizer and a reductant.

Proton reduction reaction catalyzed by homoleptic nickel bis-1,2-dithiolate complexes: Experimental and theoretical mechanistic investigations

A series of homoleptic monoanionic nickel dithiolene complexes [Ni(bdt)2](NBu4), [Ni(tdt)2](NBu4), and [Ni(mnt)2](NBu4) containing the ligands benzene-1,2-dithiolate (bdt2-), toluene-3,4-dithiolate (tdt2-) and maleonitriledithiolate (mnt2-), respectively, have been employed as electrocatalysts in the hydrogen evolution reaction with trifluoroacetic acid as proton source in acetonitrile. All complexes were active catalysts with TONs reaching 113, 158 and 6 for [Ni(bdt)2](NBu4), [Ni(tdt)2](NBu4), and [Ni(mnt)2](NBu4), respectively. Faradaic yield for hydrogen evolution reaction reaches 88 % for 2- , which also displays the minimal overpotential requirement value (467 mV) within the series. Two pathways for H2 evolution can be hypothesized that differ on on the sequence of protonation and reduction steps. DFT calculations are in agreement with experimental data and indicate that protonation at sulfur follows reduction to the dianion. Hydrogen evolves from the direduced-diprotonated form via a highly distorted nickel hydride intermediate. The effects of acid strength and concentration in the hydrogen-evolving mechanism are also discussed.

H2 evolution by a cobalt selenolate electrocatalyst and related mechanistic studies

[Co(bds)₂][nBu₄N] (where bds = 1,2-benzenediselenolate) was identified as an electrocatalyst for H₂ evolution. Mechanistic studies indicate that with acid a protonated oligomeric {[Co(bds)₂(H)_x]^x−1}_m is formed, which was found to reenter the catalytic cycle and generate H₂.