Electrocatalyst For Proton Reduction Research Articles

Redox Active Ligands for Catalyzing the Hydrogen Evolution Reaction.

Boron subphthalocyanines with chloride and fluoride axial ligands and three antimony complexes chelated by corroles that differ in size and electron-richness were examined as electrocatalysts for reduction of protons to hydrogen. Experiment- and computation-based investigations revealed that all redox events are ligand-centered and that the meso-C of the corroles and the peripheral N atoms of the subphthalocyanines are the largely preferred proton-binding sites.

Transition Metal Pyrithione Complexes (Ni, Mn, Fe, and Co) as Electrocatalysts for Proton Reduction of Acetic Acid.

A series of mononuclear first-row transition metal pyrithione M(pyr) n complexes (M = Ni(II), Mn(II), n = 2; M = Co(III), Fe(III), n = 3) have been prepared from the reaction of the corresponding metal salt with the sodium salt of pyrithione. Using cyclic voltammetry, the complexes have been shown to behave as proton reduction electrocatalysts albeit with varying efficiencies in the presence of acetic acid as the proton source in acetonitrile. The nickel complex displays the optimal overall catalytic performance with an overpotential of 0.44 V. An ECEC mechanism is suggested for the nickel-catalyzed system based on the experimental data and supported by density functional theory calculations.

Bis(benzimidazole)amino thio- and selenoether Iron(II) complexes as proton reduction electrocatalysts

Two novel Iron (II) complexes featuring tetrapodal bis(benzimidazole)amino thio- and selenoether ligands (LS and LSe) were synthesized, characterized, and tested as electrocatalysts for the hydrogen evolution reaction. The bromide complexes [Fe(LS,LSe)Br2] (1–2) are highly insoluble, but their DMSO solvates were characterized by single crystal X-ray diffraction, revealing an octahedral coordination environment that does not feature coordination of the chalcogen atoms. The corresponding triflate derivatives [Fe(LS,LSe)(MeCN)3]OTf2 (1c-2c) were employed for electrocatalytic proton reduction, with 1c exhibiting higher activity, thus suggesting that the thioether may participate as a more competent pendant ligand for proton transfer.

Electrocatalytic hydrogen evolution mediated by an organotelluroxane macrocycle stabilized through secondary interactions.

A discrete liphophilic organotelluroxane macrocycle has been found to catalyse the hydrogen evolution reaction (HER) by proton reduction efficiently. The macrocycle is synthesized via chloride abstraction from bis(p-methoxyphenyl) tellurium dichloride (p-MeOC6H5)2TeCl2 (1) by silver salts AgMX4 (MX4 = BF4-, and ClO4-) resulting in in situ generated di-cationic tetraorganoditelluroxane units; two such units are held together by two weak anions μ2-MX4, bridging to form 12-membered di-cationic macrocycles [((p-MeO-C6H4)2Te)2(μ-O)(μ2-F2BF2)2]2+ (2) and [((p-MeO-C6H4)2Te)2(μ-O)(μ2-O2ClO2)2]2+ (3) stabilized via Te-(μ2-BF4/ClO4), with secondary interactions. The charge is balanced by the presence of two more anions, one above and another below the plane of the macrocycle. Similar reaction at higher temperatures leads to the formation of telluronium salts R3TeX [X = BF4- (4), ClO4- (5)] as a major product. The BF4- anion containing macrocycle and telluronium salt were monitored using 19F NMR. HRMS confirmed the structural stability of all the compounds in the solution state. The organotelluroxane macrocycle 2 has been found to act as an efficient electrocatalyst for proton reduction in an organic medium in the presence of p-toluene sulfonic acid as a protic source.

Manganese Tricarbonyl Diimine Bromide Complexes as Electrocatalysts for Proton Reduction.

Manganese tricarbonyl diimine complexes bearing pyridine and imidazole ligands have been prepared as electrocatalysts for proton reduction using acetic acid as the proton source. The electron-donor ability of the diimine ligand is found to play an important role in determining the efficiency of the electrocatalysts with [MnBr(pybz)(CO)3] (pybz = 2-(2-pyridyl)benzimidazole) exhibiting the lowest overpotential (0.28 V) toward proton reduction. The [Mn(pybz)(CO)3(MeCN)]+ cationic complex prepared via debromination of [MnBr(pybz)(CO)3] by a silver salt has also been shown to catalyze proton reduction upon its electrochemical reduction. A neutral complex [Mn(pyridine-benzimidazolate)(CO)3(MeCN)], which can be synthesized by reacting [MnBr(pybz)(CO)3] with a strong base, has been detected using IR-SEC (infrared spectroelectrochemistry) as an intermediate species in the catalytic process. Using [MnBr(pybz)(CO)3] as the model electrocatalyst, we have carried out density functional calculations to propose a proton reduction mechanism consistent with our experimental observations.

FeFe]‐Hydrogenase models featuring dithiolato‐bridgehead functionality: Preparation, structures, and electrocatalytic proton reduction

AbstractTo further develop the active site models of [FeFe]‐hydrogenases, a new series of diiron model complexes [{(μ‐SCH2)2CHCH2OC(O)C6H4I)}Fe2(CO)6] (I is located at ‐para for 1, ‐meta for 2, and ‐ortho for 3) bearing dithiolato‐bridgehead functionality were successfully prepared by treatments of bridghead‐hydroxyl‐containing parent complex [{(μ‐SCH2)2CHCH2OH)}Fe2(CO)6] (A) and three different iodobenzoic acids (IC6H4CO2H) in the presence of 4‐dimethylaminopyridine (DMAP) as catalyst and dicyclohexylcarbodiimide (DCC) as dehydrating reagent. All these new complexes 1–3 have been fully characterized through elemental analysis, FT‐IR and NMR (1H, 13C) spectroscopies, and X‐ray crystallography. Further electrochemical and electrocatalytic properties of complexes 1–3 are studied and compared in the absence and presence of acetic acid (HOAc) as a proton source by cyclic voltammetry (CV), indicating that they may be considered as the active biomimetic electrocatalysts for proton reduction to hydrogen (H2).

A PEGylated Tin Porphyrin Complex for Electrocatalytic Proton Reduction: Mechanistic Insights into Main-Group-Element Catalysis.

Electrocatalytic proton reduction to form dihydrogen (H2 ) is an effective way to store energy in the form of chemical bonds. In this study, we validate the applicability of a main-group-element-based tin porphyrin complex as an effective molecular electrocatalyst for proton reduction. A PEGylated Sn porphyrin complex (SnPEGP) displayed high activity (-4.6 mA cm-2 at -1.7 V vs. Fc/Fc+ ) and high selectivity (H2 Faradaic efficiency of 94 % at -1.7 V vs. Fc/Fc+ ) in acetonitrile (MeCN) with trifluoroacetic acid (TFA) as the proton source. The maximum turnover frequency (TOFmax ) for H2 production was obtained as 1099 s-1 . Spectroelectrochemical analysis, in conjunction with quantum chemical calculations, suggest that proton reduction occurs via an electron-chemical-electron-chemical (ECEC) pathway. This study reveals that the tin porphyrin catalyst serves as a novel platform for investigating molecular electrocatalytic reactions and provides new mechanistic insights into proton reduction.

A PEGylated Tin Porphyrin Complex for Electrocatalytic Proton Reduction: Mechanistic Insights into Main‐Group‐Element Catalysis

AbstractElectrocatalytic proton reduction to form dihydrogen (H2) is an effective way to store energy in the form of chemical bonds. In this study, we validate the applicability of a main‐group‐element‐based tin porphyrin complex as an effective molecular electrocatalyst for proton reduction. A PEGylated Sn porphyrin complex (SnPEGP) displayed high activity (−4.6 mA cm−2 at −1.7 V vs. Fc/Fc+) and high selectivity (H2 Faradaic efficiency of 94 % at −1.7 V vs. Fc/Fc+) in acetonitrile (MeCN) with trifluoroacetic acid (TFA) as the proton source. The maximum turnover frequency (TOFmax) for H2 production was obtained as 1099 s−1. Spectroelectrochemical analysis, in conjunction with quantum chemical calculations, suggest that proton reduction occurs via an electron‐chemical‐electron‐chemical (ECEC) pathway. This study reveals that the tin porphyrin catalyst serves as a novel platform for investigating molecular electrocatalytic reactions and provides new mechanistic insights into proton reduction.

Structural and electrochemical investigations of new mononuclear nickel(II) dithiolate complexes bearing a pendant amine

To further develop the biomimetic chemistry of [FeFe]- and [NiFe]-hydrogenases for the catalytic reduction of protons to hydrogen (H2), two new mononuclear Ni(II) ethanedithiolate complexes [{(Ph2PCH2)2NR}Ni(SCH2CH2S)] (R = tert-butyl (Bu t ), 1 and n-butyl (Bu n ), 2) with pendant amine-containing diphosphines were synthesized in moderate yields from the treatments of mononickel dichlorides [{(Ph2PCH2)2NR}NiCl2] with ethanedithiol (HSCH2CH2SH) with triethylamine (Et3N) as an acid-binding agent at room temperature. The as-prepared complexes 1 and 2 have been fully characterized by means of elemental analysis, nuclear magnetic resonance (NMR) spectrum, and X-ray crystallography. Further, the electrochemical properties of 1 and 2 are investigated in the absence or presence of acetic acid (HOAc) by using cyclic voltammetry (CV), suggesting that they can be considered as the active electrocatalysts for proton reduction to H2.

Dithiolato-Bridged Nickel(II) Salicylcysteamine Complexes as Robust Proton Reduction Electrocatalysts: Cyclic Voltammetry and Computational Studies.

A series of Schiff-base nickel(II) complexes were prepared from the reaction of nickel(II) acetate with N-salicylcysteamine [HO-C6H4-CH═N(CH2)2SH] ligands. These complexes were analyzed to be dimeric nickel complexes containing two bridging thiolato ligands. Using cyclic voltammetry, they were found to be efficient homogeneous proton reduction electrocatalysts when acetic acid was used as the proton source in acetonitrile. Catalysis was triggered upon electrochemical reduction of the nickel complex. In particular, rate constants (kobs) in the range of 104 s-1 at moderate overpotentials of 0.5-0.6 V were achieved when chloro- or bromo-containing nickel complexes were used. Combined with the experimental data, density functional theory calculations lent support to an ECEC mechanism, with the first electrochemical reduction step contributing significantly to the rate-determining step.

Influence of pendant amine of phosphine ligands on the structural, protophilic, and electrocatalytic properties of diiron model complexes related to [FeFe]-hydrogenases

To explore the influence of pendant amine of phosphine ligands on the structural, protophilic, and electrochemical properties of diiron model complexes related to [FeFe]-hydrogenases, one new aminophosphine-substituted diiron oxadithiolate (odt) complex Fe2(μ-odt)(CO)5{Ph2P(CH2NMe2)} (1) and its new reference analogue Fe2(μ-odt)(CO)5{Ph2P(CH2Ph)} (2) were synthesized and characterized through elemental analysis, FT-IR as well as NMR spectroscopies, and X-ray crystallography. Notably, a comparative study on the protonation and electrochemistry of 1 and 2 is performed in the absence or presence of strong acid CF3CO2H and weak acid CH3CO2H by using in situ spectroscopic techniques (IR and NMR) and cyclic voltammetry (CV). The protonation study reveals that complexes 1 and 2 are treated with excess CF3CO2H to form a small amount of their respective Fe-protonated products [(μ-H)Fe2(μ-odt)(CO)5{Ph2P(CH2NMe2)}](CF3CO2) ([1(μH)]+) and [(μ-H)Fe2(μ-odt)(CO)5{Ph2P(CH2Ph)}](CF3CO2) ([2(μH)]+), but they are unreactive with excess CH3CO2H. The CV investigation suggests that complexes 1 and 2 are found to be active biomimetic electrocatalysts for proton reduction to hydrogen (H2) with CF3CO2H and CH3CO2H, in which complex 1 shows a slightly greater turnover frequency (TOF) value for H2 evolution in contrast to its reference 2.

Heteroleptic thiolate diamine nickel complexes: Noble-free-metal catalysts in electrocatalytic and light-driven hydrogen evolution reaction

Three mononuclear heteroleptic nickel complexes bearing the non-innocent o-aminobenzenethiolate (2-amnt) ligand and different diamine ligands, namely, [Ni(2-amnt)(o-phen)] (o-phen = o-phenylenediamine) (1), [Ni(2-amnt)(3,4-daba)] (3,4-daba = 3, 4-diamino benzoic acid) (2) and [Ni(2-amnt)(dmnt)] (dmnt = diaminomaleonitrile) (3), were synthesized and characterized. Complexes 1–3 are active homogeneous proton reduction electrocatalysts in dimethylformamide with trifluoroacetic acid as a proton source. All the three complexes are tested in light-driven hydrogen evolution reaction, indicating different catalytic efficiencies. Thus, although complex 3 indicates no catalytic action, both complexes 1 and 2 catalyze H2 evolution in water-dimethylformamide solutions using fluorescein (Fl) as a photosensitizer. Complex 2 acts as a homogeneous catalyst reached 834 turnovers (TON); whereas 1, despite being active in hydrogen evolution reaction (HER), decomposes to form nanomaterials. DFT calculations combined with electrochemical and spectroscopic data were employed to investigate the catalytic mechanism for H2 formation as well as to unravel the key factors that influence the relative catalytic efficiencies. The proposed catalytic pathway involves ligand-based reduction and protonation steps followed by the formation of a nickel-hydride intermediate that reacts with a solution proton to generate H2 via a very low energy transition state.

Front Cover: Synthesis, Characterization and Electrochemical Reductive Properties of Complexes [Fe2(CO)4(κ2‐PPh2NR2)(μ‐dithiolato)] Related to the H‐Cluster of [FeFe]‐H2ases (Eur. J. Inorg. Chem. 3/2021)

The Front Cover shows the fate of protons at a diiron site inspired by the H-cluster of [FeFe]-H2ases. Different conformations favor the capture of protons and their transfer to the diiron core, thus generating a suitable proton channel. Finding the right combinations of ligands for the design of efficient diiron electrocatalysts for proton reduction is like a 3D combination puzzle involving concerted electronic and steric effects through the choice of ligands around the dinuclear site. More information can be found in the Full Paper by C. Elleouet, P. Schollhammer and co-workers.

Assembly and Redox-Rich Hydride Chemistry of an Asymmetric Mo2S2 Platform.

Although molybdenum sulfide materials show promise as electrocatalysts for proton reduction, the hydrido species proposed as intermediates remain poorly characterized. We report herein the synthesis, reactions and spectroscopic properties of a molybdenum-hydride complex featuring an asymmetric Mo2S2 core. This molecule displays rich redox chemistry with electrochemical couples at E½ = −0.45, −0.78 and −1.99 V vs. Fc/Fc+. The corresponding hydrido-complexes for all three redox levels were isolated and characterized crystallographically. Through an analysis of solid-state bond metrics and DFT calculations, we show that the electron-transfer processes for the two more positive couples are centered predominantly on the pyridinediimine supporting ligand, whereas for the most negative couple electron-transfer is mostly Mo-localized.

Di-Iron Analogue of [FeFe]-Hydrogenase Active Site as a Molecular Electro-catalyst for Proton Reduction

With Me3NO being present, the ligand substitution reaction of [ (μ-dmedt){Fe (CO)3}2] by 4,4′-bipyridine afforded a new diiron complex (μ-dmedt)Fe2 (CO)5 (4,4′-bipyridine) 1, which was detected using IR, 1H NMR, elemental and MS analyses. Our electrochemical tests showed that compound 1 is capable to catalyze hydrogen production, which served as an efficient molecular electro-catalyst in the CH3CN solution or adsorbed on the surface of electrode for electrochemical generation of molecular hydrogen from HOAc or aqueous media. The relevance of these studies for designing efficient hydrogenase models lies in providing insights into the core of stereo-electronic features. A novel complex (μ-dmedt)Fe2 (CO)5 (4,4′-bipyridine) 1 bearing a functional groups have been prepared, which served as an efficient molecular electro-catalyst in the CH3CN solution or chemisorbed on the surface of electrode for electrochemical generation of molecular hydrogen from HOAC or aqueous media. Those will be of relevance to the natural system and crucial to the potential industrial application.

Triphos nickel(II) halide pincer complexes as robust proton reduction electrocatalysts

Three air-stable Triphos nickel(II) halide pincer complexes (Triphos = bis(2-diphenylphosphinoethyl)phenylphosphine) have been prepared and tested as proton reduction electrocatalysts using acetic acid or trifluoroacetic acid as the proton source in acetonitrile. Electrochemical studies have found them to be robust catalysts with the iodide complex exhibiting the lowest overpotential of 0.52 V in the presence of acetic acid. A proton reduction catalytic mechanism has also been proposed based on the experimental data and density functional calculations performed on a structurally-similar model complex.

Hydrogen formation using a synthetic heavier main-group bismuth-based electrocatalyst

A synthetic heavier p-block bismuth (Bi) complex with a rigid pincer ligand, is shown to be a Bi-based molecular electrocatalyst for proton reduction under weak acid conditions. High activity of the complex is observed in catalysis, with a icat/ip value of 95 with CH3CO2H. Experimental and theoretical studies indicate that the hydrogen evolution reaction (HER) mechanism involves generating a low-valent BiI complex and its reactivity toward acid to form a BiIII-hydride species. Subsequent protonolysis of the BiIII-hydride eliminates H2 through a heterolytic route. The HER cycling through the low-valent metal species and its hydride has been proposed previously in transition-metal (TM) centered catalysts but rarely validated in main-group catalytic systems. The finding brings insight into heavier main-group Bi-catalyzed H2 evolution and the role of low-valent main-group metal centre during catalysis.

Molybdenum carbonyl complexes as HER electrocatalysts

Four cyclopentadienyl and allylic molybdenum carbonyl complexes have been prepared and tested as proton reduction electrocatalysts using trifluoroacetic acid as the proton source in acetonitrile. Cyclic voltammetry studies showed that these complexes are able to function as moderate electrocatalysts with overpotentials of about 0.9 V and catalytic efficiency of up to 60%. The average turnover number and turnover frequency for each of the molybdenum catalyst measured over a five-hour period have been calcualted to be in the range of 16–24 and 3.2 h−1 to 5.4 h−1 respectively for 1 mM catalyst and 20 acid equivalent. A mechanism to account for the proton reduction process is also proposed based on the electrochemical and spectroscopic measurements.

Bis(cyclopentadienyl)nickel(II) μ-Thiolato Complexes as Proton Reduction Electrocatalysts.

Thiolato-bridged cyclopentadienylnickel dimeric complexes have been prepared and found to be efficient and robust proton reduction electrocatalysts using acetic acid as the proton source. From cyclic voltammetry studies, moderate overpotentials of around 0.6 V and ic/ip values from 7.8 to 12.2 have been determined for 20 equiv of acetic acid at a scan rate of 100 mV/s. A turnover number of around 7 has been determined for each of the nickel complexes. The thiolato substituent of the complex does not appear to influence the catalysis significantly. Each of the nickel complexes acts as a robust homogeneous catalyst that could sustain continuous proton reduction for hours. On the basis of the experimental data, an electrochemical-chemical-electrochemical-chemical mechanism describing the catalytic process has been proposed as well.

The effect of a pendant amine in phosphine ligand on the structure and electrochemical property of diiron dithiolate complexes

In order to explore the effect of a pendant amine on a phosphine ligand on the structure and electrochemical properties of diiron dithiolate complexes, this work reports the crystallographic and electrocatalytic comparisons of three diiron monophosphine complexes Fe2(μ-pdt)(CO)5{Ph2P(NHR)} [pdt = propanedithiolate (SCH2CH2CH2S); R = para-methoxycarbonylphenyl (C6H4CO2Me-p) (1), para-methoxyphenyl (C6H4OMe-p) (2) and phenyl (Ph) (3)] with a pendant amine and one reference analogue Fe2(μ-pdt)(CO)5{Ph2P(CH2Ph)} (4). While the new complex 4 has been characterized by elemental analysis and various spectroscopic techniques, the molecular structures of 3 and 4 were further determined by X-ray crystallography. In addition, the electrochemical properties of 1–4 were studied in acetonitrile (MeCN) in the absence and presence of acetic acid (HOAc) as a mild proton source using cyclic voltammetry (CV). This may demonstrate that they are found to be active electrocatalysts for proton reduction to hydrogen (H2).