Cobalt Dithiolene Research Articles

Hydrogen Evolving Activity of Dithiolene-Based Metal-Organic Frameworks with Mixed Cobalt and Iron Centers.

Electrocatalytic systems based on metal-organic frameworks (MOFs) have attracted great attention due to their potential application in commercially viable renewable energy-converting devices. We have recently shown that the cobalt 2,3,6,7,10,11-triphenylenehexathiolate (CoTHT) framework can catalyze the hydrogen evolution reaction (HER) in fully aqueous media with Tafel slopes as low as 71 mV/dec and near-unity Faradaic efficiency (FE). Taking advantage of the high synthetic tunability of MOFs, here, we synthesize a series of iron and mixed iron/cobalt THT-based MOFs. The incorporation of the iron and cobalt dithiolene moieties is verified by various spectroscopic techniques, and the integrity of the crystalline structure is maintained regardless of the stoichiometries of the two metals. The hydrogen evolving activity of the materials was explored in pH 1.3 aqueous electrolyte solutions. Unlike CoTHT, the FeTHT framework exhibits minimal activity due to a late catalytic onset [-0.440 V versus reversible hydrogen electrode (RHE)] and a large Tafel slope (210 mV/dec). The performance of the mixed-metal MOFs is adversely affected by the incorporation of Fe, where increasing Fe content results in MOFs with lower HER activity and diminished long-term stability and FE for H2 production. It is proposed that the FeTHT domains undergo alternative Faradaic processes under catalytic conditions, which alter its local structure and electrochemical behavior, eventually resulting in a material with diminished HER performance.

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.

Improving and Understanding the Hydrogen Evolving Activity of a Cobalt Dithiolene Metal-Organic Framework.

Despite the promising previous reports on the development of electrocatalytic dithiolene-based metal-organic frameworks (MOFs) for the hydrogen evolution reaction (HER), these materials often display poor reproducibility of the HER performance because of their poor bulk properties upon integration with electrode materials. We demonstrate here an in-depth investigation of the electrocatalytic HER activity of a cobalt 2,3,6,7,10,11-triphenylenehexathiolate (CoTHT) MOF. To enhance the durability and charge transport properties of the constructed CoTHT/electrode architecture, CoTHT is deposited as an ink composite (1) composed of Nafion and carbon black. We leverage here the well-established use of catalyst inks in the literature to increase adhesion of the catalyst to the electrode surface and to improve the overall electrical conductivity of the integrated catalyst/electrode. The utilization of the composite 1 leads to a significant improvement in the overpotential (η) to reach a current density of 10 mA/cm2 (η = 143 mV) compared to prior reports, resulting in the most active MOF-based electrocatalyst for the HER that contains only earth-abundant elements. Extensive density functional theory (DFT) calculations were applied to understand the structure of CoTHT and the mechanistic pathways of the HER. The computational results suggest that an AB stacking geometry is energetically favorable, where one layer is slipped by 1.6 Å relative to the neighboring one along the a and b vectors. Additionally, the DFT calculations indicate that the catalytic cycle likely involves a Volmer discharge step to generate a cobalt hydride, followed by a Heyrovsky step to form a cobalt-H2 intermediate, and finally the dissociation of H2.

Negatively charged metal-organic hosts with cobalt dithiolene species: improving PET processes for light-driven proton reduction through host-guest electrostatic interactions.

By incorporating 1,2-benzenedithiol as a chelator to construct cobalt dithiolene species, two negatively charged redox-active metal-organic hosts were obtained. By taking advantage of electrostatic interactions, cationic Ru-based photosensitizers were constrained to improve photoinduced electron transfer processes for efficient photocatalytic proton reduction.

Pyranopterin Related Dithiolene Molybdenum Complexes as Homogeneous Catalysts for CO2 Photoreduction

AbstractTwo original dithiolenes, with a pyrazine ring fused with a pyran ring carrying the dithiolene chelate, mimicking molybdopterin (MPT) present in the active site of formate dehydrogenases (FDHs), have been synthesized. The first one mimicks MPT in the dihydropyrazine form while the second mimicks MPT in the more biologically relevant tetrahydropyrazine form. Both have been structurally characterized as a ligand within a cobalt(cyclopentadienyl)(dithiolene) complex. The corresponding MoO(dithiolene)2 complexes have been also prepared and are reported as the first functional and stable catalysts inspired by the Mo center of FDHs so far: they indeed catalyze the photoreduction of CO2 into formic acid, as the major product, and carbon monoxide, achieving more than 100 turnover numbers in about 8 h.

Pyranopterin Related Dithiolene Molybdenum Complexes as Homogeneous Catalysts for CO2 Photoreduction.

Two original dithiolenes, with a pyrazine ring fused with a pyran ring carrying the dithiolene chelate, mimicking molybdopterin (MPT) present in the active site of formate dehydrogenases (FDHs), have been synthesized. The first one mimicks MPT in the dihydropyrazine form while the second mimicks MPT in the more biologically relevant tetrahydropyrazine form. Both have been structurally characterized as a ligand within a cobalt(cyclopentadienyl)(dithiolene) complex. The corresponding MoO(dithiolene)2 complexes have been also prepared and are reported as the first functional and stable catalysts inspired by the Mo center of FDHs so far: they indeed catalyze the photoreduction of CO2 into formic acid, as the major product, and carbon monoxide, achieving more than 100 turnover numbers in about 8 h.

Electrolyte Effect on Electrocatalytic Hydrogen Evolution Performance of One-Dimensional Cobalt–Dithiolene Metal–Organic Frameworks: A Theoretical Perspective

Discovering inexpensive and earth-abundant electrocatalysts to replace the scarce platinum group metal-based electrocatalysts holds the key for large-scale hydrogen fuel generation, which relies heavily on the theoretical understanding of the properties of candidate materials and their operating environment. The recent applications of the cobalt–dithiolene complex as promising electrocatalysts for the hydrogen evolution reaction have been broadened by forming low-dimensional metal–organic frameworks (MOFs) through polymerization. Using the Gibbs free energy of the adsorption of hydrogen atoms as a key descriptor, S atoms within one-dimensional MOFs are identified to be the preferred catalytic site for HERs. Our theoretical results further reveal that the activities of part S atoms can be improved by interacting with alkali metal cations from the electrolytes; specifically, the influence of cations on the performance is dependent on the electron affinity of cations. Our theoretical findings, therefore, demonstrate that the selection of electrolytes can be a promising approach to enhance the performance of electrocatalysts for HERs.

Understanding Variability in the Hydrogen Evolution Activity of a Cobalt Anthracenetetrathiolate Coordination Polymer

Here, a cobalt dithiolene coordination polymer (CP) based on 9,10-dimethyl-2,3,6,7-anthracenetetrathiolate was synthesized via an interfacial reaction and was electrochemically characterized on glassy carbon (GCE) and graphite (GR) electrodes. Double-layer capacitance measurements, electrochemical impedance spectroscopy studies, and Tafel analyses were used to understand the role of electrochemically accessible active sites, electron and charge transfer, and electrical integration between the catalyst and the support in the resultant electrocatalytic hydrogen evolving activity. Overpotentials to achieve 10 mA/cm2 ranging from 445 to 571 mV and from 388 to 527 mV for GCE|CP and GR|CP, respectively, were observed. Changes in the double-layer capacitance, which is related to electrochemically active surface area, and charge transfer resistance were determined to be the critical factors in the observed enhancement in catalytic activity, whereas bulk catalyst loading, which had been previously used to describe...

Metallic Conductivity in a Two-Dimensional Cobalt Dithiolene Metal-Organic Framework.

Two-dimensional (2D) metal-organic frameworks (MOFs) have received a great deal of attention due to their relatively high charge carrier mobility and low resistivity. Here we report on the temperature-dependent charge transport properties of a 2D cobalt 2,3,6,7,10,11-triphenylenehexathiolate framework. Variable temperature resistivity studies reveal a transition from a semiconducting to a metallic phase with decreasing temperature, which is unprecedented in MOFs. We find this transition to be highly dependent on the film thickness and the amount of solvent trapped in the pores, with density functional theory calculations of the electronic-structure supporting the complex metallic conductivity of the material. These results identify the first experimentally observed MOF that exhibits band-like metallic conductivity.

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

Analysis of a cobalt bis(dithiolate) complex reported to mediate hydrogen evolution under electrocatalytic conditions in acetonitrile revealed that the cobalt complex transforms into an electrode-adsorbed film upon addition of acid prior to application of a potential. Subsequent application of a reducing potential to the film results in desorption of the film and regeneration of the molecular cobalt complex in solution, suggesting that the adsorbed species is an intermediate in catalytic H2 evolution. The electroanalytical techniques used to examine the pathway by which H2 is generated, as well as the methods used to probe the electrode-adsorbed species, are discussed. Tentative mechanisms for catalytic H2 evolution via an electrode-adsorbed intermediate are proposed.

Noble-metal-free Co3S4-S/G porous hybrids as an efficient electrocatalyst for oxygen reduction reaction.

Developing of a new noble-metal-free catalyst to replace Pt-based catalysts of the oxygen reduction reaction (ORR) both in alkaline and acidic conditions is extremely significant for the fuel cell. In this paper, based on the pyrolysis of an inexpensive precursor cobalt dithiolene (a S4-chelate complex) on simultaneously reduced graphene oxide (GO) as a support matrix, a high-efficiency noble-metal-free hybrid for oxygen reduction reaction (ORR) consisting of Co3S4 nanoparticles encapsulated in porous sulfur doped graphene (referred as Co3S4-S/G) was fabricated. The catalyst obtained at 800 °C (Co3S4-S/G-800) manifests excellent oxygen reduction activity. Of note, the Co3S4-S/G-800 hybrids also exhibited prominent ORR activity with high selectivity (mainly 4e- reaction process) and very low H2O2 yield in acidic electrolyte. The optimal Co3S4-S/G-800 hybrid displayed much greater tolerance to methanol and higher stability than that of Pt/C. These admirable performances endorse Co3S4-S/G-800 electrocatalyst holding great potential for fuel cells. Meanwhile, this work also provides a simple and practical method to fabricate cobalt chalcogenides by using the cost-effective and easily synthesized S4-chelate complex.

One dimensional metal dithiolene (M = Ni, Fe, Zn) coordination polymers for the hydrogen evolution reaction.

Immobilization of metal complexes to electrode surfaces has emerged as an attractive strategy to combine homogeneous and heterogeneous catalysis. We recently reported the immobilization of cobalt dithiolene catalytic units via incorporation into extended one and two dimensional (1D and 2D) frameworks. We extend here this methodology to the formation of 1D nickel, iron, and zinc dithiolene coordination polymers based on benzene-1,2,4,5-tetrathiolate (BTT) frameworks and investigate their catalytic H2-evolving activities under fully aqueous conditions. The nickel dithiolene coordination polymer is an active electrocatalyst for the hydrogen evolution reaction (HER). An overpotential of 470 mV was required to reach a current density of 10 mA cm-2 at pH 1.3, making this system one of the best performing heterogenized molecular catalysts for HER. This overpotential is 90 mV lower than that of the cobalt analogue, suggesting that the nickel coordination polymer is a more efficient H2-evolving catalyst. Additionally, no decrease in activity is observed for the nickel polymer during the first hour of electrolysis, indicating that it is stable under prolonged electrolysis.

Experimental and Theoretical Insight into Electrocatalytic Hydrogen Evolution with Nickel Bis(aryldithiolene) Complexes as Catalysts.

A series of neutral and monoanionic nickel dithiolene complexes with p-methoxyphenyl-substituted 1,2-dithiolene ligands have been prepared and characterized with physicochemical methods. Two of the complexes, the monoanion of the symmetric [Ni{S2C2(Ph-p-OCH3)2}2] (3(-)) with NBu4(+) as a counterion and the neutral asymmetric [Ni{S2C2(Ph)(Ph-p-OCH3)}2] (2), have been structurally characterized by single-crystal X-ray crystallography. All complexes have been employed as proton-reducing catalysts in N,N-dimethylformamide with trifluoroacetic acid as the proton source. The complexes are active catalysts with good faradaic yields, reaching 83% for 2 but relatively high overpotential requirements (0.91 and 1.55 V measured at the middle of the catalytic wave for two processes observed depending on the different routes of the mechanism). The similarity of the experimental data regardless of whether the neutral or anionic form of the complexes is used indicates that the neutral form acts as a precatalyst. On the basis of detailed density functional theory calculations, the proposed mechanism reveals two different main routes after protonation of the dianion of the catalyst in accordance with the experimental data, indicating the role of the concentration of the acid and the influence of the methoxy groups. Protonation at sulfur seems be more favorable than that at the metal, which is in marked contrast with the catalytic mechanism proposed for analogous cobalt dithiolene complexes.

Efficient Electrochemical and Photoelectrochemical H2 Production from Water by a Cobalt Dithiolene One-Dimensional Metal-Organic Surface.

Solar-driven hydrogen evolution from water has emerged as an important methodology for the storage of renewable energy in chemical bonds. Efficient and practical clean-energy devices for electrochemical or photoelectrochemical splitting of water require the immobilization of stable and active hydrogen-evolving catalysts onto electrode or photocathode materials, which remains a significant challenge. Here we show that cobalt(II) reacts with benzene-1,2,4,5-tetrathiol in the presence of base to form a cobalt dithiolene polymer 1. The generated polymer is immobilized onto glassy carbon electrodes (GCE) to generate a metal-organic surface (MOS 1|GCE), which displays efficient H2-evolving activity and stability in acidic aqueous solutions. Moreover, the generated polymer is integrated with planar p-type Si to generate very efficient photocathode materials (MOS 1|Si) for solar-driven hydrogen production from water. Photocurrents up to 3.8 mA/cm(2) at 0 V vs RHE were achieved under simulated 1 Sun illumination. MOS 1|Si photocathodes operate at potentials 550 mV more positive than MOS 1|GCE cathodes to reach the same activity for H2 evolution from water (1 mA/cm(2)).

Two-dimensional metal-organic surfaces for efficient hydrogen evolution from water.

Hydrogen production through the reduction of water has emerged as an important strategy for the storage of renewable energy in chemical bonds. One attractive scenario for the construction of efficient devices for electrochemical splitting of water requires the attachment of stable and active hydrogen evolving catalysts to electrode surfaces, which remains a significant challenge. We demonstrate here the successful integration of cobalt dithiolene catalysts into a metal-organic surface to generate very active electrocatalytic cathode materials for hydrogen generation from water. These surfaces display high catalyst loadings and remarkable stability even under very acidic aqueous solutions.

Divergent Synthesis and Tuning of the Electronic Structures of Cobalt–Dithiolene–Fullerene Complexes for Organic Solar Cells

A pentakis(aryl)[60]fullerene cobalt trisulfide complex (1a) was obtained from the reaction of a pentakis(aryl)[60]fullerene cobalt dicarbonyl complex (2a) with triphenylmethanethiol. This reaction proceeded smoothly even at ambient temperature to produce 1a in moderate to high yield. The series of pentakis(aryl)[60]fullerene cobalt dithiolene complexes 3–9 were also synthesized by the reaction of 1a with a variety of readily available disulfide compounds. The HOMO and LUMO levels of the complexes could be tuned by making changes to the substituents on the dithiolene ligands, with changes to the electronic structure of the compounds influencing their photovoltaic performance, especially in terms of their open-circuit voltage. Small-molecule organic solar cells constructed from the naphthalene dithiolene complex 9 and tetrabenzoporphyrin showed a power conversion efficiency of 0.59%.

An organometallic dithiolene complex exhibiting electrochemically initiated hydrogen generation

We clarify the electrochemical behavior of an organometallic cobalt dithiolene complex (1-NH) with a secondary sulfonyl amide (–NHSO(2)–) substituted Cp ligand involving proton dynamics. The reductions of 1-NH and its deprotonated derivative (1-N(−)) are centered on the CpCoS(2) moiety. The 2e(−) reduction of 1-NH quantitatively gives 1-N(−) with hydrogen generation.

Computational Study of Anomalous Reduction Potentials for Hydrogen Evolution Catalyzed by Cobalt Dithiolene Complexes

The design of efficient hydrogen-evolving catalysts based on earth-abundant materials is important for developing alternative renewable energy sources. A series of four hydrogen-evolving cobalt dithiolene complexes in acetonitrile-water solvent is studied with computational methods. Co(mnt)(2) (mnt = maleonitrile-2,3-dithiolate) has been shown experimentally to be the least active electrocatalyst (i.e., to produce H(2) at the most negative potential) in this series, even though it has the most strongly electron-withdrawing substituents and the least negative Co(III/II) reduction potential. The calculations provide an explanation for this anomalous behavior in terms of protonation of the sulfur atoms on the dithiolene ligands after the initial Co(III/II) reduction. One fewer sulfur atom is protonated in the Co(II)(mnt)(2) complex than in the other three complexes in the series. As a result, the subsequent Co(II/I) reduction step occurs at the most negative potential for Co(mnt)(2). According to the proposed mechanism, the resulting Co(I) complex undergoes intramolecular proton transfer to form a catalytically active Co(III)-hydride that can further react to produce H(2). Understanding the impact of ligand protonation on electrocatalytic activity is important for designing more effective electrocatalysts for solar devices.

Cobalt-dithiolene complexes for the photocatalytic and electrocatalytic reduction of protons in aqueous solutions

Artificial photosynthesis (AP) is a promising method of converting solar energy into fuel (H(2)). Harnessing solar energy to generate H(2) from H(+) is a crucial process in systems for artificial photosynthesis. Widespread application of a device for AP would rely on the use of platinum-free catalysts due to the scarcity of noble metals. Here we report a series of cobalt dithiolene complexes that are exceptionally active for the catalytic reduction of protons in aqueous solvent mixtures. All catalysts perform visible-light-driven reduction of protons from water when paired with Ru(bpy)(3)(2+) as the photosensitizer and ascorbic acid as the sacrificial donor. Photocatalysts with electron withdrawing groups exhibit the highest activity with turnovers up to 9,000 with respect to catalyst. The same complexes are also active electrocatalysts in 11 acetonitrile/water. The electrocatalytic mechanism is proposed to be ECEC, where the Co dithiolene catalysts undergo rapid protonation once they are reduced to CoL(2)(2-). Subsequent reduction and reaction with H(+) lead to H(2) formation. Cobalt dithiolene complexes thus represent a new group of active catalysts for the reduction of protons.

A Cobalt–Dithiolene Complex for the Photocatalytic and Electrocatalytic Reduction of Protons

The complex [Co(bdt)(2)](-) (where bdt = 1,2-benzenedithiolate) is an active catalyst for the visible light driven reduction of protons from water when employed with Ru(bpy)(3)(2+) as the photosensitizer and ascorbic acid as the sacrificial electron donor. At pH 4.0, the system exhibits very high activity, achieving >2700 turnovers with respect to catalyst and an initial turnover rate of 880 mol H(2)/mol catalyst/h. The same complex is also an active electrocatalyst for proton reduction in 1:1 CH(3)CN/H(2)O in the presence of weak acids, with the onset of a catalytic wave at the reversible redox couple of -1.01 V vs Fc(+)/Fc. The cobalt-dithiolene complex [Co(bdt)(2)](-) thus represents a highly active catalyst for both the electrocatalytic and photocatalytic reduction of protons in aqueous solutions.