Cobalt Dithiolene Complexes Research Articles

Efficient Hydrogen Production at pH 7 in Water with a Heterogeneous Electrocatalyst Based on a Neutral Dimeric Cobalt-Dithiolene Complex

The development of efficient hydrogen production technologies is fundamental for replacing fossil-fuel-based energies. As such, electrocatalysts derived from Earth-abundant metal complexes are appealing, and interesting performances have typically been disclosed under acidic conditions in organic solvents. However, their applicability under relevant pH-neutral conditions has been underexplored. Herein, we demonstrate that nonionic, dimeric cobalt-dithiolene complexes supported on a multiwalled carbon nanotube (MWCNT)/carbon paper (CP) electrode are powerful electrocatalysts for hydrogen production in aqueous media at pH 7. The high turnover numbers encountered (TON up to 50980) after long reaction times (up to 16 h) are explained by the increased electroactive cobalt concentration on the modified electrode, which is ca. 4 times higher than that of a state-of-the-art cobalt porphyrin electrocatalyst. These findings point out that immobilizing well-defined, multinuclear, low-cost metal complexes on carbon material is a promising strategy to design highly electroactive electrodes enabling production of green energies.

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.

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.

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.

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.

Syntheses and reactivities of azido coordinated CpCo(dithiolene) complexes

Azido coordinated dithiolene complexes [CpCo(N 3){S 2C 2(CO 2Me) 2}( S-CHR 1R 2)], where R 1, R 2 = H ( 4a); R 1 = H, R 2 = SiMe 3 ( 4b); R 1 = H, R 2 = CO 2Et ( 4c), were synthesized by the reactions of the corresponding Cl − coordinated precursors [CpCo(Cl){S 2C 2(CO 2Me) 2}( S-CHR 1R 2)] ( 3a– 3c) with sodium azide. The Cl − coordinated complex 3d (R 1, R 2 = CO 2Me) did not produce any N 3 − coordinated complexes but formed the CR 1R 2-bridged alkylidene adduct [CpCo{S 2C 2(CO 2Me) 2}(CR 1R 2)] ( 2d; R 1, R 2 = CO 2Me). The structure of 4a was determined by X-ray diffraction study. In the molecular structure of 4a, the coordinated N 3 − ligand and CHR 1R 2 group were located at the same side with respect to the dithiolene ring ( syn form), although the corresponding Cl − precursor ( 3a; R 1, R 2 = H) was anti form. A structural conversion of syn/ anti was conceivable during the Cl −/N 3 − ligand exchange. Thermal (80 °C) and photochemical reactions (Hg lamp) of 4a– 4c were performed. Among them, 4c was relatively well reacted compared with the others to form the CR 1R 2-bridged alkylidene adduct ( 2c; R 1 = H, R 2 = CO 2Et), followed by a formal HN 3 elimination, and the reaction also produced non-adduct of the cobalt dithiolene complex [CpCo{S 2C 2(CO 2Me) 2}] ( 1). The electrochemical 1e − reduction of 4c underwent a formal N 3 − ligand elimination, and successive second reduction caused the CHR 1R 2 group elimination or reformed the CR 1R 2-bridged alkylidene adduct 2c.

ChemInform Abstract: CpCoI‐Mediated Diels—Alder Reaction Forming Dimeric 1,3‐Dithiol‐2‐one Derivative with Spiro Structure and Successive Formation of Novel Cobalt Dithiolene Complex.

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CpCoI-mediated Diels–Alder Reaction Forming Dimeric 1,3-Dithiol-2-one Derivative with Spiro Structure and Successive Formation of Novel Cobalt Dithiolene Complex

Abstract 4,5-Bis(bromomethyl)-1,3-dithiol-2-one (1) reacts with CpCo(CO)2 (2) under thermal condition to undergo a debromination of 1 and to form dimeric 1,3-dithiol-2-one derivative having a spiro structure 3 by the [4 + 2] Diels–Alder reaction, and the resulted 3 further reacts with 2 to afford new CpCo(dithiolene) complex 4.

Isostructural diamagnetic cobalt(III) and paramagnetic nickel(III) dithiolene complexes with an extended benzenedithiolate core [CpM III(bdtodt)] (M = Co and Ni)

The isostructural diamagnetic [CpCo(bdtodt)] and paramagnetic [CpNi(bdtodt)] (Cp = η 5-cyclopentadienyl, bdtodt:benzo[1,3]dithiol-2-one-5,6-dithiolato) complexes were prepared by starting from the corresponding bis(dithiocarbonate): benzo[1,2-d;4,5-d′]bis[1,3]dithiole-2,6-dione. Both Co and Ni complexes are isostructural and crystallize in the orthorhombic system, space group Pbca. The formally M III (16-electron for Co III and 17-electron Ni III) complexes were investigated by X-ray structure analyses and exhibit the same two-legged piano-stool geometry. The CV of the radical [CpNi(bdtodt)] resulted in well-defined reversible reduction and oxidation waves. On the other hand, oxidation of [CpCo(bdtodt)] leads to dimerization in CH 2Cl 2 or reaction in the more coordinating CH 3CN solvent. The absorption maximum ( λ max) of [CpNi(bdtodt)] (741 nm) showed a more red shift compared with [CpCo(bdtodt)] (595 nm) in dichloromethane solution. The structural similarities, and electrochemical, spectroscopic and magnetic differences between various [CpCo(dithiolene)] and [CpNi(dithiolene)] complexes are further analyzed.

Theoretical studies on the aromaticity of η 5-cyclopentadienyl cobalt dithiolene complexes

Some η 5-cyclopentadienyl cobalt dithiolene complexes CpCoS 2C 2R 2 have been optimized at B3LYP/6-311++G(d) level. The optimized geometries agree well with experiment. The analyses of nature bond orbital and nucleus-independent chemical shift (NICS) at B3LYP/6-311++G(d) and GIAO-B3LYP/6-311++G(d) levels reveal the aromatic character of the η 5-cyclopentadienyl cobalt dithiolene complexes. However, their aromaticity is weaker than that of the isolated CoS 2 C 2 + 1 . There are two reasons for the change of heterocyclic aromaticity of the metal dithiolene in the η 5-cyclopentadienyl cobalt dithiolene complexes with respect to that of the isolated CoS 2 C 2 + 1 . The better equalization of bond lengths in the isolated cation CoS 2 C 2 + 1 is the first reason. The other reason is that the contribution to the NICS from the metallic cobalt atom is much larger in the isolated cation CoS 2 C 2 + 1 . The planar character of cyclopentadienyl is destroyed slightly in the complexes. At the same time, the size of cyclopentadienyl (Cp) becomes bigger than the isolated Cp −1 and this is caused by the cobalt atom in the pentagon. The π-electron delocalization causes stronger aromaticity of the Cp in the complexes than that of the isolated Cp −1.

The dithioleneligand—‘innocent’ or ‘non-innocent’? A theoretical and experimental study of some cobalt–dithiolene complexes

As Jørgensen pointed out in 1966 (Coord. Chem. Rev., 1966, 1, 164), a ligand is to be regarded as 'innocent' if it allows the oxidation state of a metal in a complex to be defined. In this respect, the vast majority of ligands are 'innocent' and, therefore, ligands that are 'non-innocent' have received special attention. Dithiolenes have been regarded as 'non-innocent' ligands since it is possible to consider a ligand of this type to be present in a complex as either: (i) an ene-1,2-dithiolate dianion or (ii) a neutral dithioketone. On this basis, the electronic structure of a dithiolene complex can be described by a set of resonance structures, each of which involves the dithiolene in one of the two forms with the oxidation state of the metal centre being adjusted accordingly. The relative importance of these structures is expected to be reflected in the corresponding molecular structure and spectroscopic properties. In this paper we present a theoretical study of the pair of related 5-cyclopentadienyl cobalt dithiolene complexes, [CpCo(S2C2(H)Ph)] and [CpCo(S2C2(H)Ph)(PMe3)]. Density functional theory calculations successfully predict their different structures and NMR chemical shifts, which we have measured. These wavefunctions have been analysed, particularly in terms of Natural Bond Orbitals and Nucleus Independent Chemical Shifts in an attempt to understand how "innocence" or otherwise is reflected in the experimental data. To this end, a similar analysis is applied to the gold complexes [Au(S2C2(H)Ph)2] and [Au(S2C2(H)Ph)2].

Formations and structures of cobalt dithiolene complexes with nitrogen group-substituted cyclopentadienyl ligands

The cobalt dithiolene complex with the sulfonylamide-substituted Cp ligand [(C 5H 4-NHTs)Co{S 2C 2(COOMe) 2}] ( 1, Ts = p-SO 2C 6H 4Me) reacted with TsOH · H 2O to give [(C 5H 4-NH 2)Co{S 2C 2(COOMe)(H)}] ( 2), [(C 5H 4-NHTs)Co(S 2C 2H 2)] ( 3) and [(C 5H 4-NHTs)Co{S 2C 2(COOMe)(H)}] ( 4). Complex 1 was dissolved in a basic aqueous solution, and the anion reacted with Me 2SO 4 to form the N-methylated product [{C 5H 4-N(Me)Ts}Co{S 2C 2(COOMe) 2}] ( 5); the carboxylic acid complex [{C 5H 4-N(Me)Ts}Co{S 2C 2(COOMe)(COOH)}] ( 6) formed by a base hydrolysis. The X-ray crystal structures of complexes 4– 6 and the methylsulfonylamide-substituted Cp complex [(C 5H 4-NHMs)Co{S 2C 2(COOMe) 2}] ( 7, Ms = SO 2Me) were determined. In the crystal structures of complexes 4 and 7, intermolecular hydrogen bondings of NH⋯O (ca. 2.1 Å) and NH⋯S (ca. 3.1 Å) were observed. Complex 6 showed the OH⋯O intermolecular hydrogen bonding (ca. 1.6 Å) of COOH moiety between two molecules, and these two molecules were assembling each other. Complexes 5 and 6 showed an intramolecular π–π interaction between the aromatic cobaltadithiolene and benzene rings, and complex 5 also has intermolecular π–π interactions between two benzene rings, and between two cobaltadithiolene rings.

Electron spin resonance study of the frozen-solution equilibrium [Co{S2C2(CF3)2}2L]+ L ⇌[Co{S2C2(CF3)2}2L2][L = P(OR)3or PPh(OMe)2; R = alkyl

The ESR spectra have been recorded for a series of cobalt dithiolene complexes [Co(S2C2R2)2L][R = CF3; L = P(OMe)3, P(OEt)3, P(OBun)3 or PPh(OMe)2]. In toluene, tetrahydrofuran or CH2Cl2–1,2-C2H4Cl2 solutions containing a small excess of L, reversible spectral changes are observed between 165 and 130 K which correspond to the addition of a second ligand to form a six-co-ordinate complex. For L = P(OMe)3 in toluene, equilibrium constants were estimated from the spectra which lead to ΔH°=–10 kJ mol–1, ΔS°=–68 J K–1 mol–1 for the five-/six-co-ordinate equilibrium. Ligand addition is not observed when CF3 is replaced by Ph, 4-MeC6H4 or 4-MeOC6H4 or L = PPh3, PEt3, P(OPri)3, P(OCH2CF3)3 or P(OPh)3. Complexes with L =(Ph2P)2CH2, (Ph2PCH2)2 or [(MeO)2PCH2]2 are five-co-ordinate with no evidence for chelation. Thus both steric and electronic effects are critical to the ability of the five-co-ordinate complex to add a sixth ligand. Unlike the five-co-ordinate complexes which are significantly distorted from ideal C2v symmetry, the six-co-ordinate complexes are highly symmetrical with greater delocalization of the unpaired electron onto the phosphite ligands.

Ligand substitution at five-coordinate centers. Reactions of neutral iron- and cobalt-dithiolene complexes

Ligand substitution at five-coordinate centers. Reactions of neutral iron- and cobalt-dithiolene complexes