Dinuclear Cobalt Research Articles

Electrochemical and photochemical-driven hydrogen evolution catalyzed by a dinuclear CoII–CoII complex

The reaction of 2,6-diamino-3-[(2-carboxymethyl) phenylazo]-pyridine (L) and CoCl2·6H2O affords a dinuclear cobalt (II) complex [Co2L2Cl3]Cl 1, a molecular catalyst. Electrochemical studies indicate that 1 can electrocatalyze hydrogen evolution both from acetic acid and purely water media (pH 7.0), with a turnover frequency of 40.64 and 750.2 mol of hydrogen per mole of catalyst per hour, respectively. Sustained proton reduction catalysis occurs at glassy carbon (GC) to give H2 over a 48 h electrolysis period with 97% Faradaic yield and no observable decomposition of the catalyst. Photocatalytic experiments indicate that complex 1 in pH 4.0 aqueous solution under air, together with [Ru(bpy)3]Cl2 and ascorbic acid as a sacrificial electron donor, in the presence of blue light (λmax = 469 nm) also can produce hydrogen with a TON = 425 mol of H2 (mol of cat)−1.

Low-Coordinate Cobalt(I) Complexes Stabilized by an Extremely Bulky Amide Ligand

A series of low-coordinate, high-spin, mono- and dinuclear cobalt(I) complexes bearing an extremely bulky amide (L″ = N(Ar*)(SiPh3); Ar* = C6H2{C(H)Ph2}2Me-2,6,4) ligand have been synthesized and characterized. These include the first example of a neutral two-coordinate cobalt(I) complex, [L″Co(IPriMe)] (IPriMe = :C{N(Pri)C(Me)}2), which has a near-linear cobalt coordination geometry.

First neutral dinuclear cobalt complex formed by bridging [μ-O2P(H)R]– ligands: synthesis, X-ray crystal structure and quantum-chemical study

The reaction of cobalt dibromide hexahydrate with 2,2’-bipyridine (bpy) and 9-anthrylphosphinic acid AntP(O)(OH)H (Ant = 9-anthryl) leads to the first example of a neutral dinuclear cobalt(ii) complex {Co2Br2[μ-O2P(H)Ant]2(bpy)2} formed by two bridging [μ-O2P(H)Ant]– ligands. The complex has been characterized by X-ray diffraction analysis and quantum-chemical calculations.

Theoretical modeling of valence tautomeric dinuclear cobalt complexes. Adducts of CoIIdiketonates with cyclic redox-active tetraone ligands

The approach to the employment of the mechanism of valence tautomerism (VT) for the design of molecular 2-qubit quantum gates has been extended to adducts 5-8 of Co(II) bis-(malonates), bis-(acetylacetonates), bis-(hexafluoroacetylacetonates) and bis-(trifluoroacetylacetonates) with tetradentate tetraone (cyclic di-o-quinones) redox-active piperazine-2,3,5,6-tetraone L5 (X = NH), 3,3,6,6-tetramethylcyclohexane-1,2,4,5-tetraone L5 (X = C(CH3)2), cyclopenta[fg]acenaphthylene-1,2,5,6-tetraone L6, cyclopenta[fg]acenaphthylene-3,4,7,8-tetraone L7 and pyrene-4,5,9,10-tetraone L8 and computationally studied using the B3LYP*/6-311++G(d,p) method. The calculations reveal ferromagnetic ordering of unpaired electrons in the low-spin electromeric forms of complexes 8 (R1, R2 = H, CH3, CF3) on the basis of L8, which provides for the paramagnetic character of all three interconverting electromers of 8 and makes it possible to realize the two-step mechanism of thermally driven migration of paramagnetic centers between the pyrene-tetraone fragment and metal ions. Through the structural variation of the ancillary diketonate ligands the energy gaps between the electromeric forms of adducts 8 and energy barriers for their interconversion were adjusted to the range of values typical of thermal VT rearrangements. The best energy parameters for the occurrence of thermal two-step VT rearrangements involving all three paramagnetic electromeric forms can be achieved for complex 8 (R1 = CH3, R2 = CF3), which was shown to meet the principal conditions for compounds with a potential role of spin qubit carriers: thermal stability with respect to dissociation into the components, thermally accessible energy barriers for the interconversion of the electromers and weak coupling between their paramagnetic centers.

Crystalline-gradient polycarbonates prepared from enantioselective terpolymerization of meso-epoxides with CO2.

The development of efficient processes for CO2 transformation into useful products is a long-standing goal for chemists, since CO2 is an abundant, inexpensive and non-toxic renewable C1 resource. Here we describe the enantioselective copolymerization of 3,4-epoxytetrahydrofuran with CO2 mediated by biphenol-linked dinuclear cobalt complex, affording the corresponding polycarbonate with >99% carbonate linkages and excellent enantioselectivity (up to 99% enantiomeric excess). Notably, the resultant isotactic polycarbonate is a typical semicrystalline polymer, possessing a melting point of 271 °C. Furthermore, the enantioselective terpolymerization of 3,4-epoxytetrahydrofuran, cyclopentene oxide and CO2 mediated by this dinuclear cobalt complex gives novel gradient polycarbonates, in which the decrement of one component and the increment of the other component occur sequentially from one chain end to the other end. The resultant terpolymers show perfectly isotactic structure and have unique crystalline-gradient nature, in which the crystallinity continuously varies along the main chain.

Mechanistic Understanding of Dinuclear Cobalt(III) Complex Mediated Highly Enantioselective Copolymerization of meso-Epoxides with CO2

The desymmetrization copolymerization of meso-epoxides with CO2 using chiral catalysts or reagents is regarded as a valuable strategy for the synthesis of optically active polycarbonates with main-chain chirality. The present study demonstrates that the biphenol-linked dinuclear Co(III) complexes show unprecedented activity, enantioselectivity, and broad substrate scope for coupling CO2 with various meso-epoxides to afford the corresponding isotactic polycarbonates. This investigation also focuses on the mechanistic understanding on the origin of enantioselectivity and highly catalytic activity in the enantiopure dinuclear Co(III) complex mediated copolymerization process, using cyclohexene oxide as a model monomer of meso-epoxides. The kinetic study by in situ infrared spectroscopy revealed a first-order dependence on the catalyst concentration in the systems of biphenol-linked dinuclear Co(III) complex alone or in the presence of an ionic cocatalyst. An intramolecular bimetallic cooperation mechanism wa...

Phosphate ester hydrolysis catalyzed by a dinuclear cobalt(II) complex equipped with intramolecular β-cyclodextrins

Dinuclear metallohydrolases are ubiquitously involved in biological transformations. For understanding and application purpose, significant efforts have been dedicated to the structural or/and functional mimics. Herein, a dinuclear Co(II) complex, Co2L, with intramolecular β-CDs is presented. The β-CDs confine a hydrophobic microenvironment that contributes to the valence stability of the ligated Co(II) ions. Species of Co2L in aqueous solution is clarified according to the results of potentiometric titration and UV–vis titration experiments. A highly positive-charged species [Co2HL]4+ is present at quasi-neutral condition, which contributed significantly to the binding and activity toward bis(4-nitrophenyl) phosphate (BNPP) assisted by the hydrophobic interactions exerted by intramolecular β-CDs. The kcat/KM value of Co2L is determined to be 9.8×10−2M−1s−1 at pH 6.5 and 308K, which exceeds that of the reported zinc analog (Zn2L) by three orders of magnitude. Unexpectedly, Co2L also catalyzes hydrolysis of a phosphate monoester, 4-nitrophenyl phosphate (NPP). The observed phosphate esterase activities are rarely reported among synthetic Co(II) complexes.

Water Oxidation Catalyzed by a Dinuclear Cobalt–Polypyridine Complex

AbstractThe dinuclear Co complex [(TPA)Co(μ‐OH)(μ‐O2)Co(TPA)](ClO4)3 (1, TPA=tris(2‐pyridylmethyl)amine) catalyzes the oxidation of water. In the presence of [Ru(bpy)3]2+ and S2O82−, photoinduced oxygen evolution can be observed with a turnover frequency (TOF) of 1.4±0.1 mol(O2) mol(1)−1 s−1 and a maximal turnover number (TON) of 58±5 mol(O2) mol(1)−1. The complex is shown to act as a molecular and homogeneous catalyst and a mechanism is proposed based on the combination of EPR data and light‐driven O2 evolution kinetics.

Water oxidation catalyzed by a dinuclear cobalt-polypyridine complex.

The dinuclear Co complex [(TPA)Co(μ-OH)(μ-O2 )Co(TPA)](ClO4 )3 (1, TPA=tris(2-pyridylmethyl)amine) catalyzes the oxidation of water. In the presence of [Ru(bpy)3 ](2+) and S2 O8 (2-) , photoinduced oxygen evolution can be observed with a turnover frequency (TOF) of 1.4±0.1 mol(O2 ) mol(1)(-1) s(-1) and a maximal turnover number (TON) of 58±5 mol(O2 ) mol(1)(-1) . The complex is shown to act as a molecular and homogeneous catalyst and a mechanism is proposed based on the combination of EPR data and light-driven O2 evolution kinetics.

Polarized Neutron Diffraction study of the molecular magnetic anisotropy

The magnetic anisotropy is a prerequisite for a metal complex to behave as a single-molecule magnet (SMM). Unfortunately, today we do not fully understand the relationships between the local structural parameters and the magnetic anisotropy that results at the molecular level. This is an issue that has become recursive in this area. Out of the synthesis work which is still important, but generates a multiplication of SMMs with frustrating properties, there are various studies to understand these relationships among which most are theoretical studies. In this context, we believe that polarized neutron diffraction (PND) can provide an experimental and complementary point of view to these theoretical studies. PND is indeed well known to allow an accurate determination of the spin density in magnetic compounds and in the field of molecular magnetism it has provided unique information on the pathways and the nature of intra- or intermolecular magnetic coupling [1]. In the case of highly anisotropic paramagnetic materials, where local magnetic moments cannot be aligned by an external magnetic field, that is more tricky, but a method based on local magnetic susceptibility tensor, has been recently developed that allows now analysing the data in this case and obtaining the magnetization distribution [2]. This approach was first used for inorganic compounds. Our idea has been to use this approach to go beyond the reconstruction of spin density to study the magnetic anisotropy in molecular systems. In this paper, we present the results of such an approach applied for the first time to metal complexes that are simple mono and dinuclear cobalt(II) complexes.

Novel Dinuclear Redox‐isomeric Complexes with a Tetrapodal Pyridine‐based Ligand

AbstractTris‐o‐semiquinonato cobalt complexes react with a tetrapodal pyridine‐derived ligand to form dinuclear cobalt compounds of general formula (OMP)[CoQ2]2, where OMP = 2,2′‐(pyridine‐2,6‐diyl)bis(N1,N1,N3,N3‐tetramethylpropane‐1,3‐diamine), Q = mono‐ or dianion of 3,6‐di‐tert‐butyl‐o‐benzoquinone (complex 1) and it derivatives: 3,6‐di‐tert‐butyl‐4,5‐N,N′‐piperazino‐o‐benzoquinone (complex 2), and 3,6‐di‐tert‐butyl‐4‐Cl‐o‐benzoquinone (complex 3). Single crystal X‐ray crystallography of 1 and 3 indicates two bis‐quinonato cobalt units bound by an OMP ligand, which acts as a bridge. Each central cobalt atom is chelated by one N1,N1,N3,N3‐tetramethylpropane‐1,3‐diamine and two o‐quinonato fragments. The nitrogen atom of the pyridine ring is uncoordinated. All complexes were characterized by NIR‐IR and EPR spectroscopy, precise adiabatic vacuum calorimetry, and by variable‐temperature magnetic susceptibility measurements. All data indicate a reversible thermally driven redox‐isomeric (valence tautomeric) transformation in the solid state for all complexes.

Dinuclear phenoxo-bridged “end-off” complexes containing a piperazine that shows chemical nuclease and cytotoxic activities

Three dinuclear cobalt(II), nickel(II), and copper(II) complexes (1–3) of a phenol-based ‘end-off’ compartmental ligand, 2,6-bis[1-(N-ethyl)piperazineiminomethyl]-4-methylphenol (HL), have been synthesized and characterized by spectral analysis. The molecular structure of one of these complexes, 2,6-bis[1-(N-ethyl)piperazineiminomethyl]-4-methylphenolato-diaqua-μ-hydroxo-μ-nitrato-dicobalt(II) nitrate, [Co2(H2L)(μ-OH)(μ-NO3)(H2O)2](NO3)3] (1), was determined by single crystal X-ray crystallography. The complex exhibits a distorted octahedral geometry around cobalt with a Co–Co distance of 2.9882(8) Å. Electrochemical studies of 1–3 reveal that the redox processes are due to ligand reactions. The EPR spectrum of 3 showed a broad signal at g = 2.11 indicating magnetic interaction between the two copper ions. The μeff values for 1 and 3 are 4.94 and 1.93 BM, respectively, which indicate a spin–spin interaction between the metal ions. Complex 3 caused a cleavage of circular plasmid pBR322 DNA into nicked circular and linear forms in the presence of a co-reactant. Human epidermoid carcinoma cells, A431, were employed for in vitro cytotoxicity studies of the synthesized complexes. The IC50 value of 3 is lower than that of the other two complexes. The copper complex (3) exhibited better chemical nuclease and cytotoxic activity than the other two complexes.

Magnetic Study of a Pentanuclear {Co2IIICo3II} Cluster with a Bent {CoII3} Motif

AbstractWe have synthesised and structurally characterised a new pentanuclear mixed‐valent cobalt cluster of formula [CoII3CoIII2(OH)2(piv)6(L)2(H2O)4] (piv = trimethylacetate, H2L = salicylideneanthranillic acid) from reaction of a dinuclear cobalt pivalate precursor with a Schiff base type ligand under mild reaction conditions. The core structure can be conveniently described as two fused Co3–μ3–OH triangles with a strict unique sharing vertex point. A complete picture of the magnetic behaviour of this compound is presented. Through combined use of susceptibility, magnetisation, and EPR data as well as broken‐symmetry DFT calculations, we have supported the magnetic data that show weak and anisotropic exchange interaction between CoII ions affording an Seff = 1/2 ground state that is not completely isolated from the low‐lying excited doublets at low temperature. Under the optimum applied field of 2 kOe, a frequency‐dependent out‐of‐phase susceptibility signal can be observed below 4 K. However, no reliable relaxation rates could be extracted due to the narrow temperature range in which this behaviour was observed.

Synthesis, structural characterization, molecular docking, and urease inhibition studies of dinuclear cobalt(II) complexes derived from 3,5-bis(pyridin-2-yl)-4-amino-1,2,4-triazole

Two dinuclear Co(II) complexes, [Co2(L)2(EtOH)4]·4ClO4 (1) and [Co2(L)2(H2O)2(NO3)2]·2NO3 (2) (L = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole), have been obtained and characterized by IR, elemental analysis, and single-crystal X-ray diffraction analysis. The stabilization of their crystal lattices is maintained by strong H-bonds between counterions and host framework, which lead to various supramolecular architectures. The urease inhibitory properties of 1, 2, and L were investigated, where the two complexes revealed strong urease inhibition activities. Docking simulations of 2 have been performed with H. pylori urease (PDB code: 1E9Z) to rationalize their binding models.

Understanding of binding of a bis(imido)-bridged dinuclear cobalt(III) complex with Calf thymus DNA

Understanding of binding of a bis(imido)-bridged dinuclear cobalt(III) complex with Calf thymus DNA

Dinuclear cobalt(ii) and cobalt(iii) complexes of bis-bidentate napthoquinone ligands

The combination of bridging bis-bidentate redox-active ligands derived from 3,3-bis-2-hydroxy-1,4-naphthoquinone (bhnqH2), ancillary ligands based on tris(2-pyridylmethyl)amine (tpa) and cobalt salts has afforded a new family of dinuclear cobalt complexes. Compounds of the complexes [Co2(bhnq)(tpa)2](2+) (1), [Co2(bhnq)(Metpa)2](2+) (2), [Co2(bhnq)(Me2tpa)2](2+) (3) [Co2(bhnq)(Me3tpa)2](2+) (4), [Co2(bhnq)(tpa)2](4+) (5), [Co2(bhMenaph)(tpa)2](2+) (6) and [Co2(bhPronaph)(tpa)2](2+) (7) (Mentpa involves n = 0, 1, 2 and 3 methyl groups at the 6-position of the tpa pyridine rings; bhMenaphH4 = bis-3,4-dihydroxy-4-methoxynaphthalene-1-one; bhPronaphH4 = bis-3,4-dihydroxy-4-(2-oxopropyl)naphthalen-1(4H)-one) have been characterised by single crystal X-ray diffraction. While complexes 1-4 possess divalent cobalt centres, trivalent cobalt is evident in complexes 5-7. The bis-bidentate redox-active bridging ligand remains in the diamagnetic quinone bhnq(2-) redox state in complexes 1-5. Metal-catalysed reaction with methoxide or acetone enolate ions gives rise to the derivatised bridging ligands present in 6 and 7. The electronic properties of compounds of 1-7 have been explored in the solid state by infrared spectroscopy and variable temperature magnetic measurements and in solution by electronic absorption spectroscopy and cyclic voltammetry.

Cobalt-Mediated Decarboxylative Homocoupling of Alkynyl Carboxylic Acids

Cobalt-mediated decarboxylative Glaser-like C–C bond coupling of carboxylates has been studied in the gas phase using collision-induced dissociation (CID) multistage mass spectrometry (MSn) experiments. Both the identity of the carboxylate RCO2– (R = Me, HC≡C, MeC≡C, and PhC≡C) and the nuclearity of the complex ([CoCl(O2CR)2]– versus [Co2Cl3(O2CR)2]–) play a role in the types of reactions observed and their relative activation energies. In the first stage of CID, the mononuclear complex [CoCl(O2CMe)2]– undergoes decarboxylation, while the dinuclear [Co2Cl3(O2CMe)2]– undergoes cluster fission to yield [CoCl3]–; all acetylenic carboxylate complexes [CoCl(O2CR)2]– and [Co2Cl3(O2CR)2]– undergo decarboxylation. Isolation of the decarboxylated products followed by a second stage of CID results in a second decarboxylation event for all systems except for [CoCl(Me)(O2CMe)]–, which undergoes bond homolysis. In the final stage of CID, all acetylenic complexes undergo Glaser coupling, forming reduced Co anions. Overall dinuclear cobalt clusters are superior to mononuclear complexes at promoting decarboxylation and reductive coupling. The order of reactivity among the acetylide ligands is PhC≡C > MeC≡C > HC≡C.

Physical Organic Chemistry

Physical organic chemistry is broadly defined and of fundamental importance for the development of interdisciplinary areas that link together chemical synthesis and biological and material sciences with theoretical and physical chemistry. In its classical sense, physical organic chemistry can be defined as the study ofmechanism, reactivity, structure, and binding in organic systems, which leads to the quantitative understanding of their properties at the molecular level. Modern physical organic chemistry also encompasses a wider range of contexts and interactions, which extend beyond reaction pathways. Some important current topics include supramolecular interactions and molecular recognition, aggregation and reactivity, computation of reaction pathways, reactions and catalysis in biology, materials where molecular structure controls function, structure activity correlations, mechanisms in synthesis and catalysis, and interactions and reactivity in organised assemblies and interfaces. The program of the Gordon Research Conference on Physical Organic Chemistry (conference title: Understanding Chemical Reactivity – New Concepts and Applications), which was held in June 2013 at the Holderness School, New Hampshire, USA, spanned the wide area of modern physical organic chemistry. This Research Front on physical organic chemistry is dedicated to this highly interdisciplinary field by showcasing research that ranges from mechanistic organic and bio-organic chemistry to materials chemistry. Fluorescence spectroscopy is a powerful method for monitoring conformational changes in proteins and protein–protein associations. Petersson and co-workers describe in their review strategies for labelling proteins with fluorescence probes with a particular focus on evaluating the balance between size and utility of the fluorophores, since large sizes can be disruptive to the protein’s fold or function, but on the other hand provides valuable characteristics such as visible wavelength absorption and emission or brightness. Formation of C–C bonds mediated by transition metals are amongst the most important reactions in organic synthesis. The search for new types of reactions requires a detailed understanding of the mechanism of these reactions. O’Hair and colleagues have used multi-stage mass spectrometric techniques to obtain mechanistic insight of the cobalt-mediated Glaser-type decarboxylative homocoupling of alkynyl carboxylic acids. These studies revealed that dinuclear cobalt clusters are superior to mononuclear complexes at promoting both decarboxylation steps as well as the reductive coupling itself. Reversible bond formation and dissociation of radical species (p-dimers) in solution phase can be applied to control the mechanical motion of molecules. Nishinaga and co-workers studied the diradical character of benzoand naphtho-annelated thiophene-pyrrole mixed oligomer dications and their application in p-dimer based supramolecular chemistry. Organic semiconductors have become increasingly important as substitutes for conventional inorganic materials. Jeffries-EL and co-workers have explored the optoelectronic properties of two-dimensional cross-shaped ‘cruciform’ molecules possessing two extended conjugation axes, which enable independent manipulation of the HOMO and LUMO levels by incorporating electron-donating and -withdrawing substituents at different positions along the two axes. This selection of invited papers in this Research Front is just a snapshot of the variety of research that is currently being performed under the broad category ‘physical organic chemistry’. It is this diversity of science and the constant developments that will keep this field exciting in the future.

Magnetic and Spectroscopic Investigation of Thermally and Optically Driven Valence Tautomerism in Thioether-Bridged Dinuclear Cobalt–Dioxolene Complexes

A series of dinuclear cobalt complexes of general formula [Co(Mentpa)(diox-S-diox)Co(Mentpa)](PF6)2·MeOH (n = 0, 2, 3) was prepared through the synthesis of the bis-bidentate ligand 6,6'-((1,4-phenylenebis(methylene))bis(sulfanediyl))bis(3,5-di-tert-butyl-benzene-1,2-diol) (diox-S-diox). The ancillary ligands Mentpa are obtained by the tripodal tris(2-pyridylmethyl)amine (tpa) ligand through successive introduction of methyl groups into the 6 position of the pyridine moieties. As expected, the steric hindrance induced by this substitution modulates the redox properties of the metal acceptor, determining the charge distribution of the metal-dioxolene adduct at room temperature. Magnetic measurements and X-ray photoelectron and X-ray absorption spectroscopies indicate that the charge distributions low-spin-Co(III)-catecholate and high-spin-Co(II)-semiquinonate characterize the complexes formed by the tpa and Me3tpa tetradentate ligands, respectively. The complex formed by the Me2tpa ligand undergoes a thermal- and light-induced interconversion of the two states, in agreement with the existence of a valence tautomeric equilibrium. All complexes were stable and behaved reproducibly under X-ray irradiation. This work points out a fast and simple chemical approach to structurally and electronically modify the catechol ring while leaving its coordination capabilities unaffected. These findings afford a robust chemical method to prepare sulfur-functionalized dioxolene ligands as new molecular bricks for chemical functionalization of noble metal surfaces with this class of molecular switches.

Protonation Equilibrium and Hydrogen Production by a Dinuclear Cobalt–Hydride Complex Reduced by Cobaltocene with Trifluoroacetic Acid

A dinuclear Co complex with bis(pyridyl)pyrazolato (bpp(-)) and terpyridine (trpy) ligands, [Co(III)2(trpy)2(μ-bpp)(OH)(OH2)](4+) (1(4+)), undergoes three-electron reduction by cobaltocene in acetonitrile to produce 1(+), which is in the protonation equilibrium with the Co(II)Co(III)-hydride complex, and the further protonation of the hydride by trifluoroacetic acid yields hydrogen quantitatively. The kinetic study together with the detection of the Co(II)Co(III)-hydride complex revealed the mechanism of the hydrogen production by the reaction of 1(+) with trifluoroacetic acid.