Cobalt Center Research Articles

Formation of cobalt-oxygen intermediates by dioxygen activation at a mononuclear nonheme cobalt(ii) center.

A mononuclear nonheme cobalt(ii) complex, [(TMG3tren)CoII(OTf)](OTf) (1), activates dioxygen in the presence of hydrogen atom donor substrates, such as tetrahydrofuran and cyclohexene, resulting in the generation of a cobalt(ii)-alkylperoxide intermediate (2), which then converts to the previously reported cobalt(iv)-oxo complex, [(TMG3tren)CoIV(O)]2+-(Sc(OTf)3)n (3), in >90% yield upon addition of a redox-inactive metal ion, Sc(OTf)3. Intermediates 2 and 3 represent the cobalt analogues of the proposed iron(ii)-alkylperoxide precursor that converts to an iron(iv)-oxo intermediate via O-O bond heterolysis in pterin-dependent nonheme iron oxygenases. In reactivity studies, 2 shows an amphoteric reactivity in electrophilic and nucleophilic reactions, whereas 3 is an electrophilic oxidant. To the best of our knowledge, the present study reports the first example showing the generation of cobalt-oxygen intermediates by activating dioxygen at a cobalt(ii) center and the reactivities of the cobalt-oxygen intermediates in oxidation reaction.

Co/Ni-polyoxotungstate photocatalysts as precursor materials for electrocatalytic water oxidation.

An open-core cobalt polyoxometalate (POM) [(A-α-SiW9O34)Co4(OH)3(CH3COO)3]8−Co(1) and its isostructural Co/Ni-analogue [(A-α-SiW9O34)Co1.5Ni2.5(OH)3(CH3COO)3]8−CoNi(2) were synthesized and investigated for their photocatalytic and electrocatalytic performance. Co(1) shows high photocatalytic O2 yields, which are competitive with leading POM water oxidation catalysts (WOCs). Furthermore, Co(1) and CoNi(2) were employed as well-defined precursors for heterogeneous WOCs. Annealing at various temperatures afforded amorphous and crystalline CoWO4- and Co1.5Ni2.5WO4-related nanoparticles. CoWO4-related particles formed at 300 °C showed substantial electrocatalytic improvements and were superior to reference materials obtained from co-precipitation/annealing routes. Interestingly, no synergistic interactions between cobalt and nickel centers were observed for the mixed-metal POM precursor and the resulting tungstate catalysts. This stands in sharp contrast to a wide range of studies on various heterogeneous catalyst types which were notably improved through Co/Ni substitution. The results clearly demonstrate that readily accessible POMs are promising precursors for the convenient and low-temperature synthesis of amorphous heterogeneous water oxidation catalysts with enhanced performance compared to conventional approaches. This paves the way to tailoring polyoxometalates as molecular precursors with tuneable transition metal cores for high performance heterogeneous electrocatalysts. Our results furthermore illustrate the key influence of the synthetic history on the performance of oxide catalysts and highlight the dependence of synergistic metal interactions on the structural environment.

Oxygen vacancy engineered unsaturated coordination in cobalt carbonate hydroxide nanowires enables highly selective photocatalytic CO2 reduction

The unsaturated coordinated cobalt centers of Vo-CCO NWs as active sites were manufactured by the self-photoetching approach, which can preferably adsorb and activate CO2 molecules, effectively inhibiting hydrogen evolution.

Hydrogen bond-induced abrupt spin crossover behaviour in 1-D cobalt(II) complexes - the key role of solvate water molecules.

The magnetic properties and structural aspects of the 1-D cobalt(ii) complexes, [Co(pyterpy)Cl2]·2H2O (1·2H2O; pyterpy = 4'-(4'''-pyridyl)-2,2':6',2''-terpyridine) and [Co(pyethyterpy)Cl2]·2H2O (2·2H2O; pyethyterpy = 4'-((4'''-pyridyl)ethynyl)-2,2':6',2''-terpyridine) are reported. In each complex the central cobalt(ii) ion displays an octahedral coordination environment composed of three nitrogen donors from the terpyridine moiety, a nitrogen donor from a pyridyl group and two chloride ligands which occupy the axial sites. 1·2H2O exhibits abrupt spin-crossover (SCO) behaviour (T1/2↓ = 218 K; T1/2↑ = 227 K) along with a thermal hysteresis loop, while 2·2H2O and the dehydrated species 1 and 2 exhibit high-spin (HS) states at 2-300 K as well as field-induced single-molecule magnet (SMM) behaviour attributed to the presence of magnetic anisotropic HS cobalt(ii) species (S = 3/2). 1·2H2O exhibited reversible desorption/resorption of its two water molecules, revealing reversible switching between SCO and SMM behaviour triggered by the dehydration/rehydration processes. Single crystal X-ray structural analyses revealed that 1·2H2O crystalizes in the orthorhombic space group Pcca while 2 and 2·2H2O crystallize in the monoclinic space group P2/n. Each of the 1-D chains formed by 1·2H2O in the solid state are bridged by hydrogen bonds between water molecules and chloride groups to form a 2-D layered structure. The water molecules bridging 1-D chains in 1·2H2O interact with the chloride ligands occupying the axial positions, complementing the effect of Jahn-Teller distortion and contributing to the abrupt SCO behaviour and associated stabilization of the LS state.

Comparison of Penta and Tetra-pyridyl Cobalt-based Catalysts for Water Reduction: H2 Production Cycle, Solvent Response and Reduction Free Energy.

Understanding water reduction towards H2 generation is crucial to overcome today's renewable energy obstacles. Previous studies have shown the superior H2 production performances of Cobalt based penta-pyridyl (CoaPPy) and tetra-pyridyl (CoaTPy) complexes in solution. We investigate H2 production cycles of CoaPPy and CoaTPy complexes immersed in water solution by means of Ab-initio Molecular Dynamics and Density Functional Theory. We monitor dynamic properties of the systems, solvent response and structural changes occurring in the catalysts, by simulating all intermediate steps of the H2 production cycle. Reduction free energies and reorganization energies are calculated. Our results show that, following the first electron injection, H2 production proceeds with the singlet spin state. Following the first electron insertion, we observe a significant rearrangement of the hydrogen bonding network in the first solvation shell. The cobalt center turns out to be more accessible for the surrounding water molecules in the case of CoaTPy at all the intermediate steps, which explains its higher catalytic performance over CoaPPy. Following the first reduction reaction, a larger gain in reduction free energy is estimated for CoaTPy with respect to CoaPPy, with a difference of 0.14 eV, in line with the experiments. For the second reduction, instead, CoaPPy shows more negative reduction potential, by 0.41 eV.

Phthalocyanine pendented polyaniline via amide linkage for an electrochemical sensing of H2O2

Cobalt phthalocyanines are known to be the better electrocatalyst for a wide range of organic, inorganic, and biomolecular analytes. The synthesis of electrocatalysts through phthalocyanine (Pc) functionalization involves a single-step reaction to a multi-step reaction. The functionalization of Pc catalyst with the conducting polymer involves either simple electronic interactions or direct covalent bonding. Here we are reporting covalently connected tetra carboxy cobalt phthalocyanine (TCCoPc) pendent via amide linking to the conducting polyaniline (PANI) backbone. The synthesized electrocatalyst is extensively utilized for the sensing of biologically active H2O2 on the surface of the catalyst modified glassy carbon electrode (GCE). The synthesized molecule was characterized by various spectroscopic techniques and the detailed electrochemical activity of the synthesized molecule for the sensing of H2O2 was investigated using linear sweep voltammetry and chronoamperometry techniques, by fabricating on the GCE. The superlative catalytic activity of the synthesized molecule is might be due to the synergetic effect of pendanted macrocyclic Pc molecule with the redox-active demeanor of central cobalt metal atom and anchoring of the metal-centered Pc through amide linking with the conducting PANI.

Reversible Spin-State Switching and Tuning of Nuclearity and Dimensionality via Nonlinear Pseudohalides in Cobalt(II) Complexes.

The self-assembly of a macrocyclic tetradentate ligand, cobalt(II) tetrafluoroborate, and nonlinear pseudohalides (dicyanamide and tricyanomethanide) has led to two cobalt(II) complexes, {[Co(L)(μ1,5-dca)](BF4)·MeOH}n (1) and [Co2(L)2(μ1,5-tcm)2](BF4)2 (2) (L = N,N'-di-tert-butyl-2,11-diaza[3,3](2,6)pyridinophane; dca- = dicyanamido; tcm- = tricyanomethanido). Both complexes were characterized by single-crystal X-ray diffraction, spectroscopic, magnetic, and electrochemical studies. Structural analyses revealed that 1 displays a one-dimensional (1D) coordination polymer containing [Co(L)]2+ repeating units bridged by μ1,5-dicyanamido groups in cis positions, while 2 represents a discreate dinuclear cobalt(II) molecule bridged by two μ1,5-tricyanomethanido groups in a cis conformation. Both complexes have a CoN6 coordination environment around each cobalt center offered by the tetradentate ligand and cis coordinating bridging ligands. Complex 1 exhibits a high-spin (S = 3/2) state of cobalt(II) in the temperature range of 2-300 K with a weak ferromagnetic coupling between two dicyanamido-bridged cobalt(II) centers. Interestingly, complex 2 exhibits reversible spin-state switching associated with spin-spin coupling. Complexes 1 and 2 also exhibit interesting redox-stimuli-based reversible paramagnetic high-spin cobalt(II) to diamagnetic low-spin cobalt(III) conversion, offering an additional way to switch magnetic properties. A detailed theoretical calculation was consistent with the stated results.

The Photoactive Excited State of the B12-Based Photoreceptor CarH.

We have used transient absorption spectroscopy in the UV-visible and X-ray regions to characterize the excited state of CarH, a protein photoreceptor that uses a form of B12, adenosylcobalamin (AdoCbl), to sense light. With visible excitation, a nanosecond-lifetime photoactive excited state is formed with unit quantum yield. The time-resolved X-ray absorption near edge structure difference spectrum of this state demonstrates that the excited state of AdoCbl in CarH undergoes only modest structural expansion around the central cobalt, a behavior similar to that observed for methylcobalamin rather than for AdoCbl free in solution. We propose a new mechanism for CarH photoreactivity involving formation of a triplet excited state. This allows the sensor to operate with high quantum efficiency and without formation of potentially dangerous side products. By stabilizing the excited electronic state, CarH controls reactivity of AdoCbl and enables slow reactions that yield nonreactive products and bypass bond homolysis and reactive radical species formation.

Cobalt Metal-Organic Framework Based on Layered Double Nanosheets for Enhanced Electrocatalytic Water Oxidation in Neutral Media.

A new cobalt metal-organic framework (2D-Co-MOF) based on well-defined layered double cores that are strongly connected by intermolecular bonds has been developed. Its 3D structure is held together by π-π stacking interactions between the labile pyridine ligands of the nanosheets. In aqueous solution, the axial pyridine ligands are exchanged by water molecules, producing a delamination of the material, where the individual double nanosheets preserve their structure. The original 3D layered structure can be restored by a solvothermal process with pyridine, so that the material shows a "memory effect" during the delamination-pillarization process. Electrochemical activation of a 2D-Co-MOF@Nafion-modified graphite electrode in aqueous solution improves the ionic migration and electron transfer across the film and promotes the formation of the electrocatalytically active cobalt species for the oxygen evolution reaction (OER). The so-activated 2D-Co-MOF@Nafion composite exhibits an outstanding electrocatalytic performance for the OER at neutral pH, with a TOF value (0.034 s-1 at an overpotential of 400 mV) and robustness superior to those reported for similar electrocatalysts under similar conditions. The particular topology of the delaminated nanosheets, with quite distant cobalt centers, precludes the direct coupling between the electrocatalytically active centers of the same sheet. On the other hand, the increase in ionic migration across the film during the electrochemical activation stage rules out the intersheet coupling between active cobalt centers, as this scenario would impair electrolyte permeation. Altogether, the most plausible mechanism for the O-O bond formation is the water nucleophilic attack to single Co(IV)-oxo or Co(III)-oxyl centers. Its high electrochemical efficiency suggests that the presence of nitrogen-containing aromatic equatorial ligands facilitates the water nucleophilic attack, as in the case of the highly efficient cobalt porphyrins.

Dinuclear mixed valence cobalt(II/III) and hetero-tetranuclear cobalt(III)/Na complexes with a compartmental ligand: Synthesis, characterization and use as catalysts for oxidative dimerisation of 2-aminophenol

A N2O4 donor compartmental reduced Schiff base ligand, H2L [(2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-methoxyphenol)], obtained on 1:2 condensation of 2,2-dimethyl-1,3-propanediamine with ortho-vanillin followed by reduction with NaBH4 in methanol solution, has been used to prepare two cobalt complexes, [(N3)CoIIIL(µ-OAc)CoII(N3)] (1) and [(μ-N3)2{(AcO)CoIIILNa(CH3OH)}2]·2CH3OH (2). Complex 1 is a dinuclear mixed valence cobalt(III)/cobalt(II) complex with CoIIIO2CoII core. Complex 2, on the other hand, is a tetranuclear cobalt(III)/sodium complex with CoO2Na(N3)2NaO2Co core. Formation of complex 1 or 2 is mainly governed by the amount of cobalt(II) precursors present in the reaction mixture. Each complex has been characterized by elemental and spectral analysis. X-ray diffraction analysis has confirmed their structures. Complex 1 crystallized in a chiral space group Pna21 where both the cobalt(III) and cobalt(II) centers adopt six-coordinate distorted octahedral geometry with cobalt(III) and cobalt(II) centers residing respectivelyat inner N2O2 and outer O4 cavities of the reduced Schiff base. Complex 2 crystallized in triclinic system with P1-space group, where both cobalt(III) and sodium centers adopt distorted octahedral geometry. Oxidation states of cobalt centers have been confirmed by bond length consideration, BVS calculations as well as from room temperature magnetic moment measurement. Both complexes 1 and 2 show phenoxazinone synthase mimicking activity with kcat values 250.21 and 493.73 h−1 respectively.

6-Arylimino-2-(2-(1-phenylethyl)naphthalen-1-yl)-iminopyridylmetal (Fe and Co) Complexes as Highly Active Precatalysts for Ethylene Polymerization: Influence of Metal and/or Substituents on the Active, Thermostable Performance of Their Complexes and Resultant Polyethylenes.

A series of 6-arylimino-2-(2-(1-phenylethyl)naphthalen-1-yl)iminopyridines and their iron(II) and cobalt(II) complexes (Fe1–Fe5, Co1–Co5) were synthesized and routinely characterized as were Co3 and Co5 complexes, studied by single crystal X-ray crystallography, which individually displayed a distorted square pyramidal or trigonal bipyramid around a cobalt center. Upon treatment with either methyluminoxane (MAO) or modified methyluminoxane (MMAO), all complexes displayed high activities regarding ethylene polymerization even at an elevated temperature, enhancing the thermostability of the active species. In general, iron precatalysts showed higher activities than their cobalt analogs; for example, 10.9 × 106 g(PE) mol−1 (Co) h−1 by Co4 and 17.0 × 106 g(PE) mol−1 (Fe) h−1 by Fe4. Bulkier substituents are favored for increasing the molecular weights of the resultant polyethylenes, such as 25.6 kg mol−1 obtained by Co3 and 297 kg mol−1 obtained by Fe3. A narrow polydispersity of polyethylenes was observed by iron precatalysts activated by MMAO, indicating a single-site active species formed.

Replacement of the Cobalt Center of Vitamin B12 by Nickel: Nibalamin and Nibyric Acid Prepared from Metal-Free B12 Ligands Hydrogenobalamin and Hydrogenobyric Acid.

The (formal) replacement of Co in cobalamin (Cbl) by NiII generates nibalamin (Nibl), a new transition‐metal analogue of vitamin B12. Described here is Nibl, synthesized by incorporation of a NiII ion into the metal‐free B12 ligand hydrogenobalamin (Hbl), itself prepared from hydrogenobyric acid (Hby). The related NiII corrin nibyric acid (Niby) was similarly synthesized from Hby, the metal‐free cobyric acid ligand. The solution structures of Hbl, and Niby and Nibl, were characterized by spectroscopic studies. Hbl features two inner protons bound at N2 and N4 of the corrin ligand, as discovered in Hby. X‐ray analysis of Niby shows the structural adaptation of the corrin ligand to NiII ions and the coordination behavior of NiII. The diamagnetic Niby and Nibl, and corresponding isoelectronic CoI corrins, were deduced to be isostructural. Nibl is a structural mimic of four‐coordinate base‐off Cbls, as verified by its ability to act as a strong inhibitor of bacterial adenosyltransferase.

Replacement of the Cobalt Center of Vitamin B12 by Nickel: Nibalamin and Nibyric Acid Prepared from Metal‐Free B12 Ligands Hydrogenobalamin and Hydrogenobyric Acid

AbstractThe (formal) replacement of Co in cobalamin (Cbl) by NiII generates nibalamin (Nibl), a new transition‐metal analogue of vitamin B12. Described here is Nibl, synthesized by incorporation of a NiII ion into the metal‐free B12 ligand hydrogenobalamin (Hbl), itself prepared from hydrogenobyric acid (Hby). The related NiII corrin nibyric acid (Niby) was similarly synthesized from Hby, the metal‐free cobyric acid ligand. The solution structures of Hbl, and Niby and Nibl, were characterized by spectroscopic studies. Hbl features two inner protons bound at N2 and N4 of the corrin ligand, as discovered in Hby. X‐ray analysis of Niby shows the structural adaptation of the corrin ligand to NiII ions and the coordination behavior of NiII. The diamagnetic Niby and Nibl, and corresponding isoelectronic CoI corrins, were deduced to be isostructural. Nibl is a structural mimic of four‐coordinate base‐off Cbls, as verified by its ability to act as a strong inhibitor of bacterial adenosyltransferase.

Evidence of Octahedral Co–Mo–S Sites in Hydrodesulfurization Catalysts as Determined by Resonant Inelastic X-ray Scattering and X-ray Absorption Spectroscopy

Fundamental understanding of the active sites in CoMo/Al2O3 catalysts is crucial to improve the production of clean transportation fuels by hydrodesulfurization (HDS). In this work, the coordination state and number of active sites (“Co–Mo–S”) in sulfided CoMo/Al2O3 catalysts prepared with and without chelating additives were determined using in situ 1s2p resonant inelastic X-ray scattering (RIXS) and X-ray absorption spectroscopy (XAS). Based on the evaluation of thiophene HDS activity as a function of the Co/Mo ratio, it was determined that only for Co/Mo < 0.1, cobalt is exclusively in interaction with the MoS2 phase, making these compositions ideal for identifying the nature of the active cobalt sites for hydrogenolysis. These cobalt promoter atoms were in centrosymmetric coordination with the MoS2 phase, with six sulfur ligands coordinated to each cobalt center. The octahedral coordination of cobalt in Co–Mo–S in alumina-supported HDS catalysts contrasts the tetrahedral cobalt coordination found in Co-promoted MoS2 model systems supported on gold or carbon. We attribute the difference to more disordered MoS2 structures in technical alumina-supported HDS catalysts. The number of Co–Mo–S sites depended strongly on the catalyst preparation method. Chelating additives, such as citric acid and nitrilotriacetic acid, increased the number of Co–Mo–S sites without significantly altering their intrinsic activity. These results demonstrate that the state-of-the-art structural characterization tools, such as RIXS, identify the structure of active sites in actual catalysts and that these structures may be different from model systems. Combining such characterization tools with guided catalyst design can provide an insight into the structure of amorphous, nanostructured materials, which aid in the development of improved heterogeneous catalysts. KEYWORDS:Hydrodesulfurization Co−Mo−S RIXS active site XAS

Ligand-Mediated Spin-State Changes in a Cobalt-Dipyrrin-Bisphenol Complex.

The influence of a redox-active ligand on spin-changing events induced by the coordination of exogenous donors is investigated within the cobalt complex [CoII(DPP·2–)], bearing a redox-active DPP2– ligand (DPP = dipyrrin-bis(o,p-di-tert-butylphenolato) with a pentafluorophenyl moiety on the meso-position. This square-planar complex was subjected to the coordination of tetrahydrofuran (THF), pyridine, tBuNH2, and AdNH2 (Ad = 1-adamantyl), and the resulting complexes were analyzed with a variety of experimental (X-ray diffraction, NMR, UV–visible, high-resolution mass spectrometry, superconducting quantum interference device, Evans’ method) and computational (density functional theory, NEVPT2-CASSCF) techniques to elucidate the respective structures, spin states, and orbital compositions of the corresponding octahedral bis-donor adducts, relative to [CoII(DPP·2–)]. This starting species is best described as an open-shell singlet complex containing a DPP·2– ligand radical that is antiferromagnetically coupled to a low-spin (S = 1/2) cobalt(II) center. The redox-active DPPn– ligand plays a crucial role in stabilizing this complex and in its facile conversion to the triplet THF adduct [CoII(DPP·2–)(THF)2] and closed-shell singlet pyridine and amine adducts [CoIII(DPP3–)(L)2] (L = py, tBuNH2, or AdNH2). Coordination of the weak donor THF to [CoII(DPP·2–)] changes the orbital overlap between the DPP·2– ligand radical π-orbitals and the cobalt(II) metalloradical d-orbitals, which results in a spin-flip to the triplet ground state without changing the oxidation states of the metal or DPP·2– ligand. In contrast, coordination of the stronger donors pyridine, tBuNH2, or AdNH2 induces metal-to-ligand single-electron transfer, resulting in the formation of low-spin (S = 0) cobalt(III) complexes [CoIII(DPP3–)(L)2] containing a fully reduced DPP3– ligand, thus explaining their closed-shell singlet electronic ground states.

59Co Internal field NMR analysis of Co35Fe35Ni30 alloy synthesized via novel low cost chemical reduction technique

We have synthesized ternary cobalt alloy using a chemical reduction process using hydrazine as a reducing agent . X-Ray Diffraction (XRD) measurements shows that the synthesized cobalt ternary alloy exists in the pure face centred cubic (fcc) phase with lattice parameter of 3.5562 ± 5 Å. The SEM micrograph shows that the particles exist in the spherical shape which suggests that the particles are crystallized in fcc phase. The magnetic properties are determined using Vibrating Sample Magnetometry (VSM) and 59Co IFNMR technique. The saturation magnetization (MS) of the synthesized sample is ~80 emu/g, a slightly lower value compared to the pure cobalt metal. 59Co IFNMR technique shows that there is a broad distribution of hyperfine field (~6 T) compared to the pure fcc cobalt metal. Further, from the deconvoluted NMR peaks suggests that the variation in the nearest neighbours to the central cobalt atom leads to the different hyperfine field.

Interfacial electronic interaction of atomically dispersed IrClx on ultrathin Co(OH)2/CNTs for efficient electrocatalytic water oxidation

The exploration of highly-efficient oxygen evolution reaction (OER) electrocatalyst with well-defined structure to understand specific structure-performance relationship is crucial to meet the requirement of water-splitting. Herein, we firstly used atomically dispersed IrClx to uniformly decorate ultrathin Co(OH)2 nanosheets to dramatically improve electrocatalytic activity. The synthesized IrClx-Co(OH)2 nanosheets/carbon nanotubes (CNTs) exhibits an overpotential of 230 mV to reach 10 mA/cm2, which was much enhanced compared to that of pristine Co(OH)2/CNTs (308 mV) and commercial benchmark IrO2 (309 mV). X-ray absorption fine structure and density functional theory simulations demonstrate strong interfacial interaction between IrClx and Co(OH)2 nanosheets via the Cl-Ir-O and Ir-Cl-Co bond can efficiently boost its electronic conductivity. The accelerated charge transfer promotes the formation of more positively charged O atoms around cobalt centers, which is beneficial for the deprotonation on IrClx-Co(OH)2 and makes the catalyst facilitate OER.

Theoretical Study of VX Hydrolysis Mechanism Catalyzed by Phosphotriesterase Mutant H254R

AbstractPhosphotriesterase (PTE) is a bacterial enzyme that can catalyze the hydrolysis of V‐type chemical warfare nerve agents. Herein, the hydrolysis mechanism of VX analogs catalyzed by the H254R PTE mutant, in which the histidine 254 residue is replaced by an arginine, has been investigated using the hybrid density functional theory method B3LYP. The chemical model of the active site of H254R PTE mutant was constructed based on the high resolution X‐ray crystal structure. We have presented the energetics and the characterization of stationary points in the catalytic reaction. The calculations indicate that the catalytic process contain two SN2 displacement steps and the step of hydroxide regeneration is rate‐limiting with an overall barrier of 22.2 kcal/mol. The Arg254 plays an important role as a proton donor/acceptor for protonating leaving group and activating water molecule. Moreover, the H254R PTE mutant with binuclear cobalt center turns out to higher activity towards VX than the binuclear zinc H254R PTE.

A new homobimetallic cobalt(II) complex based on the tetradentate 3,5-bis(2-pyridyl)-1H-1,2,4-triazole ligand: Synthesis, crystal structure, Hirshfeld analysis, spectroscopic characterization, magnetic properties and antimicrobial activities

A novel homobimetallic cobalt(II) complex, [Co2(bpt)2(H2O)4](NO3)2·2H2O (bpt = 3,5-bis(2-pyridyl)-1,2,4-triazolate), has been synthesized and characterized by single crystal X-ray diffraction, Hirshfeld surface analysis, as well as UV–Visible and FTIR techniques. The crystal structure determination shows that the dinuclear complex crystallizes in the triclinic crystal system with the P1- space group. The (N′,N1,N2,N″)2 double bridging coordination mode is realised with a distorted octahedral N4O2-coordinated cobalt centre. The structure cohesion is ensured by different intermolecular hydrogen bonds, π–π stacking interactions and intermolecular lone-pair···π interactions. The structure consists of [Co2(bpt)2]2+ dimeric units with a cobalt-cobalt distance of 4.170(2) Å. Magnetic measurements carried out on the homobimetallic complex indicate a relatively weak antiferromagnetic interaction between the two cobalt centres. This complex exhibited antimicrobial activity against several strains of the fungal phytopathogens Verticillium dahliae and Fusarium oxysporum fsp. melonis, as well as against the phytopathogenic bacteria Agrobacterium tumefaciens, suggesting its use in crop protection.

Equilibria in Aqueous Cobalt(II)-Reduced Schiff Base N-(2-hydroxybenzyl)alanine System: Chemical Characterization, Kinetic Analysis, Antimicrobial and Cytotoxic Properties.

The present study describes the coordination properties of a reduced Schiff base, N-(2-hydroxybenzyl)alanine, towards cobalt(II) using potentiometric as well as spectroscopic (UV-Vis and ESI-MS) methods. The results indicate the formation of six mononuclear complexes showing high stability in aqueous solution. Coordination occurs in the {O−phenolic,N,O−carboxyl} and {N,O−carboxyl} chelation modes, depending on the degree of ligand deprotonation. Examination of the complexation equilibria at pH ca 7, which is important from a biological point of view, allowed to identify two species: [CoL] and [CoL2H]−. The kinetic analysis showed a structural change of those cobalt(II) complexes from octahedral to tetrahedral in accordance with a first-order time relationship. The antimicrobial properties of N-(2-hydroxybenzyl)alanine, cobalt(II) nitrate and of the Co(II) – ligand complexes were determined against Gram-positive bacteria (Enterococcus faecalis, Staphylococcus aureus, Staphylococcus epidermidis), Gram-negative bacteria (Pseudomonas aeruginosa, Escherichia coli, Helicobacter pylori) and a fungal strain (Candida). The results indicate that the complexes are more active for more strains than the ligand alone. Nevertheless, the complexes induce a higher decrease in the metabolic activity of cells but without damage to nuclei. Tetrahedral structures show stronger anti-cellular toxicity than octahedral complexes, which is most likely due to the higher accessibility of the cobalt(II) center.