Mononuclear Cobalt Research Articles

Synthesis, X-ray characterization and catalytic homogenous alcohol oxidation activity of Co(II)–carboxamide complex with green oxidant (H2O2) under mild conditions

Abstract In this study, a new air and moisture stable mononuclear cobalt(II)–carboxamide complex, [Co(TCrbx)2(CH3OH)2](ClO4)2, was synthesized and characterized (TCrbx = N-(4-methylpyridin-2-yl)thiophene-2-carboxamide). Complex characterization mainly was done with single crystal X-ray analysis. Ligand characterization was done with several spectroscopic techniques (Elemental Analysis, FT-IR, 1H NMR, 13C NMR). Cobalt(II) complex possesses distorted octahedral geometry coordinated with two carboxamide ligands at equatorial and two methanol ligands at axial positions and two perchlorate anions as counter ions. Synthesized complex was successfully tested as homogenous catalyst for the oxidation of benzyl alcohol with environmental friendly oxidant hydrogen peroxide (H2O2) under mild conditions. Benzaldehyde was selectively obtained with the conversion value of 99.5% in dimethyl formamide after 3-h reaction time at 50 °C with 133 TON value. Solvent and temperature effects were also investigated.

Valence tautomerism in catecholato cobalt Bis(phenolate) diamine complexes as models for Enzyme–substrate adducts of catechol dioxygenases

Mononuclear cobalt(III) complexes of 3,5-di-tert-butylcatecholato cobalt complex of the substituted bis(phenol) diamine ligands of H2LNEX (CoLNEX(DTBC) which X: BM, Bu and OB stands for 2-(tert-butyl)-4-methyl, 2,4-di-tert-butyl and 2-(tert-butyl)-4-methoxy phenols, respectively, and DTBC is an abbreviation for 2,4-di-tert-butylcatechol, as models for enzyme-substrate adducts of catechol dioxygenases, have been synthesized and characterized by different techniques. X-ray structure analysis has revealed that cobalt centers were surrounded by two phenolates, two amine nitrogens and two oxygen atoms of catechol ligand. For all three compounds, the magnetic susceptibility increases with decreasing temperature as expected for non-interacting, magnetic moments which confirm the monomeric structure of cobalt complexes and valence tautomerism between Co(II) and Co(III) oxidation state. Crystal structures confirm the nature of Co(II)/Co(III) character consistent with the magnetic experiments. In electronic absorption spectra of two complexes, the position of LMCT bands show the phenolate(π)-to- Co(II) or Co(III) (dπ*) charge transfer. The cyclic voltammetry studies, show the ligand-centered oxidation of phenolate group to phenoxyl radical.

Computer Simulation of the Structure and Magnetic Properties of Cobalt Complexes with N-Substituted Pyridinophanes and Radical-Functionalized o-Benzoquinones

The structure and energy and magnetic characteristics of mononuclear cobalt complex with N,N'-dialkyl-2,11-diaza[3.3]-(2,6)pyridinophanes and o-benzoquinone ligands containing stable radicals (nitronyl nitroxide and TEMPO) were studied by the density functional theory (DFT UTPSSh/6-311++G(d,p)). It was shown that the stability of electromeric forms of complexes and, hence, the possibility of implementing the magnetic bistability mechanisms (spin-crossover and valence tautomerism) can be controlled by varying alkyl substituents at the tetraazamacrocycle nitrogen atoms. The nature of the radical group determines the strength of exchange interactions in which it participates, but does not affect the exchange in the redox-active moiety.

Coordination environment variations in multinuclear trigonal bipyramid Co(II) complexes bearing tetradentate sulfonamide N-donors and phenoxazinone synthase activities

Most documented investigations deploying transition metal complexes as phenoxazinone synthase models only report one or two complexes, which limits understanding of structure-property correlations between varying compositions as well as the coordination environments of the active site changes and the biomimetic activities. In efforts to systematically investigate changes in the vicinity of cobalt(II), five new complexes of two tripod N,N′,N″-(nitrilotris(ethane-2,1-diyl))tris(benzenesulfonamide) ligand derivatives (1 and 2) were self-assembled with incorporation of Na+, K+ or Cs+ as bridging counter cations. All the complexes were structurally analysed and found to possess distorted trigonal bipyramid cobalt(II) centres such that an easily displaceable water molecule always occupy one of the axial positions. While ligand 1 forms (Na[Co1.w]E)n as a 1D coordination polymer, ligand 2 forms the dimeric complexes (Na[Co2.w]M)2, (Na[Co2.w])2, (K[Co2.w]A)2 and (Cs[Co2.w]M2,w2)2. Continuous Shape Measurement (CShM) calculations were carried out to analyse distortions of the obtained trigonal bipyramid geometries. The high turnover numbers of 584 h−1 for (Na[Co2.w])2, 564 h−1 for (Cs[Co2.w]M2.w2)2, 543 h−1 for (K[Co2.w]A)2 and 446 h−1 for (Na[Co2.w]M)2 represent attractive level of phenoxazinone mimicking capacity relative to literature data for mononuclear cobalt(II) complexes. It was concluded that the release of free cobalt(II) anions [Co1.w]− or [Co2.w]− via dissociations of the multinuclear complexes is necessary for the phenoxazinone synthase mimicking activities to occur and that the incorporated group I metal ions possess no influence on the observed mimicking activities. The trigonal bipyramid coordination environments of the anionic chelates [Co1.w]− or [Co2.w]− yielded better outcomes than their previously reported vacant octahedral (vOC-5, C4v) complex analogues.

Luminescence cobalt (II) complexes: Synthesis, characterization, photophysical and DFT study

Abstract Four mononuclear cobalt (II) complexes of substituted hydrazino quinoline Schiff bases 1(a–d) were synthesised and characterized by UV, IR, NMR and TGA studies. The geometry of cobalt complexes 1(a–d) unambiguously attested as distorted octahedral’ and the ligand was coordinated through NNO donor fashion of tridentate nature. Structure of the proposed complexes were optimized using Density Functional theory (DFT) with Gaussian 09/ Gauss view software. Mulliken charges, global softness and electrophilicity index were derived for the optimized structure and the energy of highest occupied orbital (HOMO) and lowest unoccupied orbital (LUMO) and energy gap were calculated. The photophysical properties of the synthesised complexes were analyzed by UV–Visible and photoluminescence spectral studies, the results revealed that the emission bands centered in the range of 445–455 nm with higher luminescence intensity and relatively large Stoke’s shift observed (198 nm–215 nm) in the absorption and emission shoed a promising novel material towards OLED’S.

Cobalt Thiolates with Organic N,N,S-Tridentate Ligand: Conversion of Thiolate Complex to Sulfinate Complex via Sulfenate

Mononuclear cobalt(III) complex with 2-[(3-aminopropyl)amino]ethanethiol (Hapaet), [Co(apaet)2]ClO4 (1) and the sulfinate complex, [Co(apaetO2)2]ClO4 (2) were synthesized and characterized by measurements of the elemental analysis, infrared and electronic spectra (UV-vis-NIR), and temperature dependence of magnetic susceptibilities (4.5—300 K), and DFT calculations. X-Ray crystal structure analysis of 1 and 2 revealed an octahedral CoS2N4 or Co(SO2)2N4 chromophore with meridional chelation of two apaet− or apaetO2 −. Conversion of 1 to the sulfinate complex 2 via the sulfenate complex was investigated by UV-vis spectra. Oxidation reaction of capsanthin with 1 was also examined.

Syntheses, X-Ray Single Crystal Structures and Biological Activities of Cobalt(III) Complexes with Reduced Schiff Base Ligands

A new mononuclear cobalt(III) complex, [Co(HL1)2]Cl (1), derived from the reduced Schiff base 2,2'-((ethane-1,2-diylbis(azanediyl))bis(methylene))diphenol (H2L1), and a new dinuclear cobalt(III) complex, [Co2(L2)2]∆2H2O (2), derived from the reduced Schiff base 6,6'-(2-hydroxypropane-1,3-diyl)bis(azanediyl)bis(methylene)bis(2-bromo-4-chlorophenol) (H2L2), were synthesized and characterized by infrared and electronic spectroscopy, and single crystal X-ray diffraction techniques. The ligands were synthesized first, and then bound to the Co(III) centre. Compound 1 contains a mononuclear [Co(HL1)2]+ cation and a chloride anion. The cationic moiety possesses crystallographic inversion center symmetry. Compound 2 contains a dinuclear [Co2(L2)2] molecule and two water hydrate molecules. The Co atoms in the complexes are in octahedral coordination. Both complexes showed potential antimicrobial activity.

Mononuclear Cobalt and Iron o-Quinone Complexes with Tetradentate N-Donor Bases: Structures and Properties

Published data on the electronic structures and magnetic behavior of the mononuclear cobalt and iron o-benzoquinone complexes with tetradentate nitrogen-containing bases are reviewed. The chosen objects are of significant interest due to their ability to manifest magnetic bistability, indicating wide prospects of the practical use of compounds of this class in molecular electronics and spintronics. The influence of structural features of the complexes on their magnetic properties is discussed on the basis of the quantum-chemical calculation results.

Mononuclear cobalt(II), nickel(II) and copper(II) complexes: Synthesis, spectral characterization and interaction study with nucleotide by in vitro biochemical analysis

A series of six novel metal complexes of the type [M(L1)2] 1a-1c and [M(L2)2] 2a-2c, where, M = Cu(II), Ni(II), Co(II), HL1 = 2-(5-(trifluoromethyl)-2-methoxyphenylimino)methyl)-4,6-dichlorophenol and HL2 = 2-(5-(trifluoromethyl)-2-methoxyphenylimino)methyl)-4-bromo-6-methoxyphenol, have been synthesized and characterized by several physicochemical techniques. Having a base of spectroscopic and analytical data, complexes 1a, 1c, 2a and 2c showed a square planar geometry, whereas, complexes 1b and 2b owned a octahedral geometry around the metal ion. Thermal decomposition process of complexes 1a-2c is nonspontaneous, i.e., the complexes are thermally stable. DNA binding (UV–Visible absorption and fluorescence) studies against calf thymus DNA (CT-DNA), revealed an intercalative mode of binding. The complexes along with corresponding ligands have been investigated for their in-vitro cytotoxic activity against two human cancer cell lines such as HeLa and A549 using cisplatin as standard drug under MTT assay. All the complexes showed higher cytotoxicity than the parent ligand and moderate to cisplatin. In addition, antimicrobial activity of all the compounds was also examined.

Heteroleptic cobalt(II) complex with nitrogen-rich macrocycles — Structure, bioactivity and DFT modelling

We herein discuss the synthesis and characterization of a novel mononuclear cobalt(II) complex CoII(TPBP)(HTMA)2 with the N-donor ligands; hexamethylenetetramine HTMA and meso-tetrakis[4-(benzoyloxy) phenylporphyrin H2TPBP. Firstly, molecular structure and supramolecular assembly have been thoroughly described. The basic core (porphyrin macrocycle) is nearly planar and the cobalt coordination centre sits in slightly distorted octahedral environment occupied by four pyrrolic nitrogen atoms (NPy) and two HTMA nitrogens (NHTMA). On the basis of quantum chemical density-functional theory calculations, metal—ligand charge-transfer interaction (MLCTI), spin density distribution and chemical reactivity character have been also highlighted. Finally, the bio-activity of free porphyrin and their cobalt complexes was evaluated in vitro, by examining their inhibitory effect against some yeasts and fungal strains, and their ability to scavenge 1,1-diphenyl-picrylhydrazyl DPPH radical. This research could provide rich insights into the structural, electronic and (bio)chemical features of analogues macrocyclic metal complexes.

Catalytic Efficacy of 2,2′-Bipyridine Cobalt(II) Complex: Hydrothermal Synthesis, X-Ray Structure and Aerobic Epoxidation of Alkenes

A mononuclear cobalt(II) complex, [Co(bpy)2(NO3)](NO3)·3H2O (1) (bpy = 2,2′-bipyridine) has been synthesized hydrothermally and the crystal structure was characterized by X-ray crystallography. Complex 1 is capable of activating aerobic oxygen at atmospheric pressure. [Co(bpy)2(NO3)](NO3)·3H2O (1) was used as an active catalyst for the aerobic epoxidaion of various alkenes with isobutyraldehyde as co-reductant in acetonitrile medium. Complex 1 catalyzes the epoxidaion reaction efficiently, which reflected in high yield of products with desired selectivity.

Nucleophilic reactivity of a mononuclear cobalt(iii)-bis(tert-butylperoxo) complex.

A mononuclear cobalt(III)-bis(tert-butylperoxo) adduct (CoIII-(OOtBu)2) bearing a tetraazamacrocyclic ligand was synthesized and characterized using various physicochemical methods, such as X-ray, UV-vis, ESI-MS, EPR, and NMR analyses. The crystal structure of the CoIII-(OOtBu)2 complex clearly showed that two OOtBu ligands bound to the equatorial position of the cobalt(iii) center. Kinetic studies and product analyses indicate that the CoIII-(OOtBu)2 intermediate exhibits nucleophilic oxidative reactivity toward external organic substrates.

Reversible thermo-induced spin crossover in a mononuclear cis-dicyanamido-cobalt(ii) complex containing a macrocyclic tetradentate ligand.

We present a unique investigation of a mononuclear cis-dicyanamido-cobalt(ii) complex, [Co(L)(N(CN)2)2] (1) (L = N,N'-di-tert-butyl-2,11-diaza[3,3](2,6)pyridinophane), which displays a reversible thermo-induced HS (S = 3/2) to LS (S = 1/2) Spin Crossover (SCO) with a T1/2 value of around 238 K. The appropriate ligand field strength around the CoN6 coordination environment is obtained using a tetradentate ligand and two dicyanamide [N(CN)2]- coligands at the cis position. Variable temperature XRD and several spectroscopic studies have been performed, which confirm the SCO behavior of 1. Interestingly, complex 1 pronounces redox stimulus responsive reversible spin-state switching from HS Co(ii) to LS Co(iii). These findings represent a rare example of the successful use of pseudohalide to design and synthesize a SCO cobalt(ii) complex.

Coordination polymers of a bis-isophthalate bridging ligand with single molecule magnet behaviour of the CoII analogue.

A linear diamine-bisisophthalate bridging linker N,N'-bis(1,3-dicarboxyphenyl-5-methylene)-1,3-dimethylpropanediamine, designed to incorporate amine/ammonium functionalities in the core of the ligand, has been isolated as the pentahydrate of its dihydrochloride salt (H6L)Cl2·5H2O. Using this compound, four new coordination polymers have been formed, namely poly-[M(H2L)]·4.5H2O (1M, where M = Co, Zn, Cd) and poly-[Cd(H2L)(OH2)]·DMF·7H2O (2). Compounds 1M are isostructural 2D coordination polymers that contain 1D channels occupied by water molecules. In the case of 1Co these form a well ordered hydrogen-bonding network as determined by single crystal X-ray studies. Compound 2, synthesised under similar conditions, is a 1D coordination polymer in which the metal is partially solvated. DC and AC magnetic studies of 1Co, which posseses a mononuclear cobalt(ii) node, revealed single molecule magnet behaviour (SMM) with an effective barrier height Ueff of 37.7 K and τ0 = 1.02 × 10-9 s, among the highest reported for CoII coordination polymers.

The first report of a tetra-azide bound mononuclear cobalt(iii) complex and its comparative biomimetic catalytic activity with tri-azide bound cobalt(iii) compounds

Three new azide-bound cobalt(iii) complexes derived from three different triamines with extensive hydrogen bonded supramolecular chain structures and the role of their structural factors in oxidative coupling of o-aminophenols have been reported.

Redox-active ligand assisted electrocatalytic water oxidation by a mononuclear cobalt complex.

In this report, the synthesis, characterization and electrocatalytic oxidation of water by a mononuclear cobalt(iii) complex, [CoIII(dpaq)(Cl)]Cl (1) featuring a redox-active pentadentate amidate ligand (H-dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate) is reported. Complex 1 has been found to be a stable and homogeneous water oxidation catalyst (WOC) in 0.1 M phosphate buffer (pH 8.0). A series of experiments (rinse test, SEM, EDX spectroscopy) confirm that this complex acts as a molecular electrocatalyst, and not a precursor of CoOx. The electrocatalytic water oxidation proceeds with high faradaic efficiency (81%) and fast rate (85 s-1). Analysis of the electrochemical reaction kinetics by foot-of-the-wave (FOWA) methodology reveals a high turnover frequency of 1.6 × 104 s-1 which is comparable to the best performing Ru-based WOCs.

Processes for Electrochemical Production of Electrolyte-free Hydrogen Peroxide

Hydrogen peroxide is the focus of growing attention for distributed manufacturing, reflecting its decentralized applications and shipping hazards. Two recent reports, one in Science by Wang and colleaugues and one in this issue of Joule by Surendranath, Hatton, and colleaugues, introduce innovative electrochemical methods to produce electrolyte-free aqueous solutions of hydrogen peroxide.

Coligand Effects on the Field-Induced Double Slow Magnetic Relaxation in Six-Coordinate Cobalt(II) Single-Ion Magnets (SIMs) with Positive Magnetic Anisotropy.

Two mononuclear cobalt(II) compounds of formula [Co(dmphen)2(OOCPh)]ClO4·1/2H2O·1/2CH3OH (1) and [Co(dmbipy)2(OOCPh)]ClO4 (2) (dmphen = 2,9-dimethyl-1,10-phenanthroline, dmbipy = 6,6'-dimethyl-2,2'-bipyridine and HOOCPh = benzoic acid) are prepared and magnetostructurally investigated. Each cobalt(II) ion is six-coordinate with a distorted octahedral CoN4O2 environment. The complex cations are interlinked leading to supramolecular chains (1) and pairs (2) that grow along the crystallographic c-axis with racemic mixtures of (Δ,Λ)-Co units. FIRMS allowed us to directly measure the zero-field splitting between the two lowest Kramers doublets, which led to axial anisotropy values of 58.3 cm-1 ≤ D < 60.7 cm-1 (1) and 63.8 cm-1 ≤ D < 64.1 cm-1 (2). HFEPR spectra of polycrystalline samples of 1 and 2 at low temperatures confirm the positive sign of D and provide an estimate of the E/D quotient [0.147/0.187 (1) and 0.052 (2)]. Detailed ac and dc magnetic studies reveal that 1 and 2 are new examples of field-induced single-ion magnets (SIMs) with small transversal anisotropy. CASSCF/NEVPT2 calculations support these results. Two Orbach processes or one Orbach plus a direct relaxation mechanism provide similar agreements with the nonlinear experimental Arrhenius plots at Hdc = 500 and 2500 G for 1. Two independent relaxation processes occur in 2, but in contrast to 1, an observed linear dependence of ln(τ) vs 1/T substantiates Orbach processes against the most widely proposed Raman and direct mechanisms. The analysis of each relaxation process in 2 provided values for Ea and τ0 that are very close to those found for 1, validating the predominant role of the Orbach relaxations in both compounds and, probably, also in other cobalt(II) SIMs. A mechanism based on a spin-phonon coupling is proposed to account for the SIM behavior in 1 and 2 with any Raman or direct processes being discarded.

Side-On Coordination of Nitrous Oxide to a Mononuclear Cobalt Center.

Despite its utility as an oxygen-atom transfer reagent for transition metals, nitrous oxide (N2O) is a notoriously poor ligand, and its coordination chemistry has been limited to a few terminal, end-on κ1-N complexes. Here, the synthesis of a mononuclear cobalt complex possessing a side-on-bound N2O molecule is reported. Structural characterization, IR spectroscopy, and DFT calculations support an η2-N,N binding mode for binding of N2O to the cobalt center.

Mononuclear cobalt, nickel and copper complexes with glycinamide: structural properties and biological activity

Mononuclear cobalt, nickel and copper complexes with glycinamide: structural properties and biological activity