Photoreduction of chlorohemin in pure pyridine

Chlorohemin (Fe(III)PPCl) undergoes photoreduction when irradiated in pure pyridine solution with 400–450 nm light. A thermal reduction is observed to occur simultaneously with the photochemical one, but after a one hour irradiation about 75% of the reduction product is formed in a photochemical way. Both five and six-coordinated species are observed to be present in solution; however, only the Fe(III)PPpy⁺ five coordinated complex is photoreducible. A mechanism is proposed whereby the primary photochemical act is an axial pyridine → iron electron transfer process yielding Fe(II)PP and py⁺ species. The Fe(II)PP moiety gives rise to the formation of the spectrophotometrically detectable Fe(II)(PP(py)₂ complex. ESR spin trapping results are consistent with the formation of 2-pyridyl radicals from py⁺ cation by fast transfer of a proton to a pyridine molecule.     

EPR studies of copper(II) complexes with poly-2-methyl-5-vinyl-pyridine and poly-2-vinylpyridine

Cu(II) complexes of poly-2-vinylpyridine (P2VP) and poly-2-methyl-5-vinylpyridine (P2M5VP), partially quaternized by dimethylsulphate, and of the analogues (2-methyl-5-ethylpyridine, 2-ethylpyridine) were studied by EPR spectroscopy in a mixture of methanol and water. Peculiarities of the complex formation reaction were observed for the polymers compared to the analogues. At ratios of [Py]:[Cu²⁺] > 40, the predominant formation of tetrapyridinate-Cu(II) species [(CuL₄)]²⁺ was found for P2M5VP. However, differences were found between the parameters of EPR-spectra for the [CuL₄]²⁺ in the polymer from that of the [Cu(2M5EPy)₄]²⁺. It was suggested that, in the polymer, [CuL₄]²⁺ complexes with structure intermediate between square planar and tetrahedral are formed. Moreover, the maximum value of the pyridine fraction forming [CuL₄]²⁺ in P2M5VP was found to be about 10% and it is appreciably less the value of the fraction in P4VP (about 40%). For P2VP at [Py]:[Cu²⁺] > 40, an insignificant amount of [CuL₁]²⁺ and [CuL₂]²⁺ are formed in the solution. It follows that the main chain position relative to the ligand nitrogen atom in these polyvinylpyridines affects profoundly the complexation between the macromolecules and Cu(II) ions. The steric hindrances due to the chain are likely to change the [CuL₄]²⁺ structure and to prevent complex formation for P2VP.     

Structure and spectroscopic characterization of the quaternary system: Copper(II), pyridine-2,6-dicarboxylate, 2,2′-bipyridyl and pyridine

Conclusion The x-ray structural results confirm what has been ascertained by thermodynamic and spectroscopic data in aqueous solution(7). It is evident that the addition of pyridine to the solution containing the mixed species [Cu(bipy)(pydca)(H₂O)] leads to the substitution of a water molecule directly bound to copper(II) ion by a pyridine molecule. This experiment also demonstrated the presence of a water molecule in the equatorial plane of the complex.The subsequent diffractometric study on single crystals derived from the copper(II)/bipy/pydca system revealed the existence in the solid state of [Cu₂(bipy)₂(pydca)₂] · 4H₂O. Thus the pydca dianion, instead of forming the statistically favoured mixed complex [Cu(bipy)(pydca)], gives rise to crystals containing two different copper(II) environments: [Cu(pydca)₂]^2– and [Cu(bipy)₂]²⁺, linked by O-carboxylate bridges. The facility with which [Cu(bipy)(pydca)(py)] can be obtained shows that the addition of pyridine prevents the formation of polynuclear species.     

Copper(II) complex based on a V-shaped ligand, 2,6- bis (2-benzimidazolyl)pyridine: synthesis,crystal structure,DNA-binding properties,and antioxidant activities

A copper(II) complex based on a V-shaped ligand, 2,6-bis(2-benzimidazolyl)pyridine (bbp), has been synthesized and characterized by elemental analysis, molecular conductivity, ¹H NMR, IR, UV-Vis spectra, and X-ray single-crystal diffraction. The crystal structure of [Cu(bbp)₂](pic)₂?·?2DMF (pic?=?picrate) shows copper is six-coordinate forming a distorted octahedron. The interaction between Cu(II) complex and DNA was investigated by spectrophotometric methods and viscosity measurement. The experimental results suggest that the Cu(II) complex binds to DNA via intercalation. Antioxidant assay in vitro also shows that the Cu(II) complex possesses significant antioxidant activities.     

Synthesis and Characterization of Polynuclear Metal Complexes with Bridging Ketoiminato and Thioiminato Schiff Base Ligands

Bimetallic and trimetallic complexes of stoichiometry [M(acacen)M′Y₂], [M(sacacen)M′Y₂], and {[M(acacen)]₂M′Y₂} have been prepared by reaction of the appropriate square-planar Schiff base metal complex with various secondary metal salts in toluene and/or absolute ethanol. Systems which are reported here include those where M = Cu(II); M′ = Cu(II), Ni(II), Co(II), Mn(lI) and Zn(II); Y^? = Cl^?, Br^?, and NO₃ ^?. Trinuclear complexes have been isolated only for {[Cu(acacen)]₂M′(NO₃)₂} where M′ = Cu(lI) or Mn(II); binuclear complexes result from all other combinations. The geometry of the chelated Cu(II) ion is square-planar in the bimetallic complexes and possibly square-pyramidal in the trimetallic compounds, while that of the secondary metal ion depends on the coordination preference of M′, the nature of Y^? and whether the bridging donor atoms are oxygen or sulfur. Probable structures of the new polynuclear complexes have been deduced from spectral, conductivity and magnetic measurements.     

Physico-chemical study of ionic equilibria for poly-2 and poly-4-vinylpyridines in alcohol-water solutions

The ionic equilibria for poly-4-vinyl pyridine (P4VP) and poly-2-vinyl pyridine (P2VP) were studied by physico-chemical techniques such as potentiometry, viscosity and NMR-¹H. The mixture of ethanol (45 per cent w.p.) and water was used as solvent to obtain the total range of ionization (0–1). It was found that the dissociation constants of pyridine residue of polymers in the absence of electrostatic interaction (pK₀ = 3·3–3·9) are lower than for the monomer analogues 4-ethylpyridine and 2-ethylpyridine (5·02) and depend on ionic strength (NaCl).A sharp decrease of pK_app at the beginning of titration and increase of specific viscosity for P4VP and P2VP are both explained by electrostatic interactions between positive charges forming during titration of the macromolecules. Most probably, these interactions act through the organic part of the macromolecule. On the other hand, it is shown by NMR-¹H that sharp changes in pK_app and specific viscosity at the beginning of the titration are not associated with changes in the average conformation of the monomer unit in the polymer. This conformation can be destroyed only when the energy of electrostatic interactions is large enough and this occurs when the mean distance between positive charges is relatively short.     

Tetrakis(μ‐benzoato‐κ2O:O)bis{[4‐(dimethylamino)pyridine‐κN1]copper(II)}

The title compound, [Cu₂(C₇H₅O₂)₄(C₇H₁₀N₂)₂], is a crystallographically centrosymmetric binuclear complex, with Cu atoms [Cu...Cu = 2.6982 (4) Å] bridged by four benzoate ligands. Each of the Cu atoms in this bunuclear copper(II) acetate hydrate analogue is present in an approximately square‐pyramidal environment, with four O atoms in a plane and the pyridine N atom at the apical site. Selected geometric parameters are compared with values for related tetrabenzoate complexes of copper(II).     

Interaction of pyridine‐2,5‐dicarboxylic acid with heavy metal ions in aqueous solutions

Interactions between pyridine‐2,5‐dicarboxylic acid and Zn(II), Ni(II), Pb(II), Cd(II), and Cu(II) were characterized in aqueous solutions (20°C; I = 0.4 (KNO₃)) by means of d.c.‐polarography, spectrophotometry, and ¹H NMR spectroscopy. Polarography was used to determine the concentration of free metal ions in the presence of 10‐fold excess ligand in weakly alkaline solutions, and to determine stability constants for the Zn(II), Cd(II), and Cu(II) complexes with pyridine‐2,5‐dicarboxylic acid. ¹H NMR spectroscopy was used to further characterize complex formation. © 2005 Wiley Periodicals, Inc. 16:285–291, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20123     

Self-reduction of chlorohemin in pyridine: Comparison with the photoreduction process

The self-reduction of chlorohemin in pure pyridine has been investigated by voltammetric techniques. The results obtained indicate that the self-reduction follows a first-order kinetic rate law. The reduction is strongly inhibited by the presence of chloride ions suggesting that the monopyridine protoporphyrin iron(III) [Fe(III)(PP)(py)]⁺ axial complex is the species undergoing self-reduction. In oxygenated solution the process is strongly retarded since the oxidation of the Fe(II)PP intermediate strongly competes with the formation of a stable Fe(II)(PP)(py)₂ complex. The results are discussed in the light of those obtained in a previous investigation on the photochemistry of chlorohemin under similar conditions.     

Interaction of pyridine‐ and 4‐hydroxypyridine‐2,6‐dicarboxylic acids with heavy metal ions in aqueous solutions

Interactions between pyridine‐2,6‐dicarboxylic acid and 4‐hydroxypyridine‐2,6‐dicarboxylic acid with Cu(II), Pb(II), and Cd(II) ions were characterized in aqueous solutions (20°C; I = 0.4 (KNO₃)) by means of dc‐polarography. In solutions with excess of ligand, Cu(II), Pb(II), and Cd(II) form 1:2 complexes with the tridentate dianion of pyridine‐2,6‐dicarboxylic acid (dipic²⁻) from weak acid to alkaline solutions. The values of log β₂ for Cu(II), Pb(II), and Cd(II) are 16.1, 11.8, and 11.0, respectively. The complexing ability of pyridine‐2,6‐dicarboxylic acid is higher in acid solutions and lower in alkaline solutions than that of 4‐hydroxypyridine‐2,6‐dicarboxylic acid. This difference is attributed to the OH‐group, which can deprotonate in basic pH. In acid solutions the OH‐group acts as an electron acceptor and reduces the electron donation available to the nitrogen atom in 4‐hydroxypyridine‐2,6‐dicarboxylic acid, whereas in alkaline solutions the OH‐group is deprotonated, and the deprotonated O₋ group acts as an electron donor and increases the coordination ability of the ligand. The triple‐deprotonated anion of 4‐hydroxypyridine‐2,6‐dicarboxylic acid (chel^3‐) forms a stable diligand complex with Cu(II), the stability constant logarithm being 21.5 ± 0.2.© 2003 Wiley Periodicals, Inc. Heteroatom Chem 14:625–632, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10203     

Reactivity of substituted copper(II) salicylates with tert‐butylperoxyl radical: Structure–reactivity relationships

Absolute rate constants (k_eff) for the chemical reactions of Cu(II)₂(3,5‐di‐iso‐propylsalicylate)₄(H₂O)₃, Cu(II)₂(3,5‐di‐tert‐butylsalicylate)₄, Cu(II)₂(3,5‐di‐tert‐butylsalicylate)₄(H₂O)₄, Cu(II)₂(3,5‐dimethylsalicylate)₄(H₂O)₃, Cu(II)₂(3‐ethylsalicylate)₄(H₂O), Cu(II)₂(3‐phenylsalicylate)₄, and Cu(II)(3,5‐di‐iso‐propylsalicylate)₂(pyridine)₂ with tert‐butylperoxyl radical were determined using kinetic electron paramagnetic resonance measurements in 10% toluene in the hexane medium at temperatures ranging from ?63°C to 2°C. These antioxidant (AO) chelates were ranked by their reactivity as follows: 2,6‐di‐tert‐butyl‐4‐methylphenol ? Cu(II)₂(3,5‐di‐tert‐butylsalicylate)₄ ? Cu(II)₂(3‐phenylsalicylate)₄ > Cu(II)₂(3,5‐di‐iso‐propylsalicylate)₄(H₂O)₃ ? Cu(II)₂(3,5‐di‐tert‐butylsalicylate)₄(H₂O)₄ ? Cu(II)₂(3,5‐dimethylsalicylate)₄(H₂O)₃ > Cu(II)₂(3‐ethylsalicylate)₄(H₂O) ? Cu(II)(3,5‐di‐iso‐propylsalicylate)₂(pyridine)₂ at 20°C. Differential pulse voltammetry was used to determine redox behavior of these chelates in CH₂Cl₂. Two types of salicylic OH groups were detected in these Cu(II) salicylates, characterized by the presence or absence of AO reactivity. One of them was coordinate covalently bonded to Cu(II) via the oxygen atoms of the salicylic OH groups, displaying oxidation peak potentials in the range from +650 to 970 mV versus Ag/Ag⁺. The second type was intramolecularly hydrogen bonded to carboxylate oxygens, with an oxidation peak potential in the range from +1100 to 1200 mV versus Ag/Ag⁺. It was concluded that non–hydrogen‐bonded salicylic OH groups are responsible for the antiperoxyl radical reactivity of these chelates, while neither Cu(II) nor salicylate ligands displayed reactivity with peroxyl radical. It has been established in this research that axially bonded electron pair donors such as pyridine and water decrease H‐donating reactivity of Cu(II) salicylates by promoting the formation of intramolecular hydrogen bonding between the salicylic OH hydrogen atoms and carboxylate oxygen atoms in the salicylic ligands. Dependences of log k_eff at 20°C and the anodic oxidation potential (E_pa) for the salicylic OH group on the difference between symmetric and asymmetric stretching frequencies of carboxylate groups (in Fourier transform infrared spectra) for the substituted Cu(II) salicylates were determined. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 42: 56–67, 2010     

(2,2′‐Bi­pyridine‐κ2N,N′)(dichromato‐κO)copper(II)

The title compound, [Cu(Cr₂O₇)(C₁₀H₈N₂)₂], a new mixed‐metal molecular compound, contains isolated molecular units, each comprised of one Cu^II atom coordinated to two 2,2′‐bi­pyridine ligands and also to an oxy­gen vertex of a dichromate anion. The Cu^II atom has an approximate trigonal–bipyramidal geometry, which is consistent with previous studies. Both enantiomers of the chiral complex mol­ecule are present and are related by inversion centers. In a reported pyridine analogue, achiral [Cu(Cr₂O₇)(pyridine)₄] chains pack in the non‐centrosymmetric space group Pna2₁. Differences in the organic ligands influence the chirality and dimensionality of the Cu—Cr₂O₇ bonding.     

Novel Imprinted Polymer for the Preconcentration of Cadmium with Determination by Inductively Coupled Plasma Mass Spectrometry

A novel Cd(II)-imprinted polymer was prepared with chemical immobilization using N-methacryloyl-L-Histidine as a vinylated chelating agent for online solid-phase extraction of Cd(II) for determination by inductively coupled plasma mass spectrometry. The Cd(II)–monomer complex was synthesized and copolymerized through bulk polymerization method in the presence of ethyleneglycoldimethacrylate cross-linker. The resulting polymer was leached with 1.0?mol?L^?1 HNO₃ to generate the cavities in the polymer for Cd(II) ions. The experimental conditions, including load pH, solution flow rate, and eluent concentration for effective sorption of Cd(II), were optimized using a minicolumn of the imprinted polymer. A volume of 5.0?mL sample 5?µg?L^?1 Cd(II) solution at pH 6.5 was loaded on the column at 2.0?mL?min^?1 using a sequential injection system followed by elution with 1.0?mL of 0.75?mol?L^?1 HNO₃. The relative selectivity coefficients of the imprinted polymer for Cd(II) were 38.5, 3.5, 3.0, 2.5, and 6.0 in the presence of Cu(II), Ni(II), Zn(II), Co(II), and Pb(II), respectively. Computational calculations revealed that the selectivity of the imprinted polymer was mediated by the stability of Cd(II)–N-methacryloyl-L-Histidine complex which was more stable than commonly used monomers including 4-vinyl pyridine, methacrylic acid, and vinylimidazole. The detection limit and relative standard deviation were 0.004?µg?L^?1 and 3.2%, respectively. The method was validated by the analysis of seawater certified reference material (CASS-4) and successfully used for the determination of Cd(II) in coastal seawater and estuarine water.     

Synthesis,structure, and ligand exchange of a copper(II)-based molecular helix with 2,6-pyridinedicarboxylates

An one-dimensional (1-D) metal–organic polymer, [Cu₂(PDA)₂(DMF)₂]_n (PDA = 2,6-pyridinedicarboxylate, DMF = dimethylformamide), was synthesized by reaction of copper(II) nitrate hemi(pentahydrate) and 2,6-pyridinedicarboxylic acid in DMF. The complex shows a molecular helix structure consisting of five-coordinate Cu(II) building blocks with distorted square pyramidal geometry. Tridentate chelating PDA, DMF, and an oxygen from the carboxylate of the adjacent Cu(II) building unit are coordinated to the copper(II) center. The weakly coordinated DMF groups in [Cu₂(PDA)₂(DMF)₂]_n easily exchange with a pyridine to generate a pyridine-coordinated non-helical 1-D metal–organic polymer with six-coordinate pseudooctahedral Cu(II) units.     

EPR studies on some polyamine copper(II) complexes in various solvents

EPR studies have been carried out on a series of copper(IIcomplexes with the general formula CuL(NCS)_xY_2-x[L = N,N,N',N″,N″-pentamethyldiethylenetriamine (Me₅den); x = 1 or 2; and Y = ClO₄, NO₃ or B?₄], dissolved in different solvents. These studies have revealed that the symmetry around copper(II) in [Cu(Me₅den)(NCS)₂] and [Cu(Me₅,den)-NCS]NO₃ is not trigonal-bipyramidal as predicted by IR, conductivity and optical data. The 4s contribution to the ground state is found to influence the isotropic contact term and bond parameters. The hyperfine line-widths observed for the copper(II) ion in solutions of these complexes dissolved in pyridine at room temperature are explained using the theory of Wilson and Kivelson. The isotropic spin—rotational relaxation contribution to the residual line-width is found to be smaller for all the complexes when they are dissolved in pyridine.     

Structurally rigid bis(pyrazolyl)pyridine Zn(II) and Cu(II) complexes: Structures and kinetic studies in ring‐opening polymerization of ε‐caprolactone

Four bis(pyrazolyl)pyridine Zn(II) and Cu(II) carboxylate complexes have been structurally elucidated and used as initiators in the ring‐opening polymerization (ROP) of ε‐carprolactone (ε‐CL). Reactions of bis(3,5‐dimethyl‐pyrazol‐1‐yl)pyridine ( L1 ) with the appropriate Zn(II) and Cu(II) carboxylates afforded the corresponding complexes; [Zn(L1)(C₆H₅COO)₂] ( 1 ), [Zn(L1)(2‐Cl‐C₆H₄COO)₂] ( 2 ), [Zn(L1)(OAc)₂] ( 3 ) and [Cu(L1)(OAc)₂] ( 4 ) in moderate to good yields. Molecular structures of compounds 1 , 2 , 3 confirmed the presence of one tridentate bound ligand L1 in the metal coordination sphere and two carboxylate anions to give five coordination number around Zn(II) and Cu(II) atoms. Complexes 1 , 2 , 3 , 4 initiated the ROP of ε‐CL at 110 °C to give polymers of moderate molecular weights. Kinetic analyses of the ROP reactions indicate pseudo ‐first‐order dependency on ε‐CL monomer and initiator. ¹H NMR and mass spectral data established a coordination insertion mechanistic pathway and behaviour of 1 , 2 , 3 , 4 as initiators in the ROP of ε‐CL. Copyright © 2016 John Wiley & Sons, Ltd.     

Electron paramagnetic resonance studies on three copper(II) complexes of 3-amino-5-methylisoxazole

EPR studies have been carried out on three copper(II) complexes of 3-amino-5-methylisoxazole(3-AMI), namely(l)Cu(3-AMI)₄Cl₂,(2)Cu(3-AMI)₂(NO₃)₂and(3)Cu(3-AMI)₂Br₂, in polycrystalline, solution and frozen solution forms in order to elucidate the stereochemistry, metal-ligand bond nature and solute-solvent interactions. The magnitude of the spectral parameters obtained from polycrystalline samples are found to represent the approximate local copper(II) environment with the ground state dominated by the d_x²-_y² orbital. The molecular species obtained in pyridine and N,N '-dimethyl formamide (DMF) solutions of Cu(3-AMI)₂(NO₃), and Cu(3-AMI)₂Br₂ are similar to the corresponding species in complexes of isoxazole and its derivatives studied earlier. But, unlike the above two complexes the spectral parameters obtained in the pyridine and DMF solutions of Cu(3-AMI)₄Cl₂ are characteristic of CuN₄ and CuO₄, chromophores respectively with strong metal-ligand σ-bonding. The 4s-character in the ground state is estimated in all the complexes to indicate the axial field strength.     

Thermochemistry of adducts of some bivalent transition metal bromides with pyridine

The compounds [MBr₂(py)₂] (where M is Mn(II), Co(II), Ni(II), Cu(II) or Zn(II); py = pyridine) were synthesized and characterized by melting points, elemental analysis, thermal analysis and electronic and IR spectroscopy. The enthalpies of dissolution of the adducts, metal(II) bromides and pyridine in 25% (v/v) 1.2 M aqueous HCl in methanol were measured and by using thermochemical cycles, the following thermochemical parameters for the adducts have been determined: the standard enthalpies for the Lewis acid/base reactions (Δ_rH^θ), the standard enthalpies of formation (Δ_fH^θ), the standard enthalpies of decomposition (Δ_DH^θ), the lattice standard enthalpies (Δ_MH^θ) and the standard enthalpies of the Lewis acid/base reactions in the gaseous phase (Δ_rH^θ(g)). The mean bond dissociation enthalpies of the M(II)-nitrogen bonds have been estimated as well as the enthalpies of the adducts formation in the gaseous phase.     

Polymeric and monomeric dipicolinate complexes with 4-hydroxymethyl pyridine: spectral, structural, thermal and electrochemical characterization

Polymeric copper(II), [Cu(μ-dpc)(μ-4-hymp)] _n (1), and monomeric nickel(II), [Ni(dpc)(4-hymp)(H₂O)₂]·H₂O (2), (dpc: dipicolinate, 4-hymp: 4-hydroxymethyl pyridine), dipicolinate complexes have been prepared and characterized by spectroscopic (IR, UV–Vis, EPR), thermal (TG/DTA), X-ray diffraction technique and electrochemical methods. In both the dipicolinate complexes, the dpc dianion acts as a tridentate ligand. In polymeric copper(II) complex, the 4-hymp and dpc ligands adopt a bridging position between the Cu(II) centers, forming the elongated octahedral geometry. The polymeric chains are linked to one another via O–H···O hydrogen bond interactions, forming the 3-D polymeric structure. The Ni(II) ion is bonded to dpc ligand through pyridine N atom together with one O atom of each carboxylate group, two aqua ligands and N pyridine atom of 4-hymp, forming the distorted octahedral geometry. The Ni(II) complexes are connected to one another via O–H···O hydrogen bonds, forming R ₄²(18) motifs in 2-D pattern. The powder EPR spectra of copper(II) complex have indicated that the paramagnetic center is in rhombic symmetry with the Cu²⁺ ion having distorted octahedral geometry. IR and UV–Vis spectroscopes all agree with the observed crystal structure.     

The effect of chloroalkane/alcohol solvent composition on the photochemical properties of copper(II) dithiocarbamate mixed-ligand complexes. EPR study

The EPR technics has been used to study the effect of solvent composition on the photochemical conversion of Cu(II) dithiocarbamate mixed-ligand complexes Cu(Et₂dtc)X (X=Cl⁻, Br⁻) and Cu(Et₂dtc)⁺…Y⁻ (Y⁻=ClO₄⁻, NO₃⁻) in chloroalkane/alcohol solutions, where chloroalkane=CCl₄, CHCl₃ or CH₂Cl₂ and alcohol=MeOH, EtOH, i-PrOH or i-BuOH. The obtained results allow to get some insight into the behaviour of the mixed-ligand complexes towards the halogen donation power of chloroalkanes and the co-ordination abilities of alcohols. The paper deals with the nature of the complexes obtained as intermediate products of photolysis.