Imidazolate-bridged Copper Research Articles

Structures and magnetic properties of three compounds from imidazolate-bridged copper(II) pyrazolylborate complexes

Three kinds of imidazolate-bridged copper(II) complexes using pz3BH anion as a capping ligand, namely, [Cu4(μ-im)4(pz3BH)4][Cu(pz3BH)2]2 (1), polymorphic [Cu4(μ-im)4(pz3BH)4] (1t), and [Cu(μ-im)(pz3BH)]n (1p) were afforded (Him=imidazole; pz3BH=hydrotris(1-pyrazolyl)borate). Their crystal structures were determined to reveal that 1 and 1t involve a tetranuclear macrocycle while 1p consists of a polymeric chain. The im nitrogen atoms occupied the copper equatorial positions. The magnetic susceptibility measurements showed that the im bridges played the role of an antiferromagnetic coupler. The Bonner–Fisher model with the Heisenberg spin Hamiltonian as H=−Σ 2JSi·Si+1 gave 2J/kB=−115.5(6)K for 1p. The magnitude of the present interaction belongs to the largest class, when compared with those of the known im-bridged copper(II) complexes.

Synthesis, structure and properties of an imidazolate bridged copper, zinc binuclear complex

A new imidazolate bridged Cu2+, Zn2+ binuclear complex [(dtma) CuImZn (dtma)]ClO4 · 2.5H2O taken as active site model for Cu, Zn-SOD has been synthesized and its crystal structure determined. All the bond lengths, bond angles and the distance between Cu and Zn atoms in Cu-Im-Zn core of the model complex are close to those in Cu, Zn-SOD. ESR parameters of the model complex as a function of pH show that the imidazolate bridge is stable in pH range 10–12, and is broken on Zn side at pH ∼9. With decreasing pH, the imidazole is released at pH ∼4 and the dtma ligand dissociates from the Cu containing fragment at pH ∼2.4.

Structures and magnetic properties of imidazolate-bridged tetranuclear and polymeric copper(II) complexes

Eight kinds of imidazolate-bridged copper(II) complexes were found to be classified into two categories from the magnetic properties. The crystal structures of [Cu(L)(μ-im)] n (Him = imidazole; L = nonane-4,6-dionate, 2,6-dimethylheptane-3,5-dionate) and [Cu(L)(μ-im)] 4 (L = nonane-4,6-dionate, 1-phenylbutane-1,3-dionate) were determined, to reveal that they consist of polymeric chains and tetranuclear cycles, respectively. Note that the nonane-4,6-dionate derivative gave the two phases. The Bonner–Fisher model (a one-dimensional antiferromagnetic chain model) was plausibly applied to [Cu(L)(μ-im)] n for the best fit, while a square model was to [Cu(L)(μ-im)] 4. The complexes with unknown crystal structures were also subjected to magnetic measurements, and the tetra- and polymeric structures could be clearly distinguished from each other by fitting the magnetic data to appropriate models. The exchange parameters were comparable for both series (2 J/ k B = −78 to −97 K) because the structurally common bridges Cu–N(eq)–N(eq)–Cu afford comparable magnitudes of couplings.

Mimicking catalase and catecholase enzymes by copper(II)-containing complexes

AbstractAn imidazolate-bridged copper(II)-zinc(II) complex (Cu(II)-diethylenetriamino-μ-imidazolato-Zn(II)-tris(2-aminoethyl)amine perchlorate (denoted as “Cu,Zn complex”) and a simple copper(II) complex (Cu(II)-tris(2-aminoethyl) amine chloride (“Cu-tren”) were prepared and immobilised on silica gel (by hydrogen or covalent bonds) and montmorillonite (by ion exchange). The immobilised substances were characterised by FT-IR spectroscopy and their thermal characteristics were also studied. The obtained materials were tested in two probe reactions: catalytic oxidation of 3,5-di-tert-butyl catechol (DTBC) (catecholase activity) and the decomposition of hydrogen peroxide (catalase activity). It was found that the catecholase activity of the Cu,Zn complex increased considerably upon immobilization on silica gel via hydrogen bonds and intercalation by ion exchange among the layers of montmorillonite. The imidazolate-bridged copper(II)-zinc(II) complex and its immobilised versions were inactive in hydrogen peroxide decomposition. The Cu(II)-tris(2-aminoethyl)amine chloride complex displayed good catalase activity; however, immobilisation could not improve it.

Speciation study of an imidazolate-bridged copper(II)–zinc(II) complex in aqueous solution

The solution equilibrium and the binding mode of the species in the five-component system containing two metal ions (copper(II) and zinc(II)) and three ligands ( A = diethylenetriamine, B = imidazole, C = tris(2-aminoethyl)amine) were investigated by pH-potentiometric titration, UV–visible spectrophotometry and EPR (electron paramagnetic resonance) spectroscopic titration in aqueous solution in the 2–11 pH range. An imidazolate-bridged heterobinuclear complex ( ACuBH − 1 ZnC) was found to evolve above pH = 7 and was stable between pH 7 and 11. The existence of the ACuBH − 1 ZnC complex (by determination of its molecular weight) was proved by mass spectrometry (ESI-MS (electrospray ionization mass spectrometry) and MALDI (matrix-assisted laser desorption/ionization) techniques). The electrochemical behaviour and the superoxide dismutase activity of this complex were also tested by cyclic voltammetry and the Riboflavin/NBT (nitro blue tetrazolium) assay, respectively.

A tris-imidazolecarboxyaldehyde copper(ii) complex with unusual carbonyl co-ordination: structure and reactivity

4-Methyl-5-imidazolecarboxyaldehyde (HL1) reacts with copper(II) perchlorate to give the complex [Cu(HL1)3](ClO4)2. Its X-ray structure reveals that three ligands are bonded to the copper(II) ion in a bidentate chelate fashion through the imidazole nitrogens and aldehyde oxygen atoms, leading to a CuN3O3 chromophore. Carbonyl co-ordination to copper(II) is not usual, in fact there are very few reported examples of structurally characterised complexes of this type. When [Cu(HL1)3](ClO4)2 is dissolved in a KOH-containing methanol solution, the polymer [Cu(L1)2]n precipitates. The magnetic properties of [Cu(L1)2]n are typical of an imidazolate-bridged copper(II) system in which the bridge involves equatorial positions on the copper(II) co-ordination polyhedra. A J value of −58.1 cm−1 was calculated by application to the experimental data of the theoretical equation for a uniform one-dimensional infinite chain system. Additionally, the co-ordinated carbonyl groups of [Cu(HL1)3](ClO4)2 react with 1,3-diaminopropane, leading to a new monomeric Schiff-base copper(II) complex. However, this reaction does not take place when the diamine is 1,2-diaminopropane, which is likely due to the sterically promoted low reactivity for imine bond formation with the –NH2 group close to the methyl substituent in 1,2-diaminopropane.

Equilibria and tyrosinase activity of a dinuclear and its analogous tetranuclear imidazolate-bridged copper(II) complexes

A copper(II) complex, [Cu(apip)(imH)] 2+, containing an imidazole ligand, in addition to a discrete tridentate imine (apip=2-[2-(2-pyridyl)ethylimino-1-ethyl]pyridine), was prepared. Both mono- and dinuclear species coexisted in aqueous solution exhibiting a pH-dependent monomer↔dimer interconversion and were monitored by different techniques. Similar data were obtained with the complex [SECu(imH)] +, where SE=2-salicylidene-aminoethane. Moreover, these species were isolated as perchlorate salts and characterized by cyclic voltammetry and spectroscopic techniques (UV–Vis, IR, Raman, and EPR). Experiments by capillary electrophoresis, in addition to parallel EPR spectra at different pH values, permitted to estimate the equilibrium constant, corresponding to the equation: 2[LCu(imH)] 2++OH −⇆[LCu(im)CuL] 3++H 2O+imH, K=(8±3)×10 7 mol −1 dm 3, in reasonable agreement to the value determined by spectrophotometric measurements, K=(0.12±0.01)×10 7 mol −1 dm 3. A further tetranuclear species, [Cu 4(apip) 4im 4] 4+, was obtained by a pH-dependent self-assembly process, separated from very alkaline solutions of the corresponding mononuclear complex. The tetranuclear and dinuclear species were observed to behave as good functional models of the tyrosinase enzyme, catalyzing the aerobic oxidation of 2,6-di- tert-butylphenol, in methanolic solution, and that of 3,4-dihydroxyphenylalanine ( l-dopa) in aqueous solution, with a biphasic behavior in the range of pH 7–11. Applying the Michaelis–Menten approach, the kinetic parameters determined indicated that the tetranuclear species is a better catalyst than the dinuclear ones, exhibiting a higher rate constant, k 3, as well as a higher K M value. The hydrophobic cavity in the polynuclear complex seems to facilitate its interaction with the phenol substrate, since the structural parameters determined for both the complexes, di- and tetranuclear species were very similar.

Synthetic, spectral and solution studies on imidazolate-bridged copper(II)-copper(II) and copper(II)-zinc(II) complexes

Synthesis, spectral and solution studies on 2-ethyl imidazolate-bridged (2-EtIm) homo-binuclear copper(II)-copper(II) and hetero-binuclear copper(II)-zinc(II) homologue are described. Magnetic moment values of homo-binuclear complexes indicate that the imidazolate group can mediate antiferromagnetic interactions. Optical spectra of hetero-binuclear complex at varying pH values suggest that the imidazolate-bridged complex is stable over the pH-range 7.15–10.0.

Synthesis, magnetic and spectral properties of binuclear imidazolate bridged copper(II)-copper(II) and copper(II)-zinc(II) complexes

Synthesis, magnetic and spectral properties of binuclear imidazolate bridged copper(II)-copper(II) and copper(II)-zinc(II) complexes

Synthesis, Characterization and Crystal Structures of Monomeric and Imidazolate-Bridged Polymeric Copper(II) Complexes of 5-Methyl-1,5,9-triazanonane

Two complexes [Cu(mtan)(H2O)(ClO4)](ClO4) (Im = imidazolate) (1) and [Cu(mtan)(Im)]n (ClO4)2n (mtan = 5-methyl-1,5,9-triazanonane) (2) have been prepared and characterized. Single-crystal X-ray analysis reveals that (1) crystallizes in the orthorhombic space group Pbcm, with a 8.906(5), b 12.255(6), c 14.821(4) Å and V 1618(1) Å3, and complex (2) crystallizes in the orthorhombic space group Pca21, with a 11.125(7), b 9.149(6), c 15.180(7) Å and 1545(1) Å3. The copper(II) atom in complex (1) is in a distorted square-pyramidal geometry, being coordinated by three nitrogen atoms from an mtan ligand and one oxygen atom from an aqua ligand at the basal plane and one perchlorate oxygen atom at the apical position. The structure of complex (2) features polymeric cationic chains composed of [Cu(mtan)(Im)]+ repeating units and discrete perchlorate anions. The Cu(1) atom in each unit has a severely distorted trigonal-bipyramidal geometry, being coordinated by three nitrogen atoms from one mtan ligand and two nitrogen atoms from two Im groups. The structural units are linked by -Im groups to give one-dimensional chains. The variable-temperature magnetic measurement on the powder sample of (2) reveals intrachain, medium antiferromagnetic interaction with g = 2.240 and J = –36.45 cm–1 through the -Im bridges.

The syntheses, crystal structure and magnetic behavior of μ-imidazolato-dicopper(II) complexes with the dinucleating hexaazamacrocycles

The synthesis, crystal structure and magnetic properties of the imidazolate-bridged dinuclear copper(II) complex [LCu2(Im)](ClO4)3(H2O) ·1/2(MeCN), (ImH=imidazole, L=bis-p-xylylBISDIEN) have been studied. Single crystal X-ray diffraction determination reveals the distorted square planar geometries of the imidazolate bridged dicopper(II) center are incorporated within the dinucleating macrocycle. The Cu—Cu separation in the complex is 6.005A. Magnetic measurements reveal an antiferromagnetic exchange interaction with a coupling constant of J=−26.52cm−1. The enzymatic activity of the title complex is 5.9 percent of that of the protein.

A novel imidazolate-bridged copper–zinc heterodinuclear complex as a Cu, Zn–SOD active site model

A novel imidazolate-bridged Cu(II)–Zn(II) heterodinuclear complex of an imidazole derivative containing two metal-binding groups has been synthesized, and its structure and properties have been clarified.

Vibrational Spectra and Normal Coordinate Analysis for an Imidazole Bridged Copper, Zinc Binuclear Complex

Abstract The normal coordinate analysis of a model compound for Cu, Zn-SOD: [(dtma)CuImZn(dtma)]CIO4 · 2.5H2O (where dtma = 4-diethylenetriamineacctate; Im = imidazolate) has been carried out by using a Urey-Bradley force field. According to the molecular structure, 240 internal coordinates were established and 174 theoretical vibrational frequencies were calculated. Due to introducing an appropriate set of internal coordinates and force constants in the course of calculation, the calculated frequencies agree well with the observed values, with the average difference 4.40cm−1 and the maximum deviation 24.2cm−1 between them, although the structure of the title compound is complicated. Some structural and spectral properties are here discussed.

Synthesis and magnetism of 6-nitrobenzimidazolate and imidazolate-bridged copper(II) complexes

Four new trinuclear copper(II) complexes, [Cu(phen)-(NBzIm)] (ClO4) (1), [Cu(bpy)(NBzIm)](ClO4) (2), [Cu-(Me2-bpy)(NBzIm)](Ac)·1/2H2O (3) and [Cu(Me2-bpy)-(Im)](ClO4)·1/2H2O (4) (phen = 1, 10-phenanthroline, bpy = 2,2′-bipyridine, NBzIm = 6-nitrobenzimidazolate ion, Im=imidazolate ion) have been prepared and characterized by variable temperature magnetic susceptibility measurements. A weak antiferromagnetic spin exchange interaction operates between copper(II) ions, exchange integrals evaluated as J =-23.82 cm-1 for (1); and J=-21.91 cm-1 for (2).

Imidazolate-bridged copper(II) complexes with infinite zigzag-chain and tetranuclear structures formed by deprotonation and self-assembly

A method of molecular design of self-assembling metal complexes has been established. The copper(II) complex with a quadridentate ligand [Cu(HL2)]ClO4 undergoes deprotonation of the imidazole proton under basic conditions to give an imidazolate-bridged polymeric compound [(CuL2)∞], where H2L2=N-3-ethoxysalicylidene-N′-imidazol-4-ylmethylenecyclohexane-1,2-diamine. The latter compound crystallizes in the monoclinic space group P21/a with a= 15.515(5), b= 9.338(5), c= 12.660(2)A, β= 95.33(2)°, and Z= 4, R= 0.078 for 1721 reflections. It has an infinite zigzag-chain structure and the variable-temperature magnetic data are well reproduced using the Ising model based on H=–2∑JijSiSj with a J value of –1.8 cm–1. The complex [Cu(HL3)Cl]ClO4 with a tridentate ligand gives an imidazolate-bridged tetranuclear complex [(CuL3)4][Clo4]4 on deprotonation at the imidazole nitrogen, where HL3=N-imidazol-4-ylmethylene-N′,N′-dimethylpropane-1,3-diamine. The latter compound crystallizes in the triclinic space group P with a= 15.216(6), b= 15.592(5), c= 15.102(5)A, α= 104.98(3)°, β= 108.44(3)°, γ= 61.12(2)°, Z= 2, and R= 0.096 for 6931 reflections. The structure consists of a cyclic tetranuclear molecule in which each imidazolate nitrogen atom co-ordinates axially to the copper(II) ion of the adjacent unit. Variable-temperature magnetic data are well reproduced using the Heisenberg model based on H=–2J(S1·S2+S2·S3+S3·S4+S4·S1) with J=–60.0 cm–1.

Synthesis, spectroscopic, magnetic properties and MMX force field study of two imidazolate-bridged dinuclear copper(II) complexes

Two new imidazolate-bridged dinuclear CuII complexes, Na[Cu2(L1)2(im)] and K[Cu2(L2)2(im)] (where H2L1=6-amino-1,3-dimethyl-5(2-carboxyphenyl)azouracil and H2L2=1,3-dimethyl-5(2-carboxyphenyl)azobarbituric acid) have been prepared and characterized by magnetic susceptibility and spectroscopic measurements. Both compounds exhibit the expected antiferromagnetic behaviour with 2J=−54.8 and −30.4 cm−1, respectively. Because of the lack of suitable crystals for single crystal X-ray analysis, we have calculated the lowest energy structures using a program based on the MMX force field. On the basis of the results, the magnitude of the magnetic interaction is discussed.

Synthesis, structure and properties of an imidazolate bridged copper, zinc binuclear complex:A model compound for Cu,Zn-SOD

A new imidazolate bridged Cu 2+, Zn 2+ binuclear complex [(dtma) CuImZn (dtma)]ClO 4· 2.5H 2O taken as an active site model for Cu,Zn-SOD has been synthesized and its crystal structure has been determined. All the bond lengths, bond angles and the distance between copper and zinc atoms in the CuImZn core of the model complex are close to those in Cu,Zn-SOD. ESR parameters of the model complex as a function of pH show that the imidazolate bridge is stable in the pH range 10–12, and is broken on the zinc side at pH ∼ 9. With decreasing pH, the imidazole is released at pH ∼ 4 and the dtma ligand dissociates from the copper-containing fragment at pH ∼ 2.4.

Isolation of a spin-frustrated imidazolate-bridged trinuclear copper(II) complex potentially relevant to the multicopper oxidases

The reaction of Cu(MeCO2)2·H2O with 1,4,7-trimethyl-1,4,7-triazacyclononane (L) and imidazole (ImH) in methanol affords, in the presence of a small amount of triethylamine and NaClO4, the blue trinuclear complex [L3Cu3(Im)3](ClO4)3 which has been characterized by X-ray crystallography, EPR and magnetic susceptibility measurements to be a spin-frustrated triangular imidazolate-bridged copper(II) complex without any µ3-X ligand.

Magnetic susceptibility studies of the native cupro-zinc superoxide dismutase and its cobalt-substituted derivatives. Antiferromagnetic coupling in the imidazolate-bridged copper(II)-cobalt(II) pair

Magnetic susceptibility studies of the native cupro-zinc superoxide dismutase and its cobalt-substituted derivatives. Antiferromagnetic coupling in the imidazolate-bridged copper(II)-cobalt(II) pair

Synthetic possibilities and strategies in the development of analogues for the dinuclear and polynuclear metal sites in enzymes and proteins

Two synthetic approaches in the synthesis of dinuclear (and polynuclear) coordination compounds are described briefly. The first method uses small external ligands (such as OH − and S 2−) that act as a bridge to bring the metal ions together. The second method uses dinucleating chelating ligands to hold two metal ions fixed together, with one (or more) bridging donor groups between the metals. Application of these methods in the synthesis of model coordination compounds for superoxide dismutase (imidazole-bridged copper and zinc systems) and for hemocyanin (copper dimers with imidazole-type ligands) is described. It is found that the most stable dinuclear systems are formed when a dinucleating chelating ligand system is used.