Pentacoordinate Copper Research Articles

Using novel open-framework copper borovanadate as an effective catalyst in the conversion of cyclohexanol to cyclohexanone

A new open-framework copper borovanadate containing two oxidation states of V4+ and V5+ cations {[Cu5(1,2-dap)10][VV2VIV10B18O54(OH)6(H2O)]}·2H3BO3·9H2O (1,2-dap: 1, 2-propylene diamine) (1) using 1,2-dap as template agent has been successfully prepared. It is composed of five coordination cations [Cu(1,2-dap)2]2+and one borovanadate anion [VV2VIV10B18O54(OH)6(H2O)]10- constructed by one {B18O30(OH)6} and two {V6O18} subunits, as well as two isolated H3BO3 molecules and nine H2O molecules. The compound exhibits for the first time penta-coordinated copper cations, uncoordinated H3BO3 molecules and 1, 2-dap ligands in the series of open-framework borovanadates. Its UV diffuse-reflectance spectrum, Mott-Schottky curve, fluorescent emission, chromaticity and second-order lifetime were investigated in detail. The stable pH range (5.8–2.3) of the borovanadate anion in DMF solution was explored by means of fluorescence and UV spectra. In particular, it showed effective catalytic activity in the conversion of cyclohexanol to cyclohexanone (conversion: 86.8 %, selectivity: 97.4 %). The possible catalytic mechanism for the catalytic reaction had been also established. The studies revealed that the borovanadate could be used as a potential stable catalyst for the selective oxidation of cyclohexanol.

Tridentate acylhydrazone copper(II) complexes with heterocyclic bases as coligands. Synthesis, spectroscopic studies, crystal structure and cytotoxicity assays

Two new penta-coordinated copper(II) hydrazone coordination compounds with 2,2́ bipyridine (I) or 1,10 phenanthroline (II) as coligands are synthesized and characterized in the solid state and in solution by spectroscopic methods (FTIR, Raman, UV–vis, EPR). Single crystal X-ray studies show that they have closely related molecular structures with the copper center in a distorted square pyramidal O2N3 environment. The N-acylhydrazone, 4-hydroxy-NŃ-(E)2-hydroxy-3-methoxybenzylidene]benzohydrazide, H2L, coordinates tridentate through its ONO donor atoms as monoanion (HL-) in the cationic complex (I), [Cu(HL)(bipy)](NO3). During the period that takes the synthesis reaction of compound (II), [Cu(L)(o-phen)] to complete, keto-amine to enol-imine tautomerization of H2L occurs and the hydrazone coordinates tridentate as dianionic ligand (L2-). The fivefold coordination sphere of the complexes is completed with the nitrogen atoms of the respective coligands. Cytotoxicity studies against bone (MG-63, IC50(I) = 5.6 ± 1.0 , IC50(II) = 3.5 ± 0.3), breast (MCF7, IC50(I) = 10.8 ± 1.9, IC50(II) = 4.0 ± 1.3), (MDA-MB-231, IC50(I) = 11.4 ± 0.6, IC50(II) = 5.3 ± 0.2) and lung cancer cells (A549, IC50(I) = 7.7 ± 0.7, IC50(II) = 7.0 ± 0.4) reveals increased effectiveness of both complexes compared with the free ligand, the copper nitrate salt and the reference metallodrug cisplatin (CDDP).

Toward Heteronuclear Molecular Re(I)-Cu(II) Boxes: Structural, Luminescent, and Magnetic Properties.

The bifunctional ligands of isonicotinic acid (Py-4-COOH) and 4-pyrid-4-ylbenzoic acid (Pybz-4-COOH) instead of polypyridines were therefore reacted with (Re(CO)4)3(C3N3S3) (C3N3S3 = cyanurate trianion), resulting in the formation of two trinuclear [(Re(CO)3)3(C3N3S3)(Py-4-COOH)3] (1) and [(Re(CO)3)3(C3N3S3)(Pybz-4-COOH)3] (2), respectively. In the meantime, both complexes 1 and 2 are connected by three bifurcated hydrogen bonds between their carboxylic acid moieties Py-4-COOH and Pybz-4-COOH to form the supramolecular trigonal-prismatic and -antiprismatic structures, respectively. It is noted that complex 1 can further react with copper(II) nitrate upon deprotonation to give nonanuclear [(Re(CO)3)3(C3N3S3)(Py-4-COO)3]2Cu3(H2O)9 (3), where two trinuclear [(Re(CO)3)3(C3N3S3)(Py-4-COO)3] moieties are connected by three penta-coordinate copper(II) ions, each coordinating to two carboxylates and three water molecules, to form the trigonal-prismatic structure. Surprisingly, addition of pyrazine (pz) in the synthetic process of complex 3 resulted in serendipitous isolation of a rare example of octadecanuclear {[(Re(CO)3)3(C3N3S3)(Py-4-COO)3]2Cu3(H2O)6(pz)2}2 (4), which can be regarded as a dimer of complex 3, connected by two bridging pz ligands. Interestingly, both complexes 3 and 4 are heteronuclear molecular Re(I)-Cu(II) boxes, constructed by a complex-as-a-ligand strategy. Furthermore, complexes 1 and 2 can exhibit respective low-energy luminescence at ca. 561 and 534 nm at room temperature upon photoexcitation, and complex 3 is found to display antiferromagnetic coupling of −127.68 and −134.70 cm–1, possibly due to multiple hydrogen bonds inducing significant Cu(II)···Cu(II) coupling.

Synthesis and characterization of penta-coordinated copper(II) complexes with hydrazido based ligand and imidazole as auxiliary ligand

The reaction of (Z)-2-(phenyl(2-(pyridin-2-yl)hydrazono)methyl)pyridine (L) and copper(II) salt in methanol yields a series of five pentacoordinated mononuclear complexes. The molecular formulations of these complexes are as [Cu(L)(ImH)2](ClO4)2(1), [Cu(L)(2-MeImH)2](ClO4)2(2), [Cu(L)(2-EthImH)2](ClO4)2(3), [Cu(L)(BenzImH)2](ClO4)2(4) and [Cu(L)(2-MeBenzImH)2](ClO4)2(5) (where ImH ​= ​Imidazole, 2-MeImH ​= ​2-Methylimidazole, 2-EthImH ​= ​2-Ethylimidazole, BenzImH ​= ​Benzimidazole, 2-MeBenzImH ​= ​2-Methylbenzimidazole). The room temperature magnetic moment values are in the range 1.79–1.81 B ​M. The conductance measurements suggest that they are electrolytic in nature. The DFT calculations were performed to get information about the structures of the complexes. The copper(II) centre in all complexes is a Penta-coordinated. The proligand has NNN donor sites viz., two pyridine N and one azomethine N atoms, whereas co-ligand coordinates through pyridine N atom forming two five-membered chelate rings. The τ5 values of these complexes are in the range 0.177–0.495. Both pro and co-ligands are neutral. X-band Epr spectral measurements have been carried out to authenticate the paramagnetic behaviours of all complexes. The stability of the copper(II) centre was examined using cyclic and differential pulse Voltammetry. The IC50 and SOD activity values for all complexes reveal that they are good SOD active complexes. The IC50 value of present complexes remains in the range 26–43 ​μM.

Halido‐bridged Copper(II) Complexes with 1‐tert‐Butyl‐1H‐tetrazole: Crystal Structure and Magnetic Properties

Complex [Cu(tbt)Cl2]n (tbt = 1‐tert‐butyl‐1H‐tetrazole) was prepared by reaction of tbt with copper(II) chloride in solution. According to single‐crystal X‐ray analysis, this complex presents 1D coordination polymer, formed at the expense of double chlorido bridges between neighboring pentacoordinate copper(II) cations. 1‐tert‐Butyl‐1H‐tetrazole acts as monodentate ligand coordinated by CuII cations via the heteroring N4 atoms. The temperature‐dependent magnetic susceptibility measurements of novel complex [Cu(tbt)Cl2]n as well as described previously 1D coordination polymer [Cu(tbt)2Cl2]n, and linear trinuclear complex [Cu3(tbt)6Br6], were carried out. Magnetic studies revealed that the copper(II) ions were weakly ferromagnetically coupled in polymeric copper(II) chloride complexes, whereas complex [Cu3(tbt)6Br6] showed antiferromagnetic coupling.

Dinuclear Ternary Copper(II) Complex: Synthesis, Characterization, Structure and DNA-Binding Studies

A dinuclear centrosymmetric copper(II) complex has been prepared and characterized via FT-IR, UV-Visible and electron spin resonance spectroscopy, electrochemical method, and powder and single crystal XRD techniques. The two copper(II) ions are connected by two μ-1,1-O atoms belonging to para-fluorophenyl acetate ligands. Each copper(II) ion is coordinated by two other carboxylate ligands in monodentate and a 2,2'-bipyridine molecule in bidentate manner. Thus the geometry around each penta-coordinated copper(II) ion is square pyramidal with distortion factor τ = 0.04. Purity and uniform crystalline nature of the complex was assessed from powder XRD spectrum. ESR signal consisted of partially resolved hyperfine splitting pattern while electrochemical studies revealed diffusion controlled electron transfer processes with diffusion co-efficient of 1.628×10-7 cm2 s-1. The results of all spectroscopic experiments support each other. The complex afforded mixed binding mode with DNA yielding DNA-binding constant values of 3.667×104and 1.438×104 M-1 using cyclic voltammetry and absorption spectroscopy, respectively. The preliminary studies heralded good structural and biological importance of the synthesized complex.

Copper(II) Cyclam Complexes with N‐­Propionic Acid Pendant Arms

AbstractFour cyclam (1,4,8,11‐tetraazacyclotetradecane) ligands with different numbers of N‐substituted propionic acid groups lead to pentacoordinate copper(II) complexes that adopt trans‐I configurations (4+1 geometry), that is, the complexes have a d ground state with significant rhombic distortion. From the structural data (X‐ray diffraction analysis and electron paramagnetic resonance, UV/Vis and IR spectroscopy), as the number of secondary amine groups of the macrocyclic ring substituted with propionic acid groups increases, the distortion from square pyramidal to trigonal bipyramidal increases, and this is expected to lead to relatively low complex stabilities. This is confirmed by in vitro studies with superoxide dismutase (SOD) and human serum challenge experiments as well as by biodistribution data with the 64Cu‐labelled complexes. The 64Cu‐labelled complexes with cyclam monopropionic and dipropionic acid show high in vitro and in vivo stabilities, and the latter provides a comparable biodistribution profile to that of 64Cu–TETA (TETA = 1,4,8,11‐tetraazacyclotetradecane‐1,4,8,11‐tetraacetic acid).

Transition Metal Complexes with 1‐Adamantoyl Hydrazones – Cytotoxic Copper(II) Complexes of Tri‐ and Tetradentate Pyridine Chelators Containing an Adamantane Ring System

AbstractFive pentacoordinate copper(II) complexes with 2‐acetylpyridine or di(2‐pyridyl) ketone 1‐adamantoyl hydrazone ligands (Adpy and Addpy, respectively) of the formulae [CuCl2(Adpy)] (1), [Cu2(μ‐Cl)2(Adpy‐H)2] (2), [Cu(NCS)2(Adpy)] (3), [Cu2(μ‐Cl)2(Addpy‐H)2] (4), and [Cu2(NCS)2(μ‐Addpy‐H)2] (5) were synthesized and characterized by spectral, electrochemical, and X‐ray structural analysis. Flow cytometry and morphological analysis confirmed that the copper(II) complexes 2 and 5 induced accumulation of a sub‐G1 phase population, and fluorescence microscopy indicated the presence of large cells in apoptosis. The interaction of the copper(II) complexes with calf thymus DNA (CT‐DNA) was monitored by changes in their UV/Vis spectra. The observed intrinsic binding constants for 2 and 5 (Kb = 1.77 × 106 and 3.58 × 106 M–1, respectively) together with ethidium displacement fluorescence experiments indicate intercalative binding. Complexes 2 and 5 showed nuclease activity against pUC19 plasmid DNA.

Copper‐Assisted Hemiacetal Synthesis: A Cu II Chain Obtained by a One‐Step in situ Reaction of Picolinaldehyde

The 1D polymer complex [Cu2(L)2(SCN)2]n (1 ) has been synthesised in a one‐step in situ reaction of picolinaldehyde with sodium thiocyanate. The complex 1 was characterised by FTIR spectroscopy, UV/Vis spectrophotometry and elemental analysis. The crystal structure of complex 1 shows that chains of dimer complexes are formed with tetra‐ and pentacoordinate copper centres alternately linked by one thiocyanato and two alkoxido bridges. Variable‐temperature magnetic measurements showed a strong antiferromagnetic interaction between the copper centres within the dimer mediated by the two alkoxido bridges with a J value of –374 cm–1, which is in agreement with the DFT‐calculated value of –284 cm–1. EPR and cyclic voltammetric analyses were also carried out, as well as DFT calculations to evaluate the packing forces and their energies.

Synthesis, Crystal Structure, and Cytotoxicity of a Copper(II) Complex With Matrine

A new copper(II) complex ligated by the naturally occurring alkaloid matrine (MT), [Cu(MT+H)2(H2O)3](ClO4)4 (1) was synthesized upon reacting matrine with Cu(ClO4)2·6H2O. Characterized of complex 1 with elemental analysis, IR spectroscopy, and single-crystal X-ray diffraction revealed the molecular structure to be a pentacoordinate copper(II) complex with two monodentate matrinium ions (MT+H) coordinated via carbonyl oxygen atoms and three aqua ligands. The positive charge of the cation [Cu(MT+H)2(H2O)3]4+ is balanced by four ClO− 4 anions. Complex 1 is cytotoxic toward MCF-7, SGC-7901, and SK-OV-3 tumor cell lines. Supplemental materials are available for this article. Go to the publisher's online edition of Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry to view the supplemental file.

Syntheses, Crystal Structures, and Properties of a Salamo‐type Tetradentate Chelating Ligand and Its Pentacoordinated Copper(II) Complex

AbstractA Salamo‐type ligand 6,6′‐diethoxy‐2,2′‐[1,1′‐(ethylenedioxydinitrilo)dimethylidyne]diphenol (H2L) and its pentacoordinate copper(II) complex [CuL(H2O)] were synthesized and structurally characterized. The molecule of H2L is located in a crystallographic inversion center and adopts E configuration with respect to the C=N bond. In the complex [CuL(H2O)], the CuII atom has a tetragonally elongated square‐pyramidal configuration with donor atoms N2O2 forming a perfect basal plane. And an infinite 2D layer supramolecular structure is formed through intermolecular C1–H1A···O5 and C2–H2A···O3 hydrogen bonding interactions.

Potentially cytotoxic new copper(ii) hydrazone complexes: synthesis, crystal structure and biological properties

A new set of penta-coordinated copper(II) hydrazone complexes containing solvated methanol were synthesized by reacting the hydrazone ligands, 2-acetylpyridine benzoyl hydrazone (HL1) and 2-acetylpyridine thiophene-2-carboxylic acid hydrazone (HL2), with [CuCl2(DMSO)2] and characterized by different spectral methods. Single crystal X-ray diffraction studies of the complexes revealed that both of them, [CuCl(L1)(MeOH)] (1) and [CuCl(L2)(MeOH)] (2), have square pyramidal geometry around the cupric ion, in which the hydrazone is coordinated through NNO atoms along with a molecule of methanol in the apical position. Interaction of the ligands HL1 and HL2 along with the corresponding copper complexes 1 and 2 with calf thymus DNA (CT-DNA) has been estimated by absorption and emission titration methods which revealed that the compounds interacted with CT-DNA through intercalation. Binding of the compounds, i.e., free ligands and complexes (1) and (2) with bovine serum albumin (BSA) protein investigated using UV-visible, fluorescence and synchronous fluorescence spectroscopic methods indicated that there occurred strong binding of copper complexes to BSA over the ligands. Further, the cytotoxicity of the compounds examined in vitro on a panel of cancerous cell lines such as a human cervical cancer cell line (HeLa), a pancreatic cancer cell line (PANC-1), an Ehrlich ascites cancer cell line (EAC) and Dalton's lymphoma ascitic cancer cells (DLA) and a normal mouse embryonic fibroblasts cell line (NIH3) demonstrated that the complexes 1 and 2 possessed superior cytotoxicity than that of well-known commercial anticancer drug cisplatin to the tumor cells but are less toxic to the normal cell line and have emerged as potential candidates for further studies.

Anomalous behavior of pentacoordinate copper complexes of dimethylphenanthroline and derivatives of terpyridine ligands: Studies on DNA binding, cleavage and apoptotic activity

Copper(II) complexes with substituted terpyridine ligands, namely [Cu(itpy)(dmp)](NO 3) 2 ( 1) and [Cu(ptpy)(dmp)](NO 3) 2 ( 2) have been synthesized and characterized. The interaction of the complexes with CT-DNA has been explored using spectroscopic techniques and viscosity. Complexes 1 and 2 bind in the grooves of DNA, interestingly 1 in the minor and 2 in the major groove. Both the complexes have been found to promote DNA cleavage; complex 1 through hydrolytic and 2 oxidative. Complexes 1 and 2 have been found to be cytotoxic and bring about apoptosis of human lung cancer cell line A549.

Ladder-like azido-bridged copper(II) complexes: Synthesis, X-ray structure and magnetic study

Two mixed-ligand copper(II) complexes [{Cu(L 1)(μ 1,3-N 3)}{Cu(L)(μ 1,3-N 3)(μ 1,1-N 3)}] n ( 1) [HL 1 = 1-( N-ortho-hydroxyacetophenimino)-2,2-dimethyl-aminoethane; L = 2-(dimethylamino)-ethylamine] and [{Cu(L 2)(μ 1,3-N 3)}{Cu(L)(μ 1,3-N 3)(μ 1,1-N 3)}] n ( 2) [HL 2 = 1-( N-5-methoxy-ortho-hydroxyacetophenimino)-2,2-dimethyl-aminoethane] have been formed upon addition of aqueous solution of sodium azide to a methanolic solution of copper nitrate trihydrate and corresponding Schiff-base ligands. The ligands, HL 1 and HL 2 undergo partial hydrolysis of their imine bond during the course of reaction. Both the complexes contain single end-to-end (μ 1,3) azido bridged 1D infinite chains (rail) which propagate parallel to the crystallographic b-axis; neighboring chains are interconnected by pairs through double asymmetric end-on (μ 1,1) azido bridges (rung) to yield a ladder-like structure. In both complexes, rungs (end-on azido bridges) do not connect copper centers of the chains like in a regular ladder; instead they connect only the alternating copper sites of the 1D chain. In a chain the coordination environment around copper(II) ions are not the same: while the {Cu(L 1)(μ 1,3-N 3)} and {Cu(L 2)(μ 1,3-N 3)} moieties have a penta-coordinated copper(II) center, the copper(II) ion of the neighboring {Cu(L 1)(μ 1,3-N 3)(μ 1,1-N 3)} or {Cu(L 2)(μ 1,3-N 3)(μ 1,1-N 3)} moiety has an octahedral coordination environment. The variable temperature (2–300 K) magnetic susceptibility measurements showed that the magnetic interaction between the metal centers in complexes 1 and 2 is dominantly antiferromagnetic. The results of magnetic model are in good agreement with the experimental data.

Two New Supramolecular Architectures of Singly Phenoxo‐Bridged Copper(II) and Doubly Phenoxo‐Bridged Manganese(II) Complexes Derived from an Unusual ONOO Donor Hydrazone Ligand: Syntheses, Structural Variations, Cryomagnetic, DFT, and EPR Studies

AbstractTwo new coordination complexes {[(L)Cu2(μ‐L)(NO3)(CH3OH)0.3(H2O)0.7]·NO3}[Cu(L)(NO3)(CH3OH)] (1) and [Mn2(μ‐L)2(H2O)(μ1‐N3)2(CH3OH)] (2) derived from (E)‐N′‐(2‐hydroxy‐3‐methoxybenzylidene)acetohydrazide [LH] have been synthesized. The new potentially tetradentate ONOO donor hydrazone ligand [LH] has shown considerable metal ion selective phenoxo bridging in 1 and 2. It has coordinated copper(II) ions in its tridentate as well as in tetradentate fashion whereas for manganese(II) ions it solely showed its tetradentate character. In 1 the two adjacent pentacoordinate copper(II) centers are connected by a rare single phenoxo linkage of the hydrazone ligand whereas 2 is a doubly phenoxo‐bridged heptacoordinated manganese(II) dinuclear species. The 3D supramolecular networks of 1 and 2 have resulted from extensive H‐bonding interactions in the respective complexes. Structural variations observed for 1 and 2 have been described by performing single‐crystal X‐ray analysis. The ligand and the complexes are characterized by elemental analyses, IR, UV/Vis and 1H NMR spectroscopy. Cryomagnetic investigations, DFT studies and EPR analysis, of 1 and 2 have also been studied in detail.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Tetraalkoxyaluminates of Nickel(II), Copper(II), and Copper(I)

The syntheses and structural details of tetraisopropoxyaluminates and tetra-tert-butoxyaluminates of nickel(II), copper(I), and copper(II) are reported. Within the nickel series, either Ni[Al(OiPr)4]2.2HOiPr, with nickel(II) in a distorted octahedral oxygen environment, or Ni[Al(OiPr)4]2.py, with nickel(II) in a square-pyramidal O4N coordination sphere, or Ni[(iPrO)(tBuO)3Al]2, with Ni(II) in a quasi-tetrahedral oxygen coordination, has been obtained. Another isolated complex is Ni[(iPrO)3AlOAl(OiPr)3].3py (with nickel(II) being sixfold-coordinated), which may also be described as a "NiO" species trapped by two Al(OiPr)3 Lewis acid-base systems stabilized at nickel by three pyridine donors. Copper(I) compounds have been isolated in three forms: [(iPrO)4Al]Cu.2py, [(tBuO)4Al]Cu.2py, and Cu2[(tBuO)4Al]2. In all of these compounds, the aluminate moiety behaves as a bidentate unit, creating a tetrahedrally distorted N2O2 copper environment in the pyridine adducts. In the base-free copper(I) tert-butoxyaluminate, a dicopper dumbbell [Cu-Cu 2.687(1) A] is present with two oxygen contacts on each of the copper atoms. Copper(II) alkoxyaluminates have been characterized either as Cu[(tBuO)4Al]2, {Cu(iPrO)[(iPrO)4Al]}2, and Cu[(tBuO)3(iPrO)Al]2 (copper being tetracoordinated by oxygen) or as [(iPrO)4Al]2Cu.py (pentacoordinated copper similar to the nickel derivative). Finally, a copper(II) hydroxyaluminate has been isolated, displaying pentacoordinate copper (O4N coordination sphere) by dimerization, with the formula {[(tBuO)4Al]Cu(OH).py}2. The formation of all of these isolated products is not always straightforward because some of these compounds in solution are subject to decomposition or are involved in equilibria. Besides NMR [copper(I) compounds], UV absorptions and magnetic moments are used to characterize the compounds.

Synthesis, Characterization, and Electrochemical Studies of Copper(II) Complexes with Tripod Ligand Tris(2‐benzimidazylmethyl)amine and α,β‐Unsaturated Carboxylate

Four pentacoordinate copper complexes of tripod ligand tris(2‐benzimidazylmethyl)amine (ntb), and four different α,β‐unsaturated carboxylate (L1–4), with compositions [Cu2(ntb)2 (L1–3)] (NO3)3 · nH2O and [Cu(ntb)(L4)](NO3) · nH2O (L1=acrylate; L2=α‐methacrylate; L3=2‐hydroxycinnamate; L4=trans‐cinnamate; n=1, 1.5, 2), were synthesized and characterized by elemental and thermal analyses, electrical conductivities, IR, and UV‐Visible measurements. Cyclic voltammograms of the copper complexes indicate a quasireversible Cu+2/Cu+ couple. Possible structures of these complexes have been proposed.

Monomeric, tetrameric, and polymeric copper di-tert-butyl phosphate complexes containing pyridine ancillary ligands.

The reaction of di-tert-butyl phosphate (((t)BuO)(2)P(O)(OH), dtbp-H) with copper acetate in the presence of pyridine (py) and 2,4,6-trimethylpyridine (collidine) has been investigated. Copper acetate reacts with dtbp-H in a reaction medium containing pyridine, DMSO, THF, and CH(3)OH to yield a one-dimensional polymeric complex [Cu(dtbp)(2)(py)(2)(mu-OH(2))](n) (1) as blue hollow crystalline tubes. The copper atoms in 1 are octahedral and are surrounded by two terminal phosphate ligands, two pyridine molecules, and two bridging water molecules. The mu-OH(2) ligands that are present along the elongated Jahn-Teller axis are responsible for the formation of the one-dimensional polymeric structure. Recrystallization of 1 in a DMSO/THF/CH(3)OH mixture results in the reorganization of the polymer and its conversion to a more stable tetranuclear copper cluster [Cu(4)(mu(3)-OH)(2)(dtbp)(6)(py)(2)] (2) in about 60% yield. The molecular structure of 2 is made up of a tetranuclear core [Cu(4)(mu(3)-OH)(2)] which is surrounded by six bidentate bridging dtbp ligands. While two of the copper atoms are pentacoordinate with a tbp geometry, the other two copper atoms exhibit a pseudooctahedral geometry with five normal Cu-O bonds and an elongated Cu-O linkage. The pentacoordinate copper centers bear an axial pyridine ligand. The short Cu.Cu nonbonded distances in the tetranuclear core of 2 lead to magnetic ordering at low temperature with an antiferromagnetic coupling at approximately 20 K (J(P) = -44 cm(-1), J(c) = -66 cm(-1), g = 2.25, and rho = 0.8%). When the reaction between di-tert-butyl phosphate (dtbp-H) and copper acetate was carried out in the presence of collidine, large dark-blue crystals of monomeric copper complex [Cu(dtbp)(2)(collidine)(2)] (3) formed as the only product. A single-crystal X-ray diffraction study of 3 reveals a slightly distorted square-planar geometry around the copper atom. Thermogravimetric analysis of 1-3 revealed a facile decomposition of the coordinated ligands and dtbp to produce a copper phosphate material around 500 degrees C. An independent solid-state thermolysis of all the three complexes in bulk at 500-510 degrees C for 2 days produced copper pyrophosphate Cu(2)P(2)O(7) along with small quantities of Cu(PO(3))(2) as revealed by DR-UV spectroscopic and PXRD studies.

Crystal structure and kinetics of the acid-catalysed decomposition of ( N, N′-bis(2-pyridylmethylene)-1,3-diamino-2,2-dicarboxyethylpropane)copper(II)perchlorate

The crystal structure of the complex [CuL(OH) 2](ClO 4) 2·H 2O; L= N, N′-bis(2-pyridylmethylene)-1,3-diamino-2,2-dicarboxyethylpropane, determined for the first time, reveals a penta-coordinated copper(II) centre in a distorted square-pyramidal geometry with a weakly bonded axial water molecule. In order to assess possible routes to the putative free ligand N, N′-bis(2-pyridylmethylene)-1,3-diaminopropane-2-carboxylic acid the behaviour (stability) of the complex towards both acid and alkaline aqueous solution has been investigated. In 1.0 M acidic chloride media at 25 °C the complex undergoes slow decomposition in two stages giving 2-pyridylmethylammonium chloride, formaldehyde and diethylmalonate in addition to aqueous Cu 2+. The initial fast stage has both H +-dependent (5.32±0.18×10 −5 M −2 s −1) and independent (1.59±0.10×10 −5 s −1) paths, the latter believed to reflect rapid chloride equilibration at the labile axial site followed by rate determining isomerisation to place a CuNH bond at the axial site leading to CuN bond cleavage. The rate constant for the H +-dependent faster stage carries a [H +] 2 dependence suggesting that here protonation of both secondary NH groups occurs within the rate-determining step prior to decomposition. Assistance from the axial site is also proposed. Absorbance changes monitored as a function of [H +] for the ∼10× slower second stage suggest that it may involve intermediates resulting from the H +-independent initial step.

Alternate substrate binding modes to two mutant (D98N and H255N) forms of nitrite reductase from Alcaligenes faecalis S-6: structural model of a transient catalytic intermediate.

High-resolution nitrite soaked oxidized and reduced crystal structures of two active site mutants, D98N and H255N, of nitrite reductase (NIR) from Alcaligenes faecalis S-6 were determined to better than 2.0 A resolution. In the oxidized D98N nitrite-soaked structures, nitrite is coordinated to the type II copper via its oxygen atoms in an asymmetric bidentate manner; however, elevated B-factors and weak electron density indicate that both nitrite and Asn98 are less ordered than in the native enzyme. This disorder likely results from the inability of the N delta 2 atom of Asn98 to form a hydrogen bond with the bound protonated nitrite, indicating that the hydrogen bond between Asp98 and nitrite in the native NIR structure is essential in anchoring nitrite in the active site for catalysis. In the oxidized nitrite soaked H255N crystal structure, nitrite does not displace the ligand water and is instead coordinated in an alternative mode via a single oxygen to the type II copper. His255 is clearly essential in defining the nitrite binding site despite the lack of direct interaction with the substrate in the native enzyme. The resulting pentacoordinate copper site in the H255N structure also serves as a model for a proposed transient intermediate in the catalytic mechanism consisting of a hydroxyl and nitric oxide molecule coordinated to the copper. The formation of an unusual dinuclear type I copper site in the reduced nitrite soaked D98N and H255N crystal structures may represent an evolutionary link between the mononuclear type I copper centers and dinuclear Cu(A) sites.