Trivalent and Tetravalent Nickel in Oxime-Imine-Amine Coordination

Model complexes of nickel(III) and nickel(IV) are furnished by hexadentate and tridentate oxime-imine-amine ligands derived by the condensation of biacetyl monoxime with amines. The synthesis, characterisation, structure, redox equilibria and reactivity of these complexes are critically reviewed. Correlations and generalisations pertaining to oxidation state stability, electroprotic equilibria and reaction mechanisms are examined. The possible significance of the present complexes in the context of the nickel-containing methanogenic enzymes is noted.     

Electrochemical and AFM study of nickel nucleation mechanisms on vitreous carbon from ammonium sulfate solutions

Reaction and nucleation mechanisms of nickel in ammoniacal solutions have been investigated as a function of nickel concentration, solution pH, deposition potential, temperature and conditioning potential. Electrochemical mechanisms of nickel reduction were found to be pH dependent, while their kinetics was concentration dependent. A surface film formed by anodic oxidation passivates nickel clusters preventing their further oxidation. Nickel nucleation on vitreous carbon, which proceeds according to the progressive nucleation model, shows a large degree of inhibition at both pH 6 and pH 9. Cluster sizes were larger when electrodeposition was carried out from solutions with higher nickel concentrations. The clusters were also larger at more negative deposition potentials and at higher solution pH. Cluster population density increased with the increasing solution temperature. Different activation energies for the nickel-aquo and nickel-ammino complexes calculated from Arrhenius diagram indicate that electroreduction of nickel-ammino complex is energetically more demanding. All electrochemical results were further verified by the atomic force microscopy investigations.     

Metallo Salicylidenetriazol Complexes Encapsulated in Zeolite-Y: Synthesis, Physicochemical Properties and Catalytic Studies

Chromium(III), zinc(II) and nickel(II) complexes of thio-Schiff base derived from salicylaldehyde and 4-amino-2,4-dihydro-1,2,4-triazole-5-thione have been encapsulated in the nanopores of zeolite-Y by a flexible ligand method. The prepared encapsulated metal complexes have been characterized by surface analysis (XRD and N₂ adsorption/desorption), spectroscopic methods, chemical and thermal analyses. The various techniques of characterization used demonstrated that these complexes were effectively encapsulated in the zeolite supercages without structural modification or loss of crystallinity of the zeolite framework. The encapsulated complexes were screened as heterogeneous catalysts for various oxidation reactions such as phenol, cyclohexene and styrene oxidation, using H₂O₂ as an oxidant. Under the optimized conditions, these catalysts exhibited high to moderate activity. After a few cycles these catalysts were found to be stable and could be reused after recovering without detectable catalyst leaching or significant loss of activity.     

Electrocatalytic Oxidation of Amines on Platinum Electrodes. Part III. Oxidation Of Ethylenediamine Via Higher Oxidation States of Nickel

The voltammetric behaviour of aqueous nickel ethylenediamine solutions has been studied. Electrocatalytic oxidation waves were obtained which are attributed to the formation of trivalent and possibly also tetravalent nickel ethylenediamine complexes as intermediates. The mechanism of the electrocatalytic process is discussed.     

Thermal properties and characteristics of thin layers of nickel dithiolene complexes

Thermal properties of near infra–red–absorbing nickel dithiolene complexes have been examined by thermal gravimetric and differential scanning calorimetry analysis. The nickel complexes with relatively low melting points gave a DRAW disc with better recording characteristics, when they were used as inhibitors of the photofading of thin layers of cyanine dyes. Thin layers of the nickel complexes on polymethylmethacrylate or glass were formed by spin coating and by the radio–frequency reactive ion plating technique. Their reflectance, absorption spectra and optical recording characteristics have been recorded.     

Lessons from isolable nickel(I) precursor complexes for small molecule activation

Small-molecule activation by transition metals is essential to numerous organic transformations, both biological and industrial. Creating useful metal-mediated activation systems often depends on stabilizing the metal with uncommon low oxidation states and low coordination numbers. This provides a redox-active metal center with vacant coordination sites well suited for interacting with small molecules. Monovalent nickel species, with their d(9) electronic configuration, are moderately strong one-electron reducing agents that are synthetically attractive if they can be isolated. They represent suitable reagents for closing the knowledge gap in nickel-mediated activation of small molecules. Recently, the first strikingly stable dinuclear β-diketiminate nickel(I) precursor complexes were synthesized, proving to be suitable promoters for small-molecule binding and activation. They have led to many unprecedented nickel complexes bearing activated small molecules in different reduction stages. In this Account, we describe selected achievements in the activation of nitrous oxide (N(2)O), O(2), the heavier chalcogens (S, Se, and Te), and white phosphorus (P(4)) through this β-diketiminatonickel(I) precursor species. We emphasize the reductive activation of O(2), owing to its promise in oxidation processes. The one-electron-reduced O(2) activation product, that is, the corresponding β-diketiminato-supported Ni-O(2) complex, is a genuine superoxonickel(II) complex, representing an important intermediate in the early stages of O(2) activation. It selectively acts as an oxygen-atom transfer agent, hydrogen-atom scavenger, or both towards exogenous organic substrates to yield oxidation products. The one-electron reduction of the superoxonickel(II) moiety was examined by using elemental potassium, β-diketiminatozinc(II) chloride, and β-diketiminatoiron(I) complexes, affording the first heterobimetallic complexes featuring a [NiO(2)M] subunit (M is K, Zn, or Fe). Through density functional theory (DFT) calculations, the geometric and electronic structures of these complexes were established and their distinctive reactivity, including the unprecedented monooxygenase-like activity of a bis(μ-oxo)nickel-iron complex, was studied. The studies have further led to other heterobimetallic complexes containing a [NiO(2)M] core, which are useful for understanding the influence of the heterometal on structure-reactivity relationships. The activation of N(2)O led directly to the hydrogen-atom abstraction product bis(μ-hydroxo)nickel(II) species and prevented isolation of any intermediate. In contrast, the activation of elemental S, Se, and Te with the same nickel(I) reagent furnished activation products with superchalcogenido E(2)(-) (E is S, Se, or Te) and dichalcogenido E(2)(2-) ligand in different activation stages. The isolable supersulfidonickel(II) subunit may serve as a versatile building block for the synthesis of heterobimetallic disulfidonickel(II) complexes with a [NiS(2)M] core. In the case of white phosphorus, the P(4) molecule has been coordinated to the nickel(I) center of dinuclear β-diketiminatonickel(I) precursor complexes; however, the whole P(4) subunit is a weaker electron acceptor than the dichalcogen ligands E(2), thus remaining unreduced. This P(4) binding mode is rare and could open new doors for subsequent functionalization of P(4). Our advances in understanding how these small molecules are bound to a nickel(I) center and are activated for further transformation offer promise for designing new catalysts. These nickel-containing complexes offer exceptional potential for nickel-mediated transformations of organic molecules and as model compounds for mimicking active sites of nickel-containing metalloenzymes.     

Polymers containing metal chelate units. I. Immobilized mono- and binuclear chelates of nickel(II) and cobalt(II)

A number of polymer carriers with chelating groups such as diketones, diacylamines, and enaminoketones has been synthesized. The immobilized mono- and binuclear chelates have been prepared by the interaction of these polymeric ligands with nickel(II) and cobalt(II) acetates in alcohol. Spectral and magnetic data confirm the tetrahedral structure of the cobalt complexes and octahedral structure of the nickel complexes. The catalytic properties of the immobilized complexes in ethylene dimerization and butadiene polymerization have been investigated.     

Stabilisation of active nickel catalysts in partial oxidation of methane to synthesis gas by iron addition

Mixed LaNi_xFe_(1−x)O₃ perovskite oxides (0≤x≤1) have been prepared by a sol–gel related method, characterised by X-ray diffraction (XRD), specific surface area measurements, transmission electron microscopy (TEM) coupled to an energy dispersive X-ray spectrometer (EDS). These systems are the precursors of highly efficient catalysts in partial oxidation of methane to synthesis gas. Studies on the state of these systems after test show the stabilisation of active nickel by increasing the amount of iron. These systems permit to control the reversible migration of nickel from the structure to the surface. The best mixed perovskite for the partial oxidation of methane is LaNi_0.3Fe_0.7O₃.     

室温固相法合成2-吡啶甲酸钴、镍配合物

利用乙酸钴(Ⅱ)、氯化镍(Ⅱ)和2-吡啶甲酸为原料,采用室温固相法合成了2-吡啶甲酸钴(Ⅱ)、镍(Ⅱ)配合物M(C5H4NCOO)2.xH2O(M=Co,Ni;x=6,3),用EDTA配位滴定、元素分析、X射线粉末衍射、红外光谱和热分析等方法对产物进行组成和结构表征。结果表明,2-吡啶甲酸中的羧基氧原子和杂环氮原子及水分子参与配位,其热分解包括失水、配体的氧化分解过程,最后完全形成金属氧化物。     

室温固相法合成2-吡啶甲酸钴、镍配合物

利用乙酸钴(II)、氯化镍(II)和2-吡啶甲酸为原料,采用室温固相法合成了2-吡啶甲酸钴(II)、镍(II)配合物M(C5H4NCOO)2?xH2O,用EDTA配位滴定、元素分析、X射线粉末衍射、红外光谱和热分析等方法对产物进行组成和结构表征。结果表明,2-吡啶甲酸中的羧基氧原子和杂环氮原子及水分子参与配位,其热分解包括失水、配体的氧化分解过程,最后完全形成金属氧化物。     

Comments on the Nature of the Oxidized Form of the Nickel Dimethylglyoxime Complex

The pulse radiolytic technique has been used in order to prove that the central nickel atom, in the oxidized form of the nickel-dimethyl-glyoxime complex, is in its trivalent oxidation state.     

A Polymer-Supported Nickel(II) Catalyst for Room Temperature Tamao–Kumada–Corriu Coupling Reactions

A nickel(II) co-ordination complex, covalently immobilised on a Merrifield resin, is an effective recyclable heterogeneous catalyst in a room temperature coupling reaction between a Grignard reagent and an organobromide. The immobilised catalyst does not leach metal into solution. Catalyst stability has been rationalised in terms of the combination of hard and soft donor atoms around the variable oxidation state nickel.     

Electrochemical reduction of perchlorate ions on platinum-activated nickel

The electrochemical reduction of perchlorate anion has been studied in a cell with a nickel working electrode and a platinum counter electrode in concentrated solutions of HClO₄. Significant reduction of perchlorate to chloride ions occurs on the nickel cathode in the potential region where hydrogen evolution occurs. It is shown that the mechanism of this process involves platinum deposited in small amounts on the cathode as a result of oxidation of the platinum anode in the perchloric acid solution. The reduction of perchlorate is accompanied by oxidation of the nickel cathode, which is attributed to chlorate ions formed in the initial step of perchlorate reduction. Changes in the surface structure of the nickel electrode have been followed using atomic force microscopy.     

Synthesis of polymer-supported metal-ion complexes and evaluation of their catalytic activities

Polymer-supported transition-metal-ion complexes of the N,N′-bis(o-hydroxy acetophenone) propylenediamine (HPPn) Schiff base were prepared by the complexation of iron(III), cobalt(II), and nickel(II) ions on a polymer-anchored N,N′-bis(5-amino-o-hydroxy acetophenone) propylenediamine Schiff base. The complexation of iron(III), cobalt(II), and nickel(II) ions on the polymer-anchored HPPn Schiff base was 83.44, 82.92, and 89.58 wt%, respectively, whereas the unsupported HPPn Schiff base showed 82.29, 81.18, and 87.29 wt % complexation of these metal ions. The iron(III) ion complexes of the HPPn Schiff base showed octahedral geometry, whereas the cobalt(II) and nickel(II) ion complexes were square planar in shape, as suggested by spectral and magnetic measurements. The thermal stability of the HPPn Schiff base increased with the complexation of metal ions, as evidenced by thermogravimetric analysis. The HPPn Schiff base showed a weight loss of 51.0 wt % at 500°C, but its iron(III), cobalt(II), and nickel(II) ion complexes showed weight losses of 27.0, 35.0, and 44.7 wt % at the same temperature. The catalytic activity of the unsupported and supported metal-ion complexes was analyzed by the study of the oxidation of phenol and epoxidation of cyclohexene in the presence of hydrogen peroxide. The supported HPPn Schiff base complexes of iron(III) ions showed a 73.0 wt % maximum conversion of phenol and 90.6 wt % epoxidation of cyclohexene, but unsupported complexes of iron(III) ions showed 63.8 wt % conversion of phenol and 83.2 wt % epoxidation of cyclohexene. The product selectivity for catechol (CTL) and epoxy cyclohexane (ECH) was 93.1 wt % and 98.1 wt % with the supported HPPn Schiff base complexes of iron(III) ions, but it was low with the supported Schiff base complexes of cobalt(II) and nickel(II) ions. The selectivity for CTL and ECH varied with the molar ratio of the metal ions but remained unaffected by the molar ratio of hydrogen peroxide to the substrate. The energy of activation for the epoxidation of cyclohexene and oxidation of phenol with the polymer-supported Schiff base complexes of iron(III) ions was 10.0 and 12.7 kJ/mol, respectively, but it was found to be higher with the supported HPPn Schiff base complexes of cobalt(II) and nickel(II) ions and with the unsupported HPPn Schiff base complexes of iron(III), cobalt(II), and nickel(II) ions. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Synthesis,characterization and antitumour studies of metal chelates of N1-isonicotinoyl-3-methyl-4-(p-hydroxybenzilidene)-2-pyrazolin-5-one

The cobalt(II), nickel(II) and copper(II) complexes of N-Isonicotinoyl-3-methyl-4-(p-hydroxybenzilidene)-2-pyrazolin-5-one (IMHBP) with different counter anions have been prepared and characterized. An octahedral geometry has been assigned to the cobalt(II) and nickel(II) complexes and a square-planar structure to the copper(II) complexes. IMHBP acts as a neutral bidentate ligand in all these complexes by coordinating through the oxygen atoms of the amide group and carbonyl at position-5. The other coordination sites are satisfied by anions alone or anions and water molecules. The ligand and the complexes were screened for their possible antitumour activity. The copper(II) complexes are appreciably active in reducing mice tumours.     

Oscillatory Behaviour During Ethane Oxidation Over Nickel and Cobalt Catalysts

Oscillatory behaviour during ethane oxidation over a nickel foil and a cobalt foil has been studied using on-line mass-spectrometry and video recording of the catalyst surface colour. It was demonstrated that during the oscillatory behaviour, periodic transitions of the catalyst surface from an oxidised state to a reduced state occurred together with a variation of the catalyst temperature. Simultaneous measurements of gas phase concentrations revealed that mostly CO₂ was produced over the oxidised state, while CO and H₂ production rates reached their maxima over the completely reduced surface. The comparison of the observed oscillations with known oscillations during methane oxidation over nickel and cobalt foils has been done. It was shown that the higher reducing ability of ethane was the reason of an increase of the frequency of the oscillations over both catalysts and a significant decrease of the temperature range of the oscillations over the cobalt foil.     

Electrochemical and spectroelectrochemical characterization of the phthalocyanines with pentafluorobenzyloxy substituents

The solution redox properties and spectroelectrochemical investigation of the novel metal-free, zinc, nickel and cobalt phthalocyanines with tetra-pentafluorobenzyloxy substituents at the periphery were studied using various electrochemical and spectroelectrochemical measurements. Cyclic voltammetry and differential pulse voltammetry studies show that while Ni(II), Zn(II) and free-phthalocyanines give up to two reduction and two oxidation processes having ligand-based diffusion controlled reversible one-electron electron transfer characters, Co(II) phthalocyanine represents one ligand-based oxidation, one metal-based reduction and one ligand-based reduction processes having diffusion controlled reversible one-electron transfer characters. Assignments of the redox couples are also confirmed by spectroelectrochemical measurements. Reduction potentials of all complexes shift to positive potentials due to the electron withdrawing tetra-pentafluorobenzyloxy substituents compared with those of the phthalocyanines bearing phenoxy derivatives. A linear variation of the first reduction and oxidation potentials versus ze/r has been obtained for zinc and nickel phthalocyanines.     

The electrochemical oxidation of manganous hydroxide

Manganous hydroxide layers have been oxidized anodically by constant potential steps. The charge uptake and current—time transients have been correlated with changes of composition and crystal structure as revealed by X-ray diffraction. Two oxidation stages have been observed, and the process is compared with the anodic oxidation of nickel hydroxide.     

Electrochemical oxidation of isobutanol to isobutyric acid at nickel oxide electrode: improvement of the anode stability

Electrooxidation processes using nickel oxide anodes in alkaline electrolyte offer good alternatives to the chemical processes used for the oxidation of alcohols. However, they suffer from the low stability of the electrodeposited layer of nickel oxide, resulting in poor current efficiency. This requires periodic reactivation of the anode. In the present work, different parameters governing the stability of the nickel oxide layer have been identified and their influence on the current efficiency for oxidation has been investigated using oxidation of isobutanol (2-methyl-1-propanol) to isobutyric acid (2-methylpropionic acid) as a model system. Alternative anode activation procedures have been studied. The procedures employed resulted in a five fold increase in the anode stability over earlier reported procedures for anode activation and electrolysis.