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Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn

X-ray photoelectron studies on some oxides and hydroxides of cobalt, nickel, and copper

X-ray photoelectron spectroscopic studies of iron oxides

Surface studies of supported model catalysts

The chemistry of copper is extremely rich because it can easily access Cu0, CuI, CuII, and CuIII oxidation states allowing it to act through one-electron or two-electron processes. As a result, both radical pathways and powerful two-electron bond forming pathways via organmetallic intermediates, similar to those of palladium, can occur. In addition, the different oxidation states of copper associate well with a large number of different functional groups via Lewis acid interactions or π-coordination. In total, these feature confer a remarkably broad range of activities allowing copper to catalyze the oxidation and oxidative union of many substrates.

Oxygen is a highly atom economical, environmentally benign, and abundant oxidant, which makes it ideal in many ways.1 The high activation energies in the reactions of oxygen require that catalysts be employed.2 In combination with molecular oxygen, the chemistry of copper catalysis increases exponentially since oxygen can act as either a sink for electrons (oxidase activity) and/or as a source of oxygen atoms that are incorporated into the product (oxygenase activity). The oxidation of copper with oxygen is a facile process allowing catalytic turnover in net oxidative processes and ready access to the higher CuIII oxidation state, which enables a range of powerful transformations including two-electron reductive elimination to CuI. Molecular oxygen is also not hampered by toxic byproducts, being either reduced to water, occasionally via H2O2 (oxidase activity) or incorporated into the target structure with high atom economy (oxygenase activity). Such oxidations using oxygen or air (21% oxygen) have been employed safely in numerous commodity chemical continuous and batch processes.3

However, batch reactors employing volatile hydrocarbon solvents require that oxygen concentrations be kept low in the head space (typically <5–11%) to avoid flammable mixtures, which can limit the oxygen concentration in the reaction mixture.4,5,6 A number of alternate approaches have been developed allowing oxidation chemistry to be used safely across a broader array of conditions. For example, use of carbon dioxide instead of nitrogen as a diluent leads to reduced flammability.5 Alternately, water can be added to moderate the flammability allowing even pure oxygen to be employed.6 New reactor designs also allow pure oxygen to be used instead of diluted oxygen by maintaining gas bubbles in the solvent, which greatly improves reaction rates and prevents the build up of higher concentrations of oxygen in the head space.4a,7 Supercritical carbon dioxide has been found to be advantageous as a solvent due its chemical inertness towards oxidizing agents and its complete miscibility with oxygen or air over a wide range of temperatures.8 An number of flow technologies9 including flow reactors,10 capillary flow reactors,11 microchannel/microstructure structure reactors,12 and membrane reactors13 limit the amount of or afford separation of hydrocarbon/oxygen vapor phase thereby reducing the potential for explosions.

Enzymatic oxidizing systems based upon copper that exploit the many advantages and unique aspects of copper as a catalyst and oxygen as an oxidant as described in the preceding paragraphs are well known. They represent a powerful set of catalysts able to direct beautiful redox chemistry in a highly site-selective and stereoselective manner on simple as well as highly functionalized molecules. This ability has inspired organic chemists to discover small molecule catalysts that can emulate such processes. In addition, copper has been recognized as a powerful catalyst in several industrial processes (e.g. phenol polymerization, Glaser-Hay alkyne coupling) stimulating the study of the fundamental reaction steps and the organometallic copper intermediates. These studies have inspiried the development of nonenzymatic copper catalysts. For these reasons, the study of copper catalysis using molecular oxygen has undergone explosive growth, from 30 citations per year in the 1980s to over 300 citations per year in the 2000s.

A number of elegant reviews on the subject of catalytic copper oxidation chemistry have appeared. Most recently, reviews provide selected coverage of copper catalysts14 or a discussion of their use in the aerobic functionalization of C–H bonds.15 Other recent reviews cover copper and other metal catalysts with a range of oxidants, including oxygen, but several reaction types are not covered.16 Several other works provide a valuable overview of earlier efforts in the field.17 This review comprehensively covers copper catalyzed oxidation chemistry using oxygen as the oxidant up through 2011. Stoichiometric reactions with copper are discussed, as necessary, to put the development of the catalytic processes in context. Mixed metal systems utilizing copper, such as palladium catalyzed Wacker processes, are not included here. Decomposition reactions involving copper/oxygen and model systems of copper enzymes are not discussed exhaustively. To facilitate analysis of the reactions under discussion, the current mechanistic hypothesis is provided for each reaction. As our understanding of the basic chemical steps involving copper improve, it is expected that many of these mechanisms will evolve accordingly.

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Aerobic Copper-Catalyzed Organic Reactions

In acidic solution, the group 4A elements form ternary heteropoly complexes with molybdophosphate, e.g., molybdotitanophosphate, molybdozirconophoaphate, and molybdohafnophosphate. Each ternary heteropoly molybdate gives a different chemical shift in its /sup 31/P NMR spectra, respectively, and its relative peak area increases with increasing metal ion concentration. Simultaneous determination of these elements was found to be possible within accuracies of 1 to 5% and detection limits for Tl, Zr, and Hf were 20, 40, and 60 ppm, respectively, under the proposed experimental condition. The small amounts of these ternary heteropoly molybdates in aqueous solution are extracted quantitatively with cyclohexanone. The optimum acidity for extraction was found to be in the pH range between 1.5 and 2.5 and its recovery in cyclohexanone was more than 95%. 14 references, 3 tables, 4 figures.

Simultaneous determination of titanium, zirconium, and hafnium in aqueous solution by phosphorus-31 nuclear magnetic resonance spectrometry

Cation effect on the polarographic reduction of nickel(II)

EXAFS (Extended X-ray Absorption Fine Structure) spectra were analyzed for various polynuclear molybdates and tungstates in order to examine the feasibility of this method for the structural study of complex polynuclear compounds. We found an unusual phenomenon that Mo–O peaks in Fourier transforms of Mo6O192− and PMo12O403− are hardly discernible while the corresponding tungstates give the W–O peaks clearly and normally.

EXAFS study on polynuclear molybdenum and tungsten compounds

The first and the second bands of the photoelectron spectra of acetic, propionic, and isobutyric acids were recorded, and the vibrational structures in the spectra were studied by comparing with the results of formic acid. The first band exhibits a νc=o, a δOH, and a ρCOOH vibrational progression, and this implies that an oxygen lone pair orbital, from which the first band of the spectra originates, contributes not only to a C=O bonding and the bonding between carbonyl oxygen and hydroxyl hydrogen, but to the settling of carboxyl group in the molecular plane. The second band exhibits a νc=o, a νc-o vibrational progression with very high frequency for the νc=o as observed in the case of formic acid. Thus the vibrational features of the second band are consistent with the ionization of an antisymmetric π orbital localized mainly on carbonyl and hydroxyl oxygen atoms. Since the 0-0 vibrational transitions for the second ionization are out of the Franck-Condon region, the second adiabatic ionization potential...

Vibrational Structures in the He(I) Photoelectron Spectra of Carboxylic Acids

A characteristic second peak appears at about 10 eV from a main peak in the XANES (X-ray Absorption Near Edge Structure) spectrum of aqueous 3d transition metal complex of divalent state. This second peak is prominent in solid samples. The peak intensity depends on the kind of central metal or counter anion in solid. Theoretical calculations based on the full multiple scattering formalism provide the information regarding the origin and property of the second peak in XANES. The second peak can be reproduced by the multiple scattering calculation considering only six first coordinating oxygen atoms. The intensity and the position of the peak vary with the charge on the first coordinating oxygen atoms which is affected by the ions or solvent molecules contacting with the metal complex.

Shigero Ikeda

Papers

Simultaneous determination of titanium, zirconium, and hafnium in aqueous solution by phosphorus-31 nuclear magnetic resonance spectrometry

Cation effect on the polarographic reduction of nickel(II)

EXAFS study on polynuclear molybdenum and tungsten compounds

Vibrational Structures in the He(I) Photoelectron Spectra of Carboxylic Acids

XANES of aqua complexes of 3d transition metals in solid and solution.