Mixed-valent copper oxides

A comprehensive report which focussed on the La2 xSrxCu O4-x/2+S ser es was published (139) in 1983 by this research group. In this broad review they reported the magnetic and electrical transport properties of these mixed-valent copper oxides in the temperature range 120-650 K. They concluded that the original semiconducting behavior in La2Cu04 transformed to semi-metallic behavior as the Cu3+ content increased with Sr-substitution. No experiments were conducted below 50 K, and therefore superconductivity was not observed. Three series of compounds, with 0.00 < x < 1.20 were... 

Assigning oxidation states of —2 to oxygen, + 3 to yttrium and + 2 to barium, one would obtain an oxidation state 7/3 for copper when jc = 0. The non-integer oxidation state of copper is interpreted as if 2/3 of the ions are present as Cu2+ and 1/3 as Cu3 +. This mixed-valent composition seems to be determinant for the occurrence of superconductivity. In fact, as noted in Table 1, all the superconducting ceramic oxides contain Cu in a non-stoichiometric composition. 

For dinuclear Cu complexes, several pathways are possible as summarized in Scheme 15 [182]. In addition, plausible alternatives involve mixed-valent Cu Cu species where only one of the Cu ions is directly involved in the electron transfer. The latter seems most hkely in cases where the substrate binds to only one of the two copper ions, and H2O2 may then form upon oxidation of the Cu Cu -semiquinone intermediate [195]. Different coordination modes of the DTBC substrate appear to be indeed possible, depending on the particular dicopper scaffold [133,196,197]. Unfortunately, detailed mechanistic studies are still quite scarce [198-203] and most proposed catalytic pathways are rather speculative. 

A series of hemocyanin and tyrosinase active site derivatives (Fig. 23) can be prepared61"66), allowing systematic variation of the binuclear copper active site and chemical perturbation for spectral studies. In the simplest derivative, met-apo, one copper has been removed and the remaining copper oxidized to the spectroscopically accessible Cu(II). Next in complexity is a mixed-valent binuclear copper site. The Cu(II), in this half-met derivative, exhibits open-shell d9 spectroscopic features and the Cu(I), though spectroscopically inaccessible, can still be studied by comparison to the met-apo derivative. Two derivatives have formally binuclear cupric sites met, which is EPR-non-detect-able, and dimer, which exhibits an intense broad EPR signal. Spectroscopic study of these derivatives has led to the present picture of the coupled binuclear copper protein active site shown at the bottom of Fig. 23. 

Cobalt sulfide and arsenide ores are often found mixed with those of nickel and copper. The mixed ore is roasted with Na2C03 and KNO3, which removes part of the sulfur and arsenic as volatile species. The residue contains the metal oxides, as well as sulfate and arsenate, and the latter are removed by leaching with water. The metal oxide mixture is then dissolved in hot HCl or H2SO4, and the individual metal oxides are fractionally precipitated using Ca(OH)2 and NaOCl. This process gives the mixed valent oxide C03O4, which is then reduced to the metal by treatment with charcoal. 

The final step in the denitrification process is carried out by the soluble enzyme nitrous oxide reductase (NoS). This enzyme has been isolated from a number of sources, and is unusual in a number of ways. In most cases, it is a homodimer of ca. 74 kDa subunits with ca. 4 Cu/subunit, but the enzyme is bright purple or pink as isolated, depending on conditions, and becomes the typical blue color expected for copper proteins only after reduction with dithionite. A variety of spectroscopic studies strongly suggest that the enzyme contains at least one mixed-valent, thiolate-bridged Cu(l)—Cu(ll) unit that may well be similar to a binuclear copper center in cytochrome c oxidase (23). The reaction catalyzed by the enzyme is deceptively simple ... 

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