Copper State of oxidation

The main by-products of the Ullmaim condensation are l-aniinoanthraquinone-2-sulfonic acid and l-amino-4-hydroxyanthraquinone-2-sulfonic acid. The choice of copper catalyst affects the selectivity of these by-products. Generally, metal copper powder or copper(I) salt catalyst has a greater reactivity than copper(Il) salts. However, they are likely to yield the reduced product (l-aniinoanthraquinone-2-sulfonic acid). The reaction mechanism has not been estabUshed. It is very difficult to clarify which oxidation state of copper functions as catalyst, since this reaction involves fast redox equiUbria where anthraquinone derivatives and copper compounds are concerned. Some evidence indicates that the catalyst is probably a copper(I) compound (28,29). 

When the radicals have p hydrogens, alkenes are formed by a process in which carbocations are probably bypassed. Instead, the oxidation and the elimination of a proton probably occur in a single step through an alkylcopper species. The oxidation state of copper in such an intermediate is Cu(III). 

The role of the two oxidation states of copper in the aryl dimerization was investigated by Cohen et al. (1974). The reaction conditions used by these authors were, however, different in several respects from those of the two cases discussed before. 

Turnes Palomino, G., Fisicaro, P., Bordiga, S. el al. (2000) Oxidation states of copper ions in ZSM-5 zeolites. A multitechnique investigation, J. Phys. Chem. B, 104, 4064. 

Pieplu, T., Poignant, F., Vallet, A. et al. (1995) Oxidation state of copper during the reduction of NOx with propane on H-Cu-ZSM-5 in excess oxygen, Stud. Surf. Sci. Catal., 96, 619. 

Cu,Zn superoxide dismutase. Essentially, these observations support a stepwise one-electron model again. Interestingly, the oxidation state of copper does not change during the catalytic reaction, i.e. the sole kinetic role of the histidine coordinated metal center is to alter the electronic structures of the substrate and 02 in order to facilitate the electron transfer process between them. 

In 1974 Cohen and coworkers145 studied the influence of the two oxidation states of copper on aryl dimerization, but not with the mixture mentioned above. 

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. 

The copper to oxygen distance will vary according to the oxidation state of copper, the degree of covalency in the bond, and the geometry about the copper atom. The data given below are the averages of reported (and theoretical "ionic") radii obtained from... 

Neither the calcium nor the strontium compound is a superconductor. A possible reason may be that the oxidation state of copper is equal, or very close, to 2+. 

Immediately after the discovery of YBa2Cus07 x, numerous groups employed iodometric titration procedures to measure the effective oxidation state of the material, and therefore the value of x. The procedure described below involves two different titrations (10X17X18) and is more accurate than a procedure in which the first titration is omitted (19). Experiment A measures the total copper content of the superconductor and Experiment B measures the total charge of the copper. The two experiments, together, give the average oxidation state of copper. 

Another type of polarizibility results from the near degeneracy of the metal levels and the oxygen 2p levels. This is directly related to the high covalency in these systems thus, this type of polarizibility will be greater for the higher oxidation states of copper and bismuth. Both of these polarizibility contributions are likely very important for theories of superconductivity based on charge fluctuations. 

A prototypical high-temperature superconductor is yttrium barium copper oxide, YBa2Cu307, in which two-thirds of the copper is in the +2 oxidation state and one-third is in the unusual +3 state. Another example is BijSrjlCaQgYoyjCujOgjys, in which the average oxidation state of copper is +2.105 and the average oxidation state of bismuth is +3.090 (which is formally a mixture of Bi3+ and Bi5+). The most reliable means to unravel these complex formulas is through wet oxidation-reduction titrations, described in this chapter. 

Redox titrations proved to be the most reliable way to measure the oxidation state of copper and thereby deduce the oxygen content of YBa2Cu30A.25 An iodometric method includes two experiments. In Experiment A, YBa2Cu30A is dissolved in dilute acid, in which Cu3+ is converted into Cu2+. For simplicity, we write the equations for the formula YBa2Cu307, but you could balance these equations for x =A 7.26... 

This is a very restricted oxidation state of copper but may be considered to occur in the polynuclear copper species Cu2, Cu3 and Cus, which have been characterized by matrix isolation techniques. Copper(O) also occurs in species formed by the reaction of copper metal vapour and carbon monoxide gas. Matrix isolation techniques have characterized a monomeric [Cu(CO)3] trigonal planar species and a dimeric [(CO)3CuCu(CO)3] species.32... 

The remarkable variety in the permutations of the R2 Dtc ligands, various oxidation states of copper, and halide ions was again demonstrated in the synthesis of the Cu2(R2Dtc)3X2 complexes with copper in the oxidation states II and III. These paramagnetic compounds (1.77-1.86 BM per formula weight, R = Me, Et X = Br, Cl) were obtained by the reaction of CuCl or CuBr and R4Tds in CHC13 (598). 

Work by Paul Beer et al. in Oxford on dithiocarbamate-functionalised resorcarenes has resulted in resorcarene trimers termed molecular loops and molecular tetrahedra composed of four resorcarene units. The molecular loops (pyridine-capped Cd(II) and Zn(II) complexes) can bind C60 which fits neatly into the 16.4 A wide central cavity.35 The slightly larger tetrahedral tetramer which has a 19.4 A edge length, compared to 19.1 A for the loop, is bridged by square planar copper(III) ions, a very unusual oxidation state of copper that is obtained from iodine oxidation of an intermediate copper(II) complex. The solid-state structures of examples of the loop and tetrahedron are shown in Figure 10.35. 

The d-d absorption of the copper complex differs in each step of the catalysis because of the change in the coordination structure of the copper complex and in the oxidation state of copper. The change in the visible spectrum when phenol was added to the solution of the copper catalyst was observed by means of rapid-scanning spectroscopy [68], The absorbance at the d-d transition changes from that change the rate constants for each elementary step have been determined [69], From the comparison of the rate constants, the electron transfer process has been determined to be the rate-determining step in the catalytic cycle. 

Deposition of copper metal Since Cu(II) is the preferred oxidation state of copper, Cu2+ salts are more stable and more available, hence, in a technical application it would be favorable to use them as starting material. We tried to reduce Cu(CF3S03)2 dissolved in [EMIM][TfO], [BMP][TfO] and [BMIM][TfO] with an argon plasma (gas pressure 100 Pa) as well as with a nitrogen plasma (100 Pa), respectively. Additional experiments with Cu(CF3SC>3)2 dissolved in [EMIM][TfO] and Ar/H2 plasmas were carried out, with the distance between the hollow cathode in the gas phase and the surface of the ionic liquid metal salt solution being 3, 45 and 100 mm. Moreover, for the 3 mm distance several experiments with different gas pressures from 50 to 500 Pa were carried out. 

Chlorides and bromides of both oxidation states of copper, as well as cuprous cyanide, are active in promoting the oxidative cleavage of the enamine double bond (Scheme 21). 

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