Disproportionation of Cu

Equation (8) also describes the kinetics of the reduction of the cupric salt, ultimately to metallic copper, which is observed in the absence of an added substrate (Macgregor and Halpern, 16). This reaction proceeds by a mechanism in which step (11) is replaced by the rapid disproportionation of Cu+ to give metallic copper ... 

Not many reduction potentials are known for copper complexes. That of the Cu2+/Cu+ couple is 0.16 V Since lo(Cu+/Cu°) is 0.52 V, the disproportionation of Cu+ to Cu° and Cu2+ is favourable. This reaction does indeed occur, which makes is impossible to study stable copper(I) solutions. Reduction potentials of copper(II)-/copper(I)-(l,10-phenanthroline)2 and a few derivatives have been calculated from a kinetic analysis of appropriate rate constants values range from 108 mV for the 5-methyl-l, 10-phenanthroline complex to 219 mV for the complex with a nitro group at the 5 position [52], Values of 0.17 V and 0.12 V are given by Phillips and Williams [53] for the phenanthroline and bipyridine complexes, respectively. Such complexes can thermodynamically catalyse both the superoxide dismutation and the one-electron reduction of hydrogen peroxide (see below). 

UV-irradiation of Cu(ll) diketonate complexes generate Cu(0), which can originate from a disproportionation of Cu(l) produced in photodegradation of /(-diketonate of Cu(ll). The quantum yields of these reactions are relatively high for UV-irradiation, and solar light is also active (2). 

The anode and cathode reactions involved in the disproportionation of Cu-Cl are presented in Equations (1) and (2) and shown pictorially in Figure 2. 

Copper(I) complexes catalyse a variety of organic reactions which are of synthetic and industrial importance.305 In such processes that involve halide abstraction from aryl or alkyl halides, the abstraction step by a Cu(I) catalyst is believed to be the rate-determining step. In order to circumvent the property of facile disproportionation of Cu, various methods of stabilising Cu(I) and influencing reaction rates were considered.306 A kinetics study of ligand (L) effects on the reactivity of Cu(I)L complexes towards C13CC02 was undertaken. The results indicated that the rate of the chlorine abstraction reaction was affected by several factors. These were the redox potential of the Cu(II/I)L couple, the hybridisation on Cu(I) in the Cu(I)L complex, steric hindrance, and electron density on the central Cu(I) cation at the binding site of the chlorine atom to be abstracted. The volume of activation,... 

This means that TI(III) salts are unstable in solution in the presence of metallic Tl. (b) The supposed disproportionation of Cu would take the following form ... 

The observation that stabilizes Cu(I), whereas amino ligands including 1,4,8,ll-tetraazacyclatetradecane catalyze the disproportionation of Cu(I) (22,23,27) suggested that linear tertiary amino ligands, for example, will also stabilize Cu(I). 

While conducting ATRP in aqueous media has both economic and environmental advantages, the equilibrium constant for disproportionation of Cu is very large in pure water K isp = 10 ), resulting in a loss of the Cu activator. 

The slow disproportionation of Cu(I), strongly affected by water content, was found to occur in protic ionic liquids by Gunawan et al. [50]. Figure 13.5 shows the representative cyclic voltammograms obtained at a GC electrode at a scan rate of 0.1 V s for reduction of Cu(II) in protic ionic liquid (PIL), ethylammonium... 

On reversing the scan direction at 0.13 V, the reduction of Cu(II) to Cu(I) is close to an ideal one-electron reversible process. Interestingly, scanning the potential to more negative values than the rl process, but prior to the onset of the rl process, an oxidation stripping peak is detected at 0.50 V on the anodic scan. This voltammetric behavior is distinctly different from that in aprotic ionic liquids (AILs), organic solvents, or aqueous media [66-70], and is attributed to the disproportionation of Cu(I)inEAN viaEq. 13.10. 

On the other hand, a very unique redox system involving Cu(l)/Cu(II)/Cu(III) oxidation states was reported by Ribas, Stahl, and coworkers [79-81]. They extensively studied the reactivity of the triazamacrocyclic ligand with Cu(II) and successfully characterized C-H activated Ar-Cu(Ill) and Cu(I) complexes. The careful investigation of the reaction stoichiometry revealed that 0.5 eq of Ar-Cu(III) and 0.5 eq of Ar-Cu(I) are formed from 1.0 eq of Cu(II), thus suggesting an disproportionation of Cu(II) into Cuflll) and Cu(I) during the C-H activation event (Eq. 46). Upon treatment of the isolated Ar-Cu(III) complex with MeOH as an oxygen nucleophile, the C-H alkoxylated product and Cu(l) salt are obtained quantitatively (Eq. 47). A similar C-N bond formation occurs when NH pyridone is used as a nitrogen nucleophile. 

Disproportionation of Cu-based Activating Complexes in Aqueous Media... 

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