Dinuclear Cu complexes

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. 

The reoxidation step of the complex takes place after coordination of 02 to the dinuclear Cu complex, forming the complex 6 (Figure 6). The p-dioxo complex is estimated to be formed on the basis of Karlin s model [75]. In the next step the phenolate anion is oxidized by a two-electron transfer to 022- via Cu(II). The resulting complex 7 is similarly transformed into compound 4 (Figure... 

Inexpensive and readily available Grignard reagents and stable dinuclear Cu complexes have been used for the first time in catalytic enantioselective conjugate addition reactions to simple acyclic a -unsaturated methyl esters.95 These reactions have provided access to highly valuable -substituted chiral esters in good yields and with excellent enantioselectivities (up to 99% ee). 

An early successful example of generation and subsequent spectroscopic study of a dioxygen adduct of a dicop-per(I) complex was reported in 1984 by Karlin and coworkers, where dioxygen was shown to bind quasireversibly to the dinuclear Cu complex [Cu2(XYL-0)]+ (Figure This... 

FIGURE 33. H NMR spectra (200 MHz) (a) of cryptand 89 in C2l)2Cl4 and (b) of its dinuclear Cu complex 91 in CD3CN (s = solvent). Reproduced by permission of Neue Schweizerische Chemische Gesellschaft from Reference 52... 

Dipyrazolate salts (299 X = O, NH) and dinuclear Zn" complexes (300 X = O, NH) were studied by C NMR. Besides, from (296) and (299 X = NH), dinuclear Cu" complexes (297) and (300 X = NH) have been isolated as crystalline solids. The x-ray crystallographic analysis of (300 X = NH) demonstrates that inside the macrocyclic cavity, the two pyrazole rings are simultaneously acting as exobidentate ligands linking both metal cations, the Cu-Cu separation being 3.9 A <94TL5723>. 

The potentially pentadentate Schiff bases of Figure 19a actually act as bis-tridentate ligands with respect to a single metal centre, giving dinuclear Cu" complexes as in Figure 19b.Furthermore,... 

Fig. 17. Proposed redox events involving non-diffusible active species for the oxidation of DNA deoxyribose by a dinuclear Cu complex in the presence of dioxygen.

In order to prepare complex 2 the appropriate amount of Cu(C104)2 6H20 was added to a methanolic solution of the free ligand. Upon refluxing, the initially green solution became colorless within minutes, indicative of the presence of a Cu complex. The formation of a dinuclear Cu complex was confirmed by NMR, UV-vis and cyclic voltammetry. The same decolorization was observed for complex 3. In this case, however, the reaction was much slower. The formed Cu complexes are stable in air in both cases. 

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