Copper compounds tetranuclear complexes

Several specific oxidative transformations of nitrogen compounds can be carried out in the presence of copper salts. Oxidation of o-phenylenediamine with molecular oxygen in the presence of a twofold excess of CuCl in pyridine results in the formation of cis,cw-mucononitrile in high yield (equation 286).618 619 The bis- i-oxo tetranuclear complex Cu4Cl402(py)4 was found to be the active species in this transformation.618 A similar procedure can be used for the selective oxidative coupling of diphenylamine to tetraphenylhydrazine by CuCl/py/02 or Cu4Cl402py4 (equation 287).619... 

Usually the stable nitroxyl radicals alone cannot directly catalyze the oxidation of alcohols with dioxygen or peroxide, so they rely on the assistance of various cocatalysts that play an important role in activating the oxidation agent. The most used cocatalysts are first row transition-metal complexes where Cu compounds with various N-donor ligands account for the prime ones. In many instances this combination serves as some kind of model to compare catalytic properties of copper compounds. For example, the performances of two asymmetric tetranuclear (with the Cu4(p—0)2(p — 0)2 404 core) and dinuclear (with the Cu2(p-0)2N202 core) copper(II) complexes were compared in the catalytic TEMPO-mediated aerobic oxidation ofbenzylic alcohols. In spite of their similarity, the complexes perform differently the tetranuclear copper(II) (R) complex is highly active leading to yields up to 99% and TONs up to 770, while the (S,R)-2 dinuclear complex is not so efficient under the same conditions. However, no solid explanation of the activity differences was proposed. 

All three M+ ions are known to form compounds with the unusual digonal linear coordination (see also section 4.9.7), though this is most common for gold. As a result, complexes R3PMX of copper and silver are normally di-and tetranuclear species with 3- or 4-coordinate metals, while the gold analogues are 2-coordinate monomers. This is the reverse of what would be expected on steric grounds [13, 14],... 

Carboxamides containing alkylamine arms like (34), synthesized from dialkylmalonate and ethylene-diamine, act as chelating dianionic N4-donors,44,45 whereas the geometric isomer (35) was prepared from 1,3-diaminopropane and chloroacetyl chloride.46 The hexadentate bis(pyrazine)-carboxamides (36) and (37) can be obtained by the addition of 1,3-propanediamine to the appropriate carboxylic acid.47 Compound (36), in the dianionic form, can be used to obtain homo- and hetero-metal bridged complexes, whereas (37) has been shown to yield tetranuclear copper(II) complexes involving opened-up dianionic ligand molecules.47... 

The syntheses and the crystal structures of the new tetranuclear P4-CO3 complexes of copper(II), [(p4-C03)(p-Br)2 Cu4(bapa)4 ]Br4 and [(p4-C03)(p-Cl)2 Cu4(bapma)4 ]Cl4-12H20, were also reported by Escuer and co-workers 62) to elucidate the multiple bridging mode of COg. The analogous compound [(p-C03)(p4-Cl)2 Cu4(bapa)4 ]Cl4 was also synthesized by them for comparative purposes. All these carbonato complexes showed very strong antiferromagnetic coupling. 

Tetranuclear copper complexes have tetrahedral, square-planar, or butterfly structures. The compound Cu4 (Pr 0)2PS2 4 has a distorted tetrahedral skeleton of the metal atoms with one sulfur atom symmetrically bonded to two copper atoms, while the other sulfur atom forms a bond with the third copper atom. Copper and silver form many clusters containing alkyl and aryl ligands (See Chapter 4). The complex Cu4(CH2SiMe3)4, like Cu4 N(SiMe3)2 4, has a square-planar structure with bridging alkyl ligands. In the alkyl compound, in contrast to the amide one, there are Cu —Cu bonds. 

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