Peroxo-bridged copper complexes

Copper(I) complexes similarly react with dioxygen to form peroxo-bridged binuclear complexes.Such complexes do not readily undergo O—O bond cleavage, apparently because the copper(III) oxidation state is not as readily attainable as the Fe(IV) oxidation state in an iron-porphyrin complex. Nevertheless, stable peroxo complexes of copper(II) have been difficult to obtain, because, as soon as it is formed, the peroxo complex either is protonated to give free hydrogen peroxide or is itself reduced by more copper(I) (Reactions 5.42 to 5.46). 

Several other peroxo bridged complexes have been lately synthesized53-54 simply by reacting appropriate precursors and dioxygen, as shown in equation 7 for di-Ir(II) complex 5, with bis(diphenylphosphinomethane) dppm, and in equation 8 for a dicopper(II) species 6. The same procedure has been also applied to obtain a peroxo bridged heme-copper complex 755 (equation 9). 

Chishiro, T., Y. Shimazaki, F. Tani, Y. Tachi, Y. Naruta, S. Karasawa, S. Hayami, and Y. Maeda (2003). Isolation and crystal structure of a peroxo-bridged heme-copper complex. Angew. Chem., Int. Ed. Engl. 42, 2788-2791. 

The importance of the ancillary multidentate ligand became apparent with the different reactivities of the Cu(I) complexes with tacn and tacn . The degree of substitution at the a-C-atom to the N-donors has an important effect on the formation of the oxidized metal species the peroxo bridge complex is favored when the tacn ligand has substituents whose a-C-atoms are tertiary, while the Cu(III)(,u-0)2Cu(III) complex is produced if one of the a-C-atoms is less substituted. Steric interactions are the obvious explanation for this effect - bulkier ligands prevent the close approach of the two copper centers that is needed to form the Cu(III)(,u-0)2Cu(III) core. A similar trend was observed in copper complexes with bidentate ligands, whereby the more stericaUy constrained systems favored formation of complexes with Cu(II)(y - / -OO)Cu(II) cores. 

A copper Xp complex with an absorbance spectrum similar to that of oxyhe-mocyanin has been reported. It was prepared with H2O2 and proposed to contain a peroxo bridge between two copper centers Kitajima, N., Koda, X., Hashimo-to, S., Kitagawa, X, Moro-oko, Y. Chem Commun 1988, 151. 

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