Peroxo-bridged complexes

These conclusions are supported by another ESR study (Jd) made on some oxygenated cobaltoximes(II), which showed that the dioxygen species finally formed is best characterized as a peroxo-bridged complex these being reversibly formed binuclear complexes. 

The formation of a double-bridged Co(III) complex from a single peroxo-bridged complex can take place by either (2.107) or (2.108), L = CHjNHj,... 

The complexes [LCo(p-02)(p-OH)CoL] [L = en, trien, dien, tetra-ethylenepentamine, or tris-(2-aminoethyl)amine] have been studied, and the new complexes [[Co(imidazole)(gly)2 202],4H20 [ Co2(imidazole)2-(gly)402 0H],3H20, and [Co(imidazole)(gly)2(02)H20] have been prepared The spectroscopic properties of various p-peroxo- and p-superoxo-cobalt(iii) complexes have been examined. The singly-bridged p-peroxo-compounds have a strong band at 300 nm, whereas this falls at 350 nm for p-peroxo-p-hydroxo-complexes and two peaks at 480 and 700 nm are observed for p-superoxo-species. The i.r. spectra of p-peroxo-bridged complexes of cobalt(iii)-cyclam have been reported. ... 

Several other peroxo bridged complexes have been lately synthesized 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)... 

This type of complex is derived from the mononuclear superoxo species via a further one-electron reduction of the dioxygen moiety. Cobalt is the only metal to form these complexes by reaction with dioxygen in the absence of a ligating porphyrin ring. Molybdenum and zirconium form peroxo-bridged complexes on reaction with hydrogen peroxide. In most cases the mononuclear dioxygen adducts of cobalt will react further to form the binuclear species unless specific steps are taken to prevent this. 

These /jl-02 complexes can often be oxidized in a one-electron step to species such as [(am)5Co02Co(am)5]5+ and (17-F-IIb). These ions were first prepared by A. Werner, who formulated them as peroxo-bridged complexes of Co111 and Co . The esr data show that the single unpaired electron is distributed equally... 

Fig. 13. Formation of a (x-peroxo-bridged complex from a nitrogen ligand complex of cobalt(II) and O2. The p,-peroxo-bridged complex is stabilized in the presence of a second bridging ligand, L, such as amino or hydroxo.

Both 0X0- and hydroxo-bridged complexes containing two TMPACu covalently linked to the porphyrinate phenyls or a tridentate derivative containing two 2-ethylpyridine links have been prepared and characterized by NMR spectroscopy. Peroxo-bridged complexes of TMPACu and porphyrin-tethered variants have also been reported. Investigation of the rates of O2 binding... 

Reibel (20j 21) has investigated the polarography of some polynuclear ammines in connection with polarographic investigations of the peroxo-bridged complexes. He found a single stage, three-electron reduction to Co" 2 for the polynuclear complexes. 

O—O bond cleaves and two equivalents of an iron(IV) oxo complex are formed (Reaction 5.38). Subsequent reactions between the peroxo-bridged complex and the Fe oxo complex produce the /x-oxo dimer (see Reactions 5.39-5.40). 

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. 

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