Superoxo complexes oxidation

The majority of the titanium ions in titanosilicate molecular sieves in the dehydrated state are present in two types of structures, the framework tetrapodal and tripodal structures. The tetrapodal species dominate in TS-1 and Ti-beta, and the tripodals are more prevalent in Ti-MCM-41 and other mesoporous materials. The coordinatively unsaturated Ti ions in these structures exhibit Lewis acidity and strongly adsorb molecules such as H2O, NH3, H2O2, alkenes, etc. On interaction with H2O2, H2 + O2, or alkyl hydroperoxides, the Ti ions expand their coordination number to 5 or 6 and form side-on Ti-peroxo and superoxo complexes which catalyze the many oxidation reactions of NH3 and organic molecules. 

The chemistry in this area (6-8) has been approached from two different directions reduction of Cr(VI) or oxidation of Cr(II) it is only recently that an overall, self-consistent picture has emerged (7,9). The key experiment is the observation that the reaction between Cr(VI) and alcohols in acid solution under an 02 atmosphere yields [(H20)5 Cijii02]2+, a Cr(III) superoxo complex,1 according to the sequence (1). 

Some nickel(II) tetraaza macrocycles have been proved to act as efficient catalysts for the electrochemical reduction of C02 in H20/MeCN medium. This indirect electroreduction occurs at potentials in the range -1.3 to -1.6 V vs. SCE and mainly produces either CO or a CO/H2 mixture, depending upon the type of complex.2854 The five-coordinate complexes [NiL] (394) formed by some deprotonated dioxopentamine macrocycles have been found to display very low oxidation potentials Nin/Nira in aqueous solution (about 0.24-0.25 V vs. SCE at 25 °C and 0.5 M Na2S04). Air oxidation of the same complexes in aqueous solution yields 1 1 NiL-02 adducts (5 = 1) which are better formulated as superoxo complexes, NimL-02 (Scheme 56). The activation of Ni-bound oxygen is such that it attacks benzene to give phenol.2855... 

Coordination of dioxygen can occur in four ways, two of which may be considered to correspond to one-electron reduction (superoxo complexes) and the others to two-electron reduction (peroxo complexes). The four possible coordination modes are illustrated in Figure 2. As can be seen, each type of complex may be further categorized as to whether it is a mono- or bi-nuclear complex. Whether a peroxo- or a superoxo-type complex is formed is dependent upon the metal involved. For mononuclear complexes, superoxo complexes would be expected with metals which readily undergo one-electron oxidation, e.g. Fe11 or Co", while peroxo complexes would be expected for metals with a preference for two-electron oxidation, e.g. Ir1, Rh1, Pt°. 

It has been shown that mononuclear cobalt superoxo complexes react with 2,4,6-tri-t-butyl-phenol. The anion [Co(CN)5(02)]3 acts as a base in the oxidation of the phenol. The neutral complex Co(salptr)(Oz) forms a peroxy adduct with 2,4,6-tri-t-butylphenol. An X-ray study on this adduct has revealed the structure shown in (10).m... 

Figure 9-29. A possible mechanism for the oxidation of phenol to 1,2-benzoquinone by dioxygen in the presence of copper(i) salts. The key steps involve the formation of a peroxo or superoxo complex, followed by electrophilic attack upon the benzene ring.

The oxidation to the superoxo complex can also be effected with 10 g. of potassium permanganate, completely dissolved in 700 ml. of nitric acid together with 1200 g. of ice. The yield in this case is reduced by ca. 20% as compared with that obtained with the Ce (IV) salt. Care must be taken to avoid excess permanganate, since the solid superoxo dicobalt permanganates are explosive. Chlorine water is also a possible oxidizing agent. 

Oxidation Catalysis by Copper Peroxo and Superoxo Complexes. Copper ions and compounds participate in or catalyze a variety of oxidation reactions that consume 02. This is one of the several key biochemical roles of copper and much of the recent work on the subject has been done in efforts to model the biological systems. In some (non-biological) cases, e.g., the Wacker process, copper(II) itself may be the actual oxidant, but usually it serves as a carrier of oxygen. 

As noted earlier, many species long considered to be in higher oxidation states have been shown to be superoxo complexes. Well-defined compounds are salts of [RhX6]2, X = F82 and Cl,83 that are paramagnetic (ca. 1.8 BM), consistent with a... 

The -amido -peroxo complex reacts with the halide to form the -amido /r-hydroxo complex and HOX which oxides two /r-amido /<-peroxo complexes to -amido ft-superoxo complexes ... 

A plausible way by which the photo-oxidation of the surface peroxo-titanium species may take place is the formation of the corresponding surface superoxo-titanium compound and its subsequent decomposition. It is to be noted, in this connection, that the superoxo complex of titanium (IV), TiOl, has been postulated to be the intermediate in the oxidation of peroxo-titanium (IV), TiOz, by cerium (IV) in perchloric acid solu-... 

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