Peroxo complex, molecular structures

It has been mentioned previously that the formation of 2 1 Co 02 -peroxodimers is generally believed responsible for the irreversible coordination of dioxygen. In apparent contrast with this affirmation, the mixed bipyridine/terpyridine ligands complex [ Co(terpy)(bipy) 2 (fi-02)]4 +, the molecular structure of which is shown in Figure 16, constitutes an example of reversible coordination of the peroxo group.22... 

As anticipated beforehand, bis( -peroxo)-rhenium complexes have been characterized in the solid state " . The noticeable feature is that the active catalyst (CH3)Re0(02)2H20 (1) does not crystallize as it is, but a modification of the seventh apical ligand is required, as indicated in compounds 11 and 12. The molecular structure of these peroxo rhenium complexes is yet again a pentagonal bipyramid, and the bond lengths are in the expected range. 

While most superoxo complexes—in contrast to peroxo compounds— have been assigned a bent, end-on coordination mode [9], the superoxide ligand of Tp Cr(02)Ph was suggested to exhibit the more unusual side-on (r] ) coordination [10]. The reactivity of the complex did not allow for the determination of its molecular structure however, close analogs could be isolated, crystallized and structurally characterized by X-ray diffraction. For example, the reaction of [Tp Cr(pz H)]BARF (pz H = 3-tert-butyl-5-methylpyrazole, BARF = tetrakis(3,5-bis(trifiuoromethyl)phenyl)borate) with O2 produced the stable dioxygen complex [Tp Cr(pz H)( ] -02)]BARF (Scheme 3, bottom), which featured a side-on bound superoxide ligand (do-o = 1.327(5) A, vo-o = 1072 cm ) [11]. Other structurally characterized... 

Fig. 3. Molecular structures of the cations in the peroxo complex [(l)Co( r-02)Co(l)](S206)Cl2 6 H20 (28) (left) and superoxo complex [(l)Co ( >02)Co(l)](S206)2Cl 10 H20 (29) (right) broken lines indicate intramolecular hydrogen bonds.

The chemistry of non-peroxo polynuclear cobalt(III) ammines is reviewed with particular emphasis on Werner s major contributions. Modern work in this area has shown that Werner s conclusions regarding the structures of these compounds are substantially correct in spite of the relatively primitive techniques he had available. There is much current interest in polynuclear cobalt(III) complexes because of their relationship to oxygen carriers and intermediates in electron transfer reactions. Modern techniques such as spectroscopy and x-ray diffraction have been used to determine the electronic and molecular structures of these compounds. 

Among coordination compounds of d transition metals with n-donor two-center ligands, peroxo complexes have been studied in the greatest detail the search for methods of molecular nitrogen fixation has stimulated studies on the structures of complexes with hydrazine derivatives. As a rule, only X-ray diffraction data are available for these complexes [10],... 

As described above, Li-peroxo iron(III) complexes are only stable at a low temperature and its crystallographic characterization is extremely difficult to achieve. To date, few X-ray structures of iron complexes containing a peroxo moiety have been reported. One of the examples is tetranuclear-ferric fi -peroxo complex, whose molecular view is presented in Fig. 2 [73]. 

Hydroperoxo, alkylperoxo, and acylperoxo copper(II) complexes While there is no direct experimental indication of involvement of dinuclear hydroperoxo copper(II) intermediate in the catalysis of tyrosinase, a dinuclear hydroperoxo copper(II) complex and related alkyl and acylperoxo complexes have been prepared either by reaction of a dinuclear jii-hydroxo complex [Cu2 (XYL-0 )(0H)] with XOOH (X=H, R, RCO) or by treating a peroxo complex [Cu2(XYL-0 )(02)] with X [138-140]. The acylperoxo complex was isolated and its molecular structure was established by X-ray crystallography (see Fig. 10). 

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. 

We have seen a quantum leap in progress in the coordination chemistry of copper-dioxygen interactions, resulting in a complete change in thinking about Cu202 structure and related protein chemistry. The problems of copper ion lability, peroxide disproportionation and air/moisture sensitivity have been overcome, and it has been proved that low-molecular-weight Cu 02 or Cu (0)2 complexes can be prepared, and that several different structural types exist. It is apparent that u-r 2 r 2-peroxo coordination is present in oxy-Hc and oxy-Tyr. 

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