Peroxo group

Peroxo- Group. When used in conjunction with the trivial names of acids, the prefix peroxo- indicates substitution of —O— by —O—O—. 

The action of hydrogen peroxide on freshly precipitated hydrated Ti(IV) oxide or the hydrolysis of a peroxide compound such as K2[Ti(02)(S0 2] yields, after drying, a yellow soHd, stable below 0°C, of composition TiO 2H2O. There is one peroxo group per titanium, but the precise stmcture is not known. The yellow soHd loses oxygen and water when heated and Hberates chlorine from hydrochloric acid. When freshly prepared, it is stable in acid or alkah, giving peroxy salts. 

As for the previous oxidation state, the chemistries of Mo" and W" are much more extensive than that of Cr" whieh is largely confined to peroxo- and fiuoro- complexes. [Cr(02)2(NH3)3], which has a dark red-brown metallic lustre, may be obtained either by treating [Cr(02)4] " with warm aqueous ammonia or by the action of H2O2 on ammoniacal solutions of (NH4)2Cr04. It has a pentagonal bipyramidal structure in which the peroxo- groups oeeupy... 

The 0-0 bond length in a peroxo-group of peroxyacid derivs, for example in the (NH4)2S208 compd, is 1.46A, which is less than the bond length in the 0 ion (1.49A)... 

Single crystals of peroxomonosulfuric acid, H2S05 (also known as Caro s acid) have been prepared by the reaction of fuming sulfuric acid and H202. In the crystal structure, the molecules are connected via hydrogen bonds.49 The distance within the peroxo group of the anion is 1.46 A. 

The electropositive metal center polarizes the peroxo group so that is much more electrophilic than free peroxide anion. Evidence has been accumulated, and it will be summarized later, that such polarization happens to the extent that nucleophilic reagents can attack a peroxo oxygen when it is coordinated to a d° metal. In other words, this situation... 

Vibrational frequencies of some titanium peroxo complexes and of solids containing peroxo and/or superoxo species are summarized in Table III. The three infrared vibrations of the triangular peroxo group in the Qv structure... 

Despite of the common reaction mechanism, peroxo complexes exhibit very different reactivities - as shown by the calculated activation energies -depending on the particular structure (nature of the metal center, peroxo or hydroperoxo functionalities, type and number of ligands). We proposed a model [72, 80] that is able to qualitatively rationalize differences in the epoxidation activities of a series of structurally similar TM peroxo compounds CH3Re(02)20-L with various Lewis base ligands L. In this model the calculated activation barriers of direct oxygen transfer from a peroxo group... 

To analyze the epoxidation activity of these peroxo complexes, we characterized for each complex TSs of oxygen transfer to the model olefin ethene. We compared the two mechanisms mostly discussed in the literature, namely insertion [2, 8, 9, 97] and a direct attack of the peroxo group on the olefin [11] (see Section 1). Direct oxygen transfer can be envisaged to occur... 

A peroxo group, formally an 022 ligand, interacts with the metal center via donation from its filled orbitals to the formally empty d orbitals of the... 

Figure 5. Frontier orbital interaction between a transition metal peroxo group and an olefin.

It is instructive to compare the properties of metal peroxo and alkyl (or hydro) peroxo groups for the case of Ti because experimental structures of both types are known [117, 119-121] and Ti compounds are catalysts for such important processes as Sharpless epoxidation [22] and epoxidation over Ti-silicalites [122], where alkyl and hydro peroxo intermediates, respectively, are assumed to act as oxygen donors. Actually, the known Ti(t 2-02) complexes are not active in epoxidation. [121-124] However, there is evidence [123] that (TPP)Ti(02) (TPP = tetraphenylporphyrin) becomes active in epoxidation of cyclohexene when transformed to the cis-hydroxo(alkyl peroxo) complex (TPP)Ti(OH)(OOR) although the latter has never been isolated. 

We considered [71] the epoxidation activity of MoVI hydroperoxo intermediates formed via opening an ti2-02 peroxo group ... 

The two oxides formed with hydrogen, H20 and H202, have already been discussed, but it should be emphasised that hydrogen peroxide and the peroxides formed from it contain the —O—O— linkage. The oxidising power of these peroxides lies in the oxygen of the peroxo-group. unlike the dioxides (see above). 

Salts with oxometallate anions function as oxidants, those with oxygen present as peroxo groups being naturally the more powerful, and separately grouped under See also peroxoacid salts... 

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