Oxo and Peroxo Ligands

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 O2 atmosphere yields [(H20)s Cr 02], a Cr(III) superoxo complex, according to the sequence (1). 

This (reversible) reaction scheme allows much of the redox chemistry of chromium to be rationalized. 

Reduction of the superoxo form with [Ru(NH3)g] (7) or electrochemi-cally (12) produces first the Cr(III) hydroperoxo complex [(H20)5 Cr02H] absorption maxima, 450, 630 nm (13). This spontaneously decomposes (7) to [Cr(OH2)6] with a half-life of ca. 10 min at room temperature, [H ] = 0.1 M (22, 24). 

The hexaaqua ion, [Cr(OH2)e] , is also the final product in the reduction process (7, 12). The complete process using Fe(II) as a reducing agent is shown in the reaction sequence (8)-(ll), where a Fenton-type (15) mechanism is proposed (7,16), with [(H20)5Cr 0] behaving as the chromium equivalent of the OH radical (16). 

If [Cr(OH2)6] is added to an air-saturated solution of the hydroperoxo product formed in (9), oxidation takes place to give the superoxo product. This is because reactions (2) and (3) generate [(H20)5Cr 0] , which reacts with the hydroperoxo form (12). 

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We consider the di(oxo)-mono(peroxo) and oxo-di(peroxo) model complexes shown in Figure 2, with one (a) or two (b) base ligands L. The metal center M is varied, M = Cr, Mo, andW. NH3 is chosen as monodentate model base ligand L. While there are no experimental structures available... 

Mononuclear peroxo compounds of known crystal structure are listed in Table 6. Except for Mo(02)4 and Mo(02)2(ttp), all structurally characterized mononuclear peroxo complexes have one Mo—Ot in addition to one or two peroxo ligands. The Mo—O, bond lengths fall in the range 1.63 to 1.73 A. The Mo—O (peroxo) distances span from 1.83 to 1.96 A. The peroxo ligand is always found cis to the oxo group and two peroxo ligands are always found cis to each... 

Sharpless et a . have conhmied this mechanism in part by labeling experiments which demonstrated that the epoxide oxygen is derived exclusively from the peroxo ligands of the complex and not from the oxo oxygen. However, the reactivity of the molybdenum complex toward olefins closely parallels that of peracids, for which a three-membered cyclic transition stale is favored. ... 

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