Transition elements peroxidation

In conclusion, it is noted that, as an empirical rule, first row transition metal elements generally form superoxide-type dioxygen complexes, whereas elements of the second and third transition series form peroxide-type dioxygen complexes. 

FIGURE 1. Nomenclature for O atoms in constitutionally asymmetric peroxides (R = alkyl, aryl R = H, metal, transition metal, group 13-17 elements)... 

A survey of crystal structures of 29 compounds (Table 8), in which the alkyl hydroperoxide anions serves as ligand to metal ions, transition metal ions or group 13-17 elements, provides a mean 0—0 bond length of 1.46 0.03 A, an O—O—C angle of 109 2.1° and a M—O—O angle of 112 6.9°. More specialized aspects that deserve to be addressed separately refer to the nature of the M—O bond, the magnitude of the dihedral angle M—O—O—C and the tetrahedral distortion of the peroxide bound C atom. 

When the central atom is a transition metal, a simple salt of that element may be mixed hot with a soluble molybdate or tungstate in a solution of appropriate pH. If the central atom must be raised to an unusual oxidation state, persulfate, peroxide or bromine water are often employed electrolytic oxidation may also be used. Alternatively, freshly precipitated hydrous metal oxides may be boiled in acidic molybdate or tungstate solutions, or coordination complexes may be decomposed in hot molybdate solutions. Free acids are prepared in several ways ... 

Metals that are capable of 2e redox changes, typically main group elements and 4d and 5d transition metals, can give heterolysis of a peroxide to form a diamagnetic oxidant that may avoid the radical pathways seen in the case of equation (14-15). O atom transfer to the substrate is possible in this way. Sharpless epoxidation provides an excellent example. In this case rBuOOH is the primary oxidant, Ti(i-OPr)4 is the catalyst precursor and a tartrate ester is the ligand that induces a high ee in the epoxy alcohol formed from an allylic alcohol. This reaction has been successfiiUy developed on an industrial scale. 

After Mo, vanadium is the second most abundant transition metal in the ocean and is certainly beneficial and probably essential for man. It is used in the form of a V prosthetic group in V-dependent haloperoxidases, which utilise hydrogen peroxide to oxidise a halide ion into a reactive electrophilic intermediate. There has been extensive debate as to whether a final transition metal ion is an essential trace element, as was originally proposed over 50 years ago — it has been widely accepted as an essential element for over 30 years. We discuss Mo and W together with V and Cr in Chapter 17. 

The most active elements for the oxygen transfer are the transition metals to the left of the Periodic Table. The order of activity of these is Mo > W > Ti > V > U > Th > Zr, Nb [465]. In addition, several nontransition metal compounds are effective in the reaction, most notably SeO2 and borate esters (See Section 11). The catalytic elements are typically in their highest attainable oxidation state, and have the essential feature of not having a readily accessible lower oxidation state. This is necessary in order not to promote the metal-catalyzed decomposition of the peroxides, which could initiate radical chain reactions. Elements such as Mn, Fe, Co, Rh, Ni, Pt, and Cu are ineffective for this reason. 

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