Transition metal peroxides peroxo complexes

In the earlier volume of this book, the chapter dedicated to transition metal peroxides, written by Mimoun , gave a detailed description of the features of the identified peroxo species and a survey of their reactivity toward hydrocarbons. Here we begin from the point where Mimoun ended, thus we shall analyze the achievements made in the field in the last 20 years. In the first part of our chapter we shall review the newest species identified and characterized as an example we shall discuss in detail an important breakthrough, made more than ten years ago by Herrmann and coworkers who identified mono- and di-peroxo derivatives of methyl-trioxorhenium. With this catalyst, as we shall see in detail later on in the chapter, several remarkable oxidative processes have been developed. Attention will be paid to peroxy and hydroperoxide derivatives, very nnconunon species in 1982. Interesting aspects of the speciation of peroxo and peroxy complexes in solntion, made with the aid of spectroscopic and spectrometric techniqnes, will be also considered. The mechanistic aspects of the metal catalyzed oxidations with peroxides will be only shortly reviewed, with particular attention to some achievements obtained mainly with theoretical calculations. Indeed, for quite a long time there was an active debate in the literature regarding the possible mechanisms operating in particular with nucleophilic substrates. This central theme has been already very well described and discussed, so interested readers are referred to published reviews and book chapters . 

The first report of transition-metal peroxides active in the BV oxidation of cyclic ketones was due to Mares and coworkers in 1978 . These authors found that Mo-peroxo complexes, containing picolinato 101 and especially dipicolinato ligands 105, are able to mediate the transformation of some cyclopentanones and cyclohexanones to the corresponding lactones by concentrated H2O2 (equation 70) with yields in the range... 

Transition metal peroxides, particularly peroxo (2), alkylperoxo (7) and hydroperoxo (8) complexes, are extremely important reactive intermediates in catalytic oxidations involving molecular oxygen, hydrogen peroxide and alkyl hydroperoxides as the oxygen source. Representative peroxo complexes are listed in Table 3, and alkylperoxo and hydroperoxo complexes are listed in Table 4 together with their reactivities. 

AH of the commercial inorganic peroxo compounds except hydrogen peroxide are described herein, as are those commercial organic oxidation reactions that are beheved to proceed via inorganic peroxo intermediates. Ozonides and superoxides are also included, but not the dioxygen complexes of the transition metals. 

Some of the early transition metals are known to form mononuclear peroxo complexes. These complexes are not formed by reacting a metal complex in a low oxidation state with dioxygen, as this usually results in the formation of metal oxo species, but rather by reaction of a metal complex in a high oxidation state, eg. TiIV, Nbv or MoVI, with the peroxide anion. This frequently leads to more than one peroxide ligand per metal centre. 

With large, low-charged cations, however, the lattice energy gain is insufficient to cause disproportionation. The peroxide ion can also be stabilised in peroxo complexes, where it acts as a ligand to transition metals, as in [Cr(V)(02)4]3-. 

As with percarboxylic acids, hydrogen peroxide usually requires metal activation.151-154 Peroxo complexes of d° transition metals can often be used to cleave olefins. Tungstic acid cleaves olefins to the carboxylic acid using 35% m/m hydrogen peroxide.154 Sodium tungstate has also been reported to... 

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