Hydroperoxo complex

Keywords EXAFS H2O2 Hydroperoxo complexes IR Raman Partial oxidations Peroxo complexes TitanosUicate TS-1 UV-Vis XANES... 

Concerning peroxo complexes, it is worth noticing that they can be formed in TS-1 by evolution of both or rf- hydroperoxo complexes upon a further deprotonation in presence ofwater with formation of H30 /H20. Very recently Bonino et al. [49] have shown, by titration in aqueous medium with NaOH, that the acidity of the TS-1/H2O system is remarkably increased by addition of H2O2 (compare full squares with full circles in Fig. 8), a feature not observed for the Ti-free silicalite-1 system (open circles and squares in Fig. 8). 

That observed for TS-1 is not peculiar for TS-1 only and can be observed on other titanosilicates like Ti-MSA, a mesoporous amorphous material that has Ti(Vl) centers exposed on the surface of the pores [124,125]. In this case, easier experiments could be performed by Prestipino et al. [50] as the peroxo/hydroperoxo complexes can be formed by dosing f-butyl hydroperoxide (which does not enter the 10-membered rings of TS-1). The XANES spectrum of Ti-MSA in vacuum is typical for almost r -like Ti(IV) centers (the intensity of the Ai T2 pre-edge peak being only 0.69, as compared with 0.91 for TS-1). Upon dosing the t-butyl hydroperoxide in decane solution on Ti-MSA, a spectrum similar to that obtained on TS-1 contacted with anhydrous H2O2 is observed on both XANES and EXAFS regions [50]. When the... 

The similarity of olefin epoxidation by TM peroxo and hydroperoxo complexes with epoxidation by dioxirane derivatives R2CO2 and percar-boxylic acids RCO(OOH) was confirmed by computational studies [73-79]. This similarity holds in particular for the spiro-type transition structure. 

Figure 15. Energies (in kcal/mol) of intermediates and TSs of ethene epoxidation by various molybdenum peroxo and hydroperoxo complexes, relative to the energy of MoOtC h tNHjh (2b) +C2H4.

For Ti, hydroperoxo complexes exhibit lower activation barriers than the corresponding peroxo species. Peroxo complexes of Ti and Cr can be considered as inert in epoxidation. Hydroperoxo species may be competitive with di(peroxo) compounds in the case of Mo. For the system MT0/H202 the di(peroxo) complex CH3Re0(02)2-H20 was found to be most stable and to yield the lowest TS for epoxidation of ethene, in line with experimental findings. However, olefin epoxidation by Re monoperoxo and hydroperoxo complexes cannot be excluded. 

The standard route to the hydroperoxo complexes in Table II is by one-electron chemical (77,78) or electrochemical (74) reduction of the superoxides. From the practical point of view, Ru(NH3)g+ proved to be an especially useful chemical reductant (72,77,78) that reacts rapidly and cleanly according to Eq. (2), and often can be used even in the presence of molecular oxygen. The Ru(NHa)g+ produced in Eq. (2) is quite unreactive and absorbs only weakly in the UV region, causing... 

The 0-0 stretch for the hydroperoxo complexes (60,86,88,90,94,95,99- ) falls in the range 780 900 cm-1. In the case of a nonheme iron complex it was found that the low-spin hydroperoxo form absorbs at a lower frequency than the high-spin peroxo complex (89,102). This trend also holds for some iron alkylperoxo complexes, and DFT calculations have shown the high-spin alkylperoxide form to have a larger activation energy for the 0-0 bond cleavage than the corresponding low-spin alkylperoxo form (104). 

Crystal structure analysis has been carried out for several hydroperoxo complexes. The 0-0 bond length (1.40-1.48 A) (79,87,90,100,103,105) is significantly longer than that in superoxo complexes and close to the 1.49 A value for hydrogen peroxide (54). 

The UV spectral data for several hydroperoxo complexes in aqueous solution are shown in Table II. Intense transitions appear for all the compounds at wavelengths that are well below the 270-290 nm maxima for the superoxo complexes. This feature is particularly useful in mechanistic studies of the complex reactions involving several forms of activated oxygen simultaneously (58). 

A variety of peroxo and hydroperoxo complexes of zirconium(IV) and hafnium(IV) have been isolated from aqueous or aqueous methanolic hydrogen peroxide solutions that contain additional ligands such as sulfate, oxalate or fluoride. Examples of recently reported complexes are listed in Table 8 along with characteristic vibrational frequencies and the pH employed in the aqueous preparations. Earlier work on peroxo compounds has been reviewed by Connor and Ebsworth180 and by Larsen.5... 

Table 8 IR and Raman Data and Synthesis Conditions (pH) for Peroxo and Hydroperoxo Complexes of...

Figure 36 The structure of the Mo 1 peroxo/hydroperoxo complex [(02)2OMo(OOH)2MoO(02)2]2 1,6...

Group VIII metal alkylperoxo and hydroperoxo complexes 347... 

In complexes (3), (7) and (8), dioxygen can be inserted between two metal atoms (bimetallic //-peroxo complexes), between one metal and one carbon atom (alkylperoxo complexes) or between one metal and one hydrogen atom (hydroperoxo complexes). 

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. 

Table 4 Reactivity of Metal-Alkylperoxo or -Hydroperoxo Complexes ...

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