W-peroxo complex

Covalent vanadium(V)-peroxo complexes of general formula V0(02)(pic)LiL2 (Li,L2 = H20 or basic ligands) and ionic complexes with the formula [V0(02)(pic)2] A (A = H" ", PPh4 ) can easily be synthesized from V20s,H202, picolinic acid and ligands. These complexes exhibit a different behavior from that of Mo - and W -peroxo complexes, ... 

The reactivity of V -peroxo complexes, which is different from and much less selective than that of Mo- and W-peroxo complexes, was tentatively attributed to a biradical —O—O ... 

Some of these W-based catalysts could be transformed into insoluble salts for example, the phosphotungstate-pyridinium salt in a toluene-tributyl phosphate (4 3) solvent mixture [328]. In this solvent the pre-catalyst is insoluble, but upon reaction with H2O2 it gives the catalytically active W-peroxo complex, which is soluble. The catalytic action is then performed under homogeneous conditions and, at the end of the reaction, H2O2 being completely consumed, the precatalyst precipitates and can be easily filtered off and recovered. The method is smart, but from an industrial point of view the separation and recovery is costly. 

Table 1. Activation barriers (in kcal/mol) calculated for various mechanisms of ethene epoxidation by model peroxo complexes of Cr, Mo and W (see Fig. 1).

For the favorable reaction mechanism, the direct front-side attack, three findings are worth noting, (i) The activation barriers for analogous species decrease along the series Cr > Mo > W. (ii) The barriers for the di(peroxo) species are lower than those of the corresponding mono(peroxo) complexes, (iii) Coordination of a second base ligand significantly increases the activation barrier. 

These findings for Re peroxo complexes are in striking contrast with Ti and V catalyzed reactions [41, 51, 52, 111, 113] in which the metal-alcoholate bond drives the allylic OH directivity. We recall that the formation of alcoholate intermediates was also rejected for epoxidations of allylic alcohols with Mo and W peroxo compounds while H-bonding (between OH and the reacting peroxo fragment) was considered consistent with kinetic data for these complexes [115]. 

While numerous peroxo complexes of Ti, V, Cr/Mo/W and Re have been experimentally isolated and well characterized, there are only few examples of alkyl peroxo complexes whose X-ray structures are known, namely (di(picolinato)VO(OOtBu)(H20) [116] and [((T 2-ter -butylperoxo) titanatrane)2 3 dichloromethane] [117], Evidence for the existence of other... 

The W(VI) oxo-peroxo complex W(C=CC=CPh)(02)(0)Cp (10 Scheme 3) is obtained by oxidation of W(C=CC=CPh)(CO)3Cp with hydrogen peroxide in strongly acidic media. Subsequent treatment with PPhs gave the dioxo-diynyl W(C=CCsCPh)(0)2Cp (11)." The terminal diynyl di-oxo complex can be prepared similar fashion." ... 

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