W-peroxo

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]. 

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 23.2 lists the oxidation states of the elements along with representative examples of their compounds. Coordination numbers as high as 12 can be attained, but those over 7 in the case of Cr and 9 in the cases of Mo and W involve the presence of the peroxo ligand or rr-bonded aromatic rings systems such as rj -CsHs or rj -C(,H(,. 

As for the previous oxidation state, the chemistries of Mo" and W" are much more extensive than that of Cr" whieh is largely confined to peroxo- and fiuoro- complexes. [Cr(02)2(NH3)3], which has a dark red-brown metallic lustre, may be obtained either by treating [Cr(02)4] " with warm aqueous ammonia or by the action of H2O2 on ammoniacal solutions of (NH4)2Cr04. It has a pentagonal bipyramidal structure in which the peroxo- groups oeeupy... 

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. 

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