Picolinato ligand

Hydroxybenzoic acid (salicylic acid) does form a complex, although its formation is not favored when compared to that of the picolinato ligand. Presumably this, at least to an extent, reflects the fact that the chelate formed with salicylate has a six-membered ring, and six-membered chelates are much less favored than the five-membered analogue. 

In view of its importance, reductive dissolution of Fe oxides has been widely studied. Reductants investigated include dithionite, thioglycolic acid, thiocyanate, hydrazine, ascorbic acid, hydroquinone, H2S, H2, Fe ", tris (picolinato) V", fulvic acid, fructose, sucrose and biomass/bacteria (Tab. 12.3). Under the appropriate conditions, reductive dissolution may also be effected photochemically. As with protonation, the extent of reduction may be strongly influenced by ligand and proton adsorption on the oxide surface. 

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... 

Although amino acids and related compounds frequently react in a monodentate fashion with bisperoxovanadate, they can react in a bidentate manner, but the products are often not bisperoxo complexes rather, one peroxide is eliminated in the condensation reaction. This is not necessarily true for all conditions, and bisperoxo-heterobidentate-ligand complexes are known, although in the solid state where a number of x-ray structures have been reported. In this case, one coordination site is an apical position, and a pentagonal bipyramidal product is formed, in the fashion of the oxalato [15] and picolinato (pyridine-2-carboxylato) [16] complexes, as represented diagrammatically in Scheme 6.3. There seems to be no detailed study of these and similar complexes in aqueous solution. Preliminary studies [17,18] of the... 

Such distortion, which is predicted by the Jahn-Teller theorem, has also been found in the structures of the Mn(L)3 compounds derived from non-Schiff base N—O bidentate ligands, such as the octahedral tris(8-hydroxyquinolato)manganese(III) CH3OH (or 0.5C6H,3OH) and the octahedral tris(a-picolinato)manganese(III) compounds (see compounds 2 and 3 respectively in Table 51). 

Aromatic hydrocarbons can be oxidized to the corresponding phenols by transition metal peroxo complexes and, in particular, vanadium(V) peroxo complexes, which act either as electrophilic oxygen transfer reagents or as radical oxidants -, depending on the nature of the ligands coordinated to the metal and on the experimental conditions. Vanadium picolinato peroxo complex (V0(02)PIC(H20)2) (39) (PIC = picoUnic acid anion) has been reported to be particularly effective in the hydroxylation of benzene and substituted benzenes (equation 50) . Accordingly, 39 smoothly oxidizes substituted benzenes 38 to the corresponding monophenols 40 in acetonitrile at room temperature. 

Oxidations of cyclic ketones to lactones by H2O2 take place in the presence of catalytic amounts of certain molybdenum complexes. The catalysts are peroxo-molybdenum complexes stabilized by picolinato- and pyridine-2,6-dicarboxyl-ato-ligands, e.g. [Mo(0)(02) CsH3N(C02)2 ]- Although the reaction is catalytic, turnover numbers are low (25 or less) and chemical yields are variable. Moreover, competing reactions are the formation of oligomeric peroxides and ring-opened products. 

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