Picolinato complexes

The remaining picolinato complex has VL stoichiometry. This complex very likely is pentacoordinate in trigonal bipyramidal geometry, as depicted in Scheme 4.13. A structure of a closely related hexamethylphosphate triamide (HMPT) derivative where the HMPT residue is located in an axial position of the complex has been determined by x-ray diffraction studies [59],... 

There is evidence from Raman studies that oxalate coordinates in a bidentate manner without elimination of a peroxo group [6], Possibly here, as for the picolinate complex, the question of the coordination could be more definitively answered by observation of 13C coordination-induced NMR chemical shifts. The x-ray structure for the oxalate complex shows that the two V0ox bond distances are very different. The VO distance to the equatorial oxalato oxygen is 2.060(4) A compared to 2.251(4) A to the apical oxygen [15], Similarly long distances are found in the picolinato complex (VOpic 2.290 A [16]) and other complexes [20], These distances are indicative of relatively weak bonds to the apical oxygens, and dissociation of that bond... 

Despite the fact that synthetic peroxovanadium complexes do not model satisfactorily the active centre of the VHPOs in the sense outlined above, most of them are effective oxidants under mild conditions, or even catalysts for, or active intermediates in the catalysis of, oxygenation reactions using hydrogen peroxide as the oxo transfer agent. Examples of 0X0 transfer reactions to organic substrates carried out via peroxovanadium complexes -the hydroxylation of alkanes, alkenes and aromatic compounds by the picolinato complex 22 (Figure 4.21) - are depicted in Scheme 4.8. 

Inorganic (top row) and organic (bottom row) vanadium compounds which are of (historical) interest as insulin mimics (sodium) vanadate (3) vanadyl (sulfate) (4) peroxovanadate (mixtures of vanadate and H2O2) (5) a peroxo-picolinato complex (6) bis(maltolato) complexes 7 R = CH3 BMOV = [VO(ma)2] = C2H5 BEOV 7b R = iPr 7c and the allixinato complex 7d and... 

Sakurai et al. have observed that vanadyl complexes with coordination modes such as VO(S4) (103) can normalize blood glucose levels (515) and are effective for normalizing both serum glucose and free fatty acid levels in streptozotocin rats and are orally active (516). Bis(picolinato)oxovanadium(IV) is also orally active against the diabe-... 

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 same N3O3 coordination sphere is exhibited by tris(picolinato)-vanadium(III) complex, rV(pic)3], the significantly distorted octahedral geometry of which is shown in Figure 7.13... 

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... 

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. 

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... 

Care should be taken however before drawing conclusions. Let us consider two homogeneous catalysts such as the pyridine-2.6-dicarboxylato (VI) and picolinato (VII) molybdenum peroxo complexes... 

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. 

The major limitation of aqueous Fenton generation of HO- is the insolubility of most organic substrates, especially hydrocarbons. The reasonable expectation that those organic solvents that are able to dissolve reasonable quantities of iron complexes are substrates for HO- has precluded their use. However, a recent study has established that the Fenton process [Eq. (5-5)] occurs in pyridine/acetic acid (2 1 molar ratio) when bis(picolinato)iron(II) [Fe l(PA)2] and HOOH are combined in a 1 1 molar ratio... 

Extension of these WL4 complexes to complexes not involving 8-quinolinol derivatives has included a number of substituted tetrakis-(picolinato)tungsten (IV) complexes (2, where Z = H, CH3, or C2H5 and the analogous 3-hydroxypicolinato complex) (3). Other species reported previously include the 2-pyrazinecarboxylato and 1-isoquino-linecarboxylato species, W(pzc)4 (3) and W(iqc)4 (4) (3), and the spectrally supported W(nq)4 and W(hqa)4 (2). Complexes not previously reported include W(tbq) 4 and the quadridentate Schiff base chelate W(dsp)2 (5 ), for which at least two isomers have been observed. 

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