Vanadyl complexes, Schiff base

Figure 20.9 (a) Protonation and reduction of an immobilized moiybdenum(Vi) nitrido complex 43 to the imido molybdenum(V) complex 44 and (b) immobilized Schiff base vanadyl complex 45. 

Interesting results have been presented by Baleizao et al. for chiral vanadyl Schiff base complex [50].120 The 29Si MAS NMR spectra have confirmed covalent grafting of the vanadyl complex into the silicate skeleton. 

Oxo-vanadium derivatives include numerous examples of Schiff base complexes, the importance of which is increasing in that they can act as insulin-enhancing agents.3 For instance, the vanadyl(IV) derivatives of TV.TV -ethylenebisCsalicylideneimine), [VO(salen)], A TV -disalicylidene-o-phenylenediamine, [VO(saloph)], and iV,Ar -ethylenebis(acetylaceto-natoimine), [VO(acen)], are shown in Scheme 1. 

Table 3 Redox potentials (V vs. SCE) for the one-electron oxidation of Schiff base complexes of vanadyl ion...

Although most oxovanadium(IV) complexes are blue, some Schiff base complexes may vary from yellow to maroon. Earlier suggestions that such colours, together with reductions in the V—O frequencies from 950-1000 to 800-850 cm-1, indicate polymerization or VO VO VO interaction are erroneous. Even for coordination environments which are very similar, the V=0 lengths may be very different, the vanadyl oxygens being exposed to the influence of the neighbouring molecules in the crystal.384... 

Spectroscopic and magnetic evidence has been presented423 in favour of a square-pyramidal structure for vanadyl complexes of the terdentate Schiff bases derived frohi 3-aldehydosalicylic acid and glycine or anthranillic acid, and an octahedral structure for the mixed complexes VOL1L2(L1 = terdentate dibasic hydrazone Schiff bases, L2 = bipy or o-phen).424... 

Vanadyl and copper(n) ions catalyse the /J-elimination reaction of O-phospho-threonine in the presence of pyridoxal.429 Equilibrium spectroscopic studies of the threonine-metal ion-pyridoxal system have identified a metal-ion complex of the amino-acid-pyridoxal Schiff base. The catalytic effect of the metal is ascribed to its electron-with drawing effecCIt was suggested that the specific catalytic effect of Cu2 + and V02+ arises from their reluctance to co-ordinate the phosphate in an axial position. Other metal ions such as nickel can also form the Schiff base complex but probably stabilize the phosphothreonine system by chelate formation. 

Allylic alcohols, for example geraniol, 2-methylallyl alcohol, 3,3-dimethylallyl alcohol, 3-buten-2-ol, l-octen-3-ol, and l-hexen-3-ol, are epoxidized with tert-butyl hydroperoxide in the presence of a vanadyl salen oxo-transfer catalyst in supercritical CO2. The metal catalyst was prepared in a simple two-step, Schiff base-type reaction to form the salen ligand, followed by complexation to the vanadyl group. The use of non-toxic supercritical CO2 in the presence of the new epoxidation vanadium catalyst led to yields and diastereoselectivities that were comparable to those resulting from the use of environmentally hazardous solvents such as CH2CI2 [59]. 

Schiff s base complexes of vanadium were encapsulated in the supercages of zeolite NaY and their catalytic activities were tested in the epoxidation of several alkenes and allylic alcohols with /er/-butylhydroperoxide The complexes investigated were VO(HPS) (vanadyl-N-(2-hydroxyphenyl)salicylideneimine) and VO(salen) (vanadyl-N,N (bis)salicylidene-imine). Particular attention was devoted to the question of leaching of vanadium during reaction. 

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