Schiff base-oxovanadium complexes

Fujita et al. used a catalytic amount of a binuclear titanium(IV) complex in an attempt to find an efficient system to oxidize sulfides with high enantioselectivity [102]. Prior to this study, they investigated other systems with several transition metals. A similar asymmetric sulfoxidation was discovered [105] using a catalytic amount of nonracemic Schiff base oxovanadium complex (Table 1.4) under atmospheric conditions at room temperature in dichloromethane. With 0.1 mol% of catalyst and cumene hydroperoxide as oxidant, oxidation produces sulfoxides in excellent yields. However, the reaction is limited to alkyl aryl sulfide substrates, and the best enantioselectivity obtained was 40% ee, for (S)-methyl p-methoxy phenyl sulfoxide. 

The modified Sharpless reagent was also successfully applied288 for the asymmetric oxidation of a series of 1,3-dithiolanes 248 to their S-monooxides 249 (equation 134). It was observed that the optical induction on sulphur (e.e. from 68 to 83%) is not significantly affected by the substituents R1 and R2. Asymmetric oxidation of a few aryl methyl sulphides by organic hydroperoxides in the presence of a catalytic amount of the optically active Schiff base-oxovanadium(IV) complexes gave the corresponding sulphoxides with e.e. lower than 40%289. 

Schiff base-oxovanadium(IV) complexes, as optically active oxidizing agents 291... 

The Schiff base-oxovanadium(IV) complex formulated as 19 was found to catalyze the asymmetric oxidation of sulfides with cumene hydroperoxide (Scheme 6C.8) [70]. Various aryl methyl sulfides were used for this process (room temperature in dichloromethane and 0,1 mol equiv. of the catalyst). Chemical yields were excellent, but enantioselectivities were not higher than 40% for the resulting methyl phenyl sulfoxide, Complex 16a, where [Ti] was replaced by VO, was also examined in the oxidation of sulfides, but the reactions gave only racemic sulfoxides [68],... 

In the same year, Fujita s group63 reported the asymmetric oxidation of aryl methyl sulfide by hydroperoxides (TBHP, CHP) and an optically active catalyst formed by a Schiff base-oxovanadium(IV) complex 32, giving (S)-sulfoxides in low ee (up to 40%) (Fig. 4). Later, they developed64 a more promising approach using 33, a binuclear Schiff base-titanium(IV) complex (4 mol% equiv) to catalyze the asymmetric oxidation of methyl phenyl sulfide by trityl hydroperoxide in methanol at 0 °C. The (ft)-methyl phenyl sulfoxide was obtained with 60% ee. 

A series of j8-diketonate Schiff base complexes (87 M = Ni, Cu, Pd, VO n=10, 12, 16) having a half disc-shape, was synthesised [164]. All the complexes, except the oxovanadium complexes, were mesomorphic (Colh), at relatively low temperatures. This approach was also applied to half-discshaped homodinuclear and heterodinuclear 1,3,5-triketonate Schiff base complexes, 88, and 1,3,5,7-tetraketonate Schiff base dicopper complexes, 89 [165]. The homodinuclear complexes 88, with M = M = Cu, X = OR, were mesomorphic showing Colh phases, whereas those with four chains (88 X = H) were not liquid-crystalline. Amongst the five heterodinuclear prepared (88 X = OR,n=14, Y = —CH2—CH2—, M = Cu, Pd, Ni, Mn, Co and M =Ni), only those with M = Cu, M = Ni and M = Pd, M =Ni, showed a mesophase, namely a Colh phase. A Colh phase was also observed for a structurally related compound, 89, (n=12, X = OR). 

Absorption and c.d. spectra of the complex formed between oxovanadium-(iv) and the Schiff base derived from (J )-l,2-propanediamine and two moles of acac indicate that the co-ordinated Schiff base moiety is close to planarity. The lack of spin-spin coupling between metal centres in iViV -propy-lenebis(salicyliminato)oxovanadium(iv) molecules (// = 1.78BM) has been discussed. Some seven new oxovanadium(iv) complexes with iV-(2-hydroxy-... 

The new series of oxovanadium(iv) complexes with Schiff bases derived from 2-aminothiophenol and substituted salicylaldehyde (40) or 2-hydroxy-naphthaldehyde has been synthesized and characterized. The ligands (40a),... 

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... 

No general review has been published since 1967.357b Reviews by Syamal358 and others722 cover only part of the published work on oxovanadium(IV) Schiff base complexes. 

Figure 31 Changes in the ordering of the oxovanadium(IV) d orbitals proposed for complexes with Schiff bases (66...

Table 39 Oxovanadium(lV) Complexes VO(SB)2 with Schiff Bases of Type (81)...

Table 41 Oxovanadium(rV) Complexes with Tridentate Schiff Bases (87)-(90)...

Table 42 Oxovanadium(IV) Complexes with Schiff Bases (97)-(99)a...

Electrochemical and structural studies of oxovanadium complexes with Schiff-base ligands attract particular attention because of their reversible redox behavior, which allows possible applications to electrocatalysts. VO(salen) and its oxidized product VvO(salen)Cl04 crystallize readily and their x-ray structures have been solved [108,109],... 

Both GC and LC behavior of metal complexes of various ligand types including salicy-laldimines and Schiff bases and fluorinated /3-diketones was reported. Metal ions included the lanthanides, transition metals, Pt, Pd and Zn. Dissociation and thermal instabilities were found to be the main limitations in the chromatography of such derivatives. The data indicate that pre-column derivatization and GC is unlikely to provide a viable method for the ultratrace determination of metal ions except in rare circumstances. On the other hand, LC of complexed metal ions was found as a valuable technique that combines the advantages of versatility, specificity and sensitivity with the capacity for simultaneous determination and speciation. Diastereoisomers of oxovanadium(IV) complexes of tetradentate Schiff bases could be resolved by both GC and LC . ... 

One subset of Schiff base complexes are the hydrazone complexes which have a R—C=N—N functional group instead of the R C=N—C functionality. These compounds have been prepared as models for bromoperoxidase389 and other biological systems.390 In the solid state, the hydrazones have coordination patterns similar to those of the Schiff bases and the majority of the complexes are oxovanadium(V) hexadentate or pentacoordinate complexes with tridentate 02N donor sets (see Table 5). While most structures contain alkoxide donors,390-403 complexes have been reported with diols and catechols,129,404,405 hydroxamic acids,288,391 hydroxy quinolinate,406, 7 and benzoylhydrazine.408 In addition, dioxo,375,409-412 oxo-bridged... 

A number of non-oxovanadium(IV) complexes have been generated by Schiff base ligands containing 02N donor sets. To date all the structurally characterized complexes contain two alkoxy or phenoxy functionalities and one imine-type functionality (48),316 617 618 The effects on varying substituents on the phenoxo groups have been investigated.620 The structure of bis[sali-cylaldehyde-2-hydroxyanilinato(2-)]VlV,316 shows the vanadium to have a nearly octahedral coordination environment. 

A typical anisotropic axial EPR spectrum of an oxovanadium(IV) complex (inset) in frozen THF. The ligand is a Schiff base derived from vanillin and tyrosine. Diamonds indicate the eight components of A (the parallel hyperfine coupling constant parallel defined by the z direction, which is the direction of the magnetic field). Arrows indicate the five inner components of Aj. Abscissa magnetic field strength (G) ordinate, intensity (arbitrary units). 

A linear correlation was also found between the a-factors and the isotropic e.p.r. A values of the complexes. Changes in A are ascribed primarily to changes in the covalency of the metal-ligand bond. A computer analysis of the electronic spectra indicated xy < xz, yz < x — y < z as the most probable order of metal d-orbital energies. Difficulties in correlating spectroscopic and structural data on oxovanadium chelates of even closely-related ligands are illustrated by the observation that within the series of Schiff bases polymeric, dimeric, and monomeric structures are encountered (see also p. 52). 

Serrette and Swager reported the mesomorphic properties of a series of oxovanadium(IV) complexes of dimeric Schiff bases 85, and 86 (Y=—CH2—CH2— —CH2—CH2—CH2— —CH2—C(Me)2—CH2— R = OCi4 H29) [163]. Despite the non-discoidal shape of these complexes, they all displayed columnar mesophases, some of the complexes showing several columnar-to-columnar phase transitions. The mesophases (hexagonal and rectangular) existed over very wide temperature-range, some of them being stable at ambient temperature. 

Magnetic exchange interactions in oxovanadium(iv)-Schiff base complexes have been studied in some detail this year. The temperature dependence of the magnetic susceptibility of NN -propylenebis(salicylaldiminato)oxovanadium-... 

Magnetic exchange interactions have also been identified in the oxovanadium-(iv) complexes with the Schiff bases derived from 2-hydroxy-l-naphthaldehyde and ethanolamine or propanolamine. Such complexes can accommodate two further donor atoms per metal and [VO(A)(L)] (A = bipy or phen HjL = Schiff base from salicylaldehyde or 2-hydroxy-l-naphthaldehyde with glycine, alanine, anthranilic acid, or o-aminophenol) complexes have been shown to involve six-co-ordinate vanadium. Similarly, VOL,B [HjL = 2-hydroxy-naphthylideneamino-acid, B = HjO, phen, or (py)2 HjL = 2-hydroxy-naphthylidene-o-aminophenol, B = phen] have normal magnetic moments ifi = 1.70—1.74 BM) at room temperature. The complex VOL (HjL = 6,ll-dimethyl-7,10-diazahexadeca-6,10-diene-2,4,13,15-tetrone) has been prepared from V0(0H)2, heptane-2,4,6-trione, and en in MeOH. The structure... 

New co-ordination complexes of oxovanadium(v) with the terdentate ONO-donor ligand derived from 2-hydroxynaphthylideneamino-acid (H2L), [VO(OMe)L]MeOH and [VO(OH)L]j, have been prepared and shown to be non-electrolytes in solution. VO(OEt)3 reacts with the Schiff bases (H2L) bis(salicylidene)ethylenediamine and salicylidene-2-hydroxypropylamine to form the corresponding VO(OEt)L complexes, which have been characterized by i.r. spectroscopy... 

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