Schiff base chelates

Complexes containing Schiff base chelating ligands have been extensively studied.137-144 Here the ligands include bidentate, tridentate and tetradentate representatives where O or S donors supplement the imine nitrogen. Examples are listed in Tables 2 and 3. Sample ligands include (2), (3) and (4). 

Beyond the 3+ oxidation state, rhodium forms a limited number of complexes in the 4+, 5+ and 6+ oxidation states. A recent review1221 gives an excellent summary of the chemistry of the higher oxidation state chemistry of Rh (as well as Ru, Os, Ir, Pd and Pt). For the 4+ oxidation state, the hexafluoro, hexachloro and trioxo dianions are well characterized. The known neutral species include RhF4 and some oxides. There are also scattered reports of Rhlv complexes containing substituted biguanides and Schiff base chelates. 

Terzis, A., Mentzafols, D., and Tajmir-Richi, H.A. (1984) Eight-coordination. Synthesis and structure of the schiff-base chelate bis(N,N -disalicyUdene-l,2-phenylenediamino)cerium(IV). Inorganica Chimica Acta, 84, 187. 

Last time syntheses of polymer bond Schiff Base chelates were reported 9 pjjg main investigation was done by Wohrie and coworkers. 

The Co chelate (56) was used as a catalyst for the oxidation of 2,6-dimethylphenol to the corresponding quinone (BQ) and diphenoquinone (DQ) in benzene at 298 Phenol oxidation is well known with low molecular Schiff Base chelates ... 

Uden, P. C., Walters, F. H. The high-pressure liquid-chromatographic separation of copper (II) and nickel(II) schiff base chelates on microparticulate silica. Anal. Chim. Acta 79,... 

A number of reactions involving pyridoxal, an amino acid, and a metal ion have been attributed to the formation of a Schiff base chelate in which a hydrogen atom on the a-carbon of the amino acid is labilized, and the Schiff base can now undergo a variety of reactions resulting in a transamination, racemization, aldol condensation, a,/3-elimination, or decarboxylation reactions (188). 

In the transamination reaction (shown in reaction scheme LVIII), a pyridoxal amino acid Schiff base chelate is first formed, and a shift of the hydrogen atom in the a-carbon takes place to give a tautomeric Schiff base, which finally undergoes hydrolytic cleavage. The result is a transamination reaction in which the amino acid is converted to a keto acid and the pyridoxal to pyridoxamine. In this type of reaction, the metal ion serves to maintain the planarity of the Schiff base chelate and exerts an electron-withdrawing action in the same direction as the heterocyclic ring (149). 

The main aspects of the proposed mechanism have been confirmed by showing that the same Schiff base chelate is formed in solution when either copper(II) or nickel(II) ions are treated with either pyridoxal and alanine or pyridoxamine and pyruvic acid (75), and also by a study of the properties of the strongly fluorescent pyridoxal-alanine-aluminum Schiff base chelate (80). 

Another example of a metal ion-induced rearrangement has been described recently in which 2,2 -bisbenzoxazoline was found to rearrange under the influence of metal ions to give the Schiff base chelate (23). Experimental evidence was given in this case to show that an equilibrium existed between the oxazoline and the Schiff base. The reaction, therefore, proceeds as shown in the scheme (LXIX). 

There are three possible routes for the formation of a Schiff base chelate (LXX) (a) There can be an equilibrium between the heterocycle and the Schiff base. In this event, a metal chelate could be formed with the Schiff base. (b) There can be an equilibrium between the heterocycle and the two starting materials. The latter can react with the metal ion in a stepwise manner to give the Schiff base chelate, (c) Alternatively, the metal ion can react directly with the heterocycle inducing a rearrangement reaction which results in the Schiff base chelate. From a kinetic study of the reactions of 2,2 -bisbenzothiazoline and its 2,2 -dimethyl derivative with Cd(II) and Zn(II), it was determined that the pathway (b) was inconsistent with the experimental results in all cases. The most probable course of the reaction involves a metal ion-induced rearrangement reaction, although it is possible in some instances that pathway (c) might simultaneously contribute to the formation of these Schiff base chelates (109). 

The related compounds, 2,2 -bisbenzothiazoline and its 2,2 -dimethyl homolog, were also found to rearrange under the influence of Zn(II), Cd(II), and Hg(II) to give the corresponding Schiff base chelates (22, 108, 109) (Section V). 

The stability of the metal chelate ring formed with Ni(II), Co(II), or Fe(II) and the donor group —N=C—C—N== (123) is responsible for the formation of Schiff base chelates of a-diketones and methylamine (reaction XCIX) instead of the polymeric condensation products obtained... 

The metal ion not only stabilizes the chelate rings and prevents the formation of undesirable products but also acts as a template. Several reactions have been described in which Schiff base chelates are formed via a metal template or a metal chelate template mechanism. In the presence of nickel(II), /3-mercaptoethylamine and a-diketones give a Schiff base chelate, whereas in the absence of metal ions, thiazolidines are obtained (198, 199). The planar metal chelate (structure CII) has the co-... 

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. 

A third approach for the synthesis of polymeric Schiff-base chelates is to prepare at first in situ a precursor chelate of the diamine and a M(II), or an o-hydroxyaldehyde and a M(I1), and to add then the second component o-hydroxyaldehyde, or a diamine [1,14,15], respectively. 

Because of the numerous interesting properties of low molecular weight tetradentate Schiff-base chelates related to catalysis and electrochemistry [16,17], some more recent papers describe the synthesis of more-soluble... 

On the other side, a totally insoluble cross-linked Schiff-base complex was prepared as follows. The glycidyl ether of bisphenol A was reacted with a Schiff-base chelate (prepared from 2,4-dihydroxybenzaldehyde and 1,3-diaminopropane followed by metallation) in a molar ratio of, for example, 6 1 at 150-200 °C to obtain the polymer 4 as shown in Eq. 6-3 (see Experiment 6-1, Section 6.6) [19]. Polymeric copper complexes in particular exhibit good thermal stabilities and good mechanical properties such as tensile strength. 

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