Ligand exchange techniques

Ligand exchange has proved to be very successful in the separation of several enantiomers. Davankov and Rogozhin (41) used chiral copper complexes bonded to silica. The enantiomeric separation is based essentially on the formation of diastereomeric mixed complexes with different thermodynamic stabilities. It is generally accepted that chiral discrimination proceeds via the substitution of one ligand in the coordination sphere of the metal ion. Ligand exchange technique is especially effective for the enantiomeric resolution of aminoacids, aminoacids derivatives, and hydroxy acids (42). 

Ultrafiltration and ion exchange techniques were used to study characteristics of the formation path of Ln(III) binding to humic acid. A competitive ligand exchange technique was employed to study the complex dissociation. In the latter, arsenazo III (Ars) was used as the competitive ligand. Eu(III) was used in the methods requiring radioactivity while Sm(III) was used in the other studies however, the chemistry of Sm(III) and Eu(III) can be considered identical within the uncertainties of these methods. 

The rate of Sm(III) dissociation from humic acid was studied using a ligand exchange technique. A solution of the Sm(III)-humate complex was equilibrated for a fixed time, then an aliquot of arsenazo III was added. The Sm(III)- arsenazo III complex is so stable (9) that it prevents reformation of the Sm(III)-humate. The rate of Sm(III)-Ars complex formation is quite rapid (9) and we found that the observed formation of the Sm(III)-arsenazo complex was determined by the rate of dissociation of the Sm(III)-humate. The formation of the Sm(III)-Ars complex was monitored spectrophotometrically by the increase in absorbance at 665 nm. Reactions were monitored either with a Milton Roy Spectronic 1201 (ti/2 ... 

Interaction with plurivalent cations via ligand exchange mechanism is one more rather widely applied crosslinking technique. The network bonds of ionic or donor-acceptor nature are located, with respect to lifetime, between the truly covalent crosslinks and physical entanglements. Generally speaking, gelation in these systems is reversible. 

The physical nature of the sulfate complexes formed by plutonium(III) and plutonium(IV) in 1 M acid 2 M ionic strength perchlorate media has been inferred from thermodynamic parameters for complexation reactions and acid dependence of stability constants. The stability constants of 1 1 and 1 2 complexes were determined by solvent extraction and ion-exchange techniques, and the thermodynamic parameters calculated from the temperature dependence of the stability constants. The data are consistent with the formation of complexes of the form PuSOi,(n-2)+ for the 1 1 complexes of both plutonium(III) and plutonium(IV). The second HSO4 ligand appears to be added without deprotonation in both systems to form complexes of the form PuSOifHSOit(n"3) +. ... 

The ligand exchange kinetics between free and coordinated ligands were studied more thoroughly for the ligands trimethylphosphite and triphenylphosphine. The kinetics were analyzed using conventional NMR line shape technique 140). 

Chiral ligand-exchange chromatography (CLEC) ° separates enantiomers by the formation of diastereomeric metal complexes. In a first instance the technique was mainly used for the separation of amino acids. Impressive results of the first separations gave rise to intensive investigation in the field and numerous publications appeared in the literature, which have been reviewed. 

Adsorption of phosphate on Fe oxides involves a ligand exchange mechanism (Par-fitt and Russell, 1977 Sigg and Stumm, 1981) and appears to be promoted by increasing the ionic strength (Bowden et al., 1980). Spectroscopic studies have not provided an entirely consistent picture of the mode of phosphate adsorption, but the consensus from studies with a range of techniques is, that phosphate adsorbs on Fe oxides predominantly as a binuclear, bidentate complex. 

Based on preliminary results from Helfferich130, further developments by Davankov and co-workers5 131 133 turned the principle of chelation into a powerful chiral chromatographic method by the introduction of chiral-complex-forming synlhetie resins. The technique is based on the reversible chelate complex formation of the chiral selector and the selectand (analyte) molecules with transient metal cations. The technical term is chiral ligand exchange chromatography (CLEC) reliable and complete LC separation of enantiomers of free a-amino acids and other classes of chiral compounds was made as early as 1968 131. 

A modification of the pyridoxal—amino acid reaction (mentioned above) has been made for automatic analysis of amino acids by ligand-exchange chromatography [95]. This technique involves separation of the amino acids prior to fluorimetric reaction and determination. As the amino acids are eluted from the column, they are mixed with the pyridoxal-zinc(II) reagent to produce a highly fluorescent zinc chelate. Amounts of as low as 1 nmole of amino acid may be detected. The first reaction involved is the formation of the pyridoxyl-amino acid (Schiff base) as in Fig.4.46. The zinc then forms a chelate which probably has the structure shown in Fig. 4.48. 

Review. Reetz1 has reviewed newer published reactions of these reagents as well as numerous unpublished results from his own research. The review emphasizes the chemo-, diastereo-, and enantioselective reactions that can be achieved. Drawbacks are that secondary and tertiary alkyltitanium compounds generally are not available, owing to /1-hydride elimination, and that ready ligand exchange makes the preparation of derivatives chiral at titanium difficult. The review includes useful suggestions for experimental techniques. 

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