Ligand Exchange Chromatography

discriminated by the action of a chiral selector only. The latter has to be involved in interaction with the enantiomers, resulting in the formation of adducts which, being diastereomeric species, may differ in their stability. These adducts in LEC are ternary complexes composed of the molecule of the chiral selector, the complexing metal ion, and the enantiomer to be recognized. Thermodynamic enantioselectivity of the system, i.e. the difference in stability of the above two diastereomeric ternary complexes which incorporate (R) or (S) enantiomers of the analyte, represents the sole source of chiral discrimination of the enantiomers in LEC. For an analytical scale resolution of enantiomers, the enantioselectivity does not need to be high, since, in principle, enantioselectivity of SAG0 = 25 J mol"1 would correspond to a separation factor a = 1.01, which is fully sufficient for two peaks to be resolved in a gas chromatographic capillary column generating about 105 theoretical plates. For a preparative scale liquid chromatography, one would prefer to deal with a separation factor of at least a = 1.5, which would correspond to enan-tioselectivity of about 1 kJmol-1. Ligand exchange often generates a discrimination effect of this order of magnitude. With the SAG0 increasing further, the column selectivity, a, rises exponentially, according to the general relation SAG0 = RTln a. In LEC systems with polydentate ligands, separation selectivities of up to a = 10 are known.

As a matter of principle, a chiral selector has to enter into at least a three-point interaction with the enantiomers, in order to be able to recognize the difference in the spatial structure of the latter. This requirement is easily met with tridentate ligands forming ternary complexes with metal ions which have a coordination number of six. Among practically important classes of organic compounds, however, are molecules with two electron-donating heteroatoms, N and/or O, situated in a chain of carbon atoms in positions 1,2 or 1,3, e.g. in 1,2- and 1,3-diamines, 1,2- and 1,3-amino alcohols, a- and amino acids and a- and ^-hydroxy acids. When forming complexes with metal ions, these compounds usually function as bidentate ligands with both heteroatoms coordinated to the same metal ion in the form of a five- or six-membered chelate ring.

It is very important that formation and dissociation of the bonds in the coordination sphere of the metal ion be fast. Otherwise, a slow rate of ligand exchange would degrade the efficiency of the chromatographic system. Therefore, only metals forming kinetically labile complexes can be employed in LEC, preferably Cu(II), Ni(II) and Zn(II). Even with the above metal ions, it has been repeatedly reported that the efficiency of ligand exchanging systems can be enhanced by raising the column temperature to about 50°C, which speeds up the ligand exchange. Ions of Cr(III), Co(III), Pd(II) and the like, form kinetically inert, stable complexes unsuitable for LEC. However, the second coordination sphere of these complexes, which actually is their solvation shell, is well organized, too, and it does exchange its ligands readily, so that an enantioselective chromatography process can also be based on outer coordination sphere ligand exchange.

Zn(II) would usually coordinate four heteroatoms with lone electron pairs in the form of a tetrahedron. The coordination number of the Ni(II) ion is six and its ligands are usually arranged in an octahedron. Copper binds four ligands in its main coordination square, but offers two additional, more remote axial positions, so that a complex with the coordination number six adopts a distorted octahedral structure.

It is worth noting, that the optical purity of a complex-forming chiral selector employed in an LEC system does not need to be necessarily 100%. It is a distinguishing feature of all direct chromatographic separations of enantiomers that even a selector contaminated by its enantiomeric form can provide a complete resolution of racemic analytes. Enan-tiomeric impurities in the chiral selector do not diminish column efficiency but merely result in the two sharp enantiomeric peaks of the analyte approaching each other and completely coalescing into one sharp peak when the enantiomeric excess (e.e.) of the selector falls to zero. This phenomenon (which is not an issue, at all, with proteins or natural carbohydrates as the chiral selectors, since these selectors are available in only one enantiomeric form) has been repeatedly proven experimentally in chiral exchanging systems involving amino acids of known e.e. as the selector.

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