Complex ions, chiral discrimination

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). 

Though precise geometries of the ion pair inclusion complexes are unknown, it is clear that the cation size plays an important role in modifying the central cavity of the ligand 54 which allows the regulation of the chiral discrimination. Larger anions can not be included in the central cavity which is evident as a lack of chiral discrimination, however, less structured diastereoisomeric ion pair complexes were observed. 

Chiral di mination studies with the tetra-amine 54 were also performed with respect to racemic molecular anions via the formation of cascade complexes. Regulation of the chiral discrimination may be achieved to some extent by the nature of the cation initially complexed. Efficient ion pairing with molecular anions is favoured in solvents of low polarity. In this respect, aqueous solutions of alkali metal salts of ( )-mandelic acid or (+)-a-hydroxy-l-naphthaleneacetic acid were extracted into a CDCI3 phase where the ligand 54 is dissolved. In case of the mandelate anion, the anion ligand ratios in the CDCI3 layer were as follows Na (0.6 1), (1 1),... 

The ability of clathrochelates to form ion pairs and covalently attached complexes is utilized in biochemistry [315-321], The stereoselectivity of the redox reactions of plastocyanine and horse heart cytochrome C with several cage complexes was reported in Ref 319. Studies on stereoselective electron transfer in different systems provide information on the importance of close ion pair association of a cage complex with protein in chiral discrimination. 

Chiral discrimination of the host-guest interaction can be measured in the gas phase. Resorcin[4]arenes bearing chiral substituents are able to select chiral quaternary ammonium ions. In these cases, complexation experiments were conducted by means of electrospray ionization (ESI) mass spectrometry. "... 

Quite a few examples of optical resolution via formation of the less soluble diastereomeric salt have been reported in which the species to be resolved and the chiral selector are both complex ions. Thus elucidation of the mode of chiral discrimination between complex cations and anions is an important factor leading to the discovery of new efficient resolving agents. As one of several approaches to this subject, we carried out very simple experiments. 

A racemic complex is loaded at the top of the column and eluted with 30% aqueous ethanol. On the column containing A-[Co(sep)] + very good resolution is achieved for/oc-[Co(P-ala)3] and/oc-[Co(a-ala)3] (ala = alaninate), while only poor resolution is attained for w r-[Co(P-ala)3] (Figure 2 )(9). On the column containing A-[Co(chxn)3]3+/ac-[Co(a-ala)3] and/ac-[Co(P-ala)3] are resolved to some extent but the elution order is reversed. In these cases [Co(ama)3] behaves as a complex anion. This reconfirms that chiral discrimination between complex cations and complex anions is attained by hydrogen bonding along the C3 or the C2 axis and that the C3+ complex favors the homochiral ion-pair, while the C2 complex favors the heterochiral ion-pair. 

In the CMPAs method, enantiomeric separation is accomplished by the formation of a pair of transient diastereomeric complexes between a racemic analyte and the CMPA. Chiral discrimination is due to the differences in the intetphase distribution ration, solvatation in the mobile phase, or binding of the complexes to the achiral/chiral stationary phase. Ion pairing, ligand exchange, inclusion complexes, and protein interactions represent the major approaches in the formation of diastereomeric complexes. 

In the preceding section it was shown that the stability of crown-ether complexes with alkylammonium salts depends on the relationship between the structures of the crown ethers and the ammonium ions. How critically this relationship determines the complex stability will become clear in this section, which deals with the discrimination between the two enantiomers of racemic salts by chiral macrocyclic ligands. 

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