SEPARATION OF CLOSE ISOMERS BY ENCLATHRATION

Dejana Vujovic1, Mino R. Caira1, Luigi R. Nassimbeni1 and Edwin Weber2

1 Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa
2 Institut für Organische Chemie, Technische Universität Bergakademie Freiberg, Leipziger Strasse 29, D-09596 Freiberg/Sachs., Germany

KEYWORDS: separations, enclathration, molecular recognition, inclusion compounds, lutidine isomers

Separation of close isomers by enclathration encapsulates two topics of supramolecular chemistry namely inclusion compounds and molecular recognition.

Inclusion compounds may be defined as those compounds in which one type of molecule is able to enclose another smaller molecule within its structure with no covalent bonding between the two species. The molecular network and enclosed species are usually referred to as the "host" and "guest" respectively1.

A host that forms complexes of different stabilities with two or more similar guests is said to show molecular recognition2. Selective complexation plays an essential role in the various aspects of supramolecular functions3. For example, if the molecular recognition is specific and the host includes one isomer from a mixture, complexation can be used in the separation of isomers and the isolation of a single compound in a pure state4.

The principles outlined above were used in an attempt to separate three lutidine isomers; 2,4-lutidine, 2,6-lutidine and 3,5-lutidine. The host compound used for the separation process is 1,4-bis(9-hydroxy-9-fluorenyl)phenyl.


2-component and 3-component competition experiments indicate that the selectivity of the host for complexation with the isomers of lutidine follows the trend: 3,5-lutidine > 2,4-lutidine > 2,6-lutidine.

Thermal analyses and crystal structure determinations were performed on the inclusion complexes of the host with three lutidine regioisomers in an attempt to gain a better understanding of the selectivity trends observed in the competition experiments. All three inclusion complexes crystallise in the space group P21/n with host to guest ratios of 1:2. The conformations of the host molecules in the crystals and the sizes of the channels in which the guest molecules are located are similar in the case of 2,4-lutidine and 3,5-lutidine inclusion complexes and they differ from those in the 2,6-lutidine inclusion complex. DSC and TG results indicate multiple step decomposition pathways in the case of all three inclusion complexes.

1. F. Vögtle, Supramolecular Chemistry, Wiley, Chichester, 1991, ch. 5.
2. Inclusion Compounds, eds. J. L. Atwood, J. E. D. Davies and D. D. MacNicol, vol. 1-3, Academic Press, London, 1984; vol. 4 and 5, Oxford University Press, Oxford, 1991.
3. Y. Aoyama in Supramolecular Chemistry, (eds) V. Balzani, L. De Cola, Kluwer Academic Publishers, Dordrecht, 1992. 4. F. Toda, Top. Curr. Chem., 140, 1987, 43.