CONTRIBUTION X-RAY INVESTIGATION OF GUEST EXCHANGE IN A UREA INCLUSION COMPOUND

A. A. Khan, S. T. Bramwell, K. D. M. Harris and M. R Truter

UCL Chemistry, London, UK

Urea inclusion compounds have been studied since the early twentieth century. Hermann and Schlenk showed their basic structure to be hexagonal at room temperature with a chiral space group P6_122 and cell parameters a = b = 8.20 Å , c = 11.01Å. This structure essentially contains linear parallel, non-intersecting channels (tunnels) formed by urea, within which the guest molecules are located. In general urea will include guest molecules based on sufficiently long chain alkanes provided the degree of substitution of the alkane is small. Our interest is geared towards a new concept in urea inclusion chemistry where we introduce a second guest molecule into the urea channels via an exchange mechanism. We show, using single crystal X-ray diffraction, that this occurs with negligible disruption to the host structure.

Our most recent and exciting discovery has been the exchange involving a single crystal of 1,8 dibromo-octane/urea inclusion complex that has one end dipped into a solution of pentadecane and the other end supported vertically by a capillary tube. We find that the alkane enters the urea channels and displaces the 1,8 dibromo-octane guest molecules from their inclusion cavities such that they exit the crystal via the other end into the capillary tube. In addition, we have used this process to successfully separate a racemic mixture of dodecan-2-ol to 95% purity. Other chiral systems have been investigated and similar results have been achieved.

It appears that the most fundamental requirement for these exchange reactions to occur is that there be an incommensurate relationship between the host and the guest substructures. This enables the activationless transfer of guest molecules through the solid crystal.