Inclusion compounds of binaphthol with picolines: structures, selectivity and kinetics of desolvation

Luigi R. Nassimbeni & Hong Su, Acta Cryst. (2001). B57, 394-398

The authors present some interesting work in the field of molecular recognition through this study of host-guest systems for which they have determined the crystal structures and thence their lattice energies. The results of further experiments allow correlation of the structures to thermal stability, energy of enclathration and the kinetics of desolvation. The choice of this bulky and rigid host, 2,2'-dihydroxy-1,1'-binaphthyl (binaphthol, BNP) follows earlier work by these authors and others and of particular interest are those systems where chiral guests have been embedded in the resolved host molecules.

The structures of the three inclusion compounds were determined from low-temperature, single-crystal X-ray data (CCD) and all show some disorder in the methyl groups of the guest molecules. The three isomers of picoline, 2-, 3- and 4-methylpyridine (2P, 3P and 4P, respectively), were selected as guest molecules and these co-crystallize with binaphthol in different space groups, C2/c, P2₁/c and Pbca. Each guest molecule relates through the picoline N atom to its host hydroxyl groups by O-H...N hydrogen bonds varying between 2.697 and 2.779 Å. The cross-sectional areas of the channels in the structures in which the guest molecules are located are calculated as 5.5 x 5.5, 5 x 6 and 7 x 8 Ų and the change in conformation of the host molecules is marked by the increase in torsion angle about the C-C bond linking the two halves of the binaphthyl group, from 83.5, to 95.3 to 103.0° for the three picoline isomers, respectively.

The thermodynamic data presented in the paper suggest that 2- and 4-picoline are preferentially enclathrated over 3-picoline, which correlates with their calculated lattice energies in the order 4>2>3. The difference between the energies calculated for 2- and 4-picoline, however, is small (3.2 kJ mol^-1) and this in turn fits with the observation of concentration dependence for the competition experiments for all three isomers, suggesting kinetic effects are also involved. TGA and DSC results on the compounds support these data, indicate a one-step desolvation and confirm the host-guest ratio of 1:2 observed in the crystal structures. The paper is well illustrated and referenced to other clathrate systems and the methodology used for the various thermodynamic calculations.

Reviewed by Judith A.K. Howard, Chemistry Dept, U. of Durham, UK