Structural influence on the intermolecular/intramolecular hydrogen bonding in solid state of substituted leflunomides: evidence by X-ray crystal structure

We report the results of an X-ray crystal structure study of nine substituted leflunomide metabolite analogs (LFM). Comparison of the hydrogen bonding characteristics exhibited by these structurally distinct LFM analogs was especially informative about the inter- and intra-molecular hydrogen bonding patterns that exist in the crystal structure of individual compounds. All compounds had the strong intramolecular hydrogen bonds. In addition, with the exception of the 2,5-difluorophenyl substituted LFM analog, all other compounds formed inter- or intra-molecular hydrogen bonds with the halogen atom and the NH group. However, we found that the presence of a fluorine atom at the 2-position on the phenyl ring of the 2,5-difluoro and 2-fluoro derivatives resulted in only one intramolecular hydrogen bond in the structural framework. Conversely, the 3,5-difluoro substituted LFM analog had an intramolecular hydrogen bond common to the other halide substituted derivatives. The anomaly exhibited by the 2,5-difluoro and the 2-fluoro substituted compounds may be owing to the smaller size of fluorine atom in comparison with the chlorine and bromine atoms in the structures of the other analogs. The presence of a fluorine at the 2-position of the phenyl ring may disrupt the intermolecular hydrogen bonding that was observed for the other derivatives due to differences in the crystal packing for these molecules.