HYDROGEN-BONDED COMPLEXES OF DIAMINOPYRIDINES AND DIAMINOTRIAZINES - OPPOSITE EFFECT OF ACYLATION ON COMPLEX STABILITIES

The association behavior of several 2,4-diamino-s-triazines, 2,6-diami nopyridines, and their acylated derivatives with uracil derivatives wa s studied. In solution H-1-NMR and IR spectroscopy were used, and in t he solid state as (co)crystals X-ray diffraction was used. Acylation o f 2,6-diaminopyridine leads to an increase of the association constant in CDCl3 of the complexes with N-propylthymine from 84 to 440-920 M(- 1), whereas acylation of diamino-s-triazines leads to a dramatic fall in the association constant of the complexes with N-propylthymine from 890 to ca. 6 M(-1). This phenomenon is related to different conformat ional preferences of these compounds. The amide groups in bis(acylamin o)pyridines prefer a trans conformation, with the carbonyl group anti with respect to the ring nitrogen and coplanar with the aromatic ring. The amides of bis(acylamino)triazines, however, reside predominantly in a cis conformation. Repulsive secondary electrostatic interactions between the cis-amide and uracil carbonyl groups are thought to be res ponsible for the low association constant of complexes of bis(acylamin o)triazines with uracils. The relatively high dimerization constants o f bis(acylamino)triazines have been rationalized by the strong tendenc y to dimerize via quadruple hydrogen bonding.