Hydrogen bonded complexes of diaminopyridines and diaminotriazines: Opposite effect of acylation on complex stabilities

The assocn. behavior of several 2,4-diamino-s-triazines, 2,6-diaminopyridines, and their acylated derivs. with uracil derivs. was studied. In soln. 1H-NMR and IR spectroscopy were used, and in the solid state as (co)crystals X-ray diffraction was used. Acylation of 2,6-diaminopyridine leads to an increase of the assocn. const. 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 assocn. const. of the complexes with N-propylthymine from 890 to ca. 6 M-1. This phenomenon is related to different conformational preferences of these compds. The amide groups in bis(acylamino)pyridines prefer a trans conformation, with the carbonyl group anti with respect to the ring nitrogen and coplanar with the arom. 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 responsible for the low assocn. const. of complexes of bis(acylamino)triazines with uracils. The relatively high dimerization consts. of bis(acylamino)triazines have been rationalized by the strong tendency to dimerize via quadruple hydrogen bondin