Ricerc@Sapienza

Ten new heterodimers were synthesized in different stoichiometry to explore the role exerted by potential proton-transfer reactions in the supramolecular structures of A-B cocrystals formed by 5-haloderivatives of uracil (halogen = F, Cl, Br, I; coformer A) coupled with aminoazines as 2-aminoadenine simulants (melamine, 2,4,6-triaminopyrimidine, 2,6-diaminopyridine; coformer B) for pyrimidine nucleobase recognition.

Seven 5-and 6-halogenated derivatives of uracil or 1-methyluracil (halogen = Cl, Br, I) were
studied by single crystal X-ray diffraction. In contrast with pure 5-halouracils, where the presence
of N-H…O and C-H…O hydrogen bonds prevents the formation of other intermolecular interactions,
the general ability of pyrimidine nucleobases to provide electron donating groups to halogen
bonding was confirmed in three crystals and cocrystals containing uracil with the halogen atom at

6-Mehyluracil, C5H6N2O2, exists in two crystalline phases: form (I), monoclinic,
space group P21/c [Reck et al. (1988). Acta Cryst. A44, 417–421]. and form (II),
monoclinic, space group C2/c [Leonidov et al. (1993). Russ. J. Phys. Chem. 67,
2220–2223]. The structure of polymorph (II) has been redetermined providing a
significant increase in the precision of the derived geometric parameters. In the
crystal, molecules form ribbons approximately running parallel to the c-axis
direction through N—H O hydrogen bonds. The radical differences observed