Competition between hydrogen bonding and donor–acceptor interactions in co-crystals of 1,3-dimethylbarbituric acid with aromatic amines

In contrast to cyclohexane-1,3-diones, 1,3-dimethylbarbituric acid (DMBA) does not enolize in crystals with formation of infinite H-bonded chains. Conversely, it forms crystals that completely lack traditional H-bond donors and hence are held together by C–H···O H-bonds and Cδ+Oδ−···Cδ+ interactions of a putative donor–acceptor nature. In view of these specific features, the DMBA molecule appeared to be a good candidate to study the competition of the various intermolecular forces in its co-crystals with traditional H-bond donors such as o-nitrophenylhydrazine, p-nitroaniline, 2,6-diamino-4-phenyl-1,3,5-triazine, 2,6-diaminopyridine and p-aminopyridine. X-Ray crystallographic results are analyzed in terms of crystal packing forces and formation of specific packing leitmotifs (supramolecular synthons). The conclusions indicate that, though a rational crystal engineering of co-crystals is far from being accomplished, a reasonable rationalization of the complex intermolecular forces acting in competition in the crystal packing is henceforth achievable.