Cocrystallization through the use of a salt: The case of thiourea with a new propanediammonium oxalate salt

Selecting molecules with functional groups such as NH and CO can facilitate the growth of cocrystals through the creation of hydrogen bond networks. However, some compounds such as oxalic acid and thiourea, yet having such functional groups, do not cocrystallize. In this paper, we show that the prior formation of salts combining the oxalic acid with a strong amine (high Delta pka) can help enhancing the electronic density of the hydrogen bond acceptor (CO groups). Using the resulting salt with thiourea can, then, lead to a cocrystal. This phenomenon is illustrated with the synthesis of a new compound (C3H12N2)(2)(C2O4)(2)center dot CH4N2S by cocrystallization of a new propanediammonium oxalate salt and thiourea. The salt and cocrystal were structurally characterized by powder and single crystal X-ray diffraction, and FT-IR spectroscopy was used to rationalize the hydrogen bonding.