In 1832, Friedrich Wo ¨hler and Justus von Liebig described the existence of polymorphism in molecular crystals for the first time in the case of benzamide. 175 years later, the crystal structure of this historical polymorph could be finally solved. Surprising similarities regarding the packing motifs between all three known forms can be found. According to Ostwald's rule, high supersaturations are needed to crystallize the most unstable Form II from solution. Herein we present a way in which all three known forms for benzamide can be concomitantly crystallized out of solution. With this trimorphic mixture at hand, a more detailed stability test could be performed and further characterization applying solid-state NMR-, Raman, and IR-spectroscopy was possible. † Dedicated to Prof. Reinhard Nesper on the occasion of his 60th birthday.

While polymorphism is a wide spread phenomenon, the number of reported polymorphic co-crystals is still very limited. Here we report the synthesis and structural characterisation of such a rare case of polymorphism with a co-crystal of benzoic acid (HBz) and sodium benzoate (NaBz). Flash evaporation yielded a new polymorph of this ionic co-crystal with a stoichiometry of 2 HBz?1 NaBz. The thermodynamic relationship between the known and the new polymorph was determined to be of enantiotropic nature. At room temperature, this new form B is metastable. While the known form A is composed of one dimensional tapes, form B is built from infinite rods. The coordination sphere of sodium, however, in both forms is octahedral and the packing around it is dense.

von Liebig discovered the first case of molecular polymorphism when crystallizing benzamide in 1832. 1 So far only microcrystalline mixtures of metastable phases with the thermodynamically stable form I were available. Optimizing the conditions for crystal growth now gave access to single crystals of the metastable form III which allowed for a detailed comparison of form I and form III by Raman and IR spectroscopy and thermoanalysis. Surprisingly, in the course of detailed DSC measurements for form I an endothermic peak prior to melting was observed. As proven by temperature dependent powder diffraction, thermal microscopy, MDsimulations, and solid-state NMR this pre-melting thermosignal is not related to an enantiotropic phase transition of form I into form III with a transition temperature close to the melting point.

Currently the synthesis of co-crystals is receiving considerable attention. We studied co-crystallisation of benzoic acid with sodium benzoate in different ratios and with different crystallisation techniques. A crystalline co-crystal, composed of two equivalents of benzoic acid and one equivalent of sodium benzoate (2 HBz?1 NaBz) could be identified and structurally characterised. By concomitant coordination of benzoate and benzoic acid the coordination needs of the cation could be satisfied. However, even with a 2 : 1 ratio, an octahedral environment could only be realised by sharing an edge in a dimer. The dimers are connected to one-dimensional tapes which in turn are packed in a distorted hexagonal arrangement. Only with a ratio of two neutral benzoic acid ligands and one benzoate can the coordination requirements of the sodium cation and charge neutrality be assured at the same time.