Polymorphism in co-crystals: a metastable form of the ionic co-crystal 2 HBz·1 NaBz crystallised by flash evaporation

Abstract

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.