Microcrystalline solid–solid transformations of conformationally-responsive solvates, desolvates and a salt of N,N′-(1,4-phenylene)dioxalamic acid: the energetics of hydrogen bonding and n/π → π* interactions

Abstract

Oxalamic acid derivatives are model molecules used in crystal engineering and molecular recognition both in solution and in the solid state. Despite their growing importance, hardly any information on polymorphism, stability and interrelations of their solid forms can be found in the literature. In this work, the molecular and supramolecular structures of three cocrystal solvates of formula H₂pOx·2S (S = DMSO, DMF, ⅓(MeOH·2W); W = H₂O) and the dimethylammonium salt of N,N′-(1,4-phenylene)dioxalamic acid (H₂pOx) are described. The nature and energetics of non-covalent interactions were explored through computational methods including the evaluation of the Hirshfeld surface, two-dimensional fingerprint plots, energy-framework diagrams and crystal lattice energies. The X-ray structural parameters were correlated with experimental solid state IR, ¹³C-CPMAS, thermal analysis and SEM to elucidate the unknown molecular and supramolecular structures of two hydromorphs and three desolvated polymorphs of H₂pOx. The role of the crystallized solvents was demonstrated to fix the conformation of H₂pOx through hydrogen bonding and n/π → π* interactions, favoring the sp–ap conformation to fit the shape of the solvent. The interconversion pathways between the ten solid phases of H₂pOx were stablished, relying on the crystal size and temperature to yield a specific solvent-free polymorph.