A gallic acid–succinimide co-crystal landscape: polymorphism, pseudopolymorphism, variable stoichiometry co-crystals and concomitant growth of non-solvated and solvated co-crystals

Abstract

The structural landscape of the binary gallic acid–succinimide combination has been explored. A recently reported dimorphic 1 : 2 co-crystal was shown to exhibit a third polymorph at a high temperature by differential scanning calorimetry (DSC). Further, seven solvated co-crystals have been obtained, including three different hydrates (gallic acid : succinimide : water in 1 : 1 : 1, 2 : 2 : 1 & 2 : 4 : 1 ratios) and solvates with 1,4-dioxane (1 : 1 : 1), tetrahydrofuran (2 : 2 : 1), acetone (2 : 2 : 1) and ethyl acetate (3 : 3 : 1). A rare phenomenon of concomitant solvation, besides concomitant polymorphism, in the 1 : 2 co-crystal is recognized, which posed difficulties in obtaining phase-pure crystal forms in bulk quantity. Based on structural insights, methods to obtain pure co-crystals and design of several solvates are suggested. Additionally, a 1 : 1 co-crystal, which had been elusive under ambient conditions, is found to show polymorphic behavior for the materials obtained under nitrogen atmosphere and by high temperature desolvation of several solvates as analyzed by powder X-ray diffraction (PXRD) and DSC.