Probing the mechanism of phase transitions in halogen-substituted solvatomorph crystals of benzimidamides via thermal and chemical stimuli

The screening of a molecule for its solid-state diversity is of importance in the chemical industry. The phenomena of phase transitions are also of interest as it allows one to understand the structural relationship that exists between the different crystal structures. This is expected to have implications in the observed properties in the substance of interest. With this background, the current study identifies two distinct phase transition phenomena in single crystals of 2-chloro-N'-(2-chlorophenyl)benzimidamide (A) and 2-bromo-N'-(2-bromophenyl)benzimidamide (B). Crystallization in DMSO produced solvatomorphs AIIα and BII at room temperature, while acetone and methanol yielded polymorphs AI and BI. The enantiotropic transition was characterized as an order-disorder phase transition in DMSO solvent, which resulted in conversion of AIIα to AIIβ, and was observed at Tc~199 K through variable-temperature SCXRD and low-temperature DSC. Since the solvent molecule ordering drives the phase transition, halogen-substituted benzimidamide interactions remain largely unchanged. However, immersing crystals of AIIα and BII in silicone oil triggered conversion to AI and BI crystals, completing the transition within 24 h, as confirmed by SCXRD studies. Thermal analysis techniques were employed to further investigate these transitions. Additionally, an in-depth examination of molecular packing was carried out to understand the underlying mechanism of the observed phase transition in these molecular crystals.