Crystal-to-crystal transformation amongst dimorphs of racemic 2,6-di-O-(p-halo benzoyl)-myo-inositol 1,3,5-orthoformates that achieves halogen bonding contacts

Abstract

Racemic 2,6-di-O-(p-halobenzoyl)-myo-inositol 1,3,5-orthoformates (bromo (1) and chloro (2)) produced two polymorphs each, thin needle type crystals (Form I) were obtained from methanol, whereas larger rectangular crystals (Form II) were produced from ethyl acetate. Both forms could be produced concomitantly on crystallization (of 1 or 2) from ethyl acetate–light petroleum ether mixture; the yield of Form II crystal was always much more compared to Form I crystals. Although, a one-dimensional isostucturality linking molecules via O–H⋯O hydrogen bonding is seen in both forms, the difference arises in linking these chains. In larger Form II crystals (of 1 and 2), the adhesions are viahalogen bonding (C–X⋯OC, X = Cl, Br) contacts, whereas in smaller Form I crystals C–H⋯X contacts join them. Interestingly, DSC and X-ray crystallographic studies confirmed the thermal crystal-to-crystal transition of Form I to Form II crystals. Transformation of a minor to major polymorph containing ‘C–Br⋯OC’ contacts, similar to the phase transition previously reported by us in the case of 2,4,6-tri-O-(p-bromobenzoyl)-myo-inositol 1,3,5-orthoformate, suggests the role of X⋯O short contacts in preferential nucleation and crystal growth.