Polymorphism and phase transformations in 2,6-disubstituted N-phenylformamides: the influence of hydrogen bonding, chloro-methyl exchange, intermolecular interactions and disorder

Abstract

The crystal structures, thermal behaviour and phase transformations of a series of 2,6-disubstituted-N-phenylformamides have been investigated. A phase transformation was only observed when chlorine was one of the substituents. Crystals of the room-temperature form of 2,6-dichloro-N-phenylformamide (1) and 2-chloro-6-methyl-N-phenylformamide (2) are isomorphous. Both compounds are orthorhombic at room temperature and transform to a monoclinic high-temperature form at 155 and 106 °C, respectively. The room-temperature structures of 1 and 2 consist of chains of N–H⋯O hydrogen-bonded molecules stacked in an alternating arrangement along the crystallographic a direction. The high-temperature forms of compounds 1 and 2 (grown by sublimation) are both monoclinic but not isomorphous, with one short axis of about 4.3 Å, and consist of chains of N–H⋯O hydrogen-bonded molecules stacked along the short axis, related by translation. When both of the chlorine substituents are replaced by methyl groups, as in 2,6-dimethyl-N-phenylformamide (3), the crystals do not undergo any phase transition on heating and only an orthorhombic form, space group P2₁2₁2₁, has been isolated. Examination of the molecular geometry and structural properties of 3 indicates that it is a hybrid structure of the low- and high-temperature forms of compounds 1 and 2. This contribution analyzes the effect of chloro-methyl exchange, the steering ability of chlorine, and the role of weak interactions on the structural and thermal properties of the compounds studied. In addition, a mechanism for the phase change in 1 is proposed and rationalized through the examination of the structures themselves together with lattice energy calculations.