Diversity in the packing modes of mesogenic diphenylpyrimidines with two chiral centres in their crystal structures: the role of interactions between the pyrimidine rings

Abstract

Various different crystal structures were found for four mesogenic diphenylpyrimidines with two chiral centres and these have been compared with the previously determined crystal structure of 2-{4-[(R)-2-fluorohexyloxy]phenyl}-5-{4-[(S)-2-fluoro-2-methyldecanoyloxy]phenyl}pyrimidine (1) which shows an isotropic mesophase. 2-{4-[(S)-2-Fluoro-2-methyldecanoyloxy]phenyl}-5-{4-[(R)-2-fluorohexyloxy]phenyl}pyrimidine (2) and 2-{4-[(R)-2-fluorohexyloxy]biphenyl-4′-yl}-5-[(S)-2-fluoro-2-methyldecanoyloxy]pyrimidine (3), which are isomers of 1 with different N positions, have a structure comprising L-shaped molecules with the chains associated between layers (MHPOBC-type) and a polar structure of stacked monolayers of parallel molecules, respectively. Two diastereomers, 2-{4-[(R)-2-fluorohexyloxy]phenyl}-5-{4-[(R)-2-butyloxypropanoyloxy]phenyl}pyrimidine (4) and 2-{4-[(S)-2-fluorohexyloxy]phenyl}-5-{4-[(R)-2-butyloxypropanoyloxy]phenyl}pyrimidine (5) with the same core as 1 but a different chain, have similar structures with crossed core moieties. It is concluded that electrostatic or dipole–dipole interactions between pyrimidine rings, maximizing the overlap of the core moieties, and the bulkiness of the chains are competing tendencies which produce the diversity of the crystal structures. In striking contrast to 1, chiral interactions are not dominant in these crystals, confirming the uniqueness of the crystal structure of 1.