Effect of intermolecular interactions on the molecular structure; theoretical study and crystal structures of 4-bromopyridinium tetrafluoroborate and diaqua(3-bromopyridine)difluorocopper(ii)

Abstract

The role of C–Br⋯F interactions in two crystal structures (4BP)BF₄ (I) and Cu(H₂O)₂(3bp)F₂ (II), (where 4BP is the 4-bromopyridinium cation and 3bp is 3-bromopyridine) is investigated. Crystal structure analysis indicates that the supramolecular assembly of I is based on symmetrical bifurcated C–Br⋯F halogen bonding and the bifurcated N–H⋯F hydrogen bonding, while that of II is based on O–H⋯F hydrogen bonding interactions. The Br⋯F distance in I is 0.13 Å less than the sum of van der Waals radii. In contrast, the Br⋯F distance in II is 0.04 Å longer than the sum of van der Waals radii, indicating that the C–Br⋯F interaction plays a minor role in developing the supramolecular structure of II. The structure of I is the first reported with perfect symmetrical bifurcated C–Br⋯F halogen bonding. II is the first reported crystal structure with C–Br⋯F–tM interactions, tM = transition metal. Theoretical calculations have shown that a charge assisted symmetrical bifurcated C–Br⋯F interaction is stronger than the corresponding linear one, whereas in the normal (not charge assisted) C–Br⋯F halogen bonding both linear and bifurcated interactions have comparable strength. This conclusion is supported by structure analysis of reported structures in this work and the published data in Cambridge Structural Database (CSD).