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

The role of C-Br center dot center dot center dot F interactions in two crystal structures (4BP)BF4 (I) and Cu(H2O)(2)(3bp)F-2 (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 center dot center dot center dot F halogen bonding and the bifurcated N-H center dot center dot center dot F hydrogen bonding, while that of II is based on O-H center dot center dot center dot F hydrogen bonding interactions. The Br center dot center dot center dot F distance in I is 0.13 angstrom less than the sum of van der Waals radii. In contrast, the Br center dot center dot center dot F distance in II is 0.04 angstrom longer than the sum of van der Waals radii, indicating that the C-Br center dot center dot center dot 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 center dot center dot center dot F halogen bonding. II is the first reported crystal structure with C-Br center dot center dot center dot F-tM interactions, tM = transition metal. Theoretical calculations have shown that a charge assisted symmetrical bifurcated C-Br center dot center dot center dot F interaction is stronger than the corresponding linear one, whereas in the normal (not charge assisted) C-Br center dot center dot center dot 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).