Secondary Bonding Interactions in Some Haloanilinium Halides

The crystal packing in the structures of various solvent-free (mono)haloanilinium halides involves with one exception the formation of approximately planar hydrophilic regions that contain classical hydrogen bonds N-HþþþX (X = halogen). Common to the substructures of all layers are rings of general graph set Rn2n(4n), involving equal numbers of chloride acceptors and NH2 moieties (two hydrogen atoms of the positively charged NH3 groups). The 2-substituted derivatives contain the shortest XþþþX contacts, and these are within the layers; it could be argued that the contacts are forced upon the system by the 2-substitution pattern. In 3-chloroanilinium chloride and its bromine analogue, the layers consist of ribbons connected by short C-HþþþX interactions. In all these compounds, the aromatic groups project from the layers and form hydrophobic regions; the layers are linked by weak interactions of the form XþþþX or CHþþþX. There is appreciable interdigitation of hydrophobic groups from adjacent layers except for the ribbon structures. 3-Iodoanilinium iodide displays a completely different structure, in which all the atoms except some hydrogen atoms of the NH3 groups lie in a crystallographic mirror plane, but without forming separate hydrophilic and hydrophobic regions. The salt 4bromoanilinium bromide could only be obtained as a hemihydrate; it too forms hydrophilic layers, but these are highly puckered.