Different solvents yield alternative crystal forms through aromatic, halogen bonding and hydrogen bonding competition

Abstract

Crystallisation of the racemic V-shaped diquinoline derivative 8 from aprotic dimethyl formamide yields a high symmetry solvent-free crystal structure in space group Fdd2. Fully eclipsed columns of enantiomerically pure molecules, joined by exo-face to endo-face C–H⋯π and C–H⋯Cl interactions, are produced. Despite most of the molecular surface being aromatic in nature, classic π⋯π associations are absent. Adjacent columns of opposite handedness are linked by means of N⋯Cl and C–H⋯Cl halogen bonds. In contrast, very different P2₁/c (8)·(solvent) adducts assemble from the protic solvents methanol, ethanol or acetic acid. These isostructural inclusion compounds contain one strong N⋯H–O hydrogen bond per host molecule, and two such (8)·(solvent) units assemble around an inversion centre to form a parallel fourfold aromatic embrace (P4AE) dimer. Its efficient internal endo,endo-facial π⋯π interaction is further supplemented by intra-dimer Cl⋯π and C–H⋯Cl associations. These P4AE units are repeated by translation and associate by means of exo,exo-facial C–H⋯Cl rather than π⋯π interactions, through Cl⋯π contacts, and by a suite of five host–guest C–H⋯O weak hydrogen bonds that supplement the N⋯H–O hydrogen bond. The (8)·(acetic acid) structure is notable for the guest carboxylic acid group acting as an alcohol mimic. In this role, the hydroxy group acts in the usual way as a hydrogen bond donor but it is the carbonyl oxygen that functions as the main acceptor atom. These observations illustrate the crucial role that crystallisation solvent choice, and the consequent competing intermolecular associations, play in the production of alternative crystal forms.