Neutral and ionic supramolecular structures of unsaturated dicarboxylic acids and acridine: Significance of molecular geometry and proton transfer

Six supramolecular structures exhibiting distinct architectures have been prepared from acridine and different dicarboxylic acids. The co-crystallization of acridine (1) and configurational isomers of dicarboxylic acids such as cis,transmuconic (a) and trans, trans-muconic acid (b), or fumaric (c) and maleic acid (d) provided strikingly different packing motifs and properties. The supramolecular structures have been rationalized based on the molecular geometry and the constituent synthons. Co-crystallization of acridine and b, c, or terephthalic acid (e) gave neutral assemblies stabilized by the O-H...N hydrogen-bond synthon I. Acridine was found to undergo proton transfer with a, d, and acetylenedicarboxylic acid (f) to form ionic supramolecular structures exhibiting synthon II, i.e. charge-assisted carboxylate-acridinium O-...H-N+-hydrogen bonding. The packing mode of 1(+).a(-) has been inferred from the corresponding acridine-isophthalic acid analog based on the superimposable structure of the acids and the common charge-assisted hydrogen bonding motif. Acetylenedicarboxylic and cis,trans-muconic acid favor the formation of antiparallel acridinium pi-stacks stabilized by alternating hydrogen bonds with two infinite anionic chains of dicarboxylic acids that also undergo head-to-tail hydrogen bonding. Because of their different structure and geometry, b, c, d, and a form less uniform packing arrangements with 1. Fumaric and trans traps-muconic acid afford 2:1 neutral co-crystals with parallel acridine rings that are stabilized by pi-pi-interactions, whereas terephthalic acid favors association of antiparallel acridine dimers to adopt a different one-dimensional network. By contrast, d undergoes intramolecular hydrogen bonding which leaves only one carboxyl group available to form a 1:1 ionic supramolecular structure with 1. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).