Comparative Supramolecular Chemistry of Coronene, Hexahelicene, and [18]Crown-6: Hydrated and Solvated Molecular Complexes of [18]Crown-6 with 5-Hydroxyisophthalic Acid and Related Di- and Tricarboxylic Acids

Abstract

The outer rim of C-H bonds of coronene (COR) and hexahelicene (HEL) is similar to that of the crown conformation of [18]crown-6 (CRO), which is exploited for crystal engineering of molecular complexes of CRO. However, although CRO does form the adduct (TMA)2 x CRO x (H2O)2 (TMA = trimesic acid = 1,3,5-benzenetricarboxylic acid), its structure does not correspond to the H-bonded, three-connected honeycomb sheet architectures of (TMA)2 x COR and (TMA)2 x HEL. Instead, porous, but noninterpenetrating, H-bonded four-connected sheets are observed, with the dihydrated, crown-shaped CRO molecules functioning as spacers rather than molecular guests. In the adduct (CHTA)2 x CRO x (H2O)5 (CHTA = cis,cis-1,3,5-cyclohexanetricarboxylic acid), the tetrahydrated CRO molecules again take up the crown conformation and act as spacers, this time within porous, noninterpenetrating H-bonded three-connected sheets. The engineering goal of CRO-filled H-bonded, hexagonal honeycomb cavities similar to the COR- and HEL-filled TMA honeycomb pores in (TMA)2 x COR and (TMA)2 x HEL was met in the adduct (HIPA)6 x CRO x (H2O)10 (HIPA = 5-hydroxyisophthalic acid), crystallized from aqueous EtOH. The crystal structure of this complex is on the one hand built up of isolated hexagonal honeycomb cavities established by six HIPA molecules cyclically linked through pairwise intercarboxylic H bonds. These cavities accommodate the crown-shaped CRO molecules, oriented such that maximally straight C-H...O contacts are enabled between its 12 equatorial H atoms and the surrounding 12 carboxylic groups of HIPA, in complete analogy to the situation prevailing in (TMA)2 x HEL and (probably) (TMA)2 xCOR. The second building block of (HIPA)6 x CRO x (H2O)10 is represented by a centrosymmetric decameric water cluster, which has the connectivity of the carbon skeleton of a bishomocubane with opposite methylene bridges, in agreement with vibrational spectroscopic evidence on gaseous (H2O)10. The crystal architecture of the adduct as a whole may either be likened to a severely distorted NaCl-type lattice, with the (HIPA)6 x CRO units replacing, for example, the Na+ ions, and the water clusters substituting the Cl- ions, or else to a system of stacked host sheets set up by C-H...O bonded (HIPA)6 macrorings, which give rise to perpendicular channels taking up guest columns of alternating, H-bonded CRO and (H2O)10 units. Crystals of another, solvated HIPA-CRO adduct of the composition (HIPA)4 x CRO x (EtOH)2 were obtained from aqueous EtOH. Their crystal structure is related to those of (TMA)2 x HEL and (TMA)2 x COR inasmuch distorted HIPA honeycomb sheets are adopted, which may be developed from the hexagonal TMA sheets by replacing one third of the pairwise intercarboxylic linkages by single interphenolic H bonds. The cavities in the HIPA sheets are thus smaller than those of the TMA honeycomb sheets and elliptically shaped. The HIPA sheets associate in pairs yielding twin cavities which take up one CRO and two EtOH molecules. The CRO molecules are suspended in the twin HIPA cages through H bonds extended from the phenolic OH groups and relayed by interposed EtOH "bridges". In keeping with the elliptic shape of the pores in (HIPA)4 x CRO x (EtOH)2, the CRO molecules are not crown-shaped, but rather adopt the more rectangular form as observed in crystalline CRO itself. The crystal structure of a dihydrate of HIPA itself was analysed, too, which assembles in a complex three-dimensional H-bonded network. It is finally concluded that hydrated CRO appears to be an avid H-bond acceptor, in particular towards carboxylic acids functioning as H-bond donors.