Supramolecular association in proton-transfer adducts containing benzamidinium cations. II. Concomitant polymorphs of the molecular salt of 2,6-dimethoxybenzoic acid with benzamidine

Abstract

Two concomitant polymorphs of the molecular salt formed by 2,6-dimethoxybenzoic acid, C9H10O4(Dmb), with benzamidine, C7H8N2(benzenecarboximidamide, Benzam) from water solution have been identified. Benzamidinidium 2,6-dimethoxybenzoate, C7H9N2+·C9H9O4−(BenzamH+·Dmb−), was obtained through protonation at the imino N atom of Benzam as a result of proton transfer from the acidic hydroxy group of Dmb. In the monoclinic polymorph, (I) (space groupP21/n), the asymmetric unit consists of two Dmb−anions and two monoprotonated BenzamH+cations. In the orthorhombic polymorph, (II) (space groupP212121), one Dmb−anion and one BenzamH+cation constitute the asymmetric unit. In both polymorphic salts, the amidinium fragments and carboxylate groups are completely delocalized. This delocalization favours the aggregation of the molecular components of these acid–base complexes into nonplanar dimers with anR22(8) graph-set motifviaN+—H...O−charge-assisted hydrogen bonding. Both the monoclinic and orthorhombic forms exhibit one-dimensional isostructurality, as the crystal structures feature identical hydrogen-bonding motifs consisting of dimers and catemers.