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

Two concomitant polymorphs of the molecular salt formed by 2,6-dimethoxybenzoic acid, C(9)H(10)O(4) (Dmb), with benzamidine, C(7)H(8)N(2) (benzenecarboximidamide, Benzam) from water solution have been identified. Benzamidinidium 2,6-dimethoxybenzoate, C(7)H(9)N(2)(+)·C(9)H(9)O(4)(-) (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 group P2(1)/n), the asymmetric unit consists of two Dmb(-) anions and two monoprotonated BenzamH(+) cations. In the orthorhombic polymorph, (II) (space group P2(1)2(1)2(1)), 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 an R(2)(2)(8) graph-set motif via N(+)-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.