4-Methoxybenzamidinium 2,6-dimethoxybenzoate

The title compound, C8H11N2O+·C9H9O4 −, was synthesized by the reaction of 4-methoxybenzamidine (4-amidinoanisole) and 2,6-dimethoxybenzoic acid. The structure consists of non-planar pairs of hydrogen-bonded 4-methoxybenzamidinium cations and 2,6-dimethoxybenzoate anions. In the cation, the amidinium group is tilted by 27.94 (10)° with respect to the benzene ring. In the anion, the sterically bulky ortho-methoxy substituents force the carboxyate group to be twisted away from the plane of the benzene ring by 73.24 (6)°. The ions are further associated in the crystal into chains along the b-axis direction by intermolecular N—H⋯O hydrogen bonds.


Comment
This Laboratory is currently engaged in the systematic structural analysis of molecular salts of benzamidine (Portalone, 2008(Portalone, , 2010. Benzamidine is a strong Lewis base and its cation can be easily anchored onto numerous inorganic and organic anions and polyanions, largely because of the presence of four potential donor sites for hydrogen-bonding. Consequently, benzamidinium ions appear to be very promising building blocks in supramolecular chemistry as multiple hydrogen bonding donor. Benzamidine has biological and pharmacological relevance (Powers & Harper, 1999;Grzesiak et al., 2000). Its cation has also been included in a number of protein structures (Marquart et al., 1983;Sprang et al., 1987;Bode et al., 1990).
The asymmetric unit of the title compound comprises a non-planar R 2 2 (8) hydrogen-bonded pair formed by one 4-methoxybenzamidinium cation and one 2,6-dimethoxybenzoate anion (Fig. 1). In the anion, the o-methoxy substituents force the carboxy group to be twisted away from the plane of the phenyl ring by 73.24 (6)°. The C-O distances of the carboxylate group range from 1.2440 (18) to 1.2509 (18) Å, indicating the delocalization of the negative charge. The pattern of bond lengths and bond angles of the phenyl ring is consistent with that reported in the structure determination of the two polymorphs of 2,6-dimethoxybenzoic acid (Swaminathan et al., 1976;Bryan & White, 1982;Portalone, 2009Portalone, , 2011, and a comparison of the present results with those obtained for similar benzene derivatives in the gas phase (Schultz et al., 1993;Portalone et al., 1998) shows no appreciable effects of the crystal environment on the ring deformation induced by substituents. In the cation, the amidinium group forms a dihedral angle of 27.94 (10)° with the phenyl ring, which is close to the values observed in neutral benzamidine (22.8°, Barker et al., 1996) as well as in protonated benzamidinium (20.7°, Papoutsakis et al., 1999;28.5 and31.9°, Portalone, 2008, 2010) and benzdiamidine (24. 5°, Jokić et al., 2001). The lack of planarity in all these systems is obviously caused by steric hindrances between the H atoms of the aromatic ring and the amidine moiety. The pattern of bond lengths and bond angles of the benzamidinium cation agrees with that reported in previous structural investigations (Papoutsakis et al., 1999;Portalone, 2008Portalone, , 2010. In particular the amidinium group, true to one's expectations, features similar C-N bonds [1.299 (2) and 1.316 (2) Å], evidencing the delocalization of the π electrons and double-bond character.
Analysis of the crystal packing (Fig. 2) shows that the pairs are associated in the crystal by extensive hydrogen bonding.
Those pairs of cations and anions are joined by intermolecular R 4 4 (8) N-H···O interactions (Etter et al., 1990;Bernstein et al., 1995;Motherwell et al., 1999) (Table 1) leading to a chain structure running along the b direction.

Experimental
The title compound was formed during cocrystallization in a 1:1 molar ratio of 2,6-dimethoxybenzoic acid (1 mmol, Aldrich at 98% purity) and 4-methoxybenzamidine (1 mmol, Fluka at 96% purity). The two components were dissolved in water (10 ml) and gently heated under reflux for 3 h. After cooling the solution to an ambient temperature, colourless crystals suitable for single-crystal X-ray diffraction were grown by slow evaporation of the solvent after two weeks.

Refinement
The amine H atoms were located in a difference Fourier map and refined freely. All other H atoms could be identified in difference Fourier maps, but were placed in calculated positions, with C-H = 0.97 Å (phenyl) and 0.99 Å (methyl), and refined as riding on their carrier atoms. The U iso values were kept equal to 1.2U eq (C) or 1.5U eq (C) for methyl H atoms. Fig. 1. The asymmetric unit of the title compound, showing displacements ellipsoids drawn the 50% probability level. H atoms are shown as small spheres of arbitrary radii. [amino(4-methoxyphenyl)methylidene]azanium 2,6-dimethoxybenzoate  Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.  (2) 170 (2) Symmetry codes: (i) −x+1, y+1/2, −z+1/2; (ii) −x+1, y−1/2, −z+1/2.