Benzamidinium 2-methoxybenzoate

The title molecular salt, C7H9N2 +.C8H7O3 −, was synthesized by reaction between benzamidine (benzenecarboximidamide) and 2-methoxybenzoic acid. In the cation, the amidinium group has two similar C—N bonds [1.3070 (17) and 1.3145 (16) Å] and is almost coplanar with the benzene ring, making a dihedral angle of 5.34 (12)°. In the anion, the methoxy substituent forces the carboxylate group to be twisted by 69.45 (6)° with respect to the plane of the aromatic fragment. In the crystal, the components are connected by two N+—H⋯O− (±)CAHB (charge-assisted hydrogen bonds), forming centrosymmetric ionic dimers with graph-set motif R 2 2(8). These ionic dimers are then joined in ribbons running along the b-axis direction by another R 4 2(8) motif involving the remaining N+—H⋯O− hydrogen bonds. Remarkably, at variance with the well known carboxylic dimer R 2 2(8) motif, the carboxylate–amidinium pair is not planar, the dihedral angle between the planes defined by the CN2 + and CO2 − atoms being 18.57 (12)°.

C 8 H 7 O 3 À , was synthesized by reaction between benzamidine (benzenecarboximidamide) and 2-methoxybenzoic acid. In the cation, the amidinium group has two similar C-N bonds [1.3070 (17) and 1.3145 (16) Å ] and is almost coplanar with the benzene ring, making a dihedral angle of 5.34 (12) . In the anion, the methoxy substituent forces the carboxylate group to be twisted by 69.45 (6) with respect to the plane of the aromatic fragment. In the crystal, the components are connected by two N + -HÁ Á ÁO À (AE)CAHB (charge-assisted hydrogen bonds), forming centrosymmetric ionic dimers with graph-set motif R 2 2 (8). These ionic dimers are then joined in ribbons running along the b-axis direction by another R 4 2 (8) motif involving the remaining N + -HÁ Á ÁO À hydrogen bonds. Remarkably, at variance with the well known carboxylic dimer R 2 2 (8) motif, the carboxylate-amidinium pair is not planar, the dihedral angle between the planes defined by the CN 2 + and CO 2 À atoms being 18.57 (12) .

Experimental
Data collection: CrysAlis PRO (Agilent, 2011); cell refinement: CrysAlis PRO; data reduction: CrysAlis PRO; program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: WinGX (Farrugia, 2012 The present study is a continuation of the work carried out in this Laboratory on proton-transfer adducts containing molecules of biological interest (Portalone, 2010(Portalone, , 2013, and deals with the single-crystal structure of the molecular salt, benzamidinium 2-methoxybenzoate, (I), obtained by a reaction between benzamidine (benzenecarboximidamide) and 2methoxybenzoic acid in water solution. Benzamidine derivatives, which have shown strong biological and pharmacological activity (Powers & Harper, 1999), are being used in this Laboratory as bricks for supramolecular construction (Portalone, 2010). Indeed, these molecules are strong Lewis base and their cations 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.
In the cation the amidinium group forms a dihedral angle of 5.34 (12) In the 2-methoxybenzoate anion the benzene ring is essentially planar and the methoxy substituent forces the carboxylate group to be twisted with respect to the plane of the aromatic fragment by 69.45 (6)°. In the anion bond lengths and bond angles of the benzene ring are in accord with corresponding values obtained for both the orthorhombic and tetragonal forms of 2,6-dimethoxybenzoic acid (Portalone, 2011 and reference therein) and 4-methoxybenzamidinium 2,6-dimethoxybenzoate (Portalone, 2012). The C-O distances of the carboxylate group, 1.2441 (16) and 1.2488 (15) Å, indicate the delocalization of the negative charge.
The molecular components of the molecular salt are connected by two N + -H···O -(±)CAHB hydrogen bonds to form ionic dimers with graph-set motif R 2 2 (8) (Bernstein et al., 1995). Furthermore, centrosymmetric ionic dimers are joined in ribbons running along the b axis by another R 2 4 (8) motif involving the remaining N + -H···Ohydrogen bonds (Fig. 2). Remarkably, at variance with the well known carboxylic dimer R 2 2 (8) motif, the carboxylate-amidinium pair is not planar, as the dihedral angle for the planes defined by the CN 2 + and CO 2atoms is equal to 18.57 (12)°.

Experimental
Equimolar amounts (0.1 mmol) of benzamidine (Fluka at 96% purity) and 2-methoxybenzoic acid (Aldrich at 99% purity) were dissolved without further purification in 6 ml of hot water and heated under reflux for 6 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
All H atoms were identified in difference Fourier maps, but for refinement all C-bound H atoms were placed in calculated positions, with C-H = 0.97 Å (phenyl) and 1.01 Å (methyl), and refined as riding on their carrier atoms. The U iso values were kept equal to 1.2U eq (C, phenyl). and to 1.5U eq (C, methyl). The hydrogen atoms of the methyl group were allowed to rotate with a fixed angle around the C-C bond to best fit the experimental electron density [HFIX 138 in the SHELX program suite (Sheldrick, 2008)]. Positional and thermal parameters of H atoms of the amidinium group were freely refined, giving N-H distances in the range 0.86 (2) -0.96 (2) Å.

Figure 1
The asymmetric unit of (I), showing the atom-labelling scheme. Displacements ellipsoids are at the 50% probability level.  Crystal packing diagram for (I), viewed approximately down a. All atoms are shown as small spheres of arbitrary radii.
For the sake of clarity, H atoms not involved in hydrogen bonding have been omitted. Hydrogen bonding is indicated by dashed lines. Special details Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes. 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 R-factors(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.