4-Methoxybenzamidinium hydrogen oxalate monohydrate

The title hydrated salt, C8H11N2O+·C2HO4 −·H2O, was synthesized by a reaction of 4-methoxybenzamidine (4-amidinoanisole) and oxalic acid in water solution. In the cation, the amidinium group forms a dihedral angle of 15.60 (6)° with the mean plane of the benzene ring. In the crystal, each amidinium unit is bound to three acetate anions and one water molecule by six distinct N—H⋯O hydrogen bonds. The ion pairs of the asymmetric unit are joined by two N—H⋯O hydrogen bonds into ionic dimers in which the carbonyl O atom of the semi-oxalate anion acts as a bifurcated acceptor, thus generating an R 1 2(6) motif. These subunits are then joined through the remaining N—H⋯O hydrogen bonds to adjacent semi-oxalate anions into linear tetrameric chains running approximately along the b axis. The structure is stabilized by N—H⋯O and O—H⋯O intermolecular hydrogen bonds. The water molecule plays an important role in the cohesion and the stability of the crystal structure being involved in three hydrogen bonds connecting two semi-oxalate anions as donor and a benzamidinium cation as acceptor.

The title hydrated salt, C 8 H 11 N 2 O + ÁC 2 HO 4 À ÁH 2 O, was synthesized by a reaction of 4-methoxybenzamidine (4-amidinoanisole) and oxalic acid in water solution. In the cation, the amidinium group forms a dihedral angle of 15.60 (6) with the mean plane of the benzene ring. In the crystal, each amidinium unit is bound to three acetate anions and one water molecule by six distinct N-HÁ Á ÁO hydrogen bonds. The ion pairs of the asymmetric unit are joined by two N-HÁ Á ÁO hydrogen bonds into ionic dimers in which the carbonyl O atom of the semioxalate anion acts as a bifurcated acceptor, thus generating an R 1 2 (6) motif. These subunits are then joined through the remaining N-HÁ Á ÁO hydrogen bonds to adjacent semioxalate anions into linear tetrameric chains running approximately along the b axis. The structure is stabilized by N-HÁ Á ÁO and O-HÁ Á ÁO intermolecular hydrogen bonds. The water molecule plays an important role in the cohesion and the stability of the crystal structure being involved in three hydrogen bonds connecting two semi-oxalate anions as donor and a benzamidinium cation as acceptor.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: KP2440).

4-Methoxybenzamidinium hydrogen oxalate monohydrate Simona Irrera and Gustavo Portalone Comment
For some time now, we have studied proton-transfer adducts containing molecules of biological interest (Portalone, 2011a;Portalone & Irrera, 2011). In this context, benzamidine derivatives, which have shown strong biological and pharmacological activity (Powers & Harper, 1999), have been used in our group as bricks for supramolecular construction (Portalone, 2010(Portalone, , 2011b(Portalone, , 2012. Indeed, the bidentate hydrogen-bonding interaction between the amidinium and the carboxylate functional groups can be a powerful organizing force in solution and in the solid state. The present study reports the single-crystal structure of the title molecular salt, 4-methoxybenzamidinium hydrogen oxalate monohydrate, (I), which was obtained by a reaction of 4-methoxybenzamidine (4-amidinoanisole) and oxalic acid in a water solution.
The asymmetric unit of the title compound comprises a non-planar 4-methoxybenzamidinium cation, a hydrogen oxalate anion and water molecule of crystallization ( Fig. 1).
In the cation the amidinium group forms dihedral angle of 15.60 (6)° with the mean plane of the phenyl ring, which agrees with the values observed in protonated benzamidinium ions [14.4 (1) -32.7 (1)°, Portalone, 2010Portalone, , 2012Irrera et al., 2012)]. 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. This conformation is rather common in benzamidinium-containing small-molecule crystal structures, with the only exception of benzamidinium diliturate, where the benzamidinium cation is planar (Portalone, 2010). Geometrical parameters of the 4-methoxybenzamidinium cation agree with those reported in previous investigations of other similar structures (Irrera et al., 2012;Portalone, 2010Portalone, , 2012Irrera & Portalone, 2012a, 2012b, 2012c. In particular the amidinium group, true to one's expectations, features similar C-N bonds [1.317 (3) and 1.302 (2) Å], evidencing the delocalization of the π electrons and double-bond character.
The semi-oxalate anion is not planar, as the dihedral angle for the planes defined by the CO 2 H and CO 2non-H atoms is 14.1 (3)°. Bond distances around atom C10 indicate a carboxylate group with delocalization of the negative charge between atoms O5 and O6. Bond distances around atom C9 are consistent with a carboxylic acid group.
In the crystal structure of (I), (Fig. 2), the hydrogen-bonding scheme is rather complex. Each amidinium unit is bound to three acetate anions and one water molecule by six distinct N-H···O intermolecular hydrogen bonds (N···O = 2.827 (2) -3.180 (3) Å, Table 1) into a one-dimensional structure. The ion pairs of the asymmetric unit are joined by two N-H···O hydrogen bonds in ionic dimers, where the carbonyl atom O3 of the semi-oxalate anion acts as a bifurcated acceptor, thus generating an R 1 2 (6) motif (Bernstein et al., 1995). These subunits are then joined through the remaining N-H···O hydrogen bonds to adjacent semi-oxalate anions into linear tetrameric chains running approximately along crystallographic b axis.
Water molecule plays an important role in the cohesion and the stability of the crystal structure: they are involved in three hydrogen bonds connecting two semi-oxalate anions as donor (O2W-H21W···O5 and O2W-H22W···O6) and a benzamidinium cation as acceptor O2W···H1B-N1 (Table 1).
Experimental 4-Methoxybenzamidine (0.01 mmol, Fluka at 96% purity) was dissolved without further purification in 6 mL of a hot aqueous solution of oxalic acid (0.01 mmol, Aldrich at 99.99% purity) and heated under reflux for 3 h. After cooling the solution to an ambient temperature, colourless crystals suitable for single-crystal X-ray diffraction separated from the solution after a week.

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 = 0.93 Å (phenyl) and 0.97 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). Positional and thermal parameters of H atoms of the amidinium and the carboxylic groups were refined, giving N-H distances in the range 0.86 (2) (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 2012); software used to prepare material for publication: WinGX (Farrugia, 2012).

Figure 1
The asymmetric unit of (I), showing the atom-labelling scheme. Displacements ellipsoids are at the 50% probability level.    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.