4-Methoxybenzamidinium hydrogen sulfate

The title salt, C8H11N2O+·HSO4 −, has been synthesized by the reaction between 4-methoxybenzamidine and sulfuric acid. The asymmetric unit comprises a nonplanar 4-methoxybenzamidinium cation and one hydrogen sulfate anion. In the cation, the amidinium group has two identical C—N bonds [1.306 (2) and 1.308 (2) Å], and its plane forms a dihedral angle of 6.49 (8)° with the mean plane of the benzene ring. The ionic components are associated in the crystal via N—H+⋯O−, resulting in chains running approximately along the b-axis direction whicg are interconnected by O—H⋯O− hydrogen bonds.

Benzamidine derivatives, which have shown strong biological and pharmacological activity (Powers & Harper, 1999;Grzesiak et al., 2000), are being used in our group as bricks for supramolecular construction (Portalone, 2010(Portalone, , 2011b(Portalone, , 2012. Indeed, these molecules are strong Lewis bases 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. The asymmetric unit of (I) comprises one non-planar 4-methoxybenzamidinium cation and one hydrogen sulfate anion ( Fig. 1). In the cation the amidinium group forms dihedral angle of 6.49 (8)° with the mean plane of the phenyl ring, which is close to the the values observed in protonated benzamidinium ions (14.4 (1) -32.7 (1)°, Portalone, 2010Portalone, , 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). The pattern of bond lengths and bond angles of the 4-methoxybenzamidinium cation agrees with that reported in previous structural investigations Portalone, 2010Portalone, , 2012. In particular the amidinium group, true to one's expectations, features identical C-N bonds within experimental error [1.306 (2) and 1.308 (2) Å], evidencing the delocalization of the π electrons and double-bond character.
Bond lengths in the slightly distorted tetrahedral hydrogen sulfate anion indicate the position of the H atom. There are three short S-O bonds of 1.4504 (13), 1.4442 (13) and 1.4464 (13) Å to terminal atoms O2, O4 and O5, respectively, and one longer bond of 1.5470 (15) Å to atom O3, which is bound to atom H3A.
The ionic components of compound (I) are joined by two N + -H···O -(±) hydrogen bonds (Table 1) to form ionic dimers with graph-set motif R 2 2 (8) (Bernstein et al., 1995). Analysis of the crystal packing of (I), (Fig. 2), shows that four N + -H···Ohydrogen bonds link the molecular components into a mono-dimensional structure. As previously mentioned, each subunit, built from the ion pairs of the asymmetric unit, forms R 2 2 (8) dimers via the bidentate interaction of the N-H and S-O groups. Adjacent ion pairs are then linked together by way of the remaining two N + -H···Ohydrogen bonds to form R 2 4 (8), resulting in chains running approximately along crystallographic b axis. These chains are then interconnected by means of the only O-H···O hydrogen bond present in the structure. slow evaporation of the solvent after one 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 group and of the hydrogen sulfate ion were freely refined, giving N-H distances in the range 0.84 (2)-0.86 (2) Å and an O-H distance equal to 0.78 (3) Å.

Figure 1
The asymmetric unit of (I), showing displacements ellipsoids drawn at the 50% probability level. The asymmetric unit     (Oxford Diffraction, 2006); empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm] 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.