4-Methoxybenzamidinium nitrate

The title salt, C8H11N2O+·NO3 −, was synthesized by a reaction between 4-methoxybenzamidine (4-amidinoanisole) and nitric acid. The asymmetric unit comprises a non-planar 4-methoxybenzamidinium cation and a nitrate anion. In the cation, the amidinium group has two similar C—N bond lengths [1.302 (3) and 1.313 (3) Å] and its plane forms a dihedral angle of 32.66 (5)° with the mean plane of the benzene ring. The nitrate–amidinium ion pair is not planar, as the dihedral angle between the planes defined by the CN2 + and NO3 − units is 19.28 (6)°. The ionic components are associated in the crystal via N—H⋯O hydrogen bonds, resulting in a three-dimensional network.

, was synthesized by a reaction between 4-methoxybenzamidine (4-amidinoanisole) and nitric acid. The asymmetric unit comprises a non-planar 4-methoxybenzamidinium cation and a nitrate anion. In the cation, the amidinium group has two similar C-N bond lengths [1.302 (3) and 1.313 (3) Å ] and its plane forms a dihedral angle of 32.66 (5) with the mean plane of the benzene ring. The nitrate-amidinium ion pair is not planar, as the dihedral angle between the planes defined by the CN 2 + and NO 3 À units is 19.28 (6) . The ionic components are associated in the crystal via N-HÁ Á ÁO hydrogen bonds, resulting in a three-dimensional network.

4-Methoxybenzamidinium nitrate Simona Irrera and Gustavo Portalone Comment
Following our on-going interest on systematic structural analysis of proton-transfer adducts containing molecules of biological interest (Portalone, 2011a;Portalone & Irrera, 2011), 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. We report here the crystal structure of the title compound, 4-methoxybenzamidinium nitrate, which was obtained by a reaction between 4-methoxybenzamidine (4-amidinoanisole) and nitric acid.
The asymmetric unit of the title compound comprises one non-planar 4-methoxybenzamidinium cation and one nitrate anion ( Fig. 1).
In the cation, the amidinium group forms dihedral angle of 32.66 (5)° with the mean plane of the phenyl ring, which is close to the the values observed in protonated benzamidinium ions (14.4 (1) -30.4 (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 group. 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 similar C-N bonds [1.302 (3) and 1.313 (3) Å], evidencing the delocalization of the π electrons and double-bond character. The molecular parameters for the nitrate ion are in the expected range.
The ionic components of compound (I) are joined by two N + -H···O -(±) hydrogen bonds to form ionic dimers with graph-set motif R 2 2 (8) (Bernstein et al., 1995). Remarkably, at variance with the well known carboxylic dimer R 2 2 (8) motif, the nitrate-amidinium pair is not planar, as the dihedral angles for the planes defined by the CN 2 + and NO 3atoms is 19.28 (6)°. Similar deviation from planarity has been previously observed in the carboxylate-amidinium pair of benzamidinium 2,6-dimethoxybenzoate .
Analysis of the crystal packing of the title compound (Fig. 2) shows that each amidinium unit is bound to three nitrate anions by five distinct N-H + ···Ointermolecular hydrogen bonds (N + ···O -= 2.850 (3) -3.174 (3) Å, Table 1). Four cation-to-anion N + ···Ointeractions form two types of hydrogen-bonded rings: the R 2 1 (4), which indicates a bifurcated hydrogen bond from N2-H2b to the two acceptors (O1, O3), and the already mentioned R 2 2 (8) supramolecular synthon. Both these ring motifs lead to the formation of ribbons approximately along crystallographic b axis. These ribbons are then interconnected by means of the remaining N + ···Ointermolecular hydrogen bond. Experimental 4-methoxybenzamidine (0.1 mmol, Fluka at 96% purity) was dissolved without further purification in 6 ml of hot water and heated under reflux for 3 h. While stirring, HNO 3 (4 mol L -1 ) was added dropwise until pH reached 2. 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 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 0.96 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 were freely refined, giving N-H distances in the range 0.84 (3) -0.91 (2) Å. In the absence of significant anomalous scattering in this light-atom study of the title compound, Friedel pairs were merged prior to refinement.

Refinement
Refinement on F 2 Least-squares matrix: full R[F 2 > 2σ(F 2 )] = 0.038 wR(F 2 ) = 0.083 S = 0.98 1253 reflections 154 parameters 0 restraints Primary atom site location: structure-invariant direct methods Secondary atom site location: difference Fourier map Hydrogen site location: inferred from neighbouring sites H atoms treated by a mixture of independent and constrained refinement w = 1/[σ 2 (F o 2 ) + (0.0527P) 2 + 0.0001P] where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.12 e Å −3 Δρ min = −0.20 e Å −3 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq O4 0.3500 (