1,4-Dimethylpiperazin-1-ium 3-hydroxy-2-naphthoate

The reaction of 1,4-dimethylpiperazine and 3-hydroxy-2-naphthoic acid gives the title 1:1 salt, C6H15N2 +·C11H7O3 −, with a singly protonated piperazinium cation. In the crystal, a single N—H⋯O hydrogen bond links the cations and anions into discrete pairs and the aromatic anions stack along the crystallographic a-axis direction. This results in layers of cations and anions alternating along the crystallographic c-axis direction. An intramolecular O—H⋯O hydrogen bond is also present.

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: LH5414).

Comment
The influence of solid-state structure on salt selection in the pharmacy industry is of ongoing interest and importance (Stahl & Wermuth, 2002;Gould, 1986;Serajuddin, 2007). As a contribution towards this we have recently investigated series of salt structures based on both protonated tertiary amines and anions derived from aryl carboxylic acids (Arlin et al., 2011;Kennedy et al., 2011). Combining these two themes we present here the structure of 1,4-dimethylpiperazinium 3-hydroxy-2-naphthoate (I).
Reaction of equimolar amounts of 1,4-dimethylpiperazine and 3-hydroxy-2-naphthoic acid in an aqueous environment gave a 1:1 salt with the base monoprotonated to give 1,4-dimethylpiperazinium, see  Table 1. This relatively limited hydrogen bonding results only in discrete cation-anion pairs being formed with no extended hydrogen bonding network. The aromatic anions stack along the crystallographic a direction in an offset manner such that the closest contact is between C2 of the carboxylate substituted ring and C11 of the non-substituted ring (C2···C11′ = 3.576 (4) Å, ′ = x + 1, y, z). The polar orientation of neighbouring anion stacks is reversed along the crystallographic c direction and this results in alternating layers of anions and cations as shown in Fig. 2.

Experimental
Addition of an equimolar amount of 1,4-dimethylpiperazine to an aqueous slurry of 3-hydroxy-2-naphthoic acid with stirring and heating to 323 K gave a clear solution. After cooling to room temperature, colourless crystals of (I) were deposited after 3 days.

Refinement
All the H-atoms were placed in geometric positions and refined in riding modes. N-H and O-H distances were set to 0.91 and 0.83 Å respectively, with the best fit orientation of the OH group to observed electron density being found by allowing rotation about the C-O bond. C-H distances of 0.93, 0.96 and 0.97 Å were adopted for CH, CH 2 and CH 3 supplementary materials sup-2 Acta Cryst. (2012). E68, o787 groups respectively. U iso = 1.2U eq of riden atom, except for CH 3 and OH where U iso = 1.5U eq of riden atom.

Figure 1
Contents of the asymmetric unit of (I). Displacement ellipsoids are drawn at the 40% probability level.  Packing of (I) viewed along the a direction. Note the stacked anions down the a direction and the layers of cations and anions that alternate along the c direction. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.14 e Å −3 Δρ min = −0.16 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.046 (5) 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.