2,4,6-Trimethylpyridinium dihydrogen phosphate

In the title compound, C8H9N+·H2PO4 −, both the cation and anion have crystallographically imposed mirror symmetry (all atoms apart from one O atom lie on the mirror plane). In the crystal, anions and cations are linked by O—H⋯O and π–π stacking interactions [centroid–centroid distance = 3.4574 (6) Å], forming chains parallel to the b axis. Adjacent chains are further connected by N—H⋯O hydrogen bonds into a two-dimensional network.

In the title compound, C 8 H 9 N + ÁH 2 PO 4 À , both the cation and anion have crystallographically imposed mirror symmetry (all atoms apart from one O atom lie on the mirror plane). In the crystal, anions and cations are linked by O-HÁ Á ÁO andstacking interactions [centroid-centroid distance = 3.4574 (6) Å ], forming chains parallel to the b axis. Adjacent chains are further connected by N-HÁ Á ÁO hydrogen bonds into a two-dimensional network.

S1. Comment
Salts of pyridine have attracted attention as phase transition dielectric materials for their applications in memory storage (Fu et al. 2007;Fu et al. 2009). With the purpose of obtaining new phase transition crystals of 2,4,6-trimethylpyridine salts, their interaction with various acids has been studied and we have elaborated a series of new materials with this organic molecule. In this study, we describe the crystal structure of the title compound, 2,4,6-trimethylpyridinium dihydrogen phosphate.
The asymmetric unit is composed of half an H 2 PO 4anion and half a C 8 H 9 N + cation ( Fig. 1), both anion and cation being located on a mirror plane. The geometric parameters are in the normal range. In the crystal structure, the anions are linked into chains parallel to the b axis by O-H···O hydrogen bonds (Table 1). The cations also are connected into chains along the b axis by π-π stacking interactions with centroid-to-centroid distances of 3.4574 (6) Å. The cationic and anionic chains further interact through N-H···O hydrogen bonds (Fig. 2), forming a two-dimensional network.

S2. Experimental
The commercial 2,4,6-trimethylpyridine (3 mmol) was dissolved in water/H 3 PO 4 (50:1 v/v) solution. The solvent was slowly evaporated in air affording colourless block-shaped crystals of the title compound suitable for X-ray analysis.
The dielectric constant of title compound as a function of temperature indicates that the permittivity is basically temperature-independent, suggesting that this compound should be not a real ferroelectrics or there may be no distinct phase transition occurred within the measured temperature range. Similarly, below the melting point (413 K) of the compound, the dielectric constant as a function of temperature also goes smoothly, and there is no dielectric anomaly observed (dielectric constant equaling to 6.6 to 8.9).

S3. Refinement
All H atoms attached to C and N atoms were fixed geometrically and treated as riding with C-H = 0.93-0.96 Å, N-H = 0.86 Å, and with U iso (H) = 1.2 U eq (C, N) or 1.5 U eq (C) for methyl H atoms. The H atom of the H 2 PO 4anion was located in difference Fourier maps and freely refined, with the O-H distance constrained to 0.86 Å..    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 > σ(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.