2,4,6-Trimethylpyridinium dihydrogen phosphate

The asymmetric unit of the title compound, C8H12N+·H2PO4 −, contains two H2PO4 − anions and two 2,4,6-trimethylpyridinium cations. In the crystal, the anions are linked by O—H⋯O hydrogen bonds, forming supramolecular chains running along the a axis; the cations are connected to the anion chains by N—H⋯O hydrogen bonds. Weak intermolecular C—H⋯O hydrogen bonding is also present in the crystal structure.

The asymmetric unit of the title compound, C 8 H 12 N + ÁH 2 PO 4 À , contains two H 2 PO 4 À anions and two 2,4,6-trimethylpyridinium cations. In the crystal, the anions are linked by O-HÁ Á ÁO hydrogen bonds, forming supramolecular chains running along the a axis; the cations are connected to the anion chains by N-HÁ Á ÁO hydrogen bonds. Weak intermolecular C-HÁ Á ÁO hydrogen bonding is also present in the crystal structure.

Comment
Salts of pyridine attracted more attention as phase transition dielectric materials for its application in memory storage (Fu et al. 2007;Fu & Xiong 2008;Fu et al. 2008;Fu et al. 2009). With the purpose of obtaining phase transition crystals of 2,4,6-trimethylpyridine salts, its 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, di-2,4,6-trimethylpyridinium dihydrogen phosphate.
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 (425 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 6.9 to 8.7).
The asymmetric unit is composed of two H 2 PO 4 anion and two C 8 H 12 N + cation ( Fig.1). Both the pyridine N atoms are protonated, thus indicating two positive charges in the pyridine N atoms. And the H 2 PO 4 anions were showing two negative charges to make the charge balance. The geometric parameters of the title compound are in the normal range.
In the crystal structure, all the H atoms of pyridinium cations and the H 2 PO 4 anions are involved in N-H···O and O-H···O hydrogen bonds. These hydrogen bonds link the ionic units into a one-dimentional chains parallel to the a-axis.

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 needle-shaped crystals of the title compound suitable for X-ray analysis.