Tetramethylammonium hydrogen terephthalate

The asymmetric unit of the title salt, C4H12N+·C8H5O4 −, contains one half of a tetramethylammonium cation and one half of a hydrogen terephthalate monoanion. The N atom of the ammonium cation lies on a twofold rotation axis and the centre of mass of the terephthalate anion is on a centre of inversion. In the crystal, the centrosymmetric terephthalate ions are linked by a very short symmetric O—H⋯O hydrogen bond [O⋯O = 2.4610 (19) Å] into a one-dimensional polymeric chain along [1-12]. The tetramethylammonium cations and terephthalate anions are then connected through a pair of bifurcated acceptor C—H⋯O hydrogen bonds, generating a three-dimensional supramolecular network. The carboxylate groups at both ends of the terephthalate anion are charge-shared with an equal probability of 0.5.

The asymmetric unit of the title salt, C 4 H 12 N + ÁC 8 H 5 O 4 À , contains one half of a tetramethylammonium cation and one half of a hydrogen terephthalate monoanion. The N atom of the ammonium cation lies on a twofold rotation axis and the centre of mass of the terephthalate anion is on a centre of inversion. In the crystal, the centrosymmetric terephthalate ions are linked by a very short symmetric O-HÁ Á ÁO hydrogen bond [OÁ Á ÁO = 2.4610 (19) Å ] into a one-dimensional polymeric chain along [112]. The tetramethylammonium cations and terephthalate anions are then connected through a pair of bifurcated acceptor C-HÁ Á ÁO hydrogen bonds, generating a three-dimensional supramolecular network. The carboxylate groups at both ends of the terephthalate anion are charge-shared with an equal probability of 0.5.

Å) in their crystal structures
The title compound (Fig. 1), [N(CH 3 ) 4 ] + [4-COOH-C 6 H 4 COO] -, consists of one half tetramethylammonium cation and one half terephthalate anion in the asymmetric unit. The cation lies on a twofold rotation axis and the anion on an inversion center. In the terephthalate anions, the two carboxyl groups are twisted from the mean plane of the benzene ring by a dihedral angle of 6.57 (2)°. Carboxyl atom O2 lies slightly farther [0.083 Å] from this plane than atom O1 [0.065 Å], owing to the strong O-H···O hydrogen bond between the terephthalate anions.
In the crystal, the terephthalate anions are linked end-to-end to form a one-dimensional polymeric chain in which adjacent ions are interconnected by a strong symmetric O-H···O (O···O distance of 2.4610 (19) Å) hydrogen bond (Table 1). Then the weak C-H···O hydrogen bonds link the ammonium cations and terephthalate anions together in a three-dimensional crystal structure. The C8-H8A···O1 and C9-H9A···O1 interactions form a pair of bifurcated acceptor bonds (Fig. 2), involving two C-H donor from an ammonium ion and an acceptor O atom from the terephthalate ion, generating an R 2 1 (6) ring motif (Etter et al., 1990;Bernstein et al., 1995).

Experimental
In an attempt to synthesize metal-organic framework materials, we obtained the title compound as a side-product.
The ligand H 2 L [5,5′-(o-Phenylene)di-1H-tetrazole] used in this work was synthesized according to literature procedures (Demko & Sharpless, 2001 octane (0.045 g, 0.4 mmol) and CH 3 OH/DMF (1/2, 15 ml) were sealed in a 25 ml Teflon-lined stainless steel autoclave, heated at 433 K for 40 h, and then cooled to room temperature over a period of 90 h. The resulting solution was filtered and the filtrate was allowed to stand in air at room temperature. After several days, colorless single crystals of the title compound were isolated.

Refinement
C-H atoms were located on a ΔF map, further idealized and finally refined in the riding model aproximation d(C-H) = 0.93Å; U(H) = 1.2U(C) eq ; d(C-H 3 ) = 0.96Å; U(H) = 1.5U(C) eq . Atom H2 is fixed by symmetry, and its isotropic displacement factor was freely refined.

Figure 1
ORTEP drawing of the asymmetric unit of title compound with the atom numbering. Displacement ellipsoids are drawn at the 50% probability level. The occupancy factor for atom H2 is 0.5. Symmetry codes: (i) -x + 1, -y + 2, -z + 1; (ii) -x + 1, y, -z + 1/2.  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.57 e Å −3 Δρ min = −0.43 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.  (10)