Dimethylammonium 3-carboxybenzoate

The asymmetric unit of the title organic salt, C2H8N+·C8H5O4 −, consists of two dimethylammonium cations and two 3-carboxybenzoate anions. The 3-carboxybenzoate anions are linked via strong intermolecular and nearly symmetrical O—H⋯O hydrogen bonds forming infinite chains parallel to [111]. Neighbouring chains are further connected by the dimethylammonium cations via N—H⋯O bonds, resulting in a double-chain-like structure. The dihedral angles of all carboxylate groups with respect to the phenylene rings are in the range 7.9 (1)–20.48 (9)°.

The asymmetric unit of the title organic salt, C 2 H 8 N + Á-C 8 H 5 O 4 À , consists of two dimethylammonium cations and two 3-carboxybenzoate anions. The 3-carboxybenzoate anions are linked via strong intermolecular and nearly symmetrical O-HÁ Á ÁO hydrogen bonds forming infinite chains parallel to [111]. Neighbouring chains are further connected by the dimethylammonium cations via N-HÁ Á ÁO bonds, resulting in a double-chain-like structure. The dihedral angles of all carboxylate groups with respect to the phenylene rings are in the range 7.9 (1)-20.48 (9) .
Data collection: APEX2 (Bruker, 2011); cell refinement: SAINT (Bruker, 2011); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXLE (Hü bschle et al., 2011) and SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2001); software used to prepare material for publication: publCIF (Westrip, 2010 The title compound, dimethylammonium 3-carboxybenzoate, was obtained as part of our investigations into the solvothermal synthesis of metal organic frameworks of magnesium with aromatic dicarboxylates. Reaction of magnesium nitrate with isophthalic acid and piperazine at 373 K did not yield an extended metal organic framework, but partial decomposition of the DMF solvent let to formation of the title dimethylammonium organic salt, which was isolated as a minor side product in the form of colourless plate-like crystals. Under the rather harsh solvothermal conditions used for the synthesis of many coordination compounds and metal organic frameworks formamides become unstable towards hydrolysis or Lewis acid catalyzed decarbonylation (Hine et al., 1981;Cottineau et al., 2011). This is also evidenced by the high number of dimethyl ammonium salts reported in the Cambridge Structural Database (CSD; Allen et al., 2002;597 entries up to Feb. 2012), counting only structures that also contain at least one metal ion. With dimethyl amine itself being a gas and being used not extensively as a reagent, it can be safely assumed that most of these structures originated from in situ hydrolysis of a dimethyl amide such as DMF. Twenty eight of these entries in the CSD with dimethyl ammonium ions also contain formate ions, the other product of DMF hydrolysis. The title compound, the dimethylammonium salt of isophtalic acid, is one such example that incorporates ammonium cations formed in situ through decomposition of a formamide.
The asymmetric unit of the title compound is composed of two hydrogen-3-carboxybenzoate anions and two dimethyl ammonium cations ( (Table 1), as is typical for very strong hydrogen bonds with very electronegative donor and acceptor atoms (Gilli & Gilli, 2009). The keto oxygen atoms of the carboxylate units, which are not involved in the O-H···O hydrogen bonds, act as acceptors for N-H···O hydrogen bonds that originate from both of the dimethylammonium cations, which double bridge the carboxylic acid and carboxylate groups of the anions into a bis(dimethylammonium)-bis(COO -···H + ··· -OOC) cluster (Fig. 3). In such a manner parallel infinite 3carboxybenzoate chains are connected into an inversion symmetric double chain like structure (Fig. 4). Supramolecular structures comprising 3-carboxybenzoates have been reported previously (Guo et al., 2010;Liu et al., 2007;Weyna et al., 2009). Similar one-dimensional chain-like structures have been reported by (Ballabh et al., 2005).

Experimental
The compound was synthesized under solvothermal conditions. In a typical synthesis, Mg(NO 3 ) 2 .6H 2 O (0.064 g, 0.25 mmol) was dissolved in a 1:1 mixture of DMF (5.0 ml) and EtOH (5.0 ml). Then, alumina (Sorbent Technologies, Atlanta, GA) (0.051 g, 0.5 mmol), isophthalic acid (0.166 g, 1.0 mmol) and piperazine (0.043 g, 0.5 mmol) were added to the reaction mixture which was stirred for one hour before transferring the mixture into a glass vial. The final mixture was heated to 373 K for 48 h. The vial was then slowly cooled to room temperature. Slow cooling of the reaction mixture yielded colourless plate-like crystals of the title compound as a minor product.

Refinement
Hydrogen atoms were placed in calculated positions with C-H bond distances of 0.95 Å (aromatic H), 0.99 Å (methyl H) or 0.88 Å (N-H). Methyl group H atoms were allowed to rotate around the C-C bond to best fit the experimental electron density. Carboxylic acid hydrogen atoms were located in difference electron density maps, but were placed in calculated positions with fixed C-O-H angles, but with the C-C-O-H dihedral angles and the O-H distances freely refined (AFIX 148 command in SHELXTL (Sheldrick, 2008)). U iso (H) values for all H atoms were constrained to a multiple of U eq of their respective carrier atom (1.2 times for aromatic and ammonium H atoms, 1.5 times for methyl and carboxylic acid H atoms).

Figure 1
View of the asymmetric unit with the atom-numbering scheme and 50% probability displacement ellipsoids.    Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'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 > 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.