2-Methylpropan-2-aminium 2-(methoxycarbonyl)benzoate

In the title compound, C4H12N+·C9H7O4 −, two C atoms and the N atom of the cation lie on a mirror plane, while all the atoms of the anion are disordered about a mirror plane. In the crystal, N—H⋯O hydrogen bonds link the components into chains along [010]. In the anion, the mean planes of the methoxycarbonyl and carboxylate groups form dihedral angles of 83.0 (2) and 83.2 (2)°, respectively, with the aromatic ring.

In the title compound, C 4 H 12 N + ÁC 9 H 7 O 4 À , two C atoms and the N atom of the cation lie on a mirror plane, while all the atoms of the anion are disordered about a mirror plane. In the crystal, N-HÁ Á ÁO hydrogen bonds link the components into chains along [010]. In the anion, the mean planes of the methoxycarbonyl and carboxylate groups form dihedral angles of 83.0 (2) and 83.2 (2) , respectively, with the aromatic ring.

Experimental
A mixture of phthalic anhydride (1.52 g, 0.01 mol) and methanol (15 ml) was refluxed for 30 min. Then tert-butylamine (0.73 g, 0.01 mol) was added to the above solution and mixed for 30 min at room temperature. The solution was kept at room temperature for 5 d. Natural evaporation gave colourless single crystals of the title compound, suitable for X-ray analysis.

Refinement
H atoms bonded to C atoms were placed in calculated positions and refined in a riding-model approximation with C-H = 0.93-0.96 Å and with U iso (H) = 1.2U eq (C) or 1.5U eq (C methyl ). H atoms bonded to N were refined independently with isotropic displacement parameters. Fig. 1. The asymmetric unit of (I), drawn with 30% probability ellipsoids. The disorder is not shown (symmetry code (a): x, -y+3/2, z).

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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.