The crystal structure of 3-ammonio- 4-aminobenzoate, C7H8N2O2 – a second polymorph

C7H8N2O2, monoclinic, P21/n (no. 14), a = 8.9884(6) Å, b = 6.0490(4) Å, c = 12.1290(8) Å, β = 95.400(2)°, V = 656.54(8) Å, Z = 4, Rgt(F ) = 0.0340, wRref(F ) = 0.0978, T = 200 K.


Source of material
The compound was obtained commercially (Aldrich). Crystals suitable for the diffraction study were obtained upon recrystallization of the compound from methanol.

Experimental details
Carbon-bound H atoms were placed in calculated positions (C-H 0.95 Å) and were included in the refinement in the riding model approximation, with U(H) set to 1.2U eq (C).
All nitrogen-bound H atoms were located on a difference Fourier map and refined freely.

Comment
Chelate ligands have found widespread use in coordination chemistry due to the increased stability of coordination compounds they can form in comparison to monodentate ligands. The stability of these compounds is enhanced further if the denticity of the ligand is increased upon incorporation of more and more potential donor sites [7]. Especially derivatives of benzoic acids offering the possibility to act as a chelate ligand are interesting in this aspect as chemical factors such as pH values could influence the protonation/deprotonation of acidic functional groups and fine-tune the denticity of the resulting ligand species. Examples would be benzoic acid derivatives bearing multiple hydroxy or amino groups. In continuation of our interest in the structures of polyfunctional carboxylic acids [8][9][10][11][12][13][14] the title compound seemed intriguing. Although the molecular and crystal structure of the latter have been reported earlier in a C centred space group [15], an initial unit cell determination of the compound available to us after recrystallization from methanol unambiguously showed the presence of a compound presenting itself in a primitive space group. To clarify the situation the structure was determined. The analysis of the data proves the presence of the 3,4-diaminobenzoic acid. The molecule is present in a zwitterionic tautomer with protonation observed on the amino group in meta position to the carboxylate substituent. Very similar C-O bond lengths of 1.2543(15) Å and 1.2734 (14)°confirm the deprotonation of the carboxyl group; moreover, these values are in good agreement with values found for other benzoate salts whose metrical parameters have been deposited with the Cambridge Structural Database [16]. Intracyclic C-C-C angles span a range of 117.05(10)-121.74(10)°with the smallest angle on the carbon atom bearing the amino group and the largest angle on the carbon atom bearing the ammonio group. The molecule is approaching planarity with an O-C-C-C dihedral angle of −22. 16(17)°for the carboxylate group.
In the crystal, classical hydrogen bonds of the N-H … O type are observed that employ every pnicogen-bound hydrogen atom as donor as well as both carboxylate oxygen atoms as acceptor. In terms of graph-set analysis [17,18], the descriptor for these hydrogen bonds is C 1 1 (7)C 1 1 (8)C 1 1 (8)R 2 2 (14)R 2 2 (14) at the unary level with both cyclic patterns being supported by hydrogen atoms of the ammonio group. In addition, C-H … π contact stems from the CH group in meta position to the carboxylate substituent. In total, the molecules are connected to undulated double sheets along [1 0 1]. π-Stacking is observed with the shortest distance measured between two centers of gravity at just 3.6442(7) Å.
Author contribution: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.