Crystal structure of a new molecular salt: 4-aminobenzenaminium 5-carboxypentanoate

The asymmetric unit of the title molecular salt, consists of half a 4-aminobenzenaminium cation and a half a 5-carboxypentanoate anion. Each ion lies about an inversion centre, the other half being generated by inversion symmetry. In the crystal, charge-assisted O—H⋯O, N—H⋯O and N—H⋯N hydrogen bonds together lead to the formation of a three-dimensional supramolecular framework.


Chemical context
p-Phenylenediamine (PPDA) has been widely used to synthesize hair dyes, engineering polymers and composites. The coordination chemistry of PPDA is well documented (Adams et al., 2011;Bourne & Mangombo, 2004). Adipic acid (AA) is an industrial chemical used to manufacture nylon and is also used in many drugs and food additives (Rowe et al., 2009). A number of salts and co-crystals involving p-phenylenediamine have been reported (Thakuria et al., 2007;Delori et al., 2016), and adipic acid is also widely known as a coformer in co-crystal formation (Swinton Darious et al., 2016;Lemmerer et al., 2012;Lin et al., 2012;Matulková et al., 2014;Thanigaimani et al., 2012). A 2:1 salt of 4-aminoanilinium (PPDAH) and sebacate, and a 1:1 salt of PPDAH and dihydrogen trimesate have been reported recently (Delori et al., 2016). We have previously reported various salts of o-phenylenediamine with aromatic carboxylic acids (Mishra & Pallepogu, 2018). Herein, we report on the synthesis and crystal structure of the 1:1 salt formed between p-phenylenediamine and adipic acid, (I).

Structural commentary
The asymmetric unit of the title salt (I), illustrated in Fig. 1, consists of half each of a 4-aminobenzenaminium cation (4-ABA) and a 5-carboxypentanoate anion (5-CP); both ions (space group P1) lie about inversion centres. Partial protonation (50%) has occurred at atom N1 of the cation, resulting in the formation of a salt with the formula unit C 6 H 9 O 4 À ÁC 6 H 9 N 2 + . One of the two adipic acid H atoms binds to atom N1 with a site-occupancy factor (SOF) of 0.5 (for atom H1NC), thereby positioned at two sites (because of inversion symmetry) in the cation. The other acid H atom (H2O) is located on an inversion center and is therefore shared equally by two O2 atoms of inversion-related anions. The C1-N1 bond length [1.4361 (13) Å ] in the 4-ABA cation is longer than literature values for a non-protonated amine (C-NH 2 ) group [cf. 1.418 (2) Å ; Czapik et al., 2010] and this can be attributed to the partial protonation with SOF = 0.50 at each site. In the 5-CP anion, the C6 O1 and C6-O2 bond lengths [1.2379 (12) and 1.2802 (11) Å , respectively] are similar to the values reported for 2-methylimidazolium hydrogen adipate monohydrate [1.244 (2) and 1.264 (2) Å , respectively; Meng et al., 2009] in which a carboxylic acid H atom is also statistically distributed between the two carboxy groups and a hydrogenbonded chain is formed. In (I), the position of this H atom (H2O) was located in a difference-Fourier map and found to be situated on an inversion centre ( 1 2 , 1 2 , 1 2 ). It is positioned symmetrically between two O2 atoms of two inversion-related 5-CP ions, which accounts for the long O-H bond length of 1.22 Å (see Table 1). The C4 ii -C4-C5-C6 torsion angle of À179.82 (9) indicates that the carbon chain in the anion is fully extended [see Fig. 1 for symmetry code (ii)].

Figure 2
A view along the c axis of the O-HÁ Á ÁO hydrogen-bonded chain of 5-CP anions (see Table 1). The H atoms (H2O; shown as grey balls) are shared between O2 atoms of inversion-related anions. The C-bound H atoms and the cations have been omitted.
A search of the CSD for salts of adipic acid (AA) with different amines yielded 67 hits. One such structure of particular interest, viz. 2-methylimidazolium hydrogen adipate monohydrate, has been reported twice, once at room temperature (BOTTOU: Meng et al., 2009), where the same type of partial disorder is observed with the carboxylic acid H atom statistically distributed between the two carboxy groups and a hydrogen-bonded chain is formed. However, the lowtemperature analysis at 120 K using synchrotron radiation (BOTTOU01: Callear et al., 2010), describes the structure as bis(2-methylimidazolium) adipate adipic acid dihydrate. In the crystal, the adipate and adipic acid molecules also form a hydrogen-bonded chain. A second structure, tetrakis(cytosinium) dihydrogen bis(adipate), also exhibits the same type of disorder of the carboxylic acid H atom (OYEREQ; Das & Baruah, 2011), and in the crystal it forms a hydrogen-bonded chain.

Synthesis and crystallization
The title molecular salt (I), was synthesized by mixing a 5 ml methanolic solution of adipic acid (AA: 0.5 mmol, 73 mg) and 3 ml of an acetonitrile solution of p-phenylenediamine (PPDA: 0.5 mmol, 54 mg). The reaction mixture was heated to 323 K with magnetic stirring for ca 30 min, and then filtered and allowed to evaporate slowly at room temperature. Purple block-like crystals of (I) were obtained after 5 d (m.p. 438 K). FTIR (KBr pellet, cm -1) : 3337, 3180, 2946, 2383, 1706, 1515, 1255, 821, 743, 501, 475.  The PXRD pattern obtained from the product of the LAG experiment, and the simulated PXRD pattern of the crystal structure of the title molecular salt. The PXRD patterns of the reactants used for the cocrystallization and LAG syntheses are also shown.

Figure 3
A partial view, normal to the ab plane, of the crystal packing of the title molecular salt (I). Hydrogen bonds are shown as dashed lines (see Table 1), and C-bound H atoms have been omitted for clarity.

Figure 4
A view along the b axis of the crystal packing of the title molecular salt (I), showing the three-dimensional supramolecular framework. Hydrogen bonds are shown as dashed lines (see Table 1), and C-bound H atoms have been omitted for clarity.
The title compound was also synthesized by liquid-assisted grinding (LAG). For this mechanochemical synthesis, equimolar amounts of AA (1 mmol, 146 mg) and PPDA (1 mmol, 108 mg) were ground for 20 min. in a mortar and pestle using 3 to 4 drops of acetonitrile. The powdered sample was collected for PXRD and the resultant pattern was scrutinized for new peaks, as evidence for the formation of the title molecular salt (I), by comparing this pattern with the simulated pattern obtained from the CIF file of salt (I). The PXRD pattern of the compound obtained from the LAG experiment matches the simulated pattern obtained for (I), formed by co-crystallization (Fig. 5).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. C-bound H atoms were placed in calculated positions and refined using a riding-model approximation: 0.95-09.99 Å with U iso (H) = 1.2U eq (C). The ammonium and carboxyl H atoms were located in difference-Fourier maps and were freely refined. The H atom (H1NC) bound to N1 of the 4-ABA cation, with an occupancy factor of 0.5, is positioned at two sites of the cation due to inversion symmetry, giving rise to a monoprotonated species. The carboxylic acid H atom (H2O) is positioned symmetrically between the two O2 atoms of inversion-related 5-CP ions (H-O = 1.22 Å ). This H atom (H2O) is located on an inversion center ( 1 2 , 1 2 , 1 2 ) with an occupancy factor of 0.5, and hence gives rise to a mono-deprotonated species.