Crystal structure of a new mol-ecular salt: 4-amino-benzenaminium 5-carb-oxy-penta-noate.

The asymmetric unit of the title mol-ecular salt (systematic name: 4-amino-anilinium 5-carb-oxy-penta-noate), C6H9N2+·C6H9O4-, consists of half a 4-amino-benzenaminium cation (4-ABA) and half a 5-carb-oxy-penta-noate anion (5-CP); the other half of each ion is generated by inversion symmetry. Protonation of p-phenyl-enedi-amine (PPDA) leads to the formation of a 1:1 salt, but scrutiny of the crystal structure reveals that both of the amine groups of PPDA are partially protonated, with a half-occupied H atom. For the 5-CP anion, an H atom is positioned on an inversion center midway between two O atoms of inversion-related 5-CP ions. In the crystal, the 5-CP anions are linked by the O-H⋯O hydrogen bond to form chains propagating along the [1-10] direction. The chains are linked via N-H⋯O and N-H⋯N hydrogen bonds involving the 4-ABA cations, forming a three-dimensional supra-molecular framework. The title salt was also prepared by mechanochemical synthesis using liquid-assisted grinding (LAG). Its PXRD pattern matches that of the simulated pattern of the crystal structure of the title mol-ecular salt.

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 co-former 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 P ) 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 C6H9O4 −·C6H9N2 +. 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–NH2) 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 hydrogen-bonded 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 (, , ). 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 C4ii—C4—C5—C6 torsion angle of −179.82 (9)° indicates that the carbon chain in the anion is fully extended [see Fig. 1 ▸ for symmetry code (ii)].

Figure 1

The mol­ecular structure of the title mol­ecular salt (I), with atom labelling for the asymmetric unit. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) −x + 1, −y + 2, −z + 2; (ii) −x + 2, −y, −z + 1; (iii) −x + 1, −y + 1, −z + 1].

Table 1

Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O2—H2O⋯O2i	1.22	1.22	2.439 (1)	180
O2—H2O⋯O1i	1.22	2.45	3.178 (1)	116
N1—H1NA⋯O1ii	0.91 (2)	1.97 (2)	2.871 (1)	171 (1)
N1—H1NB⋯O1	0.90 (2)	2.24 (2)	3.060 (1)	152 (1)
N1—H1NB⋯O2i	0.90 (2)	2.51 (2)	3.098 (1)	123 (1)
N1—H1NC⋯N1iii	0.95 (2)	1.89 (2)	2.840 (1)	174 (2)

Symmetry codes: (i) ; (ii) ; (iii) .

Supra­molecular features

In general, adipic acid (AA) forms (8) homosynthons facilitated by strong O—H⋯O inter­actions; however, it is known to form heterosynthons when co-crystallized with amines (Lemmerer et al., 2010 ▸, 2012 ▸). In the crystal structure of (I), the formation of the (8) homosynthon is not favoured because of the inversion-related arrangement of the 5-CP anions, which forms a linear chain along the [10] direction (Fig. 2 ▸, Table 1 ▸) as a result of the sharing of the H atom (H2O) between the O2 atoms of inversion-related anions as discussed above; otherwise this could be supposed to be the contribution of a strong O—H⋯O hydrogen bond. The crystal structure features inter­ionic N—H⋯O and O—H⋯O hydrogen bonds between the anions and cations (Table 1 ▸, Fig. 3 ▸). One of the H atoms (H1BA) on N1 is a bifurcated hydrogen-bond donor and the carboxyl­ate O1 atom of the anion acts as a triple acceptor, accepting H atoms from two different NH3 groups (H1NA and H1NB), as illustrated in Fig. 3 ▸. The chains of the 5-CP anions are linked via the N—H⋯O, O—H⋯O and N—H⋯N hydrogen bonds, forming a three-dimensional supra­molecular framework (Table 1 ▸ and Fig. 4 ▸).

Figure 2

A view along the c axis of the O—H⋯O 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.

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.

Database survey

A search of the Cambridge Structural Database (CSD, Version 5.38, update May 2017; Groom et al., 2016 ▸) for 4-amino­anilinium and 1,4-phenyl­enedi­ammonium salts gave 70 hits. The crystal structure of p-phenyl­enedi­amine (PPDA) itself was first reported by Povetéva & Zvonkova (1975 ▸). Perhaps the most relevant hit is for the structure of bis­(4-amino­anilinium) deca­nedioate (CSD refcode IPAPAS; Delori et al., 2016 ▸), one of the few salts formed with an aliphatic diacid. There are a number of structures reported of PPDA with mineral acids (Chandrasekaran, 1969 ▸; Marsh, 2009 ▸; Anderson et al., 2006 ▸), and co-crystals and salts of PPDA with organic acids (Thakuria et al., 2007 ▸; Delori et al., 2016 ▸). Some 1:1 co-crystals of PPDA and various diols (viz. 1,8-octane diol, 1,10-decane diol and 1,12-dodecane diol) have also been reported (Loehlin & Okasako, 2007 ▸).

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 low-temperature 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): ν 3337, 3180, 2946, 2383, 1706, 1515, 1255, 821, 743, 501, 475.

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 ▸).

Figure 5

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 co-crystallization and LAG syntheses are also shown.

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 (, , ) with an occupancy factor of 0.5, and hence gives rise to a mono-deprotonated species.

Table 2

Experimental details

Crystal data
Chemical formula	C6H9N2 +·C6H9O4 −
M r	254.28
Crystal system, space group	Triclinic, P
Temperature (K)	100
a, b, c (Å)	5.2918 (3), 7.1666 (4), 8.4205 (7)
α, β, γ (°)	92.069 (6), 104.165 (6), 97.172 (5)
V (Å3)	306.47 (4)
Z	1
Radiation type	Mo Kα
μ (mm−1)	0.10
Crystal size (mm)	0.42 × 0.38 × 0.32
Data collection
Diffractometer	Rigaku Oxford Diffraction XtaLAB Pro: Kappa dual offset/far
Absorption correction	Multi-scan (CrysAlis PRO; Rigaku OD, 2015 ▸)
T min, T max	0.924, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	3776, 1416, 1341
R int	0.015
(sin θ/λ)max (Å−1)	0.685
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.034, 0.092, 1.08
No. of reflections	1416
No. of parameters	96
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å−3)	0.39, −0.22

Computer programs: CrysAlis PRO (Rigaku OD, 2015 ▸), SHELXS97 (Sheldrick, 2008 ▸), SHELXL2016/6 (Sheldrick, 2015 ▸), OLEX2 (Dolomanov et al., 2009 ▸), Mercury (Macrae et al., 2008 ▸), PLATON (Spek, 2009 ▸) and publCIF (Westrip (2010 ▸).

Crystal structure: contains datablock(s) Global, I. DOI: 10.1107/S2056989018000737/dx2003sup1.cif

Click here for additional data file.

Supporting information file. DOI: 10.1107/S2056989018000737/dx2003Isup4.cml

Click here for additional data file.

Supporting information file. DOI: 10.1107/S2056989018000737/dx2003Isup6.mol

CCDC reference: 1816419

Additional supporting information: crystallographic information; 3D view; checkCIF report

C6H9N2+·C6H9O4−	Z = 1
Mr = 254.28	F(000) = 136
Triclinic, P1	Dx = 1.378 Mg m−3
a = 5.2918 (3) Å	Mo Kα radiation, λ = 0.71073 Å
b = 7.1666 (4) Å	Cell parameters from 5347 reflections
c = 8.4205 (7) Å	θ = 5.2–29.2°
α = 92.069 (6)°	µ = 0.10 mm−1
β = 104.165 (6)°	T = 100 K
γ = 97.172 (5)°	Block, purple
V = 306.47 (4) Å3	0.42 × 0.38 × 0.32 mm

Rigaku Oxford Diffraction XtaLAB Pro: Kappa dual offset/far diffractometer	1416 independent reflections
Radiation source: fine-focus sealed X-ray tube, Enhance (Mo) X-ray Source	1341 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.015
ω scans	θmax = 29.1°, θmin = 5.0°
Absorption correction: multi-scan (CrysAlis PRO; Rigaku OD, 2015)	h = −5→7
Tmin = 0.924, Tmax = 1.000	k = −9→8
3776 measured reflections	l = −11→10

Refinement on F2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: mixed
R[F2 > 2σ(F2)] = 0.034	H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.092	w = 1/[σ2(Fo2) + (0.0483P)2 + 0.0998P] where P = (Fo2 + 2Fc2)/3
S = 1.08	(Δ/σ)max < 0.001
1416 reflections	Δρmax = 0.39 e Å−3
96 parameters	Δρmin = −0.22 e Å−3
0 restraints	Extinction correction: (SHELXL-2016/6; Sheldrick, 2015), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.15 (2)

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.

	x	y	z	Uiso*/Ueq	Occ. (<1)
O1	0.88613 (16)	0.51078 (11)	0.73311 (10)	0.0284 (2)
O2	0.62365 (13)	0.36690 (9)	0.50333 (8)	0.0159 (2)
H2O	0.500000	0.500000	0.500000	0.079 (10)*
C4	0.90322 (17)	0.07097 (12)	0.49487 (11)	0.0140 (2)
H4A	0.867169	0.119848	0.384244	0.017*
H4B	0.735161	0.007357	0.511087	0.017*
C5	1.01023 (17)	0.23441 (12)	0.62353 (11)	0.0127 (2)
H5A	1.048177	0.183438	0.733310	0.015*
H5B	1.178973	0.295883	0.606644	0.015*
C6	0.82994 (17)	0.38253 (12)	0.62270 (11)	0.0122 (2)
N1	0.43897 (17)	0.62290 (12)	0.87167 (10)	0.0177 (2)
H1NA	0.268 (3)	0.582 (2)	0.8186 (19)	0.033 (4)*
H1NB	0.538 (3)	0.608 (2)	0.800 (2)	0.036 (4)*
H1NC	0.470 (5)	0.534 (3)	0.953 (3)	0.013 (6)*	0.5
C1	0.47080 (18)	0.81542 (13)	0.93517 (11)	0.0147 (2)
C2	0.65701 (19)	0.95024 (15)	0.89941 (12)	0.0180 (2)
H2	0.764255	0.916272	0.830590	0.022*
C3	0.68658 (19)	1.13477 (14)	0.96421 (12)	0.0179 (2)
H3	0.814280	1.227114	0.939883	0.021*

	U11	U22	U33	U12	U13	U23
O1	0.0309 (4)	0.0199 (4)	0.0270 (4)	0.0138 (3)	−0.0099 (3)	−0.0120 (3)
O2	0.0149 (4)	0.0149 (3)	0.0169 (4)	0.0069 (3)	0.0000 (3)	−0.0023 (3)
C4	0.0124 (4)	0.0124 (4)	0.0173 (5)	0.0051 (3)	0.0023 (3)	−0.0011 (3)
C5	0.0108 (4)	0.0119 (4)	0.0154 (4)	0.0034 (3)	0.0027 (3)	0.0006 (3)
C6	0.0131 (4)	0.0101 (4)	0.0144 (4)	0.0023 (3)	0.0049 (3)	0.0015 (3)
N1	0.0164 (4)	0.0225 (4)	0.0144 (4)	0.0030 (3)	0.0040 (3)	0.0012 (3)
C1	0.0137 (4)	0.0187 (5)	0.0107 (4)	0.0033 (3)	0.0009 (3)	0.0006 (3)
C2	0.0160 (4)	0.0246 (5)	0.0154 (4)	0.0041 (4)	0.0070 (3)	0.0008 (4)
C3	0.0149 (4)	0.0223 (5)	0.0172 (5)	0.0003 (4)	0.0064 (4)	0.0023 (4)

O1—C6	1.2379 (12)	N1—C1	1.4361 (13)
O2—C6	1.2802 (11)	N1—H1NA	0.914 (17)
O2—H2O	1.2197 (6)	N1—H1NB	0.906 (17)
C4—C5	1.5195 (12)	N1—H1NC	0.95 (2)
C4—C4i	1.5213 (16)	C1—C2	1.3870 (14)
C4—H4A	0.9900	C1—C3ii	1.3916 (13)
C4—H4B	0.9900	C2—C3	1.3876 (14)
C5—C6	1.5123 (12)	C2—H2	0.9500
C5—H5A	0.9900	C3—H3	0.9500
C5—H5B	0.9900
C6—O2—H2O	113.50 (6)	C1—N1—H1NA	111.1 (9)
C5—C4—C4i	111.39 (9)	C1—N1—H1NB	111.9 (10)
C5—C4—H4A	109.3	H1NA—N1—H1NB	107.8 (14)
C4i—C4—H4A	109.3	C1—N1—H1NC	114.7 (15)
C5—C4—H4B	109.3	H1NA—N1—H1NC	101.0 (18)
C4i—C4—H4B	109.3	H1NB—N1—H1NC	109.6 (18)
H4A—C4—H4B	108.0	C2—C1—C3ii	120.02 (9)
C6—C5—C4	115.04 (8)	C2—C1—N1	121.06 (8)
C6—C5—H5A	108.5	C3ii—C1—N1	118.91 (9)
C4—C5—H5A	108.5	C1—C2—C3	119.97 (9)
C6—C5—H5B	108.5	C1—C2—H2	120.0
C4—C5—H5B	108.5	C3—C2—H2	120.0
H5A—C5—H5B	107.5	C2—C3—C1ii	120.01 (9)
O1—C6—O2	123.47 (8)	C2—C3—H3	120.0
O1—C6—C5	120.06 (8)	C1ii—C3—H3	120.0
O2—C6—C5	116.47 (8)
C4i—C4—C5—C6	−179.82 (9)	C3ii—C1—C2—C3	0.09 (16)
C4—C5—C6—O1	172.41 (9)	N1—C1—C2—C3	−179.06 (9)
C4—C5—C6—O2	−8.04 (12)	C1—C2—C3—C1ii	−0.10 (16)

D—H···A	D—H	H···A	D···A	D—H···A
O2—H2O···O2iii	1.22	1.22	2.439 (1)	180
O2—H2O···O1iii	1.22	2.45	3.178 (1)	116
N1—H1NA···O1iv	0.91 (2)	1.97 (2)	2.871 (1)	171 (1)
N1—H1NB···O1	0.90 (2)	2.24 (2)	3.060 (1)	152 (1)
N1—H1NB···O2iii	0.90 (2)	2.51 (2)	3.098 (1)	123 (1)
N1—H1NC···N1v	0.95 (2)	1.89 (2)	2.840 (1)	174 (2)