The zwitterionic structure of 2-hy­droxy-4-[(2-hy­droxy­benzyl­idene)amino]­benzoic acid

The title compound, C₁₄H₁₁NO₄, exists in the solid phase in the zwitterionic form, 2-{[(4-carb­oxy-3-hy­droxy­phen­yl)iminium­yl]methyl}phenolate, with the H atom from the phenol group on the 2-hy­droxy­benzyl­idene ring transferred to the imine N atom, resulting in a strong intra­molecular N-H...O hydrogen bond between the iminium H atom and the phenolate O atom, forming a six-membered hydrogen-bonded ring. In addition, there is an intra­molecular O-H...O hydrogen bond between the carb­oxy­lic acid group and the adjacent hy­droxy group of the other ring, and an inter­molecular C-H...O contact involving the phenol group and the C-H group adjacent to the imine bond, connecting the mol­ecules into a two-dimensional network in the (10) plane. [pi]-[pi] stacking inter­actions result in a three-dimensional network. This study is important because it provides crystallographic evidence, supported by IR data, for the iminium zwitterionic form of Schiff bases.

S1. Introduction

Schiff bases, characterized by the –N═CH– (imine) group, constitute an important class of organic compounds. They are used as inter­mediates in organic synthesis or as ligands in coordination chemistry. The name Schiff bases refers to Hugo Schiff, whose studies involving aniline generated important information about the chemistry of imine compounds (Tidwell, 2008). They are used in optical and electrochemical sensors, and in biological systems like anti­biotics and anti­allergic, anti­phlogistic and anti­tumour substances (Layer, 1963). Schiff bases derived from the salicyl­aldehyde family exhibit a strong binding ability because of the presence of N/O donors, and they act as ligands in inorganic [organometallic?] chemistry (Yao et al., 2006; Le Guennic et al., 2007).

S2. Experimental

S2.1. Synthesis and crystallization

In a 100 ml flask, 4-amino­salicylic acid (10.0 mmol, 1.0 g) was dissolved in methanol (20 ml) and salicylic aldehyde (11.0 mmol, 1.1 g) was added. The mixture was stirred at room temperature for 1 h. After this time, an orange

precipitate was isolated by filtration. The solid was dissolved in hot tetra­hydro­furan (T = 333 K) and the resulting solution was set aside. After 4 d, orange crystals of (I) suitable for single-crystal X-ray diffraction analysis were collected (yield 73%; m.p. = 432 K). Elemental analysis, calculated for C₁₄H₁₁NO₄: C 65.37, H 4.31, N 5.44%; found: C 64.54, H 4.49, N 5.32%. IR spectroscopic data were obtained with a Bruker FT–IR spectrometer using KBr disks and a resolution of 4 cm^-1. The main absorption bands are (ν, cm^-1): 3426 (OH), 3049 (C—H_arom), 1642 (C═O), 1608 (C═N), 1512 (C═C), and 791 and 762 (C—Har_oop). [oop is what?]

S2.2. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. O- and N-bound H atoms were added in idealized positions and further refined according to the riding model, with O—H = 0.82 Å and N—H = 0.86 Å, and with U_iso(H) = 1.5U_eq(O) or 1.2U_eq(N). [Please confirm added text] C-bound H atoms were included in the riding-model approximation, with C—H = 0.95 Å and U_iso(H) = 1.2U_eq(C).

S3. Results and discussion

The title Schiff base, (I) crystallizes in the monoclinic space group P2₁/n with four molecules in the unit cell. Selected bond lengths and angles are presented in Table 2. The compound (Fig. 1) crystallizesin the zwitterionic form, with an anionic phenolate and a cationic iminium group.

The C8—N1 bond length [1.3067 (17) Å] agrees well with the double-bond character of imine groups in other zwitterion Schiff bases described in the literature, e.g. 1.306 (19) and 1.310 (2) Å (Makal et al., 2011; Eltayeb, Teoh, Chantrapromma & Fun, 2010). The trans configuration relative to the C8—N1 bond can be inferred from the C9—C8—N1—C5 torsion angle of -176.72 (12)°. Aromatic rings A (C2–C7) and B (C9–C14) are not coplanar and are twisted with a dihedral angle of 19.34 (7)°. The iminium H atom is close to the phenolate O atom, allowing the formation of an intra­molecular hydrogen bond (N1—H1N···O1; Table 3), in which the donor–acceptor distance is 2.5682 (15) Å, indicating a strong hydrogen bond with a highly covalent character (Makal et al., 2011). This kind of inter­action has been observed in other phenolate–imine zwitterions in which these groups are close to each other, for example, in (E)-4-hy­droxy-2-{[(2-phenyl­ethyl)­iminiumyl]methyl}­phenolate and 4-nitro-1-oxo-2-{(E)-[2-(piperidin-1-yl)ethyl]­iminio­methyl}­cyclo­hexadienide, with N···O = 2.5884 (15) and 2.674 (3) Å, respectively (Ortegon-Reyna et al., 2012; Santos-Contreras et al., 2009). Another strong intra­molecular inter­action [O2(—H2)···O4 = 2.5669 (17) Å], with the same behaviour, is observed between the carbonyl O atom of the carb­oxy­lic acid group and the phenol H atom of ring A. In both cases, six-membered rings are formed and, according to Etter's rule (Etter, 1990), this kind of intra­molecular hydrogen bond is formed in preference to an inter­molecular hydrogen bond. The reasoning behind this rule is based on the same kind of idea as the chelate effect in inorganic chemistry and is entropically favoured.

Atom O1 forms a strong inter­molecular hydrogen bond with the hy­droxy H atom of the carb­oxy­lic acid group of an adjacent molecule [O3—H3O···O1^iv = 2.5058 (15) Å; symmetry code: (iv) x - 3/2, -y + 1/2, z - 1/2]. This inter­action links the molecules into one-dimensional zigzag chains running in the [301] direction (Fig. 2). The same behaviour has been observed in similar compounds, for example, (E)-4-bromo-2-[(2-hy­droxy­phenyl)­iminio­methyl]­phenolate (Eltayeb, Teoh, Fun & Chantrapromma, 2010). The zigzag chains inter­act with each other through weak inter­molecular hydrogen bonds between the phenol O atom and the imine C—H group [C8—H8···O2ⁱⁱⁱ; symmetry code: (iii) -x, -y + 1, -z], forming a two-dimensional network parallel with the (103) plane (Fig. 2). Using graph-set notation (Etter, 1990), the hydrogen-bonding system can be described as N1 = C₃⁴(16)S(6)R₂²(14) and N2 = R₁₀¹⁴(46). These two intra­molecular inter­actions and the above-mentioned inter­molecular O3—H3O···O1^iv inter­action have the same dimensions and behaviour as those in 2-[(E)-(3-carb­oxy-4-hy­droxy­phenyl)­iminio­methyl]-4-chloro­phenolate (Farag et al., 2010). The only difference between these two structures is the presence of one chlorine substituent in the compound described in the literature. Although an additional inter­molecular inter­action is formed with the Cl atom, the same behaviour is observed, with a two-dimensional extension in zigzag chains. A three-dimensional network arises through π–π stacking inter­actions between parallel aromatic rings from different two-dimensional layers (Fig. 3). Geometric details for this inter­action are (Spek, 2009), for A···A(x - 1, y, z), centroid-to-centroid distance = 3.8393 (8) Å, perpendicular distance = 3.4862 (6) Å and ring slippage = 1.608 (1) Å, and for B···B(x - 1, y, z), centroid-to-centroid distance = 3.8393(s.u.?) Å, perpendicular distance = 3.3671 (6) Å and ring slippage = 1.845 (1) Å.

Related literature

For related literature, see: Eltayeb, Teoh, Chantrapromma & Fun (2010); Eltayeb, Teoh, Fun & Chantrapromma (2010); Etter (1990); Farag et al. (2010); Layer (1963); Le Guennic, Petit, Chastanet, Pilet, Amor & Robert (2007); Makal et al. (2011); Ortegon-Reyna, Garcias-Morales, García-Báez, Ariza-Castolo & Martínez-Martínez (2012); Santos-Contreras, Ramos-Organillo, García-Báez, Padilla-Martínez & Martínez-Martínez (2009); Spek (2009); Tidwell (2008); Yao et al. (2006).

Computing details

Data collection: CrysAlis PRO (Agilent, 2012); cell refinement: CrysAlis PRO (Agilent, 2012); data reduction: CrysAlis PRO (Agilent, 2012); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and Mercury (Macrae et al., 2008); software used to prepare material for publication: publCIF (Westrip, 2010) and WinGX (Farrugia, 2012).

Figures

	Fig. 1. A view of asymmetric unit of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.
	Fig. 2. The bidimensional molecular chain generated by O—H···O and C—H···O hydrogen bonds (dashed lines). [Symmetry codes: (iii) -x, -y + 1, -z; (iv) x - 3/2, -y + 1/2, z - 1/2.]
	Fig. 3. An illustration of the π–π stacking interactions. Spheres denote the ring centroids.

2-{[(4-Carboxy-3-hydroxyphenyl)iminiumyl]methyl}phenolate

Crystal data

C14H11NO4	F(000) = 536.00
Mr = 257.24	Dx = 1.482 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 16916 reflections
a = 3.8393 (1) Å	θ = 2.5–26.4°
b = 18.3309 (4) Å	µ = 0.11 mm^−1
c = 16.4738 (4) Å	T = 298 K
β = 96.212 (2)°	Prism, light brown
V = 1152.58 (5) Å^3	0.62 × 0.34 × 0.12 mm
Z = 4

Data collection

Agilent Xcalibur diffractometer with Atlas Gemini Ultra detector	2349 independent reflections
Radiation source: Enhance (Mo) X-ray Source	2006 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.038
ω scans	θmax = 26.4°, θmin = 2.5°
Absorption correction: analytical [CrysAlis PRO (Agilent, 2012), based on expressions derived by Clark & Reid (1995)]	h = −4→4
Tmin = 0.973, Tmax = 0.994	k = −22→22
57654 measured reflections	l = −20→20

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.037	H-atom parameters constrained
wR(F^2) = 0.112	w = 1/[σ^2(Fo^2) + (0.0601P)^2 + 0.3375P] where P = (Fo^2 + 2Fc^2)/3
S = 1.03	(Δ/σ)max < 0.001
2349 reflections	Δρmax = 0.20 e Å^−3
173 parameters	Δρmin = −0.20 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 2008)
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0074 (18)

Special details

Experimental. CrysAlisPro, Agilent Technologies, Version 1.171.35.21 (release 20-01-2012 CrysAlis171 .NET) (compiled Jan 23 2012,18:06:46) Analytical numeric absorption correction using a multifaceted crystal model based on expressions derived by R.C. Clark & J.S. Reid. (Clark, R. C. & Reid, J. S. (1995). Acta Cryst. A51, 887-897)

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 F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
O1	0.8457 (3)	0.38680 (5)	0.32616 (6)	0.0451 (3)
O2	−0.0306 (3)	0.40129 (6)	−0.08909 (6)	0.0548 (3)
H2	−0.1219	0.3728	−0.1235	0.082*
O3	−0.3555 (3)	0.18835 (6)	−0.05976 (7)	0.0517 (3)
H3O	−0.4509	0.1691	−0.1013	0.077*
O4	−0.3091 (4)	0.28165 (6)	−0.14503 (7)	0.0587 (4)
N1	0.4554 (3)	0.40605 (6)	0.19115 (6)	0.0317 (3)
H1N	0.5294	0.3784	0.2316	0.038*
C1	−0.2603 (4)	0.25408 (8)	−0.07654 (9)	0.0390 (3)
C2	−0.0840 (3)	0.29480 (7)	−0.00597 (8)	0.0320 (3)
C3	−0.0210 (4)	0.26326 (7)	0.07081 (8)	0.0350 (3)
H3	−0.0955	0.2157	0.0784	0.042*
C4	0.1491 (4)	0.30062 (7)	0.13611 (8)	0.0349 (3)
H4	0.1872	0.2788	0.1873	0.042*
C5	0.2629 (3)	0.37157 (7)	0.12417 (7)	0.0298 (3)
C6	0.1985 (4)	0.40522 (7)	0.04878 (8)	0.0338 (3)
H6	0.2709	0.4530	0.0418	0.041*
C7	0.0250 (4)	0.36713 (7)	−0.01626 (8)	0.0338 (3)
C8	0.5342 (3)	0.47523 (7)	0.19883 (8)	0.0314 (3)
H8	0.4479	0.5071	0.1575	0.038*
C9	0.7436 (3)	0.50374 (7)	0.26683 (7)	0.0303 (3)
C10	0.8951 (3)	0.45695 (7)	0.33079 (8)	0.0329 (3)
C11	1.0976 (4)	0.49017 (9)	0.39741 (8)	0.0410 (3)
H11	1.2017	0.4613	0.4397	0.049*
C12	1.1427 (4)	0.56407 (9)	0.40054 (9)	0.0442 (4)
H12	1.2740	0.5847	0.4455	0.053*
C13	0.9958 (4)	0.60951 (8)	0.33760 (9)	0.0423 (4)
H13	1.0323	0.6596	0.3406	0.051*
C14	0.7997 (4)	0.58003 (7)	0.27221 (8)	0.0360 (3)
H14	0.7013	0.6102	0.2305	0.043*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0593 (7)	0.0324 (5)	0.0399 (6)	0.0019 (4)	−0.0121 (5)	0.0054 (4)
O3	0.0714 (8)	0.0386 (6)	0.0413 (6)	−0.0119 (5)	−0.0107 (5)	−0.0078 (5)
O2	0.0902 (9)	0.0390 (6)	0.0307 (5)	−0.0055 (6)	−0.0139 (5)	0.0056 (4)
N1	0.0383 (6)	0.0286 (6)	0.0265 (5)	0.0033 (4)	−0.0045 (4)	−0.0001 (4)
O4	0.0863 (9)	0.0472 (7)	0.0368 (6)	−0.0064 (6)	−0.0194 (6)	−0.0018 (5)
C5	0.0306 (6)	0.0291 (6)	0.0286 (6)	0.0047 (5)	−0.0019 (5)	−0.0038 (5)
C2	0.0330 (7)	0.0295 (6)	0.0320 (7)	0.0038 (5)	−0.0035 (5)	−0.0035 (5)
C9	0.0319 (6)	0.0305 (6)	0.0281 (6)	0.0010 (5)	0.0015 (5)	−0.0022 (5)
C7	0.0405 (7)	0.0311 (7)	0.0285 (6)	0.0055 (5)	−0.0029 (5)	0.0014 (5)
C1	0.0423 (8)	0.0358 (7)	0.0365 (7)	0.0034 (6)	−0.0061 (6)	−0.0052 (6)
C8	0.0355 (7)	0.0299 (6)	0.0279 (6)	0.0046 (5)	−0.0007 (5)	0.0005 (5)
C3	0.0403 (7)	0.0268 (6)	0.0369 (7)	−0.0007 (5)	−0.0004 (6)	0.0001 (5)
C6	0.0418 (7)	0.0256 (6)	0.0328 (7)	−0.0001 (5)	−0.0020 (6)	−0.0004 (5)
C11	0.0414 (8)	0.0508 (9)	0.0291 (7)	0.0001 (6)	−0.0039 (6)	0.0009 (6)
C13	0.0453 (8)	0.0341 (7)	0.0473 (8)	−0.0066 (6)	0.0051 (7)	−0.0096 (6)
C14	0.0398 (7)	0.0314 (7)	0.0367 (7)	0.0013 (5)	0.0030 (6)	0.0001 (5)
C12	0.0425 (8)	0.0533 (9)	0.0356 (7)	−0.0085 (7)	−0.0011 (6)	−0.0120 (7)
C10	0.0347 (7)	0.0347 (7)	0.0287 (6)	0.0020 (5)	0.0010 (5)	0.0001 (5)
C4	0.0434 (7)	0.0303 (7)	0.0294 (7)	0.0026 (5)	−0.0027 (5)	0.0031 (5)

Geometric parameters (Å, º)

O1—C10	1.3008 (17)	C9—C10	1.4323 (18)
O3—C1	1.2975 (18)	C7—C6	1.3873 (18)
O3—H3O	0.8200	C8—H8	0.9300
O2—C7	1.3497 (16)	C3—C4	1.3786 (19)
O2—H2	0.8200	C3—H3	0.9300
N1—C8	1.3067 (17)	C6—H6	0.9300
N1—C5	1.4095 (16)	C11—C12	1.366 (2)
N1—H1N	0.8600	C11—C10	1.4126 (19)
O4—C1	1.2319 (17)	C11—H11	0.9300
C5—C6	1.3848 (18)	C13—C14	1.358 (2)
C5—C4	1.3929 (18)	C13—C12	1.400 (2)
C2—C3	1.3879 (18)	C13—H13	0.9300
C2—C7	1.4062 (19)	C14—H14	0.9300
C2—C1	1.4820 (18)	C12—H12	0.9300
C9—C8	1.4069 (18)	C4—H4	0.9300
C9—C14	1.4163 (18)
O1···N1	2.5682 (15)	O3···O1^iv	2.5058 (15)
O1···O3^i	2.5058 (15)	O3···C11^iv	3.348 (2)
O1···C1^i	3.3169 (18)	O3···C10^iv	3.2998 (17)
O2···C14^ii	3.2530 (17)	O4···O2	2.5669 (16)
O2···O4	2.5669 (17)	N1···C10^v	3.4440 (16)
O2···C8^iii	3.3725 (17)
C1—O3—H3O	109.5	C4—C3—H3	119.2
C7—O2—H2	109.5	C2—C3—H3	119.2
C8—N1—C5	127.39 (11)	C5—C6—C7	119.51 (12)
C8—N1—H1N	116.3	C5—C6—H6	120.2
C5—N1—H1N	116.3	C7—C6—H6	120.2
C6—C5—C4	120.95 (12)	C12—C11—C10	120.91 (13)
C6—C5—N1	122.00 (12)	C12—C11—H11	119.5
C4—C5—N1	117.01 (11)	C10—C11—H11	119.5
C3—C2—C7	118.54 (12)	C14—C13—C12	119.67 (13)
C3—C2—C1	121.66 (12)	C14—C13—H13	120.2
C7—C2—C1	119.80 (12)	C12—C13—H13	120.2
C8—C9—C14	119.11 (12)	C13—C14—C9	120.67 (13)
C8—C9—C10	120.99 (12)	C13—C14—H14	119.7
C14—C9—C10	119.89 (12)	C9—C14—H14	119.7
O2—C7—C6	118.00 (12)	C11—C12—C13	121.53 (13)
O2—C7—C2	121.61 (12)	C11—C12—H12	119.2
C6—C7—C2	120.39 (12)	C13—C12—H12	119.2
O4—C1—O3	123.88 (13)	O1—C10—C11	122.43 (12)
O4—C1—C2	121.55 (13)	O1—C10—C9	120.26 (12)
O3—C1—C2	114.57 (12)	C11—C10—C9	117.31 (12)
N1—C8—C9	122.95 (12)	C3—C4—C5	118.88 (12)
N1—C8—H8	118.5	C3—C4—H4	120.6
C9—C8—H8	118.5	C5—C4—H4	120.6
C4—C3—C2	121.69 (12)
C8—N1—C5—C4	−167.01 (13)	C3—C4—C5—N1	−176.00 (13)
C8—N1—C5—C6	15.2 (2)	N1—C5—C6—C7	176.29 (13)
C5—N1—C8—C9	−176.72 (12)	C5—C6—C7—O2	−179.38 (13)
O3—C1—C2—C3	1.6 (2)	N1—C8—C9—C10	2.52 (19)
O3—C1—C2—C7	−178.75 (13)	N1—C8—C9—C14	−176.53 (12)
O4—C1—C2—C3	−178.39 (15)	C8—C9—C10—O1	1.19 (18)
O4—C1—C2—C7	1.3 (2)	C14—C9—C10—O1	−179.78 (12)
C1—C2—C7—O2	0.8 (2)	O1—C10—C11—C12	−179.66 (13)
C3—C2—C7—O2	−179.45 (13)

Symmetry codes: (i) x+3/2, −y+1/2, z+1/2; (ii) −x+1, −y+1, −z; (iii) −x, −y+1, −z; (iv) x−3/2, −y+1/2, z−1/2; (v) x−1, y, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1N···O1	0.86	1.88	2.5682 (15)	136
O2—H2···O4	0.82	1.84	2.5669 (17)	147
O3—H3O···O1^iv	0.82	1.70	2.5058 (15)	167
C3—H3···O3	0.93	2.44	2.7502 (18)	100
C8—H8···O2^iii	0.93	2.50	3.3725 (17)	156

Symmetry codes: (iii) −x, −y+1, −z; (iv) x−3/2, −y+1/2, z−1/2.