organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
2. Experimental
2.1. Synthesis and crystallization
In a 100 ml flask, 4-aminosalicylic acid (10.0 mmol, 1.0 g)
was dissolved in methanol (20 ml) and salicylaldehyde
(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 tetra-
hydrofuran (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 cmꢁ1. The
main absorption bands are (cmꢁ1): 3426 (ꢁOH), 3049 (ꢁC—
Harom), 1642 (ꢁC O), 1608 (ꢁC N), 1512 (ꢁC C), and 791
and 762 (ꢂoopC—Har; oop = out of plane).
ISSN 0108-2701
The zwitterionic structure of
2-hydroxy-4-[(2-hydroxybenzyl-
idene)amino]benzoic acid
´
Anderson A. B. C. Junior, Gustavo S. G. De Carvalho,
ˆ
Lippy F. Marques, Charlane C. Correa, Adilson D. Da Silva
´
and Flavia C. Machado*
Department of Chemistry, Federal University of Juiz de Fora, Juiz de Fora, Minas
Gerais 36036-900, Brazil
Correspondence e-mail: flavia.machado@ufjf.edu.br
Received 20 June 2013
Accepted 1 July 2013
The title compound, C14H11NO4, exists in the solid phase in the
zwitterionic form, 2-{[(4-carboxy-3-hydroxyphenyl)iminium-
yl]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—Hꢀ ꢀ ꢀ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—Hꢀ ꢀ ꢀO
hydrogen bond between the carboxylic acid group and the
adjacent hydroxy group of the other ring, and an inter-
molecular C—Hꢀ ꢀ ꢀ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 (103) plane.
ꢀ–ꢀ stacking interactions result in a three-dimensional
network. This study is important because it provides crystal-
lographic evidence, supported by IR data, for the iminium
zwitterionic form of Schiff bases.
2.2. Refinement
Crystal data, data collection and structure refinement
details are summarized in Table 1. O- and N-bound H atoms
were initially located in a difference Fourier map and were
then added in idealized positions and further refined
˚
according to the riding model, with O—H = 0.82 A and
˚
N—H = 0.86 A, and with Uiso(H) = 1.5Ueq(O) or 1.2Ueq(N).
C-bound H atoms were included in the riding-model
˚
approximation, with C—H = 0.95 A and Uiso(H) = 1.2Ueq(C).
3. Results and discussion
The title Schiff base, (I) (Fig. 1), 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 crystallizes in the zwitterionic form, with an anionic
phenolate and a cationic iminium group.
Keywords: crystal structure; carboxylic acids; zwitterionic
structures; Schiff bases.
1. Introduction
˚
The C8—N1 bond length [1.3067 (17) A] agrees well with
the double-bond character of imine groups in other zwitterion
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 anti-
allergic, 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 chemistry (Yao et
al., 2006; Le Guennic et al., 2007).
Figure 1
A view of asymmetric unit of (I), showing the atom-numbering scheme.
Displacement ellipsoids are drawn at the 50% probability level.
934 # 2013 International Union of Crystallography
doi:10.1107/S0108270113018143
Acta Cryst. (2013). C69, 934–936