Crystal and molecular structures of 2-amino-4-nitrobenzoic acid and its cocrystals with 2,20-bipyridine (2/1) and bis(pyridin-2-yl)ketone (1/1)

Article abstract of DOI:10.1007/s10870-011-0114-7

The eight-membered {HOC=O}2 synthon featured in the crystal structure of 2-amino-4-nitrobenzoic acid (1) is replaced by carboxylic acidN-pyridine hydrogen bonds in its cocrystals with 2,20-bipyridine (2/1; 2) and bis(pyridin-2-yl)ketone (1/1; 3) indicating the robust nature of the latter synthon. Disruption of the three-dimensional architecture based on O-H.O and N-H.O(nitro) hydrogen bonds in (1) is evident in the cocrystals which form supramolecular tubes (2) and chains (3) based on O-H.N and N-H.O hydrogen bonding. Compound (1) crystallizes in the monoclinic space group P2 ₁/n with a = 3.6291(1) A° , b = 7.7339(3) A° , c = 26.561(1) A° , bβ = 91.385(2), and Z = 4. Compound (2) crystallizes in the monoclinic space group C2/c with a = 27.562(3) A° , bβ = 6.8300(6) A° , c = 12.923(1) A° , b = 110.593(5), and Z = 4. Compound (3) crystallizes in the monoclinic space group P2₁/c with a = 3.795(3) A° , b = 12.024(8) A° , c = 35.65(2) A° , b = 92.131(6), and Z = 4 (determined from synchrotron data). Springer Science+Business Media, LLC 2011.

Full text of DOI:10.1007/s10870-011-0114-7

J Chem Crystallogr (2011) 41:1418–1424
DOI 10.1007/s10870-011-0114-7
ORIGINAL PAPER
Crystal and Molecular Structures of 2-Amino-4-Nitrobenzoic
Acid and Its Cocrystals with 2,20-Bipyridine (2/1)
and Bis(pyridin-2-yl)ketone (1/1)
•
James L. Wardell Edward R. T. Tiekink
Received: 1 March 2011 / Accepted: 29 April 2011 / Published online: 12 May 2011
Ó Springer Science+Business Media, LLC 2011
Abstract The eight-membered {ꢀꢀꢀHOC=O}2 synthon
featured in the crystal structure of 2-amino-4-nitrobenzoic
acid (1) is replaced by carboxylic acidꢀꢀꢀN-pyridine hydrogen
bonds in its cocrystals with 2,20-bipyridine (2/1; 2) and
bis(pyridin-2-yl)ketone (1/1; 3) indicating the robust nature
of the latter synthon. Disruption of the three-dimensional
architecture based on O–HꢀꢀꢀO and N–HꢀꢀꢀO(nitro) hydrogen
bonds in (1) is evident in the cocrystals which form supra-
molecular tubes (2) and chains (3) based on O–HꢀꢀꢀN and
N–HꢀꢀꢀO hydrogen bonding. Compound (1) crystallizes in
Introduction
Investigations into the phenomenon of cocrystal formation,
that is multicomponent crystals [1], are motivated by sev-
eral imperatives. Primary amongst these are the applica-
tions in the pharmaceutical industry where the formation of
a cocrystal represents a derivativation of the original drug
by non-covalent association [2]. A key motivation behind
cocrystal formation rests with the proposal that cocrystals
formed with drugs have a reduced tendency to form
polymorphs compared with the drugs themselves [3, 4], a
consideration that has great implications for intellectual
property issues. Additional impetus revolves practical
concerns such an enhancing solubility for greater bio-
availability, stability to humidity, prolonging shelf life, etc.
[5, 6]. However, cocrystals have found uses predating these
applications, such as in organic synthesis [7], for promoting
crystal growth [8], as non linear optical materials [9], and
for the separation of enantiomers [10]. Other perhaps less
developed opportunities for cocrystals include their appli-
cations as luminescent materials [11], for stabilization of
otherwise unstable chemical species [12], including vola-
tile compounds such as perfumes [13], and for absolute
structure determination [14].
˚
the monoclinic space group P21/n with a = 3.6291(1) A,
˚
˚
b = 7.7339(3) A, c = 26.561(1) A, b = 91.385(2)°, and
Z = 4. Compound (2) crystallizes in the monoclinic space
˚
group C2/c with a = 27.562(3) A, b = 6.8300(6) A,
˚
˚
c = 12.923(1) A, b = 110.593(5)°, and Z = 4. Compound
(3) crystallizes in the monoclinic space group P21/c with
˚
˚
˚
a = 3.795(3) A, b = 12.024(8) A, c = 35.65(2) A, b =
92.131(6)°, and Z = 4 (determined from synchrotron data).
Keywords Carboxylic acid ꢀ Pyridines ꢀ Cocrystal ꢀ
Hydrogen bonding ꢀ Supramolecular chemistry
In a sense, the formation of a cocrystal goes against the
widely used technique of fractional crystallization for the
purification of organic compounds. The formation of a
cocrystal implies the resultant multicomponent crystal
features preferable supramolecular synthons compared to
those existing in the pure forms of the cocrystal formers.
These ideas lead to the concept of a hierarchy of supra-
molecular synthons where synthons are listed in order of
propensity of formation [15, 16], following on from the
ideas of Etter [17]. Amongst the most reliable supramo-
lecular synthons in crystal engineering of cocrystals and
J. L. Wardell (&)
´
´
Centro de Desenvolvimento Tecnologico em Saude (CDTS),
Fundac¸a˜o Oswaldo Cruz (FIOCRUZ), Casa Amarela, Campus de
Manguinhos, Av. Brasil 4365, Rio de Janeiro, RJ 21040-900,
Brazil
e-mail: j.wardell@abdn.ac.uk
E. R. T. Tiekink (&)
Department of Chemistry, University of Malaya,
Kuala Lumpur 50603, Malaysia
e-mail: Edward.Tiekink@gmail.com
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J Chem Crystallogr (2011) 41:1418–1424
1419
relevant to present study is that featuring a carboxylic acid-
OH hydrogen bonded to a pyridyl-N atom [18]. In con-
tinuation of ongoing studies of cocrystal formation
involving pyridyl-N atoms [19–23], in the present report,
the crystal and molecular structures of 2-amino-4-nitro-
benzoic acid, hitherto not described crystallographically,
and its 2/1 and 1/1 cocrystals with the pyridine contain-
ing bases 2,20-bipyridine and bis(pyridin-2-yl)ketone,
respectively.
CrystalClear [27] and APEX2 [28]. The structures were
solved by direct methods with SHELXS-97 [29] and
refined by a full-matrix least-squares procedure on F2 using
SHELXL-97 [29] with anisotropic displacement parame-
ters for non-hydrogen atoms and a weighting scheme of the
form w = 1/[r2(F2o) ? aP2 ? bP] where P = (Fo2 ? 2F2c)/
3. Carbon-bound hydrogen atoms were included in the final
˚
refinement in their calculated positions with C–H = 0.95 A.
The acidic hydrogen atoms were located in difference
˚
Fourier maps and refined with O–H = 0.84(1) A and
˚
N–H = 0.88(1) A, and with Uiso(H) = yUeq(parent atom);
Experimental
y = 1.5 for O and 1.2 for N. Disorder in the nitro group in (1)
could not be resolved so in the final cycles of refinement the
anisotropic displacement parameters were constrained to be
nearly isotropic with the ISOR command in SHELXL-97
[29] and the N–O bond distances were refined with N–
Synthesis
Reagents were Commercial Samples. Crystals of 2-amino-
4-nitrobenzoic acid were isolated from the slow evapora-
tion from its ethanol solution; M.pt. 270–271 °C.
˚
O = 1.220(1) A. Crystal data and refinement details are
given in Table 1. Figures 1, 2 and 3 showing the atom
labeling schemes, were drawn with 50% displacement
ellipsoids using ORTEP-3 [30], and the remaining figures
were drawn with DIAMOND [31] with arbitrary spheres.
Data manipulation and interpretation were accomplished
using WinGX [32] and PLATON [33].
2-Amino-4-nitrobenzoic acid 2,20-bipyridine (2/1)
A solution containing 2-amino-4-nitrobenzoicacid (1.0 mmol)
and 2,20-bipyridine (2.10 mmol) in ethanol (20 mL) was
maintained at room temperature and crystals of the 2:1
cocrystalline product were slowly formed and harvested;
M.pt. 197–198 °C.
Results and Discussion
A solution of 2-amino-4-nitrobenzoic acid (1.0 mmol)
and 2,20-bipyridine (1.1 mmol) in ethanol (20 mL) also
gave crystals of the 2/1 cocrystal.
Molecular Structures
The molecular structure of the acid 2-amino-4-nitrobenzoic
acid, (1), is shown in Fig. 1 and selected geometric
parameters for this structure, as well as for (2) and (3), are
displayed in Table 2. The molecule exists as the acid, a
conclusion confirmed by the disparity in the C–O bond
distances associated with the carboxylic acid residue,
Table 2. The carboxylic acid is effectively coplanar with
the aromatic ring to which it is connected, a configura-
tion assured owing to the presence of an intramolecular
N–HꢀꢀꢀO hydrogen bond formed between the proximate
amino acid and the carbonyl-O2 atom. By contrast, the
nitro group is twisted out of the plane, Table 2.
2-Amino-4-nitrobenzoic acid bis(pyridin-2-yl)ketone
(1/1)
A solution containing 2-amino-4-nitrobenzoic acid (1.0 mmol)
and bis(pyridin-2-yl)ketone (1.05 mmol) in ethanol (20 mL)
was maintained at room temperature and crystals of the 1/1
cocrystalline product were slowly formed and harvested;
M.pt. 185–188 °C.
1
In both cases the IR (in KBr) and H NMR (in Me2CO)
spectra of the respective cocrystalline product were simple
the summation of the spectra of the components as is often
observed for cocrystals [19, 20, 23].
The asymmetric unit of (2) comprises a molecule of
2-amino-4-nitrobenzoic acid in a general position and half
a 2,20-bipyridine, as this is located about a crystallographic
2-fold axis, Fig. 2. Therefore, (2) is a 2/1 cocrystal. Con-
firmation of the presence of an acid is found in the derived
geometric parameters which match closely those deter-
mined for the acid (1). By contrast to the situation in (1),
both the carboxylic acid and nitro groups are coplanar with
the aromatic ring, Table 2. There is a significant twist
about the central C12–C12i bond in the 2,20-bipyridine
molecule as seen in the value of the N3–C12–C12i–
N3i torsion angle of 157.41(17)°; symmetry operation
X-ray Crystallography
Intensity data for a bronze needle of (1) and an orange
block of (2) were collected at 120 K on a Enraf–Nonius
FR591 rotating anode CCD fitted with Mo Ka radiation.
The data sets were corrected for absorption based on
multiple scans [24] and reduced using standard methods
[25, 26]. Data for a red rod of (3) were collected at 120 K
on a Bruker SMART APEX2 CCD using synchrotron
˚
radiation with k = 0.6905 A; data processing was with
123

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J Chem Crystallogr (2011) 41:1418–1424
Table 1 Crystal data and refinement details for (1)–(3)
Compound
(1)
(2)
(3)
Empirical formula
Formula weight
Crystal habit, color
Crystal system
Space group
C7H6N2O4
C10H8N2, 2(C7H6N2O4)
C11H5N2O, C7H9N2O4
182.14
520.46
366.33
Needle, bronze
Block, orange
Rod, red
Monoclinic
Monoclinic
Monoclinic
P21/n
C2/c
P21/c
˚
a (A)
3.6291(1)
27.562(3)
3.795(3)
˚
b (A)
7.7339(3)
6.8300(6)
12.024(8)
˚
c (A)
26.561(1)
12.923(1)
35.65(2)
b (°)
Volume (A )
93.569(2)
110.593(5)
92.131(6)
3
˚
744.03(5)
2277.4(4)
1625.5(18)
Z
4
4
4
Density (calculated, g cm-3
)
1.626
1.518
1.497
Absorption coefficient (mm-1
)
0.136
0.117
0.112
F(000)
376
1080
760
Crystal size (mm)
0.05 9 0.05 9 0.25
0.11 9 0.20 9 0.40
0.005 9 0.005 9 0.06
h range for data collection (°)
Reflections collected
Independent reflections
Rint
3.1–26.5
3.1–27.5
1.1–25.0
14326
21394
12895
1547
2613
3050
0.051
0.067
0.060
Reflections with I C 2r(I)
Number of parameters
Goodness-of-fit on F2
Final R indices [I C 2r(I)]
a, b for weighting scheme
R indices [all data]
1272
1909
2413
127
181
253
1.07
1.04
1.09
R1 = 0.042, wR2 = 0.112
0.073, 0.159
R1 = 0.055, wR2 = 0.136
0.093, 0.487
R1 = 0.052, wR2 = 0.124
0.070, 0.403
R1 = 0.051, wR2 = 0.119
0.32, -0.39
R1 = 0.082, wR2 = 0.151
0.29, -0.32
R1 = 0.068, wR2 = 0.137
0.35, -0.28
-3
)
˚
Largest difference peak and hole (A
CCDC deposition no.
814981
814982
814983
i = 1 - x, y, -1/2 - z. The N atoms are orientated in
opposite directions, a conformation that allows each to
participate in significant hydrogen bonding interactions
(see below). Cocrystal (2) was also isolated from the co-
crystallization experiment containing equimolar quantities
of the reagents highlighting the stability of the carboxylic
acidꢀꢀꢀpyridine interaction.
atoms resulting in a bifurcated interaction which effec-
tively blocks off this site from interacting with the car-
boxylic acid. Hence, the isolation of the 1/1 cocrystal in
contrast to the 2/1 cocrystal formed in the case when the
base was 2,20-bipyridine from the 1:1 cocrystallization
experiment (see Experimental).
The asymmetric unit of (3) comprises a single molecule
each of 2-amino-4-nitrobenzoic acid and bis(pyridin-2-
yl)ketone, Fig. 3, and is therefore a 1/1 cocrystal. Geo-
metric parameters associated with the carboxylic acid
reside match those found in (1) as does the coplanarity and
twisting of the carboxylic acid and nitro groups, respec-
tively, out of the plane of the aromatic ring. In the base, the
dihedral angle between the pyridyl rings is 43.24(9)°
indicating a significant twist in the molecule. As with (2),
the N atoms are orientated in opposite directions to opti-
mize hydrogen bonding interactions (see below). The
hydrogen bond formed between the amino-H and the car-
bonyl-O5 brings into proximity one of the pyridine (N3)
Supramolecular Aggregation
Based on O–HꢀꢀꢀO and N–HꢀꢀꢀO(nitro) hydrogen bonding, a
three-dimensional architecture is apparent in the crystal
structure of (1); see Table 3 for geometric data describing
the most significant intermolecular forces operating in the
crystal structures of (1)–(3). Centrosymmetrically related
carboxylic acid residues associate via O–HꢀꢀꢀO hydrogen
bonds to form an eight-membered {ꢀꢀꢀHOC=O}2 synthon.
The dimeric aggregates are connected by N–HꢀꢀꢀO(nitro)
hydrogen bonds whereby each amino-H forms a connection
to a nitro-O of different molecules resulting in supramo-
lecular chains extending in the ab plane; as the amino-
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J Chem Crystallogr (2011) 41:1418–1424
1421
Fig. 3 The two molecular aggregate of (3) showing atom labeling.
Hydrogen bonds are shown as dashed lines. Displacement ellipsoids
are drawn at the 50% probability level (Color figure online)
H1n atom also forms an intramolecular N–HꢀꢀꢀO2 contact,
it is bifurcated (Fig. 4).
In (2), the familiar carboxylic acid dimer is absent as
the carboxylic acid-OH forms a hydrogen bond with the
pyridyl-N3 atom, an arrangement stabilized by a close
C–H18ꢀꢀꢀO2 contact resulting in the seven membered
{HOC=OꢀꢀꢀHCN} heterosynthon [18]. By contrast to that
found in (1), the amino-H atoms form hydrogen bonds with
carboxylic acid-O rather than nitro-O atoms with each H
atom forming a contact to one of the O atoms of the for-
mer; the persistence of the intramolecular H1nꢀꢀꢀO2
hydrogen bond is noted. The result of the hydrogen
bonding is the formation of supramolecular tubes aligned
along the c axis, Fig. 5a, b. The tubes are connected into
the three-dimensional structure via C–HꢀꢀꢀO interactions
involving the nitro-O atoms, Fig. 6.
Fig. 1 The molecular structure of (1) showing atom labeling.
Displacement ellipsoids are drawn at the 50% probability level
(Color figure online)
˚
Table 2 Selected bond lengths (A), angles (°), and torsion angles (°)
for the 2-amino-4-nitrobenzoic acid molecule in (1)–(3)
Parameter
(1)
(2)
(3)
1.346(3)
C1–O1
1.324(2)
1.331(2)
C1–O2
1.238(2)
1.469(2)
1.347(2)
121.8(1)
114.7(1)
123.5(1)
175.9(1)
-165.4(1)
1.222(2)
1.486(2)
1.354(2)
122.8(2)
113.7(1)
123.5(2)
-173.6(1)
-177.3(2)
1.238(2)
1.495(3)
1.371(3)
122.1(2)
113.5(2)
124.4(2)
-179.0(2)
164.7(2)
C1–C2
As with (2), the carboxylic acid-OH forms a hydrogen
bond with a pyridyl-N4 atom; a close C–H8ꢀꢀꢀO2 contact
closes the seven membered {HOC=OꢀꢀꢀHCN} heterosyn-
thon. The amino-H atoms participate into three hydrogen
bonding contacts. As with the previous structures, the
H1n atom forms an intramolecular hydrogen bond with the
carboxylic acid-O2 atom. The H2n atom is bifurcated,
C3–N1
O1–C1–O2
C2–C1–O1
C2–C1–O2
O1–C1–C2–C3
C4–C5–N2–O3
Fig. 2 The three molecular
aggregate of (2) showing atom
labeling for the asymmetric
unit; symmetry operation i:
1 - x, y, -1/2 - z. Hydrogen
bonds are shown as dashed
lines. Displacement ellipsoids
are drawn at the 50%
probability level (Color figure
online)
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J Chem Crystallogr (2011) 41:1418–1424
Table 3 Hydrogen bonding
˚
A
H
B
HꢀꢀꢀB
AꢀꢀꢀB
A–HꢀꢀꢀB
Symmetry operation
parameters (A–HꢀꢀꢀB; A, °)
for (1)–(3)
(1)
O1
H1o
H1n
H1n
H2n
O2
O2
O3
O4
1.80(2)
2.04(2)
2.45(2)
2.25(1)
2.653(2)
2.688(2)
2.820(2)
3.093(2)
178(1)
130(1)
106(1)
161(1)
2 - x, -y, -z
N1
N1
N1
x, y, z
1 ? x, -1 ? y, z
2‘ - x, -1/2 ? y, 1/2 - z
(2)
O1
N1
N1
N1
C8
C9
C10
H1o
H1n
H1n
H2n
H8
N3
O2
O2
O1
O2
O3
O4
1.81(2)
2.04(2)
2.49(2)
2.24(2)
2.40
2.664(2)
2.697(2)
3.088(2)
3.076(2)
3.157(2)
3.465(3)
3.405(2)
176(2)
131(1)
125(1)
159(2)
137
x, y, z
x, y, z
1 - x, y, 1/2 - z
x, 1 - y, 1/2 ? z
x, y, z
H9
2.57
157
-1/2 ? x, 1‘ - y, -1/2 ? z
-1/2 ? x, 1/2 ? y, -1 ? z
H10
2.46
176
(3)
O1
H1o
H1n
H2n
H2n
H18
H13
H16
N4
O2
O5
N3
O2
O1
O2
1.98(2)
2.07(2)
2.38(2)
2.55(2)
2.49
2.814(3)
2.769(3)
3.091(3)
3.367(4)
3.273(4)
3.353(4)
3.443(4)
170(2)
136(2)
138(2)
155(2)
140
x, 1 ? y, z
N1
x, y, z
N1
x, y, z
N1
x, y, z
C18
C13
C16
x, -1 ? y, z
-1 ? x, -1 ? y, z
-1 - x, -1/2 ? y, 1/2 - z
2.53
146
2.55
156
Fig. 4 View in projection down
the b axis of the crystal packing
in (1). The O–HꢀꢀꢀO and
N–HꢀꢀꢀO hydrogen bonds are
shown as orange and blue
dashed lines, respectively
(Color figure online)
being connected to the carbonyl-O5 and pyridyl-N3 atoms.
The result is the formation of a supramolecular chain along
the b axis, Fig. 7. Interactions of the type C–HꢀꢀꢀO
involving the carboxylic acid-O atoms exclusively link
chains into layers two molecules thick that stack along c,
Fig. 8. The nitro groups lie to the periphery of the layers,
resembling the situation seen for (2), and form weaker
C–HꢀꢀꢀO contacts but longer the standard criteria in
PLATON [33]. The shortest of these contacts involves
The characterization of cocrystals presented herein repre-
sents the first reported for 2-amino-4-nitrobenzoic acid
which has hitherto only been characterized in its salts
according to a search of the CSD [34]. There are three
examples of salts, namely ethylenediammonium 4-nitroanth-
ranilate dehydrate [35], dicyclohexylammonium 4-nitro-
anthranilate [36], and guanidinium 2-amino-4-nitrobenzoate
monohydrate [37]. In each of these the original base did not
feature a pyridine-N atom and proton transfer occurred
during the cocrystallisation experiment. The isolation of the
cocrystals (2) and (3) is a testament to the robust nature of the
seven-membered carboxylic acidꢀꢀꢀN-pyridine synthon [18].
i
i
C6–H6 and O3i [H6ꢀꢀꢀO3 = 2.62 A, C6ꢀꢀꢀO3 = 3.444
˚
˚
(3) A with an angle at H6 = 145° for i: 2 - x, 1 - y, 1 - z].
Conclusions
The presence of 2-amino-4-nitrobenzoic acid in its
cocrystals formed with 2,20-bipyridine (2/1) and bis(pyridin-
2-yl)ketone (1/1) is confirmed by comparison of geometric
parameters defining the respective carboxylic acid residues.
Supplementary Material
CCDC 814981–814983 contains the supplementary crys-
tallographic data for the three structures reported in this
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J Chem Crystallogr (2011) 41:1418–1424
1423
Fig. 5 Supramolecular tube in
(2) sustained by O–HꢀꢀꢀO and
N–HꢀꢀꢀO hydrogen bonds shown
as orange and blue dashed lines,
respectively: a side-on view and
b end-on view (Color figure
online)
Fig. 6 View in projection down
the c axis of the crystal packing
in (2). The O–HꢀꢀꢀO and
N–HꢀꢀꢀO hydrogen bonds, and
C–HꢀꢀꢀO contacts are shown as
orange, blue and green dashed
lines, respectively. The
supramolecular tubes, shown in
Fig. 5, are orientated in the
c direction and are connected by
C–HꢀꢀꢀO contacts (Color figure
online)
Fig. 7 Supramolecular chain
aligned along b in (3) with a flat
topology and sustained by
O–HꢀꢀꢀO and N–HꢀꢀꢀO hydrogen
bonds shown as orange and blue
dashed lines, respectively
(Color figure online)
123

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J Chem Crystallogr (2011) 41:1418–1424
Fig. 8 A view of the unit cell
contents in (3) shown in
projection down the a axis
highlighting the stacking of
layers along c. The O–HꢀꢀꢀO and
N–HꢀꢀꢀO hydrogen bonds, and
C–HꢀꢀꢀO contacts are shown as
orange, blue and green dashed
lines, respectively (Color figure
online)
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www.ccdc.cam.ac.uk/conts/retrieving.html [or from the
Cambridge Crystallographic Data Centre (CCDC), 12
Union Road, Cambridge CB2 1EZ, UK; fax ?44(0)1223-
336033; email: deposit@ccdc.ccam.ac.uk].
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Acknowledgments JLW acknowledges support from CAPES and
FAPEMIG (Brazil). The University of Malaya is thanked for support
this research (UMRG RG125/10AFR). The use of the EPSRC X-ray
crystallographic service at the University of Southampton, England,
Prof. W. Clegg and the synchrotron component, based at Daresbury,
and the valuable assistance of the staff at those centers are gratefully
acknowledged.
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