Sp2CH?Cl hydrogen bond in the conformational polymorphism of 4-chloro-phenylanthranilic acid

Article abstract of DOI:10.1039/c7ce00772h

Chlorine can participate in numerous interactions such as halogen bonding, hydrogen bonding, and London dispersion in the solid state. In this work, we report the influence of a chlorine substituent on the polymorphism of a potential anticancer drug, 4-ch

Full text of DOI:10.1039/c7ce00772h

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2
sp CH⋯Cl hydrogen bond in the conformational
polymorphism of 4-chloro-phenylanthranilic acid†
Cite this: DOI: 10.1039/c7ce00772h
d
Meng Liu, a Chuming Yin, a Peng Chen, a Mingtao Zhang, b Sean Parkin,
a
Panpan Zhou, c Tonglei Li, b Faquan Yu
a and Sihui Long
*
*
Chlorine can participate in numerous interactions such as halogen bonding, hydrogen bonding, and Lon-
don dispersion in the solid state. In this work, we report the influence of a chlorine substituent on the poly-
morphism of a potential anticancer drug, 4-chloro-phenylanthranilic acid (CPAA). Three polymorphs have
been discovered for this compound, and the three forms were characterized by single-crystal X-ray diffrac-
tion, power X-ray diffraction (PXRD), FT-IR, and Raman spectroscopy. Both conformational flexibility of the
2
molecule and the sp CH⋯Cl hydrogen bond seem to lead to the polymorphism of the system. The phase
behavior was investigated by differential scanning calorimetry (DSC), with the conclusion that form II con-
verts to III upon heating. A conformational scan shows the conformational minima corresponds to the
conformers existing in the polymorphs. Lattice energy calculations show energies of −106.70, −104.72, and
−194.42 kJ mol−1 for forms I to III, providing information on relative stability for each form. Hirshfeld analy-
sis revealed that intermolecular interactions such as H⋯H, C⋯H, H⋯Cl, and H⋯O contribute to the stabil-
ity of the crystal forms.
Received 24th April 2017,
Accepted 7th July 2017
DOI: 10.1039/c7ce00772h
rsc.li/crystengcomm
and some have shown great promise.2–4 In addition, FAs are
also synthetic precursors to acridones and acridines, which
1. Introduction
Non-steroidal anti-inflammatory drugs (NSAIDs) are essential
medicines in everyday life since some of them also possess
antipyretic, analgesic, and antirheumatic properties in addi-
tion to anti-inflammatory effects.1 Classic NSAIDs include
salicylic acid derivatives, pyrozolones, arylalkanoic acids [aryl-
and heteroarylacetic acids, aryl- and heteroarylpropionic
acids, and N-arylanthranilic acids (a.k.a. fenamic acids)], and
oxicams.
Fenamic acids (FAs) are nitrogen isosteres of salicylic acid.
Representative fenamic acid NSAIDs include mefenamic acid,
meclofenamic acid, chlofenamic acid, tolfenamic acid,
flufenamic acid, niflumic acid, clonixin, and flunixin. These
compounds are potent cyclooxygenase (COX) inhibitors,
which lend them anti-inflammatory properties. Like other
NSAIDs, they can also act as analgesics and antirheumatics.
Recently, FAs have also been investigated as therapeutics for
neurodegenerative and amyloid diseases as well as cancer,
present bioactive properties such as anti HIV, anticancer,
antibacterial, antifungal, and antimalarial activities.5
4-Chloro-2-phenylanthranilic acid (CPAA) (Scheme 1) is an
FA with a modest COX inhibitory effect due to the presence
of the chlorine atom on the anthranilic acid moiety [struc-
ture–activity relationship (SAR) studies of FAs indicate that
substitution on the anthranilic acid aromatic ring makes
them weak COX inhibitors].6,7 Although it has poor anti-
inflammatory properties, it shows good inhibitory effects on
the aldol-keto reductase (AKR), which likely is involved in the
development of hormone-dependent breast cancer.8 It was
also investigated as a Cl−-channel blocker in chloride trans-
porting epithelia, which has ramifications in myotonia such
as Thomsen's and Becker's disease, cystic fibrosis, hereditary
calculus of kidney, and type 3 Bartter's syndrome.9 Its affinity
to bovine albumin was also studied to help design better FA
drugs.10
Fenamic acids are diarylamines and conformationally flex-
ible, which makes them prone to show polymorphism, i.e.,
a Key Laboratory for Green Chemical Process of Ministry of Education, School of
Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan,
Hubei, China. E-mail: fyuwucn@gmail.com, sihuilong@wit.edu.cn,
longsihui@yahoo.com; Tel: +(027) 87194980, +(86) 15549487318
b Department of Industrial and Physical Pharmacy, Purdue University, West
Lafayette, Indiana, USA
c Department of Chemistry, Lanzhou University, Lanzhou, Gansu, China
d Department of Chemistry, University of Kentucky, Lexington, Kentucky, USA
† CCDC 1545269, 1545271 and 1545272. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/c7ce00772h
Scheme 1 Structure of 4-chloro-2-phenylanthranilic acid.
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existing in more than one crystal form.11 Polymorphism is
widely observed in organic compounds, particularly those
that are conformationally flexible.12 Many FAs exhibit poly-
morphism. For example, two, five, nine, two, and four forms
have been found for mefenamic acid,13 tolfenamic acid,14
flufenamic acid,15 niflumic acid,16 and clonixin,17 respec-
tively. The existence of nine forms of flufenamic acid is
reported to be the current world record for most forms hav-
ing been structurally characterized by single-crystal X-ray dif-
fraction.15 Polymorphism is of particular significance in
pharmaceuticals because different forms of the same API
may have different kinetic, thermodynamic, surface, mechan-
ical, and packaging properties, which can affect clinical for-
mulation and eventual bioavailability.18,19
Similar to the classic fenamic acid NSAIDs, CPAA is
conformationally flexible. It differs from the well-known FA
drugs, which have substituents on the aniline ring, by having
the substituent Cl on the anthranilic acid aromatic ring of
CPAA instead. Chlorine is known to have steering effect on
the spatial arrangement of the molecules due to its participa-
tion in halogen bonds.20–24 Moreover, the carboxylic acid is
able to form both a catemer motif and the more common
acid–acid dimer.25 How would the combination of conforma-
tional flexibility, variability of carboxylic acid interactions
and the presence of Cl on the anthranilic acid aromatic ring
influence the polymorphic behavior of CPAA? In this study,
we investigate the effect of the substituent, i.e., Cl on the
solid-state properties, specifically polymorphism of CPAA. We
describe three polymorphs for CPAA, their characterization,
phase behavior, and the related theoretical studies.
Scheme 2 Synthesis of CPAA.
ature water. The mixture was filtered. The crude product was
obtained by precipitation upon acidification of the filtrate
with dilute HCl. The product was purified on a silica-gel col-
umn using petroleum ether/ethyl acetate (v/v 4 : 1, Rf = 0.50)
as eluent. The product was obtained as colorless solid (1.55
g, yield%: 60).
1HNMR (400 MHz, CDCl3): δppm 9.36 (s, 1H), 7.96 (d, 1H),
7.41 (t, 2H), 7.27 (d, 2H), 7.20 (t, 1H) 7.14 (d, 1H), 6.71 (dd,
1H); 13CNMR (100 MHz, CDCl3): δppm 173.0, 150.0, 142.0,
139.7, 133.7, 129.6, 125.4, 123.9, 117.4, 113.2, 108.4; IR (KBr,
cm−1) 3330 (s), 3036–2500 (s), 1665 (s), 1559 (s), 1498(s), 1416
(s), 1330 (m), 1251 (s), 1156 (s), 1094 (s), 930 (s), 752 (s), 684
(s), 574 (s); MS (ESI): 248.24 (M + 1); mp: 204 °C.
2.3. Crystal growth
The crystal growth method of slow evaporation, slow cooling,
and quench cooling was applied for polymorph screening of
CPAA. For slow evaporation, the pure compound was
dissolved in different solvents, forming saturated solutions at
ambient temperature (∼22 °C) (Table 1). The solutions were
set for slow evaporation in a vibration-free place until single
crystals were harvested. As an example, 100 mg of CPAA was
added to 10 mL HPLC grade methanol. The mixture was
stirred overnight and the remaining solid was removed by pi-
pette filtration. A vial containing the clear solution was cov-
ered with perforated parafilm. Slow evaporation led to single
crystals in about a week. For slow cooling, a saturated solu-
tion in a beaker covered by cover glass was concentrated to
1/3 of the original volume by gentle heating on a hot plate,
followed by cooling to room temperature with the heat source
turned off. For quench cooling, a supersaturated solution of
2. Experimental section
2.1. General
All solvents and reagents were purchased from commercial
sources and used as received: 2,4-dichlorobenzoic acid, ani-
line, and Cu2O were from Aladdin; Cu, K2CO3,
2-ethoxyethanol, and the solvents used for crystal growth were
from Sinopharm Chemical Reagent Co., Ltd. The IR, Raman
and UV-vis spectra were recorded using an FT-IR Perkin-
Elmer LX10-8873, a Thermo Electron DXR Laser Confocal
Microscopy Raman Spectrometer, and a Lambda 750 UV/vis/
NIR spectrophotometer, respectively. NMR spectra were
obtained on a Varian INOVA spectrometer at an observation
frequency of 400 MHz. Thermal analyses were performed on
an SII DSC 6220 (SEIKO, Japan) apparatus, using a heating
rate of 10 °C min−1.
Table 1 Crystal growth of CPAA in a variety of solvents
Solvents
Method
Form
MeOH
Hexane
i-PrOH
CH3CN
Toluene
CHCl3
EtOAc
EtOAc
Acetone
Ether
CH2Cl2
DMSO
THF
DMF
H2O
Slow evaporation
Slow evaporation
Slow evaporation
Slow evaporation
Slow evaporation
Slow evaporation
Slow evaporation
Slow cooling
Slow evaporation
Slow evaporation
Quench cooling
Slow evaporation
Slow evaporation
Slow evaporation
Slow evaporation
Slow evaporation
Slow evaporation
I
I
I
I
I
I
I
II
II
II
II
II
II
II
2.2. Synthesis and characterization
Synthesis of CPAA (Scheme 2). 2,4-Dichlorobenzoic acid
(2.00 g, 10.0 mmol), aniline (1.02 g, 11.0 mmol), Cu (5.8 mg,
0.9 mmol), Cu2O (5.8 mg, 0.4 mmol) and K2CO3 (2.76 g, 20.0
mmol) were added to a round-bottom flask, followed by addi-
tion of 2-ethoxyethanol (13 mL) under nitrogen protection.
The resulting mixture was refluxed at 130 °C for 24 h. The
hot reaction mixture was poured into 30 mL of room temper-
No crystals formed
III
III
Benzene
CH3COOH
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CPAA in CH2Cl2 at 40 °C was rapidly cooled in a freezer with
a temperature of −20 °C. All crystallization experiments were
conducted in an unmodified atmosphere. Each experiment
was repeated multiple times. Specifically, forms I, II, and III
can be produced in chloroform, methylene chloride, and
acetic acid, respectively, and other solvents as well. The iden-
tity of the crystals was confirmed by powder X-ray diffraction
for tiny, and by single-crystal X-ray diffraction when high-
quality single crystals were obtained.
2.4. Crystal structure determination
The crystal structures of all three polymorphs of CPAA were
determined by single-crystal X-ray diffraction.
Data collection was carried out at 90 K on a Nonius
kappaCCD diffractometer with MoKα radiation (λ = 0.71073
Å).26 Cell refinement and data reduction were done using
SCALEPACK and DENZO-SMN.27 Structure solution and refine-
ment were carried out using the SHELXS and SHELXL2016 pro-
grams, respectively.28,29
Fig. 1 Labeling of CPAA.
Powder X-ray diffraction (PXRD) data for each sample were
collected on a Rigaku X-ray diffractometer with CuKα radia-
tion (40 kV, 40 mA, λ = 1.5406 Å) between 5.0–50.0° (2θ) at
ambient temperatures. The finely ground sample was placed
on a quartz plate in an aluminum holder.
Lattice energy. To calculate lattice energies for each CPAA
form, the experimental crystal structures were first optimized
with the lattice parameters kept constant. Optimization was
conducted by DFT using PW1PW functional with 6-21g** ba-
sis sets by Crystal14.30 Dispersion energy contributions to lat-
tice energies were calculated using the DFT-D3 program of
Grimme, with Becke–Johnson damping.31,32 Basis set super-
position error (BSSE) was considered by the counterpoise
method.33 The energy convergence of the optimization and
energy calculation was set to 10−7 hartree. The root-mean-
square (RMS) values for calculation convergence were set to
0.0003 and 0.0012 au for energy gradient and atomic dis-
placement, respectively. All calculations were conducted on a
Linux cluster.
2.5. Thermal analyses
Phase behavior for each of the solid forms was studied by
differential scanning calorimetry (DSC). The instrument and
procedure were aforementioned.
Spectroscopic studies
IR and Raman. For IR spectrum recording, samples were
dispersed in KBr pellets. For Raman measurement, samples
were compressed in a gold-coated sample holder.
UV-vis. For the UV-vis experiments, solutions (in chloro-
form, dichloromethane, and acetic acid) were contained in
quartz cuvettes. The concentrations of the solutions were 1 ×
10−4 mol L−1, 5 × 10−5 mol L−1 and 1 × 10−4 mol L−1 respectively.
Hirshfeld surface analysis. Hirshfeld surface analy-
ses34,35 were performed with CrystalExplorer (Version 3.1)36
to further understand the relative contributions to inter-
molecular interactions by various molecular contacts in
the polymorphs.
2.6. Computational details
Conformation search. Conformational flexibility poten-
tially derives from three bonds, i.e., C2–C14, C1–N7 and N7–
C8 (Fig. 1). However, the double-bond character of C1–N7 (C–
C = ∼1.37 Å) precludes rotation, and in spite of the essen-
tially single-bond character for C2–C14 bond (C–C = ∼1.47
Å), the carboxylic acid group in all conformers is coplanar
with the aromatic ring (Table 2). Thus, the energy of different
conformations of a single CPAA molecule defined solely by
the torsion angle, τ (C1–N7–C8–C9) was evaluated with Gauss-
ian 09 (Gaussian, Inc., Wallingford, CT). The molecule was
optimized from various initial structures in order to identify
the most stable conformation, which was then used for scan-
ning each torsion angle with all bond lengths, bond angles,
and other torsion angles fixed. Structural optimization and
conformational searches were performed at the B3LYP/6-
311+G(d, p) level of theory.
3. Results and discussion
3.1. Crystal structures
Three polymorphs (I, II, and III) of CPAA have been discov-
ered so far (Fig. 2). Form I crystals were obtained as colorless
plates from (but not limited to) ethyl acetate, form II crystals
Table 2 Bond length of C1–N7 and N7–C8 in the conformers of CPAA
Bond length (Å)
Conformer
C1–N7
N7–C8
I-A
I-B
II-A
II-B
III
1.372
1.371
1.366
1.368
1.376
1.420
1.421
1.426
1.416
1.407
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Fig. 2 Crystals of CPAA polymorphs I–III.
were grown as long yellow rods from (but not limited to) ace-
tone and form III crystals were harvested as colorless rods
from benzene or acetic acid. Structure determination by
single-crystal X-ray diffraction found form I to be triclinic,
In form I, the asymmetric unit consists of two molecules
(Z′ = 2), each having a twisted conformation with the dihe-
dral angle between the two aromatic rings of 54.84IJ5)° for
molecule A and 60.53IJ5)° for molecule B. The two molecules
form an acid–acid dimer hydrogen-bonding motif
(R22(8)).37–39 The intermolecular hydrogen bond has a bond
distance and bond angle of 1.674(10) Å and 175.9IJ2)° for
¯
¯
space group P1 (Z = 4); form II to be triclinic, space group P1
(Z = 4); form III to be orthorhombic, space group Pbca (Z =
8). Crystallographic data for each form are given in Table 3;
for complete CIF files, see the ESI.† Careful examination of
the crystallographically independent molecules in each of the
asymmetric units indicates they are conformationally differ-
ent, as suggested by the dihedral angle between the two aro-
matic rings (54.84IJ5)° for I-A and 60.53IJ5)° for I-B in form I,
64.43IJ5)° for II-A and 48.99IJ6)° for II-B in form II, and
40.76IJ10)° for form III). Conformational variability is readily
apparent in a superposition of all five experimental confor-
mations (Fig. 3).
O16B–H16B⋯O15A, and 1.70(2)
Å
and 174.2IJ2)° for
O16AH16A⋯O15B, respectively. Pairs of such dimers further
2
interact via weak sp CH⋯Cl hydrogen bonds between C12A–
H12A and Cl1A (graph set (R22(18))). The bond distance be-
tween H12A and Cl1A is 2.930 Å, bond angle of C12A–
H12A⋯Cl1A is 117.36°, and the distance between C12A and
Cl1A is 3.471 Å, which is nearly identical with the sum of
the van der Waals radii of the two atoms. Cl1B is not in-
volved in hydrogen bond. In addition to the intermolecular
hydrogen bonds, an intramolecular hydrogen bond exists
between the carboxylic acid carbonyl O and the NH bridg-
Table 3 Crystallographic data of CPAA polymorphs I–III
ing the two aromatic rings, with
a bond distance of
1.973(18) Å and bond angle 133.3IJ2)° for molecule A and
1.951(18) Å and 136.3IJ2)° for molecule B (Fig. 4).
I
II
III
Formula
Formula weight
Crystal size
C13H10ClNO2
247.67
0.50 × 0.20 ×
0.10
Triclinic
P1
9.9015(2)
10.7701(2)
11.3935(2)
90.2523(8)
103.8986IJ8)
102.7160IJ8)
4, 2
1148.40(4)
1.432
90.0(2)
C13H10ClNO2
247.67
0.20 × 0.03 ×
0.03
C13H10ClNO2
247.67
0.40 × 0.10
× 0.10
Orthorhombic
Pbca
9.85040IJ10)
7.4915(2)
31.4272(8)
90
Similar to form I, form II has two molecules in the asym-
metric unit (Z′ = 2), and both molecules show a twisted con-
formation, with a dihedral angle of 64.43IJ5)° for molecule A
and 48.99IJ6)° for molecule B. The two molecules also form
A–B acid–acid dimers. The intermolecular hydrogen bond
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
β/°
γ/°
Triclinic
¯
¯
P1
8.2249(2)
10.1269(3)
13.7993(4)
88.2266IJ11)
85.9540IJ11)
81.4168IJ11)
4, 2
1133.43(5)
1.451
90.0(2)
90
90
Z, Z′
8, 1
2319.15(9)
1.419
90.0(2)
0.317
V/Å3
Dcal g−1 × cm−3
T/K
Abs coeff
0.320
0.324
(mm−1
)
FIJ000)
512
512
1024
θ range (deg)
Limiting indices
1.84–27.48
−12 ≤ h ≤ 12
−13 ≤ k ≤ 13
−14 ≤ l ≤ 14
99.9%
1.48–27.50
−10 ≤ h ≤ 10
−13 ≤ k ≤ 13
−17 ≤ l ≤ 17
99.8%
1.30–27.47
−12 ≤ h ≤ 12
−9 ≤ k ≤ 9
−40 ≤ l ≤40
100.0%
Completeness
to 2θ
Unique
3681
3265
1420
reflections
R1[I > 2σ(I)]
wR2 (all data)
Fig.
3 Superposition of all five molecular conformations in the
0.0405
0.1163
0.0437
0.1017
0.0549
0.1778
asymmetric units of the three polymorphs of CPAA.
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Fig. 4 Crystal packing of I (a), II (b), III (c). For clarity, only intermolecular hydrogen bonds are shown (dotted line).
has a bond distance and bond angle of 1.73(2) Å and
Only one crystal form of fenamic acid has been obtained
after an exhaustive polymorph screening, and various num-
bers of polymorphs have been discovered for other
substituted FAs. It is inferred that the substituent groups
play an important role on the polymorphism of FAs.14 The
three polymorphs of CPAA, therefore, result likely from both
174.6IJ2)° for O16B–H16B⋯O15A, and 1.60(2) Å and 176.0IJ2)°
for O16AH16A⋯O15B, respectively. These acid–acid dimers
2
are further connected by two non-conventional sp CH⋯Cl hy-
drogen bonds between C13B–H13B and Cl1A, forming a di-
mer of dimers, but in a different manner than I. In form I,
2
the two sp C–H⋯Cl hydrogen bonds help form inversion-
the conformation differences of the molecules and the non-
2
related dimer between two molecules and two dimers, and in
conventional sp CH⋯Cl hydrogen bonds. The conforma-
2
2
form II, the two sp C–H⋯Cl hydrogen bonds only connect
tional variation and the presence of sp CH⋯Cl hydrogen
two dimers. The bond distance between H13B and Cl1A is
2.870 Å, the bond angle of C13B–H13B⋯Cl1A is 108.71°,
which falls slightly below the preferred minimum hydrogen
bond angle of 110°, and the distance between C13B and Cl1A
is 3.300 Å, which is less than the sum of the van der Waals
radii of C and Cl.40 Cl1B again is not involved in hydrogen
bond. An intramolecular hydrogen bond similar to that in
form I is formed between the carbonyl O of the carboxylic
acid and the anilino NH, with a bond distance of 2.01(2) Å
and bond angle 136.2IJ2)° for molecule A and 1.944(19) Å and
136.8IJ2)° for molecule B.
bonds could be mutually dependent. A methyl/Cl exchange
should shed light on the structure determining role of
chlorine.
3.2. Thermal properties
DSC was conducted to investigate the thermal properties of
the three forms, shown in Fig. 5. Form I shows only one
The form III crystallizes with one molecule in the
asymmetric unit. The molecule has a dihedral angle be-
tween the two aromatic rings of 40.76IJ10)°. Inversion-
related pairs of molecules form acid–acid homodimers
(graph set (R22(8))). The hydrogen bond has a bond dis-
sp2
tance of 1.76(3) Å and bond angle of 178IJ3)°. The
-
CH⋯Cl weak hydrogen bond between C13–H13 and Cl1
(bond distance of H13 and Cl1 is 2.945 Å, and bond an-
gle of C13–H13⋯Cl1 is 133.43°) leads to extended dimer
tapes due to the participation in hydrogen bonding of the
chlorine. The distance between C13 and Cl1 (3.664 (3) Å)
is longer than the sum of the van der Waals radii of the
two atoms. The intramolecular hydrogen bond has param-
eters of 1.96(3) Å and 137IJ2)° (Fig. 4).
Fig. 5 DSC thermograms of the polymorphs of CPAA.
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Fig. 8 Raman spectra of CPAA polymorphs.
Fig.
6 Experimental and calculated PXRD patterns of the three
polymorphs.
III, yet the energy involved is extremely small. Nevertheless,
we cannot rule out the possibility that forms I and III hap-
pen to be isoenergetic.43,44
Fig. 6 shows powder X-ray diffraction patterns of forms I,
II, and III collected at room temperature, along with patterns
calculated from the single-crystal structures determined at 90
K. The similarity of experimental and calculated patterns for
each form indicates the thermodynamic stability at room
temperature. The comparison suggests that each batch of
crystallization product was predominantly a single poly-
morph, not a mixture of multiple polymorphs.
Spectroscopic characteristics. IR spectrum of each form of
CPAA is shown in Fig. 7. All three forms show similar IR
spectra, but subtle differences are observed for the various
forms as indicated in Table 4.
For all three forms of CPAA, the Raman spectrum was
measured for comparison (Fig. 8). Differences are subtle yet
distinguishable (Table 5).
Fig. 7 IR spectra of the three polymorphs.
The vibrational spectra show the effect of the intramolecu-
lar hydrogen bond between NH and OC. The NH stretch is
at a lower frequency of around 3326, 3342, 3324 cm−1 for
forms I, II and III, respectively, the carboxyl CO stretch is
at 1619 cm−1 for forms I and II, and 1604 cm−1 for form III,
and the C–Cl stretch is at 756.0 cm−1 for forms I and II, and
759.2 cm−1 for form III.
Solvent can dictate polymorphism preference in many
polymorphic systems.45–48 The predominant conformation
in the solution may be conserved in the solid state. Since
the polymorph formation of CPAA seems to show some
solvent selectivity, we measured the UV-vis spectra in sev-
eral solvents. The data are shown in Fig. 9. All three solu-
tions have similar UV-vis curves. Two peaks are observed
in the spectrum for each solution, and the subtle
DSC peak with an onset temperature of 198.5 °C, which is
the melting of the crystals. Form II has two endothermic
DSC peaks. The first, with the onset temperature of 192.5
°C, appears to be a phase transition to a new form that
melts at approximately 199.0 °C. Form III shows only one
peak with an onset temperature of 198.8 °C, which corre-
sponds to the melting of the crystals. Based on the DSC
thermograms, we can infer that form II converts into form I
or III when heated. As mentioned before, form I has a Z′ =
2 and form II has a Z′ = 1 and most of the time, the Z′ = 1
form is more stable than the high Z′ forms.41,42 Thus, form
II likely converts into form III instead of form I, and there
is also a good chance that form I also converts into form
Table 4 Characteristic IR peaks of the three polymorphs of CPAA
N–H stretch
Carboxyl O–H stretch and aromatic C–H stretch
Carboxyl CO stretch
Aromatic CC stretch
C–Cl stretch
Form I
Form II
Form III
3328.5
3328.5
3328.5
3005.2–2560.5
3025.7–2553.9
3059.4–2553.9
1665.2
1658.6
1665.2
1570.8 1510.1 1429.7
1570.8 1513.1 1429.7
1570.8 1503.6 1422.4
756.0
756.0
759.2
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Table 5 Characteristic Raman peaks of the three polymorphs of CPAA
N–H stretch
Carboxyl O–H stretch and aromatic C–H stretch
Carboxyl CO stretch
Aromatic CC stretch
C–Cl stretch
Form I
Form II
Form III
3325.6
3342.1
3324.1
3090.9–3063.9
3084.0–3048.7
3097.4–3053.4
1600.9
1619.6
1604.5
1507.6 1465.8 1406.5
1508.1 1467.2 1403.8
1511.6 1471.1 1425.5
756.9
754.3
765.7
difference lies in the maximum absorption wavelength
and extinction coefficient of the peak corresponding to
I, its lattice energy is higher than that of form I which
might be due to the hydrogen bond difference between the
two forms, since form I has a shorter and more linear hy-
drogen bond than form II. Nonetheless, the small energy
difference between forms I and II should not be regarded
as evidence of which is more stable, since there is uncer-
tainty inherent to the computational methods employed
(which may range from a few kJ mol−1 to a few kCal mol−1,
or even higher). Also it should be pointed out that the com-
putation implicitly assumed a temperature of 0 K, whereas
the densities are based on the structures measured at 90 K,
and the DSC experiments were performed at elevated tem-
peratures, which could cause the discrepancy.
CPAA is a conformationally flexible molecule, for which
polymorphism could arise from its intrinsic conformational
flexibility. The conformational scan conducted over τ for a
single CPAA molecule is illustrated in Fig. 10. The global
minima of τ are identified at 43 and 143 due to the symme-
try of the benzene ring. The conformer of form III lies at the
exact minimum, the other four conformers from forms I and
II are located near the minima (i.e., well within the energy
well), and the conformers reside in two different energy val-
leys. Thus it appears that the polymorphic system is of con-
formational polymorphism, not merely due to conforma-
tional adjustment.49 The torsion angle of all five
conformations is listed in Table 6.
the n → π* excitation of the carboxyl CO group: λmax
=
361.9 nm and ε = 5.30 × 103 L mol−1 cm−1 in CHCl3, λmax
= 359.6 nm and ε = 7.20 × 103 L mol−1 cm−1 in CH2Cl2
and λmax = 359.6 nm and ε = 4.33 × 103 L mol−1 cm−1 in
CH3COOH, respectively; and that corresponding to π → π*
excitation of the substituted benzene ring: λmax = 288.4
nm and ε = 1.23 × 104 L mol−1 cm−1 in CHCl3, λmax
=
286.8 nm and ε =1.78 × 104 L mol−1 cm−1 in CH2Cl2, λmax
= 284.9 nm and ε = 9.90 × 103 L mol−1 cm−1 in CH3-
COOH. The UV-vis spectra seem to reflect the small con-
formational difference among the three polymorphs of
CPAA. The UV-vis spectra are a consequence of the equi-
librium of
a variety of conformations in the solution
rather than the locked conformer in the solid state.
3.3. Computational results
For forms I, II, and III, calculated lattice energies based on
the empirically augmented DFT method were −106.70,
−104.72, and −194.42 kJ mol−1, respectively. The energy dif-
ference between form III and forms I and II is much higher
than 6 kJ mol−1, which is in agreement with the observation
that conformational polymorphs have higher lattice energies
difference.43 The results suggest the stability order to be III
> I > II with form III being the most stable one, which
agrees with the observation that form II converts into form
III when heated. The density values indicate a ranking order
of forms II > I > III with form II being the densest
(Table 2). All three forms have similar calculated densities
at 90 K (I: 1.432 g cm−3; II: 1.451 g cm−3; and III: 1.419 g
cm−3). While form II has a higher density than that of form
Hirshfeld surface analysis. The Hirshfeld analysis re-
sults are shown in Fig. 11. It is evident that in all four-
teen potential molecular contacts, hydrogen–hydrogen con-
tacts predominate, contributing 38.5, 43.2 and 31.0% of
Fig. 9 UV-vis spectra of CPAA in different solvents.
Fig. 10 Conformation scan of single CPAA molecule.
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Table 6 Torsion angle of the conformers
teractions. Surprisingly, the Cl⋯Cl interaction is present
only in form III, and provides only marginal contribution.
Taken together, the crystal structures were stabilized by a
variety of interactions.
Conformer
Torsion angle (C1–N7–C8–C9/C13) (°)
I-A
I-B
II-A
II-B
III
−55.24/126.42
57.84/−125.12
−60.92/122.00
46.97/−138.13
−40.30/142.65
4. Conclusions
CPAA, a fenamic acid with good AKR inhibition, has been
found to exist in three solvent-free crystal forms so far. The
polymorphs were characterized by structure determination by
single-crystal X-ray diffraction, PXRD and spectroscopic
methods such as FT-IR and Raman. The polymorphism ap-
pears to result from the conformational flexibility of the mol-
the overall intermolecular interactions in forms I, II and
III, respectively. The second most significant inter-
molecular interaction is C⋯H, resulting in 20.7, 15.8, and
24.6% of the sum of intermolecular interactions in the in-
dividual forms. In addition, H⋯Cl and H⋯O interactions
also contribute significantly to the total intermolecular in-
ecule, as suggested by the conformers in the three modifica-
2
tions and the conformational scan and the sp CH⋯Cl
Fig. 11 Hirshfeld surface analysis of the three forms of CPAA.
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hydrogen bonds. No halogen bond was observed in any of
the three forms. Their phase behaviors were studied by DSC.
The metastable form II appears to convert into the most sta-
ble form III. Lattice energies of −106.70, −104.72, and −194.42
kJ mol−1 for forms I to III confirmed the relative stabilities of
the polymorphs, and Hirshfeld analysis indicated inter-
molecular interactions such as H⋯H, C⋯H, H⋯Cl, and
H⋯O contribute significantly to the overall stability of the
forms. The knowledge obtained in this study should be valu-
able to the selection of a suitable form for the formulation of
CPAA. We are currently investigating the effects of replacing
the chlorine atom by a methyl group.
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Acknowledgements
ML and SL thanks Natural Science Foundation of Hubei Prov-
ince (2014CFB787) and the President's Fund of Wuhan Insti-
tute of Technology (CX2016075) for financial support. PPZ ac-
knowledges the financial support by the National Natural
Science Foundation of China (Grant No. 21403097) and the
Fundamental Research Funds for the Central Universities
(lzujbky-2014-182). TL is grateful to NSF for supporting the
work (DMR1006364).
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