An investigation of the polymorphism of a potent nonsteroidal anti-inflammatory drug flunixin

Article abstract of DOI:10.1039/c9ce01619h

Flunixin [2-(3-trifluoromethyl-2-methyl-phenylamino)-nicotinic acid, FLX], a potent nonsteroidal anti-inflammatory drug widely used in veterinary medicine, was found to exist in at least two crystal forms (I and II), in contrast to clonixin [2-(3-chloro-2-methyl-phenylamino)-nicotinic acid, CLX], which exists in four solvent-free forms and multiple solvates. Form I was harvested from a variety of solvents and characterized by single-crystal X-ray diffraction, PXRD, FT-IR, and Raman spectroscopy. Its crystal structure is sustained on the strong acid-pyridine hydrogen bond. Form II was generated by thermal treatment of form I. Other aspects of this polymorphic system were investigated both experimentally and theoretically. Quantum chemistry calculations were performed to shed light on the lack of polymorphism from solution-phase crystallization. Conformational scan of the dihedral angle C2-N7-C8-C9 (τ) revealed two stable conformations, one with τ near 170°, and the other near 70°, corresponding to the molecule in the crystal. Hirshfeld analysis accounted for the major intermolecular interactions contributing to the overall stability of the crystal.

Full text of DOI:10.1039/c9ce01619h

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An investigation of the polymorphism of a potent
nonsteroidal anti-inflammatory drug flunixin†
Cite this: DOI: 10.1039/c9ce01619h
Hao Liu,a Xing Yang,a Shanyu Wu,b Mingtao Zhang,b Sean Parkin,c Shuang Cao,a
a
Tonglei Li, d Faquan Yu‡a and Sihui Long
*
Flunixin [2-(3-trifluoromethyl-2-methyl-phenylamino)-nicotinic acid, FLX], a potent nonsteroidal anti-
inflammatory drug widely used in veterinary medicine, was found to exist in at least two crystal forms (I
and II), in contrast to clonixin [2-(3-chloro-2-methyl-phenylamino)-nicotinic acid, CLX], which exists in four
solvent-free forms and multiple solvates. Form I was harvested from a variety of solvents and characterized
by single-crystal X-ray diffraction, PXRD, FT-IR, and Raman spectroscopy. Its crystal structure is sustained
on the strong acid–pyridine hydrogen bond. Form II was generated by thermal treatment of form I. Other
aspects of this polymorphic system were investigated both experimentally and theoretically. Quantum
chemistry calculations were performed to shed light on the lack of polymorphism from solution-phase
crystallization. Conformational scan of the dihedral angle C2–N7–C8–C9 (τ) revealed two stable
conformations, one with τ near 170°, and the other near 70°, corresponding to the molecule in the crystal.
Hirshfeld analysis accounted for the major intermolecular interactions contributing to the overall stability of
the crystal.
Received 14th October 2019,
Accepted 22nd November 2019
DOI: 10.1039/c9ce01619h
rsc.li/crystengcomm
therapeutic effects by inhibiting prostaglandin (PG) synthesis
through blocking the access of arachidonic acid to its binding
1. Introduction
Nonsteroidal anti-inflammatory drugs (NSAIDs) are not only
medicines for inflammation, but are also analgesic and
antipyretic ones, and they are widely used for the treatment
of a variety of human and veterinary disease conditions
involving pain and inflammation management.1,2 NSAIDs are
some of the most widely used therapeutic drugs. Classic
NSAIDs include (but not limited to) salicylates, arylalkanoic
acids, aryl- and heteroaryl acetic acids, aryl- and heteroaryl
propionic acids, N-aryl anthranilic acids (also known as
fenamic acids, FAs), oxicams, etc.3,4 These drugs exert their
site on the cyclooxygenase enzyme.5,6 N-Aryl anthranilic acids
are bio-isosteres of salicylic acid,7 with mefenamic acid
(MFA), flufenamic acid (FFA), clonixin (CLX), and flunixin
(FLX) as representatives.8 Among them, CLX and FLX are
more closely related since they are both anilinonicotinic
acids. Yet CLX is a human NSAID,9 and FLX is a potent
veterinary one widely used in horses and other livestock, in
the form of meglumine salt, for the treatment of
inflammatory diseases or colics.10,11 Currently the application
of FLX in human diseases is also explored.12 FAs are
conformationally flexible diarylamines. Polymorphism, i.e.,
the formation of more than one crystal form of a given
molecule, is widely observed in FAs and other compounds as
multiple forms have been found for them.13,14 For example,
for mefenamic acid two polymorphs (I and II) have been
a Key Laboratory for Green Chemical Process of Ministry of Education, Hubei Key
Laboratory of Novel Reactor and Green Chemical Technology, Hubei Engineering
Research Center for Advanced Fine Chemicals, School of Chemical Engineering and
Pharmacy, Wuhan Institute of Technology, 206 1st Rd Optics Valley, East Lake New
Technology Development District, Wuhan, Hubei 430205, China.
E-mail: fyuwucn@gmail.com, Sihuilong@wit.edu.cn, longsihui@yahoo.com;
Tel: +027 87194980
b Computational Center for Molecular Science, College of Chemistry, Nankai
University, Tianjin, China
c Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, USA
d Department of Industrial and Physical Pharmacy, Purdue University, West
Lafayette, Indiana 47907, USA
† Electronic supplementary information (ESI) available: Two crystal structures of
FLX in the form of crystallographic information file (CIF) were deposited in the
Cambridge Crystallographic Data Centre (CCDC) with accession codes 1957482
and 1957486. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c9ce01619h
‡ H. Liu and X. Yang contributed equally to this study.
Scheme 1 Structures of CLX and FLX.
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Table 2 Crystallographic data of form I of CLX and FLX (both LT and RT)
I (CLX)
FLX (LT)
FLX (RT)
Formula
C13H11ClN2O2 C14H11F3N2O2 C14H11F3N2O2
Formula weight
Crystal size (mm)
262.69 296.25 296.25
0.20 × 0.20 × 0.12 0.50 × 0.35 × 0.30 0.50 × 0.35 × 0.30
Crystal system
Space group
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
P21/c
Scheme 2 Synthesis of FLX.
a/Å
b/Å
c/Å
α/°
7.479(1)
14.162(2)
11.582(2)
90.00
7.5380(1)
14.0607(2)
12.3766(1)
90.00
7.68546IJ13)
14.1282(2)
12.49862IJ17)
90.00
Table 1 Polymorph screening of FLX
β/°
γ/°
101.55(1)
90.00
4, 1
1201.9(3)
1.452
90.0(2)
0.312
544
2.30–27.50
−9 ≤ h ≤ 9
−18 ≤ k ≤ 18
−15 ≤ l ≤ 15
103.3203IJ5)
90.00
4, 1
1276.50(3)
1.542
90.0(2)
102.1605IJ15)
90.00
4, 1
1326.67(4)
1.483
293(2)
Solvent
Growth conditions
Form
Z, Z′
V/Å3
Dcal/g cm−3
T/K
Ethyl acetate
Ethyl acetate
Methanol
Slow evaporation
Slow cooling
Slow evaporation
Slow cooling
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
Abs coeff (mm−1
FIJ000)
)
0.134
608
1.120
608
Methanol
Ethanol
Ethanol
Slow evaporation
Slow cooling
2θ range (deg)
Limiting indices
2.23–25.98
−9 ≤ h ≤ 9
−16 ≤ k ≤ 16
−15 ≤ l ≤ 15
98.2%
2157
0.0391
0.0971
4.4785–66.496
−7 ≤ h ≤ 8
−16 ≤ k ≤ 16
−14 ≤ l ≤ 14
99.5%
2037
0.0393
0.1074
Acetone
Acetone
Slow evaporation
Slow cooling
Acetonitrile
iso-Propanol
Water
Slow evaporation
Slow evaporation
Slow evaporation
Slow evaporation
Slow evaporation
Slow evaporation
Slow evaporation
Slow evaporation
Slow cooling
Completeness to 2θ 99.8%
Unique reflections 2264
R1[I > 2σ(I)]
wR2 (all data)
Ether
4.66
0.121
Dichloromethane
Dimethylformamide
Acetic acid
Benzene
Dimethyl sulfoxide
amino]nicotinic acid (2-MPNA), respectively.20,21 Even for CLX,
30 years after the first report of its polymorphism, we have
found that it tends to form solvates with DMF and DMF-like
solvents.8 In contrast, FLX was first synthesized in 197422 and
ever since has been widely used in the control of pain and
inflammation as a veterinary medicine and recently its effect
on the uterine mobility of equine embryos has been
investigated.23 Yet, its polymorphism has never been studied.
In this work, we attempted to shed light on the solid state
properties of FLX, a compound which closely resembles CLX
structurally. As seen, the only difference between CLX and
FLX is that Cl at the 3 position of CLX is replaced with CF3 in
FLX (Scheme 1), which is a strategy widely used in medicinal
reported.15 Nine polymorphs of FFA have been investigated,16
and tolfenamic acid has been reported to have at least five
forms.17 In the past decade, our lab has investigated the
pharmaceutical potential and polymorphism of FAs,
particularly CLX and its derivatives.8,18 A series of CLX and FA
analogues have been synthesized and their polymorphic
behavior has been studied experimentally and theoretically.19
Among them, several highly polymorphic compounds stand
out. For example, four forms have been discovered for
2-(phenylamino)nicotinic acid [2-PNA] and 2-[methylIJphenyl)-
Fig. 1 Crystals and conformer of FLX. Scale bar 0.2 mm.
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Fig. 2 Crystal packing of FLX (for clarity, hydrogens not involved in hydrogen bonding are omitted).
Fig. 3 DSC thermograms of FLX.
Fig. 4 PXRD patterns of the two forms of FLX.
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Table 3 UNI intermolecular potentials in CLX crystals
Diagrammatic
drawing
Intermolecular potentials
Number of
contributions
Crystal form
Types of interactions
(kcal mol−1
)
I
π–π stacking between two benzenes
−11.6
1
Hydrogen bond between carboxylic acid and pyridine
−10.0
−8.2
2
1
H–π stacking between H and pyridine
π–π stacking between two pyridines
−6.5
1
II (zwitterions)
π–π stacking between two benzenes and two pyridines
Hydrogen bond between carboxylate and pyridinium
π–π stacking between two benzenes and two pyridines
−17.9
−5.6
2
2
2
III
−18.7
Hydrogen bond between two carboxylic acids
Antiparallel π–π stacking between benzene and pyridine
Hydrogen bond between two carboxylic acids
−9.5
−18.8
−9.5
1
2
1
IV
chemistry.24 Intuitively, we would expect FLX to be
polymorphic, just like CLX. Yet, since CF3 is not the same as
Cl, difference in solid properties could be expected. In reality,
only one form was obtained from crystallization in all the
solvents used. Thus, we also tried to investigate the
polymorphic behavior of FLX theoretically.
solvents for crystal growth were from Sinopharm Chemical
Reagent Co., Ltd. (Shanghai, China), and were used as received.
2.2. Synthesis
FLX was synthesized by reacting 2-chloronicotinic acid with
2-methyl-3-trifluoromethyl-phenylamine with p-TsOH as
a
catalyst under a nucleophilic aromatic substitution (SNAr)
mechanism according to a literature procedure and purified
by recrystallization from MeOH (Scheme 2).20
2. Experimental section
2.1. Materials
2-Chloronicotinic acid (2.0 g, 12.7 mmol) and 2-methyl-3-
trifluoromethyl-phenylamine (1.8 g, 12.9 mmol) were
suspended in pyridine (1.1 g, 13.7 mmol), and p-TsOH (0.6 g,
3.2 mmol) in 10 mL of water was added to the mixture. The
resulting system was refluxed overnight and then it was
All chemicals were purchased from commercial sources:
2-methyl-3-trifluoromethyl-phenylamine
was
from
Bide
Pharmatech Ltd. (Shanghai, China), pyridine, 2-chloronicotinic
acid and p-TsOH were from Aladdin Industrial Corporation, and
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Table 4 UNI intermolecular potentials in FLX crystals
Crystal
form
Diagrammatic
drawing
Intermolecular potentials
Number of
contributions
Types of interactions
(kcal mol−1
)
I
π–π stacking between two benzenes
−13
1
Hydrogen bond between carboxylic acid and pyridine
−10.3
2
H–π stacking between H and pyridine
π–π stacking between two pyridines
−7.2
−5.1
1
1
cooled to room temperature. The product precipitated and
was recovered by filtration, and further purified by
crystallization in MeOH (2.4 g, yield%: 73). The purity of the
final product is over 99%.
2.5. Computational details
Gavezzotti's UNI intermolecular potentials of all the crystals
were investigated first.25,26 All the CLX dimers were extracted
from the experimental crystal structures. FLX dimers were
simulated by substituting the Cl in their corresponding CLX
forms with CF3 in order to compare and explain the difference
between CLX and FLX. All dimers discussed here were
optimized at the B3LYP/6-311G** level with dispersion
corrections (GD3BJ)27 using Gaussian16.28 Single point energy
calculations based on the optimized structures were performed
with a larger basis set (6-311+G**). For both optimization and
single point calculations, solvation was implicitly accounted for
using the SMD29 continuum solvation model. Three typical
solvents, water (H2O), benzene, and dichloromethane (DCM),
were selected based on their polarity difference. All calculations
were performed at 1 atm and 298.15 K. Stabilization energy (ΔE)
was defined as ΔE = EIJdimer) − 2 × EIJmonomer), in which
EIJmonomer) refers to the energy of the optimized single
molecule in the corresponding solvent, and EIJdimer) refers to
the energy of the optimized dimer. The sum of the stabilization
energies of the assumed dimers was used to estimate and
compare the thermodynamic stabilities of the possible
corresponding crystal forms between CLX and FLX. The relaxed
potential energy surface scan for the dihedral angle was
conducted at B3LYP/6-311+G** with a step of 10 degrees.
Hirshfeld surface analysis30,31 was carried out and its fingerprint
plot revealed intermolecular contacts within the crystal
structure, providing insight into the intermolecular interactions.
The relative contributions of various interactions to the
Hirshfeld surface were calculated using CrystalExplorer.32
1H NMR (400 MHz, DMSO-d6) δ ppm 10.30 (s, 1H), 8.34
(m, 1H), 8.27 (m, 1H), 8.11 (d, 1H), 7.19 (m, 2H), 6.87 (m,
1H), 2.33 (s, 3H); 13C NMR (125 MHz, DMSO-d6) δ ppm 169.7,
156.3, 153.1, 141.0, 140.3, 128.6, 127.7, 126.9, 126.2, 124.1,
120.6, 114.7, 108.4, 14.1; IR (KBr, cm−1) 3239 (w), 2460 (w),
1677 (s), 1510 (s), 1242 (m), 1121 (s); EI-MS (MH+) 297; mp
235 °C.
2.3. Crystallization
Since FLX and CLX are structurally similar, naturally we
expected them to behave alike during crystallization. The
same growth conditions used for CLX were applied to the
crystal generation for FLX.8 A detailed description of the
procedure can be found in the ESI.† The same form was
produced by slow evaporation in all the solvents used.
Slow cooling was also used for crystal growth (see the
ESI† for details). All crystallization experiments were
conducted in an unmodified atmosphere. The identity of
the crystals was confirmed by either single-crystal X-ray or
powder X-ray diffraction. The crystallization results are
listed in Table 1.
2.4. Characterization
The crystal structure of FLX was determined by single-crystal
X-ray diffraction (SCXRD). Powder X-ray diffraction (PXRD)
was applied for the bulk sample. The thermal behavior of
FLX was investigated by differential scanning calorimetry
(DSC). IR and Raman spectra were measured for the FLX
samples. The experimental details for each characterization
are described in the ESI.†
3. Results and discussion
3.1. Crystal structures
In contrast to the four solvent-free polymorphic forms
and one solvate found for CLX, only one crystal form
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Table 5 Stabilization energies for CLX dimers in different solvents
Original
Solvent
structures
extracted
from the
Diagrammatic
drawing
ΔE (kcal mol−1
in H2O
)
ΔE (kcal mol−1
in benzene
)
ΔE (kcal mol−1
)
in DCM
crystal form
Types of dimers
I
π–π stacking between two benzenes
−14.34
−8.85
−9.37
Hydrogen bond between carboxylic acid and pyridine
H–π stacking between H and pyridine
−12.79
−19.53
−13.77
−13.33
−8.58
−12.50
−13.40
π–π stacking between two pyridines
−6.26
NA
−7.35
−4.70
II
π–π stacking between two benzenes and two pyridines −28.21
(zwitterions)
Hydrogen bond between carboxylate and pyridinium
−19.37
0.60
−10.49
III
π–π stacking between two benzenes and two pyridines −20.47
−14.05
−16.67
−14.49
−13.90
Hydrogen bond between carboxylic acid and pyridine
−11.86
IV
Antiparallel π–π stacking between benzene and
pyridine
−22.05
−11.65
−17.22
−16.67
−16.45
−13.79
Hydrogen bond between two carboxylic acids
was obtained for FLX through crystallization in all the
solvents used. No solvate was harvested from DMF. The
The crystal structure of FLX is isostructural to that of CLX
form I as can be seen from the crystallographic data. The
molecule in the asymmetric unit is highly twisted as
suggested by the dihedral angle of 69.97 (4)° between the
pyridine ring and the benzene ring, similar to that of 68.22
(5)° in CLX-I. Due to the nonplanarity of the molecule, the
crystals are colorless and the energetically more favorable
hydrogen bond between the carboxylic acid and pyridine N is
observed (C(6) in graph-set notation).32,33 This is in
agreement with the general rule established in our recent
study regarding the formation of either the acid–acid
homosynthon or the acid–pyridine heterosynthon in 2-PNA
crystal is monoclinic, space group P21/c (Z
= 4). The
crystallographic data of form I (both LT and RT) and
CLX-I are listed in Table 2; complete CIF files are
provided in the ESI.† There is one formula unit in the
asymmetric unit. These crystallographically independent
molecules are conformationally similar to those in CLX-
I-LT as suggested by the dihedral angle between the
two aromatic rings in the molecules (68.22 (5)° for
CLX-I-LT, 69.97IJ4)° for FLX (LT) and 71.14 (5)° for FLX
(RT), respectively) (Fig. 1).
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Table 6 Stabilization energies for FLX dimers in different solvents
Solvent
ΔE (kcal mol−1
Assumed dimers
corresponding to the Diagrammatic
)
ΔE (kcal mol−1
)
ΔE (kcal mol−1
)
CLX crystal form
drawing
Types of dimers
in H2O
in benzene
in DCM
I
π–π stacking between two benzenes
−14.12
−8.43
−9.01
Hydrogen bond between carboxylic acid and
pyridine
−12.10
−12.78
−12.09
H–π stacking between H and pyridine
π–π stacking between two pyridines
−18.72
−13.60
−23.38
−18.00
−5.52
−5.98
−1.68
0.00
−6.22
−6.92
II (zwitterions)
π–π stacking between two benzenes and two
pyridines
−9.56
Hydrogen bond between carboxylate and
pyridinium
−10.91
III
π–π stacking between two benzenes and two
pyridines
−18.98
−11.39
−13.65
−16.55
−14.55
−13.80
Hydrogen bond between two carboxylic acids
IV
Antiparallel π–π stacking between benzene and
pyridine
−20.76
−11.55
−15.83
−16.57
−15.08
−13.81
Hydrogen bond between two carboxylic acids
analogues, i.e., if the dihedral angle between the pyridine
ring and the benzene ring is over 30°, the acid–pyridine
heterosynthon will be formed, otherwise the acid–acid
homosynthon will be formed.34 The hydrogen bond
parameters are 1.846 Å for the bond length and 173.73° for
the bond angle. Other than the intermolecular hydrogen
bond, there is also an intramolecular hydrogen bond in each
molecule between the NH that bridges the two aromatic rings
and the carbonyl O of the carboxylic acid (S6), with a bond
length of 1.961 Å and a bond angle of 134.79° (Fig. 2).
heating, the crystals grown from different solvents showed
two thermal events, the minor one with an onset temperature
of 220 °C which seemed to be a phase transition which led to
a new form and the major one next to it with an onset
temperature of 227 °C, which was the melting of the new
form. To confirm the formation of the new polymorph, the
sample was cooled down to room temperature when it was
heated right after the phase transition temperature, and
when the new form was heated again, only one thermal event
was observed (Fig. 3).
Fig. 4 shows the powder X-ray diffraction patterns of the
crystals grown from solvents and obtained after thermal
treatment of the original crystals, along with the PXRD
pattern of form I calculated from the single-crystal structure
determined at 293 K.
3.2. Thermal properties
We resorted to DSC to investigate the thermal properties of
FLX, and the DSC thermogram is shown in Fig. 3. Upon
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3.3. Computational results
than those for the formation of forms III and IV, which were
−42.89 kcal mol−1 and −46.69 kcal mol−1, respectively. As for
FLX, the thermodynamic difference value of form I was
−46.33 kcal mol−1, which was also more stable than those for
the formation of forms III and IV, which were −42.90 kcal
mol−1 and −43.96 kcal mol−1, respectively. Therefore, the
The only difference between CLX and FLX was that Cl at the
3 position of CLX was replaced with CF3 in FLX. CLX is
known to exist in four solvent-free forms and at least one
solvate crystal form, namely forms I, II, III, IV, and S.
However, only one crystal form has been obtained from
solution for FLX in this study. Theoretical calculations were
made to compare and explain the polymorphism difference
of CLX and FLX.
First, we tried to find out the dimers with strong
interactions in all the known crystal forms according to
Gavezzotti's UNI intermolecular potentials. The results are
listed in Tables 3 and 4. From the results we could conclude
that the hydrogen bond dimers as well as π–π stacking
dimers dominated the crystal structures.
All dimers with their relative energies are listed in
Tables 5 and 6. The total energies of the assumed crystals
were then calculated based on the corresponding dimers and
their contribution to different crystals. The summed up
interaction energies were then compared as shown in Table 7
to discuss the thermodynamic stability of the assumed
crystals.
The single molecule in CLX form II was a zwitterion;
however, zwitterions are not stable in non-polar solvents, so
form II was excluded from the non-polar solvent benzene. In
addition, the FLX dimer with an acid–pyridine hydrogen
bond found in FLX form I would rotate along the hydrogen
bond axis, so the restricted optimization was conducted for
this dimer with the dihedral angle of the hydrogen bond
plane fixed.
formation of form
I could also be explained with its
thermodynamic stability.
Our calculations reasonably explained the formation of
form IV in benzene for CLX. The thermodynamic difference
value was −51.11 kcal mol−1 for form IV in benzene, while the
values of forms I and III were −50.14 kcal mol−1 and −44.76
kcal mol−1, respectively. Intriguingly, a similar trend was
found for FLX calculations, which were not consistent with
the experimental results. The limitation of calculations may
be a possible explanation for this. A restricted optimization
was adopted to avoid the rotation of the hydrogen bond axis
when we optimized the acid–pyridine hydrogen bond dimers
for FLX, which would give a higher energy for the unstable
structure, while FLX molecules can form acid–pyridine
hydrogen bond chains experimentally with the adjacent
molecules hindering the rotation. Additionally, the growth of
long chains will decrease the polarity of the molecules and
thus further decrease the repulsion from the nonpolar
solvents. Therefore, the stability of form
I
was
underestimated by the limitation of calculations. The
thermodynamic difference for form I of FLX was calculated
to be −45.5 kcal mol−1, which was insufficient to compete
with the thermodynamic difference of −48.22 kcal mol−1 for
form IV. Moreover, there are still many other factors affecting
the crystallization including kinetic reasons which were not
investigated here.
The relaxed potential energy surface (PES) evaluation was
carried out via the flexible scanning of the dihedral angle
C2–N7–C8–C9 (τ) of FLX based on its optimized structure. It
can be found that there are two stable conformations (Fig. 5):
Our calculations reasonably explained why CLX and FLX
mainly formed form I in water from the perspective of
thermodynamics. According to the data, we could find that
form I would be 20 kcal mol−1 thermodynamically lower than
forms III and IV.
Our calculations also explained why CLX and FLX could
generate form
I
in dichloromethane. For CLX,
a
thermodynamic difference of −55.14 kcal mol−1 could be
obtained for the formation of form I, which was more stable
Table 7 The sum of the stabilization energies (ΔE) of assumed dimers of
CLX and FLX in different solvents
Corresponding
crystal form
ΔE of CLX
ΔE of FLX
Solvent
(kcal mol−1
)
(kcal mol−1
)
I
Water
Benzene
Dichloromethane
Water
Benzene
Dichloromethane
Water
Benzene
Dichloromethane
Water
Benzene
−73.22
−50.35
−55.14
−95.17
NA
−30.38
−52.80
−44.76
−42.89
−55.74
−51.11
−46.69
−70.64
−45.50
−46.33
−82.75
−3.36
II
III
IV
−40.93
−49.36
−43.84
−42.90
−53.07
−48.22
−43.96
Fig. 5 Relaxed potential energy surface (PES) scan along C2–N7–C8–
C9.
Dichloromethane
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Fig. 6 Hirshfeld surface with different orientations.
Fig. 7 a. 2D fingerprint plots for FLX; b. relative contributions to the Hirshfeld surface by the various intermolecular contacts in the crystal.
the nearly planar one with τ near 170° and the twisted one
with τ near 70° (τ = 68.3° in the crystal). The energy difference
is less than 2 kcal mol−1, and the rotation energy barrier
between them is only about 2.5 kcal mol−1. This suggests that
the two conformations could be transformed freely with a
dynamic equilibrium. Further optimizations of the two
conformations were conducted, where the energy of the
twisted conformation (τ = 70.7°) is slightly higher than that
of the nearly planar one (τ = 172.4°) by only 1.36 kcal mol−1
including zero point energy (ZPE) correction, or 2.11 kcal
mol−1 in Gibbs free energy.
In Fig. 6, it is evident that there are dominant interactions
represented by the bright red spots, which are caused by the
hydrogen bond interaction between the N atom of pyridine
and the H atom of the carboxylic acid. This dominant
interaction is also represented by two spikes in the left-
bottom region of the fingerprint plot (Fig. 7). Other close
contacts were also counted with their contributions to the
Hirshfeld surface area in Fig. 7. H–π interactions can be seen
with C⋯H contacts with 18.1% of the Hirshfeld surface.
H⋯F contacts can also be found with a proportion of 22.7%.
π⋯π stacking interactions were represented by C⋯C contacts
with proportion of 2.9%. Hirshfeld surface analysis
provided us a better understanding of the interactions of
molecules in the crystal.
a
4. Conclusions
FLX was synthesized through an SNAr reaction and two
crystal forms were obtained, one (I) through crystallization in
solvents and the other (II) through thermal phase transition.
Form I was fully characterized by SCXRD, PXRD, FT-IR and
Raman spectroscopy. The crystal structure of form I is
sustained on the acid–pyridine heterosynthon, same as that
observed in form I of CLX. Form I transforms into form II
upon thermal treatment. Theoretical calculations were
performed to explain the lack of polymorphism in different
solvents. Potential energy surface scan discloses two
energetically close conformations, one with a torsion angle of
170° and the other 70°, which corresponds to the
experimental conformation in the crystal. Hirshfeld analysis
further delineates the contribution of individual interactions
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to the overall stability of each form. Considering the close
resemblance between FLX and CLX, the “lack” of
polymorphism for FLX deserves further investigation.
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Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
SL thanks the Natural Science Foundation of Hubei Province
for financial support (2014CFB787). HL acknowledges the
sponsorship from the Innovation Fund of the Graduate
School (CX2018003). TL is grateful to NSF for supporting this
work (DMR1006364). SP is grateful to the NSF MRI program
(CHE0319176 and CHE1625732).
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