Synthesis and identification of pseudopolymorphs of 4-hexyloxybenzoic acid derivative

Article abstract of DOI:10.1016/j.molstruc.2005.03.007

Compound 1 self-assembles into two crystalline pesudopolymorphic forms 1A and 1B with tetrahydrofuran and acetonitrile solvent molecules, respectively. In both the structures, the crystallographic aspects pertaining to the influence of solvent molecules t

Full text of DOI:10.1016/j.molstruc.2005.03.007

Journal of Molecular Structure 748 (2005) 57–62
www.elsevier.com/locate/molstruc
Synthesis and identification of pseudopolymorphs
of 4-hexyloxybenzoic acid derivative
*
Akhila Jayaraman, Venkataramanan Balasubramaniam, Suresh Valiyaveettil
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
Received 5 February 2005; accepted 1 March 2005
Available online 28 April 2005
Abstract
Compound 1 self-assembles into two crystalline pesudopolymorphic forms 1A and 1B with tetrahydrofuran and acetonitrile solvent
molecules, respectively. In both the structures, the crystallographic aspects pertaining to the influence of solvent molecules towards
facilitating the formation of hydrogen bonded networks are described. The crystal structures are stabilized through acid dimer formation,
C–H/O, C–H/p and p/p interactions. The solvent molecules are stabilized in the crystal lattice through weak C–H/O and C–H/N
interactions.
q 2005 Elsevier B.V. All rights reserved.
Keywords: Weak interactions; Solid-state self-assembly; Pseudopolymorphism; Alkoxybenzoic acid
1. Introduction
Hydroxybenzoic acid gave different polymorphs owing
to the formation of multiple hydrogen bonds between
hydroxyl and carboxylic acid groups in the lattice [7]. It
can, therefore, be rationalized that the derivatives of
4-hydroxybenzoic acid such as alkoxybenzoic acids are
promising building blocks for the generation of supra-
molecular assemblies. Alkoxybenzoic acids have been
investigated for their mesomorphic properties and in the
design of supramolecular liquid crystals with aza
compounds [8]. 4-Hexyloxybenzoic acid undergoes four
crystalline transitions before forming a nematic phase [9].
The crystal structure of 4-hexyloxybenzoic acid has been
previously reported and exists in two polymorphic forms
[10]. We have recently reported the existence of
pseudopolymorphism in a 4-hydroxy benzoic acid
derivative [11]. In this paper, we report the synthesis
and X-ray crystallographic studies of a 4-hexyloxyben-
zoic acid derivative (1). Bulky biphenyl groups were
incorporated to design a V-shaped (arrow) building block
to promote additional stabilization through weak p–p
and C–H/p interactions. Alkyl chains were introduced
to explore the interplay of alkyl chain crystallization and
hydrophilic interactions in the crystal lattice. Acid 1
exists as two pseudopolymorphs (A and B) which differ
significantly in their crystal structures depending on the
solvents used for crystallization (Fig. 1).
Polymormorphism and pseudopolymorphism are a
common phenomena in biological systems and, therefore,
understanding fundamental aspects of such phenomena has
immense potential in crystal engineering studies [1].
Polymorphism is the existence of two or more different
crystal structures for the same compound and pseudopoly-
morphism refers to the formation of one or more solvated
crystalline forms of the same compound [2]. When organic
compounds are crystallized from their solutions, the solvent
is often eliminated from the system due to a combination of
entropic and enthalpic factors. However, multipoint recog-
nitions involving strong and weak hydrogen bonds between
the solvent and the solute precludes the elimination of the
solvent molecules into the bulk due to thermodynamic
factors [3]. As a result, the solvent is integrated into the
growing crystal. Based on CSD studies it has been shown
that only 15% of the organic crystals show pseudopoly-
morphism and a number of such structures are reported in
literature [3–6].
*
Corresponding author. Tel.: C65 68744327; fax: C65 67791691.
E-mail address: chmsv@nus.edu.sg (S. Valiyaveettil).
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2005.03.007

58
A. Jayaraman et al. / Journal of Molecular Structure 748 (2005) 57–62
COOH
CO2CH3
CO2H
CO2CH3
(ii)
Br
CO2CH3
(i)
(iv)
R
R
R
R
Br
Br
Br
O
O
O
O
OH
2
(CH2)5
CH3
4
(CH2)5
(CH2)5
CH3
1
CH3
3
R =
Fig. 1. Molecular structure of acid 1.
Scheme 1. Synthetic route for acid 1. Reagents and conditions: (i) C6H13Br,
K2CO3, MEK, reflux, 24 h; (ii) 4-biphenyl boronic acid, 2 N Na2CO3,
toluene, Pd(PPh3)4, reflux, 48 h; (iii) 2 N NaOH/EtOH, HC, 8 h.
2. Experimental section
All reagents were purchased from Aldrich, Fluka,
Merck and TCI and were used without further purifi-
cation unless mentioned in the paper. Tetrahydrofuran
(THF) was freshly distilled under N2 over sodium.
Methyl-3,5-dibromo-4-hydroxybenzoate (2) was pur-
chased from Sigma Aldrich. Toluene, methyl ethyl
ketone (MEK) and diethyl ether were purchased from
J.T. Baker and were purified by distillation. Silica gel 60
(0.040–0.063 mm) purchased from Merck was used for
chromatography using hexane and dichloromethane as
1
eluants to give colorless needles in 86% yield. H NMR
(300 MHz, CDCl3, d ppm): 0.88 (t, JZ6.81 Hz, 3H,
CH2), 1.01 (m,6H,-CH2), 1.26 (m,2H,-CH2) 3.90 (s, 3H,
CH3), 4.02 (t, JZ6.42 Hz, 2H, –OCH2), 8.16 (s, 2H, Ar–
H). 13C NMR (CDCl3, d ppm): 13.9, 22.4, 25.3, 29.8,
31.4, 52.4, 73.8, 118.3, 127.8, 133.9, 157.3,
164.3(iCaO). FT-IR (KBr, cmK1): 2951, 1728, 1588,
1546, 1458, 1432, 1379, 1278, 1126, 1065, 978, 762.
MS-ESI: m/z, 391 (MC) Elemental analysis calcd
C14H18Br2O3 (391.96): C, 42.67; H, 4.60, Br, 40.55.
Found: C, 42.86; H, 4.64; Br, 40.94.
1
column chromatography purifications. H and 13C NMR
spectra were recorded using Bruker ACF 300 instrument.
IR spectra were recorded using BIO-RAD FT-IR
spectrophotometer. Elemental analysis was performed
using Perkin Elmer CHNS Auto analyzer. Mass spectral
measurements were done using Finnigan TSQ 7000
spectrometer with ESI ionization capabilities. X-ray
diffraction data on single crystals were collected on a
Bruker AXS SMART CCD 3-circle diffractometer with
2.1.2. Synthesis of compound (4)
To a vertical three necked flask equipped with a
condenser was added 3 (5 g, 0.014 mol), 4-biphenyl
boronic acid (7.01 g, 0.036 mol), toluene (75 ml) and 2 N
potassium carbonate (75 ml). The flask was degassed
three times before the catalyst, tetrakispalladium triphe-
nylphosphine (10 mol%) was added in the absence of
light under argon and contents were refluxed under inert
atmosphere for 48 h. The reaction mixture was cooled,
filtered, concentrated and extracted with diethyl ether.
The resulting crude compound was purified using column
chromatography with hexane/dichloromethane as eluent
to afford 4 as a white solid in 71% yield. 1H NMR
(300 MHz, CDCl3, d ppm): 0.71 (t, JZ7.2 Hz, 3H), 0.97
(m, 6H), 1.21 (m, 2H), 3.35 (t, JZ6 Hz, 2H), 3.95 (s,
3H), 7.36 (m, 6H), 7.68 (m, 12H), 8.14 (s, 2H, Ar–H).
13C NMR (75.4 MHz, CDCl3, d ppm): 13.8, 22.3, 25.2,
29.6, 31.1, 52, 73.5, 125.7, 126.7, 126.9, 127.3, 128.7,
129.7, 131.5, 135.7, 136.9, 140.1, 140.6, 158.3 (Ar-C),
166.6 (iCaO). FT-IR (KBr, cmK1): 2955, 1727, 1554,
1441, 1373, 1282, 1115, 1007, 964, 905, 733, 598. MS-
ESI: m/z, 540 (MC). Elemental analysis calcd C38H36O3
(540.27): C, 84.41; H, 6.71. Found: C, 84.03; H, 6.59.
˚
Mo Ka radiation (lZ0.71073 A) at 23 8C. The software
used was SMART [12] for collecting frames of data,
indexing reflections and determining lattice parameters;
SAINT [12] for integration of intensity of reflections and
scaling; SADABS [13] for absorption corrections; and
SHELXTL [14] for space group determination, structure
solution and least square refinements on F2. The
structures were solved by direct methods. Non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were
introduced at fixed distances from carbon atoms.
2.1. Synthesis
The synthesis of acid 1 is given in Scheme 1.
2.1.1. Methyl hexyloxy-3,5-dibromo benzoate (3)
Methyl-3,5-dibromo-4-hydroxybenzoate (2) (10 g,
0.032 mol) was dissolved in 200 ml MEK and stirred
with potassium carbonate (13.3 g, 0.096 mol) and 6-
bromohexane (10.6 g, 0.064 mol). The mixture was
refluxed for 24 h, filtered and concentrated at reduced
pressure. The residue was extracted with ether washed
with water, dried over anhydrous sodium sulphate and
concentrated. The crude compound was purified by flash
2.1.3. Synthesis of compound (1)
A mixture of compound (4) (2 g, 0.004 mol) and sodium
hydroxide (0.01 mol, 0.4 g) in ethanol–water mixture (2:1,
50 ml) was refluxed for 8 h. The solution was then acidified

A. Jayaraman et al. / Journal of Molecular Structure 748 (2005) 57–62
59
with 5 N hydrochloric acid, concentrated and extracted with
3.2. Solid state self-assembly of 1A
The crystal lattice of1A belongs to the triclinic systemwith
diethyl ether. The ether extract was washed with water
and dried over sodium sulphate. The solvent was evaporated
to give a white solid which was used for further
ꢀ
a space group P1. The ORTEP diagram shows the asymmetric
1
characterization and crystallization, yield 96%. H NMR
unitcontainingtwomoleculesofthe acidwith onemoleculeof
disorderedTHF(Fig. 2a). Thebiphenylringsare twistedout of
plane with respect to each other. The selected torsion angles
between phenyl rings are: C3–C4–C10–C11: 47(5)8; C7–C6–
C22–C23: 49.1(6)8; C40–C41–C47–C48: 131.1(5)8; C44–
C43–C59–C60: K139.8(4)8. The molecules are assembled
into 1D chains via O–H/O hydrogen bonds from the
(300 MHz, CDCl3, d ppm): 0.69 (t, 3H, JZ7.2 Hz, CH3),
0.93 (m, 6H, CH2), 1.20 (m, 2H, -CH2), 3.35 (t, 2H, JZ
6 Hz, –OCH2), 7.35 (m, 6H, Ar–H), 7.66 (m, 12H, Ar–H),
8.19 (s, 2H, Ar–H). 13C NMR (75.4 MHz, CDCl3, d ppm):
13.8, 22.3, 25.2, 29.6, 31.1, 73.6, 124.8, 126.8, 127, 127.3,
128.7, 129.7, 132.1, 135.9, 136.7, 140.2, 140.6, 159.1
(Ar-C), 171.3(iCaO). FT-IR (KBr, cmK1): 3028, 2930,
1686, 1595, 1485, 1422, 1377, 1338, 1267, 1227, 996, 844,
761, 729, 692. MS-ESI: m/z, 526 (MC). Elemental analysis
calcd C37H34O3 (526.3): C, 84.38; H, 6.51. Found: C, 84.32;
H, 6.54.
˚
carboxylic acid groups [O2–H2/O4: d(O/O): 2.65(4) A,
˚
:O2–H2/O4: 1788; O5–H5/O1: d(O/O): 2.63(4) A,
:O5–H5/O1: 1708 at (xK1, yC1, z) and (xC1, yK1, z),
respectively]. In two dimensions, these infinite chains of
molecules are stacked into staggered layers with a slight offset
in a head-to-head fashion (Fig. 2b). There is no cavity in the
structure and the solvent molecules establish only weak
interactions with the host. The oxygen atom of the THF acts as
H-bond acceptor. The interactions of the acid with the THF
molecule involve C–H/O hydrogen bonds which stabilizes
the lattice. The interaction parameters are C8–H8/O2S at
3. Results and discussion
3.1. Self-assembly of pseudopolymorphs 1(A–B)
Two solvated crystals of 1 were obtained from a 1:1
mixture of tetrahydrofuran (THF)/methanol (MeOH) (1A)
and chloroform (CHCl3)/acetonitrile (CH3CN) (1B) via
slow evaporation method. The crystal structures of 1A and
1B are comparable with similar unit cell parameters and
space group symmetry; however, there are differences in the
nature, position and interactions of the solvent molecules in
the lattice and a higher packing density for 1A. The
important crystal structure parameters and refinement data
are given in Table 1.
˚
(1Cx, y, z): C/O: 3.15(11) A, :C–H/O: 1398; C31–
˚
H31/O2S at (1Cx, K1Cy, z): C/O: 3.21(10) A, :C–H/
O: 144 8, Fig. 2c). The phenyl rings in the alternate layers are
twisted with respect to each other and the distances between
˚
the layers are ca. 6 and 5 A. The layers run parallel to each
otherandarestabilizedviaC–H/pinteractionsasrevealedin
Fig. 2d and Table 2. All C–C bonds in the alkyl chain are in
anti-conformation except for the C35–C36 and C70–C71
bonds which have a gauche conformation. Alkyl chains of
Table 1
Crystal data and structure refinement parameters for pseudopolymorphs 1A and 1B
Form
1A
1B
Empirical formula
Crystal system
Space group
C37H34O3$(C4H8O)0.5
Triclinic
ꢀ
P1
C37H34O3$(C2H3N)0.25
Triclinic
ꢀ
P1
˚
a (A)
˚
b (A)
˚
c (A)
11.0254(1)
14.888(2)
19.0877(2)
82.462(2)
87.878(2)
85.493(2)
4
10.3284(1)
14.6317(1)
20.7169(2)
78.213(3)
76.91(3)
82.987(2)
4
a (8)
b (8)
g (8)
Z
3
˚
V (A)
3095.4(5)
1.207
2975.7(5)
1.198
Dcalc (g cm3)
Absorption coefficient (mmK1
F(000)
)
0.076
0.075
1200
1142
Crystal size (mm3)
Index ranges
R(int)
0.50!0.44!0.30
K13%h%13 K17%k%17 K22%l%22
0.1178
0.50!0.22!0.16
K12%h%12 K16%k%17 K15%l%24
0.0514
R1
0.0856
0.0822
wR2
0.1786
0.2102
GOF
0.972
0.972
q range (8)
1.64–25.00
33,103
1.43–25.00
16,981
Reflns collected
Independent reflns
Solvent
10,904
10,478
THF/methanol
Colorless/rod
CHCl3/CH3CN
Colorless/rod
Color/crystal shape

60
A. Jayaraman et al. / Journal of Molecular Structure 748 (2005) 57–62
Fig. 2. (a) ORTEP diagram showing labelling scheme for form 1A thermal ellipsoids are drawn at 30% probability level (b) 2D layer arrangement of molecules
with slight offset. No interdigitation of alkyl chains was observed (c) C–H/O interactions between THF and 1. C8–H8/O2S at (1Cx, y, z); C31–H31/O2S
at (1Cx, K1Cy, z) (d) interconnected C–H/p interactions shown by blue dotted lines. Symmetry operators: (x, y, z); (x, 1Cy, z); (1Kx, 1Ky, 1Kz); (Kx,
1Ky, 1Kz). Alkyl chains and some hydrogen atoms attached to the ring carbon atoms have been removed for clarity. Color codes: O, red; C, grey; H, green; O,
red. (For interpretation of the reference to color in this legend, the reader is referred to the web version of this article.)
the neighbouring molecules in the layers do not interdigitate
but pack antiparallel to each other (Fig. 2b) and are involved in
C–H(alkyl)/p interactions with the phenyl rings of the
adjacent molecules.
fashion with slight offset (Fig. 3b). The alternate layers are
˚
spaced at a distance of ca. 5 A. The molecules in the layers
are stabilized by multiple C–H/p interactions (Fig. 3c).
Additionally, the p/p interactions organize the phenyl
˚
rings at a distance of 4 A and in the process stabilize the 3D
network. The network is further stabilized by weak C–H/O
hydrogen bond involving hydrogen atom of C59
3.3. Solid state self-assembly of 1B
˚
[C59–H59/O5 at 1Kx, Ky, Kz: d(O/O): 3.418(6) A,
The single crystal structure of form 1B also belongs to
ꢀ
the triclinic system with a P1 space group. Each asymmetric
:C–H/O: 1488] (Fig. 3d). The short alkyl chains do not
favour interdigitation. C33–C34 and C70A–C71A are in
gauche conformation. All other C–C bonds in the alkyl
chain are in anti-conformation. The C–H(alkyl)/p inter-
actions (2.81–3.20 A) between the aliphatic hydrogens and
adjacent biphenyl rings are observed in the crystal lattice.
unit consists of two molecules of 1 and a fraction of the
solvent, acetonitrile. The ORTEP representation of 1B
shows two symmetry independent acid molecules (Fig. 3a).
The hexyl chains in both the molecules are disordered
(50/50). Similar to form 1A, the phenyl rings are twisted out
of plane with respect to each other. Each molecule of the
acid is hydrogen bonded to another equivalent molecule
through dimer formation by the carboxylic acid groups (O1–
˚
Table 2
˚
X–H/Cg (p-Ring) interactions (A, 8) for 1A
˚
H1/O2 at 1Kx, 1Ky, Kz: O/O: 2.665(4) A, :O–H/O:
X–H/Cg
d(H/Cg)
d(X/Cg)
:X–H/Cg
˚
1708; O4–H4/O5 at Kx, 1Ky, Kz: O/O: 2.610(4) A,
C11–H11/Cgai
C19–H19/Cgbi
C20–H20/Cgcii
C70–H70/Cgfiv
2.91
2.89
2.96
2.90
3.579(4)
3.643(6)
3.838(5)
3.661(5)
129
137
157
136
:O–H/O: 1808). The solvent acetonitrile is weakly
bonded to one of the hydrogen atoms of the alkyl chain
through C–H/N interactions [C37C–H37C/N1S:
d(H37C/N1S): 1.25 A, :C37C–H37C/N1S: 1278;
C37D–H37D/N1S: d(H37D/N1S): 0.97 A, :C37D–
˚
Symmetry codes: (i) x, y, z; (ii) x, 1Cy, z; (iii) 1Kx, 1Ky,1Kz; (iv) Kx,
1Ky, 1Kz [Cga: C63–C68; Cgb: C57–C62; Cgc: C51–C56; Cgd: C39–C44; Cge
26–C31; Cgf: C8–C13] Cg refers to the ring centre of gravity and the
alphabets represents the ring involved in interactions.
˚
:
H37D/N1S: 1408]. As seen in form 1A, the chains of
acid dimers are arranged into layers in a head to head
C

A. Jayaraman et al. / Journal of Molecular Structure 748 (2005) 57–62
61
Fig. 3. (a) ORTEP diagram of form 1B (thermal ellipsoids are drawn at 30% probability level). Color codes: C, grey; H, green; O, red; N, blue (b) 2D layer
arrangement with slight offset (c) interconnected C–H/p interactions shown by blue dotted lines. The adjacent layers of the acid are shown by different colors.
(d) Stabilization of the layers through C–H/O interactions, (C59–H59/O5) at 1Kx, Ky, Kz. Also shown is the p/p interactions between neighbouring
˚
phenyl rings, 4 A at x, y, z. Alkyl chains have been removed for clarity. (For interpretation of the reference to color in this legend, the reader is referred to the
web version of this article.)
4. Comparison of the pseudopolymorphs
aza compounds such as pyrazine, bipyridine and 1,10-
phenanthroline but instead crystallized as solvates in the
solid lattice. Acid dimer formation is the main hydrogen
bonding motif inside the crystal lattice. It is expected that
several other solvated forms besides the ones discussed in
the paper may exist. However, repeated efforts to obtain
other forms have not been successful so far. Investigations
of the other possible pseudopolymorphs are currently in
progress.
Both 1A and 1B have similar packing characteristics
except for the nature of the included solvent molecule. Self-
recognition assisted acid dimer formation is the major
hydrogen bonding motif observed in the crystal lattice. In
form 1A, the oxygen atom of THF is bonded to the acid
groups through C–H/O interactions. There are two
symmetry independent units in 1B and the acetonitrile
molecules are involved in C–H/N interactions with one of
the hydrogen atoms of the alkyl chain. Due to the short
length of the alkyl chains, no interdigitation was observed in
the lattice of 1A and 1B and the alkyl chains from the
neighbouring stacks interact laterally to pack into the
available space. When 1 was cocrystallized with azacom-
pounds such as bipyridine and pyrazine from THF solution,
only 1A was obtained. Attempts to obtain co-crystals with
aza compounds in various solvents proved futile. This
signifies the importance of structure regulating interactions
and thermodynamic factors involved in the formation of
such pseudopolymorphs [15].
Acknowledgements
We thank Prof. Koh Lip Lin and Ms Tan Goek Kheng at
the X-ray diffraction laboratory for their assistance in data
collection and structure refinement. SV and AJ thank the
National University of Singapore for funding support and
research scholarship, respectively. All technical support
from various laboratories at the Department of Chemistry,
National University of Singapore is acknowledged.
Appendix. Supplementary data
5. Conclusion
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2005.
03.007
Acid 1 represents a good example of pseudopolymorphic
systems. The acid failed to form molecular complexes with

62
A. Jayaraman et al. / Journal of Molecular Structure 748 (2005) 57–62
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