Cocrystals of caffeine with formylphenoxyaliphatic acids: Syntheses, structural characterization, and biological activity

Article abstract of DOI:10.1016/j.molstruc.2012.10.033

Three organic cocrystals namely, caffeine:p-formylphenoxyacetic acid [(caf)(p-fpaa)] (1) caffeine:o-formylphenoxyacetic acid monohydrate [(caf)(o-fpaa)]H₂O (2) and caffeine:p-formylphenoxypropionic acid [(caf)(p-fppa)] (3) were synthesized and studied by FT-IR, NMR, and single crystal XRD studies. The crystal system of cocrystal [(caf)(p-fpaa)] (1) is monoclinic with space group P21/n and Z = 16, that of cocrystal [(caf)(o-fpaa)]H₂O (2) is triclinic with space group P - 1 and Z = 2, and that of cocrystal [(caf)(p-fppa)] (3) is monoclinic with space group P21/c and Z = 4. The imidazole-carboxylic acid synthon is observed in all the three cocrystals. The intermolecular hydrogen bonds, OH···N and π-π interactions together play a major role in stabilizing the crystal structure of all the three cocrystals. The biological activities of crystals 1-3 were studied.

Full text of DOI:10.1016/j.molstruc.2012.10.033

Journal of Molecular Structure 1034 (2013) 302–309
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Cocrystals of caffeine with formylphenoxyaliphatic acids: Syntheses, structural
characterization, and biological activity
G.S. Suresh Kumar a, P.G. Seethalakshmi b, N. Bhuvanesh c, S. Kumaresan a,
⇑
a Department of Chemistry, Manonmaniam Sundaranar University, Tirunelveli 627 012, Tamilnadu, India
b Department of Chemistry, A.P.C. Mahalaxmi College for Women, Thoothukudi 628 002, Tamilnadu, India
c Department of Chemistry, Texas A&M University, College Station, TX 77842, USA
h i g h l i g h t s
"
"
"
"
"
Imidazole–carboxylic acid synthon is present in all the three (1–3) cocrystals.
Cocrystals (1–3) are stabilized by intermolecular H-bonding and interactions.
Caffeine and the corresponding acid are directly connected in 1 and 3.
Cocrystal 2 contains a polymeric motif.
Cocrystals 1–3 have slightly higher DPPH radical scavenging activity than caffeine.
p–p
a r t i c l e i n f o
a b s t r a c t
Article history:
Three organic cocrystals namely, caffeine:p-formylphenoxyacetic acid [(caf)(p-fpaa)] (1) caffeine:o-form-
ylphenoxyacetic acid monohydrate [(caf)(o-fpaa)]H2O (2) and caffeine:p-formylphenoxypropionic acid
[(caf)(p-fppa)] (3) were synthesized and studied by FT-IR, NMR, and single crystal XRD studies. The
crystal system of cocrystal [(caf)(p-fpaa)] (1) is monoclinic with space group P21/n and Z = 16, that of
cocrystal [(caf)(o-fpaa)]H2O (2) is triclinic with space group P ꢀ 1 and Z = 2, and that of cocrystal
[(caf)(p-fppa)] (3) is monoclinic with space group P21/c and Z = 4. The imidazole–carboxylic acid synthon
Received 13 September 2012
Received in revised form 15 October 2012
Accepted 15 October 2012
Available online 8 November 2012
Keywords:
Caffeine
Cocrystal
Formylphenoxyaliphatic acids
Synthon
is observed in all the three cocrystals. The intermolecular hydrogen bonds, OAHꢁꢁꢁN and
p–p interactions
together play a major role in stabilizing the crystal structure of all the three cocrystals. The biological
activities of crystals 1–3 were studied.
Ó 2012 Elsevier B.V. All rights reserved.
X-ray diffraction
Hydrogen bonding
1. Introduction
electronic, and optical materials [12], as well as media for conduct-
ing controlled solid-state organic syntheses [13].
The term cocrystal was first coined in the perspective of com-
plexes between nucleic bases [1] and later it was subsequently
popularized by Etter [2]. The definition of cocrystal is often a topic
of debate [3,4]. Cocrystals have gained a lot of recent attention ow-
ing to their amenability to design and their ability to tailor physio-
chemical properties. Cocrystals were recognized as valuable
materials [5,2,6,7] during the late 1990s. The great potential of
cocrystals resides in the improvement of physical properties of
cocrystals like solubility, dissolution rate, melting point, color,
etc. with respect to those of the co-formers [8–10]. The reported
uses of cocrystals include pharmaceutical materials [11],
Caffeine (methylxanthine) is a natural alkaloid found in various
plants, commonly used as a formulation or food additive and as a
pharmaceutical model compound [14]. It is a metabolic stimulant
[15] of the central nervous system. Caffeine is present in medica-
tions for asthma, apnea in newborns [11], and Alzheimer’s disease
(AD) [16–20]. Aryloxyacetic acid derivatives possess a wide array
of diverse bioactivities such as antimicrobacterial-, anti-inflamma-
tory-, antioxidant-, antibacterial-, analgesic-, antisickling-, antip-
aemic-, antiplatelet-, non-prostanoid prostacyclin mimetic-,
diuretic-, and growth regulators [21].
Earlier we have synthesized some cocrystals/molecular salts
[22–24] of organic acidic and basic components. In continuation
of our work in organic cocrystals, herein we report the synthesis
and the crystal structures of three cocrystals namely, caffeine:p-
formylphenoxyacetic acid [(caf)(p-fpaa)] (1), caffeine:o-formyl-
⇑
Corresponding author. Tel.: +91 9443182502; fax: +91 462 2334363.
E-mail address: skumarmsu@yahoo.com (S. Kumaresan).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.10.033

G.S. Suresh Kumar et al. / Journal of Molecular Structure 1034 (2013) 302–309
303
phenoxyacetic acid monohydrate [(caf)(o-fpaa)]H2O (2), and caf-
feine:p-formylphenoxypropionic acid [(caf)(p-fppa)] (3).
[(caf)(p-fpaa)] (1) C17H18N4O6: C, 54.54; H, 4.81; N, 14.97%. Found:
C, 54.44; H, 4.61; N, 14.77%, for [(caf)(o-fpaa)]H2O (2) C17H20N4O7:
C, 52.04; H, 5.102; N, 14.28%. Found: C, 52.00; H, 5.08; N, 14.22%,
and for [(caf)(p-fppa)] (3) C18H20N4O6: C, 55.67; H, 5.15; N,
14.43%. Found: C, 55.60; H, 5.08; N, 14.32%.
2. Experimental
2.1. General
2.3. Antimicrobial activity
The FT-IR spectra were recorded in pellet form with spectral
grade KBr on a JASCO FT-IR 410 spectrometer in the range 4000–
400 cmꢀ1. The 1H NMR spectra were recorded on a BRUKER spec-
trometer operating at 300 MHz using TMS as internal standard
and DMSO-d6 as solvent. The crystal structures of cocrystals caf-
feine:p-formylphenoxyacetic acid [(caf)(p-fpaa)] (1), caffeine:o-
formylphenoxyacetic acid monohydrate [(caf)(o-fpaa)]H2O (2),
and caffeine:p-formylphenoxypropionic acid [(caf)(p-fppa)] (3)
were determined using a BRUKER APEX 2 X-ray (three-circle) dif-
fractometer. The data reduction was done with the program APEX2
[25]. The absorption correction was employed using the program
SADABS [26]. The structure solution was obtained using SHELXTL
(XS) [27] and refined on F2 to convergence [27,28]. Absence of
additional symmetry was verified using PLATON (ADDSYM) [29].
Caffeine was purchased from commercial source. Formylphenoxy-
aliphatic acids were synthesized by literature method [30].
The in vitro biological screening effects of all the three synthe-
sized cocrystals were tested against the bacteria: Staphylococcus
aureus, Streptococcus sps., Enterobacter sps., Escherichia coli, Pseudo-
monas aeruginosa, Proteus sps., and Klebsiella sps., by well diffusion
method [31] using Muller Hinton agar nutrient as the medium. The
antifungal activities of the compounds were also evaluated by the
well-diffusion method against the fungi viz., Candida sps., Aspergil-
lus sps., cultured on antimitotic medium. The compounds were
tested at a concentration of 50 lg/0.01 mL in aqueous solution. In
the typical procedure [32], a well was made on the agar medium
inoculated with microorganisms. The well was filled with the test
solution using a micropipette and the plate was incubated 24 h for
bacteria and 72 h for fungi at 35 °C. During this period, the test
solution diffused and the growth of the inoculated microorganisms
was affected. The concentration was noted at which the inhibition
zone was developed. The percentage of inhibition was calculated
as 100(C ꢀ T)/C, where C is the average diameter of bacteria/fungal
growth on the control plate and T is the average diameter of bacte-
ria/fungal growth on the test plate. The susceptibility zones were
measured in diameter (mm). The susceptibility zones measured
were the clear zones around the disks killing the bacteria.
2.2. Synthesis of [(caf)(p-fpaa)] (1), [(caf)(o-fpaa)]H2O (2), and
[(caf)(p-fppa)] (3)
Caffeine (0.097 g, 0.5 mmol) and 0.5 mmol of formylphenoxy-
aliphatic acids (p-fpaa and o-fpaa 0.090 g, and p-fppa 0.097 g) were
dissolved separately in 1:1 v/v aqueous methanol. One solution is
gradually added into the other with stirring and allowed to stand
at room temperature. Slow evaporation of the mixture under ambi-
ent conditions yielded colorless crystals of the compounds
in 3 days (Yield: 1, 85%, 2, 83% and 3, 80%). Anal. Calcd. for
2.4. DPPH radical scavenging assay
Scavenging effect of 2,2-diphenyl-1-picrylhydrazyl (DPPH) rad-
ical was measured according to the procedure described by Blois
with a slight modification [33]. 0.1 mL of 1 mM methanol solution
Table 1
Crystal data and structure refinement parameters for caffeine:p-formylphenoxyacetic acid [(caf)(p-fpaa)] (1), caffeine:o-formylphenoxyacetic acid monohydrate
[(caf)(o-fpaa)]H2O (2) and caffeine:p-formylphenoxypropionic acid [(caf)(p-fppa)] (3).
Empirical formula
C17H18N4O6
C17H20N4O7
C18H20N4O6
Formula weight
Temperature
374.35
110(2) K
392.37
110(2) K
388.38
110(2) K
Wavelength
Crystal system
Space group
0.71073 Å
Monoclinic
P21/n
a = 16.047(8) Å
b = 26.395(13) Å
c = 17.137(9) Å
0.71073 Å
Triclinic
1.54178 Å
Monoclinic
P21/c
a = 16.430(2) Å
b = 6.6440(11) Å
c = 17.785(2) Å
P ꢀ 1
Unit cell dimensions
a = 16.430(2) Å
b = 6.6440(11) Å
c = 17.785(2) Å
a
=
c
= 90°
a
= 78.979(2)°
a = c = 90°
b = 113.803(5)°
b = 71.473(2)°
b = 113.871(7)°
c
= 65.741(2)°
Volume
6641(6) Å3
884.8(3) Å3
1775.3(4) Å3
Z
16
2
4
Density (calculated)
Absorption coefficient
F(000)
Crystal size
h Range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to h = 27.50°
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
Extinction coefficient
Largest diff. peak and hole
1.498 Mg/m3
1.473 Mg/m3
1.453 Mg/m3
0.116 mmꢀ1
0.116 mmꢀ1
0.934 mmꢀ1
3136
412
816
0.57 ꢂ 0.52 ꢂ 0.38 mm3
0.57 ꢂ 0.54 ꢂ 0.32 mm3
0.15 ꢂ 0.08 ꢂ 0.02 mm3
1.47–27.50°
1.97–27.46°
2.94–61.01°
ꢀ20 6 h 6 20, ꢀ34 6 k 6 34, ꢀ22 6 l 6 22
73,511
15138 [R(int) = 0.0619]
99.1%
Semi-empirical from equivalents
0.9574 and 0.9370
Full-matrix least-squares on F2
15138/0/990
ꢀ12 6 h 6 12, ꢀ12 6 k 6 12, ꢀ14 6 l 6 14
10,191
3960 [R(int) = 0.0312]
97.9%
Semi-empirical from equivalents
0.9638 and 0.9367
Full-matrix least-squares on F2
3960/0/258
ꢀ17 6 h 6 18, ꢀ7 6 k 6 7, ꢀ20 6 l 6 19
22,301
2603 [R(int) = 0.0759]
95.8%
Semi-empirical from equivalents
0.9816 and 0.8725
Full-matrix least-squares on F2
2603/0/258
1.031
1.035
1.043
R1 = 0.0639, wR2 = 0.1605
R1 = 0.0962, wR2 = 0.1768
0.00056(11)
R1 = 0.0437, wR2 = 0.1163
R1 = 0.0486, wR2 = 0.1212
0.019(4)
R1 = 0.0377, wR2 = 0.0885
R1 = 0.0534, wR2 = 0.0921
0.0026(4)
0.643 and ꢀ0.452 e Åꢀ3
0.557 and ꢀ0.531 e Åꢀ3
0.326 and ꢀ0.199 e Åꢀ3

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G.S. Suresh Kumar et al. / Journal of Molecular Structure 1034 (2013) 302–309
of DPPH was incubated with various concentrations of each
cocrystal (100 and 200 g/mL). After 30 min incubation period
l
at room temperature, absorbance of the resulting solution was
recorded at 517 nm. DPPH radical scavenging activity was ex-
pressed as the inhibition percentage was calculated as absor-
bance of control minus absorbance of sample/absorbance of
control ꢂ 100. Standard butylatedhydroxyanisole (BHA) and caf-
feine were used for comparison. The principle involved in this
method is that the antioxidants react with the stable DPPH free
radical (deep violet color) and convert it to 2,2-diphenyl-1-pic-
rylhydrazine with discoloration. The degree of discoloration indi-
cates the scavenging potential of the antioxidant sample [34].
3. Results and discussion
3.1. Crystal structure
Crystallographic data and structural refinement details for the
cocrystals [(caf)(p-fpaa)] (1), [(caf)(o-fpaa)]H2O (2), and [(caf)(p-
fppa)] (3) are presented in Table 1. Their geometrical parameters
are listed in Table 2. Some of the weak intermolecular interac-
tions are listed in Table 3. The complete sets of structural param-
eters for the three cocrystals 1–3 are deposited in CCDC with
deposition numbers 889465, 889466, and 889464 respectively.
3.1.1. Crystal structure of caffeine:p-formylphenoxyacetic acid
[(caf)(p-fpaa)] (1)
The asymmetric unit of 1 contains four molecules of caffeine
and four molecules of p-formylphenoxyacetic acid. The crystal
structure shows that there is no proton transfer from the car-
boxyl group of p-fpaa to the caffeine in the asymmetric unit
[35].
A predicted intermolecular hydrogen bonding motif
O3AH3Aꢁ ꢁ ꢁN3 is observed between the carboxyl group of p-fpaa
and the imidazole nitrogen of caffeine as in many cocrystals of
caffeine [36]. Excepting this strong OAHꢁ ꢁ ꢁN hydrogen bonding,
there is no weak CAHꢁ ꢁ ꢁO hydrogen bonding between the car-
bonyl oxygen of ACOOH group of p-fpaa and the acidic proton
(C16H) in the imidazole moiety of caffeine as in many cases
[37,21]. The CAO bond lengths [C9AO2: 1.202(3) Å, C9AO3:
1.318(3) Å; C9AAO2A: 1.207(3) Å, C9AAO3A: 1.323(2) Å;
C9BAO2B: 1.194(3) Å, C9BAO3B: 1.317(3) Å; C9CAO2C:
1.208(2) Å, C9CAO3C: 1.320(2) Å; Table 2] confirm the asymmet-
ric structure of the ACOOH moiety.
Each p-fpaa combines with one unit of caffeine through
O3AH3ꢁ ꢁ ꢁN3 hydrogen bonding with Etter’s [38,39,2] graph
set designator D forming discrete molecules. Table 3 shows
the hydrogen bonding geometry of cocrystal 1. This hydrogen
bond is essentially linear, with an angle about hydrogen
atom 176.03° for O(3)AH(3A)ꢁ ꢁ ꢁN(3), 177.75° for O(3A)A
H(3AB)ꢁ ꢁ ꢁN(3A), 177.28° for O(3B)AH(3BA)ꢁ ꢁ ꢁN(3B), and 176.11°
Table 3
Hydrogen bond geometry (Å and °).
DAHꢁ ꢁ ꢁA
D(DAH)
d(Hꢁ ꢁ ꢁA)
D(Dꢁ ꢁ ꢁA)
\DAHꢁ ꢁ ꢁA
Cocrystal 1
O(3)AH(3A)ꢁ ꢁ ꢁN(3)
O(3A)AH(3AB)ꢁ ꢁ ꢁN(3A)
O(3B)AH(3BA)ꢁ ꢁ ꢁN(3B)
O(3C)AH(3CA)ꢁ ꢁ ꢁN(3C)
0.840
0.841
0.840
0.839
1.818
1.829
1.827
1.830
2.657
2.669
2.667
2.668
176.03°
177.75°
177.28°
176.11°
Cocrystal 2
O(3)AH(3A)ꢁ ꢁ ꢁN(3)
Cocrystal 3
O(3)AH(3)ꢁ ꢁ ꢁN(3)
0.840
0.839
1.795
1.852
2.633
2.690
172.58°
176.43

G.S. Suresh Kumar et al. / Journal of Molecular Structure 1034 (2013) 302–309
305
for O(3C)AH(3CA)ꢁ ꢁ ꢁN(3C) (Table 3) (Fig. s1, vide Supporting infor-
mation for the packing diagram of [(caf)(p-fpaa)]).
of [(caf)(o-fpaa)]H2O (2) contains one molecule of caffeine, one
molecule of o-formylphenoxyacetic acid, and a molecule of water.
Cocrystal hydrates are formed with inclusion of water molecules in
their crystal structures. These materials receive high stability from
high humidity conditions [40]. The crystal structure of 2 shows
that there is no proton transfer from the carboxyl group of o-fpaa
to the caffeine in the asymmetric unit. The CAO bond lengths
[C9AO2: 1.2149(18) Å, C9AO3: 1.3190(17) Å, Table 2] indicate that
the acid moiety is present as ACOOH. An intermolecular hydrogen
bonding motif O3AH3ꢁ ꢁ ꢁN3 [O3AH3A = 0.84 Å, O3ꢁ ꢁ ꢁN3 = 2.633 Å,
O3AH3Aꢁ ꢁ ꢁN3 = 172.58°] is observed between the carboxyl group
of o-fpaa and the imidazole nitrogen of caffeine [35,36]. In addition
to this strong OAHꢁ ꢁ ꢁN hydrogen bonding, there is no weak
CAHꢁ ꢁ ꢁO hydrogen bonding between the carbonyl oxygen of
ACOOH group of o-fpaa and the acidic proton in the imidazole
moiety of caffeine as in other cases [37,21]. But the carbonyl
The carboxyl group O3AC9AO2 of p-fpaa lies almost on the
same plane of the phenyl ring (C1AC6), since it makes a dihedral
angle of 9.27° with the phenyl ring of p-fpaa. The caffeine molecule
and the p-fpaa molecule are nearly coplanar as their mean plane
angle is only 2.97°.
The crystal structure is stabilized by the face-to-face p–p stack-
0
ing force (3.639 ÅA) which is found between the parallel imidazole
moiety of caffeine and the phenyl ring of p-fpaa (Fig. s2, vide Sup-
porting information).
3.1.2. Crystal structure of caffeine:o-formylphenoxyacetic acid
monohydrate [(caf)(o-fpaa)]H2O (2)
Fig. 1c shows the ORTEP diagram of [(caf)(o-fpaa)]H2O (2) along
with the adopted atomic numbering scheme. The asymmetric unit
Fig. 1. ORTEP view for (a) [(caf)(p-fpaa)] (1), (b) [(caf)(o-fpaa)]H2O (2), and (c) [(caf)(p-fppa)] (3).

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G.S. Suresh Kumar et al. / Journal of Molecular Structure 1034 (2013) 302–309
Fig. 1. (continued)
oxygen of ACOOH group forms H-bonding with the water mole-
cule OwAH1wꢁ ꢁ ꢁO2 [OwAH1w = 0.87 Å, Owꢁ ꢁ ꢁO2 = 2.968 Å,
OwAH1wꢁ ꢁ ꢁO2 = 179.86°, Table 3].
Each o-fpaa molecule is connected with two molecules of caf-
feine. With one caffeine it is directly connected by hydrogen bond-
ing with the imidazole nitrogen [O3AH3Aꢁ ꢁ ꢁN3, H-bond donor],
Fig. 2. 1D chain of [(caf)(o-fpaa)]H2O (2).
Fig. 3. Tube-like structure of [(caf)(o-fpaa)]H2O (2).

G.S. Suresh Kumar et al. / Journal of Molecular Structure 1034 (2013) 302–309
307
Fig. 4. Packing along b-axis in [(caf)(p-fppa)] (3).
whereas with the second caffeine it is connected by hydrogen
bonding through water molecule [OwAH1wꢁ ꢁ ꢁO2, H-bond accep-
tor]. The second hydrogen atom of water forms H-bonding with
the carbonyl oxygen of caffeine OwAH2wꢁ ꢁ ꢁO6. So, each caffeine
is also connected with two molecules of o-fpaa, one directly and
another through water molecule.
This mode of connection gives rise to 1D chain with Etter’s
[38,39,2] graph set designator C33ð12Þ. In the formation of the chain,
all the caffeine molecules are lying on one side and all the o-fpaa
molecules are lying on the other side (Fig. 2).
In the crystal packing of [(caf)(o-fpaa)]H2O (2), the 1D chains
are arranged antiparallel to each other so as to form a tube-like
chain (Fig. 3) in which the caffeine molecules are present outside
the tube and o-fpaa molecules are present inside the tube.
The caffeine molecule and the o-fpaa molecule are almost par-
allel to each other, since the dihedral angle between them is
8.33°. The carboxyl group of o-fpaa lies in the same plane of the
phenyl ring of o-fpaa as revealed by the dihedral angle of 1.07°.
caffeine through O3AH3ꢁ ꢁ ꢁN3 hydrogen bonding with Etter’s
[38,39,2] graph set designator D forming discrete molecules. The
O3ꢁ ꢁ ꢁN3 distance is 2.690 Å. The \DAHꢁ ꢁ ꢁA 176.43° shows that this
bond is linear. When the discrete molecules are packed in the crys-
tal, they form zigzag chains when viewed along b axis (Fig. 4).
The carboxyl group O3AC9AO2 of p-fppa lies almost on the
same plane of the phenyl ring (C1AC6), since it makes a dihedral
Table 4
1H NMR (DMSO-d6) data of cocrystals 1–3.
No. of protons, splitting,
type of proton
Proton numbering as
mentioned in ORTEP
Chemical shift
(d ppm)
Cocrystal (1)
3H, s, NACH3
3H, s, NACH3
3H, s, NACH3
2H, s, OCH2
2H, d, ArAH
2H, d, ArAH
1H, s, ArAH (caf)
1H, s, CHO
14
15
17
8
2, 6
3, 5
16
7
3.19
3.38
3.85
4.83
7.10
7.85
7.98
9.86
The crystal structure is stabilized by a
p–p stacking interaction
(3.582 Å) which is present between the phenyl ring of o-fpaa and
the imidazole ring of caffeine (Fig. s3, vide Supporting
information).
Cocrystal (2)
3H, s, NACH3
3H, s, NACH3
3H, s, NACH3
2H, s, OCH2
1H, t, ArAH
1H, d, ArAH
1H, t, ArAH
1H, d, ArAH
1H, s, ArAH (caf)
1H, s, CHO
14
15
17
8
3
2
4
5
16
7
O3
3.21
3.40
3.87
4.90
7.10
7.15
7.64
7.71
8.00
10.44
13.21
3.1.3. Crystal structure of caffeine:p-formylphenoxypropionic acid
[(caf)(p-fppa)] (3)
The ORTEP diagram of the cocrystal 3 is shown in Fig. 1b. The
asymmetric unit of 3 contains one molecule of caffeine and a mol-
ecule of p-formylphenoxypropionic acid. The crystal structure
shows that there is no proton transfer from the carboxyl group of
p-fppa to the caffeine in the asymmetric unit. The CAO bond
lengths [C9AO2: 1.201(2) Å, C9AO3: 1.337(2) Å, indicate that the
acid moiety is not deprotonated.
As in the cocrystals 1 and 2 an intermolecular hydrogen bond-
ing motif O3AH3---N3 [O3AH3 = 0.839 Å, O3ꢁ ꢁ ꢁN3 = 2.690 Å,
O3AH3ꢁ ꢁ ꢁN3 = 176.43°] is observed between the carboxyl group
of p-fppa and the imidazole nitrogen of caffeine. The weak
CAHꢁ ꢁ ꢁO hydrogen bonding between the carbonyl oxygen of
ACOOH group of p-fppa and the acidic proton (C15AH15) in the
imidazole moiety of caffeine is not found in this cocrystal 3 as in
the cocrystals 1 and 2. Each p-fppa combines with one unit of
1H, s, COOH
Cocrystal (3)
2H, t, CH2ACO
3H, s, NACH3
3H, s, NACH3
3H, s, NACH3
2H, s, OACH2
2H, d, ArAH
2H, d, ArAH
1H, s, ArAH (caf)
1H, s, CHO
8
2.74
3.21
3.40
3.87
4.28
7.13
7.86
8.00
9.87
12.46
17
16
18
7
2, 6
3, 5
15
10
O4
1H, s, COOH

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G.S. Suresh Kumar et al. / Journal of Molecular Structure 1034 (2013) 302–309
Table 5
frequency in cocrystals is due to the involvement of carboxyl group
in hydrogen bond formation [41–43].
Antibacterial activity of cocrystals 1–3.
Cocrystals
Bacterial species
3.3. 1H NMR of caffeine:p-formylphenoxyacetic acid
[(caf)(p-fpaa)] (1), caffeine:o-formylphenoxyacetic acid monohydrate
[(caf)(o-fpaa)]H2O (2), and caffeine:p-formylphenoxypropionic acid
[(caf)(p-fppa)] (3)
a
b
c
d
e
f
g
[(caf)(p-fpaa)] (1)
[(caf)(o-fpaa)]H2O (2)
[(caf)(p-fppa)] (3)
++ +
++
++
++ +
++
++
++ +
ꢀ
ꢀ
ꢀ
++ +
++
ꢀ
++ +
ꢀ
++
++
++
++
++ +
++
a. Staphylococcus aureus, b. Streptococcus sps., c. Enterobacter sps. d. E. coli, e. Pseu-
domonas aeroginosa, f. Proteus sps., g. Klebsiella sps.
1H NMR spectra of cocrystals 1–3 (Fig. s6a–c, vide Supporting
information) confirms the 1:1 stoichiometric ratio of caffeine with
formylphenoxyaliphatic acids. The details of the 1H NMR spectra
are given in Table 4.
Table 6
Antifungal activity of cocrystals 1–3.
3.4. Thermogravimetric studies of [(caf)(p-fpaa)] (1), [(caf)(o-
fpaa)]H2O (2), and [(caf)(p-fppa)] (3)
Cocrystals
Fungal species
a
b
[(caf)(p-fpaa)] (1)
[(caf)(o-fpaa)]H2O (2)
[(caf)(p-fppa)] (3)
ꢀ
ꢀ
TGA measurement (Fig. s7a–c, vide Supporting information)
shows that the cocrystals 1 and 3 undergo gradual decomposition
after 175 °C and the cocrystal 2 remains intact until 125 °C. The
first weight loss of 6.15% (calculated 4.587%) for the cocrystal 2
may be due to the removal of a molecule of water. There is a grad-
ual decomposition after 240 °C. The weight loss process in all the
three cocrystals is an endothermic process which has been identi-
fied from their DTA curves.
ꢀ
ꢀ
++
++
a. Candida sps., b. Aspergillus sps. Inhibition zone diameter mm (% inhibition): +, 6–
10 (27–45%); ++, 10–14 (45–64%); ++ +, 14–18 (64–82%); ++ ++, 18–22 (82–100%).
(ꢀ) = No inhibition zone. Percent inhibition values are relative to inhibition zone of
standard antibacterial (chloramphenicol) and standard antifungal (Nystatin), con-
sidered as 100% inhibition, tested under the same conditions as the new compounds
reported here.
3.5. Biological activity
Table 7
DPPH radical-scavenging activity of cocrystals 1–3.
3.5.1. Antimicrobial activity
The in vitro biological screening effects of all the three cocrys-
tals were tested against the bacteria: S. aureus, Streptococcus spe-
cies, Enterobacter species, E. coli, P. aeruginosa, Proteus species,
and Klebsiella species by the well-diffusion method [31] using
Muller Hinton agar nutrient as the medium. The antifungal activi-
ties of the compounds were also evaluated by the well-diffusion
method against the fungi viz., Candida species, Aspergillus species
cultured on anti mytotic medium. The compounds were tested at
Samples
DPPH radical scavenging (%)
100 g/mL 200 lg/mL
l
[(caf)(p-fpaa)] (1)
[(caf)(o-fpaa)]H2O (2)
[(caf)(p-fppa)] (3)
Standard (caffeine)
BHA
56.32
50.32
57.36
47.70
95.74
54.12
50.28
55.34
52.78
94.02
a concentration of 50 lg/0.01 mL in methanol solution. The results
are tabulated in Tables 5 and 6 respectively.
From the antibacterial activity result (Table 5) it is clear that
[(caf)(p-fpaa)] (1) has good activity against five tested bacteria
and has no activity against E. coli and Pseudomonas aeroginosa.
[(caf)(o-fpaa)]H2O (2) has moderate activity against five tested
bacteria and has no activity against Enterobacter sps. and Klebsiella
sps. Whereas [(caf)(p-fppa)] (3) has good activity against Entero-
bacter sps., moderate activity against five tested bacteria but no
activity against Proteus sps.
From the antifungal activity result (Table 6) it is evident that
[(caf)(p-fpaa)](1) and [(caf)(o-fpaa)]H2O (2) are not active against
the tested two fungi whereas [(caf)(p-fppa)] (3) has moderate
activity towards the tested two fungi.
angle of 7.03° with the phenyl ring of p-fppa. The caffeine molecule
and the p-fppa molecule are lying nearly parallel because their
mean plane angle is 14.88°. The crystal structure is stabilized by
a
p–p stacking interaction (3.482 Å) which is present between
the phenyl ring of o-fppa and the imidazole ring of caffeine
(Fig. s4, vide Supporting information).
3.2. IR spectrum of caffeine:p-formylphenoxyacetic acid
[(caf)(p-fpaa)] (1), caffeine:o-formylphenoxyacetic acid monohydrate
[(caf)(o-fpaa)]H2O (2) and caffeine:p-formylphenoxypropionic acid
[(caf)(p-fppa)] (3)
Compounds (1–3) exhibit broad bands between 3442 and
3563 cmꢀ1 indicating the intermolecularly H-bonded OH group
[41–43] (Fig. s5a–f, vide Supporting information). In the IR spectra
of cocrystals [(caf)(p-fpaa)] (1) and [(caf)(o-fpaa)]H2O (2), three in-
3.5.2. Free radical-scavenging activity
The free radical-scavenging activity for the cocrystals
[(caf)(p-fpaa)] (1), [(caf)(o-fpaa)]H2O (2), and [(caf)(p-fppa)] (3)
was evaluated using DPPH model system [33,34] and the results
are presented in Table 7. From the result it is clear that the free
radical-scavenging activities of cocrystals are slight greater than
that of caffeine.
tense bands were displayed at 1745 cmꢀ1
,
1700 cmꢀ1 and
1656 cmꢀ1 for [(caf)(p-fpaa)] (1) and at 1706 cmꢀ1, 1681 cmꢀ1
,
and 1660 cmꢀ1 for [(caf)(o-fpaa)]H2O (2). Cocrystal [(caf)(p-fppa)]
(3) displayed two intense bands at 1706 cmꢀ1 and 1655 cmꢀ1. It
is presumed that the stretching frequencies of the carbonyl group
of caffeine (N(CH3)AC@O, 1700 cmꢀ1), and that of the carbonyl
4. Conclusions
(AC@O, 1717 cmꢀ1
) and the carboxylic carbonyl (ACOOH,
1754 cmꢀ1) of the free formylphenoxyaliphatic acids merge to-
gether and appear as these new bands in the cocrystals. The forma-
tion of the new compounds could be confirmed by the changes in
the carbonyl frequencies in the crystals. The change in carbonyl
Three organic cocrystals [(caf)(p-fpaa)] (1), [(caf)(o-fpaa)]H2O
(2), and [(caf)(p-fppa)] (3) were synthesized and their structures
were determined from FT-IR, NMR, TGA/DTA, and single crystal
XRD studies. All the three cocrystals form two-component

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