Supramolecular architecture of picric acid and pyrazoles: Syntheses, structural, computational and thermal studies

Article abstract of DOI:10.1080/10610278.2012.655276

The salts comprised of picric acid [(OH)(NO₂)₃C ₆H₂] and different ditopic pyrazoles Pz^R1,R2H (where R₁ =R₂ =H for 1, R₁ =R₂ =Me for 2, R₁ =Ph, R

Full text of DOI:10.1080/10610278.2012.655276

This article was downloaded by: [McGill University Library]
On: 19 October 2012, At: 02:28
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,
37-41 Mortimer Street, London W1T 3JH, UK
Supramolecular Chemistry
Publication details, including instructions for authors and subscription information:
http://www.tandfonline.com/loi/gsch20
Supramolecular architecture of picric acid and
pyrazoles: syntheses, structural, computational and
thermal studies
Udai P. Singh a , Nidhi Goel a , Gurdip Singh b & Pratibha Srivastava b
a Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee, 247 667, India
b Department of Chemistry, DDU Gorakhpur University, Gorakhpur, 273000, India
Version of record first published: 23 Apr 2012.
To cite this article: Udai P. Singh, Nidhi Goel, Gurdip Singh & Pratibha Srivastava (2012): Supramolecular architecture of
picric acid and pyrazoles: syntheses, structural, computational and thermal studies, Supramolecular Chemistry, 24:4, 285-297
To link to this article: http://dx.doi.org/10.1080/10610278.2012.655276
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Any substantial or systematic
reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to
anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the contents
will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should
be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims,
proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in
connection with or arising out of the use of this material.

Supramolecular Chemistry
Vol. 24, No. 4, April 2012, 285–297
Supramolecular architecture of picric acid and pyrazoles: syntheses, structural, computational
and thermal studies
Udai P. Singha*, Nidhi Goela, Gurdip Singhb and Pratibha Srivastavab
aDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India; bDepartment of Chemistry, DDU Gorakhpur
University, Gorakhpur 273000, India
(Received 28 September 2011; final version received 22 December 2011)
The salts comprised of picric acid [(OH)(NO2)3C6H2] and different ditopic pyrazoles PzR1,R2H (where R1 ¼ R2 ¼ H for 1,
R1 ¼ R2 ¼ Me for 2, R1 ¼ Ph, R2 ¼ Me for 3) and 3,30,5,50-tetramethyl-4,40-bipyrazole (Me4bpz) 4 are reported. Due to
presence of different substituents on pyrazole, each salt contains infinite three-dimensional structure held together by
primary NZH· · ·O, OZH· · ·N, OZH· · ·O hydrogen bonds and secondary CZH· · ·O interactions. The structure and
harmonic vibration frequencies of the complexes were calculated in terms of the density functional theory. The orientation
of molecule remains same in both the solid phase and the gaseous phase. The thermal decomposition of these salts was
studied by thermogravimetry and differential thermogravimetric analysis. Kinetic parameters were evaluated using
model-fitting and isoconversional methods.
Keywords: picric acid; pyrazoles; hydrogen bond interaction energy; thermolysis; ignition delay
1. Introduction
attractive features, the pyrazole and its derivatives have
been used for co-crystallisation with picric acid to see the
effect of substituents present on pyrazole ring on
supramolecular network and thermal stability of picric
acid. The present paper reports the co-crystallisation of the
picric acid and pyrazole synthons as well the thermolysis
of these salts.
In chemistry and material science, the role of molecule
with different functional groups has become an important
area of research (1–3). One can achieve materials with
desired physical and chemical properties by using different
functionalities in the building units (4, 5). Picric acid with
three nitro groups forms crystalline picrates of various
organic molecules through ionic, non-covalent and p–p
interactions as these interactions have been widely utilised
as a supramolecular heterosynthon in the design of co-
crystals (6, 7). It is known that picric acid acts not only as
an acceptor but also as an acidic ligand to form strong
hydrogen bonds with nitrogen atoms in heteroaromatic
rings (8). Bonding of electron donor/acceptor picric acid
molecules strongly depends on the nature of secondary
organic moieties. In the past, picric acid has been used for
co-crystallisation (9–16) by several workers. However, the
literature on co-crystallisation using pyrazole as one
component and picric acid as other component is very
limited (17–19). The pyrazole nucleus both thermally and
hydrolytically is very stable and occupies a position
similar to that of pyridine or ammonia in spectrochemical
series. As a ligand, it coordinates to metals and metalloids
through 2-N but after deprotonation, the formed pyrazolate
anions can coordinate through both nitrogen atoms as an
exobidentate ligand of C2v symmetry. The nucleophilicity
of the nitrogen and their steric accessibility may be varied
through appropriate ring substitution. Due to these
2. Experimental
2.1 Materials
All manipulations were performed in air using commercial
grade solvents after pre-dried by appropriate drying agents
(20). Picric acid (2,4,6-trinitrophenol), pyrazole (PzH) and
3,5-dimethylpyrazole (PzMe2H) were purchased from
Aldrich Chemical Company (St. Louis, MO, USA).
3-phenyl-5-methylpyrazole (PzPh,MeH) and 3,30,5,50-tetra-
methyl-4,40-bipyrazole (Me4bpz) were prepared by the
methods available in the literature (21, 22).
2.2 Instrumentation
Crystallised salts were carefully dried under vacuum for
several hours prior to elemental analysis on Elementar
Vario EL III analyzer. IR spectra were obtained on a
Thermo Nicolet Nexus FT-IR spectrometer in KBr. 1H and
13C NMR spectra were recorded on Bruker-D-Avance 500
spectrometer with Fourier transform technique using
*Corresponding author. Email: udaipfcy@iitr.ernet.in
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2012 Taylor & Francis
http://dx.doi.org/10.1080/10610278.2012.655276
http://www.tandfonline.com

286
U.P. Singh et al.
where E*a is the activation energy for thermal ignition, A is
the pre-exponential factor and T is the absolute
temperature. E*a was determined from the slope of a plot
of ln(Di) versus 1/T.
tetramethylsilane as internal standard. The melting points
were determined on a JSGW melting point apparatus.
2.2.1 X-ray crystallography
The X-ray data collection was performed on a Bruker Kappa
Apex four-circle CCD diffractometer using graphite-
2.2.3.4 Percent oxygen balance. The percent oxygen
balance (OB) was calculated by following equation
suggested by Martin and Yallop (35):
˚
monochromated MoKa radiation (l ¼ 0.71070 A) at
100 K. In the reduction of data Lorentz and polarisation
corrections, empirical absorption corrections were applied
(23). Crystal structures were solved by direct methods.
Structure solution, refinement and data output were carried
out with the SHELXTL program (24, 25). Non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were
placedingeometricallycalculatedpositionsbyusingariding
model. Images and hydrogen bonding interactions were
created in the crystal lattice with DIAMOND and MER-
CURY software (26, 27).
ꢂꢀ
ꢁꢃ
z 2 2x 2 ðy=2Þ 100
OB ¼
;
n
where x, y and z are the respective number of atoms of C,
H, N, respectively, and n is the total number of atoms in the
molecule.
2.2.4 Kinetics analysis of isothermal TG data
2.2.2 Computational study
Kinetic analysis of solid state decomposition is usually
based on a single-step kinetic equation (36):
Geometry optimisation of different species involved during
the course of present investigation was done using density
functional methods with 6-31G(d,p) basis set, with Gaussian
03suiteofprogram(28, 29). Theinput forthe simulationwas
the Z-matrix generalised byGaussianview thatwas alsoused
for visualisingthemoleculeswithoptimisedgeometries. The
frequency calculation was also performed.
da
¼ kðTÞfðaÞ;
ð1Þ
dt
where t is the time, T is the temperature, a is the extent of
conversion (0 , a , 1), k(T) is the rate constant and f(a)
is the reaction model (37), which describes the dependence
of the reaction rate on the extent of reactions. The value of
a is experimentally derived from the global mass loss in
TG experiments. The reaction model may take various
forms; the temperature dependence of k(T) can be
satisfactorily described by the Arrhenius equation, whose
substitution into equation (1) yields
2.2.3 Thermal analysis
2.2.3.1 Non-isothermal thermogravimetry. Non-isother-
mal thermogravimetry (TG) of salts 1–4 was recorded in
the temperature range 10–8008C at a heating rate of
108C min21 in static air using PerkinElmer’s (Pyris
Diamond) (Woodland, California, USA) thermogravi-
metric analyzer. The accuracy of the furnace was ^18C.
ꢄ
ꢅ
da
dt
2E
¼ A exp
ꢀfðaÞ;
ð2Þ
RT
where A is pre-exponential factor, E is activation energy
and R is the gas constant.
2.2.3.2 Isothermal TG. The isothermal TG studies
(wt 0.033 g, 100–200 mesh) of salts 1–4 were also
performed at appropriate temperatures (200–2408C) in
static air using indigenously fabricated TG apparatus (30)
fitted with temperature cum controller.
2.2.4.1 Model-fitting method. Rearrangement and inte-
gration of equation (1) for isothermal conditions give
gjðaÞ ¼ kjðTÞt;
½fðaÞꢁ21da is the integrated form of the
ð3Þ
2.2.3.3 Ignition delay measurements. The ignition delay
(Di) data were recorded using tube furnace technique (31)
(mass 0.020 g, 100–200 mesh) in the temperature range
340–3808C (^18C). Each run was repeated five times and
the mean Di values are calculated. The Di data were found
to fit in the following equation (32–34):
Ð
a
where gðaÞ ¼
0
reaction model. The subscript j has been introduced to
emphasise that substituting a particular reaction model in
equation (3) results in the evaluation of the corresponding
rate constant, which is determined from the slope of a plot
of gj(a) versus t. For each reaction model selected, the rate
constants are evaluated at several temperatures Ti and
Arrhenius parameters are determined using the Arrhenius
*
a
Di ¼ AeE =RT
;

Supramolecular Chemistry
287
equation (4) in its logarithmic form:
3.40; N, 21.53. Found: C, 40.43; H, 3.29; N, 21.39. IR
(KBr, cm21): 3191, 3126, 1576, 1469, 1336, 1298, 1154,
1032, 1008, 835, 737. 1H NMR (DMSO-d6) d: 8.49 (s, 2H,
PA2), 5.74 (s, 1H, pz), 2.12 (s, 6H, CH3), 12.03 (s, br, 2H,
NH). 13C NMR (DMSO-d6) d: 159.59, 140.94, 144.68,
126.53, 125.15, 103.18, 11.85. m.p. 1558C.
Ej
In kjðTiÞ ¼ In Aj 2
:
ð4Þ
RTi
Arrhenius parameters were evaluated for isothermal
experimental data by the model-fitting method.
2.3.3 Synthesis of [PA2·PzPh,MeH2þ·CH3OH] (3)
2.2.4.2 Isoconversional method. This method allows the
activation energy to be evaluated without making any
assumptions about the reaction model. Additionally, the
method evaluates the effective activation energy as a
function of the extent of conversion which allows one to
explore multistep kinetics.
Salt 3 was prepared by same procedure as outlined above
for 1 using 3-phenyl-5-methylpyrazole (0.16 g, 1.0 mmol)
in methanol (10 ml) with 71.8% (0.30 g, 0.72 mmol) yield.
Anal. Calcd (%) for C17H17N5O8 (419.36): C, 48.69; H,
4.08; N, 16.70. Found: C, 48.49; H, 3.97; N, 16.53. IR
(KBr, cm21): 3308, 3102, 1860, 1627, 1533, 1431, 1341,
1260, 1151, 1082, 834, 780, 702, 534. 1H NMR (DMSO-
d6) d: 8.48 (s, 2H, PA2), 6.44 (s, 1H, pz), 2.25 (s, 3H,
CH3), 7.26–7.75 (m, 5H, Ph), 12.39 (s, br, 2H, NH), 3.16
(s, 3H, OCH3). 13C NMR (DMSO-d6) d: 159.60, 140.93,
126.57, 125.16, 148.21, 141.02, 133.93, 128.60, 127.24,
124.91, 100.19, 48.59, 11.04. m.p. 1208C.
The basic assumption of the isoconversional method
(40) is that the reaction model as defined in equation (1) is
not dependent on temperature or heating rate. Under
isothermal conditions, on combining equations (3) and (4),
we get
ꢆ
ꢇ
Aa
Ea
2In ta;i ¼ In
2
;
ð5Þ
gðaÞ
RTi
2.3.4 Synthesis of [2PA2·2Me4bpzHþ·CH3CN] (4)
where Ea is evaluated from the slope of the plot of –ln ta,i
against T2i 1. Thus, Ea at various ai for salts 1–4 was
evaluated.
Salt 4 was prepared by same procedure as outlined above
for 1 using 3,30,5,50-tetramethyl-4,40-bipyrazole (0.19 g,
1.0 mmol) in an acetonitrile–methanol mixture (v/v%, 1:4,
10 ml) with 68.9% (0.61 g, 0.68 mmol) yield. Anal. Calcd
(%) for C34H37N15O14 (879.79): C, 46.41; H, 4.23; N,
23.88. Found: C, 46.37; H, 4.19; N, 23.73. IR (KBr, cm21):
3347, 1830, 1626, 1581, 1536, 1426, 1313, 1165, 1072,
2.3 Syntheses of organic salts
2.3.1 Synthesis of [2PA2·2PzH2þ·OH2] (1)
1
907, 812, 710, 616. H NMR (DMSO-d6) d: 8.46 (s, 2H,
Picric acid (0.22 g, 1.0 mmol) and pyrazole (0.06 g,
1.0 mmol) were mixed in a water–methanol mixture
(v/v%, 1:4, 10 ml). The resulting solution was stirred for
6 h and filtered through celite. The filtrate was evaporated
to dryness under vacuum and the yellow solid obtained
was redissolved in methanol. The yellow crystals of salt 1
in 69.0% (0.42 g, 0.69 mmol) yield, suitable for X-ray data
collection, were obtained by slow evaporation of solvent at
room temperature. Anal. Calcd (%) for C18H16N10O15
(612.41): C, 76.49; H, 2.37; N, 23.56. Found: C, 76.35; H,
2.31; N, 23.47. IR (KBr, cm21): 3218, 3141, 2996, 1843,
1611, 1567, 1539, 1494, 1360, 1321, 1278, 1162, 1134,
PA2), 1.95 (s, 12H, CH3), 12.20 (s, br, 3H, NH), 2.07 (s,
3H, CH3CN). 13C NMR (DMSO-d6) d: 159.58, 148.30,
140.92, 137.37, 126.54, 125.21, 125.14, 120.99, 117.91,
66.68, 54.04, 1.03. m.p. 1808C.
3. Results and discussions
The reaction of picric acid and corresponding pyrazoles
(PzH/PzMe2H/PzPh,MeH/Me4bpz) in appropriate mixture of
solvent resulted in the formation of ionic salts (1–4) as
shown in Scheme 1. The different formulations for ionic
salts 1–4 were confirmed by elemental analysis, IR, NMR
and crystallographic structure analyses. The asymmetric
and the symmetric stretching vibrations of ZNO2 group
show bands at 1536 and 1334 cm21, respectively (11, 38).
The shift of the nas(NO2) vibration to lower frequency
(1581–1530 cm21) in the spectrum of salts (1–4)
compared with the free picric acid (1607 cm21) suggested
the large electron density on the picric acid. The NH
stretching vibration is normally observed at 3500–
3400 cm21 which is shifted to lower wave number due
to the attraction of the ZNH protons by the picrate anion
1
1077, 909, 769, 711, 608, 543. H NMR (DMSO-d6) d:
8.48 (s, 2H, PA2), 7.61 (t, 1H, pz), 6.26 (d, 2H, pz), 12.64
(s, br, 2H, NH). 13C NMR (DMSO-d6) d: 159.61, 140.95,
133.15, 126.56, 125.17, 104.21. m.p. 1308C.
2.3.2 Synthesis of [PA2·PzMe2H2þ] (2)
Salt 2 was prepared by same procedure as outlined above
for 1 using 3,5-dimethylpyrazole (0.09 g, 1.0 mmol) in
methanol (10 ml) with 63.2% (0.20 g, 0.63 mmol) yield.
Anal. Calcd (%) for C11H11N5O7 (325.25): C, 40.62; H,

288
U.P. Singh et al.
N
NH
Salt-1
H3C
CH3
N
NH
NO2
Salt-2
Salt-3
Salt-4
O2N
OH
H3C
NO2
N
NH
CH3
H3C
N
NH
N
HN
CH3
H3C
Scheme 1. General method for preparation of salts 1–4.
leading to an increase in ZNH bond length in salts (1–4)
(39). The shifting towards lower frequency in both cases is
due to the hydrogen-bonded non-covalent interactions
between donor (ZNH protons) and acceptor (picrate
anion) (40). The crystallographic and the selected
hydrogen bonding data are given in Tables 1 and 2,
respectively.
(Figure S1, available online). Due to the presence of
various interactions, picrate and pyrazolate ions are self-
assembled in a 3D square-shaped channels and these
channels act as host molecules for water. The guest
molecules (water) are situated between the two channels
˚
through NZH· · ·O [N8-H8A· · ·O15, 1.783(24) A; N9-
˚
H9B· · ·O15, 1.921(19) A] as well as OZH· · ·O [O15-
˚
H15A· · ·O8, 1.969(24); O15-H15A· · ·O14, 2.259(22) A;
˚
O15-H16A· · ·O1, 2.037(23) A] non-covalent interactions
(Figure 2).
3.1 Structure description
3.1.1 Crystal structure of [2PA2·2PzH2þ.OH2] (1)
The salt 1 crystallises in the triclinic crystal system with
P-1 space group. As shown in Figure 1, the unit cell
consists of two molecules of protonated pyrazole with four
NH group, two molecules of deprotonated picric acid (i.e.
the acidic hydrogen of the hydroxyl group on picric acid
has been transferred to the nitrogen atom of the pyrazole)
and a water molecule. Both deprotonated OH groups of
picric acid and protonated nitrogen atoms of both pyrazole
molecules form a cationic–anionic acid–base pair via
3.1.2 Crystal structure of [PA2·PzMe2H2þ] (2)
The unit cell of 2 contains one molecule of anionic
deprotonated picric acid and one molecule of cationic
protonated 3,5-dimethylpyrazole (Figure 3). It also
crystallises in the triclinic crystal system with P-1 space
group and contains NZH· · ·O [N4-H4A· · ·O1,
˚
˚
1.678(21) A; N4-H4A· · ·O7, 2.298(24) A; N5-H5· · ·O7,
˚
2.362(4) A] as well as CZH· · ·O [C2-H2· · ·O6,
˚
NZH· · ·O [N7-H7A· · ·O1, 1.852(23) A; N10-H10A· · ·O8,
˚ ˚
2.603(2) A; C7-H7A· · ·O1, 2.606(3) A; C9-H9· · ·O4,
˚
1.930(21) A] and CZH· · ·O [C13-H13A· · ·O2, 2.581(1) A;
˚
˚
2.639(4)A; C11-H11B· · ·O5, 2.478(4) A] non-covalent
˚
˚
C18-H18· · ·O9, 2.532(1) A] intermolecular interactions
interactions which are responsible for the formation of

Supramolecular Chemistry
Table 1. Crystal data and structure refinement parameters of salts 1–4.
289
Salts
(1)
(2)
(3)
(4)
Formula
C18H16N10O15
612.41
C11H11N5O7
325.25
Triclinic
P-1
7.8020(3)
8.2452(3)
10.9454(4)
101.136(2)
92.809(3)
90.568(2)
689.89(4)
2
C17H17N5O8
419.36
Monoclinic
P 21/c
14.7308(10)
7.0891(5)
17.5439(12)
90
93.955(3)
90
1827.7(2)
4
Block
296(2)
1.524
C34H37N15O14
879.79
Monoclinic
P 21/c
16.076(2)
21.176(3)
11.5913(15)
90
94.078(7)
90
3935.9(9)
4
Needle
296(2)
1.485
Formula weight
Crystal system
Space group
Triclinic
P-1
˚
a (A)
7.6790(2)
11.8274(2)
13.5296(3)
98.5490(10)
101.7860(10)
91.8420(10)
1187.04(5)
2
˚
b (A)
˚
c (A)
a (0)
b (0)
g (0)
3
˚
V(A )
Z
Crystal habit
Temperature
DCalc (g/cm3)
m(MoKa) (cm21
F(000)
Square
296(2)
1.713
0.152
628
Block
296(2)
1.566
0.133
336
)
0.123
872
0.118
1832
Crystal size
0.27 £ 0.19 £ 0.13
1.56–30.93
7439
0.27 £ 0.21 £ 0.17
2.52–31.85
4679
0.34 £ 0.29 £ 0.23
1.39–28.26
4457
0.23 £ 0.19 £ 0.13
1.27–28.40
9793
Theta for data collection (o)
No. of measured reflections
No. of observed reflections
Data/restraints/parameters
Goodness-of-fit
6133
3140
3194
7226
7439/0/412
0.813
0.0380
4679/0/214
0.775
0.0495
4457/0/282
0.977
0.0406
9793/0/601
1.327
0.0564
a
Final R indices[I . 2(I)] R1
b
wR2
0.1094
0.1383
0.1163
0.1843
a R1 ¼ SkFoj 2 jFck=SjFoj,
b
wR2 ¼ {S½wðF2o 2 F2cÞ ꢁ=SwðF2oÞ }1=2
:
2
2
sheet with rectangular cavities in 3D views (Figure 4(a),
4(b)). This cavity can be used for trapping the appropriate
˚
size of guest molecule (about 1.2 A).
3.1.4 Crystal structure of [2PA2·2Me4bpzHþ·CH3CN] (4)
The salt 4 with two molecules of picric acid, two
molecules of 3,30,5,50-tetramethyl-4,40-bipyrazole and one
molecule of acetonitrile in unit cell crystallises in the
monoclinic system with P 1 21/c space group (Figure 7).
The NZH and CZH group of pyrazole in 4 is
non-covalently interacted with O2, O8, O11, O13 and
O14 atoms of picrate acceptor, forming a charge-assisted
3.1.3 Crystal structure of [PA2·PzPh,MeH2þ·CH3OH] (3)
The co-crystallisation of picric acid and 3-phenyl-5-
methylpyrazole resulted in the formation of salt 3 which
crystallises in monoclinic system with P 1 21/c 1 space
group. The methanol solvent present in lattice binds
one molecule of picrate anion and one molecule of 3-
phenyl-5-methylpyrazolate ion (Figure 5). The presence
of different non-covalent interactions, viz. NZH· · ·O
þNZH· · ·O2 [N13-H13A· · ·O14, 2.792(25) A; N14-
˚
˚
H1AA· · ·O8, 1.800(25) A; N14-H1AA· · ·O14, 2.284
(25) A] and þCZH· · ·O2 [C19-H19B· · ·O11, 2.575(2) A;
˚
˚
˚
C22-H22A· · ·O11, 2.605(5) A; C30-H30A· · ·O2, 2.685
˚
(7) A; C30-H30C· · ·O5, 2.761(9) A; C32-H32A· · ·O8,
˚
˚
˚
2.914(2) A; C33-H33B· · ·O14, 2.600(3) A] hydrogen
bonds. The presence of different non-covalent interactions
resulted in the formation of host–guest structure where the
host assembly was present with a rectangular-shaped
cavity. This cavity is formed by the presence of various
non-covalent interactions between the picrate and
3,30,5,50-tetramethyl-4,40-bipyrazolate ions. The guest
acetonitriles are located inside the cavity through
˚
˚
[N1-H1· · ·O1, 1.782(23) A; N1-H1· · ·O2, 2.315(21) A;
˚
N2-H2A· · ·O8, 1.664(22) A], CZH· · ·O [C1-H1C· · ·O2,
˚
˚
2.355(2) A; C6-H6· · ·O7, 3.149(2) A; C6-H6· · ·O8,
˚
˚
2.859(2) A; C7-H7· · ·O7, 2.592(3) A; C17-H17B· · ·O5,
˚
2.687(3) A; C17-H17C· · ·O6, 3.020(2) A; C17-H17C· · ·
˚
˚
O7, 2.636(2) A], OZH· · ·O [O8-H8A· · ·O1, 2.151(3) A;
˚
˚
O8-H8A· · ·O6, 2.225(3) A] cause the formation of host–
˚
CZH· · ·N [C2-H2· · ·N15, 2.433(9) A; C21-H21C· · ·N15,
˚
2.413(3) A] intermolecular interactions where the solvent
guest structure (Figure S2, available online). Due to
these interactions, cation and anion form the cross-wires
which are stacked one above the other and form an oval-
shaped cavity between the two sets of cross-wires for
guest methanol molecule as shown in Figure 6.
molecule behaves as both donor and acceptor (Figure S3,
available online). All these interactions form the 3D zigzag
railway tracks like perspective view (Figure 8).

290
Table 2. Non-covalent interactions for 1–4 (A and ).
U.P. Singh et al.
o
˚
S. N
DZH· · ·A
d(DZH)
d(HZA)
d(DZA)
,(DHA) .
1.
[2PA2·2PzH2þ·OH2] (1)
O15-H16A· · ·O1
O15-H16A· · ·O7
O15-H15A· · ·O8
O15-H15A· · ·O14
N7-H7A· · ·O1
0.847(22)
0.847(22)
0.831(25)
0.831(25)
0.877(22)
0.920(24)
0.869(19)
0.851(22)
2.037(23)
2.317(24)
1.969(24)
2.259(22)
1.852(23)
1.783(24)
1.921(19)
1.930(21)
2.804(2)
2.904(2)
2.721(2)
2.837(2)
2.693(2)
2.679(2)
2.724(2)
2.764(2)
150.23(228)
126.77(201)
150.29(215)
126.94(198)
160.07(210)
163.91(219)
152.96(197)
166.39(220)
N8-H8A· · ·O15
N9-H9B· · ·O15
N10-H10A· · ·O8
2.
[PA2·PzMe2Hþ2 ] (2)
N4-H4A· · ·O1
N4-H4A· · ·O7
N5-H5· · ·O7
0.954(21)
0.954(21)
0.860(2)
0.930(1)
0.960(2)
0.930(2)
0.959(2)
1.678(21)
2.298(24)
2.362(4)
2.603(2)
2.606(3)
2.639(4)
2.478(4)
2.596(4)
2.834(4)
2.837(5)
3.418(2)
3.374(5)
3.516(6)
3.420(5)
166.44(218)
114.87(162)
115.19(9)
C2-H2· · ·O6
146.62(8)
C7-H7A· · ·O1
C9-H9· · ·O4
C11-H11B· · ·O5
137.17(12)
157.53(9)
167.26(12)
3.
[PA2·PzPh,MeH2þ·CH3OH] (3)
N1-H1· · ·O1
N1-H1· · ·O2
0.912(23)
0.912(23)
1.014(22)
0.820(1)
0.820(1)
0.960(2)
0.931(2)
0.929(2)
0.930(2)
0.960(3)
0.960(2)
0.960(2)
1.782(23)
2.315(21)
1.664(22)
2.151(3)
2.225(3)
2.355(2)
3.149(2)
2.859(2)
2.592(3)
2.687(3)
3.020(2)
2.636(2)
2.647(4)
2.888(4)
2.660(3)
2.846(4)
2.900(3)
3.170(3)
3.600(4)
3.728(4)
3.323(4)
3.577(5)
3.730(3)
3.593(3)
157.43(206)
120.65(176)
166.12(197)
142.64(9)
139.92(10)
142.32(13)
111.83(12)
156.29(12)
135.81(13)
154.55(14)
131.81(14)
174.87(14)
N2-H2A· · ·O8
O8-H8A· · ·O1
O8-H8A· · ·O6
C1-H1C· · ·O2
C6-H6· · ·O7
C6-H6· · ·O8
C7-H7· · ·O7
C17-H17B· · ·O5
C17-H17C· · ·O6
C17-H17C· · ·O7
4.
[2PA2·2Me4bpzHþ·CH3CN] (4)
N13-H13A· · ·O14
N14-H1AA· · ·O8
N14-H1AA· · ·O14
C2-H2· · ·N15
0.947(23)
0.901(25)
0.901(25)
0.930(2)
0.960(2)
0.960(3)
0.960(2)
0.960(3)
0.960(4)
0.960(4)
0.959(4)
2.792(25)
1.800(25)
2.284(25)
2.433(9)
2.575(2)
2.413(3)
2.605(5)
2.685(7)
2.761(9)
2.914(2)
2.600(3)
2.989(9)
2.679(8)
2.776(11)
3.193(11)
3.471(3)
3.240(4)
3.491(5)
3.349(11)
3.592(13)
3.649(6)
3.331(6)
143.64(22)
164.75(227)
114.67(34)
138.85(16)
155.35(14)
144.19(19)
153.47(14)
126.77(14)
145.26(15)
134.26(14)
133.18(22)
C19-H9B· · ·O11
C21-H21C· · ·N15
C22-H22A· · ·O11
C30-H30A· · ·O2
C30-H30C· · ·O5
C32-H32A· · ·O8
C33-H33B· · ·O14
The above structural studies reveal that on increasing
the substituents of pyrazole ring, the number of non-
covalent interactions varies from salt 1 to 4, which in turn
causes different type of 3D packing view for each salt.
The presence of vacant cavity in salt 2 may be used for
trapping suitable size of cations/anions.
optimised structure was the global minimum on the
potential energy surface. Single-point energy calcu-
lations were performed, and zero-point corrected total
energies for various species were recorded. In addition
to the characterisation of these salts, the gas phase
geometries, harmonic vibrational frequencies and bind-
ing energies of a series of pyrazole and their substituted
derivatives were computed. The B3LYP predicted
structure of salts as shown in Figure 9 and geometrical
parameters are given in Table S1 (available online).
The hydrogen bond interaction energies were deter-
mined according to the following equation:
3.2 Computational study
The optimised structural parameters of all individual
pyrazoles, acids and their salts were calculated at
B3LYP/6-31G(d,p) basis sets and are summarised in
Table S1 (available online). Each optimised geometries
showed positive vibrational frequencies showing that
DE ¼ Esalt 2 ðEpyrazole 2 EacidÞ;

Supramolecular Chemistry
291
Figure 1. Molecular structure of salt 1.
where Esalt, Epyrazole and Eacid are the zero-point corrected
total energies of salt, pyrazole and acid calculated at
DFT(B3LYP)/6-31G(d,p) level of theory. We have removed
the solvent and water molecules to check the relative
stability.
energy is still greater than 1 and 2 as the number of non-
covalent interactions are high but the steric hindrance of
pyrazole also plays an important role. The more steric
hindered groups on the pyrazole ring reduce the hydrogen
bond interaction energy. This is important to point out
that the salt 3 shows the more hydrogen bond interaction
energy than salt 4 due to the fact that the two pyrazole
rings create the much more steric hindrance in salt 4 as
compared to salt 3 which reduces the hydrogen bond
interaction energy. Along with this, the negligible
difference in the energy of the optimised structure of
The trend observed for the hydrogen bond interaction
energy is given in Table 3. In case of salts 1 and 2, the
number of non-covalent interactions are same as both
have almost same amount of hydrogen bond interaction
energy. On the other hand, in salts 3 and 4, the interaction
Figure 2. Host-guest complex formed due to NZH· · ·O,
CZH· · ·O, OZH· · ·O intermolecular interactions in salt 1. (The
H2O molecules occupying the cavity as guest molecules between
the alternate channels are represented in space-fill model).
Figure 3. Molecular structure of salt 2.

292
U.P. Singh et al.
Figure 4. (a) Various non-covalent interactions and (b) Sheet like packing via NZH· · ·O, CZH· · ·O interactions, forming a rectangular
cavity in 2.
DTA thermogram for salt 2 indicates that it also undergoes
two stages of decomposition. The first stage (,28.2% mass
loss) corresponds to endotherm at 1028C. In this step, one
molecule of 3,5-dimethylpyrazolate (92–2008C) leaves out.
In the second stage (200–2348C), picrate ion leaves
(,68.5% mass loss) which corresponds to exotherm at
2788C. Three steps of decomposition take place in salt 3. In
the first step, methanol molecule releases in the 101–2258C
temperature range (,7.1% mass loss) which corresponds to
endotherm at 1628C, while in next second and third steps,
both 3-phenyl-5-methylpyrazolate and picrate ions release
in 225–2958C (,52.8% mass loss) and 505–6668C
(,35.9% mass loss) temperature range, respectively. The
DTA peak at 2818C and 5898C is exothermic for both steps,
respectively. TG curve of salt 4 exhibited three well-
separated weight loss stages. In first step, acetonitrile
molecule releases in 99–1718C temperature ranges (,4.6%
mass loss). In second step (240–2898C), protonated Me4bpz
leaves out (,49.7% mass loss) exothermically at 2938C,
while picrate ion (,45.1% mass loss) also releases (289–
6898C) exothermically at 6868C in third step.
Figure 5. Molecular structure of salt 3.
salts and the crystal structure suggests that the orientation
and interaction remain almost the same from the gaseous
to the solid phase (Table S2, available online). The
optimised bond lengths and angles for salts 1–4 are given
in Table S1 (available online), which again show that the
molecules are arranged in same fashion in both solid and
gaseous phases.
The kinetics of thermal decomposition of salts 1–4
was evaluated using 14 mechanisms based on kinetic
models as given in Table S3 (available online). The set of
reaction models (36) were used to analyse the isothermal
TG data in the range of 200–2408C for 1–4 (Figure. 10) to
calculate the Ea values for thermal decomposition, which
were done in the range of decomposition/vapourisation
using indigenously fabricated TG apparatus. The acti-
vation energy values are reported in Table 5. In the model-
fitting method, the kinetics is analysed by choosing a
‘best-fit’ model based on the value of the correlation
coefficient r close to 1. Average values 55.5, 64.3, 67.0 and
55.0 kJ mol21 have been obtained as activation energies for
isothermal decomposition of salts 1–4. The isoconver-
sional method is known to permit estimation of activation
energy independent of the model used. This method is being
used to establish a relation between activation energy and
extent of conversion (a) of the sample. According to
Figure 11, for a particular salt, each activation energy has a
3.3 Thermal analysis
These salts are stable at room temperature and their thermal
stability is demonstrated by a thermogravimetric (TG),
derivative thermogravimetric (DTG) and differential
thermal analysis (DTA). The thermoanalytical data for
salts 1–4 are listed in Table 4, and the non-isothermal TG–
DTA curves of salts 1–4 are shown in Figures S4–S7
(available online). Salt 1 shows two steps of decomposition.
In the first step (71–2058C), one water molecule and
protonated pyrazole leave out (,23.4% mass loss) which
corresponds to endotherm at 1598C in DTA thermogram,
while in the second step (205–2798C) one picrate ion
(,72.8% mass loss) leaves out exothermically at 3508C.
Beyond this temperature, the explosion of resulting mass
occurs with the formation of gaseous products. The TG–

Supramolecular Chemistry
293
Figure 6. Host-guest complex formed due to NZH· · ·O, CZH· · ·O intermolecular interactions in salt 3. (The CH3OH molecules
occupying the oval shaped cavity as guest molecules between the cross wires are represented in space-fill model).
separate value at different a. Although these salts are stable
at room temperature but ignite when subjected to sudden
high temperature. Further to evaluate the sensitivity of these
salts, their ignition delay measurement was carried out in
temperature ranges 340–3808C for salts 1–4 (Table 6).
For salts 1–4, E*a was determined from the slope of a plot of
ln(Di) versus 1/T as shown in Figure 12. The energy of
activation for ignition is 28.6, 29.3, 46.4 and 37.7 kJ mol21
for salts 1–4, respectively. From the ignition delay
measurements, it is clear that the salt 3 is highly thermal
stable than 1, 2 and 4 as it has the highest activation energy
of ignition delay. The activation energy (calculated by
isothermal TG) and ignition delay measurement have
different values that may be due to the different temperature
ranges. The OB values reported in Table 6 suggest that these
salts belong to low explosives.
4. Conclusion
In summary, we prepared the hydrogen bonded supramo-
lecular architecture of picric acid with different substituted
pyrazole having different 3D packing views. Different
factors such as variation of substituents, number and type
of interactions, and the orientation of the molecules in 3D
spaces are responsible for the change in the packing. Our
structure analysis reveals that in salts 1–4 the host
molecules self-assembled due to various types of non-
covalent interactions and form different shaped cavity. On
the other hand, the solvent molecules as water, methanol
and acetonitrile behave as the guest molecule and occupied
the cavity in salts 1, 3 and 4. The present study
Figure 7. Molecular structure of salt 4.

294
U.P. Singh et al.
Figure 8. Host-guest complex formed due to NZH· · ·O, CZH· · ·O, CZH· · ·N intermolecular interactions in salt 4. (The CH3CN
molecules occupying the rectangular shaped cavity as guest molecules are represented in space-fill model).
Figure 9. Optimised geometry of salts (a) 1, (b) 2, (c) 3 and (d) 4.
demonstrated that the variation of substituent groups on
Table 3. Hydrogen bond interaction energy (kcal/mol).
pyrazole ring plays an important role in controlling the
structure of the salts. Theoretical studies also suggested
that in both solid and gaseous phase, structure is same and
hydrogen bond interaction energy largely depends on the
functional moieties that are being involved in the
interaction. From the ignition delay measurements, it is
clear that the salt 3 is highly thermal stable than others as it
has the highest activation energy of ignition delay.
Hydrogen bond
interaction energy
(kcal/mol)
S. N
Salts
1.
2.
3.
4.
[2PA2·2PzH2þ·OH2] (1)
[PA2·PzMe2Hþ2 ] (2)
13.39
14.51
26.56
21.96
[PA2·PzPh,MeH2þ·CH3OH] (3)
[2PA2·2Me4bpzHþ·CH3CN] (4)

Supramolecular Chemistry
295
Table 4. TG–DTA phenomenological data of salts 1–4 under air atmosphere.
TG
DTA
DTG
Peak temp. (8C)
Salts
Stage
T range /8C
Observed mass loss (%)
Peak temp. (8C)
Nature
[2PA2·2PzH2þ·OH2] (1)
I
II
I
II
I
II
III
I
II
III
71–205
205–279
92–200
200–324
101–225
225–295
505–666
99–151
23.4
72.8
28.2
68.5
7.1
52.8
35.9
4.6
159
280
102
278
162
281
589
154
293
686
endo
exo
endo
exo
endo
exo
exo
endo
exo
exo
135
268
190
271
161
275
592
147
282
678
[PA2·PzMe2Hþ2 ] (2)
[PA2·PzPh,MeH2þ·CH3OH] (3)
[2PA2·2Me4bpzHþ·CH3CN] (4)
240–289
289–689
49.7
45.1
Figure 10. Isothermal TG in static air atmosphere for salts 1–4 (a) 1, (b) 2, (c) 3 and (d) 4.

296
U.P. Singh et al.
Table 5. Arrhenius parameters for isothermal decomposition of salts 1–4.
(1)
(2)
(3)
(4)
Salts
Modela
Ea
r
Ea
r
Ea
r
Ea
r
1
2
3
4
5
6
7
8
61.5
61.3
60.9
58.5
57.4
56.2
58.6
59.1
58.3
57.5
54.5
58.0
59.0
55.5
0.9718
0.9722
0.9728
0.9756
0.9765
0.9762
0.9750
0.9740
0.9757
0.9364
0.9754
0.9767
0.9737
0.9770
66.0
65.8
65.4
64.3
64.1
64.6
64.6
65.4
65.1
64.6
64.7
64.6
65.9
64.3
0.9983
0.9984
0.9986
0.9991
0.9990
0.9990
0.9989
0.9989
0.9990
0.9991
0.9575
0.9991
0.9988
0.9987
62.8
62.9
63.3
65.0
65.8
66.3
64.7
64.4
64.9
65.6
67.9
65.2
64.3
67.0
0.9643
0.9650
0.9663
0.9728
0.9752
0.9764
0.9716
0.9702
0.9720
0.9743
0.9566
0.9732
0.9698
0.9784
60.5
60.2
59.4
55.0
53.1
53.9
57.4
58.1
57.1
55.0
50.4
55.6
59.0
51.2
0.9484
0.9490
0.9501
0.9533
0.9531
0.9516
0.9515
0.9499
0.9507
0.9521
0.8888
0.9522
0.9483
0.9512
9
10
11
12
13
14
Note: Ea ¼ kJ/mol.
a Enumeration of the model is as given in Table S3 (available online).
Figure 12. Graph of ln Di vs 1/T.
Figure 11. Dependence of activation energy (Ea) on the extent
of conversion (a) for salts 1–4.
Table 6. Activation energy for thermal ignition (E ) and correlation coefficient (r) for salts 1–4.
*
Di/s at temperature (8C)
Salts
340 ^ 1
350 ^ 1
360 ^ 1
370 ^ 1
380 ^ 1
E*/kJmol21 (kJ/mol)
r
OB
(1)
(2)
(3)
(4)
109
97
107
112
99
85
89
93
93
79
79
80
82
74
68
77
78
67
61
70
28.58
29.30
46.45
37.75
0.9933
0.9931
0.9973
0.9742
257.60
266.17
279.78
271.50
5. Supporting Information Available
www.ccdc.cam.ac.uk). The additional DFT calculations
and figures are available in PDF formats.
CCDC numbers 844776 – 844779 contain the supplemen-
tary crystallographic data (CIF) for this article. These data
can be obtained free of charge from the Director, CCDC,
12 Union Road, Cambridge CB2 1EZ, UK (Fax: þ44-
1223-336-033; e.mail: deposit@ccdc.cam.ac.uk or http://
Acknowledgements
The authors gratefully acknowledge CSIR, New Delhi, India, for
their financial assistance in the form of SRF to Nidhi Goel.

Supramolecular Chemistry
297
(20) Perrin, D.D.; Armarego, W.L.; Perrin, D.R. Purification of
Laboratory Chemicals, 2nd ed. Pergamon: New York, 1980.
(21) Puerta, D.T.; Cohen, S.M. Inorg. Chim. Acta 2002, 337,
459.
(22) Mosby, W.L. J. Chem. Soc. 1957, 3997.
¨
(23) Sheldrick, G.M.; University of Gottingen: Germany, 1996.
(24) Sheldrick, G.M. Acta Crystallogr. 1990, A46, 467.
¨
(25) Sheldrick, G.M. SHELXTL-NT; University of Gottingen:
Germany, 2000.
(26) Brandenburg, K. DIAMOND; Crystal Impact GbR: Bonn,
Germany, 2000.
(27) MERCURY, Cambridge Crystallographic Data Centre.
12 Union Road, Cambridge CB2 1EZ: UK. Available from:
Website: http://www.ccdc.cam.ac.uk/.
(28) Gaussian 03, Rev. C.02; Guassian, Inc. Wallingford CT,
2004.
References
(1) Desiraju, G.R. Angew. Chem., Int. Ed. 1995, 34, 2311.
(2) Moulton, B.; Zaworotko, M.J. Chem. Rev. 2001, 101, 1629.
(3) Beatty, A.M. Cryst. Eng. Comm. 2001, 51, 1.
(4) Reger, D.L.; Debreczeni, A.; Horger, J.J.; Smith, M.D.
Cryst. Growth Des. 2011, 11, 4068.
¨
(5) Porada, J.H.; Neudorfl, J.; Blunk, D. Cryst. Growth Des.
2011, 11, 3648.
(6) Srinivasan, P.; Kanagasekaran, T.; Gopalakrishnan, R.;
Bhagavannarayana, G.; Ramasamy, P. Cryst. Growth Des.
2006, 6, 1663.
(7) Yamaguchi, S.; Goto, M.; Takayanagi, H.; Ogura, H.
Bull. Chem. Soc. Jpn. 1988, 61, 1026.
(8) In, Y.; Nagata, H.; Doi, M.; Ishida, T.; Wakahara, A.
Acta Crystallogr. 1997, C53, 367.
(9) Dobrowolska, W.S.; Bator, G.; Sobczyk, L.; Grech, E.;
Scheibe, J.N.; Pawlukojc, A.; Wuttke, J. J. Mol. Struct.
2010, 975, 298.
(10) Jin, S.; Zhang, W.; Wang, D.; Gao, V.; Zhou, J.Z.; Chen, R.;
Xu, X. J. Chem. Crystallogr., 2009, 40, 87.
(29) Hariharan, P.C.; Pople, J.A. Theo. Chem. Acta 1972, 28,
213.
(30) Singh, G.; Singh, R.R. Res. Ind. 1978, 23, 92.
(31) Singh, G.; Vasudeva, S.K.; Kapoor, I.P.S. Indian J. Tech.
1991, 29, 589.
(11) Chandramohan, A.; Bharathikannan, R.; Kandhaswamy,
M.A.; Chandrasekaran, J. Cryst. Res. Technol. 2008, 43, 93.
(12) Yathirajan, H.S.; Narayana, B.; Swamy, M.T.; Sarojini,
B.K.; Bolte, M. Acta Crystallogr. 2008, E64, o119.
(13) Narayana, B.; Sarojini, B.K.; Kamath, K.P.; Yathirajan,
H.S.; Bolte, M. Acta Crystallogr. 2008, E64, o117.
(14) MacDonald, J.C.; Yigit, M.V.; Mychajlonka, K. Cryst.
Growth Des. 2005, 5, 2248.
(15) Muthamizhchelvan, C.; Saminathan, K.; Fraanje, J.;
Peschar, R.; Sivakumar, K. Anal. Sci. 2005, 21, x61.
(16) Smith, G.; Wermuth, U.D.; Healy, P.C. Acta Crystallogr.
2004, E60, o1800.
(32) Zinn, J.; Rogers, R.N. J. Phys. Chem. 1962, 66, 2646.
(33) Semenov, N. Chemical Kinetics: Chain Reactions;
Clarendon Press: Oxford, Chapter 18, 1935.
(34) Freeman, E.S.; Gorden, S. J. Phys. Chem. 1956, 60, 867.
(35) Martin, A.R.; Yallop, H.J. Trans Faraday Soc. 1958, 54,
257.
(36) Brown, M.E.; Dollimore, D.; Galway, A.K. Reaction in the
Solid State. Comprehensive Chemical Kinetics; Elsevier:
Amsterdam, 1960; Vol. 22, pp. 1–340.
(37) Singh, G.; Felix, P.S.; Pandey, D.K. Thermochim. Acta
2004, 411, 61.
(38) Ghazaryan, V.V.; Fleck, M.; Petrosyan, A.M. Spectrochim.
Acta 2011, A78, 128.
´
(17) Llamas-Saiz, A.L.; Foces-Foces, C.; Echevarrıa, A.;
Elguero, J. Acta Crystallogr. 1995, C51, 1401.
(18) Infantes, L.; Foces-Foces, C.; Claramunt, R.M.; Lopez, C.;
(39) Ikeda, C.; Nagahara, N.; Motegi, E.; Yoshioka, N.; Inoue, H.
Chem. Commun. 1999, 43, 1759.
´
Elguero, J. J. Heterocycl. Chem. 1999, 36, 595.
´
(40) Silverstein, R.M.; Bassler, G.C.; Morrill, T.C. Spectro-
metric Identification of Organic Compounds, 5th ed. Wiley:
New York, 1991.
(19) Zaderenko, P.; Gil, M.S.; Lopez, P.; Ballesteros, P.;
Fonseca, I.; Albert, A. Acta Crystallogr. 1997, B53, 961.
ERRATUM
The version of this article, originally published online on 23 April 2012, has been corrected to include missing figures.
Taylor & Francis wishes to apologise for this oversight.