Effect of electron donating substituents on supramolecular structure of salts having phenylphosphonic acid and pyrazoles

Article abstract of DOI:10.1007/s11224-011-9781-4

The reactions of phenylphosphonic acid [(OH)₂POC ₆H₅] (PPA) with a variety of ditopic pyrazoles Pz ^R1,R2H (where R₁ = R₂ = H for 1, R₁ = R₂ = Me for 2, R₁ = Ph, R₂ = Me for 3 and R₁ = R₂ = iPr for 4) produce salts 1-4. Due to the presence of different substituents on pyrazole, each salt shows different structure. This study demonstrates that the presence of different substituents on the pyrazole moieties provides sufficient non-covalent interactions which are responsible for the different supramolecular structures adopted. Theoretical studies reveal that the proton affinity increases on the nitrogen having lone pair of electron with the increase of electron density on pyrazole ring but the orientation remains the same in both the solid and gaseous phase.

Full text of DOI:10.1007/s11224-011-9781-4

Struct Chem (2011) 22:931–941
DOI 10.1007/s11224-011-9781-4
ORIGINAL RESEARCH
Effect of electron donating substituents on supramolecular
structure of salts having phenylphosphonic acid and pyrazoles
•
Udai P. Singh Sujata Kashyap Ray J. Butcher
•
•
•
Hari Ji Singh Bhupesh Kumar Mishra
Received: 2 November 2010 / Accepted: 11 March 2011 / Published online: 29 March 2011
Ó Springer Science+Business Media, LLC 2011
Abstract The reactions of phenylphosphonic acid [(OH)2
POC6H5] (PPA) with a variety of ditopic pyrazoles
PzR1,R2H (where R1 = R2 = H for 1, R1 = R2 = Me for 2,
R1 = Ph, R2 = Me for 3 and R1 = R2 = iPr for 4) pro-
duce salts 1–4. Due to the presence of different substituents
on pyrazole, each salt shows different structure. This study
demonstrates that the presence of different substituents on
the pyrazole moieties provides sufficient non-covalent
interactions which are responsible for the different supra-
molecular structures adopted. Theoretical studies reveal
that the proton affinity increases on the nitrogen having
lone pair of electron with the increase of electron density
on pyrazole ring but the orientation remains the same in
both the solid and gaseous phase.
Introduction
Due to increased interest in the design and synthesis of new
smart materials, the field of supramolecular chemistry has
progressed to the point where one can obtain the desired
materials or supramolecular structure with specific chemi-
cal and physical properties via crystal engineering [1–11].
There are several factors which control the properties of
product like reaction time, reaction conditions, existence of
different covalent and non-covalent interactions, geometry,
molecular size and the stability of the final product, etc.
The phosphonic acid functional group (R–P(O)(OH)2)
has several similarities with carboxylic acid group and both
are very useful in designing the new materials. The use of
phosphonic acid in the construction of 2D or 3D hydrogen
bonded supramolecular material and crystal engineering is
less developed in comparison to carboxylic acid [12–14]
and has been limited mainly to metal phosphonates,
organic–inorganic hybrid materials, co-crystals and organic
salts. Different phosphonic acids have been utilized for the
construction of organic salts and co-crystals with a number
of organic bases and neutral molecules especially with
aniline, ethylenediamine, p-phenylazoaniline, hexamethy-
lenediamine, 2,20-bipyridine, 4,40trimethylene-dipyridine,
hexamacrocyclic crown ethers, 4,40-bipyridine, p-dimeth-
ylaminopyridine and guanidinium chloride [11, 15–20]. In
addition, they are widely used for the preparation of metal
complexes too [21–30]. The importance of (P)O–HꢀꢀꢀO=P
bonds in nucleic acids and biological molecules is also well
documented.
Keywords Phosphonic acid ꢀ Pyrazole ꢀ Crystal
structure ꢀ Supramolecular assembly
Electronic supplementary material The online version of this
article (doi:10.1007/s11224-011-9781-4) contains supplementary
material, which is available to authorized users.
U. P. Singh (&) ꢀ S. Kashyap
Department of Chemistry, Indian Institute of Technology
Roorkee, Roorkee 247667, India
e-mail: udaipfcy@iitr.ernet.in
R. J. Butcher
Department of Chemistry, Howard University, 525 College
Street NW, Washington, DC 20059, USA
The ditopic pyrazole ligand with various substituents at
3 or 5 position is an excellent candidate for the crystal
engineering and design of new materials. The example of
co-crystals using pyrazole as one component are very
limited in literature [31, 32]. However, the effect of the
H. J. Singh ꢀ B. K. Mishra
Department of Chemistry, DDU Gorakhpur University,
Gorakhpur 273001, India
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Struct Chem (2011) 22:931–941
substituent on the various components of crystalline
materials such as salts, co-crystals and coordination poly-
mers has been reported recently [33–36].
Functional Methods with 6-31G(d,p) basis set with
Gaussian 03 suite of program [45, 46]. The input for the
simulation was the z-matrix generalized by Gauss view that
was also used for visualizing the molecules with optimized
geometries [47].
We are interested in exploring the co-crystallization of the
phosphonic acid and pyrazole synthons as well as in the
orientation of both in the solid and gaseous phase. Here we
report the crystal structures of four salts that demonstrate
how the size of substituents allows for different structure/
packing of 1–4. We are of the opinion that understanding of
structural correlations in salts 1–4 may facilitate the design
of new solid materials using different pyrazoles as one
component.
Preparation of organic salts
Preparation of [PzH2 ꢀPPA-] (1)
?
Phenylphosphonic acid (0.15 g, 1.0 mmol) and pyrazole
(0.06 g, 1.0 mmol) were mixed in an acetonitrile–methanol
mixture (v/v %, 1:4, 5 mL). The resulting solution was
stirred for 6 h and filtered through Celite. Colourless
crystals of salt 1 suitable for X-ray data collection were
obtained by slow evaporation at room temperature. Yield:
64% (0.14 g). Anal. Calcd for C9H11N2O3P: C 47.79, H
4.90, N 12.39; found: C 47.42, H 4.79, N 12.60. IR (KBr,
cm-1): 3652, 3137, 2758, 1836, 1593, 1528.
Experimental
Materials and methods
All manipulations were performed in air using commercial
grade solvents after predried by appropriate drying agents
[37]. Phenylphosphonic acid (PPA), pyrazole (PzH), and
3,5-dimethylpyrazole (PzMe2H) were purchased from
Aldrich Chemical Company, USA. 3-Phenyl-5-methylpyr-
azole (PzPh,MeH) and 3,5-diisopropylpyrazole (PziPr2H)
were prepared by methods available in literature [38, 39].
Crystallized 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
Nikolet Nexus FT-IR spectrometer in KBr. 1H-NMR spectra
were recorded on Bruker-D-Avance 500 spectrometer with
Fourier transform technique using TMS (tetramethylsilane)
as internal standard. Powder XRD data were collected using
Bruker Advance D8 XRD diffractometer.
Preparation of [PzMe2H2 ꢀPPA-] (2)
?
Salt 2 was prepared and crystallized by the method
described for the preparation of salt 1. Yield: 67% (0.18 g).
Anal. Calcd for C11H15N2O3P: C 51.97, H 5.95, N 11.02;
found: C 52.07, H 5.85, N 11.15. IR (KBr, cm-1): 3639,
3129, 2684, 1870, 1616, 1547.
?
Preparation of [PzPh,MeH2 ꢀPPA-] (3)
Salt 3 was prepared and crystallized by the method
described for the preparation of salt 1. Yield: 42% (0.32 g).
Anal. Calcd for *C32H33N4O6P2: C 60.85, H 5.27, N 8.87;
found: C 60.95, H 5.33, N 9.00. IR (KBr, cm-1): 3473,
3133, 2664, 1850, 1613, 1589.
X-ray crystallography
* The single hydrogen in the crystal is distributed over
two phosphonic acid.
The X-ray data collection were performed on a Bruker Kappa
Apex four circle-CCD diffractometer using graphite mono-
?
˚
chromated MoKa radiation (k = 0.71070 A) at 100 K. In the
Preparation of [PziPr2H2 ꢀPPA-] (4)
reduction of data Lorentz and polarization corrections,
empirical absorption corrections were applied [40]. Crystal
structures were solved by Direct method. Structure solution,
refinement and data output were carried out with the SHEL-
XTL program [41, 42]. Non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were placed in geometri-
cally calculated positions using a riding model. Images and
hydrogen bonding interactions were created in the crystal
lattice with DIAMOND and MERCURY software [43, 44].
Salt 4 was prepared and crystallized by the method
described for the preparation of salt 1. Yield: 71% (0.22 g).
Anal. Calcd for C15H23N2O3P: C 58.05, H 7.47, N 9.03;
found: C 58.15, H 7.36, N 9.16. IR (KBr, cm-1): 3589,
3173, 2967, 1847, 1614, 1537.
Results and discussion
Computational study
The reaction of phenylphosphonic acid and corresponding
pyrazoles (PzH/PzMe2H/PzPh,MeH/PziPr2H) in acetonitrile–
methanol mixture resulted in the formation of organic
ionic salts (1–4) as shown in Scheme 1. The different
Geometry optimization of different species involved during
the course of present investigation were done using Density
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Struct Chem (2011) 22:931–941
933
significant downfield shift as compared to the free ligands
suggested that the two constituent of salts are well hydro-
gen bonded in the solution. The crystallographic and
selected hydrogen bonding data are given in Tables 1
and 2, respectively.
N
NH
Salt-1
H3C
CH3
Crystal structure of salts 1–4
N
NH
Salt-2
Salt-3
The salt 1 crystallizes in the monoclinic crystal system with
P21 space group. As shown in Fig. 1, the unit cell consists
of two molecules of protonated pyrazole with four NH
groups and two molecules of deprotonated phenylphos-
phonic acid with two phosphonic acid groups. The struc-
ture of phenylphosphonic acid (alone) consists of hydrogen
bonded layers aligned parallel to the ab plane and the
phenyl groups pointing out from the layers as reported in
the literature [48]. The phosphonic acid group acts as both
hydrogen donor through the O1 atom and acceptor through
O2 and O3 atoms. The deprotonated phosphonic acid
groups and protonated nitrogen atom of both pyrazoles
form a cationic anionic acid–base pair, where all the four
NH groups are available for hydrogen bonding. The
organic salt 1 shows N–HꢀꢀꢀO as well as C–HꢀꢀꢀO interac-
tion (Fig. 2). The OH group of phosphonic acid forms a
O–HꢀꢀꢀO interaction with neighbouring phosphonic acid
O
H3C
P
OH
N
NH
OH
CH3
H
CH3
CH3
H
H3C
N
NH
Salt-4
Scheme 1 Preparation of Salt 1–4
formulations for ionic salts 1–4 were confirmed by ele-
mental analysis and crystallographic structure analyses.
The solution behaviour of the hydrogen bonded salts was
studied by 1H-NMR spectroscopy. All salts show a
˚
molecule (O1–H1AꢀꢀꢀO5, 1.795; O4–H4AꢀꢀꢀO3, 1.751 A)
resulting in a three-dimensional dendrite structure with
Table 1 Crystallographic data and structure refinement details for 1–4
Salt
1
2
3
4
Empirical formula
Formula weight
Crystal system
Space group
C9H11N2O3P
226.17
Monoclinic
P21
C11H15N2O3P
254.22
C32H33N4O6P2
631.56
C15H23N2O3P
310.32
Trigonal
P31
Orthorhombic
Cmc21
Triclinic
ꢀ
P1
˚
A (A)
8.422(6)
16.201(1)
8.656(6)
90.00
16.198(4)
16.198(4)
12.408(5)
90.00
17.583(2)
11.975(2)
14.674(2)
90.00
9.686(4)
10.378(1)
16.581(17)
86.52(4)
88.76(4)
78.01(4)
1627.04(3)
4
˚
B (A)
˚
c (A)
a (deg)
b (deg)
c (deg)
111.10(4)
90.00
90.00
90.00
120.00
90.00
3
˚
V (A )
1101.99(1)
4
2819.55(15)
9
3090.10(6)
4
Z
D
calc/Mg/m3
1.363
1.347
1.358
1.267
T, K
100
100
100
100
F(000)
l, mm-1
472
1206
1324
664
0.239
0.218
0.192
0.180
GOF(F2)
1.014
1.142
1.068
1.031
R1, wR2 [I [ 2r(I)]a
0.0370, 0.0751
0.0847, 0.1958
0.0927, 0.2008
0.0462, 0.0858
0.0623, 0.0935
0.0463, 0.1076
0.0690, 0.1176
R1, wR2 (all data)a
0.0556, 0.0817
P
P
P
P
R1 = kFo| - |Fck/ |Fo|, wR2 = { [w(F2o - Fc2) ]/ w(Fo2)2}1/2
2
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Struct Chem (2011) 22:931–941
˚
Table 2 Non-covalent interactions for 1–4 (A and °)
D–HꢀꢀꢀA
d(D–H)
d(H–A)
d(D–A)
\(DHA)[
1
N1–H1BꢀꢀꢀO2
N2–H2BꢀꢀꢀO6
N3–H3BꢀꢀꢀO5
N4–H4BꢀꢀꢀO3
O1–H1AꢀꢀꢀO5
O4–H4AꢀꢀꢀO3
C16–H16ꢀꢀꢀO2
C17–H17ꢀꢀꢀO4
C18–H18ꢀꢀꢀO6
C18–H18ꢀꢀꢀO6
2
0.880(3)
0.880(2)
0.880(2)
1.750(5)
1.657(3)
1.829(2)
2.626(7)
2.526(4)
2.672(3)
172.3
168.9
159.7
170.0
179.2
167.7
132.1
111.9
112.0
157.0
0.870(34) 1.744(34) 2.605(3)
0.830(22) 1.795(2)
2.634(3)
2.578(3)
3.185(4)
3.152(6)
3.172(4)
3.334(7)
0.840(3)
0.950(3)
0.950(3)
0.950(3)
0.950(3)
1.751(2)
2.469(3)
2.672(6)
2.691(2)
2.439(6)
N1D–H1DDꢀꢀꢀO3A 0.881(4)
N2D–H2DAꢀꢀꢀO1A 0.880(7)
1.872(4)
1.680(5)
1.681(3)
1.814(0)
1.697(3)
1.849(4)
1.819(4)
1.729(4)
1.745(3)
2.512(3)
2.520(1)
2.418(4)
2.599(2)
2.684(5)
2.556(8)
2.545(6)
2.660(4)
2.573(5)
2.667(6)
2.514(7)
2.552(5)
2.541(5)
3.425(7)
3.443(6)
3.360(7)
3.520(6)
152.4
173.7
166.1
160.6
174.5
153.7
138.9
165.7
157.2
161.0
157.0
171.0
156.5
N1E–H1EDꢀꢀꢀO2C
N2E–H2EAꢀꢀꢀO1C
N1F–H1FDꢀꢀꢀO1B
2F–H2FꢀꢀꢀO2B
0.880(5)
0.880(3)
0.879(4)
0.880(5)
O2A–H2ABꢀꢀꢀO3A 0.841(5)
O3B–H3BꢀꢀꢀO2B
O3C–H3CꢀꢀꢀO1C
C3F–H3FAꢀꢀꢀO3C
C5F–H5FBꢀꢀꢀO2C
C3E–H3EAꢀꢀꢀO2A
0.840(3)
0.840(3)
0.950(5)
0.980(6)
0.950(6)
Fig. 1 ORTEP view of salt 1 at 30% probability
C1EF–H1EAꢀꢀꢀO3C 0.980(6)
3
N1–H1DꢀꢀꢀO3
N2–H2AꢀꢀꢀO1
O2–H2ꢀꢀꢀO4
O4–H4AꢀꢀꢀO2
C1–H1AꢀꢀꢀO2
C6–H6AꢀꢀꢀO3
C12–H12AꢀꢀꢀO4
C1–H1Aꢀꢀꢀp
C18–H18ꢀꢀꢀp
4
0.957(33) 1.72(34)
0.892(28) 1.74(28)
2.672(3)
2.632(3)
2.507(7)
2.507(7)
3.389(4)
3.601(3)
3.733(4)
3.604(3)
4.342(3)
168.4
177.0
158.1
162.8
159.5
146.0
136.1
116.6
125.3
0.840(2)
0.840(2)
1.709(2)
1.692(2)
0.980(13) 2.453(2)
0.950(3)
0.950(4)
0.980(3)
0.951(3)
2.773(2)
2.989(2)
3.050(0)
3.723(3)
N1C–H1CDꢀꢀꢀO2B
0.880(7)
1.837(8)
2.671(4)
157.5
N2C4–H2CBꢀꢀꢀO2A 0.880(17) 1.652(32) 2.531(49) 176.1
N1D–H1DDꢀꢀꢀO1B 0.881(14) 1.700(21) 2.659(34) 162.0
N2D–H2DBꢀꢀꢀO1A 0.880(13) 1.809(22) 2.565(35) 166.2
O3A–H3AꢀꢀꢀO2B
O3B–H3BꢀꢀꢀO1A
0.833(12) 1.828(20) 2.656(31) 168.9
0.841(9) 1.837(19) 2.671(28) 171.2
Fig. 2 Different non-covalent interactions in salt 1
channels (Fig. S1). The presence of different non-covalent
interactions resulted in host–guest structure (Fig. 3) where
the anionic host assembly was present with diamond-
shaped cavity formed by the phosphonic acid molecules
and the cationic guest pyrazole molecules are located
inside the cavity. It is important to point out that the
structure of phenylphosphonic acid alone consists of
hydrogen bonded layers and is converted to a cuboid
structure in the presence of pyrazole.
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Struct Chem (2011) 22:931–941
935
(Fig. 4). In comparison to the number of interactions
present in salts 1, 2 contain some additional interactions
such as the C–HꢀꢀꢀO due to methyl C–H of pyrazole
(Fig. 5). The presence of additional interaction as well as
the two methyl substituents on pyrazole ring causes the
formation of anionic host assembly of phosphonic acid
with a triangular cavity. The cationic guest pyrazole mol-
ecules are not exactly residing in the cavity unlike to the
salt 1 (Fig. 6) but is helping in the cavity formation. The
presence of different non-covalent interactions among
the molecular components of salt 2, i.e., phenylphosphonic
acid and 3,5-dimethylpyrazole causes the formation of the
dendrimer structure along c-axis as shown in Fig. S2.
The co-crystallization of phenylphosphonic acid and
3-phenyl-5-methylpyrazole resulted in the formation of salt 3
which crystallizes in orthorhombic system with Cmc21 space
group. The asymmetric unit contained two half phosphonic
acid moieties on a crystallographic symmetry element and
one protonated pyrazole base (Fig. 7). The H was distributed
over two O’s of two different acid moieties with the occu-
pancies of 0.54 and 0.46. The dominant O–HꢀꢀꢀO hydrogen
bond is produced via the interaction of OH group of one
phosphonic acid with the oxygen (P=O) of adjacent phos-
phonic acid and vice versa forming a dimer, analogous to
benzoic acid, which results in the discrete one-dimensional
chain (O2–H2ꢀꢀꢀO4, 1.709; O4–H4ꢀꢀꢀO2, 1.692) (Fig. 8). The
neighbouring one-dimensional chains are stacked in a
parallel fashion through C–HꢀꢀꢀO contacts involving the
Fig. 3 Host-guest complex in salt 1
The unit cell of salt 2 contains three molecules of
anionic deprotonated phenylphosphonic acid and three
molecules of cationic protonated 3,5-dimethylpyrazole
forming an ionic salt held together through hydrogen bonds
Fig. 4 ORTEP view of salt 2
at 30% probability
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Struct Chem (2011) 22:931–941
Fig. 5 Different non-covalent interactions in salt 2
Fig. 6 a Three dimensional
view of host framework.
b Enlarged view of a fraction
showing encapsulated 3,5-
dimethylpyrazole in salt 2
interaction of phenylic hydrogen of pyrazole and phosphonic
oxygen as well as the phenylic hydrogen of phosphonic acid
with the oxygen of its neighbouring phosphonic acid
(C1–H1AꢀꢀꢀO2, 2.453(2); C6–H6AꢀꢀꢀO, 2.773(2), C12–
H12AꢀꢀꢀO4; 2.989(2)). Along with above interactions the p
electron of the phenyl group of pyrazoles forms C–Hꢀꢀꢀp
interaction with the methylic hydrogen of pyrazoles as well
as the phenylic hydrogen of phosphonic acid (C1–H1Aꢀꢀꢀp,
3.050(0); C18–H18ꢀꢀꢀp, 3.723(3)). The presence of different
types of non-covalent interaction between phenylphosphonic
acid and 3-phenyl-5-methylpyrazole in the unit cell gave an
entirely different packing for this salt in comparison to 1 and
2 (Fig. 9). The resulting heteromeric synthon connects the
acid and the base into an infinite two-dimensional mat-like
structure as shown in Fig. S3.
The salt 4 with two molecules of phenylphosphonic
acid and two molecules of 3,5-diisopropylpyrazole in the
ꢀ
unit cell crystallizes in the triclinic system with P1 group
(Fig. 10). It is important to note that due to steric hin-
drance, isopropyl groups are not involved in hydrogen
bonding (Fig. 11). Also, this bulky group leads to the
formation of head-to-tail hydrogen bonding resulting in a
stack of phosphonic acid with alternate O–HꢀꢀꢀO interac-
tion, i.e., OH group of one phosphonic acid forms
O–HꢀꢀꢀO interaction with P=O of the neighbouring phos-
phonic acid and the OH of the same forms interaction
with next adjacent phosphonic acid, resulting in an
entirely different type of three-dimensional packing as
alternate channels of phenylphosphonic acid and pyra-
zoles (Fig. 12).
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Struct Chem (2011) 22:931–941
937
Fig. 7 ORTEP view of salt 3
at 30% probability
Fig. 8 Different non-covalent
interactions in salt 3
Structural diversity and effect of substituents
structure in the presence of pyrazole. The change in the
stacking pattern of phosphonic acid from dimeric nature
in 3 to head-to-tail in 4 may be due to the differences in
orientation of the both components with the dihedral
angle 78.21° and 71.45° in 3 and 4, respectively. The
present structural studies revealed that the number of
non-covalent interactions vary from salt 1 to salt 4 which
in turn causes different type of three-dimensional packing
for each salt, i.e., cuboid for 1, triangular cavity for 2
The spatial arrangement of different substituent groups
on the 3,5 position of pyrazole ring governs the nature,
type, number and strength of hydrogen bonds as well as
short contacts which ultimately direct the crystal struc-
ture and its packing [34]. It is important to point out that
the structure of phenylphosphonic acid alone consists of
hydrogen bonded layers and is converted to a cuboid
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Struct Chem (2011) 22:931–941
Fig. 9 a Dimeric analogue of benzoic acid. b Alternate channels of host framework formed by phosphonic acid. c Host with guest 3-phenyl-5-
methylpyrazole in salt 3
Fig. 10 ORTEP view of salt 4
at 30% probability
and the alternate channels of the two component for 3
and 4.
simulated ones generated from single-crystal X-ray dif-
fraction (Fig. S4(a–d)).
Computational study
Powder XRD
The optimized structural parameters of all individual pyra-
zoles, acids and their salts were calculated at DFT(B3LYP)/
6-31G(d,p) level and are summarized in Table S1 (Fig. 13).
Each optimized geometry showed positive vibrational fre-
quencies suggesting that the optimized structure was the
We also examined the structural homogeneity of bulk
samples of 1–4 through a comparison of experimental and
simulated powder X-ray diffraction (XRD) patterns. The
experimental patterns correlate favourably with the
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Struct Chem (2011) 22:931–941
939
Fig. 11 Different non-covalent
interactions in salt 4
Fig. 12 a Dimeric analogue of benzoic acid. b Alternate channels of host framework formed by phosphonic acid. c Host with guest 3,5-
diisopropylpyrazole in salt 4
Fig. 13 Optimized structure
of salt 1–4
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Struct Chem (2011) 22:931–941
global minimum on the potential energy surface. Single
point energy calculations were performed and zero point
corrected total energies for various species were recorded.
Characterization of gas phase geometries and binding ener-
gies of salt of pyrazole and its substituted derivatives were
computed.
Conclusion
In the present work, the reaction of phenylphosphonic acid
with various ditopic pyrazole resulted in the formation of a
series of polar ionic salts with different three-dimensional
packing. A number of factors may be responsible for
the change in the packing most notably the variation of
substituents, number and type of interaction and the ori-
entation of the molecules in three-dimensional space. The
structural details reveal that the presence of a maximum
number of non-covalent interactions in salt 1 and 2 may
be responsible for the formation of a cavity. In salt 1, the
plane pyrazole moiety resides in the cavity whereas
in salt 2, 3,5-dimethylpyrazole helps in making the cavity.
As the substitution increases at 3 and 5 position of pyrazole
in salts 3 and 4, the number of non-covalent interaction
between the two components of the co-crystal decreases.
The cavity disappears and chains of the both cationic and
anionic component run in a parallel fashion with slight
differences. Both salts 3 and 4 have the same number of
non-covalent interactions and same type of supramolecular
structure. In conclusion, we may suggest that the variation
of substituent groups on the pyrazole ring play an impor-
tant role in controlling the structure of the salts and has not
been demonstrated so far by other workers with pyrazole
system. Theoretical studies also suggested that both solid
and gaseous phase structures are the same and hydrogen
bond interaction energy largely depends on the functional
moieties which are being involved in the interaction.
Table 3 shows that the proton affinity increases with the
increase of electron donating groups on pyrazole ring. The
proton preferably binds to the lone pair of nitrogen present
in the plane of the ring. The presence of higher substitution
makes the pyrazole ring more and more electron rich
thereby increases the proton affinity, i.e., as the electron
donating ability is increased, the proton affinity is
enhanced.
The trend observed for the hydrogen bond interaction
energy is given in Table 4. In case of salt 2 the presence of
two methyl group increases the possibility of hydrogen
bonded structure as compared to salt 1 and the hydrogen
bond interaction energy is almost twice for 2. On the other
hand in salts 3 and 4, the more sterically hindered groups
on the pyrazole ring reduce the hydrogen bond interaction
energy. However, the interaction energy for 3 and 4 is still
greater than 1. It is important to point out that for salts 3
and 4, the number of non-covalent interactions is same as
both have almost same amount of hydrogen bond interac-
tion energy.
Along with this, the negligible difference in the energy of
the optimized structure of salts and the crystal structure
suggest that the orientation and interaction remain almost
same in both the gaseous and solid phase (Table S2). The
optimized bond lengths and angles for the co-crystals 1–4 are
given in Table S1, which again shows that the molecules are
arranged in same fashion in both solid and gaseous phases.
Supplementary material
CCDC-788148 (1), -788149 (2), -788150 (3) and -788151
(4) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge at www.
ccdc.cam.ac.uk/conts/retrieving.html [or from the Cam-
bridge Crystallographic data Center (CCDC), 12 Union
Road, Cambridge CB2 1EZ, UK: fax: ?44(0)1223-336033;
email: deposit@ccdc.cam.ac.uk].
Table 3 Trend for the proton affinity (kcal/mol)
S. no.
Salt
Proton affinity
(kcal/mol)
1
2
3
4
[PzH2 ꢀPPA-] (1)
219.68
230.27
231.96
233.64
?
?
[PzMe2H2 ꢀPPA-] (2)
?
[PzPh,MeH2 ꢀPPA-] (3)
?
[PziPr2H2 ꢀPPA-] (4)
Acknowledgment The authors gratefully acknowledge CSIR, New
Delhi, India for financial assistance in the form of SRF to SK.
Table 4 Hydrogen bond interaction energy (kcal/mol)
S. no.
Salt
Hydrogen bond
interaction energy
(kcal/mol)
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