Anion directed supramolecular architecture of pyrazole based ionic salts

Article abstract of DOI:10.1039/c0ce00820f

The reaction of 3,5-diphenylpyrazole (Pz^Ph2H) with different inorganic acids affords salts viz., Pz^Ph2H₂ ⁺·H₂PO₄^- (1), Pz ^Ph2H₂⁺·NO₃ <

Full text of DOI:10.1039/c0ce00820f

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PAPER
Anion directed supramolecular architecture of pyrazole based ionic salts†
a
Udai P. Singh, Sujata Kashyap,a Hari Ji Singhb and Ray J. Butcherc
*
Received 6th November 2010, Accepted 22nd March 2011
DOI: 10.1039/c0ce00820f
The reaction of 3,5-diphenylpyrazole (PzPh2H) with different inorganic acids affords salts viz.,
PzPh2H2+$H2PO4ꢀ (1), PzPh2H2+$NO3ꢀ$H2O (2), PzPh2H2+$Clꢀ (3), 8PzPh2H2+$4HSO4ꢀ$2SO4ꢀ2$3H2O
(4) and PzPh2H2+$ClO4ꢀ$H2O$CH3OH (5) with different structures. The present study demonstrates
that the formation of hydrogen bond between protonated pyrazoles and anions provides a sufficient
driving force for the directed assembly of an extensive supramolecular frame work. Theoretical studies
reveal that with the increase in the strength of the inorganic acid, the hydrogen bond interaction energy
increases. Using Gaussview to analyse the optimized geometries obtained at DFT(B3LYP)/6-31G(d,p)
level of theory further revealed that the orientation of molecules remained the same both in the solid
and gaseous phases.
performed by density functional theory (DFT). The present
Introduction
paper reports the synthesis, crystal structure and DFT calcula-
The development of the receptor for anions has become a chal-
tion for the pyrazole based ionic salts and demonstrates the
lenging area of research because of its importance for molecular
significant role played by anions in deciding the hydrogen bond
recognition in biological systems as well as in supramolecular
interaction energy and supramolecular structure.
chemistry.1–6 A series of neutral amine, amide, urea, pyrrole,
indole and imidazolium receptors have been reported in the
literature.7–13 The ditopic pyrazole ligand with various substitu-
ents at the 3 or 5 position is an excellent candidate for molecular
engineering as various substituents can increase or decrease the
electron density on pyrazole ring. Being a positional isomer of
imidazole, pyrazole is ubiquitous in biochemical structure and its
function.14,15 Pyrazoles also play significant role in various fields
such as in catalysis, pharmacological, antimicrobial, fungicidal,
herbicidal, coordination and supramolecular chemistry.16–20
Although the various metal complexes of pyrazole have been
widely used for both anion as well as cation receptors,21–29 its use
as an anionic sensor has not been explored significantly expect in
a few recent work.30–33
Results and discussion
Salts 1–5 were prepared by the reaction of PzPh2H with corre-
sponding acid, i.e., H3PO4, HNO3, HCl, H2SO4 and HClO4 in
methanol–water solution (Chart 1). The colourless crystals
suitable for X-ray data collection were obtained by slow evap-
oration of resultant reaction mixture. The different formulations
were confirmed by elemental analysis, IR, NMR and crystallo-
graphic structure analyses of these salts. 3,5-diphenylpyrazole
In earlier studies the anion binding was studied both in solu-
tion and the solid state (crystallographic data).31,32,34,35 Presently
the computational power and algorithm has reached to such
a level that reasonably accurate details of binding energy for
isolated species in the gas phase can be obtained using Gaussian
03 and methods implemented. Rao and Sastry36 have reported
the proton affinity of pyrazole systems based on the calculation
aDepartment of Chemistry, Indian Institute of Technology Roorkee,
Roorkee, 247667, India. E-mail: udaipfcy@iitr.ernet.in
bDepartment of Chemistry, DDU Gorakhpur University, Gorakhpur,
273001, India
cDepartment of Chemistry, Howard University, 525 College Street NW,
Washington, DC, 20059, USA
† Electronic supplementary information (ESI) available. CCDC
reference numbers 800167–800171. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c0ce00820f
Chart 1
4110 | CrystEngComm, 2011, 13, 4110–4120
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similar to other hetero-aromatics containing N–H group and
shows N–H stretching vibration in the region of 3600–3300 cmꢀ1
a cavity as shown in Fig. 2. This cavity is occupied by two
dihydrogen phosphate ion through various non-covalent inter-
actions (Table 2). The trapped dihydrogen phosphate anions
.
In this region the position of the vibrations largely depend on the
extent of hydrogen bonding. These vibrations are shifted to lower
frequency (longer wavelengths) side with band widening in all the
five salts as compared to the free 3,5-diphenylpyrazole. The
hydrogen bonding characteristic of salts 1–5 in the solution was
studied by 1H-NMR spectroscopy. The prominent downfield
chemical shift in the NMR spectra observed in each case with
respect to the free ligand [((CD3)2CO, 270 MHz) 7.12 (s, 1 H, pz),
7.30–7.50 (m, 6 H, Ph), 7.85–7.95 (m, 4 H, Ph), 12.52 (s, br, 1 H,
NH)] suggested the existence of hydrogen bonding in the solu-
tion. The crystallographic data and structure refinement
parameters are given in Table 1. The selected hydrogen bonding
data are summarized in Table 2. Optimized Cartesian coordi-
nates and Mulliken charges for each atom are provided in Table
S1 (ESI).†
2
self-dimerized in R2 (8) motif through O–H/O interaction
ꢀ
(O1–H1/O3, 1.80 A, i.e. OH group of one dihydrogen phos-
phate with the oxygen of the other dihydrogen phosphate and
vice-versa) as reported in other cases (Fig. S1).†3,37–40 The C–H/
ꢀ
p interaction (3.21–3.37 A) among these pyrazoles also formed
small cavity which was empty (Fig. 3). It is important to mention
here that these two different type of cavities are present in salt 1.
The larger one is occupied by the dimer of the dihydrogen
phosphate, whereas the smaller one is vacant and can be used for
trapping other suitable ions. To the best of our knowledge, the
existence of two different type of cavities in the same salt using
pyrazole and phosphoric acid is not available in the literature
even today. The presence of different non-covalent interactions
resulted in cationic and anionic host–guest structure where the
pyrazole acts as host and dihydrogen phosphate anion as a guest
molecule (Fig. 3).
ꢀ
PzPh2H2+$H2PO4
(1)
PzPh2H2+$NO3ꢀ$H2O
(2)
Salt 1 crystallizes in orthorhombic with the space group
P21212. The unit cell contains one protonated PzPh2H2+ and one
dihydrogen phosphate anion (Fig. 1) bonded together through
a variety of hydrogen bonds (Fig. S1).† In the unit cell, depro-
tonated phosphoric acid and protonated pyrazole form
a cationic anionic pair where both NH groups of pyrazole are
involved in the hydrogen bonding with the anion. The geometry
around the phosphorus in dihydrogen phosphate is almost
regular tetrahedron with an average bond angle of 109.25ꢁ
(O1–P1–O2, 106.4; O2–P1–O4, 105.1; O4–P1–O3, 115.5; O3–P1–
O1, 110.0). The four PzPh2H2+ interacted with each other through
the C–H/p interaction (C12–H12/p, 3.17; C14–H14/p,
+
The unit cell of salt 2 (Fig. 4) consists of protonated PzPh2H2
,
nitrate anion and one mole of crystalline water. The geometry
around the nitrogen in nitrate ion is trigonal planar with an
average bond angle of 119.9ꢁ (O1–N3–O2, 118.9; O2–N3–O3,
121.4; O3–N3–O1, 119.5). Similar to the salt 1, the cavity
formation also occurs in salt 2 through C–H/p interactions
+
(C6–H6/p, 3.01; C8–H8/p, 3.87 A) (Fig. 5) of four PzPh2H2
.
ꢀ
+
In the cavity, the nitrate anion and water are linked with PzPh2H2
through N–H/O interaction involving four NH of two diagonal
pyrazoles and oxygen atoms of nitrate/water (N1–H1/O1W,
1.79; N2–H2A/O1, 1.91 A). The water molecule present in the
ꢀ
ꢀ
3.61 A) between the p electron density of phenyl ring present on
pyrazole and adjacent phenylic CH, resulting in the formation of
lattice also forms O–H/O interaction (O(1W)–H(1W1)/O(1),
Table 1 Crystal data and structure refinement parameters of salts 1–5a
1
2
3
4
5
Empirical formula
Formula weight
Crystal system
Space group
C15H15N2O4P
318.26
Orthorhombic
P21212
16.115(3)
18.888(4)
4.9329(12)
90.00
1501.5(6)
4
1.408
C15H15N3O4
301.30
Monoclinic
P21/n
6.2530(2)
14.4187(5)
15.9581(6)
93.3700(10)
1436.30(9)
4
C15H13ClN2
256.72
Monoclinic
P21/n
8.1846(2)
17.0013(4)
9.4108(2)
99.5070(10)
1291.52(5)
4
C120H114N16O27S6
2404.63
Monoclinic
P21
14.7258(7)
20.1708(8)
20.1033(7)
104.689(10)
5776.1(4)
2
C16H19ClN2O6
370.78
Orthorhombic
Pbca
18.4041(17)
8.4011(7)
22.587(2)
90.00
3492.3(5)
8
1.410
ꢀ
a/A
ꢀ
b/A
ꢀ
c/A
b/ꢁ
ꢀ
V/A
3
Z
Dc/g cmꢀ3
1.393
0.103
1.320
0.278
1.383
0.202
m/mmꢀ3
0.203
0.254
Crystal size/mm
q range/ꢁ
Reflections collected
Independent reflections
Parameters
0.28 ꢂ 0.27 ꢂ 0.25
1.66–28.25
3721
3229
196
0.27 ꢂ 0.25 ꢂ 0.24
1.90– 28.33
3530
2868
205
0.32 ꢂ 0.29 ꢂ 0.26
2.40– 28.33
3177
2570
163
0.23 ꢂ 0.21 ꢂ 0.18
1.05– 28.33
27489
17694
1545
0.19 ꢂ 0.15 ꢂ 0.16
1.80– 26.68
3687
2640
244
Flack parameter
GOF (F2)
R1; wR2 [I > 2s(I)]
R1; wR2 (all data)
ꢀ0.06(9)
—
1.034
—
1.032
0.0363; 0.0907
0.0480; 0.0986
0.28(4)
1.022
0.0588; 0.1144
0.1041; 0.1235
—
1.056
0.0727; 0.1002
0.2103; 0.2369
1.037
0.0356; 0.0802
0.0455; 0.0854
0.0372; 0.0939
0.0498; 0.1026
P
P
P
P
a
R1 ¼ kFo| ꢀ |Fck/ |Fo|, wR2 ¼ { [w(Fo ꢀ Fc )2]/ w(Fo2)2}1/2
.
2
2
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Table 2 Selected hydrogen bond parameters (A, ꢁ) for salts 1–5
ꢀ
D–H/A
d(D–H)
d(H/A)
d(D/A)
:(DHA)
1
O(1)–H(1)/O(3) #1
O(2)–H(2)/O(4) #2
N(1)–H(1A)/O(3)
N(2)–H(2B)/O(4) #2
C(12)–H(12A)/p #3
C(14)–H(14A)/ p #4
0.84
0.84
0.88
0.88
0.95
0.95
1.80
1.89
1.84
1.81
3.17
3.61
2.622(19)
2.673(2)
2.659(19)
2.659(19)
4.042(8)
4.529(9)
167
155
154
161
153
161
Symmetry transformations used to generate equivalent atoms: #1 ꢀx + 2, ꢀy + 1, z #2 x, y, z ꢀ 1 #3 3/2 ꢀ x, 1/2 + y, ꢀz #4 x ꢀ 1/2, 1/2 + y, 1 ꢀ z
2
O(1W)–H(1W1)/O(1) #1
O(1W)–H(1W2)/O(2) #2
O(1W)–H(1W2)/O(3) #2
N(1)–H(1)/O(1W)
N(2)–H(2A)/O(1)
C2–H2/O2 #3
C5–H5/O3
C11–H11/O1W
C12–H12/O2 #1
C14–H14/O1 #4
C6–H6/p #5
0.83
0.80
0.80
0.88
0.88
0.95
0.95
0.95
0.95
0.95
0.95
0.95
2.01
2.24
2.35
1.79
1.91
2.53
2.50
2.50
2.66
2.60
3.01
3.87
2.847(13)
2.975(14)
3.094(13)
2.648(14)
2.704(13)
3.449(2)
3.364(2)
3.382(2)
3.286(2)
3.506(1)
3.901(1)
4.487(3)
176
150
152
163
148
162
153
155
124
163
159
129
C8–H8/p #6
Symmetry transformations used to generate equivalent atoms: #1 ꢀx + 1, ꢀy + 1, ꢀz + 1 #2 ꢀx, ꢀy + 1, ꢀz + 1 #3 1/2 ꢀ x, 1/2 + y, 3/2 ꢀ z #4 3/2 ꢀ x,
y ꢀ 1/2, 3/2 ꢀ z #5 x ꢀ 1/2, y ꢀ 1/2, 3/2 ꢀ z #6 x ꢀ 3/2, 3/2 ꢀ y, 1/2 + z
3
N(1)–H(1A)/Cl(1)
0.88
0.88
0.95
0.95
2.17
2.17
2.80
2.80
3.039(10)
3.033(12)
3.688(15)
3.632(15)
168
165
155
146
N(2)–H(2B)/Cl(1) #1
C(11)–H(11A)/Cl(1)
C(15)–H(15A)/Cl(1) #2
Symmetry transformations used to generate equivalent atoms: #1 ꢀx + 1, ꢀy + 1, ꢀz #2 x + 1/2, ꢀy + 1/2, z + 1/2
4
O(1C)–H(1C)/O(3B)
0.84
0.84
0.84
0.84
0.84
0.82
0.82
0.82
0.80
0.82
0.80
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.95
0.95
0.88
0.95
0.94
0.95
0.95
0.95
0.88
0.95
0.94
0.95
1.63
1.66
1.70
1.71
2.64
1.94
1.92
2.01
1.96
1.99
1.97
1.78
1.84
1.80
1.84
1.87
2.63
1.88
1.85
1.91
1.84
1.85
1.82
1.76
1.82
1.78
1.82
1.86
2.47
2.66
2.32
1.75
2.35
2.49
2.38
2.66
2.46
1.84
2.52
2.37
2.52
2.450(3)
2.499(3)
2.533(4)
2.545(4)
3.153(4)
2.753(4)
2.743(4)
2.828(3)
2.769(4)
2.803(3)
2.761(4)
2.638(3)
2.713(3)
2.635(3)
2.691(4)
2.716(3)
3.064(4)
2.722(3)
2.706(3)
2.777(4)
2.689(4)
2.696(3)
2.660(4)
2.631(4)
2.648(4)
2.644(4)
2.666(4)
2.662(4)
3.192(4)
3.601(4)
3.244(4)
2.632(4)
3.264(5)
3.436(4)
3.309(4)
3.569(5)
3.380(4)
2.713(4)
3.413(5)
3.313(4)
3.424(4)
164
176
170
174
121
169
172
171
177
168
169
163
170
157
163
162
111
160
163
170
160
160
158
170
156
167
161
151
139
170
163
170
158
171
164
172
162
170
155
170
159
O(1D)–H(1D)/O(2A) #1
O(1E)–H(1E)/O(4C) #2
O(1F)–H(1F)/O(3A)
O(1F)–H(1F)/O(4A)
O(1W)–H(1W1)/O(2D)
O(1W)–H(1W2)/O(4F)
O(2W)–H(2W1)/O(2B)
O(2W)–H(2W2)/O(4E)
O(3W)–H(3W1)/O(1B)
O(3W)–H(3W2)/O(4E)
N(1A)–H(1AA)/O(2F)
N(2A)–H(2AB)/O(4A)
N(1B)–H(1BA)/O(1B) #3
N(2B)–H(2BB)/O(2W) #3
N(1C)–H(1CA)/O(2C) #3
N(1C)–H(1CA)/O(3C) #3
N(2C)–H(2CB)/O(4B) #3
N(1D)–H(1DA)/O(4D) #2
N(2D)–H(2DA)/O(1A)
N(1E)–H(1EA)/O(3W)
N(2E)–H(2EB)/O(2B)
N(1F)–H(1FA)/O(1W)
N(2F)–H(2FB)/O(3F)
N(1H)–H(1HA)/O(3C)
N(2H)–H(2HB)/O(3E) #1
N(1G)–H(1GA)/O(3D) #2
N(2G)–H(2GB)/O(1A)
N(2G)–H(2GB)/O(4A)
C(2C)–H(2CA)/O(1D)
C(2B)–H(2B)/O(4F)
N(2F)–H(2FB)/O(3F)
C(5F)–H(5FA)/O(3F)
C(2D)–H(2DB)/O(1E)
C(2F)–H(2FA)/O(1C)
C(9F)–H(9FA)/O(1C)
C(5H)–(5AA)/O(4A)
N(2A)–H(2AB)/O(4A)
C(12H)–H(12H)/O(2B)
C(2E)–H(2EA)/O(3A)
C(5D)–H(5DA)/O(1A)
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Table 2 (Contd.)
D–H/A
d(D–H)
d(H/A)
d(D/A)
:(DHA)
C(2B)–H(2BA)/p
C(15E)–H(15F)/p
p/p
0.95
0.94
—
3.59
3.88
3.55
3.413(4)
3.580(4)
—
71
64
—
Symmetry transformations used to generate equivalent atoms: #1 x + 1, y, z #2 x ꢀ 1, y, z #3 x, y, z ꢀ 1
5
O(1S)–H(1S)/O(1B) #1
O(1S)–H(1S)/O(1A) #1
O(1W)–H(1W1)/O(1A) #2
O(1W)–H(1W1)/O(1B) #2
O(1W)–H(1W1)/O(3B) #2
O(1W)–H(1W2)/O(2A)
O(1W)–H(1W2)/O(3B)
N(1)–H(1A)/O(1S)
0.84
0.84
0.81
0.81
0.81
0.82
0.82
0.88
0.88
0.94
0.95
1.90
2.20
2.03
2.27
2.50
2.02
2.14
1.81
1.83
3.31
3.58
2.743(8)
3.045(7)
2.837(6)
3.078(8)
3.040(8)
2.776(5)
2.950(8)
2.670(4)
2.689(4)
3.719(3)
4.282(4)
177
178
177
171
125
153
170
165
164
107
132
N(2)–H(2B)/O(1W)
C(6)–H(6)/p
C(7)–H(7)/p
Symmetry transformations used to generate equivalent atoms: #1 x, y + 1, z #2 ꢀx + 1, ꢀy, ꢀz + 1
2.01; O(1W)–H(1W2)/O(2), 2.24; O(1W)–H(1W2)/O(3),
salt 3 forms different three dimensional packing as compared to
salts 1 and 2 because of different orientation of the pyrazoles and
the chloride ions. Unlike the salts 1 and 2 the molecular
components of salt 3 form pseudo cavity for trapping chloride
ion (Fig. 8). The C–H/p distance in salt 3 is found to be in the
ꢀ
2.35 A) with the oxygen of nitrate ion (Fig. S2).† Interestingly,
the formation of a small size cavity is not observed in salt 2
during the packing as the C–H/p distance between the phenyl
ꢀ
group of those pyrazoles lie in the range of 9.505–10.550 A
ꢀ
(which are too large to form such a cavity) (Fig. 6).
range of 12.53 A, which seems to be too large for the formation
of a true cavity.
PzPh2H2+$Clꢀ
(3)
8PzPh2H2+$4HSO4ꢀ$2SO42ꢀ$3H2O
(4)
The hydrochloride salt of PzPh2H (salt 3), crystallizes in the
monoclinic crystal system with P21/c space group and its ORTEP
view is shown in Fig. 7. The unit cell consists of one PzPh2H2+ and
one chloride ion. The proton gets transferred from the HCl to
PzPh2H and forms protonated pyrazole. Both the pyrazol-1yl NH
and the protonated NH are available for hydrogen bonding and
involved in the principle N–H/Cl interaction (N1–H1A/Cl1,
ꢀ
2.17; N2–H2B/Cl1, 2.17 A) (Fig. S3).† The chloride ion shows
the primary N–H/Cl as well as secondary C–H/Cl interactions
as reported earlier.41–45 The cationic host pyrazole assembly in
Fig. 2 Formation of cavity through C–H/p interaction between
cationic pyrazoles trapping two anionic dimers with various N–H/O
and C–H/O interaction in salt 1. Colour code: C ¼ grey; P ¼ cyan; O ¼
red; N ¼ blue.
Fig. 1 ORTEP drawing with 50% probability of salt 1.
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Fig. 3 Three dimensional packing of salt 1, entrapping anionic dihydrogen phosphate dimers in the pyrazole cage. Colour code: C ¼ grey; P ¼ cyan;
O ¼ red; N ¼ blue.
+
The salt 4 has been found to contain seventeen moieties viz.,
eight PzPh2H2+, four bisulphate, two sulphate and three water
molecules in a unit cell (Fig. 9). The possible source of bisulphate
is the deprotonation of sulphuric acid which may further
undergo deprotonation resulting in the formation of sulphate
anion. The tetrahedral geometry were observed around the
sulphur atom in both bisulphate as well as in sulphate ion with an
average bond angle of 108.9 and 109.85 (for bisulphate, O1C–
S1C–O2C, 108.8; O2C–S1C–O3C, 110.4; O3C–S1C–O4C, 112.3;
O4C–S1C–O1C, 104.1; for sulphate, O1A–S1A–O2A, 109.2;
O3A–S1A–O1A, 109.3; O2A–S1A–O4A, 109.6; O4A–S1A–
only six PzPh2H2 show different N–H/O and C–H/O inter-
actions (N(2A)–H(2AB)/O(4A), 1.84; N(1B)–H(1BA)/O(1B),
1.80; N(2C)–H(2CB)/O(4B),1.88; N(2D)–H(2DA)/O(1A),
1.91;
N(2E)–H(2EB)/O(2B),1.85;
(1A),1.86; C(5H)–(5AA)/O(4A), 2.46; N(2A)–H(2AB)/
O(4A), 1.84; C(12H)–H(12H)/O(2B), 2.52; C(2E)–H(2EA)/O
N(2G)–H(2GB)/O
O3A, 111.3ꢁ). Out of eight PzPh2H2+, the six PzPh2H2 form two
+
sets of three each (blue and red) and the rest two lies as such in
the lattice (green). During packing the three partial overlapped
+
PzPh2H2 (blue and red) form the continuous chain via C–H/p
and p/p interaction (C(2B)–H(2BA)/p, 3.59; C(15E)–H
ꢀ
(15F)/p, 3.88; p/p, 3.55 A). These blue and red chains are
connected to the green through other interactions. Out of eight,
Fig. 5 Cationic pyrazole cage in salt 2 trapping nitrate anions through
N–H/O and C–H/O interactions. Colour code: C ¼ grey; O ¼ red;
N ¼ blue; H2O ¼ green.
Fig. 4 ORTEP drawing with 50% probability of salt 2.
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Fig. 6 Three dimensional packing of salt 2. Guest nitrate anions are entrapped in the cage formed by the host cationic pyrazole assembly. Colour code:
C ¼ grey; O ¼ red; N ¼ blue; H2O ¼ green.
ꢀ
(3A), 2.37; C(5D)–H(5DA)/O(1A), 2.52 A) with the sulphate
anions (Fig. S5).† The bisulphate anion also shows the N–H/O
and C–H/O interactions with different atoms of nearby
PzPh2H2+ (Table 2, Fig. S6).† The water molecules are involved in
O–H/O interactions through its hydrogen atom with the
oxygen of sulphate as well as bisulphate anions in the range 1.92–
ꢀ
1.97 A. The cluster of sulphate, bisulphate and water molecules
are interlinked together through O–H/O interaction and the
two groups of these cluster within a chain are linked through
+
PzPh2H2 (green) (Fig. S7).† The different primary (N–H/O,
O–H/O) and secondary (C–H/O) non-covalent interactions
link the sulphate, bisulphate and water molecule on either side of
the chain formed by the protonated PzPh2H2+ (Fig. 10). The larger
+
distance between the PzPh2H2 of two parallel chains (5.721 A) is
ꢀ
found to be responsible for the non formation of cavity in salt 4.
The structure of this salt is unique in the sense that both sulphate
as well as the bisulphate anions are simultaneously linked with
+
different PzPh2H2 in one unit cell.
Fig. 8 Space-fill view of chloride anions encapsulated by protonated pyr-
azoles forming pseudo cavity. Colour code: C ¼ grey; N ¼ blue; Cl ¼ cyan.
Fig. 7 ORTEP drawing with 50% probability of salt 3.
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PzPh2H2 are arranged in one dimensional chain where the two
adjacent PzPh2H2+ are linked through C–H/p interaction between
phenylic CH and p electrons of phenyl group on neighbouring
pyrazoles in head to tail fashion (C6–H6/p, 3.31; C7–H7/p,
+
PzPh2H2+$ClO4ꢀ$H2O$CH3OH
(5)
The salt 5 crystallizes in the orthorhombic with Pbca space group.
+
Its unit cell contains one protonated PzPh2H2 , water, methanol and
ꢀ
3.58 A). The parallel chains are linked by each other with perchlo-
perchlorate anion (Fig. 11). The tetrahedral geometry was observed
around chlorine atom in perchlorate ion with an average bond angle
of 109.47ꢁ (O1A–Cl1–O2A, 109.3; O2A–Cl1–O3A, 109.8; O3A–
Cl1–O4A, 109.6; O4A–Cl1–O1A, 109.2). During packing the
rate, methanol and water molecules present in the lattice through C–
H/O and N–H/O interactions (Table 2). Both the pyrazol-1-yl
NH as well as the protonated NH are involved in the principle N–
H/O interaction with the oxygen of water and methanol (N(1)–H
Fig. 9 ORTEP drawing with 50% probability of salt 4.
Fig. 10 Alternate channels of host framework formed by the self-assembly of the cationic protonated pyrazoles with guest sulphate anions, pink;
bisulphate, yellow; water, cyan on both side in salt 4.
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Anion selectivity
Since PzPh2H forms salt with different anions, an attempt has
been made to test the selectivity of this ligand by performing
a particular experiment under the same reaction condition in
which salts 1–5 were prepared. Thus, an aqueous methanolic
solution containing one equivalent of the PzPh2H was added to
a solution containing one equivalent (1.0 mmol) of each H3PO4,
HNO3, HCl, H2SO4 and HClO4 at room temperature (25 ꢁC,
pH ¼ 1.63). Suitable colourless crystals were obtained by slow
evaporation and subjected to characterization by single-crystal
X-ray method. The single-crystal X-ray data clearly indicated
ꢀ
(a ¼ 14.345(5); b ¼ 20.187(7); c ¼ 20.111(5) A; a ¼ 90.00; b ¼
ꢁ
3
ꢀ
106.01(6); g ¼ 90.00 ; V ¼ 5779.2(3) A ) that in the mixture of the
acids, the ligand was superiorly selective for sulphate and bisul-
phate anions simultaneously because of the extensive electro-
static as well as non-covalent interactions.1,2
DFT calculations
Fig. 11 ORTEP drawing with 50% probability of salt 5.
In Gauss view the position of the anion and protonated pyrazoles
were fixed in such a way so that the distance between the two will
be in the approximate range as were in the solid phase (obtained
from the crystallographic data) before submitting to the Gauss
03 suite of programs and then allowed them for optimization of
minimum potential energy. The internal degree of freedom were
allowed to relax during the calculation. The optimized structural
parameters of all individual acid, pyrazole and their salts were
calculated at B3LYP/6-31G(d,p) basis sets and tabulated in
ꢀ
(1A)/O(1S), 1.81; N(2)–H(2B)/O(1W), 1.83 A). The perchlorate,
methanol and water molecules are interlinked through O–H/O
interaction among each other (O(1S)–H(1S)/O(1B), 1.90; O(1S)–H
(1S)/O(1A), 2.20; O(1W)–H(1W1)/O(1A), 2.03; O(1W)–H
(1W1)/O(1B), 2.27; O(1W)–H(1W1)/O(3B), 2.50; O(1W)–H
ꢀ
(1W2)/O(2A), 2.02; O(1W)–H(1W2)/O(3B), 2.14 A) as shown in
Fig. S9.† Unlike the structure of 4, the salt 5 consists of continuous
discrete chain of perchlorate, methanol and water that runs parallel
on either side of the chain formed by the pyrazoles. The presence of
perchlorate, methanol and water molecules may be responsible for
the larger separation between two parallel chain of pyrazoles (7.95–
Table 3 Hydrogen bond interaction energy
ꢀ
8.52 A) and resulting in the absence of a cavity (Fig. 12).
Hydrogen bond interaction
energy (Kcal molꢀ1
)
It is important to point out that the orientation as well as short
range C–H/p distances in salts 1 and 2 play important role in
the formation of cavity for trapping dihydrogen phosphate and
nitrate anions, respectively. On the other hand in the case of salts
3–5, the long C–H/p distances and orientation of PzPh2H2 do
not permit the formation of cavity and these salts show entirely
different type of three dimensional structures.
S. No
Salt
ꢀ
1
2
3
4
5
PzPh2H2+$H2PO4
9.28
PzPh2H2+$NO3
13.69
18.23
19.39
24.52
ꢀ
PzPh2H2+$Clꢀ
+
PzPh2H2+$HSO4
ꢀ
PzPh2H2+$ClO4
ꢀ
Fig. 12 Ball and stick view of stacked cationic pyrazoles forming host assembly trapping perchlorate, methanol and water molecule in between the
ꢀ
channels in salt 5. Colour code: C ¼ grey; N ¼ blue; ClO4 ¼ green; CH3OH ¼ blue; H2O ¼ cyan.
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Fig. 13 Optimized geometry of salts (a) 1 (b) 2 (c) 3 (d) 4 (e) 5.
Table S2.† Each optimized geometry showed positive vibrational
Conclusion
frequencies showing that the optimized structure was the global
minimum on the potential energy surface. Single point energy
calculations were performed and zero point corrected total
energies for various species were recorded. In addition to the
characterization of these salts, the gas phase geometries,
harmonic vibrational frequencies and binding energies of a series
of pyrazole salts were computed. The hydrogen bond interaction
energies were determined using the following equation,
In the present work we have reported the anion directed supramo-
lecular frame work which involves a change in the number and type
of interaction and the orientation of the molecules in the three
dimensional space. In salts 1–2, the dihydrogen phosphate and
nitrate anions reside in the cavity, whereas in salt 3, PzPh2H2+ forms
only a pseudo cavity for chloride ion. In case of salts 4–5, the cavity
disappears and chain of pyrazoles and anions run in a parallel
fashion. To the best of our knowledge this is the first example of the
PzPh2H used as receptor for both sulphate as well as bisulphate
simultaneously. The presence of phenyl groups on 3 and 5 position of
pyrazole ring as two wings play important role in trapping the anions
of different size. Also, both phenyl groups arrange themselves in such
a way to form a cavity through C–H/p interaction. In conclusion,
we have demonstrated that the variation of anion plays a significant
role in controlling the structure of salts. The theoretical calculation
reveals that increase in the strength of the acid increases the hydrogen
bond interaction energy as the perchloric acid has maximum
hydrogen bond interaction energy. The results show that both in
solid and gaseous phase, the structure of salts remain the same.
DE ¼ ESalt ꢀ (E
+ EAcid)
Pyrazole
Where ESalt, EPyrazole and EAcid are the zero point corrected total
energies of salt, pyrazole and acid. We have removed the solvent
and water molecules to check the relative stability.
The DFT calculation shows that with the change of the acid, the
hydrogen bond interaction energy changes for each salt (Table 3).
Greater is the strength of the acid, larger would be the hydrogen
bond interaction energy which is in accordance with the value of
first protonation constant for different acids as given below.46
ꢀ
PzPh2H2+$ClO4ꢀ > PzPh2H2+$HSO4 > PzPh2H2+$Clꢀ
ꢀ
ꢀ
>PzPh2H2+$NO3 > PzPh2H2+$H2PO4
Experimental
General
The comparison of bond lengths and angles of the salts
(obtained from theoretical calculation and crystallographic data)
are given in Table S2.† The negligible difference in the energy
between the optimized structure and crystal structure of these
salts suggest that the orientation and interaction remain almost
the same both in gaseous and the solid phase (Fig. 13, Table S3).†
All manipulations were performed in air using commercial grade
solvents, pre-dried by the literature method.47 PzPh2H was
prepared by the procedure reported earlier.48 Hydrochloric acid
(HCl, 35%), nitric acid (HNO3, 65%) and sulphuric acid (H2SO4,
98%) were purchased from Rankem (Laboratory reagent grade)
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ꢀ
whereas the orthophosphoric acid (H3PO4, 85%) and perchloric
acid (HClO4, 70%) were from S. D. Fine-Chem Limited. Crys-
tallized 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 FTIR
spectrometer in KBr. 1H-NMR spectra were recorded on Bruker-
D-Avance 500 spectrometer with Fourier transform technique
using TMS (tetramethylsilane) as internal standard.
monochromated Mo-Ka radiation (l ¼ 0.71070 A) at 100 K. In
the reduction of data Lorentz and polarization corrections,
empirical absorption corrections were applied.49 Crystal struc-
tures were solved by Direct methods. Structure solution, refine-
ment and data output were carried out with the SHELXTL
program.50,51 Non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were placed in geometrically calculated posi-
tions by using a riding model. Images and hydrogen bonding
interactions were created in the crystal lattice with DIAMOND
and MERCURY software.52,53
Synthesis of [PzPh2H2+$H2PO4ꢀ]. To the 10 mL methanol–
water solution of PzPh2H (0.22 g, 1.0 mmol), 0.5 mL of H3PO4
(v/v%, 10 : 0.5 mL) was added and the resulting solution was
stirred for 6 h. The colourless crystals of salt 1 in 56% (0.18 g,
0.56 mmol) yield, suitable for X-ray data collection were
obtained by slow evaporation of solvent at room temperature.
1H-NMR ((CD3)2CO, 500 MHz, ppm): 7.18 (s, 1 H, pz), 7.33–
7.52 (m, 6 H, Ph), 7.87–7.99 (m, 4 H, Ph), 12.55 (s, br, 2 H, NH).
Anal. calcd for C15H15N2O4P (318.28): C 56.61, H 4.75, N
8.80%. Found: C 56.42, H 4.60, N 8.63%. IR (KBr/cmꢀ1): 3560w,
3420m, 2386b, 1634s, 1478s, 906m, 764m, 678m.
Computational study
Geometry optimization of different species involved during the
course of present investigation were done using density func-
tional method (B3LYP) with 6-31G(d,p) basis set as imple-
mented in the Gaussian 03 suite of program.54,55 The input for the
simulation was the z-matrix generated by Gauss view56 that was
also used for visualizing the optimized structures of molecules.
ChemCraft, version 1.5 software was used for comparing the
optimized structure with the crystallographic one.
Synthesis of [PzPh2H2+$NO3ꢀ$H2O]. Salt 2 was prepared by
same procedure as outlined above for 1 using HNO3. Yield: 70%
(0.21 g, 0.69 mmol). 1H-NMR ((CD3)2CO, 500 MHz, ppm): 7.18
(s, 1 H, pz), 7.35–7.54 (m, 6 H, Ph), 7.87–7.99 (m, 4 H, Ph), 12.57 (s,
br, 2 H, NH). Anal. calcd for C15H15N3O4 (301.30): C 59.79, H
5.02, N 13.95%. Found: C 60.10, H 5.12, N 13.57%. IR (KBr/
cmꢀ1): 3764w, 3691w, 2725b, 1625s, 1317m, 1104w, 1040m, 830m.
Acknowledgements
The authors gratefully acknowledge CSIR, New Delhi, India for
financial assistance in form of SRF to SK.
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