The association of like-charged ions in tunable protic pyrazolium salts

Article abstract of DOI:10.1016/j.molstruc.2021.130684

The association of like-charged ions is an elusive concept due to the lack of supporting experimental evidence. In the present work, 3,5-dimethyl-1-(p-substitutedphenyl)pyrazolium hexafluorophosphate (p-Cl (2a), p-Br (2b), p-OCH₃ (2c)) tunable

Full text of DOI:10.1016/j.molstruc.2021.130684

Journal of Molecular Structure 1242 (2021) 130684
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
The association of like-charged ions in tunable protic pyrazolium salts
Melek Canbulat Özdemira,∗, Ebru Aktanb, Onur S¸ ahinc
a Department of Environmental Engineering, Middle East Technical University, Ankara 06800, Turkey
b Department of Chemistry, Faculty of Science, Gazi University, Ankara 06500, Turkey
c Department of Occupat Health & Safety, Faculty of Health Sciences, Sinop University, Sinop 57000, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
The association of like-charged ions is an elusive concept due to the lack of supporting experimental ev-
idence. In the present work, 3,5-dimethyl-1-(p-substitutedphenyl)pyrazolium hexafluorophosphate (p-Cl
(2a), p-Br (2b), p-OCH3 (2c)) tunable protic pyrazolium salts have been synthesized and characterized.
The association of like-charged ions in the synthesized salts was observed, as evidenced by X-ray struc-
ture analysis of single crystals. The effect of the p-substituent on the association of like-charged ions has
been investigated. DFT-D3 studies were performed to unravel the links between the experimentally ob-
tained X-ray results for the solid phase and theoretically calculated results for an isolated system in the
gas phase. Additionally, infrared analysis and molecular electrostatic potential analysis were conducted
using DFT-D3 to get further information.
Received 10 February 2021
Revised 7 May 2021
Accepted 8 May 2021
Available online 18 May 2021
Keywords:
Tunable protic salts
Pyrazolium
Like-charge attraction
Crystal structure
DFT-D3 calculations
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
their cation [16–22]. The strength of interactions present between
the ions of ILs, such as Coulomb forces, hydrogen bonding, and
The electrostatic attraction of oppositely charged ions that gives
rise to the formation of ion pairs is a well-known theory in chem-
istry. Contrarily, the formation of ion pairs between like-charged
ions appears as a rare and contradictory concept for which limited
experimental evidence exists [1,2]. The pairing of like-charged ions
was detected for aqueous mixed electrolyte solutions of tetram-
ethylammonium ions [3], for guanidinium ions in water [4], for
metastable colloidal crystallites [5], and certain ionic liquids (ILs)
[6–9]. In principle, the cation–cation interactions of organic species
might be probable by sufficient delocalization of charge in the
cation and the existence of weakly coordinating anions [6,10]. The
self-association of simple organic cations through hydrogen bond-
ing was first reported for 4-oxopiperidinium salts with weakly co-
ordinating anions, as evidenced by their X-ray structure analysis
[10].
dispersion forces, typically determines their structures and prop-
erties [23]. The effect of hydrogen bonding on the properties of
ionic liquids/salts, has been studied in detail [24–28]. Besides, re-
cent ab initio molecular dynamic calculations of 1,2,4-triazolium
and 1,3-dimethylimidazolium cations have confirmed the existence
of π-type interactions between ionic species [29–31]. The π-π
ring stacking interactions between the cations of ionic liquids/salts
have also been observed experimentally by examining their single-
crystals through X-ray diffraction analysis. Accordingly, these inter-
actions have been detected for imidazo-pyridine based ionic liquid
([ImPr][TFA], melting point 83–85 °C), for imidazolium picrate salt
(mmimPic, melting point 147.5 °C), and for anilinium-based salts
(2a, 3a, and 3c with melting points 116.9 °C, 131.5 °C, and 125.2 °C,
respectively) [32–34 ]. Further, head-to-tail dimers of cations have
been observed for the tailored palladate tunable aryl alkyl ionic
liquid (3d, melting point 108 °C) due to π-stacking of the neigh-
boring phenyl rings [35].
Ionic liquids are organic salts consisting entirely of ions and
find particular applications in various fields due to their unique
properties [11–15]. Protic ionic liquids (PILs) or protic molten salts
(with melting points up to 200 °C), the subclass of ILs, are readily
obtained by transferring a proton from particular Brønsted acid to
a suitable Brønsted base. Despite their facile synthesis, PILs show
complex structural behaviors due to the exchangeable proton on
The aim of the present study is to assess the interactions be-
tween ions of pyrazolium based tunable protic salts. In our pre-
vious work, pyrazolium based tunable protic ionic liquids/salts
were synthesized, and solid-state structures of two salts (2a
and 3d) were determined by X-ray diffraction analysis [36].
Thus, in continuation of our studies, new 3,5-dimethyl-1-(p-
substitutedphenyl)pyrazolium based tunable protic salts (2a–2c),
which all include weakly coordinating hexafluorophosphate anion,
∗
Corresponding author.
E-mail address: ozmelek@metu.edu.tr (M. Canbulat Özdemir).
https://doi.org/10.1016/j.molstruc.2021.130684
0022-2860/© 2021 Elsevier B.V. All rights reserved.

M. Canbulat Özdemir, E. Aktan and O. S¸ ahin
Journal of Molecular Structure 1242 (2021) 130684
Table 1
Crystal data and structure refinement parameters for 2a–2c.
have been synthesized. The structures of the salts were confirmed
by appropriate spectroscopic methods (1H, 13C, and 19 F NMR,
FTIR) and elemental analysis. The experimental evidence of the
association of like-charged ions in synthesized salts was demon-
strated through single-crystal X-ray diffraction analysis (SCXRD).
Dispersion-corrected density functional theory (DFT-D3) studies
were also conducted to compare experimentally obtained and the-
oretically calculated results [37].
2a
2b
2c
Empirical formula
Formula weight
Crystal system
Space group
C
22H23N4Cl2PF6
C22H23N4Br2PF6
648.23
C24H29N4O2PF6
550.48
559.31
Monoclinic
P21/n
Monoclinic
C2/c
Monoclinic
C2/c
˚
a (A)
8.9958 (6)
23.5782 (17)
11.7593 (9)
91.057 (2)
2493.8 (3)
4
8.9324 (7)
21.0010 (18)
13.4307 (11)
94.227 (3)
2512.6 (4)
4
9.4422 (11)
21.107 (3)
13.3039 (13)
96.654 (5)
2633.6 (5)
4
˚
b (A)
˚
c (A)
β (°)
2. Experimental section
3
˚
V (A )
Z
Dc (g cm−3
)
1.490
1.714
1.388
2.1. Materials and instrumentation
μ (mm−1
)
0.39
3.35
0.18
θ range (°)
Measured refls.
3.1–26.2
41,730
3.0–28.3
20,454
3.6–26.2
27,769
All the reagents and solvents were supplied commercially
and handled as received. Acetic acid (glacial), acetylacetone,
acetonitrile, 4-chlorophenylhydrazine hydrochloride (for synthe-
sis), 4-bromophenylhydrazine hydrochloride (for synthesis), 4-
methoxyphenylhydrazine hydrochloride (for synthesis), ethyl ac-
etate, ethanol (absolute), sodium chloride, n-hexane were obtained
from Merck. Sodium bicarbonate, sodium sulfate, and hexafluo-
rophosphoric acid (~55 wt.% in H2O) were purchased from Sigma
Aldrich.
Independent refls.
4579
2327
2730
Rint
0.073
0.066
0.054
S
1.06
1.13
1.15
R1/wR2
˚
ꢀρmax/ꢀρmin (eA
0.104/0.219
0.57/–0.47
0.070/0.128
0.52/–0.41
0.065/0.153
0.21/–0.28
−3
)
3,5-dimethyl-1-(p-methoxyphenyl)pyrazolium hexafluorophosphate
(2c): FTIR (ATR, cm−1) νmax= 3444, 3144, 2970, 1598, 1513, 1453,
1318, 1257, 1171, 1039, 848, 555. 1H-NMR (CDCl3, ppm): δ = 2.28
(s, 3H, -CH3), 2.35 (s, 3H, -CH3), 3.88 (s, 3H, p-(OCH3)-pH), 6.19 (s,
1H, -CH), 7.0–7.03 (d, 2H, pH, J = 8.9 Hz), 7.29–7.32 (d, 2H, pH,
J = 8.9 Hz). 13C-NMR (CDCl3, ppm): δ = 11.83, 12.06, 55.70, 107.44,
114.89, 127.15, 128.51, 143.54, 147.65, 160.53. 19 F-NMR (CDCl3,
ppm): −70.79, −73.32. Analysis: calcd for C24H29F6N4O2P: C 52.36
H 5.31 N 10.18. Found: C 52.30 H 5.28 N 10.20. Yield: 90 % (white
crystal), M.p: 133 °C.
The synthesis of 3,5-dimethyl-1-(p-substitutedphenyl)-1H-
pyrazole derivatives (1a-1c) was carried out under microwave
irradiation by the multimode oven (Microsynth-Milestone). NMR
spectra (1H, 13C, 19 F) of the salts (2a–2c) were recorded on a
Bruker Avance 300 Ultra-Shield spectrometer. FTIR spectra of all
compounds were acquired with a “Thermo Fischer Scientific Nico-
let iS10 FTIR” spectrometer. Their melting points were measured
ꢀꢀ
by the “Electrothermal 9200 melting point apparatus and were
not corrected. The elemental analysis of the salts was obtained by
ꢀꢀ
the “LECO, CHNS-932 elemental analyzer.
2.2. Synthesis and characterization of the pyrazolium based tunable
protic salts (2a-2c)
2.3. X-ray diffraction analysis
The data collection was performed on a D8-QUEST diffractome-
ter equipped with graphite-monochromatic Mo-Kα radiation by se-
lecting suitable crystals of 2a-2c at 296 K. The structures of the
salts were solved by direct methods using SHELXS-2013 [38] and
refined by full-matrix least-squares methods on F2 using SHELXL-
2013 [39]. All non-hydrogen atoms were refined with anisotropic
parameters. The H atom of the N atom was located in a difference
map and refined freely. The other H atoms were located from dif-
ferent maps and then treated as riding atoms with a C-H distance
The
appropriate 3,5-dimethyl-1-(p-substitutedphenyl)−1H-
pyrazole compound (1a-1c) (5 mmol), which was synthesized
under MW irradiation by following a reported procedure [36], was
dissolved in ethanol (5 mL). Hexafluorophosphoric acid (5 mmol,
55% in water) was added dropwise to the solution stirring at room
temperature, and continued to stir for 30 min. The solid formed
was collected by filtration, dissolved in acetonitrile (10 mL), and
refluxed with active charcoal. After filtration, the solvent was evap-
orated. Their single crystals were acquired by slow evaporation of
ethanol solutions of the salts.
˚
of 0.93-0.96 A. The following procedures were implemented in our
analysis: data collection: Bruker APEX2 [40]; the program used for
molecular graphics were as follows: MERCURY programs [41]; soft-
ware used to prepare material for publication: WinGX [42]. Details
of data collection and crystal structure determinations are given in
Table 1.
1-(p-chlorophenyl)-3,5-dimethylpyrazolium hexafluorophosphate
(2a): FTIR (ATR, cm−1) νmax= 3424, 3115, 2987, 2927, 1601, 1550,
1495, 1413, 1093, 846, 555. 1H-NMR (CDCl3, ppm): δ = 2.33 (s,
3H, -CH3), 2.43 (s, 3H, -CH3), 6.27(s, 1H, -CH), 7.40–7.43 (d, 2H,
pH, J = 8.8 Hz), 7.51–7.54 (d, 2H, pH, J = 8.8 Hz). 13C-NMR (CDCl3,
ppm) δ = 11.66, 11.88, 108.30, 127.05, 130.14, 133.06, 136.46,
144.31, 147.85. 19 F-NMR (CDCl3, ppm): −70.58, −73.11. Analysis:
calcd for C22H23Cl2F6N4P: C 47.24 H 4.14 N 10.02. Found: C 46.74
H 4.11 N 9.93. Yield: 80 % (white crystal), M.p: 117 °C.
2.4. Quantum chemical calculations
1-(p-bromophenyl)−3,5-dimethylpyrazolium hexafluorophosphate
(2b): FTIR (ATR, cm−1) νmax= 3423, 3144, 3107, 2971, 2925, 1597,
1549, 1491, 1408, 1071, 843, 555. 1H-NMR (DMSO–d6, ppm):
δ = 2.17 (s, 3H, -CH3), 2.30 (s, 3H, -CH3), 6.09 (s, 1H, -CH), 7.45–
7.48 (d, 2H, pH, J = 8.7 Hz), 7.66–7.69 (d, 2H, pH, J = 8.7 Hz).
13C-NMR (DMSO–d6, ppm) δ =12.56, 13.56, 108.11, 120.27, 126.41,
132.47, 138.96, 140.25, 148.64. 19 F-NMR (CDCl3, ppm): −70.41,
−72.97. Analysis: calcd for C22H23Br2F6N4P: C 40.76 H 3.58 N 8.64.
Found: C 40.25 H 3.68 N 8.54. Yield: 82 % (white crystal), M.p: 141
°C.
The molecular geometry optimizations and the maps of
molecular electrostatic potential (MEP) of the 3,5-dimethyl-1-(p-
substitutedphenyl)pyrazolium hexafluorophosphates (2a–2c) were
carried out using a 6–31G(d) basis set in which B3LYP-D3 func-
tional were implemented in the gas phase. Each optimized struc-
ture of the salts was confirmed to be local minima on the po-
tential energy surface through vibrational frequency analysis to
prevent imaginary frequencies. The geometry optimizations of the
salts were conducted based on their crystal structures using the
Gaussian 09 program package [43].
2

M. Canbulat Özdemir, E. Aktan and O. S¸ ahin
Journal of Molecular Structure 1242 (2021) 130684
Fig. 1. The synthetic path for the preparation of the pyrazolium based tunable protic salts (A) Acetic acid, MW, 120 C; (B) Ethanol, HPF (55% in water), R.T.
º
6
Table 2
3. Results and discussion
˚
Hydrogen bonds parameters for 2a–2c (A, °).
3.1. Synthesis and characterization
D-HꢀA
D-H
HꢀA
DꢀA
D-HꢀA
2a
The syntheses of pyrazolium based tunable protic salts (2a–2c)
were performed via a two-step reaction, as outlined in Fig. 1. In
the first step, proper arylhydrazine hydrochloride and acetylace-
tone were used as starting materials to synthesize 1a–1c com-
pounds under MW irradiation. In the second step, the salts were
obtained by a Brønsted acid-base reaction between the 1a–1c and
hexafluorophosphoric acid in a stoichiometric ratio at room tem-
perature.
The NMR (1H, 13C, 19 F), FTIR, elemental analysis, and single-
crystal X-ray diffraction analysis were performed to elucidate the
chemical structures of the salts. The NMR and FTIR spectra of the
salts are given in Supplementary Materials. The FTIR spectra of
the compounds show typical bands belonging to cations of 2a-2c
C6-H6ꢀF2
C19-H19ꢀF6i
N2-H2AꢀN4
C22-H22CꢀCg(1)
2b
C8-H8ꢀF3ii
C11-H11AꢀCg(1)i
2c
0.93
2.50
3.165 (7)
3.322 (8)
2.755 (7)
3.583 (7)
129
161
168
126
0.93
2.43
0.91 (7)
0.96
1.86 (7)
2.93
0.93
0.96
2.53
2.99
3.277 (7)
3.866 (7)
137
152
C9-H9ꢀF3ii
C11-H11CꢀO1iii
0.93
0.96
2.44
2.49
3.230 (3)
3.360 (4)
143
150
Symmetry code: (i) −x + 1/2, y + 1/2, −z + 1/2; Cg(1)=C1-C6 for 2a;
(i) 1-x,1-y, 1-z; (ii) x + 1, y, z; Cg(1)=C7/C9-N2-N1 for 2b; (ii) x − 1,
y, z; (iii) −x + 3/2, −y + 1/2, −z + 1 for 2c.
Table 3
(ν(C = N) and ν(C = C) stretching vibrations: 1601–1408 cm−1
)
˚
πꢀπ interactions distances for 2a-2c (A).
and hexafluorophosphate anion (ν(PF): 848–843 cm−1). 1H-NMR
spectra of the salts show the signals compatible with the aliphatic
and aromatic protons of the pyrazolium cations. Besides, the peaks
of carbon atoms, observed in 13C-NMR spectra, are compatible
with the expected structures of the synthesized compounds. Their
19 F-NMR spectra show typical doublets at ca. δ (−70,15 ppm)-
(−73.32 ppm) due to the octahedral geometry of hexafluorophos-
phate anion.
Cg(I)
Cg(J)
Cg-Cg
Perpendicular distance
2a
Cg(1)
2b
Cg(1)i
Cg(2)i
Cg(1)i
3.772 (3)
3.993 (3)
4.023 (2)
3.355
3.570
3.679
Cg(2)
2c
Cg(1)
Symmetry codes: (i) 1-x, 1-y, 1-z; Cg(1)=C1-C6 for 2a; (i)
1/2-x, 1/2-y, 1-z; Cg(2)=C1-C6 for 2b; (i) 3/2-x, 1/2-y, 1-z;
Cg(1)=C1-C6 for 2c.
3.2. Crystal structures of the pyrazolium based tunable protic salts
The association of like-charged ions in the pyrazolium based
tunable protic salts has been observed by single-crystal X-ray
diffraction analysis. The SCXRD experiment has established that
the 2a crystallizes in the monoclinic crystal system with space
group P21/n (Table 1). The asymmetric unit of 2a consists of one
cation, one neutral pair, and one hexafluorophosphate anion, as
shown in Fig. 2a. The cation-neutral pairs produce N-HꢀN hydro-
gen bond (Table 2), and they are also part of a hydrogen bond net-
work connecting all building units to an extensive one-dimensional
hydrogen-bonded lattice. The combination of the N-HꢀN hydrogen
bond between the cation and neutral pair and C-HꢀF hydrogen
bond between the cation and [PF6−] anion produces a chain along
[010] direction (Fig. 2b). As illustrated in Fig. 2c, the molecules
of 2a are connected by C-Hꢀπ and πꢀπ interactions (Table 3).
The asymmetric unit also includes [PF6−] anion, which charge-
counterbalances the cation molecule.
in the molecule at (x, y, z) acts as a hydrogen-bond donor to
the C7/C9-N2-N1 ring in the molecule at (x + 1, y, z), so form-
ing a centrosymmetric R22(8) ring motifs (Table 2). Besides, an in-
termolecular πꢀπ interaction occurs between the two symmetry-
related phenyl rings of neighboring molecules (Table 3). Thus, the
combination of C-Hꢀπ and πꢀπ interactions produce a chain
along [110] direction. In the crystal of 2c, unlike from 2b, C-HꢀO
hydrogen-bonds produce a centrosymmetric R22(18) ring motifs
because of the methoxy substituent at the para position of the
phenyl ring. The combination of πꢀπ interactions, C-HꢀO hydro-
gen bonds between the cations and C-HꢀF hydrogen bonds be-
tween the cations and anions produce a chain along [101] direction
(Fig. 4b, Tables 2 and 3).
3.3. Geometric optimizations of pyrazolium based tunable protic salts
The 2b and 2c compounds crystallize in the monoclinic crystal
system with space group C2/c (Table 1). In the crystals of 2b and
2c, the asymmetric units consist of one cation molecule and half
hexafluorophosphate anion (Fig. 3a and 3b). Additionally, in 2a, all
atoms are located in general positions, while the phosphor atom of
hexafluorophosphate anion is located on a 2-fold inversion center
in 2b and 2c.
In the optimized geometry of 2a, calculated at the B3LYP-D3/6–
31G(d) level in the gas phase, classical hydrogen bonding interac-
tions between flor atoms of [PF6−] anion and the hydrogen atoms
of pyrazolium cation were observed (Fig. 5a). Based on the ob-
tained results, the hydrogen bond lengths of F52ꢀH8, F53ꢀH26,
˚
˚
˚
and F54ꢀH8 were found as 2.36 A, 2.39 A, and 2.32 A, respec-
tively. Additionally, the N-HꢀN hydrogen bond produced by the
cation–neutral pairs, detected in the SCXRD experiment, was also
As shown in Fig. 4a, the cations of 2b compound are con-
nected by C-Hꢀπ and πꢀπ interactions. The H11A atom of C11
3

M. Canbulat Özdemir, E. Aktan and O. S¸ ahin
Journal of Molecular Structure 1242 (2021) 130684
Fig. 2. (a) The molecular structure of 2a showing the atom numbering scheme (b) Crystal structure of 2a, showing the formation of a chain along [010] generated by N-HꢀN
and C-HꢀF hydrogen bonds (c) Crystal structure of 2a, showing the formation of C-Hꢀπ and π ꢀπ interactions.
Fig. 3. (a) The molecular structure of 2b showing the atom numbering scheme. [(i) -x, y, -z + 3/2] (b) The molecular structure of 2c showing the atom numbering scheme.
[(i) -x + 2, y, -z + 1/2].
˚
˚
observed theoretically. The hydrogen bond distance of H26ꢀN51
lated as 2.60 A and 2.32 A, respectively. The corresponding ex-
˚
˚
˚
was calculated as 1.76 A (exp. 1.86 A), and the angle value of N25-
H26ꢀN51 was calculated as 175° (exp. 168°).
perimental value of H20…O63 length was observed as 2.49 A.
The theoretical angle value of C41-H42ꢀO27 was found as 155°
(exp. 150°). The X-ray crystal structure and DFT calculations at the
B3LYP/6–31+G(d) level of theory, including D3 dispersion correc-
tion, show that hydrogen bonding between the cations of the 2c
overcomes the repulsive Coulomb forces [6,8].
As seen in Fig. 5b, the optimized geometry of 2b is con-
sistent with its crystal structure depicted in Fig. 6. The bond
˚
lengths of C19-H20 and C44-H45 were found to be 0.93 A in
˚
the SCXRD experiment and 1.08 A in theoretical calculations. The
H20ꢀF1, H28ꢀF1, and H45ꢀF3 hydrogen bond lengths were cal-
˚
˚
˚
˚
culated 2.32 A (exp. 2.53 A), 2.27 A and 2.30 A, respectively. The
theoretical angle value of C19-H20ꢀF1 was found as 134° (exp.
137°). According to theoretical calculations, C8-C17-N30-C21 and
C33-C42-N55-C46 dihedral angles were 41°.
3.3.1. Vibrational spectra
The vibrational frequencies of the salts (2a, 2b and 2c) were
obtained theoretically and experimentally in the spectral region
of 4000 to 500 cm−1. The calculated frequencies must be multi-
plied by the scaling factors since the calculated harmonic frequen-
cies are higher than those observed experimentally. The differences
between calculated and experimental frequencies arise from basis
set defects, neglecting anharmonic effects, and electron correlation
[44–46]. Thus, the B3LYP-D3/6–31G(d) calculated vibrational fre-
quencies are calibrated with the scaling factor 0.9642. The experi-
The optimized geometry of 2c calculated at the B3LYP-D3/6–
31G(d) level in the gas phase is given in Fig. 5c. The hydrogen bond
˚
length of H67ꢀF31 was calculated as 1.71 A. The calculated bond
length of hydrogen-bonded N-H (N64-H67) was found as 1.034 A,
˚
˚
and this value was higher than free N-H (N28-H66) (1.026 A), as
expected. The distances of H20ꢀO63 and H65ꢀO27 were calcu-
4

M. Canbulat Özdemir, E. Aktan and O. S¸ ahin
Journal of Molecular Structure 1242 (2021) 130684
Fig. 4. (a) Crystal structure of 2b, showing the formation of a chain along [110] generated by C-Hꢀπ and πꢀπ interactions (b) Crystal structure of 2c, showing the formation
of a chain along [101] generated by πꢀπ interactions, C-HꢀO and C-HꢀF hydrogen bonds.
Fig. 5. (a) The optimized geometry of 2a, (b) the optimized geometry of 2b, (c) the optimized geometry of 2c, calculated at B3LYP-D3/6–31G(d) level in the gas phase.
3444 cm−1 (calc. 3157 cm−1) for 2c (N64-H67ꢀF31). The hydrogen-
mentally scaled vibrational frequencies and assignments of funda-
mental vibrational bands of the synthesized compounds are given
in Supp. Table 1.
bonded N-H was not observed for 2b.
The CH3 asymmetric stretching vibrations were observed ex-
perimentally as very weak and weak bands between 2987 cm−1
–
The free NH and hydrogen-bonded NH stretching bands of the
salts were observed at two different wavenumbers in their FTIR
spectra (Supp. Table 1). The free N-H stretching vibrations of the
2a, 2b and 2c were observed as weak intensity bands at 3749 cm−1
(calc.no observation), 3750 cm−1 (calc. 3353 cm−1), and 3711 cm−1
(calc. 3211 cm−1), respectively. On the other hand, the hydrogen-
bonded NH stretchings were observed as very weak intensity
bands at 2927 cm−1 (calc. 2797 cm−1) for 2a (N25-H26ꢀN51) and
2970 cm−1 and theoretically between 3029 cm−1–2991 cm−1. The
C = C, C = N, and N = N stretching vibrations appear in the region
1605 cm−1–1414 cm−1 (exp.1601 cm−1–1408 cm−1) and show usu-
ally mixing with the CH3 scissoring. The O-CH3 symmetric stretch-
ings of compound 2c assigned at 1257 cm−1 as a very strong and
at 1249 cm−1 as a weak intensity band (corresponding calculated
values are 1264 cm−1 and 1248 cm−1). The C-Cl stretching was ob-
5

M. Canbulat Özdemir, E. Aktan and O. S¸ ahin
Journal of Molecular Structure 1242 (2021) 130684
Fig. 6. Crystal structure of 2b, showing the formation of C-H···F hydrogen bonds.
Fig. 7. MEP plots of 2a, 2b, and 2c obtained at the B3LYP-D3/6–31G(d) level of the theory (For interpretation of the references to color in this figure, the reader is referred
to the web version of this article.).
served at 506 cm−1 (calc. 502 cm−1) for 2a as a medium band.
color) are located on NH hydrogen atoms of the 3,5-dimethyl-1-(p-
The band at 647 cm−1 (calc. 680 cm−1) can be assigned as a C-
substitutedphenyl)pyrazolium cations.
Br stretching band for 2b. The very strong bands observed at 846
cm−1 for 2a (calc. 857 cm−1), 843 cm−1 for 2b (calc. 891 cm−1
and 848 cm−1 for 2c (calc. 863 cm−1) can be assigned as F-P-F
)
Conclusion
In this work, we report the association of ions in 3,5-
symmetric stretchings.
dimethyl-1-(p-substitutedphenyl)pyrazolium hexafluorophosphate
tunable protic salts by X-ray analysis of single crystals. Based on
the obtained results, the hydrogen bonding and π-type interac-
tions between the ions of the synthesized salts have significant
effects on the arrangement of their crystal structures. It is also
observed that the combination of intermolecular interactions that
lead to the association of the cations varies depending on the p-
substituent at the phenyl ring of the pyrazolium cation. Accord-
ingly, for the 2a; C-Hꢀπ and πꢀπ interactions and the N-HꢀN
3.3.2. Molecular electrostatic potential (MEP) maps
The maps of the molecular electrostatic potential surface for the
ground state geometry of the molecules provide information about
their electrophilic and nucleophilic regions [47,48]. The MEP maps
of the pyrazolium based tunable protic salts were obtained at the
B3LYP-D3/6–31G(d) level of theory. As seen in Fig. 7, the negative
potential sites (red color) of the salts are located on [PF6−] anion
and distribute on fluorine atoms. The positive potential sites (blue
6

M. Canbulat Özdemir, E. Aktan and O. S¸ ahin
Journal of Molecular Structure 1242 (2021) 130684
hydrogen bond between the cation and neutral pair, for the 2b;
C-Hꢀπ and πꢀπ interactions between the two symmetry-related
phenyl rings of neighboring cations, and for the 2c; πꢀπ interac-
tions and C-HꢀO hydrogen bonds between the cations are respon-
sible for the association of the cations of the salts. The theoretical
calculations based on the crystal structures of the salts also show
that the obtained bond length and dihedral angles are consistent
with the experimental results.
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Crystallographic data for the structural analysis have been de-
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[15] X. Zhang, W. Xiong, Z. Tu, L. Peng, Y. Wu, X. Hu, Supported ionic liquid mem-
branes with dual-site interaction mechanism for efficient separation of CO2,
ACS Sustain. Chem. Eng. 7 (2019) 10792–10799, doi:10.1021/acssuschemeng.
9b01604.
This research did not receive any specific grant from funding
agencies in the public, commercial, or not-for-profit sectors.
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Declaration of Competing Interest
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relationships and expanding applications, Chem. Rev. 115 (2015) 11379–11448,
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All authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
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Melek Canbulat Özdemir: Investigation, Writing
- original
draft, Methodology, Data curation, Formal analysis, Visualization.
Ebru Aktan: Investigation, Writing - original draft, Data curation,
Visualization. Onur S¸ ahin: Investigation, Writing - original draft,
Data curation, Visualization.
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Acknowledgments
The authors acknowledge to Scientific and Technological Re-
search Application and Research Center, Sinop University, Turkey,
for the use of the Bruker D8 QUEST diffractometer. The theoreti-
cal calculations reported in this paper were performed at TUBITAK
ULAKBIM, High Performance and Grid Computing Center (TRUBA
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found, in the online version, at doi:10.1016/j.molstruc.2021.130684.
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