Influence of the anion nature and alkyl substituents in the behavior of ionic liquids derived from phenylpyridines

Article abstract of DOI:10.1016/j.molstruc.2017.10.062

Quaternary alkyl 2-phenylpyridinium and 2-(2,4-difluorophenyl)pyridinium amines with iodide, hexafluorophosphate and bis(trifluoromethylsulfonyl)imide anions have been fully characterized by ¹H NMR, FT-IR and MALDI mass spectroscopic methods an

Full text of DOI:10.1016/j.molstruc.2017.10.062

Accepted Manuscript
Influence of the anion nature and alkyl substituents in the behavior of ionic liquids
derived from phenylpyridines
Paulina Dreyse, Antonia Alarcón, Antonio Galdámez, Iván González, Diego Cortés-
Arriagada, Francisco Castillo, Andy Mella
PII:
S0022-2860(17)31408-4
10.1016/j.molstruc.2017.10.062
MOLSTR 24433
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 22 August 2017
Revised Date: 16 October 2017
Accepted Date: 17 October 2017
Please cite this article as: P. Dreyse, A. Alarcón, A. Galdámez, Ivá. González, D. Cortés-Arriagada,
F. Castillo, A. Mella, Influence of the anion nature and alkyl substituents in the behavior of
ionic liquids derived from phenylpyridines, Journal of Molecular Structure (2017), doi: 10.1016/
j.molstruc.2017.10.062.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
INFLUENCE OF THE ANION NATURE AND ALKYL
SUBSTITUENTS IN THE BEHAVIOR OF IONIC LIQUIDS DERIVED
FROM PHENYLPYRIDINES
Paulina Dreyse a,*, Antonia Alarcón a, Antonio Galdámez b, Iván González c,d, Diego
Cortés-Arriagada e, Francisco Castillo d, Andy Mella f
aDepartamento de Química, Universidad Técnica Federico Santa María, Avenida España 1680, Valparaíso,
Casilla 2390123, Chile. e-mail: paulina.dreyse@usm.cl
.
bFacultad de Ciencias, Departamento de Química, Universidad de Chile, Las Palmeras 3425, Ñuñoa,
Santiago, Chile.
cFacultad de Ciencias de la Salud, Universidad Central de Chile, Lord Cochrane 417, Santiago, Chile.
dFacultad de Química, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Macul,
Santiago, Chile.
ePrograma Institucional de Fomento a la Investigación, Desarrollo e Innovación. Universidad Tecnológica
Metropolitana. Ignacio Valdivieso 2409, P.O. Box 8940577, San Joaquín, Santiago, Chile.
fDepartamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Blanco Encalada
2008, Santiago, Casilla 487-3, Chile.
1

ACCEPTED MANUSCRIPT
Abstract
Quaternary alkyl 2-phenylpyridinium and 2-(2,4-difluorophenyl)pyridinium amines with
iodide, hexafluorophosphate and bis(trifluoromethylsulfonyl)imide anions have been fully
characterized by 1H-NMR, FT-IR and MALDI mass spectroscopic methods and studied by
quantum chemistry calculations. The compounds with bis(trifluoromethylsulfonyl)imide
anion can be classified as ionic liquids, because they melt at room temperature. The
quaternary amines with iodide and hexafluorophosphate anions are solid at 25°C. The X-
ray diffraction characterization of the 2-(2,4-difluorophenyl)-1-methylphenylpyridinium
hexafluorophosphate
and
1-ethyl-2-(2,4-difluorophenyl)phenylpyridinium
hexafluorophosphate show an extensive series of C-H···F, C-F···π and P-F···π
intermolecular interactions, which give rise to a supramolecular network. The relationship
between the solid-state structures and the melting points is discussed by the evaluation of
the thermal behavior based on experimental data from Differential Scanning Calorimetry
(DSC) studies, and also using the analysis of the ion pairs binding energies. These new
compounds based on phenylpyridine allow us to grow the diversity of ionic liquids and
their crystalline salts, increasing the knowledge about the chemical and physical properties
of these ionic species.
Keywords: Ionic Compounds; Ionic Liquids; Phenylpyridiniums; Ion Pairs Binding
Energies.
2

ACCEPTED MANUSCRIPT
1. Introduction
One of the most promising areas of “green chemistry” is the research of Ionic
Liquids (ILs) [1, 2]. The ILs correspond to liquid salts composed entirely of ions; an
organic cation and an organic or inorganic anion [3-5]. These salts have melting points
around or below 100°C, and negligibly vapor pressure, hence, they do not emit volatile
organic compounds (VOCs) [2]. In this sense, ILs can be considered as green solvents in
comparison with the typical organic solvents. These kinds of salts are characterized by the
presence of a flexible anion and an asymmetric cation, therefore, the attractive forces
between the ions are weaker than the conventional ionic salts, being the liquid state
thermodynamically favored [6-9].
The first report of IL appeared in 1914, and it was related to the synthesis of
ethylammonium nitrate [CH3CH2NH3][NO3] [10]. Afterwards, during the 70's and 80's,
efforts were made to obtain ILs derived of imidazolium with chloroaluminate as anion, but
these were not air stable [11, 12]. In the 90's, air and water stable ILs with 1-ethyl-3-
-
-
-
-
methylimidazolium cation combined with different anions (NO3 , NO2 , BF4 , MeCO2 and
-2
SO4 ) were synthetized [13]. The ionic liquid feature in these compounds is explained by
the presence of hydrogen bonding between the hydrogen atom at position two of the
imidazole ring with the high electronegative atom of the anion (C-H···X) [13]. In these ILs
the C-H···X bonding appears strong enough, but not maximized enough to generate a close
packing, consequently, the melting points are lower compare to conventional salts [13].
This report did not get much attention when it was published [14], however, from 2000 to
date, the number of publications related to ILs is extremely extensive, and the wide
diversity of ILs prepared allow us to know interesting physical and chemical properties of
3

ACCEPTED MANUSCRIPT
these salts, such as high thermal and chemical stability, high solvation capacity of several
organic and/or inorganic compounds, low volatility and a good ionic conductivity (10-1
S/m) [15-21]. According with these characteristics, ILs are used as green solvents in
chemical and electrochemical reactions [15, 22], in nanoparticles synthesis [23], as active
ingredients in pharmaceutical compounds [24, 25], and also in electrochemical devices,
including solar cells [26], electrochromic windows [27] and light-emitting systems [28].
Several ILs based on alkylimidazolium are known [14, 29]; depending on the
substituents in the imidazole core and the anion nature, imidazolium ILs at room
temperature and ionic liquid crystals (ILCs) have been obtained [30-34]. For example, there
are outstanding publications about 1-butyl-3-methylimidazolium hexafluorophosphate
[BMIM][PF6] ionic liquid [35, 36] where their spectroscopic properties [37] and
applications in light emitting devices [38-40] are described. Also, the optimized synthetic
procedure of different ILs based on 1-alkyl-3-methylimidazolium with halide and
bis(trifluoromethanesulfonyl) imide ([NTf2]-) has been reported [41]; and the use of IL with
bis-imidazolium cation in the stripping of mercury from ionic liquid/aqueous two-phase
systems [42]. In the case of ILCs, 1-dodecyl-3-methylimidazolium iodide has been used as
the hole transport layer in dye sensitized solar cells, promoting a high light to electricity
conversion efficiency [30].
The variety of reports related to ILs derived from polypyridinic systems such as
phenanthrolinium and bipyridinium is less extensive. In the case of phenanthrolinium-IL, a
N-methyl-1,10-phenanthrolinium based ionic liquid with [NTf2]- has been reported, in
which is highlighted their ability to solvate metal complexes for applications in catalysis
and in synthesis of nanoparticles [43]. The behavior of different ILs based on monoalkyl-
4

ACCEPTED MANUSCRIPT
and dialkylbipyridinium cations have been also studied, in combination with iodide,
bromide, and [NTf2]- anions [44]. This study shows that longer alkyl chains (hexyl and
decyl) in the case of monoalkylbipyridinium salts, and asymmetric alkyl chains in the case
of dialkylbipyridinium salts, promote ILs with lower final melting points [44]. The ILs
derived from phenylpyridinium are much less numerous. A sample is 1-dodecyl-4-
phenylpyridinium bis(pentafluoroethylsulfonyl)imide, an hydrophobic ionic liquid [45],
which was studied by second harmonic generation measurements, analyzing the difference
in the orientations of the cation at the IL/water and the IL/air interfaces [45].
In order to increase the library of ILs based on phenylpyridiums we report the
synthesis and characterization of a new series of ionic liquids derived for 2-phenylpyridine
(ppy) and 2-(2,4-difluorophenyl)pyridine (F2ppy) which are shown in Figure 1. These
phenylpyridines were quaternizated with methyl and ethyl iodides; the iodides were
exchanged by [PF6]- and [NTf2]- anions, obtaining a series of twelve salts. Some of these
salts were obtained as liquid at room temperature, qualifying as ionic liquids. For the other
salts, Differential Scanning Calorimetry (DSC) experiments were carried out and the
tendencies of the melting points were explained on the basis of X-ray diffraction results and
theoretical calculations of the ion pair binding energy.
2. Experimental
2.1. Synthesis of salts derived from phenylpyridines
A general scheme for the synthetic procedures to obtain the salts derived from
phenylpyridines and their acronyms are shown in Figure 1.
5

ACCEPTED MANUSCRIPT
Fig. 1 Scheme general of synthesis of the ionic compounds based on phenylpyridines.
Quaternization reactions were performed as described in literature [43] with some
modifications. One equivalent of phenylpyridine and 1.2 equivalents of iodoalkane were
dissolved in 10 mL of acetonitrile using a round bottom flask covered with aluminum foil.
The mixtures were refluxed with magnetic stirring for four days for the reactions with
iodomethane, and six days for iodoethane, and monitored by thin layer chromatography.
The solvent of the mixtures was evaporated and then the product was dissolved in
chloroform, and was precipitated by slow addition of hexane. The resulting mixtures were
allowed to stand for one night to promote the precipitation of the expected product. Finally,
the pure quaternized species were separated by decantation. The [1a][I] and [1b][I] salts
have been previously used by Xiao et al. as precursor in hydrogenation transfer catalysis
[46, 47].
For metathesis reactions, a method described in literature was used [43]. This
method involves the dissolution of the phenylpyridinium derivative in 2.5 mL of distilled
water, and a dropwise addition of an aqueous solution of the corresponding salt. In the
metathesis reaction with LiNTf2 salt, the mixtures were reacted for five hours with
6

ACCEPTED MANUSCRIPT
magnetic stirring at 60°C. The formation of two phases was observed, and the liquid salts
were separated using liquid-liquid extraction with chloroform. In the metathesis with KPF6
salt, the reaction was performed with magnetic stirring for twelve hours at 60°C, where a
white precipitate appeared. The solid salts obtained were isolated by filtration and dried
under vacuum. 1-methyl-2-phenylpyridinium hexafluorophosphate [1a][PF6] has been
previously used by Xu et al. in host-guest supramolecular chemistry [48].
The yields obtained for synthesized compounds, with the corresponding
descriptions of their spectroscopic characterizations (FT IR, 1H-NMR and MALDI-MS) are
1
reported below. For H-NMR characterization, each proton is labeled according to the
scheme showed in the supplementary material (Figure S1). The characterization by
MALDI-MS was performed in both positive and negative ion modes, assigned the subscript
1 and 0, according to the ionic fragment analyzed. The obtained 1H-NMR and MALDI-MS
spectra are available in the supplementary material.
1-methyl-2-phenylpyridinium iodide, [1a][I]. Yield: 0.86 g (2.90 mmol, 94%). FT IR
(KBr, cm-1):
MHz, Chloroform-
(m, H3), 7.89 (d, = 7.9 Hz, H4), 7.69 – 7.50 (m, H5-H9), 4.35 (s, -CH3 methyl).
1-methyl-2-phenylpyridinium hexafluorophosphate, [1a][PF6]. Yield: 0.10 g (0.32
ν
3008 (C-H, N-CH3),
ν
1627, 1568, 1516 (C=Carom),
ν
732 (I-). H NMR (400
1
d)
δ
9.46 (d, = 6.0 Hz, H1), 8.62 (td,
J
J = 7.7, 0.8 Hz, H2), 8.23 – 8.08
J
mmol, 63%). FT IR (KBr, cm-1):
ν
3102 (C-H, N-CH3),
ν
1631, 1583, 1517 (C=Carom),
= 6.2 Hz, H1), 8.62 (t, J
ν
833,
698, 557 (PF6 ). 1H NMR (400 MHz, Acetone-d6
)
δ
9.02 (d,
J
= 7.8
-
Hz, H2), 8.10 (t,
CH3 methyl).
J
= 6.9 Hz, H3), 8.04 (d,
J
= 7.9 Hz, H4), 7.67 – 7.54 (m, H5-H9), 4.25 (s, -
7

ACCEPTED MANUSCRIPT
1-methyl-2-phenylpyridinium bis(trifluoromethylsulfonyl)imide, [1a][NTf2]. Yield:
0.14 g (0.31 mmol, 97%). FT IR (KBr, cm-1):
(C=Carom),
8.73 (d,
7.9 Hz, H4), 7.63 – 7.52 (m, H6-H8), 7.47 (m, H5, H9), 4.12 (s, -CH3 methyl). MALDI-MS m/z
ν
3102 (C-H, N-CH3), ν1631, 1581, 1518
-
ν
1352 (S=O), 1196, 1136, 1056 (C-F, NTf2 ). 1H NMR (400 MHz, Chloroform-
d
)
δ
J = 6.2 Hz, H1), 8.42 (t, J = 7.8 Hz, H2), 7.90 (t, J = 6.9 Hz, H3), 7.80 (d, J =
:
calc. 170.09; found 170.08 [(C12H12N)1(C2F6NO4S2)0], calc. 279.92; found 280.33
[(C12H12N)0(C2F6NO4S2)1].
1-ethyl-2-phenylpyridinium iodide, [1b][I]. Yield: 0.20 g (0.64 mmol, 21%). FT IR (KBr,
cm-1):
MHz, Chloroform-
H3), 7.81 (d, = 7.9 Hz, H4), 7.48 (s, H5-H9), 4.61 (q,
Hz, -CH3 ethyl).
1-ethyl-2-phenylpyridinium hexafluorophosphate, [1b][PF6]. Yield: 0.05 g (0.15 mmol,
ν
3031 (C-H, N-CH2-CH3),
ν
1625, 1578, 1517 (C=Carom),
= 7.8 Hz, H2), 8.11 (t,
= 7.3 Hz, -CH2 ethyl), 1.37 (t,
ν
702 (I-).1H NMR (200
= 7.0 Hz,
= 7.3
d
)
δ
9.47 (d, = 5.8 Hz, H1), 8.61 (t,
J
J
J
J
J
J
52%). FT IR (KBr, cm-1):
ν
3097 (C-H, N-CH2-CH3),
ν
1629, 1573, 1509 (C=Carom),
ν838,
-
697, 557 (PF6 ). 1H NMR (200 MHz, Chloroform-
d
)
δ
8.92 (d, = 5.9 Hz, H1), 8.46 (t, J =
J
7.9 Hz, H2), 8.10 (t,
J
= 6.5 Hz, H3), 7.81 (d,
J
= 7.7 Hz, H4), 7.65 (m, H6-H8), 7.54 (bs, H5,
H9), 4.59 (q, = 7.3 Hz, -CH2 ethyl), 1.49 (t,
J
J
= 7.1 Hz, -CH3 ethyl). MALDI-MS m/z: calc.
184.11; found 184.13 [(C13H14N)1(PF6)0], calc. 144.96; found 144.95 [(C13H14N)0(PF6)1].
1-ethyl-2-phenylpyridinium bis(trifluoromethylsulfonyl)imide, [1b][NTf2]. Yield: 0.14
g (0.30 mmol, 93%). FT IR (KBr, cm-1):
(C=Carom), 1352 (S=O, NTf2), 1197, 1134, 1058 (C-F, NTf2 ). H NMR (400 MHz,
Chloroform-
7.83 (d, = 7.9 Hz, H4), 7.62 (m, H6-H8), 7.50 (d,
ν3098 (C-H, N-CH2-CH3), ν1626, 1578, 1514
-
1
ν
d)
δ
8.89 (d,
J
= 6.2 Hz, H1), 8.48 (t,
J
J
= 7.8 Hz, H2), 8.04 (t,
J = 7.0 Hz, H3),
J
= 7.5 Hz, H5, H9), 4.54 (q,
J
= 7.3 Hz, -
8

ACCEPTED MANUSCRIPT
CH2 ethyl), 1.45 (t,
J
= 7.3 Hz, -CH3 ethyl). MALDI-MS m/z: calc. 184.11; found 184.11
[(C13H14N)1(C2F6NO4S2)0], calc. 279.92; found 280.34 [(C13H14N)0(C2F6NO4S2)1].
2-(2,4-difluorophenyl)-1-methylpyridinium iodide, [2a][I]. Yield: 0.92 g (2.76 mmol,
92%). FT IR (KBr, cm-1):
733 (I-). 1H NMR (400 MHz, Chloroform-
ν
3014 (C-H, N-CH3),
ν
1627, 1576, 1523 (C=Carom),
9.68 (d, = 5.9 Hz, H1), 8.58 (t,
= 7.9 Hz, H4), 7.89 (d, = 7.8 Hz, H9), 7.15
J = 9.1 Hz, H6), 4.36 (s, -CH3 methyl). MALDI-MS m/z: calc.
ν1485 (C-
Farom),
ν
d
)
δ
J
J =
7.8 Hz, H2), 8.25 (t,
J
= 6.9 Hz, H3), 8.02 (q,
J
J
(t, = 8.2 Hz, H8), 7.03 (t,
J
206.08; found 206.07 [(C12H10F2N)1(I)0], calc. 126.90; found 126.88 [(C12H10F2N)0(I)1].
2-(2,4-difluorophenyl)-1-methylphenylpyridinium hexafluorophosphate, [2a][PF6].
Yield: 0.12 g (0.34 mmol, 16%). FT IR (KBr, cm-1):
ν
3110 (C-H, N-CH3),
ν
1632, 1597,
9.21
= 8.5, 6.5
J = 8.5 Hz, H6), 4.12 (s, -CH3 methyl).
-
1581 (C=Carom),
(d, = 6.1 Hz, H1), 8.70 (t,
Hz, H9), 7.68 (td, = 10.5, 2.3 Hz, H8), 7.45 (t,
ν
1499 (C-Farom),
ν
826, 557 (PF6 ). 1H NMR (400 MHz, DMSO-d6
)
δ
J
J = 7.8 Hz, H2), 8.27 – 8.19 (m, H3- H4), 7.79 (td, J
J
MALDI-MS m/z: calc. 206.08; found 206.08 [(C12H10F2N)1(PF6)0], calc. 144.96; found
144.96 [(C12H10F2N)0(PF6)1].
2-(2,4-difluorophenyl)-1-methylphenylpyridinium bis(trifluoromethylsulfonyl)imide,
[2a][NTf2]. Yield: 0.53g (1.09 mmol, 92%). FT IR (KBr, cm-1):
1352 (S=O, NTf2), 1195, 1139, 1057 (C-F, NTf2). 1H NMR
ν3102 (C-H, N-CH3),
ν
1631, 1596, 1581 (C=Carom), ν
(400 MHz, Chloroform-
d
)
δ
9.08 (d,
J
= 6.1 Hz, H1), 8.69 (t,
J
= 7.9 Hz, H2), 8.20 (t,
J =
7.0 Hz, H3), 8.08 (d, = 7.7 Hz, H4), 7.83 – 7.75 (m, H9), 7.34 (d,
J
J
= 8.8 Hz, H8), 7.27 –
7.20 (m, H6), 4.34 (s, -CH3 methyl). MALDI-MS m/z: calc. 206.08; found 206.04
[(C12H10F2N)1(C2F6NO4S2)0], calc. 279.92; found 280.31 [(C12H10F2N)0(C2F6NO4S2)1].
9

ACCEPTED MANUSCRIPT
1-ethyl-2-(2,4-difluorophenyl)phenylpyridinium iodide, [2b][I]. Yield: 0.27 g (0.78
mmol, 30%). FT IR (KBr, cm-1):
ν
3057 (C-H, N-CH2-CH3),
ν
1624, 1592, 1577 (C=Carom),
9.75 (s, H1), 8.55 (d,
= 7.2 Hz, H9), 7.17 (t, = 8.0
= 65.0 Hz, -CH2 ethyl), 1.55 (t, = 6.8 Hz, -CH3
1
ν
1485 (C-Farom),
ν
735 (I-). H NMR (400 MHz, Chloroform-
d
)
δ
J =
6.1 Hz, H2), 8.29 (s, H3), 7.97 (q,
J
= 7.6 Hz, H4), 7.85 (d,
J
J
Hz, H8), 7.06 (t, = 8.9 Hz, H6), 4.71 (d,
J
J
J
ethyl). MALDI-MS m/z: calc. 220.09; found 220.10 [(C13H12F2N)1(I)0], calc. 126.90; found
126.91 [(C13H12F2N)0(I)1].
1-ethyl-2-(2,4-difluorophenyl)phenylpyridinium hexafluorophosphate, [2b][PF6].
Yield: 0.05 g (0.14 mmol, 49%). FT IR (KBr, cm-1):
ν
3105 (C-H, N-CH2-CH3),
ν
1628,
836, 557 (PF6 ). 1H NMR (400 MHz, Chloroform-
= 7.8 Hz, H2), 8.12 (t, = 7.0 Hz, H3), 7.81 (d,
= 8.2 Hz, H9), 7.18 (t, = 8.4 Hz, H8), 7.06 (t, = 9.0 Hz, H6), 4.49
= 7.3 Hz, -CH2 ethyl), 1.51 (s, -CH3 ethyl). MALDI-MS m/z: calc. 220.09; found 220.10
[(C13H12F2N)1(PF6)0]. calc. 144.96; found 144.96 [(C13H12F2N)0(PF6)1].
-
1596, 1579 (C=Carom),
8.93 (d, = 6.2 Hz, H1), 8.48 (t,
8.0 Hz, H4), 7.66 (q,
(q,
ν1493 (C-Farom), ν
d
)
δ
J
J
J
J =
J
J
J
J
1-ethyl-2-(2,4-difluorophenyl)phenylpyridinium
bis(trifluoromethylsulfonyl)imide,
[2b][NTf2]. Yield: 0.13 g (0.26 mmol, 92%). FT IR (KBr/cm-1):
ν3094 (C-H, N-CH3),
ν
1628, 1596, (C=Carom),
MHz, Chloroform- 8.96 (d,
H3), 7.86 (d, = 7.9 Hz, H4), 7.59 (td,
(m, H6), 4.45 (h, J = 7.1, 6.6 Hz, -CH2 ethyl), 1.44 (t,
ν
1351 (S=O, NTf2), 1193, 1136, 1055 (C-F, NTf2). 1H NMR (400
= 6.2 Hz, H1), 8.51 (t, = 7.8 Hz, H2), 8.08 (t, = 6.9 Hz,
= 8.4, 6.1 Hz, H9), 7.21 – 7.11 (m, H8), 7.08 – 7.02
= 7.3 Hz, -CH3 ethyl). MALDI-MS m/z
d
)
δ
J
J
J
J
J
J
:
calc. 220.09; found 220.07 [(C13H12F2N)1(C2F6NO4S2)0], calc. 279.92; found 280.33
[(C13H12F2N)0(C2F6NO4S2)1].
10

ACCEPTED MANUSCRIPT
2.2. Equipment and methods
All reagents and solvents were analytic grade and were used without further
purification. The 1H NMR spectra of compounds were collected on a Bruker AVANCE 400
MHz spectrometer at room temperature. The infrared spectra were obtained using a Bruker
Vector-22 FT-IR spectrometer on KBr pellets.
The samples were prepared in a 3 mg/mL concentration of in chloroform and were
measured by MALDI mass spectroscopy. The spectra were obtained from mixtures of the
samples with a matrix of
with isopropyl alcohol) in a 1:1 ratio. An aliquot of 2
carrier plate sample micro scout (Bruker Daltonics Inc., MA, USA). In addition, 1
α
-cyano-4-hydroxycinnamic acid (CHCA, 10 mg/mL prepared
L of these mixtures were placed in a
L of
ꢀ
ꢀ
each sample was measured without matrix. The acquisition of spectra was performed using
MALDI-TOF Microflex equipment (Bruker Daltonics Inc., MA, USA) in positive and
negative ion mode by reflection detection, coupled with flexControl 3.0 program (Bruker
Daltonik GmbH, Germany). The equipment was previously calibrated to obtain the samples
spectra, with an external standard corresponding to a mixture of peptides of 200–3000 Da.
The final spectra correspond to a total of 15 scans of 30 laser impacts (450 laser impacts in
total) applied at different points of the samples taken randomly. The spectrum analysis was
performed using flexAnalysis 2.2 program (Bruker Daltonik GmbH, Germany).
The X-ray data were collected at 297(2) K with a Bruker CCD diffractometer using
MoKα radiation [49]. The structure was solved by direct methods and refined with the full-
matrix least-squares method on F2 with the use of SHELX-97 program package [50]. All H-
atoms were refined freely with isotropic displacement parameters. DIAMOND and
PLATON programs were used to prepare crystallographic data for publication [51, 52].
11

ACCEPTED MANUSCRIPT
Crystallographic data (excluding structure factors) for all structures in this paper have been
deposited with the Cambridge Crystallographic Data Centre as supplementary publication
no CCDC 1520330 and 1520331. Copies of the data can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax: +44-(0)1223-
336033 or e-mail: deposit@ccdc.cam.ac.uk).
Differential Scanning Calorimetry (DSC) experiments were carried out using a
Mettler-Toledo 822e thermic analyzer, at a heating range of 25-500°C (10°C/min), under
nitrogen atmosphere. The DSC plots are depicted in the supplementary material.
2.3. Computational methodology
Quantum chemistry calculations were performed at the density functional theory
(DFT) level in Gaussian09 [53] program; crystallographic structures were used as input
geometries for geometry optimizations when they were available. The PBE [54] functional
was used in combination with the all-electron Def2-SVP basis sets [55]. The DFT-D3
method (with the Becke-Johnson damping function) was used to include corrections for
dispersion forces [56-58]; in this case, the dispersion correction (Edisp) is added to the SCF-
DFT energies (ESCF DFT), which are obtained with the exchange-correlation functional.
−
Hence, the total DFT-D3 energies are expressed as a sum of electronic and dispersion
contributions: EDFT
= ESCF
+ Edisp. Vibrational frequency calculations were
−DFT
−D3
performed for all the molecular systems; in all the systems, only positive frequencies were
associated with all the vibrational modes, indicating that all the molecular systems
correspond to energy minimums and they are stable. Total ion-pair binding energies
(IPBE's) were obtained as:
12

ACCEPTED MANUSCRIPT
IPBE = Ecation −anion − (Ecation + Eanion
)
(1)
where, Ecation, Eanion and Ecation-anion corresponds to the total energies of the cation, anion,
and anion-cation systems, respectively, which also include the DFT-D3 dispersion
corrections and the zero point energy (ZPE) correction. The more negative the IPBE values,
the more stable cation-anion systems are. Basis set superposition errors were corrected by
means of the geometrical counterpoise correction [59]. According to the DFT-D3 method
noted above, total IPBE's can be also decomposed into the sum of electronic and dispersion
contributions: IPBE = IPBESCF-DFT + IPBEdisp. IPBESCF-DFT is the electronic contribution
and would be the total IPBE without dispersion corrections (entirely due to electronic
contributions); IPBEdisp is the dispersion contribution to IPBE, which can be obtained as
IPBEdisp = Edisp(cation-anion)
− (Edisp(cation) − Edisp(anion)), where Edisp(i) are the DFT-D3 dispersion
corrections of the isolated fragments and the cation-anion system.
3. Results and discussion
3.1. Synthesis
The general experimental procedures to obtain the salts derived from
phenylpyridines involves a quaternization followed by a metathesis step (see Figure 1). As
starting materials, two commercial phenylpiridines were used: 2-phenylpyridine (ppy) and
2-(2,4-difluorophenyl)pyridine (F2ppy). For the quaternization reaction, iodomethane and
iodoethane were used, and for the metathesis procedures KPF6 anf LiNTf2 salts were used.
Details of the synthetic procedures are described in experimental section.
The quaternization reactions with CH3-I were more efficient than the reactions with
C2H5-I. With CH3-I the reaction times were shorter and the products yields were around
13

ACCEPTED MANUSCRIPT
93%. The quaternization reactions with C2H5-I required longer reaction times; obtaining
lower yields (about 26%). This behavior can be attributed to the rotation in the Csp2-Csp2
bond between the pyridine and phenyl (or difluorophenyl) rings, which could have affected
the efficient binding of a longer alkyl [60, 61]. In similar quaternization reactions with 2,2´-
bipyridine the monocation is obtained by addition of butyl-iodide in a process of low
efficiency (15% conversion) at 80°C [62].
Concerning the phenylpyridines used (ppy or F2ppy), it is possible to observe that
their nature is not a factor that influence in the quaternization reaction; thus, the nature of
alkyl iodide is the determining condition for the reaction efficiency. The quaternization
process to obtain iodide salts gave the expected products, which were verified by the
1
signals of the alkyl protons in the H NMR spectra and by the IR spectra, which shows a
signal associated with a quaternary amine.
The metathesis reactions with [PF6]- were of lower yield than the ones with [NTf2]-.
This behavior is explained by the larger hydrophobicity of the ionic liquid with [NTf2]- in
-
comparison with the compound obtained with PF6 [5]. Therefore, in the last case, some
percentage of the compound could be lost in the aqueous phase of the metathesis reaction.
These reactions gave the expected products, which is evidenced by the IR and negative-
mode MALDI-MS spectra that show the characteristic signals of the anions. In addition, the
1H-NMR spectra of studied compounds show chemical shifts to high field for protons
associated with the pyridine ring and alkyl substituents, when iodide is exchanged by [PF6]-
or [NTf2]-. The most significant shift is observed for
α-proton of pyridine, which is
attributed to the increment of the C-H···anion interaction strength, as been previously
determined in vinyl imidazolium salts [63]. In these systems, the chemical shift of the
14

ACCEPTED MANUSCRIPT
carbenic proton is very sensitive to the anion nature, in fact, the chemical shifts of this
proton are δ 10.56 ppm with [I]- and δ 8.86 ppm in the case with [PF6]- [63], this gap is
comparable with the shifts obtained for phenylpyridinium salts studied in this work, which
can be observed in Figure 2 for [2b]+ compounds. Similar behaviors were observed for
other compounds.
-
-
Fig. 2 Comparative 1H RMN spectra of [2b][X] with X = I- (A), PF6 (B) and NTf2 (C).
The iodide salts were obtained as yellow solids, while the [PF6]- salts were obtained
as white solids. In the metathesis reactions with [NTf2]-, the liquid salts at room
temperature were obtained in all the cases; hence, ionic liquids were produced with this
anion. This behavior is expected due to the high delocalization of the negative charge in the
15

ACCEPTED MANUSCRIPT
[NTf2]- compared to the [PF6]-, favoring an ionic liquid behavior at room temperature [64,
65]. The appearance of the salts obtained with [2a]+ cation can be observed in Figure 3.
Other synthesized salts display the same color and physical state.
Fig. 3 Appearance of A. [2a][I], B. [2a][PF6] and C. [2a][NTf2] compounds.
3.2. X-ray crystallography characterization
In the case of salts derived from F2ppy cation and [PF6]- anion single crystals were
isolated. The molecular structure of [2a][PF6] and [2b][PF6] with their atom numbering
scheme are shown in Figure 4. Crystal data and details of the structure determination are
shown in Table 2. Selected bond distances and bond angles are shown in Table 3.
The differences in the molecular conformation of [2a][PF6] and [2b][PF6] can be
defined in terms of the torsion angles of N1-C12-C13-C14 [66.0(3)°] and N1-C10-C11-C5
[106.7(4)°], respectively. This significant difference in the dihedral angle values that
contain the C-C bond between the pyridine and difluorophenyl rings, may be related to a
larger distortion in the case of [2b][PF6] due to the inclusion of a slightly longer alkyl chain
(ethyl compared to methyl). In [2b][PF6] there is a restricted rotation between the rings due
to the steric hindrance generated by a longer carbon chain, promoting an increment in the
dihedral angle. This fact is in agreement with the low yield of the quaternization reactions
with iodoethane as was described in the synthesis section.
16

ACCEPTED MANUSCRIPT
The C12-C13 and C10-C11 bond distances of the corresponding bridge are 1.480(3)
Å and 1.477(4) Å. The C-F bond distances are similar to the average values reported for a
C
sp2-F bond (Table 3) [66]. Other relevant structural parameters (bond distances and
angles) are in reasonable agreement with its reported analogues [66]. The P-F bond lengths
in [2a][PF6] ranged from 1.573(3) to 1.5956(19) Å [F6-P-F4 = 90.49(13)° and F5-P-F8 =
177.91(11)°]. The octahedral volume is 5.312 Å3 with angle variance of 0.76 deg2 [67].
Moreover, P-F bond lengths in [2b][PF6] ranged from 1.543(4) to 1.587(4) Å [F6-P-F4 =
80.0(2)° and F5-P-F3 = 177.99(13)°]. The octahedral volume of [2b][PF6] is 5.319 Å3 with
angle variance of 2.14 deg2 which is slightly distorted than [2a][PF6].
Fig. 4 A view of the asymmetric unit showing the organic molecules and [PF6]- anion: A. [2a][PF6] and B.
[2b][PF6]. Displacement ellipsoids are drawn at the 50% probability level and H-atoms have been omitted for
clarity.
17

ACCEPTED MANUSCRIPT
Table 2. Crystal data and details of the structure determination.
Compound
Formula
[2a][PF6]
C12 H10 F2 N, PF6
351.18
[2b][PF6]
C13 H12 F2 N, PF6
365.21
Formula weight
Crystal shape / color
Crystal size (mm)
Crystal system
Space group
a (Å)
Polyhedron/Colorless
0.34 x 0.25 x 0.20
Monoclinic
P21/c
Plate/Colorless
0.47 x 0.42 x 0.14
Monoclinic
P21/c
9.839(2)
11.152(2)
11.083(2)
12.303(3)
103.14(3)
1480.8(5)
4
b (Å)
14.242(3)
11.102(2)
114.67(3)
1413.6(5)
4
c (Å)
β (°)
V (Å3)
Z
Dcalc (g/cm-3)
1.650
1.638
0.71073
0.71073
Wavelength, Mo Kα(Å)
T (K)
297(2)
297(2)
F(000)
704
736
θ
-range (°)
2.28 <θ< 27.89
1.18 <θ< 25.00
hkl-range
-12: 12 ; -18: 18 ; -13: 14 -13: 13 ; -13: 13 ; -14: 14
µ (mm-1)
0.279
0.270
Reflections collected / Rint
11601/ 0.022
2301
10423/0.0247
2614
Observed data [I> 2.0 σ(I)]
Parameters
239
213
R, wR2[F2> 2σ(F2)]
0.0483, 0.1468
0.0755, 0.22183
1.08
Goodness-of-Fit on F2 (GooF = S) 1.03
largest diff peak and hole (e Å-3)
-0.23/ 0.31
-0.29/ 0.49
Table 3. Selected bond lengths (Å) and angles (°).
[2a][PF6]
C14-F1
C19-F2
1.350(3) C13-C15-C21 120.5(3)
1.357(3) N1-C17-C16
121.7(3)
118.8(3)
120.8(2)
C13-C14 1.386(3) F2-C19-C21
N1-C18
N1-C12
1.486(4) C12-N1-C17
1.358(4)
[2b][PF6]
C9-F1
1.310(4) C3-C4-C11
1.346(4) N1-C1-C2
120.5(3)
121.5(4)
118.9(4)
120.8(4)
C19-F2
C10-C11 1.476(4) F2-C7-C6
N1-C11
N1-C1
1.356(4) C10-N1-C1
1.337(4)
18

ACCEPTED MANUSCRIPT
In the crystal packing of [2a][PF6] and [2b][PF6], the molecules are linked by
C-H···F intermolecular interactions (Fig. 5, Table 4) [68]. The combination of C-H···F
(diflurophenyl ring), C-H···F (methyl-pyridinium ring) and C-H···F (ethyl-pyridinium
ring) leads to the formation of a supramolecular array. The three-dimensional network in
[2a][PF6] is reinforced by P-F···π [F7-Cg1-C16 = 84.9(3)°] and C-F···π [F2-Cg1-C16 =
96.1(2)°] interactions (Fig. 6, Table 4). These intermolecular interactions are above or
below the plane of methyl-pyridinium group. The P-F···π connections can be ascribed to C-
+
-
-
Hδ ···Fδ -P contacts between organic molecules and the fluorine atoms of the PF6 anions
+
-
(anion-π intermolecular interactions) [69]. The distance H δ ···F δ involved in the
supramolecular structure is shorter than the sum of the van der Walls radii. In addition,
C-F···π in [2a][PF6] of the F2 atom of one molecule with pyridinium ring of another is
observed (Fig. 6, Table 4). These interactions have been described in organic crystal
structures by Parasanna and Rybalova et al. [70, 71].
In contrast, the supramolecular assembly in [2b][PF6] exhibits only C-H···F
interactions with H···F bond distance of 2.41 and 2.33Å (Fig. 5b). Thus, the molecules are
linked by non-classical H-bond giving rise to chains runs along bc plane with a graph-set
descriptor C22 (7) motif.
19

ACCEPTED MANUSCRIPT
Table 4. Hydrogen-bond and intermolecular contact interaction geometry (Å, º).
D-X···A
d(D-X) d(X···A) d(D···A) <(DXA)
[2a][PF6]
C11-H11···F5i
C16-H16···F3ii
C17-H17···F3iii
C18-H18c···F8iv 0.92(3)
C21-H21···F3v
0.92(3)
0.89(2)
0.94(3)
1.02(3)
2.47(3)
2.54(3)
2.47(3)
2.55(3)
2.53(3)
3.320(3)
3.167(3)
3.210(4)
3.364(4)
3.401(4)
162(2)
124(2)
129(2)
149(3)
157(2)
134(2)
146(1)
C19-F2···Cg1vi 1.357(3) 3.314(2) 4.372(3)
P1-F7···Cg1
1.590(2) 3.283(2) 4.683(2)
[2b][PF6]
C1-H1···F4vii
C2-H2···F3viii
0.88(4)
1.00(5)
2.41(4)
2.33(5)
3.229(5)
3.325(4)
155(4)
173(4)
Symmetry codes: (i) 1-x, -1/2+y, 3/2-z; (ii) 1-x, 1/2+y, 3/2-z; (iii) 1+x, 1/2-y, 1/2+z; (iv) 1-x, -y, 1-z; (v) x, y,
z; (vi) x, -1/2+y, 3/2-z ; (vii) 1-x, -1/2+y, 1/2-z; (viii) -1+x, y, -1+z
Cg1 represent the centroid of the N1/C12/C22/C20/C16/C17 ring in [2a][PF6].
Fig. 5 Part of the crystal structure of A. [2a][PF6] and B. [2b][PF6] showing the formation of the C-H···F
intermolecular interactions. Dashed lines represent intermolecular interactions. Some H-atoms not involved in
intermolecular interactions have been omitted for clarity.
20

ACCEPTED MANUSCRIPT
Fig. 6 Part of the crystal structure of [2a][PF6] showing the formation of the C-F···π and P-F···π
intermolecular interactions. Dashed lines represent intermolecular interactions.
3.3. Differential Scanning Calorimetry characterization
The thermal behavior of iodide and [PF6]- salts were performed by DSC
experiments. The results are summarized in Table 1; DSC plots can be observed in the
supplementary material.
Table 1. Melting (Tm) and decomposition (Td) points.
Cation Anion Tm/ °C Td/ °C
[1a]+
[1b]+
[2a]+
[2b]+
[I]-
[PF6]-
[I]-
[PF6]-
[I]-
[PF6]-
[I]-
[PF6]-
150
170
70
240
430
235
425
210
425
225
410
130
195
130
210
145
21

ACCEPTED MANUSCRIPT
In all the solid compounds, the first signals between 70-210 °C are attributed to
melting points. The second signals, 225 °C for iodide salts and 420 °C for [PF6]- salts,
can be attributed to chemical decomposition. According to these data, it is possible to
observe higher melting points tendency for salts with F2ppy cation than salts with ppy,
independent of the anion nature. This behavior is attributed to the predominance of
electrostatic interactions, favored by the fluorine atoms in the F2ppy moieties as described
below in the DFT calculations section. Besides, more intermolecular interactions in F2ppy
based salts could be expected compared to salts with ppy cations. This explanation can be
supported by C-F···π interactions observed in the crystal packing of the [2a][PF6] salt.
[1a][I] and [1b][I] salts have lower melting point in comparison with the analogous
compounds with [PF6]-, which is in agreement with previous studies for the liquid crystal
phase of 1-vinyl-3-alkylimidazolium [63]. In these cases, a better packing of the [PF6]- salts
compared to the halide salts causes a higher melting point for the former [63]. This
behavior is also evidenced by theoretical calculations (Fig. 7) where in the [1a][I] there are
less electrostatic interactions in comparison with the [1a][PF6]; then, in the first case the
predominance of the dispersion interactions favor the stability of cation-anion interaction.
In opposition, the [2a][I] and [2b][I] salts have higher melting points than the counterparts
with [PF6]-. In this case, F2ppy cations have a better packing but the higher melting points
with iodide anions are attributed to a higher polarizable character of [I]- than [PF6]- [72].
Regarding the decomposition points, it is possible to observe that the [PF6]- salts
have the highest values ( 420 °C, [PF6]- salts vs 225 °C, [I]- salts). Therefore, the [PF6]
salts show a greater thermal stability.
22

ACCEPTED MANUSCRIPT
3.4. Theoretical Calculation
According to Bernard et al., the ratio between the total ion-pair binding energy
(IPBE) and its dispersion component (IPBEdisp) correlates well with melting point of
several ILs [73] whose interactions are dominated by cation-anion interactions. Because of
the change in the Gibbs free energy at the melting point (Tm) is 0, Tm can be expressed as
Tm=
∆Hf/∆Sf, where
∆Hf and ∆Sf are the enthalpies and entropies of fusion respectively. In
this framework,
∆
Hf is expected to be proportional to the total IPBE; while, the change in
entropy is a result of rotational and translational motions allowed by a decrease in the
dispersive interactions when the melting takes place. In this regard, a large gain in entropy
(this is a large decrease in the dispersion interactions) at melting results in a lower melting
point. Therefore, Bernard et al. proposed the Tm values correlates as: Tm ≈ IPBE/IPBEdisp
,
where IPBEdisp is the dispersion component of the total IPBE, and indicates that the
magnitude of melting point is determined by the interplay between electrostatic and
dispersive interactions. This ratio has been correlated with the Tm of a series of ILs paired
with [Br]-, [PF6]-, [NTf2]-, among others [61]. To get insight about the trends in the melting
point of the synthetized salts, dispersion-corrected DFT-D3 calculations were performed at
the PBE-Def2-SVP level of theory for the studied cation-anion pairs, and their IPBE,
IPBEdisp and IPBE/IPBEdisp values are displayed in Table 5. The molecular structures of
cation-anion pair of the studied salts are shown in the supplementary material.
23

ACCEPTED MANUSCRIPT
Table 5. Total ion-pair binding energy (IPBE) and its dispersion contribution (IPBEdisp) for all the studied
ionic liquids (cation-anion pairs), and the IPBE/IPBEdisp ratio. Energies are in kcal/mol. Calculations were
performed at the PBE-D3/Def2-SVP level of theory.
Cation Anion
IPBE IPBEdisp. IPBE/IPBEdisp
one cation-anion pairs
[1a]+
[1b]+
[2a]+
[2b]+
[I]-
[PF6]-
[NTf2]-
[I]-
[PF6]-
[NTf2]-
[I]-
[PF6]-
[NTf2]-
[I]-
[PF6]-
[NTf2]-
-93.97
-78.23
-80.71
-92.47
-76.89
-81.76
-95.80
-78.85
-87.10
-95.25
-79.27
-84.76
-3.24
-4.97
-10.38
-3.07
-5.38
-8.46
-3.34
-5.42
-9.46
-3.18
-5.60
-9.83
28.97
15.75
7.78
30.17
14.29
9.67
28.71
14.54
9.21
29.97
14.17
8.62
two cation-anion pairs
[1a]+
[2a]+
[I]-
[PF6]-
[I]-
-192.26
-159.47
-178.43
-152.18
-17.17
-15.22
-15.85
-17.42
11.20
10.48
11.26
8.74
[PF6]-
According to the different cations, the IPBE values appear in the order of [I]− >
[NTf2]− > [PF6]−; conversely, the magnitude of the dispersion component appears in the
order of [NTf2]− > [PF6]− > [I]−. It is clear that the cation-anion interaction is largely favored
by dispersive interactions when the [NTf2]− anion is used, which causes lower
IPBE/IPBEdisp ratios compared to [I]− and [PF6]−, promoting the lowest melting points for
the salts with [NTf2]−, in agreement with the experimental data, due to these compounds are
liquid at room temperature (consistent with a behavior of ionic liquid), while, the melting
point of compounds with [I]− should be higher than they with [PF6]− according to their
IPBE/IPBEdisp ratios. However, the correlation between the IPBE/IPBEdisp ratios and the
24

ACCEPTED MANUSCRIPT
experimental melting points are consistent for cation-anion pairs involving [2a]+ and [2b]+
cation, indicating that for these systems the electrostatic interaction is favored by the
fluorine atoms in the F2ppy cations. In order to have a congruence with the melting points
of the salts with [1a]+ and [1b]+ cations, and for a deeper understanding of this behavior,
the IPBE, IPBEdisp and IPBE/IPBEdisp values were computed for two cation-anion pairs
involving [1a]+, [2a]+, [I]− and [PF6]−, which were taken as representatives of the studied
series. The molecular structures of these two cation-anion pairs are shown in Figure 7.
Fig. 7 Optimized molecular structures of two cation-anion pairs. Structures were obtained at the PBE-
D3/Def2-SVP level of theory. Color code: White (H); grey (C); blue (N); orange (P), cyan (F), purple (I).
As noted in Table 5, the difference in the IPBE/IPBEdisp ratios between [1a][I] and
[1a][PF6] salts is of 13 when one cation-anion pairs are considered in the calculation;
while, the difference in the IPBE/IPBEdisp ratios decreases to 0.3 when two cation-anion
units are considered. Conversely, in the case of the [2a][I] and [2a][PF6] salts, the
25

ACCEPTED MANUSCRIPT
difference in the IPBE/IPBEdisp ratios is always at least 3. Thus, the electrostatic part is
still the main contribution to the interaction in these cases with F2ppy cations. Therefore,
the lower melting point of the [1a][I] salt than [1a][PF6] (and their analogues with [1b]+)
would be a result of the decreased electrostatic interactions, thus dispersive interactions
plays a key role in the stability of the cation-anion interaction. Due to the increment in the
contribution of dispersion forces, the change in entropy is larger and the melting point
decreases. Otherwise, in the case of the salts containing [PF6]-, this anion with more
fluorine moieties is capable to increase the electrostatic component of the total IPBE,
compensating the increase in the dispersive interactions. The latter is clearly noted from the
optimized molecular structures of the two cation-anion pairs (Fig. 7): in the [1a][I] system,
the [I]− anions does not allow the π-π interactions between the aromatic rings in the cations,
which is associated with a decreased electrostatic interaction as compared with the other
systems. In the case of the [1a][PF6], the electrostatic interaction is favored between the
fluorine atoms of the [PF6]− units and the cations, favoring also the π-π stacking between
the phenyl rings. On the other hand, both the [2a][I] and [2a][PF6] pairs show similar
structural reordering with a π-π stacking between the phenyl rings, which is also favored by
the fluorine atoms in the phenyl rings. Note that to quantify these behaviors, more cation-
anion pairs could be used in the calculations, which increase the computational cost. This
work is being explored by our group.
26

ACCEPTED MANUSCRIPT
4. Conclusions
A series of phenylpyridinium-based (ppy and F2ppy) salts with [I]-, [PF6]- and
[NTf2]- anions have been synthesized and characterized. Regarding to the synthetic
procedures, the length of iodide alkyl chain is the determining condition for the reaction
efficiency, being at advantage the quaternization reactions with the shorter alkyl chain
(CH3-I) in comparison with the larger one (C2H5-I). This result is in agreement with the
long time reaction and low yield reported in quaternization reaction of 2,2´-bipyridine with
butyl-iodide at 80°C [62]. In the case of compounds with [NTf2]-, ionic liquids are
obtained. Other salts were obtained as crystalline solids. According with this behavior, the
nature of the anions is the only factor that determines the physical state of the ionic
compound obtained. This is explained because as the anion size increases, the
delocalization negative charge in the anion increase and, therefore, to decrease the ability to
the interactions cation-anion [74, 75].
The crystal structure of [2a][PF6] and [2b][PF6] was analyzed according with the X-
ray crystallographic data. In the case of [2b]+ fragment, the dihedral angle that contain C-C
bond between pyridine and phenyl pyridine rings is higher than [2a]+, which in an
additional evidence of the distortion generated for and longer alkyl chain binding to
nitrogen atom.
For the salts with iodide and [PF6]- anions, differences in the melting points were
discussed on the basis of the DSC experiments, X-ray crystallography and theoretical
calculations. These results shows that in the case of F2ppy based salts, the electrostatic
cation-anion interactions are predominant, which show higher melting points compared
with ppy based salts where dispersive interactions are predominant in the stability of the
salts. This explanation is supported by the non-classical H-bonds found in the crystal
27

ACCEPTED MANUSCRIPT
organizations of [2a][PF6] and [2b][PF6]. In addition, this behavior is reinforced by
IPBE/IPBEdisp ratios calculated when two cation-anion pairs are considered as an
approximation to the behavior in the packing crystal.
On the other hand, the salts with [PF6]- show higher decomposition melting points
than iodide salts, which indicate that the [PF6]- salts are thermally more stable.
The studies of the physical and chemical properties shown in this work, allows the
community to know the potential applications that this series of compounds can offer. It is
interesting to continue studying these new cations and the possibility to increase the
number of the ionic liquids, using for example methyl or ethylsulfate flexible anions in
future metathesis reactions in order to extent the applications of ILs and/or ILCs.
Acknowledgements
This work was supported by FONDECYT, Chile [grant numbers 11130221 and 3160285];
Universidad Técnica Federico Santa María [grant number 216.13.1]; Fondo de Innovación
para la Competitividad del Ministerio de Economía, Fomento y Turismo, Chile [grant
numbers 120082 and RC 130006 project (CILIS)]. AM acknowledges Masters Scholarship
[grant number RC 130006 (CILIS)]. IG acknowledges Instituto de Investigación e
Innovación en Salud of Universidad Central de Chile. The authors acknowledge Dr.
Ricardo Tapia of Pontificia Universidad Católica de Chile and Dra. Sheila Lascano of
Universidad Técnica Federico Santa María for the help on the experimental measurements
and discussions.
28