Synthesis, Characterization, Solution Behavior, and Density Functional Theory Analysis of Some Pyridinium-Based Ionic Liquids

Article abstract of DOI:10.1002/jsde.12034

Some picolinium ionic liquids (IL) [α/γ-PicC_n][Br/NO₃] (n = 3, 5, 7) were synthesized and characterized by ¹H NMR data. The surface tension and density of all the IL were determined. The aggregation behavior of these IL in

Full text of DOI:10.1002/jsde.12034

J Surfact Deterg (2018)
DOI 10.1002/jsde.12034
ORIGINAL ARTICLE
Synthesis, Characterization, Solution Behavior, and Density
Functional Theory Analysis of Some Pyridinium-Based Ionic
Liquids
1
1
1
1
Ashish Tiwari · Mira Sahoo · Pratima Soreng · Bijay K. Mishra
Received: 18 July 2017 / Revised: 5 December 2017 / Accepted: 15 January 2018
©
2018 AOCS
Abstract Some picolinium ionic liquids (IL) [α/γ-PicCn]
Br/NO ] (n = 3, 5, 7) were synthesized and characterized
Introduction
[
3
1
by H NMR data. The surface tension and density of all the
IL were determined. The aggregation behavior of these IL
in aqueous medium and in dichloromethane was assessed
from the variation of electrical conductivity in these media.
The critical aggregation concentrations of these IL in aque-
ous medium were found to decrease significantly by the
addition of cetyltrimethylammonium bromide. The struc-
tural features and the conformation of these IL were further
investigated by using density functional theory calcula-
tions. The bromide ion was found to be inclined asymmet-
rically to one side of the pyridinium nucleus, while the
nitrogen of the nitrate group lies close to the nitronium ion
of the pyridinium nucleus.
Room-temperature ionic liquids (RTIL) are generally
organic salts with melting points at or below 373 K
(Stark & Seddon, 2007). These are composed of a bulky
organic cation with an alkyl chain and a variable number of
carbon atoms, and an inorganic or organic anion. The elec-
trostatic anion–cation interactions are relatively weak
because of the inefficient packing of the large, irregular
organic cation with the inorganic or organic counteranion,
resulting in the existence of the salt in the liquid state at
room temperature (Rilo, Vila, García-Garabal, Varela, &
Cabeza, 2013). Compared to classical organic solvents, IL
exhibit some typical properties like low melting point, low
volatility and flammability, high chemical and thermal sta-
bility, high electrical conductivity, a broad electrochemical
window, and high polarity (Rilo et al., 2013; Welton,
Keywords Room-temperature ionic liquid Á Contact ion
pair Á Critical aggregation concentration Á DFT Á Alkyl
pyridinium bromide
1
999). Mjalli, Naser, Jibril, Alizadeh, and Gano (2014)
reported the synthesis and characterization of some novel
IL-based deep eutectic solvents from tetrabutylammonium
chloride and polyhydroxy compounds. Because of the vari-
J Surfact Deterg (2018).
+
ety of side chains that can be connected to the central N
ion, many quaternary ammonium IL could be prepared to
tune the physical properties by the selection and modifica-
tion of the anion and the cation. Numerous articles on IL
describing general chemical aspects of their applications
have already appeared in the literature (Benavides-Garcia &
Monroe, 2009; Danten, Cabaço, & Besnard, 2010; Fujimori
et al., 2007; Kiefer & Pye, 2010; Olivier-Bourbigou,
Magna, & Morvan, 2010; Ong & Ceder, 2010; Verma &
Banerjee, 2010; Xiao et al., 2010).
Electronic supplementary material The online version of this article
(doi:10.1002/jsde.12034) contains supplementary material, which is
available to authorized users.
*
Bijay K. Mishra
bijaym@hotmail.com
While synthesizing alkyl γ-picolinium bromides as pre-
cursors for the synthesis of some polymethine dyes, we
observed that the salts with alkyl chains having an odd
1
Centre of Studies in Surface Science and Technology, School
of Chemistry, Sambalpur University, Jyoti Vihar, Sambalpur
7
68 019, India
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in the micro range. The surface tension of neat IL was mea-
sured by a manual tensiometer (model ST 500-man; Nima
Technologies, Coventry, UK) using the Wilhelmy plate
R
R
N+ R'
N
R'
Br
Br-
−1
method with an uncertainty of Æ0.03 mN m at 302 K.
Where R= o-CH , p-CH . R'= C H
(n = 3, 5, 7)]
3
3
n
2n+1
Densities of the liquids were measured by a pycnometer.
Scheme 1 Reaction of picoline with alkyl bromide
Synthesis of the IL [α/γ-PicC ][Br]
n
number of carbon atoms (propyl and amyl groups) are liq-
uid at room temperature (Patel, Panda, Kuanar, & Mishra,
In a sealed tube, equimolar amounts of alkyl halide and α/-
γ-picoline were heated in an oil bath for 24–30 h at differ-
ent temperatures. After completion of the heating, a brown
colored liquid was obtained, which was washed with ether
several times till the ether extract did not contain the start-
ing materials (Scheme 1). The compounds synthesized
using this protocol are shown in Table 1.
2004). Subsequently, many alkyl picolinium salts have
been synthesized, mostly having an alkyl chain with an
even number of carbon atoms, and established as IL
(Cornellas et al., 2011; Singh & Kumar, 2007, 2008). Some
novel ether-based pyridinium IL capable of absorbing SO2
were synthesized by Wang et al. (2014). Blends of mono-
ethanolamine and some pyridinium IL were found to be
effective absorbents of carbon dioxide (Huang, Zhang,
Zhang, Dong, & Zhang, 2014). Some of these compounds
aggregate in an aqueous medium above a certain concentra-
tion, called the critical aggregation concentration (CAC)
Synthesis of IL [α/γ-PicC ][NO ]
n
3
To a solution of 0.5 mL α/γ-picolinium alkyl bromide in
mL Millipore water, a solution of 0.5 g AgNO in 10 mL
5
3
water was added slowly till the precipitation was complete.
The solution was then warmed in a water bath for 30 min.
The precipitate of silver bromide was filtered off, and the
water from the filtrate was removed under reduced pres-
sure. The syrupy liquid obtained was kept in a desiccator.
The compounds synthesized by the above procedure are
listed in Table 2.
(Bandres, Meler, Giner, Cea, & Lafuente, 2009; Galgano &
El Seoud, 2010; Sastry et al., 2012). Copolymerization of
vinyl pyridinium IL has been carried out by immobilizing
these IL on a magnetic core to obtain an effective catalyst
for biodiesel synthesis (Liang, 2014). Cai, Huang, Chen,
Hu, and Wu (2014) used an ion-exchange method to
replace the halide by a hydroxide and obtained IL with high
purity.
Measurement of CAC
Here we report the synthesis of some picolinium IL from
2
/4-methyl pyridine and alkyl bromides and their behavior
The CAC values were determined by the electrical conduc-
tance method. The conductivities of the solutions of the IL
in water, dichloromethane (DCM), and an aqueous solution
of cetyltrimethyl ammonium bromide (CTAB) were moni-
tored at different concentrations, and the specific conduc-
tance values were plotted against the concentration of IL
in both organic and aqueous media. Structural characteris-
tics of the IL were also investigated through density func-
tional theory (DFT) analysis.
Experimental
(
[
Fig. 1). A break point in the plot of the conductance versus
IL] indicated the onset of formation of the aggregates, and
Materials and Methods
this point of intersection was taken as as the CAC of the IL
Table 3).
(
The α/γ-picoline and alkyl bromides were obtained from
Spectrochem and Sisco Chemicals (Mumbai, India). Milli-
pore water was used for the preparation of IL solutions.
The melting points of compounds were measured by the
open capillary method and the boiling points using Siwo-
loboff’s method (Sahay, Mishra, Behera, & Shah, 1988).
The structure of the products was established using NMR
Table 1 Structure, boiling point, density, and surface tension of [α/-
n
γ-PicC ][Br] IL
IL
Reaction
Boiling
point ( C)
Density
(g cm )
Surface
tension
(mN m )
ꢀ
−3
temperature
ꢀ
–1
( C)
[
[
[
[
[
[
γ-PicC
α-PicC
γ-PicC
α-PicC
γ-PicC
α-PicC
3
][Br]
145
130
145
130
180
180
200–202
190–192
220–222
210–212
232–235
210–213
1.432
1.42
1.45
1.5
47.8
56.2
44.3
42.6
41.8
37.9
1
spectral data. The H-NMR spectra were recorded on a
3
][Br]
][Br]
4
00-MHz Bruker NMR spectrometer in D O and CDCl as
2 3
5
solvents for the different compounds. Different concentra-
tions of IL were prepared and their conductivities in the
aqueous medium were recorded by a direct reading conduc-
tivity meter (type 303; SYSTRONICS, Ahmedabad, India)
5
][Br]
][Br]
7
1.52
1.47
7
][Br]
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n 3
Table 2 Structure and physical characteristic of [α/γ-PicC ][NO ] IL
ꢀ
−3
–1
IL
Structure
Boiling point ( C)
Density (g cm
1.36
)
Surface tension (mN m )
[γ-PicC
3
][NO
3
]
148–150
52.8
50.7
N+
-
NO
3
[α-PicC
3
][NO
3
]
145–148
1.42
N+
NO -
3
[
γ-PicC
5
][NO
3
]
155–159
151–154
1.38
1.41
51.6
49.1
+
N
-
NO3
[α-PicC
5
][NO
3
]
N+
NO -
3
[
γ-PicC
7
][NO
3
]
174–177
169–172
1.40
1.41
48.5
47.3
N+
-
NO
3
[α-PicC
7
][NO
3
]
N+
-
NO
3
Results and Discussion
for the pyridinium protons were found at ~9.325δ and
.89δ (Fig. S1) with J = 6.4. The singlet of the pyridinium
7
Like imidazolium salts, pyridinium salts have relatively
low melting points to be considered as RTIL. Substitution
of the methyl group in the pyridinium moiety contributes to
the lowering of the melting point by increasing the nonpo-
lar characteristic of the molecule, thereby decreasing the
intermolecular dipole–dipole interactions of the compound.
N-Alkyl picolinium bromides were prepared by the Mench-
utkin reaction of picoline with an excess of neat alkyl bro-
mide to ensure complete quaternization of the entire
picoline (Scheme 1). The unreacted alkyl bromides were
removed by washing with ethyl ether several times till a
clear NMR spectrum of the molecule was obtained. Among
a series of alkyl bromides, propyl, pentyl, and heptyl bro-
mides alone could produce RTIL from the quaternization
reaction.
methyl group appeared at 2.564δ. With variation of chain
length in the N-alkyl group, these peaks suffered slight
upfield shift for the 2-proton and downfield shift for the 3-
proton of the pyridinium ring. The singlet of the methyl
group also showed an upfield shift. However, when the
counterion changed from bromide to nitrate, the changes in
the chemical shifts were significant around the probable
localized site of the counterion. In [γ-PicC ][NO ], the
peak for 2-proton suffered a upfield shift of 0.86δ, while
that of the 3-proton was to an extent of 0.28δ (Fig. S7).
The peaks of pyridinium methyl protons were not affected
much, with a shift of 0.07δ only.
For 2-methyl substituted pyridinium moieties, four peaks
appeared in the aromatic region, each one distinctive of
each proton as two doublets and two triplets. The doublet
at the extreme downfield region may be assigned to the 6-
proton of the pyridinium nucleus and appearing around
3
3
In the typical NMR spectral data of [γ-PicC ][Br], i.e.,
n
[
γ-PicC ][Br], the characteristic doublets of two proton area
3
(
a)
(b)
1
1
1
4
2
0
8
6
4
2
0
20
1
1
5
0
5
0
0
0.005
0.01
0.015
0
0.005
0.01
0.015
Conc in mM
Conc in mM
Fig. 1 Plots of conductance versus concentration of (a) [α-PicC
3
5
][Br] and (b) [α-PicC ][Br]
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Table 3 CAC of IL (in mM) in the absence and presence of CTAB
IL
CAC in water
CAC in DCM
CAC of mixed surfactant (CTAB:IL)
1
:1
2:1
4:1
6:1
8:1
10:1
12:1
[
[
[
[
[
[
γ-PicC
α-PicC
γ-PicC
α-PicC
3
][Br]
45.45
45.45
45.25
38.46
50.00
50.00
21.37
25.25
25.26
18.50
32.46
11.47
5.31
5.74
5.32
5.10
5.00
5.26
4.46
4.58
4.31
4.33
4.38
4.42
3.70
3.87
3.90
4.06
3.90
3.93
3.70
3.90
3.79
3.70
3.82
3.90
3.61
3.66
3.73
3.87
3.78
3.78
3.57
3.67
3.62
3.76
3.70
3.70
3.62
3.63
3.60
3.64
3.75
3.76
3
][Br]
][Br]
5
5
][Br]
][Br]
γ-PicC
α-PicC
7
7
][Br]
2
9.27δ for bromide salts, while that around 8.55δ was for
IL R was 0.93. The surface tension of [α-PicC ][Br] was
3
corresponding nitrate salts. The peak for C-5 proton
appeared as a triplet around 8.4δ for bromide and 8.2δ for
the corresponding nitrate. The protons at C-3 and C-4 posi-
tions have chemical shift values of 7.67–8.00δ for both bro-
mide and nitrate salts.
found to be more than that of the corresponding [γ-PicC3]
[Br], while the trend was opposite for [PicC ][Br] and
5
[PicC ][Br] IL.
7
CAC of IL
The aliphatic protons of the N-alkyl chain also exhibited
a typical triplet for the terminal methyl group at 0.98δ for
the propyl chain and 0.81δ for the heptyl chain in 4-methyl
The amphiphilic characteristics of the IL are due to the
hydrophilic pyridinium group and the hydrophobic alkyl
group. Characteristically, amphiphilic compounds assemble
in an aqueous medium at a particular concentration known
as the critical micelle concentration (CMC) when the
assemblies lead to the formation of micelles. The possibil-
ity of aggregation of these IL was explored through con-
ductivity measurements. In the aqueous solution, the
conductivity of the IL increased with increasing concentra-
tion of the IL, and the corresponding plot was found to be
bilinear. The transition point of the bilinearity may be con-
sidered as the CAC of the IL. In most of cases, CAC
appeared at around 0.06 M of the IL (Fig. 1).The CAC
values of 4-methyl IL increased with increase in the alkyl
chain length, while that of 2-methyl IL decreased
significantly.
The addition of CTAB (CTAB:CMC = 0.9 mM) to the
aqueous solution of IL was found to decrease the CAC sig-
nificantly. At equimolar concentration of both CTAB and
IL, the CAC values decreased from ~50 to 5 mM. The non-
ideal change in the CAC may be ascribed to a mixed aggre-
gation process in which CTAB played the dominant role.
Rodrigues, Ito, and Sabadini (2011) observed a similar
nonideal change in the CMC of CTAB with other small
cationic surfactants and attributed the result to a synergistic
effect.
pyridinium salts. The peak for the N-CH proton appeared
2
as a triplet at 4.82δ for the N-propyl group and at 4.86δ for
N-heptyl group with bromide as the counterion. With
change in the position of methyl group in the pyridinium
nucleus, no significant change in the peak position was
observed. The other methylene units showed peaks at ~1.3δ
and 1.9δ. The change in the counterion, i.e., from bromide
to nitrate, induced a significant upfield shift in the peak of
N-CH2.
The significant changes in the chemical shift values of
proton at the 2- and 6-positions of picolinium units and the
N-CH group clearly suggest that the pyridinium salts exist
2
as tight ion pair with the counterion. In 2-methyl pyridi-
nium compounds, the 6-proton suffered more shift (0.72δ)
compared to the pyridinium methyl group (0.30δ), indicat-
ing the closeness of the counterion toward the C-6 proton
compared to the methylene group.
Physical Properties of IL
The density values of the IL were found to be in the range
−3
1
.4–1.5 g cm . There was no specific trend for the change
of density with the structure of the IL. The surface tension
values were found to decrease with increase in the chain
length of the pyridinium bromide salts. In a review on rheo-
logical behavior of IL, it was reported that with increasing
chain length in the alkyl group linked to N or C of imidazo-
lium groups, the surface tension of the corresponding IL
decreases (Sedev, 2011). In the case of 4-methyl pyridi-
nium compounds, a plot of surface tension versus the num-
ber of carbon atoms in the alkyl chain was found to be
Quantum Mechanical Calculations
The physical and chemical properties of IL are highly
dependent on their molecular structure (Fumino, Wulf, &
Ludwig, 2009). For example, in imidazolium-type IL, the
position and length of alkyl chains or type of the inorganic
anion affect the ability to form hydrogen bonding, and
2
linear with R = 0.99, while in case of 2-methyl pyridinium
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n 3
Table 4 Energy, absorbance, and dipole moment of [α/γ-PicC ][Br/NO ] IL
Alkyl chain
Energy (a.u.)
E
HOMO (a.u.)
ELUMO (a.u.)
ΔE (kCal)
λmax (nm)
M (Debye)
[
[
[
[
α-PicC
n
][Br]
C3
C5
C7
C3
C5
C7
C3
C5
C7
C3
C5
C7
−2977.8470
−3056.4770
−3135.1023
−2977.8491
−3051.3761
−3129.4455
−686.4124
−765.0411
−843.6686
−686.4237
−765.0427
−843.6687
−0.1615
−0.1612
−0.1617
−0.1617
−0.1612
−0.1617
−0.1738
−0.1739
−0.1738
−0.1727
−0.1721
−0.1719
−0.0982
−0.0981
−0.0949
−0.0951
−0.0954
−0.0949
−0.0876
−0.0849
−0.0869
−0.0832
−0.0824
−0.0822
39.746
39.659
41.918
41.780
41.300
41.918
54.0976
55.8609
54.4741
56.1558
56.3064
56.3253
720.92
682.08
684.33
692.34
682.08
719.33
528.51
511.82
524.85
509.14
507.77
507.60
13.68
13.55
13.52
γ-PicC
n
][Br]
13.73
13.68
13.56
α-PicC
n
][NO
3
]
10.797
11.303
11.330
11.512
11.485
11.498
n 3
γ-PicC ][NO
]
consequently affect their physical and chemical properties
Huddleston et al., 2001). Thus, investigation of their inter-
molecule and the interactions of functional groups was con-
ducted. Optimized geometries of IL and the type and possi-
ble positions for hydrogen bonds from van der Waals
atomic radii overview are shown in Figs. 2 and S8A–J.
Hydrogen bonds are denoted by dashed lines, and the cor-
responding HÁ Á ÁX and OÁ Á ÁH distances are labeled.
(
actions at the molecular level with counterion and with var-
ious solutes is of great importance to develop new
applications. Molecular dynamics simulation is a powerful
tool for correlating the physical and chemical properties
with structural parameters. However, the result quality
of every simulation depends on the force field, which
should reproduce system properties accurately enough over
a wide range of thermodynamic states. Theoretical calcula-
tions were also performed to explore the structure and inter-
actions of the counterion in the synthesized IL. Recently,
Kirchner (2009) reviewed the theoretical investigations of
IL. Combined with the experimental work, these studies
have encouraged researchers to make more informed deci-
sions regarding the properties to be expected for a given
class of IL.
From the calculations, it was found that the variation of
chain length and the position of the methyl group in the
pyridine ring have no significant effect on the dipole
−
moment. The Br ion remains on one side of the pyridine
ꢀ
ring with the dihedral angle of 20–22 ( N C H Br).
The dipole moments of the picolinium nitrates are found to
be less than those of the corresponding bromide salts,
which may be due to the position of the nitrate ion with
respect to pyridinium nucleus. In [γ-PicC ][Br], there are
3
−
weak nonconventional hydrogen bonds between Br and
C H(7) and C H(11) with the atomic distance of 2.302
The synthesized RTIL structures were drawn using the
GaussView program. The geometries of the compounds
were optimized by using DFT considering the Becke’s
three-parameter hybrid exchange functional and the Lee–
Yang–Parr correlation functional (B3LYP) (Becke, 1993;
Lee, Yang, & Parr, 1988) by using 6-31G (d) basis set. All
calculations were carried out with the Gaussian 03 program
and 2.531 Å, respectively. In [α-PicC ][Br], there are three
hydrogen bonds with bond distances 2.3, 2.498, and
3.068 Å for C H(7), C H(11), and C H(18),
3
(Frisch et al., 2003).
After optimizing the molecular structure with Gaussian
03, the quantum chemical parameters were obtained from
the output file (Table 4). Typically, the van der Waals force
increases as the molecular weight increases. The electric
charge on individual atoms and the shortest hydrogen-bond
distance can be predicted from quantum mechanical calcu-
lations. The common quantum chemical descriptors such as
dipole moment (μ), energy of the highest occupied molecu-
lar orbital (EHOMO), and energy of the lowest unoccupied
molecular orbital (ELUMO) were determined. A thorough
hydrogen-bonding analysis involving the geometry of the
2
.
5
3
0
9
9
3
Fig. 2 Optimized structure of [γ-PicC ][Br]
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respectively. The proximity of NC H in [α-PicC3]
Br] when compared to [γ-PicC ][Br] may be ascribed to
682–721 nm for the bromide salts and 507–528 nm for the
nitrate salts. The change in counterion from bromide to
nitrate increases the bandgap significantly. Bai, Zhu, and
Chen (2011) have also observed the HOMO–LUMO gap
for imidazolium bromide and nitrate salts to be almost in
the same range.
[
3
the steric effect of the 2-methyl group, which repels the
−
flexible terminal methyl group toward Br . This is sup-
ported by the increase in bond angle of C N C in
[
α-PicC ][Br] than [γ-PicC ][Br]. In the corresponding
3
3
nitrate salt, the central nitrogen atom is found to remain
above the plane of the picolinium (bond angles of C N N
ꢀ
being 76–105 ) aligned to the π-electron cloud with slight
Conclusion
distortion. The oxygen atoms of nitrate ion in [γ-PicC3]
[
NO ] may be linked to C H(8), C H(13), and C H
N-Alkyl pyridinium salts are found to be IL, but with rela-
tively high melting temperatures compared to imidazolium
IL. Furthermore, the alkyl chain with an odd number of car-
bon atoms is found to have low melting points compared to
those with an even number of carbon atoms. Quantum
mechanical studies suggest the orientation of the counterion
to be unsymmetrical with respect to the organic cation.
The odd number of carbon atoms in the alkyl chain and the
asymmetric orientation of the counterion probably contrib-
ute to the liquid characteristics of the molecules. Pyridi-
nium ions are more promising substrates than imidazolium
ions, with the possibility of different types of substitution
reactions of the pyridinium nucleus, and hence can be con-
sidered for task-specific IL.
3
(12) with interatomic distances of 2.230, 2.229, and
2
.816 Å, respectively. The liquid characteristics, thus, may
be ascribed to the spatial orientation of the anion, which
deviates from the symmetry, leading to the relatively weak
packing of molecules.
The molecular charge distributions on individual atoms of
the IL are tabulated in Table S1. The charge on the adjacent
C of N in the pyridine ring is assumed to have a positive
value, while that of alkyl chain has a negative value. The rea-
soning seems to be obvious because the former atoms are
connected to the electron-deficient N through π-bond and the
electron clouds are polarized away from the C atoms, while
in the latter case, due to the σ-bond, the inductive effect
increases the electron cloud on the carbon atom connected to
the quaternary N atom. The charge distribution on the hydro-
gen atoms close to the counterion supports the sidedness of
the counterion. In a symmetrical compound like [γ-PicC3]
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