Cluster Formation through Hydrogen Bond Bridges across Chloride Anions in a Hydroxyl-Functionalized Ionic Liquid

Article abstract of DOI:10.1002/cphc.201801206

Several recent studies of hydroxyl-functionalized ionic liquids (ILs) have shown that cation-cation interactions can be dominating these materials at the molecular level when the anion involved is weakly interacting. The hydrogen bonds between the like ions led to the formation of interesting chain-like, ring-like, or distinct dimeric (i. e. two ion pairs) supermolecular clusters. In the present work, vibrational spectroscopy (ATR-IR and Raman) and density functional theory (DFT) calculations of the hydroxyl-functionalized imidazolium ionic liquid C₂OHmimCl indicate that anion-cation hydrogen bonding interactions are dominating, leading to the formation of distinct dimeric ion pair clusters. In this arrangement, the Cl^? anions function as a bridge between the cations by establishing bifurcated hydrogen bonds with the OH group of one cation and the C(2)-H of another cation. Cation–cation interactions, on the other hand, do not play a significant role in the observed clusters.

Full text of DOI:10.1002/cphc.201801206

Accepted Article
Title: Cluster formation through hydrogen bond bridges across chloride
anions in a hydroxyl-functionalized ionic liquid
Authors: Sumit Kumar Panja, Boumediene Haddad, Mansour Debdab,
Johannes Kiefer, Yassine Chaker, Serge Bresson, and
Annalisa Paolone
This manuscript has been accepted after peer review and appears as an
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the VoR from the journal website shown below when it is published
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To be cited as: ChemPhysChem 10.1002/cphc.201801206
Link to VoR: http://dx.doi.org/10.1002/cphc.201801206
A Journal of
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ChemPhysChem
10.1002/cphc.201801206
Cluster formation through hydrogen bond bridges across chloride anions in
a hydroxyl-functionalized ionic liquid
a
b
c
d
Sumit Kumar Panja, Boumediene Haddad, Mansour Debdab, Johannes Kiefer, * Yassine
c
e
f
Chaker, Serge Bresson, Annalisa Paolone
a
b
c
Dr. Sumit Kumar Panja, Dr. Boumediene Haddad, Dr. Mansour Debdab, Prof. Dr. Johannes
d
c
e
f
Kiefer, * Dr. Yassine Chaker, Dr. Serge Bresson, Dr. Annalisa Paolone
a
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore
60012, Karnataka, India.
5
b
Department of Chemistry, Dr Moulay Tahar University of Saida, Algeria.
Laboratoire de Synthèse et Catalyse Tiaret LSCT, University of Tiaret, Algeria.
c
d
Technische Thermodynamik, Universitꢀt Bremen, Badgasteiner Str. 1, 28359 Bremen,
Germany.
e
Laboratoire de Physique des Systèmes Complexes, Université Picardie Jules Verne, 33 rue
St Leu 80039 Amiens cedex, France.
f
Consiglio Nazionale delle Ricerche, Instituto dei Sistemi Complessi (CNR-ISC), University
of Rome La Sapienza, Piazzale A. Moro 5, 00185 Roma, Italy.
1
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ChemPhysChem
10.1002/cphc.201801206
ABSTRACT
Several recent studies of hydroxyl-functionalized ionic liquids (ILs) have shown that cation-
cation interactions can be dominating these materials at the molecular level when the anion
involved is weakly interacting. The hydrogen bonds between the like ions led to the formation
of interesting chain-like, ring-like, or distinct dimeric (i.e. two ion pairs) supermolecular
clusters. In the present work, vibrational spectroscopy (ATR-IR and Raman) and quantum
chemical theory (DFT) of the hydroxyl-functionalized imidazolium ionic liquid C OHmimCl
2
indicate that anion-cation hydrogen bonding interactions are dominating and lead to the
-
formation of distinct dimeric ion pair clusters. In this arrangement the Cl anions function as a
bridge between the cations by establishing bifurcated hydrogen bonds with the OH group of
one cation and the C(2)-H of another cation. Cation-cation interactions, on the other hand, do
not play a significant role in the observed clusters.
TOC GRAPHICS & TEXT
Peculiar ion pair dimers: A combined theoretical and experimental study of a hydroxyl‐functionalized
ionic liquid provides strong evidence for the formation of distinct ion‐pair dimers in the condensed‐
phase material. The strongly-interacting chloride anions act as a hydrogen bonding bridge between
the cations, and cation-cation interactions are negligible.
KEYWORDS: Ionic liquid; H-bonding Interaction; Ion pair; Cluster formation; Density
Functional Theory (DFT)
2
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ChemPhysChem
10.1002/cphc.201801206
Introduction
Ionic liquids (ILs) are fascinating materials and have received considerable attention in
academia as well as industry over the past 20 years. The tunability of their properties via
combining suitable cations and anions makes them attractive for applications in a variety of
fields including organic synthesis, catalysis, electrochemistry, and separation.[1-4] These
designable properties (e.g. viscosity, electrical conductivity, and non-volatility) are determined
by the molecular interactions between the ions. Therefore, unravelling the structure-property
relationships and finding universal correlations that can be utilized for a rational design, i.e.
without the need for extensive trial-and-error testing, is an important goal.[5, 6] In fact, a full
understanding of how the chemical structure of the ions and the interionic forces effect the
macroscopic properties of an IL will be a game changer.
The molecular interactions inside ILs are rather complicated compared to other classes of
materials. There are long-range coulombic forces due to the charges of the ions, dispersion
forces between aliphatic chains, as well as hydrogen bonding. The interplay of these
interactions yields a local structure and produces micro/mesoscopic local domains. Studying
how these phenomena affect the macroscopic properties has been and still is a hot topic in
chemistry and chemical engineering. A common approach is to investigate series of ILs with a
systematically changing structure in one ion while keeping the other ion constant. For example,
varying the alkyl chain length in 1,3-dialkylimidazolium ILs or changing the anion was
[
7-14]
performed in many studies.
Studying ILs with functionalized side chains is another highly active area of research that aims
at understanding the structure-property relationships. In particular, substituting alkyl chains by
hydroxyl-functionalized moieties is currently very popular as this adds a further group that can
potentially form hydrogen bonds. These hydroxyl-functionalized ILs were developed for a
number of interesting applications. For example, they exhibit a remarkable capability of
reversibly capturing green-house gases.[15-17] Moreover, the introduction of the hydroxyl group
[
18]
was found to enhance the hydrophilicity and enzyme activity, to improve the solubility of
[
19]
inorganic salts, and to make suitable reaction media for metal oxide powders with specified
[20, 21]
size.
Very recently, a number of spectroscopic and theoretical studies focused on the molecular
interactions of hydroxyl-functionalized ILs.[22-26] In these papers, a large variety of cations, e.g.
imidazolium-, ammonium-, piperidinium-, pyrrolidinium-, and pyridinium-based ones, was
functionalized
with
an
OH-containing
group.
The
counterions
included
bis(trifluoromethylsulfonyl)imide (NTf
2
), tetrafluoroborate (BF
4
), and picrate. Interestingly, in
all these studies the experimental and theoretical results led to the conclusion that the OH-
functionalized ILs share strong cation-cation interactions. The hydrogen bonding between the
OH groups of one cation and either the OH group of another one or the C(2)-H proton of the
imidazolium ring of a second cation was found to overcome the coloumbic repulsion of the like
ions. However, experimentally, only the OH…OH hydrogen bonds could be observed
unambiguously. In some ILs, a hydrogen bonding network with chain or ring-like structures
[
24]
[27, 28]
was found, which is very similar to the observations in simple aliphatic alkyl alcohols.
Due to all these studies pointing into the same direction, it was reasonable to speculate/conclude
[
26]
that cation-cation interactions are a universal phenomenon in OH-functionalized ILs.
However, a comparative study of the Ludwig group[29] showed that the cation-cation
interactions are only dominating in the case of weakly interacting anions. In ILs with strongly
3
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ChemPhysChem
10.1002/cphc.201801206
interacting anions, on the other hand, cation-anion interactions via the hydroxyl group were
observed as an important mechanism.
In this study we show an example of an OH-functionalized IL, which forms distinct dimeric
clusters through cation-anion interactions. For this purpose, 1-(hydroxyethyl)-3-
methylimidazolium chloride (C OHmimCl) was studied theoretically and experimentally. The
2
schematic structure of the cation and anion is depicted in Chart 1 together with the atom
numbering scheme.
Chart 1. Structure and atom numbering scheme of title compound.
Materials and Methods
Synthesis
and
characterization
of
1-(hydroxyethyl)-3-methylimidazolium
chloride(C
2
OHmimCl). All materials were of analytical grade and procured from Sigma
Aldrich. This includes solvents for the synthesis (ultrapure water, acetonitrile, dichloromethane,
ethanol) and D O and CDCl for the NMR experiments. The detailed synthesis procedure is
described in a previous paper. In brief, a mixture of 2-chloroethanol (0.67 mL, 10 mmol) and
- methylimidazole (1.1 g, 10 mmol) was heated for 24 h at 120 C. The crude product, which
crystallized upon cooling, was finely crushed, washed with Et O (3x80 mL), and dried under
2
3
[
30]
1
2
reduced pressure for 10 h. In order to remove volatile components and water, the IL was dried
under constant stirring at a temperature of 298 K for about 24 h at a reduced pressure of 2 Pa;
there was no detectable mass loss of a 2 g sample during this period of time. Finally, the water
content was below 100 ppm as proved by Karl Fischer titration. After drying, the samples were
handled under argon atmosphere.
1
C
2
OHmimCl: State: Pale yellow powder; mp 56 C; Yield: 1.8 g (94.5%); H-NMR:(600 MHz,
2
1
1
1
2
2
D O):  (ppm) 8.78 (s, H), 7.54 (s, H), 7.49 (s, H), 4.34 (t, H, J = 4.9 Hz), 3.96 (t, H, J = 4.9
Hz), 3.93 (s, H); C-NMR (150.93 MHz, D O):  (ppm) 136.4,123.6, 122.4, 59.8, 51.5, 35.7.
3
13
2
Instrumentation. Infrared Spectroscopy. For the study of the vibrational properties, attenuated
total reflectance Fourier transform infrared (ATR/FTIR) spectra were acquired on a Vertex 70-
RAM II Bruker spectrometer (Bruker Analytical, Madison, WI) operating with a Golden Gate
-1
TM diamond ATR accessory (Specac Ltd., Slough, UK). FTIR/ATR spectra (600-3500 cm )
-1
were collected with a nominal resolution of 1 cm by co-adding 64 scans for each spectrum.
Raman Spectroscopy. FT-RAMAN spectra were acquired by means of a Vertex 70-RAM II
Bruker FT-RAMAN spectrometer with a Nd:YAG laser (yttrium aluminium garnet crystal
doped with triply ionized neodymium) having a wavelength of 1064 nm and a maximum power
of 1.5 W. The measurement accessory is pre-aligned; only the Z-axis of the scattered light is
4
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ChemPhysChem
10.1002/cphc.201801206
adjusted to set the sample in the appropriate position regarding the focal point. The RAM II
spectrometer is equipped with a liquid-nitrogen cooled Ge detector and FT-RAMAN spectra
-1
-1
(
200-3500 cm ) were collected with 1 cm nominal resolution by co-adding 128 scans for each
spectrum at room temperature.
Quantum Chemical Calculations. Structure Calculations. DFT (B3LYP and CAM-B3LYP
method) calculations were employed to calculate the optimized energies and to compute the
geometries for different possible conformers of the ion pair, and clusters of two ion pairs. The
new hybrid exchange-correlation functional with long-range-corrected (LC) exchange
functional CAM-B3LYP was chosen to obtain a better understanding of the intermolecular
interactions as the standard B3LYP does not include a long-range correlation function. The high
level basis set 6-311++G(d, p) was used in all the calculations, as it performed well in the
prediction of molecular interactions.[31] The Gaussian 09 suite of programs was used for all
the computations, and the GaussView 5.1 program was used to visualize the structures.[33]
[32]
Wavenumber Calculations. The vibrational wavenumbers of all conformers of the cation, the
anion, and the ion pair configurations of IL along with their IR and Raman intensities were also
calculated using the Gaussian 09 program. All the calculations were performed without
applying any constraints. In every case, the optimized structures finally converged into the
minima of the potential surface, which was confirmed from the absence of negative (imaginary)
wavenumbers for any normal mode.
Binding Energy Calculation. Binding energy calculation of ion-pair was also performed using
DFT calculations with the B3LYP/6-311++G(d,p) basis set using Gaussian 09 linked to the
GaussView 5.1 program. The energy of the ion-pair formation (ΔE) was estimated using Eqn
1), according to Turner et al.[
6]
(
푀푖푛
푀푖푛
−
∆
E = 퐸퐴푋 – 퐸퐴
+
– 퐸푋
+ 퐸BSSE + ΔZPVE
(1)
where ΔE is the energy of the ion-pair formation and EAX, EA+, EX- are the energy of ionic
pair, the isolated cation, and anion, respectively. The interaction energies were corrected by the
basis set super-position errors (BSSE) correction with the counterpoise procedure method
[
34]
advanced by Boys and Bernardi
at B3LYP/++G(d,p) level. Zero-point energy (ZPE)
correction calculated using the unscaled B3LYP/6-311+G(d,p) frequencies was also taken into
account for the calculated interaction energies.
Results and Discussion
In the first step, an isolated ion pair was considered in the theoretical analysis. Figure 1
illustrates four stable configurations establishing a number of different molecular interaction
mechanisms. The lowest energy structure (Fig 1a) is characterized by the chloride anion
forming interactions with the protons of the hydroxyl and the C(2)-H groups. In addition, a
+
-
-

···Cl interaction is observed. The other conformers show bifurcated CH···Cl interactions at
the aromatic and aliphatic CH groups. A more detailed description and analysis of the isolated
ion pair configurations is provided in the supplementary material.
5
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ChemPhysChem
10.1002/cphc.201801206
1
mol
a: ∆퐸 = 0 kcal 1b: ∆퐸 = 10.09
푟푒푙
푟푒푙
-1
-1
kcal mol
1
d:
1
mol
c: ∆퐸 = 15.28 kcal ∆퐸 = 11.98 kcal
푟푒푙
푟푒푙
-1
-1
mol
Figure 1 (a-d): Optimized structure of various ion pair configurations for C
gas phase.
2
OHmimCl in the
In the next step, two ion pairs were considered at the same time. The optimized geometry of the
resulting ion pair dimer is illustrated in Figure 2. Interestingly, in contrast to the previously
[
26]
studied OH-functionalized ILs, where the cations formed a central unit stabilized by strong
cation-cation hydrogen bonding, the current C OHmimCl shows the opposite behavior. The
2
two chloride anions take a central role acting a hydrogen bonding bridge between the cations.
This yields a highly symmetric structure, in which each chloride ion establishes three different
+
-
interactions: 1) an H-bond with the C(2)-H of one cation and 2) a  ···Cl interaction plus 3) an
H-bond with the OH group of the other cation. In other words, if at all cation-cation interactions
play only a minor role here.
Figure 2. Optimized dimeric ion pair of C OHmimCl in gas phase.
2
In order to verify these theoretical findings, the experimental vibrational spectra were analyzed.
Figures 3a and 3b show the CH/OH stretching region of the FTIR and Raman spectra,
respectively. The full spectra are shown in the supplementary material for completeness. The
experimental data were decomposed by a spectral fitting routine in order to identify the
individual vibrational contributions to the overall band. The band assignments are summarized
in Table 1 together with the corresponding theoretically predicted wavenumbers for the lowest
energy configurations.
6
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ChemPhysChem
10.1002/cphc.201801206
The previous studies of other OH-functionalized ILs indicated that hydrogen bonding between
the OH groups of different cations and C(2)-H···OH hydrogen bonding are a dominating
mechanisms. This resulted in OH vibrations similar to those found in alkyl alcohols.[26] The
individual contributions to the OH stretching band in the normal aliphatic alkyl alcohols are
-
1 [27]
-1
commonly found at wavenumbers larger than 3200 cm : one peak at ~3250 cm from the
-1
twofold H-bonded OH groups and one peak at ~3350 cm single H-bonded groups. The
vibrational spectra of C OHmimCl, however, do not show any characteristic signals in this
2
range. Such signatures would be expected to appear as rather narrow peaks due to the highly
defined hydrogen bonding state in the clusters. Instead the OH signatures are found at around
-1
3
100 cm , which can be attributed to the strong interactions with the chloride ions. This
supports the theoretical predictions. Another corroborating feature can be found in the
-1
fingerprint region (see supplementary material). The presence of a strong peak at 603 cm plus
-1
the absence of a peak around 625 cm suggests that the C OH chain more or less exclusively
2
exists in its gauche form, which is a strong indicator of the predicted ion pair dimer
configuration. Moreover, none of the spectral signatures suggests significant cation-cation
interactions.
a)
b)
Figure 3: Experimental ATR-FTIR (a) and Raman (b) spectra of C
2
OHmimCl.
Table 1: Selected Vibrational band assignments of C OHmimCl.
2
Band
Assignment
Experimental: Theory:
FTIR /Raman Ion-pair
Theory:
7
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Dimeric
IP
휈.(퐶4,5 ꢀ 퐻) 3140/(3151,
142)
3191/3173 3191/3173
3
휈 (퐶 ꢀ 퐻) -/3127
3205
3158
3051
3067
3221
3051
.
2
휈.(푂 ꢀ 퐻) 3096/3101
휈 (푁
퐴푆
3051/3054
ꢀ
퐶퐻3)
휈 (푁
퐴푆
3034/3035
3015
3008
ꢀ
퐶퐻2)
휈.(퐶 = 퐶) 1570/1570
1597
1595
−
1
−1
For the C-H region 3300-2700 cm , the N-CH
reference peak for scaling the theoretical wavenumbers and the scaling factor is found to be
.968.
3
stretching at 3051 cm was selected as a
0
Conclusions
This work presented a combined computational and experimental analysis of 1-(hydroxyethyl)-
3
-methylimidazolium chloride. The formation of distinct dimeric clusters from two ion pairs
was found. Interestingly, these clusters are characterized by strong cation-anion interactions
and a geometrical configuration, in which the chloride anions act as a hydrogen bonding bridge
between the cations. Cation-cation interactions, which were found to be the dominating
mechanism in hydroxyl-functionalized ionic liquids with weakly coordinating anions, do not
play a significant role in the clusters observed herein. Our work shows that further theoretical
and experimental effort is necessary to finally unravel the structure-property relationships of
this fascinating class of materials. One specific point that should be addressed in future studies
are temperature effects, which may cause significant changes to the observed interaction
mechanisms.
Conflicts of interest
There are no conflicts to declare.
Acknowledgment
SKP thanks Prof. Puspendu K. Das for his suggestions and SERB for N-PDF post-doc
fellowship (Scheme: N-PDF/2016/000041) at the Department of Inorganic and Physical
Chemistry (IPC), Indian Institute of Science (IISc), Bangalore-560012, India. The authors
gratefully acknowledge support (financial support and instrument facilities) from the
Department of Chemistry at the Dr. Moulay Tahar University of Saida, Algeria and MESRS.
8
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ChemPhysChem
10.1002/cphc.201801206
BH thanks Dr. Khonati for fruitful discussions and Q. Arnould for performing the FT-Raman
measurements at the Walloon Agricultural Research Centre (CRA-W).
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