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A Combined Experimental/Computational Study of Dicationic Ionic Liquids with Bromide and Tungstate Anions

Authors:

Abstract

A panel of dicationic ionic liquids (DILs) with different rigid xylyl (ortho, meta, para) spacers and different anions (bromide and tungstate) has been synthetised and characterised through different experimental and computational techniques. Differences and analogies between the systems are analysed using information derived from their DFT structures, semiempirical dynamics, thermal behaviour, and catalytic properties versus the well-known reaction of CO2 added to epichlorohydrin. A comparison between the proposed systems and some analogues that present non-rigid spacers shows the key effect displayed by structure rigidity on their characteristics. The results show an interesting correlation between structure, flexibility, properties, and catalytic activity.
Molecules 2024, 29, 2131. https://doi.org/10.3390/molecules29092131 www.mdpi.com/journal/molecules
Article
A Combined Experimental/Computational Study of Dicationic
Ionic Liquids with Bromide and Tungstate Anions
Guelber Cardoso Gomes
1,†
, Claudio Ferdeghini
1,†
, Luca Guglielmero
2,
*, Felicia D’Andrea
1
, Lorenzo Guazzelli
1
,
Andrea Mezzea
1
and Christian Silvio Pomelli
1,
*
1
Department of Pharmacy, University of Pisa, Via Bonanno 33, 56126 Pisa, Italy;
guelbercardoso@gmail.com (G.C.G.); claudioferdeghini@gmail.com (C.F.); felicia.dandrea@unipi.it (F.D.);
lorenzo.guazzelli@unipi.it (L.G.); andrea.mezzea@unipi.it (A.M.)
2
Classe di Scienze, Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
* Correspondence: luca.guglielmero@sns.it (L.G.); christian.pomelli@unipi.it (C.S.P.)
These authors contributed equally to this work.
Abstract: A panel of dicationic ionic liquids (DILs) with dierent rigid xylyl (ortho, meta, para)
spacers and dierent anions (bromide and tungstate) has been synthetised and characterised
through dierent experimental and computational techniques. Dierences and analogies between
the systems are analysed using information derived from their DFT structures, semiempirical
dynamics, thermal behaviour, and catalytic properties versus the well-known reaction of CO
2
added
to epichlorohydrin. A comparison between the proposed systems and some analogues that present
non-rigid spacers shows the key eect displayed by structure rigidity on their characteristics. The
results show an interesting correlation between structure, exibility, properties, and catalytic
activity.
Keywords: ionic liquids; tungstate ion; DFT; catalysis; dicationic ionic liquids
1. Introduction
Academic interest in the study of ionic liquids (ILs) has grown constantly during
recent decades [1]. The peculiar set of physical–chemical properties displayed by ILs,
arising from their structure composed only of ions, has made them valuable systems in a
wide variety of applications, including dissolution, fractionation, and valorisation of
biomasses [2–6]; organic synthesis and catalysis [7–9]; electrochemistry; energy storage
devices [10–14]; and, more recently, biological and pharmaceutical applications [15,16].
Furthermore, ILs typically display negligible vapour pressure, high thermal stability, and
low ammability, which have gained this neoteric class of solvents a generally accepted
reputation for being relatively green media [17–22]. Finally, the great number of possible
IL structures oers the possibility of designing and preparing task-specic systems
characterized by nely tuned properties by selecting an appropriate couple of ions [23].
Dicationic ionic liquids (DILs), a class of ILs that display a positive ion consisting of two
cationic moieties covalently connected to each other, can oer an even higher structural
variability [24–28]. The possibility of tuning the physical–chemical properties of DILs by
changing the type and the length of the spacer chain has been assessed and explored by
various studies in the literature [25,28–32]. DILs can be useful also as charged tags for the
study of reactivity in ILs through use of ESI mass techniques [33,34]. The nature of the
spacer (length, type of chain) has been reported to have a strong eect on the thermal
stability, melting point, electrochemical window, and solubility characteristics of DILs
[25,29,31,32,35–37]. The possibility of undergoing specic thermally induced
rearrangements has also been pointed out to be directly determined by the length of the
spacer [38,39]. Furthermore, the linker chain structure was also identied to be an
Citation: Cardoso Gomes, G.;
Ferdeghini, C.; Guglielmero, L.;
D’Andrea, F.; Guazzelli, L.;
Mezzea, A.; Pomelli, C.S. A
Combined Experimental/
Computational Study of Dicationic
Ionic Liquids with Bromide and
Tun gst ate A nio ns. Molecules 2024, 29,
2131. hps://doi.org/10.3390/
molecules29092131
Academic Editor: Kenneth Laali
Received: 15 March 2024
Revised: 10 April 2024
Accepted: 24 April 2024
Published: 3 May 2024
Copyright: © 2024 by the authors.
Submied for possible open access
publication under the terms and
conditions of the Creative Commons
Aribution (CC BY) license
(hps://creativecommons.org/license
s/by/4.0/).
Molecules 2024, 29, 2131 2 of 23
important factor in the determination of the catalytic activity of imidazolium DILs toward
the reaction of cyclocarbonation of epoxides [36,40,41]. The use of ILs for the dissolution
of metal ions and complexes has already gained a certain popularity in research elds
such as electrochemistry and metal recovery from waste materials [42–47]. In parallel, the
inclusion of catalytically or electrochemically active metal-based systems in ILs structures
is aracting growing interest due to the exibility and good potentialities of this approach
[48–50]. Bragato et al. and Calmanti et al. respectively reported the use of molybdate and
tungstate-based ILs as eective catalysts for the cycloaddition of CO2 to epoxides [51,52].
Other authors have also reported encouraging results concerning the use of tungstate-
based ILs for the catalysis reaction of CO2 with substrates like diamines, propargylic
amines, aminophenols, aminothiobenzenes, and cyanoanilines for the preparation of
corresponding cyclocarbonated compounds [53–57] or for the conversion of cyclic
carbonates into linear ones [58]. In addition to this, tungstate salts have shown interesting
catalytic performances in the catalysis of oxidative reactions like the preparations of
carboxylic acids and aldehydes as well as sulfones and epoxides [59,60]. Concerning
epoxidations, apart from classic tungstate salts, the use of tungstate ILs has also been
investigated, producing positive outcomes [53,56]. Nevertheless, to the best of our
knowledge, no papers in the literature have yet considered tungstate-based DILs. In the
present study, we report the synthesis and characterization of a panel of bromide and
tungstate-based DILs featuring imidazolium charged heads and rigid aromatic spacers.
Imidazolium systems have been chosen for this study due to their great popularity, thus
making possible a more direct comparison with structurally related DILs. Furthermore,
the possibility of C2-H hydrogens forming hydrogen bonds with the anions considered in
this work as well as with other molecules makes imidazolium DILs interesting both from
the point of view of structural investigation and catalytic activity assessment [61]. The
structures of the double-charged cations considered are reported in Figure 1. The eect of
dierent spacers on the thermal properties of the proposed compounds has been assessed
and a computational study of the relationship between the system conformations and the
type of linker moiety has been carried out. An analysis of possible conformation of the
minimal ion pairs with a comparison between bromide and tungstate DILs is reported in
the computational part. Furthermore, we report some results of semiempirical molecular
dynamics studies of these systems. Considering the aention given in the literature to the
use of tungstate systems for CO2 activation reactions, we performed a proof-of-concept
study of the applicability of the synthesised DILs in the catalysis of CO2’s addition to
epoxides. Finally, the interactions occurring between tungstate anions and carbon dioxide
in DILs–CO2 clusters were further explored by computational means.
Figure 1. Bromide and tungstate DILs synthesised and studied in this paper.
Molecules 2024, 29, 2131 3 of 23
2. Results and Discussion
2.1. Synthesis
A panel of six DILs featuring a xylyl semirigid spacer with bromide and tungstate
anions was considered in this study (Figure 1). The prepared compounds have been
characterised through 1H-NMR, 13C-NMR (Figures S1–S12, Supplementary Materials),
and ATR-FTIR (Figures S13–S18, Supplementary Materials).
The synthesis of bromide DILs 13 was performed following a literature procedure
[62]. N-methyl imidazole and the selected di-halogenated xylenes were reacted in
acetonitrile at solvent reux temperature overnight, obtaining the desired compounds in
an excellent yield (Scheme 1). Tungstate DILs 46 were prepared from the bromide DILs
(Scheme 2). Diering from the synthesis of compounds 13, the preparation of tungstate
DILs 46 presented few diculties. The rst aempted approach relied on the metathesis
reaction with Na2WO4, but it proved unsuccessful. The specic solubility properties of
Na2WO4, of bromide DILs, and of the byproducts resulting from the ion metathesis made
it impossible to nd a suitable solvent for the ion exchange. A second aempt was made
using Ag2WO4, prepared right before use, utilising the extremely low Kps of the AgBr salt
to push the equilibrium in favour of the desired products. This method proved to be only
partially successful due to the presence of silver impurities in the obtained tungstate DILs,
which we found impossible to remove. Eventually, the use of a procedure based on an ion
exchange resin proved fully successful. Compounds 13 were rstly converted into the
corresponding hydroxides using the ion exchange column, and the desired tungstate DILs
were quantitatively obtained by reacting the hydroxide intermediates with H2WO4. The
complete substitution of the halide anions during the preparation of the hydroxide
intermediate was veried by a silver assay, as described in Section 3.5. IR analysis can be
used as an eective method of verifying the formation of the desired tungstate product.
Upon comparing the IR spectra of the DILs with bromide anions (Figures S13–S15,
Supplementary Materials) and the spectra of the tungstate DILs (Figures S16–S18,
Supplementary Materials), an intense peak associated with the WO42 group (Figure S19,
Supplementary Materials) appeared in the last three spectra in the region at about 800
cm1 (consistent with what was reported by Calmanti et al.) [52]. On the other hand, the
signals characteristic of H2WO4, i.e., the peak at 940 cm1 and the broad peak centred at
about 650 cm1, are not visible in the spectra of tungstate DILs 46, indicating the absence
of unreacted tungstic acid in the prepared compounds (Figure S20, Supplementary
Materials).
Scheme 1. Synthesis of the bromide DILs (13).
Molecules 2024, 29, 2131 4 of 23
Scheme 2. Synthesis of tungstate DILs (46).
2.2. Thermal Characterization
The thermal stability of compounds 16 was rstly investigated by
thermogravimetric analysis (Figures S21–S26, Supplementary Materials). From the
obtained data (Table 1), a direct connection was evident between the anion type and the
thermal stability of the DIL. Compounds 13, featuring bromide anions, displayed a
degradation temperature almost 100 °C higher than their tungstate counterparts 46. At
the same time, the degradation prole itself appeared greatly inuenced by the anion
type. Bromide DILs (13) were characterised by a single degradation peak, even if
sometimes they showed shoulder peaks or minor degradation peaks at higher
temperatures (Figures S21–S23, Supplementary Materials). On the other hand, tungstate
DILs displayed much more complicate proles, with multiple degradation steps
overlapping in the region between 200 °C and 400 °C, indicating a strong eect of the
tungstate anion on the thermal degradation mechanisms (Figures S24–S26,
Supplementary Materials). The nature of the linker chain also proved to strongly aect
the thermal stability properties of compounds 16 (Table 1). o-xylenes spacers were found
to provide compounds with a lower thermal stability when compared with m- and p-
xylenes linkers, with this eect being more noticeable in tungstate DILs. Finally, the type
of spacer was found to deeply aect the degradation prole of the studied compounds.
Concerning bromides DILs, compound 1 (featuring the o-xylene linker) displayed a single
and steep degradation step, while DIL 2 (featuring the m-xylene linker) still exhibited a
quite simple degradation prole, mainly degrading in a single step but with a well-
pronounced shoulder observable on the left side of the main degradation peak in the
dweight%/dT plot (Figures S21 and S22, Supplementary Materials). Finally, the p-xylene
DIL (3) displayed an intermediate behaviour between 1 and 2 (Figures S23,
Supplementary Materials). It can therefore be concluded that rigid xylyl linker chains
exert a much stronger eect on the thermal degradation proles of DILs compared to the
eect reported in literature for alkyl linker chains [29]. Concerning tungstate DILs 46, the
inuence of the spacer on the thermal degradation prole was found to be even more
noticeable. Compound 6, featuring a p-xylene spacer, was found to degrade in a single
step (Figure S26, Supplementary Materials). On the other hand, compounds 4 and 5
displayed a very complex degradation prole (Figures S24 and S25, Supplementary
Materials).
Table 1. The Tstart 5%, Tonset, and Tpeak of compounds 16. Tstart 5% is dened as the temperature at which
the weight loss of the sample is equal to 5% of the initial mass; Tonset is dened as the temperature at
the onset of the weight (%) vs. temperature curve; and Tpeak is dened as the temperature of the
highest peak in the dweight%/dT plot.
Compound 1 2 3 4 5 6
Tpeak (°C) 334.2 361.5 355.2 214.8 251.2 260.3
Tonset (°C) 310.7 296.0 328.7 205.3 233.2 248.1
Tstart 5% (°C) 281.2 284.5 299.7 217.2 240.2 253.1
2.3. Computational Results—DFT Studies
We chose to consider the minimum neutral clusters (dications reported in Figure 2
plus 2 Br or WO42) as a single supermolecular (or superionic) system given the fact that
the strong electrostatic interactions involved lead to signicant deformations of the
geometry of the system. The optimised structures of compounds 16 are reported in
Figure 3 (bromides 13) and Figure 4 (tungstates 46). Relative and absolute energies are
reported in Table 2 (bromides 13) and Table 3 (tungstates 46).
Systems with bromide anions are discussed rst. For each isomer, we found two local
minima with structural analogies, the rst where anions are located between the two
Molecules 2024, 29, 2131 5 of 23
imidazolium rings (internal) and a second one (external) where at least one anion is
outside the space between the rings.
Figure 2. Dications with rigid spacers studied in this paper. From left to right: ortho, meta and para
isomers. The labels indicate the C2, C4, CB[enzylic], CPh[enyl] positions used in this section.
Table 2. Energetic and selected geometrical quantities related to minimal neutral clusters with
bromide anions. The carbon atoms labels are dened in Figure 2. Distances are reported in Å. E +
ZPE are relative to the most stable isomer (meta) in the most stable conformation (internal). ∆∆E +
∆∆ZPE are the dierences between the two conformations of the same isomer. The sum of hydrogen
and bromide van der Waals radii uctuate in the range 2.97–3.15 Å according to dierent
compilations proposed in literature [63].
Isomer Ortho Meta Para
Conformer Internal External Internal External Internal External
Energies
Abs. Energies (Ha) 5988.938627 5988.939888 5988.945324 5988.927851 5988.936105 5988.930137
E + ZPE (KJ/mol) +17.58 +14.27 0.00 +45.87 +24.20 +39.87
∆∆E + ∆∆ZPE(KJ/mol) 3.31 +45.87 +15.67
distances/Å
H(C2)-Br1 2.49 - 2.66 - 2.48 2.97
H(C2)-Br2 - 2.50 3.38 2.25 - 3.01
H(C2)-Br1 - 2.41 2.42 2.89 2.48 2.36
H(C2)-Br2 2.43 - 3.95 - - -
H(C4)-Br1 2.80 - - 2.53 - -
H(C4)-Br2 2.63 - - - - -
H1(CB)-Br2 2.65 2.64 2.79 - - -
H1(CB)-Br1 2.89 2.48 - - - -
H2(CB)-Br2 - 2.70 - - - -
H1(CM)-Br1 - - - - 2.98 2.63
H2(CM)-Br2 - - - - 2.84 -
H1(CM)-Br1 - - - - 2.98 2.86
H2(CM)-Br2 - - - - 2.85 -
Table 3. Top: energetic and selected geometrical quantities related to neutral minimal neutral
clusters with tungstate anions. The carbon atom labels are dened in Figure 2. Distances are
reported in Å.
Isomer Ortho Meta Para
Abs. energies (Ha) 1209.542873 1209.586818 1209.573244
E + ZPE (KJ/mol) +115.38 0.00 +35.64
C2-H 1.08 1.16 1.09
C2-H 1.10 1.08 1.13
Molecules 2024, 29, 2131 6 of 23
H(C2)-W 2.78 2.82
H(C2)-W 2.67 2.80 2.80
H(C4)-W 3.13
H(C5)-W 3.21
H(CPh)-W 2.91
W-O (average) 1.78 1.79
(a) Ortho–internal (+17.58KJ/mol) (b) Ortho–external (+14.27KJ/mol)
(c) Meta–internal (0.00KJ/mol) (d) Meta–external (+45.87KJ/mol)
(e) Para–internal (+24.20KJ/mol) (f) Para–external (+39.87KJ/mol)
Molecules 2024, 29, 2131 7 of 23
Figure 3. Optimised geometries of the minimal neutral clusters with bromide anions (13). Colour
code for the atoms: hydrogen: white; carbon: grey; nitrogen: blue; bromine: red.
The diverse geometry of the linker leads to dierences in the spatial arrangement in
terms of relative orientation and distances between the two imidazolium pendants (Figure
3). Concerning the ortho structures in 1, in the internal conformer, the reduced distance
between the two imidazolium rings forces the two anions to move apart. Conversely, in
the external conformer, the dierent orientation of the rings allows a larger energetic
stabilization. This is the only isomer where the external structure is more stable than the
internal one. In the meta case 2, there is enough space between the cationic headgroups to
accommodate both bromide ions inside, leading to the most energetically stable
conformation of the series and to the larger internal/external energetic gap. In the para
isomer 3, the two imidazolium pendants are diametrically opposite and the dierence
between the two conformers is therefore less well dened. In the symmetric internal
structure, the anions can interact eciently with both the imidazolium rings (Figure 3e).
These structures are coherent with those reported by Verma et al. [64] where conformers
very similar to the internal structures are reported for bromides and other single-charged
anions.
The scenario changes drastically with the tungstate DILs. While in the bromide case
there are two independent anions, in the tungstate case, there is a single, spherical top and
a sterically rigid dianion paired with a sterically exible dication. In this case only, the
analogous of the internal conformers is possible.
This dierent electrostatic and geometric scenario leads to remarkable dierences in
the geometrical arrangement of the dicationic structure depending on the type of anionic
counterpart. From the electrostatic point of view, the C2-H moieties of the imidazolium
rings seek energetic stabilization by pointing toward the negative charge centre. When the
space between the two rings is small, like in the ortho tungstate 4 (Figure 4a), this is not
possible and one of the rings must ip, pointing to its less polar backside (C4 and C5)
towards the anion. On the other hand, with the same dicationic structure with two
independent anions, as in the bromide case 1, the conformation of the minimal neutral
cluster adapts to a Yin–Yang-like structure where the two C2-H are anti-parallelly
arranged, pointing to the two dierent bromides. In this case (both ortho structures in
Figure 3a,b), there is a small energetic dierence because the two structures are not so
dierent. In the meta and para cases (5 and 6), there is enough space to host the WO42
anion with the rings aligned accordingly to the most energetically favourable
arrangement.
Looking at the energetic quantities, the pocket between the rings in the meta isomer
is the most suitable for hosting both kinds of anionic counterparts, while the ortho is too
small and the para too large. The internal meta/bromides (2, Figure 3c) and the
meta/tungstate bromides (5, Figure 4b) are able to ip the benzene ring of the spacer in
order to allow one hydrogen to point toward the anions. With respect to the other isomers
where the benzene ring is parallelly oriented, this arrangement can stabilise the structure
through the polarization of the aromatic system and by enhancing the dispersive ring–
anion(s) interactions.
Molecules 2024, 29, 2131 8 of 23
(a) Ortho (+115.38 KJ/mol) (b) Meta (0.00 KJ/mol)
(c) Para (+35.64 KJ/mol)
Figure 4. Optimised geometries of the minimal neutral clusters with tungstate anions (46). Colour
code for the atoms: hydrogen: white; carbon: grey; nitrogen: blue; bromine: red; tungsten: cyan.
2.4. Computational Results—Molecular Dynamics Using the GFN2 Force Field
The dynamics of the bromide and tungstate DILs, the subject of this study (46), were
assessed using the program xtb [65]. The parameters for the optimization were set to the
standard procedure of the program; moreover, for the dynamics, the temperature was set
to 298.15 K during a 2 ns simulation with a 2 fs step, and one structure was saved every
100 fs using the GFN2 force eld [66]. The simulation was carried out in an NVT
environment with a shake algorithm for all atoms.
These calculations have been performed to check the reliability of the force eld for
these kinds of systems and the stability of the above presented structures in a less static
framework.
The RMDS analysis, reported in Figure 5, shows a very dierent behaviour for sys-
tems with dierent anions. The bromide-based systems (13), which at the ab initio level
showed two stable conformers, are more exible and show larger oscillations, while the
tungstate ones are more rigid. A similar behaviour can be concluded from the energetic
proles available in the Supplementary Materials (Figure S27).
Molecules 2024, 29, 2131 9 of 23
Molecules 2024, 29, 2131 10 of 23
Figure 5. RMSD along the simulation time to the IL using the bromide anion 1-3 (top, ortho, meta
and para conformations) and WO42 anion 4-6 (boom, ortho, meta and para conformations).
The reactivity of tungstate systems (4-6) with carbon dioxide was tested using the
same level of theory. From the study, it emerged that tungstate DILs + CO2 systems are
more energetically stable than their components for all the conformations. Interestingly,
it was observed that a high variation in energy occurred in the ortho conformation cluster
after 1.5 ns of simulation, and meta and para conformations displayed a variation in
energy during the considered time of the analysis. However, this energy reduction was
much lower when compared with the ortho cluster, and it was generated by a smaller
change in conformation that created a more stable system (Figure 6).
Figure 6. Energy variation along the simulation time to the IL using the tungstate anion (4-6) with
the CO2 molecule system (conformations ortho, meta and para). Energies were shifted by 53,500
Kcal/mol in order to handle smaller numbers.
Molecules 2024, 29, 2131 11 of 23
An RMSD analysis was performed to study the dynamic of the system and to gain a
beer understanding of the stability of the system. These data displayed characteristics
like the ones observed for energy: the systems with the WO42 anions were more stable
and presented a lower variation when compared with the Br.
This fact is due to dierent reasons:
1. The electrostatic eect of the double-charged WO42 anion is stronger. This has
several eects on the geometry of the dications, the most evident being the stretching
of the C2-H and C2-H bonds (Table 3).
2. Dierently from the WO42 systems, in the bromide case, there are two dierent
anions that can be displaced independently. The dication/2Br systems therefore has
a larger number of interionic degrees of freedom than the dication/WO42 ones. This
was also demonstrated by the existence of the external conformations in the rst case
only.
3. The WO42 ion is bulkier than the bromide one. This lead to a greater sterical
hindering and to a more rigid structure.
As a further evidence of the structural rigidity and stability of tungstate DIL
structures, the computational results concerning the interaction of these systems with CO2
showed only minimal structural variations with respect to the DIL before the
coordination. Following the insertion of the carbon dioxide molecule, it interacted with
the tungstate anion by forming an oxygen bridge, resulting in a carbonate-like structure,
as depicted in Figure S31 (Supplementary Materials). The clusters where the CO2
interacted with the WO42 anion presented a stable RMSD for the meta system, the ortho
system presented a small variation during the simulation, and the para system presented
three dierent congurations (Figure 7).
Concerning the Br- system, the para conformation cluster presented a high variation
after the rst 400 ps and presented relative stability between 800 ps and 1.4 ns; then, the
system returned to having high movement. This is due to the fact that the rigid internal
conformation (Figure 3, boom left) that presented several interactions between both the
imidazolium rings (via H(C2), H(C2), H1(CM), H2(CM), H1(CM) and H2(CM) polar
hydrogens) and the anions was very rigid, and some of these bonds were disrupted when
the geometry changed. This was only partially balanced by the simultaneous formation of
some weaker electrostatic interactions. The ortho conformation presented an instability
before 600 ps; however, it gained a more stable conformation during the remaining time,
with some minor changes between two dierent states over time. The meta conformation
provided the most stable simulation, presenting a lower variation and not manifesting
any notable change in conformations during that time (Figure 5). is the laer was also the
case when the internal/external energy gap was larger.
Concerning the WO42 systems, they proved to be stable over time, with the ortho
presenting a change in its conformation between 1.6 ns and 1.8 ns, (this conformation was
observed only in this time range, and later it changed back to the initial conguration).
This time, stability could be related to energy stability. Since the tungstate anion presented
a high volume and at least four points that could create bonds, we observed more and
stronger interactions between the cation and the anions which stabilised and made the
structure more rigid and less mobile (Figure 6). The CO2 cluster was found to be stable for
the ortho and the meta conformations, while the para system presented two variation
points, creating three dierent relatively stable conformations (Figure 7). The rst and the
second conformations occurred before 800 ps, while after this time, the system evolved
toward a third conguration which remained stable. These conformations were generated
by a modication of the imidazolium rings’ positioning to accommodate the CO2
molecule. On the other hand, the molecule of CO2 was stable and did not change in con-
formation during any of the simulations.
Molecules 2024, 29, 2131 12 of 23
(a) (b)
(c)
Figure 7. RMSD along the simulation time to the IL using the tungstate anion and CO2 molecule. (a)
red: ortho DIL tungstate (4), black: ortho DIL tungstate (4) with CO2, green: ortho dication with CO2;
(b) red: meta DIL tungstate (5), black: meta DIL tungstate (5) with CO2, green: meta dication with
CO2; (c) red: para DIL tungstate (6), black: para DIL tungstate (6) with CO2, green: para dication with
CO2.
Finally, to understand the changes that occur in these systems in the dynamic
process, the dierences between HOMO and LUMO in the rst and the last geometrical
structures of the MD runs are here reported. The structures of the DILs featuring bromide
anions did not present signicant dierences (and thus are reported in the Supplementary
Materials Figures S28–S30), while the tungstate ones are reported in Figure 8. Regarding
the systems featuring the anion WO42, the charge distribution changesd during the
simulation and created a new HOMO-LUMO conguration. At the starting point, the
HOMO region was localised on the imidazolium ring on the region between the nitrogen
atoms, while the LUMO region was found on the external oxygen atom of the WO42 anion,
meaning that the oxygen did not interact with the cation. For the last point of the
simulation, it has been observed that the HOMO region moved from the imidazolium ring
to the tungstate anion in all the considered conformations, and that the LUMO region
shifted to the central ring of the cation, indicating that the system changed its charge
conguration as a result of the interaction with the anion. Moreover, it has been observed
Molecules 2024, 29, 2131 13 of 23
that the interaction of the oxygen atom created a stable conguration with both the
imidazolium rings.
Figure 8. HOMO-LUMO analyses for the DILs with tungstate anion (46) in the ortho, meta, and
para congurations (from left to right). Colour code for the atoms: hydrogen: white; carbon: grey;
nitrogen: blue; bromine: red; tungsten: cyan.
The same analysis was performed on the systems with CO
2
(Figure 9). Regarding
these systems, at the beginning of the simulation, it was observed that not all the
congurations presented the HOMO-LUMO regions in the same positions at the start
point of the analysis. For the ortho conguration, the HOMO and the LUMO regions were
concentrated on the central ring (the LUMO was located in the other half of the ring with
respect to the HOMO), making it very reactive. The meta and para congurations instead
presented the HOMO region on the CO
2
molecule and the LUMO region on the external
oxygen atom of the WO
42
anion (the same conguration observed on the systems without
CO
2
molecules).
Figure 9. HOMO-LUMO analyses for the DILs with tungstate anion (46) with carbon dioxide in
ortho, meta, and para congurations (from left to right). Colour code for the atoms: hydrogen: white;
carbon: grey; nitrogen: blue; bromine: red; tungsten: cyan. Alternative versions of the last point
structures without orbitals are reported in Supplementary Materials (Figures S28–S30).
At the end of the simulation, the HOMO region was concentrated on the tungstate
anion (a behaviour already observed when the CO
2
molecule was not incorporated in the
studied system), and the LUMO region was concentrated on the oxygen atom of the CO
2
molecule, demonstrating the presence of an interaction between the molecule and the
anion. This observation is consistent with literature data, and the good agreement with
previous studies underlines the possibility of simulating this type of interaction [67]. The
molecular dynamics using the GFN method demonstrated a good capability to simulate
DIL systems both using a small anion (as the bromide) and a heavy anion (as the
tungstate) and the interactions between them and other molecules.
Molecules 2024, 29, 2131 14 of 23
The energy analyses demonstrated that the increase in the anion size created a more
stable system when combined with a dicationic IL and that the conformation of the
molecule played a crucial role in the stabilization of the system. Concerning the bromide
anion (13), the increase in the inter-ring space created a higher conformational energy
and increased the movement of the system because of the increase in the mobility of the
anion. However, for the tungstate anion (46), the increase in the inter-ring space created
a lower conformational energy and reduced the movement due to the increase in the
number of bonds and the beer distribution of the charge throughout the system. When
a carbon dioxide molecule was added to the tungstate system, it was observed that the
system reorganised to favour the interactions between the anion of the DIL and the CO2
molecule. As observed in the literature for similar compounds, a partial charge transfer
took place between the carbon dioxide and the tungstate [52,68]. This interaction can
activate the carbon dioxide molecule toward various reactions, and it is therefore
interesting for the design of catalysts for CO2 valorisation processes like the synthesis of
cyclic carbonates, cyclic carbamates, and urea derivatives [52,55].
2.5. Proof-of-Concept Test of Compounds 16 as Catalysts for the of Cycloaddition of CO2 to
Epoxides
Considering the good CO2 activation potential that emerged from the computational
study on the proposed tungstate DILs, compounds 46 (and their parent bromide DILs 1
3) were tested as catalysts for the cycloaddition of CO2 to epoxides (CCE reaction) (Scheme
3). Epichlorohydrin was chosen as the substrate due to its widespread use as a benchmark
epoxide and the consequent possibility of comparing it with the results in the literature
[51]. The reactions were performed with a CO2 pressure of 10 bar, a catalyst load of 1
mol%, a reaction time of 2 h, and temperatures of 80 °C and 100 °C.
Scheme 3. Schematic representation of the CCE under investigation.
The obtained results show a clear relationship between the type of spacer and the
catalytic activity of the DILs (Figure 10). In both the bromide (13) and the tungstate (46)
DILs series, the compounds featuring an o-xylene (1, 4) linker displayed the best
performances in the CCE reaction, with 1 achieving a quantitative yield after 2 h at 100 °C
and 4 reaching a yield of 82% in the same experimental conditions. All the reactions
exhibited a complete selectivity, and no byproducts were observable by NMR
spectroscopy. On the other hand, m-xylene (2, 5), and p-xylene (3, 6) DILs displayed a
lower catalytic activity when compared to 1 and 4, featuring an o-xylene spacer. Yields of
78% and 75% were observed, respectively, for bromides DILs 2 and 3, while yields of 50%
and 57% were observed, respectively, for tungstate DILs 5 and 6. The signicant dierence
in reactivity between the DILs featuring an o-xylene spacer and the other ones points out
the great inuence of the proximity of the imidazolium groups on the catalytic activity of
the compound. Concerning the inuence of the anion, bromide DILs proved more active
than their tungstate counterparts, irrespectively of the type of linker moiety. These data,
together with the computational results, provide some useful insights into the role of both
imidazolium rigid dications and tungstate anions in catalysing the CCE reaction. From
the calculations discussed in the previous section, an eective interaction between the
tungstate anion and the carbon dioxide emerged. Nevertheless, if it is true that the energy
data of DILs–CO2 clusters points out an activation eect exerted by the WO42 group on
the CO2 molecule, it is evident that this activation alone is not sucient to lead to a good
catalytic activity. Moreover, it was noted that the dication conformations (meta and para)
Molecules 2024, 29, 2131 15 of 23
leading to the most stable dication–WO42 systems were also the ones leading to lower
cyclic carbonate yields. The obtained results can be rationalised considering that the key
role in the catalytic cycle is actually played by the activation of the epoxide. As reported
in the literature, the activation of the epoxide ring is usually due to its interaction with a
nucleophile, often assisted by the interaction with the oxygen of the epoxide ring with an
electrophile (Scheme 3) [40]. Concerning the role of imidazolium systems in the
electrophilic aack, the relatively acidic Im-H2 hydrogen (on the C2 of the imidazolium
ring) is the atom more directly interacting with the epoxide. On the other hand, ionic
liquid anions are well reported in the literature to eectively act as nucleophiles in the
CCE reaction [36,40]. A nal aspect to be considered concerns the capability of both the
nucleophile and the electrophile to take part in the catalytic cycle; it is evident that
hindered groups will less eectively activate the epoxide ring. With respect to their meta
and para counterparts, ortho DILs manifest more accessible Im-H2 hydrogens according
to our computational results (Figures 3 and 4). This is particularly evident for the tungstate
series, where 4 (Figure 4a), is the only tungstate DIL with a Im-H2 hydrogen pointing
outward the ionic liquid structure. Furthermore, the higher mobility displayed by ortho
structures 1 and 4 in the computational study (and thus their higher adaptability to their
substrates) may represent another reason for their higher catalytic activity. The lower
performances displayed by tungstate DILs 46 with respect to their bromide counterparts
13 can be rationalised considering both the nucleophilicity of the anion and the eect of
WO42 on the structures of DILs 46. In fact, the divalent anion WO42 confers a higher
degree of rigidity to tungstate DILs (as discussed above in the computational section). In
the second instance, the calculations display that the WO42 anion, bulkier than the Br-
anion, tends to occupy a good part of the concavity of the dication, yielding a packed
structure where the accessibility to the groups involved in the catalysis of the CCE reaction
was hindered (Figure 4). A second set of reactions was performed under the same
experimental conditions but lowering the temperature to 80 °C in order to gain an insight
into the eect of temperature on the catalytic activity of compounds 13. Furthermore, the
data collected from the reaction performed at 80 °C are directly comparable to data from
the literature related to DILs featuring exible linker chains, thus allowing us to evaluate
the eect of rigid xylene spacers on the catalytic activity of bromide DILs. Unfortunately,
compounds 13 were found to be only partially soluble in the reaction medium at this
temperature, heavily aecting the results. The addition of epichlorohydrin carbonate (20
mol%) to the reaction mixture in order to promote the solubilisation of the catalysts only
partially solved the problem, and the reacted mixture displayed considerable amounts of
catalysts 1 and 3 still undissolved, while only 2 was found to be fully solubilised. This
behaviour is in contrast with that reported in the literature regarding DILs with linear
spacers, where the same amount of epichlorohydrin carbonate was sucient to fully
solubilise the least soluble catalyst reported in the study. It is evident that the presence of
a rigid xylene-based spacer greatly aects the solubility of imidazolium DILs in
epichlorohydrin, especially at a lower temperature Concerning the catalytic activity, the
yield obtained using the DIL 2 as catalyst in the aforementioned experimental conditions
(60%) appears consistent with that reported in the literature for bromide DILs featuring
exible propyl, butyl, pentyl, and hexyl linker chains in their structures (with yields of
57%, 62%, 67%, and 70%, respectively) [36].
Molecules 2024, 29, 2131 16 of 23
Figure 10. Results obtained for the CCE reaction performed on epichlorohydrin with bromide
catalysts 1-3 (red) and tungstate catalysts 4-6 (blue). Reaction conditions: 1 mol% catalyst, 10 bar
CO2, 100 °C, 2 h.
3. Materials and Methods
All the employed reagents and solvents were used without further purications. α,α′-
dibromo-o-xylene, α,α′-dibromo-m-xylene, and α,α′-dibromo-p-xylene were purchased
from TCI Chemicals—Europe N.V., Zwijndrecht, Belgium. N-methylimidazole, tungstic
acid, and epichlorohydrin were obtained from Alpha Aesar (Thermo Fisher—Dreieich,
Germany). Acetonitrile, ethyl acetate, methanol-d4 and sodium hydroxide were purchased
from Merck Life Science S.r.l.—Darmstadt, Germany.
Fourier transform infrared spectroscopy (FTIR) spectra were registered using a Cary
600 Series FTIR spectrometer (Agilent Technologies, Santa Clara, CA, USA). Analyses
were performed on a window of diamond with a length spanning from 5000 cm1 to 550
cm1, with 32 scans and resolution of 4 cm1. 1H-NMR and 13C-NMR spectra were recorded
with a Bruker 400 UltraShield operating at 400 MHz (1H-NMR) and 100 MHz (13C-NMR).
The chemical shifts δ are referenced to either residual CD3OD (δH 3.31, δC 49.0) or D2O (δH
4.80), and J-values are given in Hz. The following abbreviations are used: s = singlet, d =
doublet, dd = double doublet, m = multiplet. The intensity of the signal corresponding to
the proton Im-H2 is heavily aected by the fast exchange of this proton in MeOD and
D2O.
Thermal gravimetric analyses were performed using a TA Instruments Q500 TGA
(weighing precision ± 0.01%, sensitivity 0.1 µg, baseline dynamic drift < 50 µg). The Curie
point of nickel standard was used to perform the temperature calibration, while weight
standards of 1000, 500, and 100 mg were used for mass calibration. All the standards were
supplied by TA Instruments Inc. Each sample (7–14 mg) was heated in a platinum crucible
at 60 °C in N2 (100 mL/min) for 20 min. Then, the sample was heated from 40 to 700 °C
(compounds 13) or to 800 °C (compounds 46) at the heating rate of 10 °C/min under
nitrogen (50 mL/min). Mass change was recorded as a function of temperature and time.
The thermal behaviour of the ionic liquids was analyzed using a dierential scanning
calorimeter DSC 250 (with a temperature accuracy of ± 0.05 °C, temperature precision of
± 0.008 °C, and enthalpy precision of ± 0.08%). The temperature calibration was performed
considering the dependence of the heating rate on the onset temperature of the melting
peak of indium. About 2–4 mg of sample was loaded into hermetic aluminium pans. Based
Molecules 2024, 29, 2131 17 of 23
on the results of TG analyses, DSC analyses were performed at 10 °C/min in N2 (50
mL/min) in a temperature range spanning from 90 to 150 °C. The sample underwent
cycles of cooling and heating to the selected temperatures using scanning rates of 10
°C/min.
An EYELA PROCESS STATION PPV-4060 reactor (StepBio, Bologna, Italy), equipped
with four autoclaves HIP-60 of stainless steel, was used for the cycloaddition of CO2 to
epoxides. Each autoclave was equipped with a glass liner of 150 mL with a magnetic
stirrer.
The geometrical structure of minimal neutral clusters was optimised using a
Gaussian suite of programs with the B3LYP function, using the basis set with eective core
potential LanL2DZ that allowed us to handle heavy atoms like tungsten and bromide at a
reasonable computational cost [69,70]. All calculations were performed using the
Gaussian16 package [71]. Some molecular dynamics studies with the xtb package, using
the GFN2 tight-binding DFT [72], were performed on the six clusters studied above at the
ab initio level. All the runs were 2000 ps in length. The starting structures corresponded
to the optimised energies at the ab initio level.
3.1. General Procedure for the Synthesis of 1,1-(1,n-Phenylenebis(methylene))bis
(3-methylimidazolium) Bromides (13)
A variation of the synthetic procedure reported by Magill et al. was followed for the
preparation of compounds 13 [73]. In a ask containing a magnetic stirrer, 3.79 mmol
(1.000 g) of the selected α,α′-dibromoxylene and 4 mL of CH3CN were added. To this
mixture, a solution of 7.96 mmol (0.654 g) of N-methylimidazole in 6 mL of CH3CN was
added dropwise, and the resulting mixture was heated to reux overnight (80 °C). After
cooling, the solution was concentrated, and the resulting solid was washed with 3 portions
of 15 mL of EtOAc each then dried under reduced pressure.
3.2. 1,1-(1,2-Phenylenebis(methylene))bis(3-methylimidazolium) Bromide (1)
The title compound, prepared according to the general procedure using α,α′-
dibromo-o-xylene, was recovered in a quantitative yield as a hygroscopic white solid.
1H-NMR (MeOD) δ: 9.06 (s, 2H, 2 × Im-H2), 7.64, 7.62 (2d, each 2H, Jvic = 2.0 Hz, 2 ×
Im-H4, 2 × Im-H5), 7.54, 7.41 (2m, each 2H, 4 × Ar-H), (m, 2H, Ar-H), 5.69 (s, 4H, 2 ×
PhCH2N), 3.97 (s, 6H, 2 × NCH3). 13C-NMR (MeOD) δ 138.1 (2 × Im-C2), 133.6 (2 × Ar-C),
131.5, 131.4 (4 × Ar-CH), 125.3, 123.8 (2 × Im-C4, 2 × Im-C5), 51.6 (2 × PhCH2N), 36.9 (2 ×
NCH3).
FTIR (cm1): 3136.3, 3073.8, 2938.8, 1645.0, 1560.4, 1453.6, 1427.9, 1362.9, 1334.9, 1156.7,
1091.6, 829.7, 754.8, 737.5, 660.0, 622.0.
The obtained NMR results (1H, 13C) were consistent with those reported in literature
in DMSO [73].
3.3. 1,1-(1,3-Phenylenebis(methylene))bis(3-methylimidazolium) Bromide (2)
The title compound, prepared according to the general procedure using α,α′-
dibromo-m-xylene, was recovered in a quantitative yield as a hygroscopic white solid.
1H-NMR (MeOD) δ: 9.15 (s, 2H, 2 × Im-H2), 7.68 (m, 3H, 2 × Im-H4 or 2 × Im-H5 and
Ar-H), 7.61(d, 2H, Jvic = 2.0 Hz, 2 × Im-H4 or 2 × Im-H5), 7.51 (s, 3H, Ar-H), 5.50 (s, 4H, 2 ×
PhCH2N), 3.96 (s, 6H, 2 × NCH3). 13C-NMR (MeOD) δ 138.3 (2 × Im-C2), 136.5 (2 × Ar-C),
131.4, 130.5, 130.2 (4 × Ar-CH), 125.3, 123.7 (2 × Im-C4, 2 × Im-C5), 53.5 (2 × PhCH2N), 36.8
(2 × NCH3).
FTIR (cm1): 3138.2, 3063.4, 3024.2, 2952.5, 1651.4, 1577.3, 1557.3, 1452.4, 1427.4, 1361.6,
1337.9, 1170.1, 865.5, 787.1, 753.3, 737.1, 620.1.
The obtained NMR results (1H, 13C) were consistent with those reported in literature
in DMSO [73].
Molecules 2024, 29, 2131 18 of 23
3.4. 1,1-(1,4-Phenylenebis(methylene))bis(3-methylimidazolium) Bromide (3)
The title compound, prepared according to the general procedure using α,α′-
dibromo-p-xylene, was recovered in a quantitative yield as a hygroscopic white solid.
1H-NMR (MeOD) δ: 9.10 (s, 2H, 2 × Im-H2), 7.65, 7.61 (2d, each 2H, Jvic = 2.0 Hz, 2 ×
Im-H4, 2 × Im-H5), 7.55 (s, 4H, 4 × Ar-H), 5.48 (s, 4H, 2 × PhCH2N), 3.95 (s, 6H, 2 × NCH3).
13C-NMR (MeOD) δ 138.2 (2 × Im-C2), 136.4 (2 × Ar-C), 130.7 (4 × Ar-CH), 125.3, 123.7 (2 ×
Im-C4, 2 × Im-C5), 53.5 (2 × PhCH2N), 36.8 (2 × NCH3).
FTIR (cm1): 3109.0, 3037.9, 2936.7, 1645.7, 1572.0, 1557.6, 1361.0, 1323.8, 1161.5, 842.9,
762.3, 732.4, 618.3.
The obtained NMR results (1H, 13C) were consistent with those reported in literature
in DMSO [74].
3.5. General Procedure for the Synthesis of 1,1-(1,N-phenylenebis(methylene))bis
(3-methylimidazolium) Tungstates (46)
Compounds 46 were synthetised following a procedure previously reported by our
research group [29]. Amberlite IRA 400 ion exchange resin (20 g) was rstly activated by
stirring in NaOH 1M for 1 day at room temperature with a tilting stirrer. The resin was
then packed in a column and washed several times with water until the excess of NaOH
was eliminated. Prior to use, the column was conditioned with a solution of 75:25 (v/v)
MeOH-H2O.
In a ask containing a magnetic stirrer, the selected bromide DIL (1.18 mmol, 0.505
g) was dissolved in 50 mL of a solution of 75:25 (v/v) MeOH-H2O and owed through the
column packed with the activated and conditioned IRA 400 resin. At the end of each
passage through the column, the completion of the ion exchange was tested by AgNO3
essay to reveal the presence of residual halide anions. The test was performed by collecting
a few drops of the solution after the column and diluting them up to 0.5 mL with deionised
water. The obtained sample was neutralised by adding a drop of nitric acid (the
neutralisation was checked with pH paper, and an excess of acid was found to not aect
negatively the outcome of the essay). Finally, a drop of an aqueous solution of AgNO3 (5%
w/w) was added to the sample. The formation of a white turbidity in the rst seconds after
the addition of the silver solution indicated the presence of residual halide anions. The
DILs solution was recovered and owed through the column until the test turned
negative. A stoichiometric amount of H2WO4 (0.59 mmol, 0.147 g) was then added to the
solution of DIL hydroxide and reacted for a night at 50 °C. The desired products were
obtained in excellent yields after removal of the solvent under reduced pressure. The ion
exchange resin was later regenerated with 250 mL NaOH 1M.
3.6. 1,1-(1,2-Phenylenebis(methylene))bis(3-methylimidazolium) Tungstate (4)
The title compound was obtained as a hygroscopic pale yellow glassy solid with a
yield of 95.1%.
1H-NMR (D2O) δ: 8.66 (s, 2H, 2 × Im-H2), 7.64 (m, 2H, 2 × Ar-H), 7.50, 7.40 (2d, each
2H, Jvic = 2.0 Hz, 2 × Im-H4, 2 × Im-H5), 7.48 (m, 2H, 2 × Ar-H), 5.54 (s, 4H, 2 × PhCH2N), 3.89
(s, 6H, 2 × NCH3). 13C-NMR (D2O) δ: 136.2, (2 × Im-C2), 131.5 (2 × Ar-C), 131.1, 130.6 (4 ×
Ar-CH), 123.8, 122.2 (2 × Im-C4, 2 × Im-C5), 50.0 (2 × PhCH2N), 35.8 (2 × NCH3).
FTIR (cm1): 3139.3, 3066.0, 1650.0, 1570.7, 1452.9, 1159.5, 793.2.
3.7. 1,1-(1,3-Phenylenebis(methylene))bis(3-methylimidazolium) Tungstate (5)
The title compound was obtained as a hygroscopic pale yellow glassy solid (92.9%
yield).
1H-NMR (D2O) δ: 8.79 (s, 2H, 2 × Im-H2), 7.57 (m, 1H, Ar-H), 7.46 (m, 7H, 2 × Im-H4, 2
× Im-H5 and 3 × Ar-H), 5.44 (s, 4H, 2 × PhCH2N), 3.92 (s, 6H, 2 × NCH3). 13C-NMR (D2O) δ:
136.1 (2 × Im-C2), 134.6 (2 × Ar-C), 130.2, 129.2, 128.5 (4 × Ar-CH), 123.8, 122.2 (2 × Im-C4, 2
× Im-C5), 52.4 (2 × PhCH2N), 35.8 (2 × NCH3).
Molecules 2024, 29, 2131 19 of 23
FTIR (cm1): 3138.4, 3065.0, 2849.7, 1648.0, 1560.3, 1449.4, 1164.9, 792.9.
3.8. 1,1-(1,4-Phenylenebis(methylene))bis(3-methylimidazolium) Tungstate (6)
The title compound was obtained as a hygroscopic white solid (94.9% yield).
1H-NMR (D2O) δ: 8.76 (s, 2H, 2 × Im-H2), 7.47–7.44 (m, 8H, 2 × Im-H4, 2 × Im-H5 and 4
× Ar-H), 5.42 (s, 4H, 2 × PhCH2N), 3.88 (s, 6H, 2 × NCH3). 13C-NMR (D2O) δ: 136.1 (2 × Im-
C2), 134.5 (2 × Ar-C), 129.3 (4 × Ar-CH), 123.9, 122.3 (2 × Im-C4, 2 × Im-C5), 52.3 (2 × PhCH2N),
35.8 (2 × NCH3).
FTIR (cm1): 3138.0, 3091.4, 1650.3, 1557.7, 1454.1, 1430.0, 1154.5, 785.4, 723.6.
3.9. General Procedure for the DILs Catalyzed Cycloaddition of CO2 to Epichlorohydrin
Epichlorohydrin (86.1 mmol, 7.966 g) and the catalyst (1% mol) were stirred at the
prexed temperature and at 10 bar of CO2 pressure for 2 h (the details of the employed
reaction conditions will be described and discussed in the results and discussion section).
The yield was evaluated by 1H-NMR on the crude reaction mixture as an average of three
runs. The uncertainty of the reported yields is always within ±3%.
4. Conclusions
In this paper, the chemical space of dicationic ionic liquids (DILs) was further
expanded by the synthesis, characterization, and analysis of a new class of compounds
composed of dierent dications with aromatic spacers coupled with bromide and
tungstate anions.
The isomerism of the rigid spacers here considered (ortho, meta and para) leads to
dierent potential conformations of the dications and dierent potential relative positions
of the cationic and the anionic moieties. By computational means, it emerged that meta
and para isomers presented a pocket between the two imidazolium rings that could host
the bromide anions (or the double-charged tungstate anion), while in the case of the ortho
isomer, this was not sterically possible. The molecular dynamics studies showed a good
complexity and several potential organizations of these systems. Moreover, the
computational study pointed out that the double-charged tungstate anion, when it could
be well accommodated in the dication pocket, led to more stable structures. The lower
thermal stability manifested by both the ortho DILs with respect to their meta and para
counterparts is coherent with these computational results.
The use of these systems as catalysts in the cyclic carbonate reaction provided strong
evidence of the inuence of the conformation and of the system mobility on their chemical
properties. On one hand, the anion type proved decisive in dening the catalytic activity
of the studied DILs, with bromide systems 13 clearly outperforming their tungstate
counterparts 46. On the other hand, the type of spacer proved equally important in both
the series of DILs, with the ortho DILs showing a remarkably higher activity when
compared with the corresponding meta and the para isomers. Based on the insights into
the structural conformations of the proposed systems obtained in the computational
study, it can be noted that the catalytic activity appeared intimately connected with the
structural exibility and adaptability of the catalyst. More specically, the good catalytic
performances of the ortho DILs were associated with lower cluster stabilities at the
computational level. Furthermore, the computational data showed that in the ortho
structures, the pocket formed by the imidazolium heads was too small to properly host
the anions, allowing both the anions and the cationic heads to be more freely accessible
by the epoxide substrate. The computational results nally indicated that the studied
tungstate DILs could also have an activating eect on carbon dioxide, with the obtained
data highlighting the existence of eective interactions between the WO42 anion and the
molecule of CO2. Nevertheless, the experimental evidence obtained in this study shows
that the catalytic role of the tested DILs is mainly determined by their interactions with
the epoxide ring. The formation of the tungstate DILs—CO2 clusters still represents an
Molecules 2024, 29, 2131 20 of 23
interesting result, and the proposed systems may display good potential in applications
in which the importance of the direct activation of carbon dioxide is preponderant.
Supplementary Materials: The following supporting information can be downloaded at:
hps://www.mdpi.com/article/10.3390/molecules29092131/s1, Figure S1. 1H-NMR spectrum of 1,1ʹ-
(1,2-phenylenebis(methylene))bis(3-methylimidazolium) bromide (1). Figure S2. 13C-NMR
spectrum of 1,1ʹ-(1,2-phenylenebis(methylene))bis(3-methylimidazolium) bromide (1). Figure S3.
1H-NMR spectrum of 1,1ʹ-(1,3-phenylenebis(methylene))bis(3-methylimidazolium) bromide (2).
Figure S4. 13C-NMR spectrum of 1,1ʹ-(1,3-phenylenebis(methylene))bis(3-methylimidazolium)
bromide (2). Figure S5. 1H-NMR spectrum of 1,1ʹ-(1,4-phenylenebis(methylene))bis(3-
methylimidazolium) bromide (3). Figure S6. 13C-NMR spectrum of 1,1ʹ-(1,4-
phenylenebis(methylene))bis(3-methylimidazolium) bromide (3). Figure S7. 1H-NMR spectrum of
1,1ʹ-(1,2-phenylenebis(methylene))bis(3-methylimidazolium) tungstate (4). Figure S8. 13C-NMR
spectrum of 1,1ʹ-(1,2-phenylenebis(methylene))bis(3-methylimidazolium) tungstate (4). Figure S9.
1H-NMR spectrum of 1,1ʹ-(1,3-phenylenebis(methylene))bis(3-methylimidazolium) tungstate (5).
Figure S10. 13C-NMR spectrum of 1,1ʹ-(1,3-phenylenebis(methylene))bis(3-methylimidazolium)
tungstate (5). Figure S11. 1H-NMR spectrum of 1,1ʹ-(1,4-phenylenebis(methylene))bis(3-
methylimidazolium) tungstate (6). Figure S12. 13C-NMR spectrum of 1,1ʹ-(1,4-
phenylenebis(methylene))bis(3-methylimidazolium) tungstate (6). Figure S13. ATR-FTIR spectrum
of 1,1ʹ-(1,2-phenylenebis(methylene))bis(3-methylimidazolium) bromide (1). Figure S14. ATR-FTIR
spectrum of 1,1ʹ-(1,3-phenylenebis(methylene))bis(3-methylimidazolium) bromide (2). Figure S15.
ATR-FTIR spectr um of 1,1ʹ-(1,4-phenylenebis(methylene))bis(3-methylimidazolium) bromide (3).
Figure S16. ATR-FTIR spectrum of 1,1ʹ-(1,2-phenylenebis(methylene))bis(3-methylimidazolium)
tungstate (4). Figure S17. ATR-FTIR spectrum of 1,1ʹ-(1,3-phenylenebis(methylene))bis(3-
methylimidazolium) tungstate (5). Figure S18. ATR-FTIR spectrum of 1,1ʹ-(1,4-
phenylenebis(methylene))bis(3-methylimidazolium) tungstate (6). Figure S19. ATR-FTIR spectrum
of sodium tungstate. Figure S20. ATR-FTIR spectrum of tungstic acid. Figure S21. Thermal
gravimetric analysis and derivative of 1,1ʹ-(1,2-phenylenebis(methylene))bis(3-methylimidazolium)
bromide (1). Figure S22. Thermal gravimetric analysis and derivative of 1,1ʹ-(1,3-
phenylenebis(methylene))bis(3-methylimidazolium) bromide (2). Figure S23. Thermal gravimetric
analysis and derivative of 1,1ʹ-(1,4-phenylenebis(methylene))bis(3-methylimidazolium) bromide (3).
Figure S24. Thermal gravimetric analysis and derivative of 1,1ʹ-(1,2-phenylenebis(methylene))bis(3-
methylimidazolium) tungstate (4). Figure S25. Thermal gravimetric analysis and derivative of 1,1ʹ-
(1,3-phenylenebis(methylene))bis(3-methylimidazolium) tungstate (5). Figure S26. Thermal
gravimetric analysis and derivative of 1,1ʹ-(1,4-phenylenebis(methylene))bis(3-methylimidazolium)
tungstate (6). Figure S27. Energy variation along the simulation time to the IL using the Bromide
anion (Conformations Ortho, Meta and Para). Figure S28. HOMO-LUMO analyses for the IL with
bromide anion in the conguration ortho. Figure S29. HOMO-LUMO analyses for the IL with
bromide anion in the conguration meta. Figure S30. HOMO-LUMO analyses for the IL with
bromide anion in the conguration para. Figure S31. Structure of cluster with CO2. Ortho
conguration. Figure S32. Structure of cluster with CO2. Meta conguration. Figure S33. Structure
of cluster with CO2. Para conguration.
Author Contributions: Conceptualization, C.S.P. and L.G. (Lorenzo Guazzelli); methodology, C.F.
and G.C.G.; software, G.C.G. and C.S.P.; investigation, C.F., L.G. (Luca Guglielmero), G.C.G. and
C.S.P.; data curation, C.F. and G.C.G.; writing—original draft preparation, L.G. (Luca Guglielmero);
writing—review and editing, L.G. (Lorenzo Guazzelli), L.G. (Luca Guglielmero), F.D., A.M. and
C.S.P.; visualization, C.S.P. and L.G. (Luca Guglielmero).; supervision, C.S.P. All authors have read
and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data are contained within the article or the Supplementary Materials.
Conicts of Interest: The authors declare no conicts of interest. The funders had no role in the
design of the study; in the collection, analyses, or interpretation of data; in the writing of the
manuscript; or in the decision to publish the results.
Molecules 2024, 29, 2131 21 of 23
References
1. Singh, S.K.; Savoy, A.W. Ionic liquids synthesis and applications: An overview. J. Mol. Liq. 2020, 297, 112038.
https://doi.org/10.1016/j.molliq.2019.112038.
2. Silva, S.S.; Mano, J.F.; Reis, R.L. Ionic liquids in the processing and chemical modification of chitin and chitosan for biomedical
applications. Green Chem. 2017, 19, 1208–1220. https://doi.org/10.1039/c6gc02827f.
3. Brandt-Talbot, A.; Gschwend, F.J.V.; Fennell, P.S.; Lammens, T.M.; Tan, B.; Weale, J.; Hallett, J.P. An economically viable ionic
liquid for the fractionation of lignocellulosic biomass. Green Chem. 2017, 19, 3078–3102. https://doi.org/10.1039/c7gc00705a.
4. Toledo Hijo, A.A.C.; Maximo, G.J.; Costa, M.C.; Batista, E.A.C.; Meirelles, A.J.A. Applications of Ionic Liquids in the Food and
Bioproducts Industries. ACS Sustain. Chem. Eng. 2016, 4, 5347–5369. https://doi.org/10.1021/acssuschemeng.6b00560.
5. Berga, L.; Bruce, I.; Nicol, T.W.J.; Holding, A.J.; Isobe, N.; Shimizu, S.; Walker, A.J.; Reid, J.E.S.J. Cellulose dissolution and
regeneration using a non-aqueous, non-stoichiometric protic ionic liquid system. Cellulose 2020, 27, 9593–9603.
https://doi.org/10.1007/s10570-020-03444-8.
6. Isik, M.; Sardon, H.; Mecerreyes, D. Ionic Liquids and Cellulose: Dissolution, Chemical Modification and Preparation of New
Cellulosic Materials. Int. J. Mol. Sci. 2014, 15, 11922–11940. https://doi.org/10.3390/ijms150711922.
7. Chiappe, C.; Pomelli, C.S. Point-Functionalization of Ionic Liquids: An Overview of Synthesis and Applications. Eur. J. Org.
Chem. 2014, 2014, 6120–6139. https://doi.org/10.1002/ejoc.201402093.
8. Hallett, J.P.; Welton, T. Room-Temperature Ionic Liquids: Solvents for Synthesis and Catalysis. 2. Chem. Rev. 2011, 111, 3508–
3576. https://doi.org/10.1021/cr1003248.
9. Ni, B.; Headley, A.D. Ionic-Liquid-Supported (ILS) Catalysts for Asymmetric Organic Synthesis. Chem. A Eur. J. 2010, 16, 4426–
4436. https://doi.org/10.1002/chem.200902747.
10. Piatti, E.; Guglielmero, L.; Tofani, G.; Mezzetta, A.; Guazzelli, L.; D’Andrea, F.; Roddaro, S.; Pomelli, C.S. Ionic liquids for
electrochemical applications: Correlation between molecular structure and electrochemical stability window. J. Mol. Liq. 2022,
364, 120001. https://doi.org/10.1016/j.molliq.2022.120001.
11. Huie, M.M.; DiLeo, R.A.; Marschilok, A.C.; Takeuchi, K.J.; Takeuchi, E.S. Ionic Liquid Hybrid Electrolytes for Lithium-Ion
Batteries: A Key Role of the Separator–Electrolyte Interface in Battery Electrochemistry. ACS Appl. Mater. Interfaces 2015, 7,
11724–11731. https://doi.org/10.1021/acsami.5b00496.
12. Watanabe, M.; Thomas, M.L.; Zhang, S.; Ueno, K.; Yasuda, T.; Dokko, K. Application of Ionic Liquids to Energy Storage and
Conversion Materials and Devices. Chem. Rev. 2017, 117, 7190–7239. https://doi.org/10.1021/acs.chemrev.6b00504.
13. Jónsson, E. Ionic liquids as electrolytes for energy storage applications—A modelling perspective. Energy Storage Mater. 2020,
25, 827–835. https://doi.org/10.1016/j.ensm.2019.08.030.
14. Noack, J.; Roznyatovskaya, N.; Herr, T.; Fischer, P. The Chemistry of Redox-Flow Batteries. Angew. Chem. Int. Ed. 2015, 54, 9776–
9809. https://doi.org/10.1002/anie.201410823.
15. Egorova, K.S.; Gordeev, E.G.; Ananikov, V.P. Biological Activity of Ionic Liquids and Their Application in Pharmaceutics and
Medicine. Chem. Rev. 2017, 117, 7132–7189. https://doi.org/10.1021/acs.chemrev.6b00562.
16. Demurtas, M.; Onnis, V.; Zucca, P.; Rescigno, A.; Lachowicz, J.I.; De Villiers Engelbrecht, L.; Nieddu, M.; Ennas, G.; Scano, A.;
Mocci, F.; et al. Cholinium-Based Ionic Liquids from Hydroxycinnamic Acids as New Promising Bioactive Agents: A Combined
Experimental and Theoretical Investigation. ACS Sustain. Chem. Eng. 2021, 9, 2975–2986.
https://doi.org/10.1021/acssuschemeng.1c00090.
17. Cao, Y.; Mu, T. Comprehensive Investigation on the Thermal Stability of 66 Ionic Liquids by Thermogravimetric Analysis. Ind.
Eng. Chem. Res. 2014, 53, 8651–8664. https://doi.org/10.1021/ie5009597.
18. Fox, D.M.; Gilman, J.W.; Morgan, A.B.; Shields, J.R.; Maupin, P.H.; Lyon, R.E.; De Long, H.C.; Trulove, P.C. Flammability and
Thermal Analysis Characterization of Imidazolium-Based Ionic Liquids. Ind. Eng. Chem. Res. 2008, 47, 6327–6332.
https://doi.org/10.1021/ie800665u.
19. Kazemiabnavi, S.; Zhang, Z.; Thornton, K.; Banerjee, S. Electrochemical Stability Window of Imidazolium-Based Ionic Liquids
as Electrolytes for Lithium Batteries. J. Phys. Chem. B 2016, 120, 5691–5702. https://doi.org/10.1021/acs.jpcb.6b03433.
20. Cimini, A.; Palumbo, O.; Simonetti, E.; De Francesco, M.; Appetecchi, G.B.; Fantini, S.; Lin, R.; Falgayrat, A.; Paolone, A.
Decomposition temperatures and vapour pressures of selected ionic liquids for electrochemical applications. J. Therm. Anal.
Calorim. 2020, 142, 1791–1797. https://doi.org/10.1007/s10973-020-10334-5.
21. Pedro, S.N.; Freire, C.S.R.; Silvestre, A.J.D.; Freire, M.G. The Role of Ionic Liquids in the Pharmaceutical Field: An Overview of
Relevant Applications. Int. J. Mol. Sci. 2020, 21, 8298. https://doi.org/10.3390/ijms21218298.
22. Xu, C.; Cheng, Z. Thermal Stability of Ionic Liquids: Current Status and Prospects for Future Development. Processes 2021, 9,
337. https://doi.org/10.3390/pr9020337.
23. Renier, O.; Bousrez, G.; Yang, M.; Hoelter, M.; Mallick, B.; Smetana, V.; Mudring, A.-V. Developing design tools for introducing
and tuning structural order in ionic liquids. CrystEngComm 2021, 23, 1785–1795. https://doi.org/10.1039/d0ce01672a.
24. Majhi, D.; Seth, S.; Sarkar, M. Differences in the behavior of dicationic and monocationic ionic liquids as revealed by time
resolved-fluorescence, NMR and fluorescence correlation spectroscopy. Phys. Chem. Chem. Phys. 2018, 20, 7844–7856.
https://doi.org/10.1039/c7cp08630j.
25. Anderson, J.L.; Ding, R.; Ellern, A.; Armstrong, D.W. Structure and Properties of High Stability Geminal Dicationic Ionic
Liquids. J. Am. Chem. Soc. 2005, 127, 593–604. https://doi.org/10.1021/ja046521u.
26. Davis, J.H. Task-Specific Ionic Liquids. Chem. Lett. 2004, 33, 1072–1077. https://doi.org/10.1246/cl.2004.1072.
Molecules 2024, 29, 2131 22 of 23
27. Giernoth, R. Task-Specific Ionic Liquids. Angew. Chem. Int. Ed. 2010, 49, 2834–2839. https://doi.org/10.1002/anie.200905981.
28. Javaherian, M.; Saghanezhad, S.J. Synthesis, Characterization and Applications of Dicationic Ionic Liquids in Organic Synthesis.
Mini-Rev. Org. Chem. 2020, 17, 450–464. https://doi.org/10.2174/1570193x16666190508091231.
29. Guglielmero, L.; Mezzetta, A.; Guazzelli, L.; Pomelli, C.S.; D’Andrea, F.; Chiappe, C. Systematic Synthesis and Properties Eval-
uation of Dicationic Ionic Liquids, and a Glance into a Potential New Field. Front. Chem. 2018, 6, 612.
https://doi.org/10.3389/fchem.2018.00612.
30. Shirota, H.; Mandai, T.; Fukazawa, H.; Kato, T. Comparison between Dicationic and Monocationic Ionic Liquids: Liquid Den-
sity, Thermal Properties, Surface Tension, and Shear Viscosity. J. Chem. Eng. Data 2011, 56, 2453–2459.
https://doi.org/10.1021/je2000183.
31. Vieira, J.C.; Villetti, M.A.; Frizzo, C.P. Thermal stability and decomposition mechanism of dicationic imidazolium-based ionic
liquids with carboxylate anions. J. Mol. Liq. 2021, 330, 115618. https://doi.org/10.1016/j.molliq.2021.115618.
32. Bender, C.; Kuhn, B.; Farias, C.; Ziembowicz, F.; Beck, T.; Frizzo, C. Thermal Stability and Kinetic of Decomposition of Mono-
and Dicationic Imidazolium-Based Ionic Liquids. J. Braz. Chem. Soc. 2019, 30, 2199–2209. https://doi.org/10.21577/0103-
5053.20190114.
33. Bortolini, O.; Chiappe, C.; Fogagnolo, M.; Massi, A.; Pomelli, C.S. Formation, Oxidation, and Fate of the Breslow Intermediate
in the N-Heterocyclic Carbene-Catalyzed Aerobic Oxidation of Aldehydes. J. Org. Chem. 2017, 82, 302–312.
https://doi.org/10.1021/acs.joc.6b02414.
34. Bortolini, O.; Chiappe, C.; Fogagnolo, M.; Giovannini, P.P.; Massi, A.; Pomelli, C.S.; Ragno, D. An insight into the mechanism
of the aerobic oxidation of aldehydes catalyzed by N-heterocyclic carbenes. Chem. Commun. 2014, 50, 2008–2011.
https://doi.org/10.1039/c3cc48929a.
35. Mezzetta, A.; Guglielmero, L.; Mero, A.; Tofani, G.; D’andrea, F.; Pomelli, C.S.; Guazzelli, L. Expanding the Chemical Space of
Benzimidazole Dicationic Ionic Liquids. Molecules 2021, 26, 4211. https://doi.org/10.3390/molecules26144211.
36. Guglielmero, L.; Mezzetta, A.; Pomelli, C.S.; Chiappe, C.; Guazzelli, L. Evaluation of the effect of the dicationic ionic liquid
structure on the cycloaddition of CO2 to epoxides. J. CO2 Util. 2019, 34, 437–445. https://doi.org/10.1016/j.jcou.2019.07.034.
37. Zhang, H.; Li, M.; Yang, B. Design, Synthesis, and Analysis of Thermophysical Properties for Imidazolium-Based Geminal Di-
cationic Ionic Liquids. J. Phys. Chem. C 2018, 122, 2467–2474. https://doi.org/10.1021/acs.jpcc.7b09315.
38. Ferdeghini, C.; Mezzetta, A.; D’andrea, F.; Pomelli, C.S.; Guazzelli, L.; Guglielmero, L. The Structure–Property Relationship of
Pyrrolidinium and Piperidinium-Based Bromide Organic Materials. Materials 2022, 15, 8483.
https://doi.org/10.3390/ma15238483.
39. Ferdeghini, C.; Guazzelli, L.; Pomelli, C.S.; Ciccioli, A.; Brunetti, B.; Mezzetta, A.; Ciprioti, S.V. Synthesis, thermal behavior and
kinetic study of N-morpholinium dicationic ionic liquids by thermogravimetry. J. Mol. Liq. 2021, 332, 115662.
https://doi.org/10.1016/j.molliq.2021.115662.
40. Anthofer, M.H.; Wilhelm, M.E.; Cokoja, M.; Drees, M.; Herrmann, W.A.; Kühn, F.E. Hydroxy-Functionalized Imidazolium Bro-
mides as Catalysts for the Cycloaddition of CO2 and Epoxides to Cyclic Carbonates. ChemCatChem 2015, 7, 94–98.
https://doi.org/10.1002/cctc.201402754.
41. Liu, M.; Liang, L.; Liang, T.; Lin, X.; Shi, L.; Wang, F.; Sun, J. Cycloaddition of CO2 and epoxides catalyzed by dicationic ionic
liquids mediated metal halide: Influence of the dication on catalytic activity. J. Mol. Catal. A Chem. 2015, 408, 242–249.
https://doi.org/10.1016/j.molcata.2015.07.032.
42. Bahadori, L.; Boyd, R.; Warrington, A.; Shafeeyan, M.; Nokian, P. Evaluation of ionic liquids as electrolytes for vanadium redox
flow batteries. J. Mol. Liq. 2020, 317, 114017. https://doi.org/10.1016/j.molliq.2020.114017.
43. Zhang, D.; Liu, Q.; Shi, X.; Li, Y. Tetrabutylammonium hexafluorophosphate and 1-ethyl-3-methyl imidazolium hexafluoro-
phosphate ionic liquids as supporting electrolytes for non-aqueous vanadium redox flow batteries. J. Power Sources 2012, 203,
201–205. https://doi.org/10.1016/j.jpowsour.2011.10.136.
44. Bahadori, L.; Hashim, M.A.; Manan, N.S.A.; Mjalli, F.S.; AlNashef, I.M.; Brandon, N.P.; Chakrabarti, M.H. Investigation of Am-
monium- and Phosphonium-Based Deep Eutectic Solvents as Electrolytes for a Non-Aqueous All-Vanadium Redox Cell. J.
Electrochem. Soc. 2016, 163, A632–A638. https://doi.org/10.1149/2.0261605jes.
45. Guglielmero, L.; Langroudi, M.M.; Al Khatib, M.; de Oliveira, M.A.C.; Mecheri, B.; De Leo, M.; Mezzetta, A.; Guazzelli, L.;
Giglioli, R.; Epifanio, A.D.; et al. Electrochemical and spectroscopic study of vanadyl acetylacetonate–ionic liquids interactions.
Electrochim. Acta 2021, 373, 137865. https://doi.org/10.1016/j.electacta.2021.137865.
46. Wang, K.; Adidharma, H.; Radosz, M.; Wan, P.; Xu, X.; Russell, C.K.; Tian, H.; Fan, M.; Yu, J. Recovery of rare earth elements
with ionic liquids. Green Chem. 2017, 19, 4469–4493. https://doi.org/10.1039/c7gc02141k.
47. Salminen, J.; Blomberg, P.; Mäkinen, J.; Räsänen, L. Environmental aspects of metals removal from waters and gold recovery.
AIChE J. 2015, 61, 2739–2748. https://doi.org/10.1002/aic.14917.
48. Ahmad, M.G.; Chanda, K. Ionic liquid coordinated metal-catalyzed organic transformations: A comprehensive review. Co-ord.
Chem. Rev. 2022, 472, 214769. https://doi.org/10.1016/j.ccr.2022.214769.
49. Guglielmero, L.; Mero, A.; Mezzetta, A.; Tofani, G.; D’Andrea, F.; Pomelli, C.; Guazzelli, L. Novel access to ionic liquids based
on trivalent metal–EDTA complexes and their thermal and electrochemical characterization. J. Mol. Liq. 2021, 340, 117210.
https://doi.org/10.1016/j.molliq.2021.117210.
50. Liu, F.; Yu, J.; Qazi, A.B.; Zhang, L.; Liu, X. Metal-Based Ionic Liquids in Oxidative Desulfurization: A Critical Review. Environ.
Sci. Technol. 2021, 55, 1419–1435. https://doi.org/10.1021/acs.est.0c05855.
Molecules 2024, 29, 2131 23 of 23
51. Bragato, N.; Perosa, A.; Selva, M.; Fiorani, G.; Calmanti, R. Molybdate ionic liquids as halide-free catalysts for CO2 fixation into
epoxides. Green Chem. 2023, 25, 4849–4860. https://doi.org/10.1039/d2gc04475g.
52. Calmanti, R.; Selva, M.; Perosa, A. Tungstate ionic liquids as catalysts for CO2 fixation into epoxides. Mol. Catal. 2020, 486,
110854. https://doi.org/10.1016/j.mcat.2020.110854.
53. Schmidt, F.; Zehner, B.; Korth, W.; Jess, A.; Cokoja, M. Ionic liquid surfactants as multitasking micellar catalysts for epoxidations
in water. Catal. Sci. Technol. 2020, 10, 4448–4457. https://doi.org/10.1039/d0cy00673d.
54. Kimura, T.; Sunaba, H.; Kamata, K.; Mizuno, N. Efficient [WO4]2–-Catalyzed Chemical Fixation of Carbon Dioxide with 2-Ami-
nobenzonitriles to Quinazoline-2,4(1H,3H)-diones. Inorg. Chem. 2012, 51, 13001–13008. https://doi.org/10.1021/ic302110a.
55. Kamata, K.; Kimura, T.; Sunaba, H.; Mizuno, N. Scope of chemical fixation of carbon dioxide catalyzed by a bifunctional mon-
omeric tungstate. Catal. Today 2014, 226, 160–166. https://doi.org/10.1016/j.cattod.2013.09.054.
56. Calmanti, R.; Sargentoni, N.; Selva, M.; Perosa, A. One-Pot Tandem Catalytic Epoxidation—CO2 Insertion of Monounsaturated
Methyl Oleate to the Corresponding Cyclic Organic Carbonate. Catalysts 2021, 11, 1477. https://doi.org/10.3390/catal11121477.
57. Hu, J.; Ma, J.; Zhu, Q.; Zhang, Z.; Wu, C.; Han, B. Transformation of Atmospheric CO2 Catalyzed by Protic Ionic Liquids: Effi-
cient Synthesis of 2-Oxazolidinones. Angew. Chem. Int. Ed. 2015, 54, 5399–5403. https://doi.org/10.1002/anie.201411969.
58. Wu, S.; Huang, J.; Wang, Y.; Tao, H.; Yu, Z.; Zhang, Y. Bisimidazolium Tungstate Ionic Liquids: Highly Efficient Catalysts for
the Synthesis of Linear Organic Carbonates by the Reaction of Ethylene Carbonate with Alcohols. Catal. Lett. 2023, 153, 62–73.
https://doi.org/10.1007/s10562-022-03969-6.
59. Jordão, A.K.; Pinheiro, T.D.N.; Barbosa, E.G. Sodium Tungstate Dihydrate (Na2WO4·2H2O): A Mild Oxidizing and Efficient
Reagent in Organic Synthesis. SynOpen 2022, 06, 208–210. https://doi.org/10.1055/a-1924-8008.
60. Noyori, R.; Aoki, M.; Sato, K. Green oxidation with aqueous hydrogen peroxide. Chem. Commun. 2003, 34, 1977–1986.
https://doi.org/10.1039/b303160h.
61. Mero, A.; Guglielmero, L.; Guazzelli, L.; D’andrea, F.; Mezzetta, A.; Pomelli, C.S. A Specific Interaction between Ionic Liquids’
Cations and Reichardt’s Dye. Molecules 2022, 27, 7205. https://doi.org/10.3390/molecules27217205.
62. Ganesan, K.; Alias, Y. Synthesis and Characterization of Novel Dimeric Ionic Liquids by Conventional Approaches. Int. J. Mol.
Sci. 2008, 9, 1207–1213. https://doi.org/10.3390/ijms9071207.
63. Batsanov, S.S. Van der Waals Radii of Elements. Inorg. Mater. 2001, 37, 871–885. https://doi.org/10.1023/a:1011625728803.
64. Verma, P.L.; Gejji, S.P. Electronic structure and spectral characteristics of alkyl substituted imidazolium based dication-X2 (X =
Br, BF4, PF6 and CF3SO3) complexes from theory. J. Mol. Liq. 2019, 293, 111548. https://doi.org/10.1016/j.molliq.2019.111548.
65. Bannwarth, C.; Caldeweyher, E.; Ehlert, S.; Hansen, A.; Pracht, P.; Seibert, J.; Spicher, S.; Grimme, S. Extended tight-binding
quantum chemistry methods. WIREs Comput. Mol. Sci. 2021, 11, e1493. https://doi.org/10.1002/wcms.1493.
66. Bannwarth, C.; Ehlert, S.; Grimme, S. GFN2-xTB—An Accurate and Broadly Parametrized Self-Consistent Tight-Binding Quan-
tum Chemical Method with Multipole Electrostatics and Density-Dependent Dispersion Contributions. J. Chem. Theory Comput.
2019, 15, 1652–1671. https://doi.org/10.1021/acs.jctc.8b01176.
67. Anthony, J.L.; Anderson, J.L.; Maginn, E.J.; Brennecke, J.F. Anion Effects on Gas Solubility in Ionic Liquids. J. Phys. Chem. B
2005, 109, 6366–6374. https://doi.org/10.1021/jp046404l.
68. Wang, M.-Y.; Ma, R.; He, L.-N. Polyoxometalate-based ionic liquids-promoted CO2 conversion. Sci. China Chem. 2016, 59, 507–
516. https://doi.org/10.1007/s11426-016-5560-9.
69. Wadt, W.R.; Hay, P.J. Ab initio effective core potentials for molecular calculations. Potentials for main group elements Na to Bi.
J. Chem. Phys. 1985, 82, 284–298. https://doi.org/10.1063/1.448800.
70. Hay, P.J.; Wadt, W.R. Ab initio effective core potentials for molecular calculations. Potentials for K to Au including the outer-
most core orbitals. J. Chem. Phys. 1985, 82, 299–310. https://doi.org/10.1063/1.448975.
71. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Petersson, G.A.;
Nakatsuji, H.; et al. Gaussian 16, Revision C.01; Gaussian, Inc.: Wallingford, CT, USA, 2016.
72. Spicher, S.; Grimme, S. Robust Atomistic Modeling of Materials, Organometallic, and Biochemical Systems. Angew. Chem. Int.
Ed. 2020, 59, 15665–15673. https://doi.org/10.1002/anie.202004239.
73. Magill, A.M.; McGuinness, D.S.; Cavell, K.J.; Britovsek, G.J.; Gibson, V.C.; White, A.J.; Williams, D.J.; White, A.H.; Skelton, B.W.
Palladium(II) complexes containing mono-, bi- and tridentate carbene ligands. Synthesis, characterisation and application as
catalysts in CC coupling reactions. J. Organomet. Chem. 2001, 617618, 546–560. https://doi.org/10.1016/s0022-328x(00)00720-8.
74. Ibrahim, H.; Koorbanally, N.A.; Ramjugernath, D.; Bala, M.D.; Nyamori, V.O. Synthesis and Characterization of Imidazolium
Salts Bearing Fluorinated Anions. Z. Anorg. Allg. Chem. 2012, 638, 2304–2309. https://doi.org/10.1002/zaac.201200260.
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