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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 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 CO
2
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.
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 flammability, 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 offers the possibility of designing and preparing task-specific systems
characterized by finely 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 offer 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 effect on the thermal
stability, melting point, electrochemical window, and solubility characteristics of DILs
[25,29,31,32,35–37]. The possibility of undergoing specific 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 identified 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 fields
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 flexibility 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 effective 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 effect of
different 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 1–3 was performed following a literature procedure
[62]. N-methyl imidazole and the selected di-halogenated xylenes were reacted in
acetonitrile at solvent reflux temperature overnight, obtaining the desired compounds in
an excellent yield (Scheme 1). Tungstate DILs 4–6 were prepared from the bromide DILs
(Scheme 2). Differing from the synthesis of compounds 1–3, the preparation of tungstate
DILs 4–6 presented few difficulties. The first aempted approach relied on the metathesis
reaction with Na2WO4, but it proved unsuccessful. The specific solubility properties of
Na2WO4, of bromide DILs, and of the byproducts resulting from the ion metathesis made
it impossible to find 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 1–3 were firstly 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 verified by a silver assay, as described in Section 3.5. IR analysis can be
used as an effective 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
cm−1 (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 cm−1 and the broad peak centred at
about 650 cm−1, are not visible in the spectra of tungstate DILs 4–6, indicating the absence
of unreacted tungstic acid in the prepared compounds (Figure S20, Supplementary
Materials).
Scheme 1. Synthesis of the bromide DILs (1–3).
Molecules 2024, 29, 2131 4 of 23
Scheme 2. Synthesis of tungstate DILs (4–6).
2.2. Thermal Characterization
The thermal stability of compounds 1–6 was firstly 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 1–3, featuring bromide anions, displayed a
degradation temperature almost 100 °C higher than their tungstate counterparts 4–6. At
the same time, the degradation profile itself appeared greatly influenced by the anion
type. Bromide DILs (1–3) 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 profiles, with multiple degradation steps
overlapping in the region between 200 °C and 400 °C, indicating a strong effect of the
tungstate anion on the thermal degradation mechanisms (Figures S24–S26,
Supplementary Materials). The nature of the linker chain also proved to strongly affect
the thermal stability properties of compounds 1–6 (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 effect being more noticeable in tungstate DILs. Finally, the type
of spacer was found to deeply affect the degradation profile 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 profile, 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 effect on the thermal degradation profiles of DILs compared to the
effect reported in literature for alkyl linker chains [29]. Concerning tungstate DILs 4–6, the
influence of the spacer on the thermal degradation profile 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 profile (Figures S24 and S25, Supplementary
Materials).
Table 1. The Tstart 5%, Tonset, and Tpeak of compounds 1–6. Tstart 5% is defined as the temperature at which
the weight loss of the sample is equal to 5% of the initial mass; Tonset is defined as the temperature at
the onset of the weight (%) vs. temperature curve; and Tpeak is defined 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 significant deformations of the
geometry of the system. The optimised structures of compounds 1–6 are reported in
Figure 3 (bromides 1–3) and Figure 4 (tungstates 4–6). Relative and absolute energies are
reported in Table 2 (bromides 1–3) and Table 3 (tungstates 4–6).
Systems with bromide anions are discussed first. For each isomer, we found two local
minima with structural analogies, the first 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 defined 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 differences between the two conformations of the same isomer. The sum of hydrogen
and bromide van der Waals radii fluctuate in the range 2.97–3.15 Å according to different
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 defined 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 (1–3). Colour
code for the atoms: hydrogen: white; carbon: grey; nitrogen: blue; bromine: red.
The diverse geometry of the linker leads to differences 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 different 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 difference
between the two conformers is therefore less well defined. In the symmetric internal
structure, the anions can interact efficiently 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 flexible dication. In this case only, the
analogous of the internal conformers is possible.
This different electrostatic and geometric scenario leads to remarkable differences 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 flip, 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 different bromides. In this case (both ortho structures in
Figure 3a,b), there is a small energetic difference because the two structures are not so
different. 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 flip 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 (4–6). 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 (4–6), 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 field [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 field 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 different behaviour for sys-
tems with different anions. The bromide-based systems (1–3), which at the ab initio level
showed two stable conformers, are more flexible and show larger oscillations, while the
tungstate ones are more rigid. A similar behaviour can be concluded from the energetic
profiles 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 different reasons:
1. The electrostatic effect of the double-charged WO42− anion is stronger. This has
several effects on the geometry of the dications, the most evident being the stretching
of the C2-H and C2′-H bonds (Table 3).
2. Differently from the WO42− systems, in the bromide case, there are two different
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 first 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 different configurations (Figure 7).
Concerning the Br- system, the para conformation cluster presented a high variation
after the first 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 different 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 configuration).
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 different relatively stable conformations (Figure 7). The first and the
second conformations occurred before 800 ps, while after this time, the system evolved
toward a third configuration which remained stable. These conformations were generated
by a modification 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 differences between HOMO and LUMO in the first and the last geometrical
structures of the MD runs are here reported. The structures of the DILs featuring bromide
anions did not present significant differences (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 configuration. 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
configuration 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 configuration with both the
imidazolium rings.
Figure 8. HOMO-LUMO analyses for the DILs with tungstate anion (4–6) in the ortho, meta, and
para configurations (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
configurations presented the HOMO-LUMO regions in the same positions at the start
point of the analysis. For the ortho configuration, 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 configurations 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 configuration observed on the systems without
CO
2
molecules).
Figure 9. HOMO-LUMO analyses for the DILs with tungstate anion (4–6) with carbon dioxide in
ortho, meta, and para configurations (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 (1–3), 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 (4–6), 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 1–6 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 4–6 (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 (1–3) and the tungstate (4–6)
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 significant difference
in reactivity between the DILs featuring an o-xylene spacer and the other ones points out
the great influence of the proximity of the imidazolium groups on the catalytic activity of
the compound. Concerning the influence 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 effective 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 effect exerted by the WO42− group on
the CO2 molecule, it is evident that this activation alone is not sufficient 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 effectively act as nucleophiles in the
CCE reaction [36,40]. A final 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 effectively 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 4–6 with respect to their bromide counterparts
1–3 can be rationalised considering both the nucleophilicity of the anion and the effect of
WO42− on the structures of DILs 4–6. 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 effect of temperature on the catalytic activity of compounds 1–3. Furthermore, the
data collected from the reaction performed at 80 °C are directly comparable to data from
the literature related to DILs featuring flexible linker chains, thus allowing us to evaluate
the effect of rigid xylene spacers on the catalytic activity of bromide DILs. Unfortunately,
compounds 1–3 were found to be only partially soluble in the reaction medium at this
temperature, heavily affecting 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 sufficient to fully
solubilise the least soluble catalyst reported in the study. It is evident that the presence of
a rigid xylene-based spacer greatly affects 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
flexible 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 purifications. 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 cm−1 to 550
cm−1, with 32 scans and resolution of 4 cm−1. 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 affected 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 1–3) or to 800 °C (compounds 4–6) 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 differential 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 effective 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 (1–3)
A variation of the synthetic procedure reported by Magill et al. was followed for the
preparation of compounds 1–3 [73]. In a flask 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 reflux 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 (cm−1): 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 (cm−1): 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 (cm−1): 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 (4–6)
Compounds 4–6 were synthetised following a procedure previously reported by our
research group [29]. Amberlite IRA 400 ion exchange resin (20 g) was firstly 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 flask 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 flowed 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 affect
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 first seconds after
the addition of the silver solution indicated the presence of residual halide anions. The
DILs solution was recovered and flowed 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 (cm−1): 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 (cm−1): 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 (cm−1): 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
prefixed 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 different dications with aromatic spacers coupled with bromide and
tungstate anions.
The isomerism of the rigid spacers here considered (ortho, meta and para) leads to
different potential conformations of the dications and different 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 influence of the conformation and of the system mobility on their chemical
properties. On one hand, the anion type proved decisive in defining the catalytic activity
of the studied DILs, with bromide systems 1–3 clearly outperforming their tungstate
counterparts 4–6. 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 flexibility and adaptability of the catalyst. More specifically, 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 finally indicated that the studied
tungstate DILs could also have an activating effect on carbon dioxide, with the obtained
data highlighting the existence of effective 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 configuration ortho. Figure S29. HOMO-LUMO analyses for the IL with
bromide anion in the configuration meta. Figure S30. HOMO-LUMO analyses for the IL with
bromide anion in the configuration para. Figure S31. Structure of cluster with CO2. Ortho
configuration. Figure S32. Structure of cluster with CO2. Meta configuration. Figure S33. Structure
of cluster with CO2. Para configuration.
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.
Conflicts of Interest: The authors declare no conflicts 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
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