A decatungstate-based ionic liquid exhibiting a very low dielectric constant suitable for acting as a solvent and a catalyst for the oxidation of organic substrates

Article abstract of DOI:10.1039/d1nj01214b

In this contribution, a new POM-based ionic liquid, namely (P6,6,6,14)4[W10O32], was fully characterized. Its viscosity and its very low dielectric constant make this hybrid ionic liquid suitable to be used as a solvent for organic transformations. As a proof of concept, this unique ionic liquid having both solvent and catalyst properties was tested for the catalytic oxidation of various alcohols and alkenes in the presence of H2O2.

Full text of DOI:10.1039/d1nj01214b

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Yellowish and blue luminescent graphene oxide quantum dots
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DOI: 10.1039/D1NJ01214B
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A decatungstate-based Ionic liquid exhibiting very low dielectric
constant suitable for acting as solvent and catalyst for oxidation
of organic substrates
Received 00th January 20xx,
Accepted 00th January 20xx
Yohan Martinetto,a,b Bruce Pégot,*a Catherine Roch,a Mohamed Haouas,a Betty Cottyn-Boitte,b
Franck Camerel,c Jelena Jeftic,c Denis Morineau,d Emmanuel Magnier,a and Sébastien Floquet.*a
DOI: 10.1039/x0xx00000x
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properties such as viscosity and dielectric constant are very rarely
investigated.3
Considering the potential catalytic properties of the POMs and the
solvent properties of Ionic liquids, the use of recyclable POM-ILs
could resolve both issues mentioned above and we are convinced
that such compounds are a promising answer to the problematics
In this contribution, a new POM-based ionic liquid, namely
(P6,6,6,14)4[W10O32], was fully characterized. Its viscosity and its very
low dielectric constant make this hybrid ionic liquid suitable to be
used as solvent for organic transformations. As proof of concept,
this unique ionic liquid combining solvant and catalyst properties
was tested for catalytic oxidation of various alcohols and alkenes in
presence of H2O2
encountered in homogeneous catalysis. However,
a better
understanding of the physical properties of POM-IL is needed for this
purpose. While the use of IL-POM dispersed on surfaces or hybrid
POMs solubilized in ILs are reported, the use of liquid POM-ILs is still
surprisingly rare in catalytic reactions3,9 and to the best of our
knowledge, only one example reports the use of a room temperature
POM-IL for the oxidation of alcohols and alkenes into carboxylic
acids.10
To bring new insights for the knowledge of physical properties of
POM-ILs and evaluate the interest of such material for homogeneous
catalysis, the purpose of this work is to synthetize and to fully
characterize a new POM-IL, liquid at room temperature and
exhibiting the required properties to act both as a good solvent and
as a catalyst for oxidizing organic molecules.
To design the POM-IL, the choice of phosphonium counter cations is
of great interest. Indeed, such type of cation yields the lowest
melting points when combined to a POM, even if it is not the most
common cations for ionic liquids.3,11
In this study, the decatungstate POM [W10O32]4-, known to possess
catalytic and photocatalytic properties,12-14 was associated with four
trihexyltetradecylphosphonium cations as described in the ESI.
The material was isolated as viscous colorless oil for which the FT-IR,
TGA, EDX, ESI-MS, Ion chromatography and NMR experiments agree
well with the expected formula (P6,6,6,14)4[W10O32].
Polyoxometalate (POM) compounds constitute a wide family of
anionic inorganic compounds which can be finely tuned at the
molecular level. The choice of the constituents, including the counter
cations, is determinant to design the final architectures. Therefore,
owing to their structural and compositional versatility, they possess
a vast range of properties and potential applications, notably in
catalysis.1-4 In this domain, POMs have been widely used for the
oxidation of various organic substrates, very often in the presence of
water solutions of hydrogen peroxide.5,6 They can be used in
heterogenous or homogeneous catalytic processes. However,
homogeneous catalytic processes display serious shortcomings of
catalyst recovery or reaction mixture separation. Besides, due to
detrimental effects in the environment, research for alternative
reaction media instead of conventional organic solvents has rapidly
grown. For these purposes, a particular attention has been given to
reusability of catalysts and use of ionic liquids as solvents in the
reaction systems.
The coupling of POMs with some organic cations allows the design of
hybrid compounds at the boundary between molecular chemistry
and chemistry of materials.7 Among these hybrid materials, most of
them are solid salts or materials with a high or even indeterminate
melting points and cannot be formally called ionic liquids, or more
precisely POM-based ionic liquids, abbreviated POM-ILs.8
Interestingly, among the hundreds of hybrids POMs reported in the
literature, only a few dozen of them can be considered as real POM-
ILs, the others has been suggested to be labelled IL-POMs3 Besides,
a close look reveals that only very few members of this restricted
POM-ILs family are liquid at room temperature and their physical
a. Institut Lavoisier de Versailles, UMR 8180, Université de Versailles St-Quentin en
Yvelines, CNRS, Université Paris-Saclay, 78035 Versailles, France.
b. Institut Jean-Pierre Bourgin, INRAE, Agro Paris Tech, Université Paris Saclay, 78000
Versailles, France.
c. Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) - UMR 6226,
35000 Rennes, France.
d. Institut de Physique de Rennes, UMR 6251, Université de Rennes 1, 35042 Rennes,
France.
Figure 1: 183W NMR (16.7 MHz) spectrum of (P6,6,6,14)4[W10O32] in CD3CN. Inset:
Representation of the POM-IL (P6,6,6,14)4[W10O32]. Arrows corresponds to the
assignments of the 183W NMR spectrum.
†. Electronic Supplementary Information (ESI) available: Experimental section, FT-IR
spectra, TGA analysis, additional NMR spectra, details on rheological properties,
details on catalytic procedures. See DOI: 10.1039/x0xx00000x
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훾
evolution of the shear stress () as a function of the shear rate ( ) at
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DOI: 10.1039/D1NJ01214B
various temperatures. From the observed straight-line flow curves, it
훾
appears that the compound is a Newtonian fluid ( =  x , where 
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is the viscosity) and that the viscosity (i.e. the slope of the curves)
gradually decreases with the increase of the temperature. This was
confirmed by the direct measurement of the viscosity as a function
of the temperature (Fig. 2c). The viscosity measured around 2200
Pa.s at room temperature drastically and rapidly decreases when
temperature increases. The viscosity is below 100 Pa.s above 40 °C
and reaches 0.3 Pa.s at 120 °C. For comparison, the viscosity of ricin
oil is 0.98 Pa.s at room temperature. The loss modulus (G”) and the
storage modulus (G’) have also been measured as a function of the
temperature between 20 and 120 °C (Fig. 2d). Both modulus values
decrease with the temperature increase. G” is at least two orders of
magnitude larger than G’ over the whole temperature range
explored, meaning that this compound is always in a quasi-liquid
state. Generally, G’ concerns elastic properties of the material, while
G” represents the viscous nature of the compound, meaning that this
compound exhibits a rather viscous behavior. It can also be noticed
that the signal associated to the storage modulus (G’) is very weak
and noisy, as expected for a liquid with a mainly viscous character.
These results demonstrate that this compound is a highly viscous
room-temperature ionic liquid (RTIL) which becomes more and more
fluid as the temperature increases. This is fully compatible with fluid
behavior needed for catalysis experiments.
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To go further, we investigated the dielectric constant of this POM-LI,
a property which is very rarely studied for POM-LIs. The evaluation
of the solvation properties of the solvent is not unique and could be
refined depending on the nature of the particular applications
targeted.16 Indeed, for ionic liquids, it was pointed out that the
Figure 2 : (a) DSC curves of the (P6,6,6,14)4[W10O32] measured between 100 °C
and -170 °C (Top: 2nd heating curve, bottom: 1st cooling curve, scan rate: 10 determination of polarity scales was influenced by the specificities of
°C.min-1; *:artefact due to the N2 pumping); (b) Polarized optical microscopy
probe-solvent local interactions, thus challenging the relationship
images showing the edge of a drop of the compound between glass slides at
between polarity and static dielectric constant, which has been
-20°C and at +20°C after removal of the top cover slide; (c) Viscosity as a
empirically established for conventional molecular solvents.17 In our
(훾
function of the temperature between 20 °C and 120 °C
= 10 s-1); c)
휀 ∗ (푓) = 휀′(푓) ―푖휀′′(푓)
Temperature dependence of G’ and G’’ (20% strain, f = 1Hz).
case, the complex dielectric function
was
investigated from 10-1 Hz to 106 Hz where f denotes the frequency,
´ and ´´ the real and loss part of the complex dielectric function.
The measurements were acquired on cooling from T = 70 °C to T = -
150 °C with temperature steps of 5 °C. Depending on the
temperature, different contributions were observed in the
instrumental frequency window, as shown in Figure 3. At high
temperature, e.g. T = 70 °C (Figure 3a), the intensity of the dielectric
loss was dominated by a power law term (-1/f), which is commonly
attributed to ionic conductivity.18 On cooling, the conductivity
contribution gradually decayed and two peaks emerged. They were
attributed to dielectric relaxation modes, and denoted mode I and
mode II, respectively (Figures 3b and 3c). The comprehensive
depiction of the evolution of the dielectric loss for a wide range of
temperatures is provided in Supplementary Information (Fig. S8-10).
The dielectric measurements were analysed quantitatively by fitting
the complex dielectric function by a model including two Havriliak
and Negami functions (HN-model) and a conductivity term19
In particular, 183W NMR (Fig. 1 and Fig S6, SI) corresponds completely
1
to the POM [W10O32]4-,14 while the H NMR spectrum (Fig. S4, ESI)
shows that the signals of the methylenic groups of the counter-
cations are significantly shifted because of the POM association,
suggesting a strong interaction between the two components.
To demonstrate the true ionic liquid nature of this compound and to
use it as a fluid solvent capable of solubilizing organic substrates,
rheological and dielectric constant measurements were performed.
Thermogravimetric analyses (Fig S3, ESI) show that the compound is
stable up to 200 °C without any degradation and a clear glass
transition centered around Tg = -10 °C is observed on the DSC heating
curves (Fig. 2a). Polarized Optical Microscopy observations show
that, below Tg, the material is a brittle solid in an isotropic glassy
state, while above Tg, the compound is viscous, isotropic and
malleable material that spreads on a surface (Fig. 2b).15
To unambiguously confirm the liquid nature of this compound above
room
temperature,
temperature-dependent
rheological
휀
according to eq. 1, where ∞ is the sample permittivity in the limit of
∆휀
measurements were performed. The Figure S7 in ESI represents the
휏
high frequency, 1,2 and
are the dielectric strength and the
퐻푁1,2
2 | J. Name., 2012, 00, 1-3
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preliminary study, to evaluate the efficiency of this new POM-IL as
DOI: 10.1039/D1NJ01214B
both solvent and catalyst. The oxidation of 4-fluorobenzyl alcohol,
relaxation time of the mode I (resp. II). 0 stands for the DC
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conductivity of the sample, where the exponent s=1 applies for
Ohmic mechanisms. i is the imaginary unit.
performed in a two-phase system (POM-IL phase+organic substrate
/ aqueous phase containing H2O2), was firstly optimized according to
procedures described in ESI (Tables S1, ESI). As a first indication of
the efficiency of our system, the alcohol was converted entirely into
the corresponding carboxylic acid in several hours (93% isolated
yield) as the sole compound. The aldehyde, which is certainly one of
the intermediate, could be observed in minor proportion by reducing
reaction time. Control reactions, without H2O2 or POM-IL, confirm
the importance of the catalyst and its co-oxidant. Encouraged by this
positive result, more challenging compounds were tested in the
previous condition. The results are gathered in Table 1.
The process works efficiently on primary benzylic alcohols (entry 2)
and also on secondary alcohols (entry 3). Interestingly, with
cyclohexanol, the adipic acid, product of over-oxidation and C-C
cleavage, was isolated as the major product without formation of the
corresponding ketone (entry 4). This is in stark contrast with Guo
results, who evidenced the oxidation of cyclohexanol to
cyclohexanone at 90-100 °C for 2 hours with 89.1% yield by adding
hexadecyltrimethylammonium decatungstate to the alcohol in
presence of aqueous H2O2.12
This reinforces the idea that the oxidative cleavage of alkene is
possible with such ionic liquid system. The first tests on cyclohexene
under the previous conditions gave only traces of diacid. The heating
mode was modified with the use of microwaves as activation which
has shown some efficiency in that kind of catalysis.20 Gratifyingly, in
2 hours with only 5 equivalents of H2O2, encouraging yields between
15 and 37% of the isolated main product could be obtained (entries
5 to 8). This microwave activation allowed the improvement of both
the efficiency and the selectivity of the transformation with a
concomitant drastic diminution of the quantity of hydrogen
peroxide, reduced by a factor of ten. The moderate yields could be
explained by the several intermediates formed during the oxidation
processes (epoxide, diol, cyclic α-hydroxy ketone, α-hydroxy
lactones).6 Further optimizations of the reaction conditions are
currently under development in our laboratory to push forward
oxidation and get complete conversion into acids.
∆휀푘
휎0
휀 ∗ (휔) = 휀∞
+
―푖
휔푠휀0
푘
∑
(1)
훾
)
훽
푘 = 1,2
푘
(
)
1 + 푖휔휏퐻푁푘
(
According to the formalism of the HN-model, the exponent
parameters  and  are introduced to account for the symmetric and
the asymmetric broadening of the complex dielectric function with
respect to the Debye one, which is recovered when  =  =1.
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Figure 3. Dielectric loss as a function of the frequency at three different
temperatures (a) 70°C, (b) -50°C and (c) -115°C. Solid lines denote fits by the
Havriliak-Negami equations to the corresponding data (see text for details).
Although the application of these fractional parameters significantly
improved the quality of the fits, their exact values were ambiguous
because of the small value of the dielectric strength and the
numerical degeneracy due to the coupling between   and the
휏
relaxation time 퐻푁. Therefore, we computed the value of the
휏
푚푎푥
relaxation time
corresponding to the maximum frequency of
the relaxation peak in order to overcome this limitation, according to
relation (2):
―1
1
휋훽
휋훽훾
훽
훽
휏
푚푎푥 = 휏퐻푁 sin
sin (
)
]
(
)
]
[
(2)
[
2 + 2훾
2 + 2훾
The resulting fits are shown as solid lines in Figure 3 for a selection of
three temperatures. Equally good fits were obtained for all the
studied temperatures. The real part of the dielectric permittivity was
shown to be weakly temperature dependent, and estimated from a
휀
휀
Cole-Cole plot (not-shown) to be ∞ = 2.9 and 푠= 3.2 in the limit of
high and low frequency, respectively, which agrees with viscosity
measurements (see Fig. S10, ESI). This very low value is surprising if
we consider the electrostatic nature of the assembly of the POM with
the four phosphonium cations and confirms the strong interaction
between these two components according to NMR data. These value
compares well with organic solvents such as dioxane or diethyl ether
( = 2.25 and 4.33 respectively) and thus appear as good solvent for
organic substrate despite its ionic nature.
Table 1. scope of oxidation with (P6,6,6,14)4[W10O32] as catalyst
Entry
1
Substrate
Product
Yield (%)b
93
COOH
OH
F
F
COOH
OH
2
3
89
62
N
N
OH
O
OH
COOH
COOH
4
15
15
COOH
COOH
5c
COOH
This POM-IL exhibits the properties required to be used as a fluid
solvent of organic transformation, i.e. a very low viscosity over 40°C,
a dielectric permittivity around 3 and a thermal stability up to 200°C.
The second part of this work was thus focused on the investigation
of the catalytic behaviour of such a system. As a proof of concept,
the oxidation reaction of alcohols and alkenes was studied in a
6c
7c
8c
37
28
26
F
F
COOH
COOH
COOH
OH
aReaction conditions: alcohol (1 eq.), H2O2 30 wt% (50 eq.), (P6,6,6,14)4[W10O32]
b
(0.1 eq.), 90°C, 16h. Isolated yield after catalyst/product separation by a
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reusable steric polymer column. cReaction conditions: alcohol (1 eq.), H2O2 30
wt% (5 eq.), (P6,6,6,14)4[W10O32] (0.1 eq.), MW irradiation, 75 °C, 90 min.
Universitaire de France are also gratefully acknowledged for their
DOI: 10.1039/D1NJ01214B
financial support. DM acknowledges support from Rennes
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Metropole and Europe (FEDER Fund – CPER PRINT2TAN). Ms. S.
Basset, Ms S. Guiheneuf, Dr N. Leclerc and Ms A. Damond are
gratefully acknowledged for their help, notably for the ion
chromatography, EDX and ESI-MS experiments.
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The traditional methods of extraction with organic solvents
commonly used with ionic liquids, or even separation by column
chromatography on silica gel, proved to be ineffective to separate
the products from the POM-IL phase as well as to recycle the catalyst.
Therefore, we demonstrated that the catalyst can be successfully
separated from the mixture of organic products at the end of the
reaction thanks to a reusable steric exclusion column. After elution
of the reaction medium on this column, the POM-IL, non-retained on
the solid phase thanks to its size, was firstly recovered and directly
reused. The oxidation products are isolated secondly from the
column and separated by classical chromatography on silica to get
yields. The POM-IL phase was reused over several cycles as shown in
Table 2. Over the first three cycles no loss of activity was observed:
the isolated yield was around 90%. Nevertheless, after 4 cycles, a
slight loss of yield was observed. This phenomenon was partly
explained by the formation of a less reactive species Lindqvist-type
POM [W6O19]2- as the number of cycles increased (see Figure S11,
183W NMR, in ESI). Accordingly to the literature, the active species is
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Notes and references
1
A.V. Anyushin, A Kondinski, T.N. Parac-Vogt, Chem Soc Rev.,
2020, 49, 382; Y.-R. Wang, Q. Huang, C.T. He, Y. Chen, J. Liu,
F.-C. Shen, Y.Q. Lan, Nature Comm., 2018, 9, article n°4466;
V.Y. Evtushok, A.N. Suboch, O.Y. Podyacheva, O.A. Stonkus,
V.I. Zaikovskii, Y.A. Chesalov, L.S. Kibis, O.A. Kholdeeva, ACS
Catal., 2018, 8, 1297.
2
3
4
A. Misra, K. Kozma, C. Streb, M. Nyman, Angew. Chem.-Int.
Edit. 2019, 59, 2, 596.
Y. Martinetto, B. Pégot, C. Roch-Marchal, B. Cottyn-Boitte, S.
Floquet, Eur. J. Inorg. Chem. 2020, 228.
N. V. Izarova, M. T. Pope, U. Kortz, Angew. Chem.-Int. Edit.,
2012, 51, 9492; H. N. Miras, J. Yan, D. L. Long, L. Cronin, Chem.
Soc. Rev., 2012, 41, 7403; A. Proust, B. Matt, R. Villanneau, G.
Guillemot, P. Gouzerh, G. Izzet, Chem. Soc. Rev., 2012, 41,
7605; T. Zheng, G. Y. Yang, Chem. Soc. Rev., 2012, 41, 7623.
E. Antonelli, R. D’Aloisio, M. Gambaro, T. Fiorani, C.
Venturello, J. Org. Chem., 1998, 63, 7190; K. Kamata, K.
Yonehara, Y. Sumida, K. Yamaguchi, S. Hikichi, N. Mizuno,
Science, 2003, 300, 964; P.R. Makgwane, S.S Ray, Catal.
Commun., 2014, 54, 118; P. Jin, Z. Zhao, Z. Dai D. Wei, M. Tang,
X. Wang, Catal. Today, 2011, 175, 619; Y. Deng, Z. Ma, K.
Wang, J. Chen, Green Chem., 1999, 12, 275.
likely
a
peroxotungstate species.6 The formation of such
5
intermediate active species involves the opening of the
decatungstate POM moiety during the catalytic process, which can
either lead to the initial product or to the Lindqvist product.
Table 2. Recycling of catalyst (P6,6,6,14)4[W10O32]. Isolated yields obtained for
runs 1-5. Reaction conditions: 4-fluorobenzyl alcohol (1 eq.), H2O2 30 wt% (50
eq.), (P6,6,6,14)4[W10O32] (0.1 eq.), 90 °C, 16 h.
6
K. Sato, M. Aoki, R. Noyori, Science, 1998, 281, 1646; R.
Noyori, M. Aoki, K. Sato, Chem. Commun., 2003, 1977.
B. Li, W. Li, H. Li, L. Wu, Accounts Chem. Res., 2017, 50, 1391.
POM-ILs, are defined as salts with a melting point < 100 °C.
Salts exhibiting melting point above 100°C does not
correspond to the definition of ionic liquids and can be
abbreviated IL-POMs.
Entry
1
Run 1
93%
Run 2
92%
Run 3
86%
Run 4
73%
Run 5
68%
7
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In summary, in this contribution, we synthetized a new compound
of formula (P6,6,6,14)4[W10O32]. The latter was unambiguously
characterized as a newtonian ionic liquid with a glass transition at -
10°C and rheological measurements allowed to evidence a good
fluidity when temperature increased and a very low dielectric
constant comparable with those obtained for apolar organic
solvents. Used as solvent for organic molecules, the addition of an
aqueous phase containing H2O2 evidenced the potentialities of such
system for oxidation of alcohols and alkenes, in particular with C-C
and C=C bond activations. Besides, we developped also an elegant
way for recycling the POM-IL phase by using steric exclusion gel.
This system appears to be a promising powerful catalyst and
prompts us now to investigate its activity towards valorizable
biopolymers considered so far as wastes by industry.
9
Y. Qiao, Z. Hou, Curr. Org. Chem. 2009, 13, 1347.
10 Z. P. Pai, A. G. Tolstikov, P. V. Berdnikova, G. N. Kustova, T. B.
Khlebnikova, N. V. Selivanova, A. B. Shangina, V. G.
Kostrovskii, Russ. Chem. Bull., 2005, 54, 1847.
11 P. G. Rickert, M. R. Antonio, M. A. Firestone, K.-A. Kubatko, T.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgments
This work is supported by a public grant overseen by the French
National research Agency as part of the « Investissement d’Avenir »
program, through the "IDI 2017" project funded by the IDEX Paris-
Saclay, ANR-11-IDEX-0003-02. INRAE, CNRS, UVSQ and Institut
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