Triggering the Chemical Instability of an Ionic Liquid under High

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Triggering the Chemical Instability of an Ionic Liquid Under High Pressure Luiz Felipe de Oliveira Faria, Marcelo Medre Nobrega, Marcia Laudelina Arruda Temperini, Roberto Bini, and Mauro Carlos Costa Ribeiro J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b06246 • Publication Date (Web): 28 Jul 2016 Downloaded from http://pubs.acs.org on July 29, 2016

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The Journal of Physical Chemistry

Triggering the Chemical Instability of an Ionic Liquid under High Pressure Luiz F. O. Fariaa, Marcelo M. Nobregaa,b, Marcia L. A. Temperinia, Roberto Binib,c, Mauro C. C. Ribeiroa*

a

Laboratório de Espectroscopia Molecular, Departamento de Química Fundamental,

Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes 748, 05508000, Brazil b

LENS, European Laboratory for Nonlinear Spectroscopy, Via Nello Carrara 1, 50019

Sesto Fiorentino (FI), Italy c

Dipartimento di Chimica“Ugo Schiff”dell’Università degli Studi di Firenze, Via della

Lastruccia 3, 50019 Sesto Fiorentino (FI), Italy

* E-mail address: [email protected] ; Tel.:+55(11) 30912057

1 ACS Paragon Plus Environment


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ABSTRACT Ionic liquids are an interesting class of materials due to their distinguished properties allowing their use in an impressive range of applications, from catalysis to hypergolic fuels. On the other hand, the reactivity triggered by the application of high pressure can give rise to a new class of materials which are not achieved under normal conditions. Here, we report the high pressure chemical instability of the ionic liquid 1allyl-3-methylimidazolium dicyanamide, [allylC1im][N(CN)2], probed by both Raman and IR techniques and supported by quantum chemistry calculations. Our results show a reaction occurring above 8 GPa involving the terminal double bond of the allyl group giving rise to an oligomeric product. The results presented herein contribute to our understanding on the stability of ionic liquids, which is of paramount interest for engineering applications. Moreover, gaining insight into this peculiar kind of reactivity could suggest new or alternative synthetic routes to achieve, for example, poly(ionic liquids).


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I. INTRODUCTION

Ionic liquids, molten salts with melting point below 100°C, have been the focus of many physical and chemical investigations due their distinguished properties allowing for applications in catalysis, electrochemical devices, tribology, etc. In this sense, understanding the ionic liquid stability threshold under different conditions is fundamental for choosing the more suitable liquid for a specific task. Thermal stability and decomposition mechanisms of ionic liquids have been discussed in the literature.1-3 Ionic liquids have negligible vapor pressure and usually thermal degradation is verified before vaporization.1-3 However, it has been shown that some ionic liquids can be distilled in conditions of low pressure without decomposing.4 Several studies also elucidated the stability of ionic liquids to radiolytic degradation compared to molecular solvents.5-8 The chemical stability of ionic liquids related to other compounds has been explored because of their extensive use as solvents, catalysts, and reagents in organic synthesis.9,10 For instance, some studies focused specifically on their reactivity when used as hypergolic fuels, i.e. when combined with an oxidizer they spontaneously ignite, eliminating the need for an additional ignition source.11-13 In this work, we present the chemical instability of the ionic liquid 1-allyl-3methylimidazolium dicyanamide, [allylC1im][N(CN)2] (see Figure 1) under high pressure monitored by Raman and infrared (IR) spectroscopies and supported by DFT calculations. This ionic liquid shows low viscosity (42 cP at 25 ºC) and thermal decomposition at 207 °C.13 It was verified that the hypergolic reactivity of [allylC1im][N(CN)2] is mainly due to the [N(CN)2]- anion,11,13 although a recent study supports the chemical stability of [N(CN)2]- because it does not show any reactivity with a variety of neutral reagents.12 Thermal decomposition of a similar ionic liquid



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based on the 1-butyl-3-methylimidazolium cation, [C4C1im][N(CN)2], was claimed to happen through the reaction between cation and anion leading to the formation of neutral molecules.14 To the authors knowledge, this is the first study probing the chemical instability of an ionic liquid under high pressure. The results presented here indicated that while [N(CN)2]- is stable up to ca. 12 GPa, a pressure-induced oligomerization occurs through the double bond of the allyl group in [allylC1im]+. The irreversible formation of such structure is evidenced by the persistence of its Raman and IR characteristic features during decompression. For comparison purpose, in a previous high pressure study it has been found that [C4C1im][BF4] remains stable even after being compressed up to 30 GPa.15

H2C

N

N

CH3

N C

C

N [allylC1im]+

N

[N(CN)2]-

Figure 1. Structure of 1-allyl-3-methylimidazolium dicyanamide, [allylC1im][N(CN)2].

II. EXPERIMENTAL AND COMPUTATIONAL DETAILS

The [allylC1im][N(CN)2] sample (> 98% purity) was purchased from Iolitec and the liquid was dried under high vacuum (below 10−5 mbar) for 24 h prior the measurements. Raman high pressure measurements at room temperature were obtained by loading the [allylC1im][N(CN)2] into a piston-cylinder diamond anvil cell (DAC) (model DiacellLeverDAC Maxi, Almax-EasyLab Technologies Ltd, Belgium) equipped 4 ACS Paragon Plus Environment

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with a 400 µm culet size, IIa type diamonds, and stainless steel gasket pre-indented to ca. 150 µm and a 150 µm hole. A ruby chip was placed in the sample chamber for in situ pressure measurements16,17 and no pressure transmitting medium was used. Raman spectra with a resolution of 4 cm-1 were obtained in a Renishaw Raman imaging microscope (Reflex inVia) with a Leica microscope and a CCD detector, by using a 20 X lens (Olympus SM Plan, N.A. 0.40). Laser lines at 632.8 nm (Renishaw He-Ne laser) with 8.5 mW and at 785 nm (Renishaw diode laser) with 100 mW were used as excitation lines for spectral acquisition and to excite the ruby fluorescence (with power reduction). IR experiments were performed by using a MDAC (membrane diamond anvil cell) equipped with a stainless steel gasket with a 150 µm hole. In order to reduce the strong sample absorption the optical path was reduced by pressing KBr in the sample chamber producing a pellet whose surface was successively scratched. Afterwards, the ionic liquid (sample thickness of about 15 µm) and a ruby chip were added above the pellet. FT-IR absorption measurements were recorded with an instrumental resolution of 1 cm-1 using a Bruker-IFS 120 HR spectrometer modified for high-pressure measurements.18 The ruby fluorescence was excited using few milliwatts of a 532 nm laser line from a Nd:YAG laser source. In both the Raman and the IR measurements, the pressure was varied virtually instantaneously in small steps of increasing pressure, then taking a relatively long time while recording the spectrum at each pressure. Quantum chemistry calculations were performed using the Gaussian03 package19 to obtain optimized structures and vibrational frequencies for the ionic pair and the oligomeric cation structures. Density functional theory (DFT) using the Becke’s three-parameter hybrid exchange functional and Lee−Yang−Parr correlation functional (B3LYP)20,21 with 6-311++G(d,p) basis set was used. No imaginary vibrational



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frequencies were obtained indicating that the vacuum geometries were at the minimum of the potential surface, except in the case of oligomeric structures with the anions.

III. RESULTS AND DISCUSSION

The chemical structure of the [allylC1im]+ cation consists of an imidazole ring with charge delocalization, substituted in the 1 and 3 positions by an allyl and methyl groups, respectively, while [N(CN)2]- anion presents two triple bonds. Thus, both the cation and anion could act as reaction sites under high pressure. The first evidence that a reaction of [allylC1im][N(CN)2] occurred upon increase of pressure was gained by the changes of the Raman spectra background. The fluorescence increases at ca. 8 GPa making almost impossible to acquire a Raman spectrum. After decompression of the system from ~ 12 GPa, the sample remained fluorescent as indicated by the spectrum background shown in Figure 2.A. Raman bands then become barely observable, except for the strong band assigned to the totally symmetric CN vibration of the anion, νs(CN), at 2195 cm-1. Further evidence for the chemical instability of [allylC1im][N(CN)2] under high pressure is the change of ruby luminescence bandwidth, Γ(P). The ruby can act like a microscopic probe of stress heterogeneity in the sample probing differences in the local environment. The method of measuring Γ(P) is commonly used to determine the glass transition pressure, Pg, of samples in the DAC.22,23 The Figure 2.B indicates change of slope of the Γ(P) plot suggesting Pg ∼ 2.0 GPa for [allylC1im][N(CN)2]. Reproducibility of this plot is assured as results shown in Fig. 2.B are a collection of data obtained along measurements performed on different samples. It is worth mentioning that Pg at room temperature for ionic liquids is indeed observed typically between 1–3 GPa.24,25 We recall there is no pressure transmitting medium in our


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experiments, so that the [allylC1im][N(CN)2] sample becomes a non-hydrostatic medium as the pressure is increased above the glass transition at 2.0 GPa. Further increase of pressure shows a sudden change of the Γ(P) evolution with pressure just above 7 GPa (blue area in Fig. 2.B), suggesting that a transformation of [allylC1im][N(CN)2] starts around this pressure.

Figure 2. (A) Raman spectra excited at 632.8 nm of [allylC1im][N(CN)2] obtained during the process of compression and decompression. The strong Raman band of diamond was removed for better visualization. (B) Pressure dependence of the ruby luminescence linewidth, Γ(P), with respect to the atmospheric pressure value. The break of slope of red lines marks the pressure of glass transition, Pg ~ 2 GPa, and the blue area indicates the pressure range in which the reaction likely initiates.

More detailed information about the chemical changes was achieved by the analysis of the vibrational bands in Raman and IR spectra after the compressiondecompression cycle. Figure 3.A shows the appearance of a Raman band at 867 cm-1 (green square), which turns out to be a fingerprint of the reaction product as discussed



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below. The decrease in Raman intensity of the ν(C=C) band (inset of Fig. 3.A) after the compression-decompression cycle suggests a possible reaction involving the terminal double bond of allyl group of [allylC1im]+. Figure 3.B shows intensity decrease of Raman bands with contributions of ν(C-H) modes of the allyl group (green arrows) according to vibrational frequencies calculated for an optimized ionic pair of [allylC1im][N(CN)2] (green bars). The structure of the ionic pair obtained by DFT calculation is shown in Figure 4. It is indeed well-known that double bonds become reactive sites when submitted to high pressures leading to the formation of oligo/polymeric materials.26-33 On the other hand, a high pressure study of imidazole (C3N2H5) showed that the ring is stable up to 20 GPa, and no polymerization or other reaction was observed.34 The Raman bands assigned to imidazolium ring are all recovered after decompression, e.g. the band at 1023 cm-1 and the ν(C-H) mode of the imidazolium ring at 3166 cm-1. Therefore, the Raman spectra suggest that the [allylC1im][N(CN)2] would have undergone an oligomerization reaction involving the terminal double bond of the allyl group.

Figure 3. Two spectral regions of Raman spectra excited at 785 nm of [allylC1im][N(CN)2] obtained during compression and decompression processes.


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Spectrum baseline was corrected and normalized by the intensity of the most suitable band in each region indicated by asterisks. (A) The 600-1200 cm-1 region normalized by the band at 1023 cm-1. The inset shows the ν(C=C) band. The Raman band inside the green square is a fingerprint of the reaction product. (B) The spectral range of CH stretching modes normalized by the band at 2959 cm-1. Green arrows indicate bands that have their intensity decreased. Vibrational frequencies and relative intensities calculated for ν(C-H) modes of [allylC1Im]+ in the optimized ion pair structure (see Figure 4) are indicated by bars. Green bars indicate ν(C-H) modes related to allyl group whereas blue bars indicate the othersν(C-H) modes. A scaling factor of 0.965 was used for theoretical frequencies.

Figure 4. DFT optimized structure of [allylC1im][N(CN)2] ionic pair in two different views.

One of the most straightforward reaction that can be realized involving the C=C group of cations is a dimerization forming a dication. Dicationic ionic liquids are very stable and thermodinamic, structural, and dynamical properties of this class of ionic liquids have been discussed.35-40 The effect of dication alkyl linkage chain was reported in a recent vibrational spectroscopy study of imidazolium-based ionic liquids. The 9 ACS Paragon Plus Environment


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fingerprint of longer alkyl linkage chain in bis-methylimidazoliumpropylidene dication is the Raman band at 866 cm-1, which is absent in bis-methylimidazoliummethylidene dication having only one CH2 group linkage.40 This band represents the main signature of the reaction triggered by pressure in the Raman spectrum shown in Fig. 3.A. Figure 5.A shows optimized structures containing two constitutional units (olig2) of the oligomer that is possibly formed by a reaction between [allylC1im]+ cations, either isolated or interacting with [N(CN)2]- anions. Vibrations with strong Raman activities in the optimized structures of isolated olig2 and olig2 + anions were calculated at 842 and 838 cm-1, respectively. Figure 5.A shows that these vibrations belong to deformations of the alkyl linkage chain. Figure 5.B shows that Raman spectra calculated for structures containing three or four constitutional units, olig3 and olig4, also exhibit several intense Raman bands in this spectral region. The optimized structures of olig2, olig3, and olig4 are shown in Figure 6.

Figure 5. (A) Displacement vectors of characteristic normal modes at 842 and 838 cm-1 of optimized structures for isolated olig2 and olig2 interacting with two [N(CN)2]anions. (B) Calculated Raman spectra of the [allylC1im]+ cation, olig2, olig3, and olig4 structures in the region of 800-900 cm-1. Band shapes were modeled by using Gaussian functions with bandwidth of 10 cm-1. 10 ACS Paragon Plus Environment

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Figure 6. DFT optimized structures of olig2, olig3, and olig4. The hydrogen atoms were omitted to facilitate the visualization.

One remaining issue is related to the reactivity of the [N(CN)2]- anion. Figure 7 shows Raman spectra in the region of symmetric and antisymmetric CN stretching modes, νs(CN) and νas(CN). Vibrational frequency values of νs(CN) and νas(CN) are fully reversible during decompression, but a new band appears in between them. One of the simplest explanation for this new band is that [N(CN)2]- anions are probing different environments on the reacted liquid. The optimized olig2 structure with anions (see Fig. 5.A) helps explaining the issue of anions probing different environments. According to calculated Raman spectra shown in the inset of Figure 7, the frequencies of CN modes in olig2 structure shift towards the region between the CN frequencies of the ionic pair. The shift of νs(CN) and νas(CN) frequencies to opposite directions can be clarified by the [N(CN)2]- electronic structure.41 It has been argued that the [N(CN)2]- electronic structure is better represented as the resonance hybrid -N=C=N–C≡N ↔ N≡C–N=C=N-, 11 ACS Paragon Plus Environment


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rather than N≡C–N-–C≡N.41 In fact, this explains νs(CN) being observed at higher frequency than νas(CN) as indicated by polarized and depolarized Raman spectra.42 Thus, it can be reasoned there is a density of states filling the spectral region between

νas(CN) and νs(CN) because of anions probing reacted structures with different extension of oligomerization.

Figure 7. Raman spectra in the region of CN stretching modes of [allylC1im][N(CN)2] obtained during compression and decompression processes. Intensities were normalized by the νs(CN) mode at 2195 cm-1. The inset shows Raman spectra calculated for [N(CN)2]- interacting with the [allylC1im]+ cation and the olig2 structure. Band shapes were modeled by Gaussian functions with bandwidth of 10 cm-1.

Further evidences supporting the above conclusions are obtained from the analysis of IR spectra of [allylC1im][N(CN)2] after the compression-decompression cycle. Figure 8.A shows that two new bands at 1502 and 1607 cm-1 appear in the IR spectrum of the recovered sample. According to IR spectra calculated for oligomer structures shown in Figure 9, these bands can be assigned to CH2 angle deformation 12 ACS Paragon Plus Environment

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modes related to the alkyl chain linkage in the reacted product. The background intensification in the 1200-1700 cm-1 region of Fig. 8.A is in accordance with a distribution of oligomers as indicated by the calculated IR spectra. The reaction occurring through the allyl group of [allylC1im]+ is confirmed by the high frequency region of the IR spectrum. Figure 8.B shows that the IR spectrum of the recovered sample clearly exhibit significant enhancement of bands at 2800 and 2930 cm-1, which characterize C-H stretching modes of saturated carbon atoms in the alkyl chain linkage of oligomeric products. Furthermore, the IR spectrum also excludes the possibility of photochemical activation of the reaction, as verified for other compounds with double bonds when using the laser to obtain the Raman spectrum under high pressure.43

Figure 8. IR spectra in the region of 600-1200 cm-1 (A) and 2600-3250 cm-1 (B) of [allylC1im][N(CN)2] initially at atmospheric pressure (black) and recovered (red) after a compression-decompression cycle up to 16.5 GPa. The arrows indicate characteristic IR bands of the reaction product.



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Figure 9. IR spectra calculated for the [allylC1im]+ cation, olig2, olig3, and olig4 structures in the region of 1200-1700 cm-1. Band shapes were modeled by using Gaussian functions with bandwidth of 10 cm-1.

IV. CONCLUSIONS

Raman and IR spectra, supported by quantum chemistry calculations, confirmed the chemical instability of [allylC1im][N(CN)2] under high pressure. Despite of several possible reactive sites in the ion structures, we concluded that the reaction occurred selectively through the allyl group of [allylC1im]+. Nowadays, it is well-known the occurrence of nanoscale structural heterogeneity in ionic liquids. Such structural heterogeneity results from segregation of polar domains by the charged part of cation and the anion, and nonpolar domains by the alkyl chains in the cation.44 Most probably this structural heterogeneity in [allylC1im][N(CN)2] could favor the oligomerization reaction through the allyl group within nonpolar domains. The results indicate that the reaction of [allylC1im][N(CN)2] is not complete, so that the extension of reaction and the role played by other parameters, e.g. temperature, deserve further investigations. 14 ACS Paragon Plus Environment

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However, this first report on chemical instability of an ionic liquid under high pressure opens the perspective of many studies concerning the singular physical and chemical properties of ionic liquids. The understanding of ionic liquids stability is definitely important for engineering applications. Moreover, this knowledge may open new or alternative synthetic routes, for example, for poly(ionic liquids).

AUTHOR INFORMATION Corresponding author *E-mail adress: [email protected]; Tel.: +55(11) 30912057

Notes The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. No competing financial interests have been declared.

ACKNOWLEDGMENT The authors acknowledge the Brazilian agencies CNPq and FAPESP (Grant Nos. 2014/15107-8, 2015/09763-2, 2015/05803-0, and 2012/13119-3) for fellowships and financial support.



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Triggering the Chemical Instability of an Ionic Liquid under High

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