Study of the Stability of 1-Alkyl-3-methylimidazolium

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A study of the stability of 1-alkyl-3-methylimidazolium hexafluoroan-timonate (V)-based ionic liquids using X-Ray Photoelectron Spectroscopy (XPS) Luiz S. Longo Jr, Emily F Smith, and Peter Licence ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00919 • Publication Date (Web): 09 Aug 2016 Downloaded from http://pubs.acs.org on August 13, 2016

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A study of the stability of 1-alkyl-3-methylimidazolium hexafluoroantimonate (V)-based ionic liquids using X-Ray Photoelectron Spectroscopy (XPS) Luiz S. Longo Jr,1,3 Emily F. Smith,2 Peter Licence3* 1 Instituto de Ciências Ambientais, Químicas e Farmacêuticas, Universidade Federal de São Paulo, Rua Prof. Artur Riedel, 275, Diadema-SP, 09972-270, Brazil 2 NMRC – Nanoscale and Microscale Research Centre, School of Chemistry, The University of Nottingham, Nottingham NG7 2RD, United Kingdom 3 The GlaxoSmithKline Carbon Neutral Laboratory, School of Chemistry, The University of Nottingham, Nottingham NG7 2RD, United Kingdom KEYWORDS. X-Ray photoelectron spectroscopy, ionic liquids, hexafluoroantimonate (V) * [email protected] ABSTRACT: A series of highly pure 1-alkyl-3-methylimidazolium hexafluoroantimonate (V) ionic liquids was firstly investigated by XPS. Reliable binding energy values for all elements within the samples were obtained by applying a C 1s fitting model previously employed for other imidazolium ionic liquids, based on setting the C 1s aliphatic component to 285.0 eV when alkyl chains are longer than 8 carbons. We also observed a straightforward X-ray-mediated photoreduction of Sb (V) to Sb (III) species, i.e. reduction of [SbF6]- anion to volatile SbF3. Thus, binding energies values observed at 542.5 eV and 533.1 eV were attributed to Sb 3d3/2 (V) and Sb 3d5/2 (V), respectively, whereas values at 541.0 eV and 531.7 eV were found for Sb 3d3/2 (III) and Sb 3d5/2 (III), respectively. The SbF3 being formed can be lost either by evaporation and/or redistribution to the bulk. Providing the sample is cooled to freezing temperatures, the SbF3 is prevented from escaping the ionic liquid and it accumulated on the surface of cooled [C4C1Im][SbF6] and [C8C1Im][SbF6]. Particularly in the case of prolonged X-ray exposure of a cooled [C4C1Im][SbF6], an unprecedented further reduction of Sb (III) to Sb (0) was noticed. This study provides fundamental knowledge on the XPS data for antimony compounds as well as providing a step towards the surface analysis of materials composed by antimony species, notably the hexafluoroantimonate (V) anion.

INTRODUCTION Ionic liquids (ILs), low temperature molten salts composed entirely of mobile ions, are often referred to as designer solvents due to the possibility of combining anions and cations to fine-tune their properties. The plethora of anions and cations available and their combinations give rise to ionic liquids which have their own set of physical or chemical properties, suitable for specific applications in several fields such as electrochemistry,1 CO2 absorption,2 gas chromatography stationery phases,3 catalysis4-8 and biocatalysis.9 In regard to the enormous range of opportunities for ion combinations that can render different and tunable ILs, sometimes unjust or unrealistic assumptions of trends in both physical and chemical properties can be made. To predict the behavior of any given ionic liquid for a specific application, changes to the chemical architecture and composition as well as their effects upon cation–anion interactions must be explored both, theoretically and experimentally. X-Ray Photoelectron Spectroscopy (XPS) is a well stablished technique to study physical and chemical properties of ionic liquids as well as quantitative elemental composition (purity) of a sample.10 So far, useful sets of information on bulk and surface chemical structure of ionic liquids from different types,11-17 in situ reaction monitoring,18,19 cation-anion interactions of neat as well as mixtures of ILs,20,21 as well as the study of the properties of halometallate-based ionic liquids22,23 were published elsewhere. Due to their low vapor pressure, ionic liquids are suitable materials to be studied by XPS and 1

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characteristic binding energies for each element within the sample can be correlated with charge transfer from the anion to the cation, which is dependent of the chemical environment and related to the anion basicity.

Figure 1. Survey XP spectrum for [C8C1Im][SbF6], showing all photoemission lines expected for the elements within the sample. No contamination with halides, adventitious carbon or silicon was observed, proving the high purity of the ionic liquids used in this study.

So far, the main class of ionic liquids studied by XPS has been the dialkylimidazolium based ionic liquids bearing standard anions such as halides (mostly Cl- and Br-), tetrafluoroborate [BF4]−, hexafluorophosphate [PF6]−, and bis[(trifluoromethane)sulfonyl]imide [Tf2N]−.11 Less common ionic liquids with the hexafluoroantimonate (V) [SbF6]− anion have been used as solvents for several organic transformations, including Friedel-Crafts alkylation24 and alkenylation,25 isobutene alkylation,26 olefin metathesis,27 asymmetric hydrogenation of quinolones,28 and cyanosilylation of carbonyl compounds.29 Kim et al.30 reported an interesting activation effect upon catalytic activity of Lewis acids (i.e. Sc(OTf)3, InCl3 and TMSOTf) due to anion exchange with non-coordinating [SbF6]- anion from ionic liquids. Thus, DielsAlder reaction of naphthoquinone and cyclohexadiene using 0.2 mol% of Sc(OTf)3 and 1 equiv. of [N4444][SbF6] in dichloromethane was completed within 30 min, whereas the same reaction was very slow without this additive. XPS studies of compounds containing antimony are mainly focused on antimony oxides and halides rather than on [SbF6]− containing compounds.31,32 First reports from early 1970s showed uncertain binding energy values for antimony 3d photoemission peaks, probably due to limitations imposed by equipment and data processing at that time (i.e., deconstruction of the Sb 3d peak due to different oxidation states of antimony and overlapping with O 1s peak).33-36 To the best of our knowledge, to date, there is no report on studies of ionic liquids with [SbF6]− anion using XPS. Thus, in this work we present our findings on the study of 1-alkyl-3-methylimidazolium hexafluoroantimonate (V) ionic liquids of general formula [CnC1Im][SbF6] (where n = 4, 8 and 12) using XPS as well as the study of the stability of hexafluoroantimonate (V) anion upon X-ray irradiation.

EXPERIMENTAL SECTION All ionic liquids used in this study were synthesized in high purity, using standard methods described in the literature.37,38 XP spectra were recorded using a Kratos Axis Ultra Spectrometer with a monochromated Al Kα source (1486.6 eV), hybrid magnetic/electrostatic optics, hemispherical analyzer and multi-channel plate and delay line detector (DLD) with a X-ray incident angle of 30o and a collection angle of 0o, both relative to the surface normal. X-ray source was operated at 10 mA emission current and 12 kV anode potential. All spectra were recorded using an entrance aperture of 300 x 700 micron with pass energy of 80 eV for survey scans and 20 eV for high resolution scans. The instrument sensitivity was 7.5 x 105 counts s−1 when measuring the Ag 3d5/2 photoemission peak for a clean Ag sample recorded at pass energy of 20 eV and 450 W emission power. Ag 3d5/2 full width at half maximum (FWHM) was 0.55 eV for the same instrument settings. Binding energy (BE) calibration was made using Au 4f7/2 (83.96 eV), Ag 3d5/2 (368.21 eV) and Cu 2p3/2 (932.62 eV). The absolute error in the acquisition of binding energies was ± 0.1 eV, as quoted by the instrument manufacturer (Kratos). Therefore, any binding energies within 0.2 eV can be assumed as equivalent. Charge neutralization, when used, was applied employing a standard Kratos charge neutralizer consisting of a filament, coaxial with the electrostatic and magnetic 2


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transfer lenses, and a balance plate which creates a potential gradient between the neutralizer and sample. Charge neutralization was carried out at 1.9 A filament current and 3.3 V balance plate voltage. Sample stubs were earthed via the instrument stage using a standard BNC connector. The preparation method for each sample was dependent upon the nature of the material to be analyzed.

Figure 2. XP high resolution spectra for (a) F 1s, (b) Sb 3d, (c) N 1s and (d) C 1s regions for [C8C1Im][SbF6]. Antimony 3d region shows only Sb (V) components of [SbF6]− anion present in the surface of the IL. All XP spectra were charge-corrected by referencing the aliphatic C 1s component to 285.0 eV. Intensities were normalized to the N 1s photoemission peak.

Room temperature ionic liquid samples were prepared by placing a small drop (ca. 10 mg) of the ionic liquid onto a stainless steel multi-sample bar; solid samples were fixed to the bar using a small piece of a double-sized adhesive tape. Long exposure experiments were carried out by placing a small amount of the solid, or liquid sample, in a stainless steel or gold-coated stub. All samples were pre-pumped overnight to pressures lower than 1 x 10-6 mbar before being transferred into the main analytical chamber, where pressure of < 1 x 10-8 mbar were maintained throughout the analysis. XP survey and high resolution scans, with all expected photoelectron and Auger lines for each element, were recorded to demonstrate elemental composition and purity of the ILs. Samples were generally run at ambient temperatures (≈ 300 K) without charge neutralization and approx. 310 K when charge neutralization was required. Where samples were cooled, dry N2 is passed through an exchange coil in an external vessel of liquid N2 and then passed through the stage, cooling to ≈150 K, allowing samples to be solidified in-situ.

RESULTS AND DISCUSSION At the outset of this work our motivation was to characterize relatively simple [SbF6]- containing ionic liquids by XPS, initial experiments investigated [C8C1Im][SbF6]. The basis for this choice was simple charge correction that we had previously shown to be robust for [C8C1Im]-based systems.11 An example of a wide scan, survey, spectrum is showed in Figure 1. Inspection of the spectrum reveals that the all expected photoelectron and Auger lines for F 1s, Sb 3d, N 1s and C 1s are 3



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present and that the sample is not contaminated by either adventitious carbon, or indeed silicones, which are often observed at the surface.39-41 It should also be noted that there was no evidence of residual sodium or bromide ions which may be carried over from the metathesis step in the ionic liquid synthesis. Experimental stoichiometries extracted from high resolution XP spectra for each element, are presented in Table 1, the values are generally within accepted error of the nominal values (between 10 and 20%).11 The high-resolution spectra for all elements present in [C8C1Im][SbF6] are displayed in Figure 2. A four component model [i.e. C2, (C4+C5), (C6+C7) and Caliphatic (i.e. from C8 to C14)] was applied for deconstruction of the C 1s peak, as previously described.11 Charge-referencing of binding energies for all elements within the ionic liquid was achieved using the value of 285.0 eV for the Caliphatic component, when the alkyl chain is longer than 8 carbons (n = 8 and 12). The same charge correction model was applied for [C12C1Im][SbF6] Table 1. Experimental and nominal (in brackets) stoichiometries for [CnC1Im][SbF6] ionic liquids. Atomic % (excluding H) F 1s

Sb 3d

N 1s

C 1s

1.000

8.627

0.477

0.278

[C4C1Im][SbF6]

32.9 (35.3)

6.4 (5.9)

11.4 (11.8)

49.4 (47.1)

[C8C1Im][SbF6]

26.3 (28.6)

5.3 (4.8)

9.4 (9.5)

58.9 (57.1)

[C12C1Im][SbF6]

19.5 (24.0)

4.2 (4.0)

7.7 (8.0)

69.5 (64.0)

a

RSF

a

Relative sensitivity factors (RSF) taken from the Kratos Library (relative to RSF for F 1s = 1.0).

Previous studies indicate that the length of the alkyl chain has a negligible impact upon the BE of the cationic head when the anion is kept the same. Thus, for [C4C1Im][SbF6], a charge correction was done by setting the measured BE of the N 1s peak equal to that obtained for its long chain counterparts (i.e. 402.2 eV). Using those charge correction models, BE values obtained for F 1s, N 1s, Sb 3d and C 1s for all [CnC1Im][SbF6] studied were reproducible within the experimental error of ± 0.1 eV (Table 2). In addition, a good correlation was observed between binding energies for C 1s components with those previously reported for their [CnC1Im][PF6] analogues.11 The main difference between these two sets of ionic liquids relies on the nature of the anion. Both hexafluoro-coordinated anions can lead to different electronic environments for the central atom as well as for the fluorine atom, anticipating different binding energies for F 1s. Thus, the F 1s peak for all [CnC1Im][SbF6] appeared at 686.2 eV, a small shift from the value observed for F 1s in 1-alkyl-3methylimidazolium hexafluorophosphate ILs (i.e. 686.7 eV). This is in agreement with a weaker Sb-F bond, in which a more electron rich fluorine atom is present. Although such a difference was observed for the F 1s peak, N 1s binding energies of ca. 402.2 eV for all [CnC1Im][SbF6] ionic liquids studied herein remained the same (within the error of ± 0.1 eV). Earlier observations for a series of 1-alkyl-3-methylimidazolium ionic liquids with [N(CN)2]−, [BF4]−, [PF6]−, [OTf]− and [Tf2N]− anions corroborated the finding that less charge is transferred from the anion to the cation when the anions are less basic and less coordinating species such as [SbF6]−.11 Antimony photoelectron emission lines need a more careful analysis. The most intense photoemission line excited by Al Kα for this element is the Sb 3d doublet, which was fitted by taking into account the 3d3/2/3d5/2 spin orbit coupling of 9.3 ± 0.1 eV.33 Thus, Sb 3d3/2 (V) and Sb 3d5/2 (V) components for [C8C1Im][SbF6] were observed at 542.5 eV and 533.1 eV, respectively (Figure 2b). These values are in relatively good accordance with those reported earlier by Birchall et al. for [N2222][SbF6].36 It is noteworthy that the Sb 3d5/2 (V) peak can overlay with the O 1s peak. Nevertheless, the relative area ratio for antimony 3d3/2/3d5/2 peaks (i.e. 2/3, respectively) was not significantly perturbed, suggesting minimal contamination by oxygen containing materials, again indicative of the high purity of the ionic liquids investigated.

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Figure 3. High resolution XP spectra of the Sb 3d region with fittings for (a) [C4C1Im][SbF6] and (b) [C12C1Im][SbF6]. Both oxidation state species, Sb (V) and Sb (III), are present in the surface of the ILs. Intensities were normalized to the N 1s photoemission peak. XP spectrum of [C12C1Im][SbF6] was charge-corrected by setting the measured binding energy of Caliphatic = 285.0 eV; XP spectrum of [C4C1Im][SbF6] was charge-corrected to N 1s = 402.2 eV.

Upon closer inspection of the Sb 3d spectra of this series of samples, it became evident that an interesting mixed oxidation-state system for antimony was present, this was most pronounced in the spectra of both 1-butyl and 1-dodecyl-3methylimidazolium hexafluoroantimonate (V), see Figure 3. The Sb 3d photoemission envelopes were clearly complex, consisting of the expected Sb 3d3/2 and Sb 3d5/2 components assigned to antimony (V) and a second, smaller component which we assign to antimony (III) at lower binding energies (Figure 3). We rationalize this as the X-ray promoted photoreduction of Sb (V) to Sb (III).35 Table 2. Experimental binding energies obtained for [CnC1Im][SbF6] ionic liquids.a Binding energy (eV)b Cation

Anion

N 1s

C2 1s

(C4+C5) 1s

(C6+C7) 1s

Caliphatic 1s

F 1s

Sb 3d3/2 (V)

Sb 3d5/2 (V)

Sb 3d3/2 (III)

Sb 3d5/2 (III)

[C4C1Im][SbF6]

402.2

287.9

287.0

286.7

285.2

686.2

542.5

533.1

541.0

531.7

[C8C1Im][SbF6]

402.2

287.7

286.9

286.6

285.0

686.2

542.4

533.1

--

--

[C12C1Im][SbF6]

402.2

287.8

287.1

286.6

285.0

686.3

542.5

533.1

541.1

531.8

a

Charge referencing achieved by setting the Caliphatic component of C 1s peak to 285.0 eV for [C8C1Im][SbF6] and b [C12C1Im][SbF6], and N 1s peak to 402.2 eV for [C4C1Im][SbF6]; The associated experimental error is ± 0.1 eV.

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Figure 4. XP spectra for (a) F 1s and (b) Sb 3d photoemission lines for NaSbF6. Sample was exposed do X-ray irradiation at room temperature at the same position over 20 hours. The scan 0 was recorded after 50 min of X-ray irradiation exposure and then scans 1 to 19 were taken in 1 hour interval. Intensities were normalized to the Na 1s photoemission peak. Charge referencing was achieved by setting the adventitious carbon photoemission peak to 285.0 eV. Some lines were omitted for clarity.

By application of the same fitting model applied to Sb (V) components, it was possible to identify binding energies of 541.0 eV and 531.7 eV for Sb 3d3/2 (III) and Sb 3d5/2 (III) components respectively, which could be attributed to SbF3 formed upon reduction of [SbF6]− present at the surface of the ionic liquid. Providing good agreement with an earlier study of SbF3 by Morgan et al.33 who noted binding energies at 541.2 ± 0.3 eV and 531.9 ± 0.3 eV for Sb 3d3/2 (III) and Sb 3d5/2 (III), respectively. The reliability of our fitting model for the antimony 3d region was further confirmed by measuring the level separation of [Sb 3d3/2 (V) – Sb 3d3/2 (III)] peaks as well as [Sb 3d5/2 (V) – Sb 3d5/2 (III)] peaks, which remained constant at 1.40 ± 0.1 eV for both, [C4C1Im][SbF6] and [C12C1Im][SbF6]. Keppler et al.42 studied the changes in the near-surface chemical composition of [C2C1Im][NTf2] upon prolonged time exposure to monochromated and non-monochromated Al Kα X-rays. While minor damage was observed in peak shapes when monochromated source was employed, even after 5 days of exposure, extensive damage on [Tf2N]− anion was noticed with non-monochromated source, producing cleavage fragments originated from CF3 groups which were identified by quadrupole mass spectroscopy (QMS). It is noteworthy that a 1.3 times higher maximum photon flux and larger exposure area were attributed to non-monochromated X-ray source. In addition, the authors concluded that [Tf2N]− anion seems to suffer higher levels of degradation compared to the damage on the imidazolium cation. Keeping in mind a possible damage on [SbF6]− anion upon continuous X-ray exposure, we next examined the stability of the ILs studied so far. Then, a relatively simple series of experiment were carried out with samples of pure [C4C1Im][SbF6] and [C8C1Im][SbF6], each placed in a different stainless steel stub, being continuously irradiated by the X-ray beam (at the same position). Survey spectra as well as high resolution scans for F 1s, Sb 3d, N 1s and C 1s were then recorded at 1 hour 6


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intervals over a total experiment time of 20 hours. We could observe that lineshapes of F1s and Sb 3d were not modified over the duration of the experiment. In fact, the XP high resolution spectra for all elements were very similar,43 consistent binding energy values and compositions were obtained across all elements in each scan. Based upon these results in isolation, one could assume that these ionic liquids are very stable and they do not undergo photoreduction by X-rays. However, we had observed reduction of antimony in earlier experiments. In principle, the reduction of Sb (V) could potentially give rise to relatively volatile reaction products which could be dynamically removed from the analysis by the high volume pumping system which is intrinsic to the XPS experiment. SbF3, if formed upon reduction of Sb (V) could be redistributed to the bulk of the sample or indeed evaporated. To test this hypothesis, we submitted a solid sample of commercially available NaSbF6 to continuous X-ray irradiation over a similar 20 hours period. Again we recorded survey and high resolution spectra for Na 1s, F 1s, Sb 3d and adventitious carbon (C 1s) in 1 hour intervals (see Figure 4 for F 1s and Sb 3d peaks over time). As the sample was now a solid salt, diffusion of reduction products away from the surface, into the bulk, of the sample was no longer possible. In principle we should now see the accumulated concentration of Sb(III) at the surface. The experiment was even better in practice, in addition to observing photoemission lines for both Sb 3d (V) and Sb 3d (III), we also observed the growth of a new fluorine containing species confirming the presence of at least 2 new reaction products (SbF3 and fluoride; see further discussion). This accumulation is possible because the material acted as a solid support for the SbF3 being formed. Binding energies for all elements of NaSbF6 and SbF3 were obtained and reported in Table 3, which highlights three methods of charge correction based upon both adventitious carbon (method A) and two independent internal references (methods B and C). The measured binding energies are comparable to those reported by Morgan et al.33 Table 3. Experimental binding energies obtained for NaSbF6. Binding energy (eV)a Charge-correction modelsb Element Na 1s

A

B

C

1072.2

1071.9

1071.9

F 1s SbF6−

686.5

686.2

686.3

F 1s SbF3 / F−

684.7

684.4

684.5 542.5

Sb 3d3/2 (V)

542.7

542.4

Sb 3d5/2 (V)

533.4

533.1

533.2

Sb 3d3/2 (III)

541.0

540.7

540.7

Sb 3d5/2 (III)

531.6

531.3

a

531.4

The associated experimental error is ± 0.1 eV; [SbF6]− = 686.2 eV; (C) Sb 3d3/2 (V) = 542.5 eV.

b

Charge correction models: (A) adventitious C 1s = 285.0 eV; (B) F 1s from

Next, we questioned if the extra F 1s component was due to SbF3 itself or to another compound formed during reduction of [SbF6]− anion. Unfortunately, there is no reported binding energy value for F 1s from SbF3. We envisioned that the fluorine atoms present in SbF3 would be broadly similar to the fluorine atoms present in the [SbF6]− anion, and consequently give rise to coincidental photoemission lines. Hence the main component of the, F 1s photoemission could refer to both [SbF6]− and SbF3 while the “new” F 1s assigned F- which we propose is produced upon photoreduction of Sb (V) (Eq. 1). SbF6− → SbF3 + F2 (g) + F−

(Eq.1)

This hypothesis is supported by the observation that the total fluorine content (the sum of F 1s from [SbF6]− and from F ) decreases as the scans proceed and the sample is irradiated for longer periods. This observation is consistent with a loss of fluorine atoms as F2 gas, which is rapidly removed by the XPS pumping system. −

Furthermore, the measured binding energy values for Na 1s shifts by ca. 0.5 eV over the period of 20 hours when NaSbF6 was irradiated by X-rays (Figure 5). This observation is consistent with the presence of a hard basic fluoride ion and consequently, the formation of sodium fluoride, shifting the BE for Na 1s to lower values (ca. 1071.4 eV, which is in good agreement with published BE data for Na 1s in NaF).34,44-46 It is noteworthy that the binding energies for all other elements and components of NaSbF6 were within the experimental error associated with this experiment.47

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Figure 5. Shift in the binding energy of Na 1s upon prolonged time of X-ray exposure. The scan 0 was recorded after 50 min of X-ray irradiation exposure and then scans 1 to 19 were taken in 1 hour interval. Charge correction applied is model C (Sb 3d3/2 (V) = 542.5 eV). The associated experimental error is ± 0.1 eV.

Primed by this experiment, we then designed an additional experiment to investigate if the same reduction of Sb (V) was observed in our ionic liquid samples. It was proposed that we would only see the products of reduction when the sample was present in the solid state. A sample of pure [C8C1Im][SbF6] was placed onto a clean stainless steel stub, which was then cooled to around 150 K once pumped into the analytical chamber. Survey and high resolution scans for all elements within the IL were recorded at one hour intervals for a total experiment time of 20 hours (20 scans). As the IL solidified to form a glass (scan 4), perturbation of the F 1s photoemission was noted, this is assigned to the production of fluoride upon reduction of Sb (V) to Sb (III) (Figure 6a; from scan 4 to scan 8; solid state), and is similar to that observed previously for solid NaSbF6. Similarly, the XP spectra for the antimony region also revealed an increase in the formation of Sb (III) species (Figures 6b; from scan 4 to scan 8; solid state). Binding energies for all components for F 1 s and Sb 3d region were in agreement (within the experimental error of ± 0.1 eV) with the previous values obtained in experiments carried out at room temperature (Tables 2 and 3). This experiment had indeed indicated that photoreduction was occurring when our sample was irradiated. Considering the RSF/TF corrected intensities for the F 1s and Sb 3d peaks the F:Sb ratio is ~ between 5.5 - 6:1 for the liquid samples, consistent with their structure. In both solid samples we observed the same correlation for the Sb (V) form. The peak positions and shapes were very similar for both solid samples where a second set of peaks also develop. For NaSbF6 there is an antimony 3d Sb (III) shoulder already present at the start of the experiment. The ratio of F(shoulder):Sb(III) is ~ 1:1 initially, rising to 3:1 after 4 hours. In the solidified OMIMSbF6 F(shoulder):Sb(III) was only ~2:1 from the start of solidification. Whilst we would expect a ratio of 3:1 for SbF3, it is not clear why this should change, be lower in ratio or be at such a low BE. Therefore, it is unclear if the F 1s in the low BE peak is just fluoride or if there is a second component at similar energy related which is SbF3. The larger F 1s peak does not change in intensity relative to the Sb (V) suggesting that the Sb (III) form is not contributing to this peak. Spectra for the F 1s and Sb 3d peaks after ~ 5 hours irradiation are shown in SI. Overall, intensities dropped for both solid samples due to adventitious carbon. This might be expected on the cooled OMIMSbF6, but it occurred on the NaSbF6 suggesting that the photoelectron emission and reduction of the antimony causes the surface to adsorb species from the vacuum. To test a further suggestion that mobility of these photoreduction products in the liquid state prevented their detection by XPS, the N2(l) cooling stream was removed. Within the time frame of a single series of scans (scan 9), the sample melted and the new photoemission lines, corresponding to both F− and Sb (III), were no longer observed. This observation confirmed that accumulation of SbF3 is only possible on the surface of a solid sample of [C8C1Im][SbF6]. When the sample is in the liquid state, any SbF3 that is formed is dynamically redistributed to the bulk and/or lost by evaporated, and cannot consequently be detected by XPS. Whilst our chamber was set up to investigate the impact of cooling our sample, we decided to investigate the extended exposure of cooled samples of [C4C1Im][SbF6] with continuous X-ray irradiation. Acquisition of survey and high resolution scans for all elements within the IL were recorded as previously described, in this experiment 10 scan cycles were competed at 150 K, total exposure time being 10 hours. Cooling of the sample was carried out slowly and the X-ray irradiation started only after solidification of the IL (scan 0). As expected, new photoemissions were observed in the case of F and Sb (Figure 7a-c, exemplified for scans 0, 4, and 8). Closer inspection of the N 1s and C 1s regions revealed degradation of the cation such that the photoemission envelopes broadened.41 As this lack of definition could impact upon the use of Caliphatic as an internal charge reference, the F 1s component of the [SbF6]− anion was used as an alternative. The binding energy of the anion had been shown to be robust and 8


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easily identified from a simple deconstruction model, even when F- was noted as a photoreaction product. Consequently the binding energy of principle component of the F 1s photoemission, i.e., [SbF6]- was set equal to 686.2 eV. Analysis of the Sb XP spectra for the pre-cooled [C8C1Im][SbF6], again shows an increase in concentration of Sb 3d (III) components in the near surface region of our glassy sample.

Figure 6. XP spectra for (a) F 1s and (b) Sb 3d regions for scans 4 (280 min), 5 (340 min), 6 (400 min), 7 (460 min), 8 (520 min) and 9 (580 min), obtained at the same position for a sample of [C8C1Im][SbF6]. Data from scan 9 were obtained when the ionic liquid was in the liquid state again (no more freezing). The scan 0 was recorded after 40 min of X-ray irradiation exposure and then scans 1 to 19 were taken in 1 hour interval. Intensities were normalized to the N 1s photoemission peak. XP spectra were charge-corrected by setting the binding energy of N 1s to 402.2 eV. Envelopes were omitted for clarity.

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As the sample was irradiated further (from scan 6 onwards) we also observed a new distinct set of Sb 3d photoemissions at a lower binding energy (Sb 3d 5/2 at 528.1 ± 0.4 eV) suggesting a third Sb environment which we attributed to Sb (0) (Figure 8c, exemplified for scan 8). We suggest that upon freezing the sample, SbF3 is prevented from escaping from the solid surface by either diffusion or evaporation, “fixed” Sb (III) is then susceptible to further reduction to Sb (0), as shown in Equation 2. 2 SbF3 → 2 Sb(0) + 3 F2 (g)

(Eq.2)

In this experiment, it was not possible to obtain accurate binding energies for all elements, taking into account the experimental error of ± 0.1 eV. In fact, BE values varied from 0.5 to 1.0 eV from scans 0 to 10, depending on the element as well as the component. For instance, Sb 3d3/2 (0) and Sb 3d5/2 (0) components were observed at 537.4 ± 0.4 eV and 528.1 ± 0.4 eV, respectively.48 A plausible explanation relies on the fact that, upon formation of high amounts of SbF3 and elemental Sb, the composition as well as chemical environment of the sample changed substantially. In addition, we may anticipate changes in conductivity of the sample as metallic antimony is now present.11 It is also worthy to mention that formation of metallic Sb was not observed with cooled sample of [C8C1Im][SbF6], probably due to the orientation of the long alkyl chains towards the vacuum phase, which forms a thick hydrocarbon blanket somewhat protecting the [SbF6]− anion from extensive photoreduction. A similar long hydrocarbon-based shielding effect was also observed on previous study of the cation–anion interactions of constrained ammonium and phosphonium ionic liquids.15

Figure 7. XP spectra for F 1s region of cooled [C4C1Im][SbF6] upon prolonged period of exposition to the X-ray: (a) scan 0 (40 min), (b) scan 4 (280 min) and (c) scan 8 (520 min). The sample was maintained in the solid state during all scans. Intensities are not normalized. XP spectra were charge-corrected by setting the binding energy of F 1s from [SbF6]− equal to 686.2 eV. Envelopes were omitted for clarity.

10


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Figure 8. XP spectra for Sb 3d region of cooled [C4C1Im][SbF6] upon prolonged period of exposition to the X-ray: (a) scan 0 (40 min), (b) scan 4 (280 min) and (c) scan 8 (520 min). The sample was maintained in the solid state during all scans. Intensities are not normalized. XP spectra were charge-corrected by setting the binding energy of F 1s from [SbF6]− equal to 686.2 eV. Envelopes were omitted for clarity.

CONCLUSION We have successfully analyzed a series of 1-alkyl-3-methylimidazolium hexafluoroantimonate (V) ionic liquids using XPS and the electronic environments of all elements within the samples were identified and characterised. The binding energies values for all elements/components were obtained applying a C 1s fitting model previously employed for other imidazolium ionic liquids. X-ray-mediated photoreduction of Sb (V) to Sb (III) was also described for the first time for these ionic liquids. A stability study of [C4C1Im][SbF6] and [C8C1Im][SbF6] is presented, demonstrating that Sb (V) from [SbF6]- anion is continuously photoreduced to Sb (III) as SbF3 upon prolonged X-ray exposure even at room temperatures. We suggest that SbF3 as a product of photoreduction can be “lost” from XPS analysis either by evaporation or redistribution to the bulk. SbF3 formation has been observed by restricting the degrees of freedom within the sample by cryo-cooling to form a glass. Our experiments showed that accumulation of Sb (III) is possible on the surface of the solid ionic liquid. In the case of extended X-ray exposure of cooled [C4C1Im][SbF6] a further reduction of Sb (III) to Sb (0) was noted along with damage to the cationic backbone of this ionic liquid. The data set presented in this study may be useful for XPS databases of antimony compounds as it provides a step towards the surface analysis of materials containing antimony species. We have demonstrated that care should be exercised when conducting analysis of the Sb 3d region of the XP spectrum. Coincidental overlap of Sb 3d and O 1s photoemissions can cause over quantification of Sb, particularly in the case of surface aggregated poly-silicone based contaminants that may not be observed in other solution based bulk sensitive methods including NMR. Furthermore we demonstrate new insight into photochemistry of the Sb itself, which could have wide reaching impacts upon composition, physical and chemical properties and indeed the chemical activity of the surface itself.

ASSOCIATED CONTENT Full experimental procedures, characterization data, and copies of NMR spectra for all compounds. Details of all XPS spectra and data analysis for all SbF6-ionic liquids as well as NaSbF6. Supplementary tables and figures for the prolonged X-ray exposure experiments carried out in this study. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author 11



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*[email protected].

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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Authors gratefully acknowledge School of Chemistry of The University of Nottingham and EPSRC (EP/K005138/1 and EP/D073014/1) for financial support. Luiz S. Longo Jr also acknowledges Science without Borders/CAPES Brazilian Program for a postdoctoral fellowship (EBX 9741-13-5) as well as Ana R. Santos for fruitful discussions on XPS data processing and artwork. We gratefully acknowledge our reviewers for useful comments regarding peak assignments and relative intensities.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

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H.; Roh, E. J.; Choi, J. H. Scandium(III) Triflate Immobilised in Ionic Liquids: A Novel and Recyclable Catalytic System for Friedel–Crafts Alkylation of Aromatic Compounds with Alkenes. Chem. Commun. 2000, 1695. Yoon, M. Y.; Kim, J. H.; Choi, D. S.; Shin, U. S.; Lee, J. Y.; Song, C. E. Metal Triflate-Catalyzed Regio- and Stereoselective Friedel– Crafts Alkenylation of Arenes with Alkynes in an Ionic Liquid: Scope and Mechanism. Adv. Synth. Cat. 2007, 349, 1725. Xing, X.; Zhao, G.; Cui, J.; Zhang, S. Isobutane Alkylation Using Acidic Ionic Liquid Catalysts. Cat. Commun. 2012, 26, 68. Kim, J. H.; Park, B. Y.; Chen, S.-W.; Lee, S.-G. In Situ Activation of a Latent Ruthenium–Carbene Complex in Ionic Liquid and Its Application in Ring-Closing Metathesis. Eur. J. Org. Chem. 2009, 2239. Ding, Z.-Y.; Wang, T.; He, Y.-M.; Chen, F.; Zhou, H.-F.; Fan, Q.-H.; Guo, Q.; Chan, A. S. C. 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30. Kim, J. H.; Lee, J. W.; Shin, U. S.; Lee, J. Y.; Sang-Gi, L. B.; Song, C. E. Activation of Lewis Acid Catalysts in the Presence of an Organic Salt Containing a Non-Coordinating Anion: Its Origin and Application Potential. Chem. Commun. 2007, 4683. 31. Tang, X.; van Welzenis, R. G.; van Setten, F. M. Oxidation of the InSb Surface at Room Temperature. Semicond. Sci. Technol. 1986, 1, 355. 32. Orchard, A. F.; Thornton, G. Evidence of “Mixed-valency” Character in the X-Ray Photoelectron Spectra of a-Diantimony Tetraoxide and Barium Bismuth Trioxide. J. Chem. Soc., Dalton Trans. 1977, 1238. 33. Morgan, W. E.; Stec, W. J.; Vanwazer, J. R. Inner-Orbital Binding-Energy Shifts of Antimony and Bismuth Compounds. Inorg. Chem. 1973, 12, 953. 34. Tricker, M. J. X-Ray Photoelectron Spectroscopic Studies of Halide Complexes of Antimony(III). Inorg. Chem. 1974, 13, 742. 35. Burrough, P; Hamnett, A.; Orchard, A. F. A Study of the Mixed-valency Compound CszSbCI, by X-Ray Photoelectron Spectroscopy. J. Chem. Soc., Dalton Trans. 1974, 565. 36. Birchall, T.; Connor, J. A.; Hillier, I. H. High-energy Photoelectron Spectroscopy of some Antimony Compounds. J. Chem. Soc., Dalton Trans. 1975, 2003. 37. Bonhôte, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundaram, K.; Grätzel, M. Hydrophobic, Highly Conductive AmbientTemperature Molten Salts. Inorg. Chem. 1996, 35, 1168 38. Xing, X.; Zhao, G.; Cui, J.; Zhang, S. Isobutane Alkylation Using Acidic Ionic Liquid Catalysts. Cat. Commun. 2012, 26, 68. 39. Smith, E. F.; Villar-Garcia, I. J.; Briggs, D.; Licence, P. Ionic Liquids in vacuo; Solution-Phase X-Ray Photoelectron Spectroscopy. Chem. Commun. 2005, 5633. 40. Smith, E. F.; Rutten, F. J. M.; Villar-Garcia, I. J.; Briggs, D.; Licence, P. Ionic Liquids in Vacuo: Analysis of Liquid Surfaces Using Ultra-High-Vacuum Techniques. Langmuir 2006, 22, 9386. 41. Lovelock, K. R. J; Smith, E. F.; Deyko A.; Villar-Garcia, I. J.; Licence, P.; Jones, R. G. Water Adsorption on a Liquid Surface. Chem. Commun. 2007, 4866. 42. Keppler, A.; Himmerlich, M.; Ikari, T.; Marschewski, M.; Pachomow, E.; Höfft, O.; Maus-Friedrichs, W.; Enders, F.; Krischok, S. Phys. Chem. Chem. Phys. 2011, 13, 1174. 43. Waterfall plots of F 1s and Sb 3d photoemission lines processed in 2 hour intervals as well as tables with binding energies values and stoichiometry compositions for all elements/components for [C4C1Im][SbF6] and [C8C1Im][SbF6] can be seen in the Supporting Information. 44. Nefedov, V. I.; Salyn, Y. V.; Leonhardt, G.; Scheibe, R. A Comparison of Different Spectrometers and Charge Corrections Used in XRay Photoelectron Spectroscopy. J. Electron Spectrosc. Relat. Phenom. 1977, 10, 121. 45. Wagner, C. D. Chemical Shifts of Auger Lines, and the Auger Parameter. Discuss. Faraday Soc. 1975¸60, 291. 46. Morgan, W.E.; Van Wazer, J. R.; Stec, W. J. Inner-Orbital Photoelectron Spectroscopy of the Alkali Metal Halides, Perchlorates, Phosphates, and Pyrophosphates. J. Am. Chem. Soc. 1973, 95, 751. 47. Please, refer to Supporting Information for plots with shifting in binding energies of all elements/components for NaSbF6 versus number of scans (time of X-ray exposure). 48. Binding energy values described in ref. 20 for Sb 3d3/2 (0) and Sb 3d5/2 (0) photoemission lines are 537.8 ± 0.3 eV and 528.5 ± 0.3 eV, respectively (charge correction applied: adv. carbon C 1s = 285.0 eV).

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FOR TABLE OF CONTENTS USE ONLY A study of the stability of 1-alkyl-3-methylimidazolium hexafluoroantimonate (V)-based ionic liquids using X-Ray Photoelectron Spectroscopy (XPS) Luiz S. Longo Jr, Emily F. Smith, Peter Licence*

SYNOPSIS TOC A series of hexafluoroantimonate (V)-based ionic liquids are analyzed using X-Ray Photoelectron Spectroscopy (XPS) and a X-ray-mediated photorreduction of Sb(V) to Sb(III) species is described.

Table of Contents graphic

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Study of the Stability of 1-Alkyl-3-methylimidazolium

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