Physical and Electrochemical Properties of Thioether-Functionalized

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Physical and Electrochemical Properties of Thioether-Functionalized Ionic Liquids Angel A. J. Torriero,†,‡ Amal I. Siriwardana,†,‡ Alan M. Bond,†,‡,* Iko M. Burgar,§ Noel F. Dunlop,| Glen B. Deacon,†,‡ and Douglas R. MacFarlane†,‡ School of Chemistry and ARC Special Research Centre for Green Chemistry, Monash UniVersity, Clayton, Victoria 3800, Australia, CMSE DiVision, CSIRO, Clayton, Victoria 3800, Australia, and Orica Ltd., 1 Nicholson Street, Melbourne, Victoria 3000, Australia ReceiVed: May 19, 2009; ReVised Manuscript ReceiVed: July 7, 2009

The preparation and characterization of a series of ionic liquids based on S-alkyl thiolonium, S-alkyl thiotetrazolium, or S-alkyl thiobenzolium cations coupled with bis(trifluoromethanesulfonyl)amide, trifluoromethanesulfonate, alkyl phosphate, chloride, and hexafluorophosphate anions are reported. All are liquid at room temperature, except the chloride salt, which has a melting point of 92 °C. The electrochemical characteristics of this class of ionic liquid have been determined by cyclic voltammetry. Potential windows of the ionic liquids have been obtained at glassy carbon, platinum, and gold electrodes and found to be the largest at glassy carbon, but are limited by oxidation of the thioether-functionalized cation. The voltammetry of IUPAC reference potential scale systems, ferrocene/ferrocenium, cobaltocenium/cobaltocene, and decamethylferrocene/decamethylferrocenium have been evaluated, with the last being most widely applicable. Nonadditivity of Faradaic current is found in the voltammograms of decamethylferrocene in the presence of ferrocene and cobaltocenium. Diffusion coefficient, viscosity, ionic conductivity, double layer capacitance, and other physical properties have also been measured. The dependence of the diffusion coefficient vs viscosity follows the Stokes-Einstein relationship. The properties of the ionic liquids are compared with the related imidazolium family of ionic liquids. 1. Introduction Ionic liquids (ILs) have generated considerable interest as alternative solvents for a wide range of applications. Usually, suggestions for use of ILs as replacement solvents are based on advantageous chemical and physical properties related to vapor pressure or toxicity for chemical, thermal, or electrochemical stability reasons, or because of their ability to dissolve a wide range of organic and inorganic compounds.1-4 Also of common importance are solvent properties that may include miscibility, or lack thereof, with other solvents, solubility with respect to reactants and products, low viscosity, high conductivity, and direct chemical effects associated with the participation of the solvent species in the reaction of interest. Consequently, it is of increasing importance to establish the physicochemical properties of new classes of ionic liquids. From an electrochemical perspective, ILs have been advocated for use as both the solvent and electrolyte in metal deposition, batteries and electrosynthesis.5-8 In the pioneering work of Dai et al.9 and Rogers and co-workers,10 ILs have been used as alternatives to traditional organic diluents for solvent extraction of metal ions.9-14 For example, Dai and co-workers9 examined the extraction of alkali and alkaline earth metals into ILs using a crown ether as an extractant. Their findings showed that the crown ether present in ILs achieved higher extraction efficiencies than when present in ordinary organic diluents. Ligands containing donor atoms such as N or S have been explored for the extraction of heavy metals through the use of * Corresponding author. E-mail: [email protected]. Fax: 61 3 9905 4597. Phone: 61 3 9905 1177. † School of Chemistry, Monash University. ‡ ARC Special Research Centre for Green Chemistry, Monash University. § CMSE Division, CSIRO. | Orica Ltd.

thioether,15,16 thiourea,17 and urea18 functional groups. Rogers and co-workers19 developed novel ILs incorporating metal coordination groups. These “tunable” ILs or “designer solvents” exhibited high extraction efficiencies when used as an extracting phase alone or when doped into nonspecific 1-alkyl-3-methylimidazolium-based ILs.20 Water immiscible ionic liquids which incorporated hydrophobic anions such as [PF6]- or [NTf2]- were used in these studies. These biphasic extractions mainly focused on the removal of metal ions from the water phase to an ionic liquid phase. Methimazole (1-methyl-2-mercaptoimidazole), MMI, which is of interest in this study, is a thiourea derivative which contains a thiol group. It is used as a drug to manage hyperthyroidism associated with Grave’s disease, through inhibition of iodine and peroxidase from their normal interactions with thyroglobulin to form thyroxine and triiodothyronine (two thyroid hormones).21 Additionally, MMI is a molecule of considerable interest in inorganic and organometallic chemistry by virtue of its prolific use as a precursor in the synthesis of novel poly(azolyl)borate ligands, where the sulfur atom, often termed a “tame thiolate”, acts as a soft donor toward a variety of transition metals.22-24 Moreover, this thiolate binding has been utilized in efforts to model the active binding site of a variety of zinc-based proteins.23-25 In the context of ionic liquids, the importance of MMI lies with its direct analogy to the ubiquitous 1-methylimidazole, with the distinction that the somewhat acidic C2 proton is replaced by a thiol linkage.26 MMI exists in a tautomeric equilibrium between the 2-thione 1a and 2-thiol 1b forms (Scheme 1) and has been observed to react in both guises, depending upon the conditions and substrates employed.27-32 Thus, in seeking to “alkylate” this molecule by addition of an alkyl halide (RX), two possible scenarios (Scheme 1) are immediately obvious, viz. (i) alkylation

10.1021/jp9046769 CCC: $40.75  2009 American Chemical Society Published on Web 07/23/2009

Thioether-Functionalized Ionic Liquids

J. Phys. Chem. B, Vol. 113, No. 32, 2009 11223

SCHEME 1: Tautomeric Equilibrium between the 2-Thiol 1a and 2-Thione 1b Forms

SCHEME 2: Structure and Abbreviations of the Cations and Anions Employed in this Work

of MMI at sulfur to afford the 2-alkylthiolonium imidazole 2a; and (ii) alkylation of MMI at nitrogen to afford the imidazolium thiol 2b.26 Both cations have extensive potential for tautomerism and might conceivably exist either as a resonance hybrid or as one discrete canonical form.26 Metzger et al. reported the reaction of MMI with MeI to afford 2-methylthiolonium imidazole.33 However, details of the reaction are limited and the structure was not established. Despite their well-recognized fundamental and practical significance, systematic accounts of the electrochemical properties of thioether-functionalized ILs, akin to those widely available for imidazole-based ILs, are yet to be reported. In this paper, we report the outcome of a systematic electrochemical study of S-alkyl thiolonium, S-alkyl thiotetrazolium, and S-alkyl thiobenzolium based ILs (Scheme 2). Ten ILs were synthesized and electrochemically characterized. Electrochemical properties surveyed include their potential windows, the availability of potential reference scales based on use of ferrocene, decamethylferrocene and the cobaltocenium cation, and some practical issues such as the relationship between their diffusion coefficients and viscosity. It is our hope that provision of this fundamental and useful information may provide a platform for extension of electrochemical and other physicochemical studies on this class of ionic liquids, which to date are known to have potential technical importance but without use in industrial applications as yet.

2. Experimental Section 2.1. Chemicals. The thioether functionalized ionic liquids were synthesized from methimazole (>99%), 5-mercapto-1methyltetrazole (98%), 2-mercaptobenzothiazole (97%), iodoethane (99%), 1-chlorobutane (99%), potassium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)amide, triethyl phosphate (>99.8%), tributyl phosphate, and silver trifluoromethanesulfonate from Aldrich. All of these chemicals were used as received from the manufacturer. Other chemicals used were HPLC grade acetonitrile (CH3CN, Merck), acetone (Merck), dichloromethane (CH2Cl2, Merck), cobaltocenium hexafluorophosphate ([Co(η5-C5H5)2][PF6] (CcPF6), Strem, MA), decamethylferrocene ((Fe(η5-C5(CH3)5)2 (DmFc), Aldrich), and ferrocene (Fe(η5-C5H5)2 (Fc), BDH). These also were used as supplied by the manufacturer. Tetrabutylammonium hexafluorophosphate (Bu4NPF6) was purchased from GFS and recrystallized twice from ethanol before use. 2.2. Synthesis. Dialkyl phosphate based ionic liquids were synthesized by using a slightly modified literature procedure.34 1-Methyl-2-ethylthiotetrazolium Diethyl Phosphate [mtzSEt][P(O)2(OEt)2]. With vigorous stirring, P(O)(OEt)3 (4.30 mL, 25.0 mmol) was added to 5-mercapto-1-methyltetrazole (2.90 g, 25.0 mmol). The mixture was heated slowly until all solid material was dissolved. Next, the mixture was refluxed for additional 7 h. The resulting IL was then taken into acetone and stirred over activated carbon at 50 °C for 20 h. After filtration through Celite, the solvent was removed at reduced pressure. The

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mixture was dried in a vacuum at 70 °C for 2 days. Yield: 98%. IR: 3573, 3232, 3153, 2965, 2938, 2878, 1947, 1582, 1487, 1467, 1346, 1328, 1179, 1132, 1052, 923 cm-1; 1H NMR (200 MHz, [D6]DMSO) δH 12.2 (s, br, 1H, +NH), 3.90 (q, J ) 7.3 Hz, 4H, 2 × OCH2CH3), 3.89 (s, 3H, NCH3), 3.25 (q, J ) 7.4 Hz, 2H, SCH2CH3), 1.34 (t, J ) 6.4 Hz, 3H, SCH2CH3), 1.19 (t, J ) 6.3 Hz, 6H, OCH2CH3); 13C NMR (75 MHz, CDCl3) δC 154.3 (CS), 62.5 (SCH2CH3), 34.1 (NCH3), 27.9 (OCH2CH3), 16.7 (SCH2CH3), 15.4 (OCH2CH3). Anal. Calcd for C8H19N4O4PS: C, 32.21; H, 6.42; N, 18.78; P, 10.38; S, 10.75. Found: C, 32.20; H, 5.96; N, 18.63; P, 9.89; S, 11.20. MS (ESI) calcd for [C4H9N4S]+ m/z 145.0; found m/z 145.0; calcd for [C4H10O4P]- m/z 153.0; found m/z 153.0. 2-Ethylthiobenzolium Diethyl Phosphate [bzSEt][P(O)2(OEt)2]. With vigorous stirring, P(O)(OEt)3 (4.30 mL, 25.0 mmol) was added to 2-mercaptobenzothiazole (4.18 g, 25.0 mmol). Heating and working up as above gave the IL. Yield: 98%. IR: 3561, 3229, 3062, 2981, 2931, 2908, 2871, 2236, 1679, 1582, 1456, 1426, 1394, 1374, 1309, 1236, 1164, 1125, 1100, 1023, 989, 972 cm-1; 1H NMR (200 MHz, [D6]DMSO) δH 13.6 (s, br, 1H, +NH), 7.98 (d, J ) 7.6 Hz, 1H), 7.83 (d, J ) 6.3 Hz, 1H), 7.42 (m, 2H), 3.90 (q, J ) 7.2 Hz, 4H, OCH2CH3), 3.34 (q, J ) 7.3 Hz, 2H, SCH2CH3), 1.40 (t, J ) 6.6 Hz, 3H, SCH2CH3), 1.20 (t, J ) 6.3 Hz, 6H, OCH2CH3); 13C NMR (75 MHz, [D6]DMSO) δC 168.4, 153.4, 135.2, 126.3, 124.4, 121.6, 121.1, 64.0, 63.7, 28.2, 16.3, 16.2, 14.7. Anal. Calcd for C13H20NO4PS2: C, 44.69; H, 5.77; N, 4.01; P, 8.86; S, 18.35. Found: C, 44.51; H, 5.93; N, 4.33; P, 8.89; S, 18.20. MS (ESI) calcd for [C9H10NS2]+ m/z 196.0; found m/z 196.0; calcd for [C4H10O4P]- m/z 153.0; found m/z 153.0. 1-Methyl-2-butylthiolonium Dibutyl Phosphate [mimSBu][P(O)2(OBu)2]. As above, P(O)(OBu)3 (6.80 mL, 25.0 mmol) and methimazole (2.85 g, 25.0 mmol) gave the ionic liquid. Yield: 98%. IR: 3461, 3217, 3052, 2957, 2933, 2908, 2873, 1691, 1583, 1495, 1460, 1416, 1379, 1277, 1232, 1193, 1149,1122, 1062, 1025, 975 cm-1; 1H NMR (200 MHz, [D6]DMSO) δH 12.9 (s, br, 1H, +NH), 7.27 (d, J ) 1.6 Hz, 2H, 2 x NCH)), 3.93 (q, J ) 6.6 Hz, 4H, 2 x OCH2CH2CH2CH3), 3.66 (s, 3H, NCH3), 3.24 (q, J ) 7.3 Hz, 2H, SCH2CH2CH2CH3), 1.67-1.37 (m, 12H, 2 x OCH2CH2CH2CH3, SCH2CH2CH2CH3), 0.94-0.87 (m, 9H, 2 x OCH2CH2CH2CH3, SCH2CH2CH2CH3); 13C NMR (75 MHz, [D6]DMSO) δC 142.2, 126.7, 122.5, 67.6, 65.9, 34.4, 34.0, 32.7, 31.8, 21.8, 18.9, 13.9. Anal. Calcd for C16H33N2O4PS: C, 50.51; H, 8.74; N, 7.36; O, 16.82; P, 8.14; S, 8.43. Found: C, 50.41; H, 8.93; N, 7.33; P, 8.49; S, 8.20. MS (ESI) calcd for [C8H15N2S]+ m/z 171.0; found m/z 171.0; calcd for [C8H18O4P]m/z 209.0; found m/z 209.0. Hexafluorophosphate anion based ionic liquids were synthesized by using a slightly modified literature procedure.35,36 1-Methyl-2-butylthiolonium Hexafluorophosphate [mimSBu][PF6]. With vigorous stirring, KPF6 (4.91 g, 26.7 mmol) was added to an aqueous solution (50 mL of H2O) of [mimSBu][Cl] (5.013 g, 24.3 mmol). The mixture was stirred overnight. Work up as above gave the IL. Yield: 95%. IR: 3636w (br), 3343, 3159, 2962, 2936, 2874, 1582, 1487, 1377, 1278, 1233, 1166 cm-1; 1H NMR (200 MHz, [D6]DMSO) δH 13.4 (s, br, 1H, + NH), 7.76 (d, J ) 8.5 Hz, 2H, 2 x NCH)), 3.78 (s, 3H, NCH3), 3.13 (t, J ) 7.3 Hz, 2H, SCH2CH2CH2CH3), 1.51-1.33 (m, 4H, SCH2CH2CH2CH3), 0.85 (t, J ) 7.2 Hz, 3H, SCH2CH2CH2CH3); 13C NMR (75 MHz, [D6]DMSO) δC 140.7, 126.0, 121.8, 35.6, 34.9, 31.9, 21.5, 13.9. Anal. Calcd for C8H15F6N2PS: C, 30.38; H, 4.78; F, 36.04; N, 8.86; P, 9.79; S, 10.14. Found: C, 30.25; H, 4.56; F, 35.87; N, 8.73; P, 9.49; S, 10.30. The K+ content was 3.1 ppm. The Li+ content was 2.1 ppm. MS (ESI)

Torriero et al. calcd for [C8H15N2S]+ m/z 171.0; found m/z 171.1; calcd for [PF6]- m/z 144.9; found m/z 145.0. 1-Methyl-2-ethylthioloniumTrifluoromethanesulfonate[mimSEt][F3CSO3]. Under dinitrogen, a Schlenk flask was charged with [mimSEt][I] (2.70 g, 10.0 mmol), and dissolved in dry dichloromethane (25 mL). To this was added Ag[F3CSO3] (2.66 g, 10.3 mmol), which after several minutes began to dissipate and give rise to a precipitate. The mixture was stirred in the dark for 8 h and then filtrated; the solvent was removed under reduced pressure to afford a liquid. Yield: 94%. IR: 3493, 3137, 3056, 2979, 2933, 2877, 2754, 1583, 1491, 1454, 1381, 1275, 1235, 1221, 1155, 1060, 1025, 968 cm-1; 1H NMR (200 MHz, CDCl3) δH 13.5 (s, br, 1H, +NH), 7.34 (m, 2H), 3.82 (s, 3H, NCH3), 3.23 (q, J ) 6.3 Hz, 2H, SCH2CH3), (1.35 (t, J ) 6.4 Hz, 3H, SCH2CH3), 13C NMR (75 MHz, CDCl3) δC 142.4, 124.7, 121.3, 120.5 (q, J ) 317 Hz), 35.5, 29.4, 14.8; Anal. Calcd for C7H11F3N2O3S2: C, 28.76; H, 3.79; F, 19.50; N, 9.58; S, 21.94. Found: C, 29.90; H, 3.95; F, 19.23; N, 9.63; 0, 16.79; S, 21.76. MS (ESI) calcd for [C6H11N2S]+ m/z 145.0; found m/z 145.0; calcd for [F3CSO3]- m/z 148.9; found m/z 149.0. Other Ionic Liquids. The synthesis of [mimSBu][Cl], [mimSEt][I], [mimSBu][F3CSO3], [mimSBu][NTf2], and [mimSEt][NTf2] has been previously described.26 2.3. Instrumentation. 1H NMR and 13C NMR spectra were recorded at 21 ( 1 °C with a Bruker Avance AV 200 spectrometer and chemical shifts (ppm) are reported relative to an external tetramethylsilane (TMS) standard. IR spectra of neat liquids were obtained with a Bruker Equinox 55 spectrometer and positive and negative ion electrospray mass spectra with a Micromass Platform 2 ESI-MS instrument. The water content of the ionic liquids was determined with a model 756 Karl Fischer Coulometer (Metrohm) using hydranal Coulomat AG as the titration solution. Duplicate measurements agreed to within 5%. Microanalysis were carried out at the Campbell Microanalytical Laboratory, University of Otago, New Zealand. Density values were obtained with an accuracy of (1% by measuring the weight of the sample in a 5 mL volumetric flask at room temperature (21 ( 1 °C). The temperature dependent phase behavior of each IL over the range of -150 to 120 °C was obtained by differential scanning calorimetry using a T.A Q100 series instrument with 10-20 mg of sample. Thermal scans below room temperature were calibrated using the cyclohexane solid-solid transition and melting points of -87 and 6.5 °C, respectively. Higher than room temperature data were calibrated against the melting point of indium (156.6 °C). Transition temperatures were reported as the peak maximum of the thermal transition. Thermogravimetric analysis was conducted in a flowing dry nitrogen atmosphere (50 mL min-1) using a temperature range of 25 and 500 °C at a heating rate of 10 °C min-1 using a PerkinElmer Pyris TGA 1 instrument. Calibration was achieved using the Curie points of four reference materials: Alumel, perkinalloy, iron and nickel. Platinum pans were used in all experiments with sample weights between 5-10 mg. Viscosity was determined from triplicate measurements carried out in a glovebox for a fixed volume to flow through a narrow orifice in a calibrated glass AMVn viscometer. The ionic conductivity of the liquid samples was obtained at 25 °C by measuring the complex impedance at a frequency between 0.1 Hz and 1 MHz with a Solarton (Farnborough, UK) 1260 immediate response analyzer. A locally designed conductivity cell containing two platinum wire electrodes was used



SCHEME 3: Synthesis of Thioether-Functionalized ILs

experiments in CH3CN employed the same cell configuration, as described above, except that an Ag/Ag+ (CH3CN, 0.01 M AgNO3) reference was employed instead of the Ag wire QRE. The ionic liquids were dried in vacuum at 70 °C for two days prior to measurement, and the electrochemical experiments were conducted inside a homemade nitrogen-filled glovebox. A ME36S microbalance (Sartorius) was employed to weigh microgram amounts of Fc, DmFc, and CcPF6 for preparation of their solutions. All voltammetric experiments were carried out at ambient temperature (21 ( 1 °C).

for the measurements. The cell constant was determined by calibration against aqueous 0.01 M KCl solution. All voltammetric experiments at stationary macrodisk working electrodes were undertaken with a BAS 100B/W electrochemical workstation (Bioanalytical System, West Lafayette, IN). Values of uncompensated resistance, were measured in a potential region where no Faradaic reaction occurs,37 using the RC time constant method available with the BAS instrument. The glassy carbon (GC), gold (Au), and platinum (Pt) working disk electrodes were from Cypress (Cypress Systems, Inc., Lawrence, KS). Prior to each experiment, the working electrodes were polished with 0.30 µm alumina (Buehler, Lake Bluff, IL) on a clean polishing cloth (Buehler), sequentially rinsed with distilled water and acetone, and then dried with lint free tissue paper. Effective electrode areas of 7.24 × 10-3 cm2 for the GC electrode, 7.78 × 10-3 cm2 for the Au electrode, and 7.34 × 10-3 cm2 for the Pt electrode were determined from the peak current for the oxidation of a 1.00 mM Fc solution in CH3CN (0.10 M Bu4NPF6) degassed with N2 and use of the Randles-Sevcik relationship. A diffusion coefficient of 2.30 × 10-5 cm2 s-1 for Fc was used in these calibrations.38 A standard three electrode arrangement was used in voltammetric studies with a Pt wire counter electrode and a silver (Ag) wire, separated from the test solution with a frit, was employed as the quasi-reference electrode (QRE). Voltammetric

3. Results and Discussion New thioether-functionalized ionic liquids were synthesized by two different methods, the simple metathesis reaction of the corresponding chloride or iodide with the silver or potassium salt of the anion (reaction 1 in Scheme 3) or by direct reaction of methimazole with trialkyl phosphates to give halogen free dialkyl phosphate ionic liquids (reactions 2-4 in Scheme 3). All new ionic liquids gave satisfactory microanalysis and had IR, NMR, and mass spectra consistent with the proposed structures. 3.1. Physicochemical Characterization of Thioether-Functionalized ILs. All the thioether-functionalized ionic liquids were dried at 70 °C under a high vacuum for two days, and

TABLE 1: Physical Properties of the Thioether-Functionalized Ionic Liquids Synthesized in this Study abbreviation

δ (g mol-1)

ηa (mPa s)

σa (mS cm-1)

Λ (S cm2 mol-1)

Tgb (°C)

[emim][NTf2] [mimSBu][NTf2] [mimSEt][NTf2] [mimSBu][PO2(OBu)2] [mimSBu][Cl] [bzSEt][PO2(OEt)2] [mtzSBu][PO2(OEt)2] [mimSEt][F3CSO3] [mimSBu][F3CSO3] [mimSEt][PF6]

1.39 1.47 1.61 1.31

34.7 98.5 94.2 117

4.39 0.55 0.51 0.09

1.24 0.17 0.14 0.03

–136 –79 –79 –79

1.23 1.21 1.41 1.38 1.71

54.7 36.9 145 152 222

0.09 0.41 0.64 0.61 0.30

0.02 0.10 0.13 0.14 0.06

–95 –92 –76

Tmc (°C)

92

Tdd (°C) 357e 354e 352 209e 378 396 312 314e

–67

Viscosity (η) and conductivity (σ) data were obtained at 25 °C. Glass transition temperature. Melting point. d Thermal decomposition was determined by thermogravimetricanalysis (TGA) of ionic liquids, which were dried in a vacuum at 70 °C. e Values obtained from ref 26. a

b

c

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Torriero et al. TABLE 2: Electrochemical Potential Windows Obtained at GC, Au, and Pt for the Thioether-Functionalized Ionic Liquidsa ionic liquid [emim][NTf2] [mimSBu][NTf2] [mimSEt][NTf2] [mimSBu][PO2(OBu)2] [mimSBu][Cl]

Figure 1. Walden plot of thioether-functionalized ILs assuming full ionization. (s) data obtained for aqueous 0.01 M KCl solution. (- - - -) Represents 1 order of magnitude below the aqueous KCl line, implying that ILs are only about 10% ionized.

then characterized by NMR spectroscopy, microanalysis, IR, ESI mass spectrometry, and viscosity (Table 1). These data showed that the isolated materials were pure, free from MMI and metal salt starting materials. The water content of dried samples, determined via Karl Fischer titration, was S-alkyl thiolonium > S-alkyl thiobenzothiazolium ≈ S-alkyl thiotetrazolium. The anion stability toward oxidation follows the order: Cl- ≈ [PO2(OBu)2]- ≈ [PO2(OEt)2]- < [F3CSO3]< PF6- < [NTf2]. This anionic stability trend is consistent with that previously published,3 but in this case, the thioether is more easily oxidized than the very stable anions. 3.3.2. DeWelopment of a Reference Potential Scale. In order to compare voltammetric data obtained in ILs, it is necessary to use either a reference electrode of known potential against a standard reference electrode or reference all data to a process whose reversible potential is assumed to be independent of the ionic liquid.49 The ferrocene/ferrocenium (Fc0/+) couple is commonly used to provide a reference potential scale in voltammetric studies in organic solvents containing added supporting electrolyte.50,51 However, ferrocene is not suitable for this purpose in some thioether-functionalized ILs because

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Torriero et al.

Figure 5. Cyclic voltammograms obtained for (A) 3.00 mM ferrocene and (B) 5.00 mM DmFc in [mimSBu][PO2(OBu)2] at a glassy carbon electrode (1 mm diameter) at scan rates of 0.10, 0.20, 0.30, 0.40, 0.50, 0.70, and 1.00 V s-1. Inset to figure (B) represent the dependence of square root of the scan rate on peak current for oxidation of DmFc.

the Fc0/+ process is very close to the positive potential window limit and also is not reversible under all conditions. Figure 5A contains a series of cyclic voltammograms for the Fc0/+ process vs the Ag wire QRE in [mimSBu][PO2(OBu)2]. Only for scan rates above 0.50 V s-1 does the Fc0/+ process exhibit partial chemical reversibility. The mid-point potential (Em) calculated from the average of the oxidation and reduction peak positions [(Epox + Epred)/2] at fast scan rates is 0.77 V vs Ag wire QRE. The voltammetric behavior indicates that Fc+ reacts with the [mimSBu][PO2(OBu)2] system, as in the case for Fc+ in [mtzSEt][PO2(OEt)2]. The nature of the chemical reaction between Fc+ and these ILs is presently unknown. The decamethyl substituted Ferrocene, DmFc, has a reversible E°′ value for the DmFc0/+ process that is ca. 0.50 V less positive than that for Fc0/+ (Table 3) in organic solvents. This is also true in ILs, and hence this process provides a redox couple wellremoved from the solvent oxidation. The DmFc0/+ process was therefore investigated as an alternative reference process. The cyclic voltammogram obtained for oxidation of DmFc at all scan rates (Figure 5B) are close to ideal, because it is well-

TABLE 3: Differences in Reversible Potential Values for the Cc+/0, DmFc0/+, and the Fc0/+ Processes in Acetonitrile and Ionic Liquids

acetonitrile/0.1 M TBA.PF6 [emim][NTf2] [mimSBu][NTf2]

∆E°′(Cc-DmFc) (V)

∆E°′(DmFc-Fc) (V)

∆E°′(Cc-Fc) (V)

0.83

0.51

1.34

0.82 0.83

0.52 0.51

1.33 1.34

removed from oxidation of [mimSBu][PO2(OBu)2] and is chemically reversible because the magnitude of the oxidation peak current (Ipox) to reduction peak current (Ipred) ratio is close to unity (Table 4). Data obtained for oxidation of DmFc in all the thioetherfunctionalized ILs are summarized in Table 4 as a function of scan rate. At all scan rates, peak-peak potential separations (∆Ep ) Epox - Epred) lie in the range of 50 to 100 mV, and hence are close to the theoretical value of 56 mV expected for a one electron reversible couple at 20 °C. Furthermore, E°′ is independent of scan rate, as expected for a reversible process.47



TABLE 4: Cyclic Voltammetric Data Obtained from Oxidation of DmFc and Fc and Reduction of Cc+ in Thioether-Functionalized Ionic Liquidsa DmFc ionic liquid [emim][NTf2]

[mimSBu][NTf2]

[mimSEt][NTf2]

[mimSBu][PO2(OBu)2]

[mimSBu][Cl]

[bzSEt][PO2(OEt)2]

[mtzSEt][PO2(Oet)2]

[mimSEt][F3CSO3]

[mimSBu][F3CSO3]

[mimSEt][PF6]

υ (mV s-1)b

∆Ep(mV)

ipox/ipred

100 300 500 1000 100 300 500 1000 100 300 500 1000 100 300 500 1000 100 300 500 1000 100 300 500 1000 100 300 500 1000 100 300 500 1000 100 300 500 1000 100 300 500 1000

62 62 65 65 68 77 86 89 63 72 84 85 79 82 88 98 61 67 86 86 80 89 92 98 63 63 63 69 69 81 89 93 66 73 76 82

1.04 1.02 1.00 1.00 0.98 0.97 0.97 0.96 0.97 0.98 0.97 0.98 0.98 0.98 0.97 0.97 0.95 0.97 0.97 0.98 1.00 0.98 0.97 0.96 1.05 1.05 1.00 1.00 0.99 0.98 0.98 0.97 1.00 0.99 0.98 0.99

e

e

e

e

e

e

e

e

Cc+

Fc ×107 D (cm2 s-1) 4.19

0.70

0.69

0.60

0.85

∆Ep (mV)

ipox/ipred

59 59 62 68 70 82 94 103 65 71 74 78 139 142 143 139

0.99 0.98 0.98 0.98 0.97 0.96 0.95 0.94 0.97 0.96 0.96 0.94 0.29 0.33 0.35 0.32

d

d

d

d

d

d

d

1.90

1.17

0.35

0.40

c

85 87 89 95 95 103 106 114 68 75 83 89 60 64 69 72 67 67 68 70

×107 D (cm2 s-1) 5.09

1.35

0.99

c

c

d

1.00 0.97 0.97 0.95 0.35 0.36 0.37 0.39 1.02 1.00 0.98 0.98 0.97 0.97 0.98 0.98 1.01 1.01 1.03 1.00

1.98

c

∆Ep(mV)

ipox/ipred

52 54 61 59 73 85 97 100 73 84 86 88 59 65 68 68 69 78 78 87

0.97 0.97 0.98 0.98 0.98 0.97 0.98 0.97 0.98 0.98 0.98 0.98 0.95 0.97 0.98 0.99 0.96 0.97 0.97 0.99

d

d

d

d

d

d

d

d

f

f

f

f

f

f

f

0.48

0.61

0.32

65 73 81 86 65 69 76 84 65 75 85 97

×107 D (cm2 s-1) 3.04

0.41

0.30

0.30

0.60

c

c

f

0.99 0.98 0.97 0.98 0.98 0.98 0.97 0.98 0.97 0.97 0.96 0.95

0.22

0.27

0.14

a The midpoint potential (Em) for the Cc+/0 process calculated from the average of the oxidation (Epox) and reduction (Epred) peak potentials, Em ) (Epox + Epred)/2, is always -1.34 ( 0.01 V versus Em for the Fc0/+ process. Furthermore, Em for the DmFc0/+ process is always -0.51 ( 0.01 V versus Em for the Fc0/+ process. E°′ ) Em for reversible processes, assuming equal diffusion coefficient for oxidized and reduced species. T ) 21 ( 1 °C except for [mimSBu][Cl], where T ) 100 °C. b Scan rate. c Not determined. d Reduced potential window prevents observation of Faradaic processes. e DmFc insoluble in this IL. f Process not chemically reversible.

Minor departures from ideal theoretical values are attributed to a small level of uncompensated IR drop. A plot of Ip vs square root of scan rate (V1/2) exhibits a linear dependence, as expected for diffusion controlled process (Figure 5B, inset). Reduction of cobaltocenium (Cc+) to cobaltocene (Cc), like oxidation of Fc to Fc+, is recommended by IUPAC for use as a potential reference scale standard.50,51 Cyclic voltammetric data (Table 4) confirm that the one electron Cc+/0 couple is almost ideal in most of the ILs at scan rates up to 1.00 V s-1 (limit of scan rate examined). ∆Ep values in these ILs again showed a small dependence on scan rate, but lie close to the ideal value of 56 mV predicted by theory. The small differences between the experimental and theoretical values for a reversible process are attributed to a small level of uncompensated IR drop. The difference in E°′ values for the Cc+/0 and the Fc0/+ processes is 1.34 ( 0.01 V and hence comparable to that found in acetonitrile (Table 3) and values reported in other ILs.52 A plot of Ip vs V1/2 for the reduction of Cc+ was found to be linear (r ) 0.999), again indicative of a fully diffusion-controlled process. No evidence of a contribution of electron hopping53 to the diffusion

coefficient was detected. Cc+ is not a suitable internal standard for use in [mtzSEt][PO2(OEt)2], because the Cc+/0 process occurs at potentials close to the negative potential window limit and is not reversible at all scan rates studied (Table 4). Despite the fact that DmFc0/+, Fc0/+, and Cc+/0 processes are ideally reversible in most of the thioether-functionalized ILs, when the cyclic voltammograms of DmFc was studied in the presence of the Fc and Cc+, the peak currents for all processes were found to be significantly higher than when studied individually in the same ionic liquids. Thus, the apparent diffusion coefficient (Dapp) values calculated for DmFc, Fc, and Cc+ were found to be between 14 and 38% larger than the values determined individually in the same ionic liquids (Table 5). This result is in accordance with the observations previously obtained in 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3methylimidazolium hexafluorophosphate, and others.54 These data support the concept that nonadditivity of Faradaic processes in ionic liquids is fairly general. 3.3.3. Double Layer Capacitance. Kornyshev55,56 and Oldham57 in their theoretical studies have stressed that one cannot

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Torriero et al.

TABLE 5: Dapp for DmFc, Fc, and Cc+ in Their Individual and Mixed Solutions in [emim][NTf2] and [mimSBu][NTf2] Ionic Liquids Determined by Cyclic Voltammetry at GC Electrode at 21 ( 1 °C ionic liquid [emim][NTf2]

[mimSBu][NTf2]

CDmFca CFcb CCc+c ×107 DDmFc ×107 DFc ×107 DCc+ (mM) (mM) (mM) (cm2 s-1) (cm2 s-1) (cm2 s-1) 5.00 0.00 0.00 5.00 5.00 0.00 0.00 5.00

0.00 5.00 0.00 5.30 0.00 5.00 0.00 5.90

0.00 0.00 5.00 5.00 0.00 0.00 5.00 5.00

a Concentration of DmFc. of Cc+.

b

4.19 5.09 4.79 0.70

5.87

3.04 3.92

1.35 0.95

1.60

0.41 0.57

Concentration of Fc. c Concentration

apply the classical Gouy-Chapman-Stern theory to ionic liquid systems. An alternative theory was suggested that took into account some of the constraints associated with ion packing in ILs, as the so-called lattice saturation effect.56 Traditionally, it is assumed Ic ) CdlV. In this study, double layer capacitance per unit area (Cdl) values were obtained by plotting the background or charging currents (Ic) over a selected potential range where no Faradic process is evident, versus scan rate (V). A linear Ic vs V relationship was found to exist and so Cdl may be calculated using the slope of the plot. The double layer capacitance values obtained at a GC electrode for [mimSEt][F3CSO3] (31.8 ( 0.20 µF cm-2), [mtzSBu][PO2(OEt)2] (30.4 ( 0.40 µF cm-2), [mimSBu][NTf2] (27.6 ( 0.30 µF cm-2), [mimSBu][PO2(OBu)2] (26.3 ( 0.30 µF cm-2), [mimSBu][F3CSO3] (24.9 ( 0.40 µF cm-2), [emim][NTf2] (24.6 ( 0.10 µF cm-2), [mimSEt][PF6] (22.1 ( 0.20 µF cm-2), and [bzSEt][PO2(OEt)2] (20.7 ( 0.20 µF cm-2) are similar to values reported for the protic ionic liquid diethanolammonium trifluoromethane sulfonate of 27.0 µF cm-2.58 The capacitance value of 16.6 ( 0.20 µF cm-2 for [mimSEt][NTf2] is comparable to that reported at a GC electrode in aqueous solution with supporting electrolyte, e.g., 15.1 µF cm-2 for aqueous 0.10 M KCl and 14.6 µF cm-2 for aqueous 3.00 M H2SO4, and to values of 10.6 - 12.4 µF cm-2 for a range of [emim]+-based ILs.59 These data suggest that the magnitudes are not too dissimilar from that found in conventional models. 3.3.4. Stokes-Einstein BehaWior. The Stokes-Einstein relationship60 (eq 5) predicts a linear dependence of D on the reciprocal of viscosity (η). Thus

D)

kBT 6πηR

(5)

where kB is the Boltzmann constant, T is the temperature, and R is the hydrodynamic radius of the diffusing species (assuming the molecule is spherical). To assess if the Stokes-Einstein relationship applies in thioether-fuctionalized ILs for the diffusion coefficient of DmFc, we analyzed a plot of D vs 1/η. As seen in Figure 6, excellent linearity is found in nine ionic liquids, which suggests, at least in the sense that D ∝ 1/η, that the Stokes-Einstein relationship holds well for DmFc in thioetherfunctionalized ILs. Furthermore, it confirms that the DmFc0/+ couple may be considered as a “model” system in these media. The Stokes-Einstein relationship is also followed for N,N,N’,N’tetramethyl-p-phenylenediamine (TMPD), Fc, and Cc+ in several ILs61,62 and is commonly obeyed in conventional solvents,60 although for very small molecules, such as H2,63 SO2,64 and O2,65 this relationship does not apply in ILs.

Figure 6. Stokes-Einstein plot of D vs 1/viscosity (η-1) for DmFc. D values were obtained from the Randles-Sevcik relationship in Table 4.

4. Conclusions A series of thioether-functionalized ionic liquids has been synthesized and electrochemically characterized. The structures of the synthesized ionic liquids have been confirmed via microanalysis, IR, NMR, and mass spectral data. Some of them have ionic conductivity and fluidity sufficient for electrochemical applications. The electrochemical properties are strongly influenced by the cation and anion combination. The potential windows (

Physical and Electrochemical Properties of Thioether-Functionalized

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