Solvent-Solute Interactions in Ionic Liquid Media: Electrochemical

Chapter 34

Downloaded by COLUMBIA UNIV on July 18, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch034

Solvent-Solute Interactions in Ionic Liquid Media: Electrochemical Studies of the FerriceniumFerrocene Couple M. Cristina Lagunas, William R. Pitner, Jan-Albert van den Berg, and Kenneth R. Seddon The QUILL Research Centre, The Queen's University of Belfast, Belfast, BT9 5AG, Northern Ireland, United Kingdom

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Electrochemical studies of the ferricenium/ferrocene ([Fc] /[Fc]) redox couple have been carried out in ionic liquids containing the 1-butyl-3-methylimidazolium cation [C mim] and the anions [CF SO ] , [N(SO CF ) ] and [PF ] . The [Fc] /[Fc] couple shows electrochemical reversibility in all cases, and controlled-potential electrolysis of ferrocene to the monocation is chemically reversible. The diffusion coefficients and Stokes-Einstein products of both the oxidized and the reduced species have been determined by using cyclic voltammetry, normal pulse voltammetry and rotating disk electrode voltammetry with both platinum and glassy carbon working electrodes. The solvodynamic radii of the diffusing species in each ionic liquid were estimated using the measured diffusion coefficients and a corrected version of the StokesEinstein equation. A dependency of the solvodynamic radii upon the anion of the ionic liquid was observed. +

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American Chemical Society In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction The use of ionic liquids as green reaction media in industrially relevant processes has increased dramatically in die last years. The haloaluminate(III) ionic liquids, initially developed in the late 1940s by Hurley and Wier [i], have found infrequent use as solvent systems due to their moisture and air-sensitivity. The report in 1992 by Wilkes and Zaworotko [2] of air- and moisture-stable ionic liquids opened the door for their application in conventional chemical reactions, many of which are metal-mediated processes. Because metal complexes are expected to behave differently in ionic liquids than in conventional solvents, fundamental questions about solvent-solute interactions in these media must be addressed. Valuable information on solvent-solute interactions can be obtained through electrochemical studies of the transport properties of metal complexes in ionic liquids. Unfortunately, very little fundamental information concerning the electrochemical properties of ionic liquids, especially the non-haloaluminate(III) ionic liquids, has yet been produced. Two recent investigations probed the fundamental nature of mass transport in ionic liquids [3,4], With the aim of developing ionic liquids as components of electrochemical gas phase reactors and gas sensor systems, Schroder et ah [3] demonstrated die effect of water on the diffusion of ionic and neutral species and proposed a model for the nanoscale structure of water in ionic liquids. Quinn et al. [4] investigated the partition of water and the ionic liquid in biphasic mixtures. There is a demonstrable need for more investigations into the fundamental nature of mass transport in ionic liquids.

The Ferricenium/Ferrocene Couple +

The ferricenium/ferrocene ([Fc] /[Fc]) redox couple has been thoroughly studied in aqueous and non-aqueous solvents [5]. Ferrocene is widely used as an internal standard for reporting electrode potentials, due to the assumption that the standard electrode potential of the [Fc] /[Fc] couple remains invariant, irrespective of the nature of the solvent [6]. Fundamental studies on metal complexes in chloroaluminate(lll) ionic liquids were initiated by Chum et al. [7] whose investigations included ferrocene. Interest in studying the behaviour of the [Fc] /[Fc] model system in ionic liquids has continued [8-12]. Cyclic voltammetric studies showed that the couple was electrochemically stable and reversible in acidic (1:2 molar ratio) mixtures of JV-ethylpyridinium bromide:aluminium(III) chloride and JV-butylpyridinium chloride:aluminium(III) chloride systems of various compositions (from basic 1:0.75 to acidic 1:2 mixtures) [9]. The ferrocene oxidation potential (0.24 V vs. the A1(III)/A1 reference electrode [9]) was reported to be independent of solvent acidity, indicating that no significant interaction occurred between the ionic liquid and ferrocene. More detailed electrochemical studies in mixtures of N-butylpyridinium chloride:aluminium(III) chloride showed that several metallocenes, including ferrocene, exhibited complicated behaviour dependent on solvent acidity [8b 10]. +

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In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

423 Both ferrocene and ferricenium were stable in neutral mixtures. In acid mixtures, ferrocene was oxidised by traces of moisture and dioxygen. In basic mixtures, ferricenium reacted with excess chloride. In addition, the electron-transfer rate constant for the couple in basic mixtures of l-ethyl-3-methylimidazolium chloride:aluminium(III) chloride was found to decrease with increased solvent basicity (*.e., increased viscosity) [8c], This has been related to the slow relaxation of the highly associated basic ionic liquid, rather than to solvation effects, given the constancy of the formal potential of the system over the solvent composition range. In the first investigation of electrochemical systems in non-haloaluminate(III) ionic liquids, Fuller et al. [11] examined the behaviour of ferrocene in the l-ethyl-3methylimidazolium tetrafluoroborate ionic liquid. The [Fc] /[Fc] couple was found to be chemically stable and electrochemically reversible. Hultgren et al, have recently looked at a number of metallocene derivatives, including ferrocene, in the 1butyl-3-methylimidazolium hexafluorophosphate ionic liquids, in an effort to provide a method for calibrating reference potentials in ionic liquids [12]. In this paper the [Fc] /[Fc] redox system was characterized electrochemically in ionic liquids containing the same cation (l-butyl-3-methylimidazolium [Cîm]*) and three different anions (trifluoromethansulfonate [OTf]", bis(trifluoromethanesulfonyl)imide [NTf ]" and hexafluorophosphate [PF ]").


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Solvent-Solute Interactions Electrochemistry provides a convenient method for investigating solvent-solute interactions. Many electrochemical techniques, such as cyclic voltammetry, rotating disc electrode voltammetry and normal pulse voltammetry, allow the determination of the diffusion coefficient of electrochemically active species. From the diffusion coefficient, the solvodynamic radius of the diffusion species can be estimated [13]. The solvodynamic radius provides dimensional information about the solute and any solvating molecules or ions. Stokes' law [14] (Equation 1) F=πbηvr

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(1)

relates the drag force F opposing a macroscopic sphere moving in an ideal hydrodynamic medium with the viscosity of the medium η and the solvodynamic radius r and. velocity ν of the macroscopic sphere. The numerical constant b usually has a value of 6 or 4, depending on whether 'stick' or 'slip' boundary conditions between the moving sphere and the fluid in contact with it apply, respectively. The 'stick' condition applies for a large spherical particle in a solvent of low relative molecular mass. The 'slip' condition applies when a molecule diffuses through a medium consisting of molecules of comparable size. The application of Stokes' law to the diffusional movements of particles gives the Stokes-Einstein relation (Equation 2), s


424 D = k T/n b rçr B

(2)

s

where k is the Boltzmann's constant, F is the absolute temperature and D is the diffusion coefficient of the analyte. Equation 2 is sometimes expressed in the form of the Stokes-Einstein product D η IΓ (Equation 2a). B

ΒηΙΤ

= k /n

br

B

(2a)

s

The Stokes-Einstein product should be constant for a given analyte provided that the solvodynamicradius,r , of the moving particle does not vary and that Equation 1 is valid. In spite of its approximate nature, Equation 2 holds reasonably well on the molecular level, and it provides a useful starting point for discussing the dimensions of solvated solute molecules. It has been shown that the Stokes-Einstein products of ferrocene in various chloroaluminate(UI) ionic liquids remain almost constant, with values lying in a narrow interval: 6.0-7.7 χ 10' g cm s' K" [7-10]. Considering the different methods employed in the determination of the diffusion coefficients and in the measurements of the viscosity of the solvents, the constancy of the Stokes-Einstein product is remarkable. This clearly demonstrates that the same molecular species (with the same r ) is in motion in all the chloroaluminate(III) ionic liquids previously studied. As no evidence of strong solvent-solute interactions has been found, this diffusion species is assumed to be unsolvated ferrocene. From Equation 2, if the Stokes-Einstein product for ferrocene lies between 6.07.7 χ 10" g cm s" K" , the solvodynamic radius of unsolvated ferrocene would then be in the range 0.98-1.22 A, for b = 6, or 1.47-1.83 A, for b = 4. These values are unrealistically low when compared with the known crystallographic radius of ferrocene (3.5 A) [IS]. Robinson and Stokes [16] demonstrated that while Equation 2 is applicable for particles greater than ca. 5 A in radius, it gives radii that are considerably too small when applied to particles smaller than this. They suggested a correction factor be introduced as the ratio r/r , where r represents the radius estimated from molecular volumes or other models, and r the radius calculated by Equation 2. Thus, the corrected solvodynamic radius, r , would be given by Equation 3:


s

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s

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s

s

c

r = (k T/n c

(3)

bηD)(r/r )

B

s

If an analogous approach is to be used in the case of ferrocene in ionic liquids, then when r is substituted by the crystallographic radius of ferrocene (3.5 A), and r by the mean radius calculatedfromthe Stokes-Einstein products (ie. 1.10 or 1.65 A for b = 6 or 4, respectively), the solvodynamic radius can be approximated by Equation 3a. s

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r = k T/2n c

B

ηϋ


(3a)

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Experimental [C mim]Cl was prepared by the reaction of chlorobutane with 1-methylimidazole in ethanenitrile and recrystallised with ethyl ethanoate [17]. [C mim]Cl was used as a precursor for all the ionic liquids used in this work [2]. Its reaction with Na[OTf] in propanone, or with Li[NTf ] or H[PF ] in water gave the corresponding ionic liquids [C mim][OTfJ, [C mim][NTf ] and [QmimJEPFJ, respectively. Residual chloride was removed by dissolving the ionic liquids in dichloromethane and washing the mixture with water until the aqueous washings did not turn cloudy in the presence of silver nitrate. In order to remove residual water and organic solvents, the liquids were first dried under reduced pressure on a rotoevaporator at 60 °C and then dried under high vacuum for 5-12 h at 60 °C. Ferrocene was sublimed prior to use. [Fc][PF ] was prepared by oxidation of ferrocene with H S 0 in water in the presence of K[PF ] [18]. Both products were stored under a dry dinitrogen atmosphere. The ferrocene and ferricenium solutions used for electrochemical experiments were prepared in three different ways. Method 1 involved dissolving a weighed amount (ca. 10-15 mg) of solute (ferrocene or [Fc][PF ]) in the corresponding mass (ca. 10-15 g) of ionic liquid. Method 2 involved introducing an exact volume (2-3 cm ) of a 0.02 mol Γ solution of the solute in dry dichloromethane (CH C1 ) into a flask, removing the CH C1 by evaporation under vacuum and adding an exact mass of the ionic liquid (ca. 10-15 g) to the solute residue. In both Method 1 and 2, the mixtures were stirred under vacuum (2-5 h) until homogeneous orange (ferrocene) or dark blue (ferricenium) solutions were formed. Method 3 involved the bulk electrolysis (oxidation) of a solution of ferrocene in [C mim][PF ] to produce a solution of [Fc][PF ], and will be discussed in detail below. All ionic liquids and ionic liquid solutions of ferrocene and [Fc][PF ] were stored either under dinitrogen or under vacuum. Table 1 summarises for the preparation of the solutions used in this investigation. 4

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Table 1. Preparation of solutions for electrochemical investigation. Solution Label A Β C D Ε F G

Ionic Liquid [C4mim][PF ] [C mim][PF ] [C mim][PF ] [C mim][PF ] [C mim][PF ] [C mim][OTf] 6

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[C mim][NTf2j 4

Solute [Fc] [Fc] [Fc] [Fcf [Fc] [Fc] [Fc]

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Preparation Method 1 2 2 3" 1 2 2

Concentration /mol Γ' 0.0048 0.0043 0.0039 0.0039 0.0051 0.0053 0.0051

electrolysis of Solution B.


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426 In all cases, the purity of the ionic liquid was determined by viscosity measurements using an LVDV-II Brookueld cone and plate viscometer. These values were used in the calculation of the diffusion coefficients and solvodynamic radii. Densities of the ionic liquids were measured with an Anton Paar DMA 4500 density meter. The electronic absorption spectra were recorded on a Perkin Elmer Model Lambda2 UV/VIS spectrometer. All electrochemical experiments were carried out with an EG&G PARC Model 283 potentiostat/galvanostat connected to a PC through an IEEE-488 bus and controlled using EG&G Pare Model 270/250 Research Electrochemistry version 4.23 software. Positive feedback iR compensation was employed to eliminate errors due to solution resistance. Voltammetric experiments were performed at 21 ± 2 °C either inside a controlled-atmosphere box or on a bench under a dry dinitrogen flow, using a threeelectrode system. The non-aqueous reference electrode (BAS) was a silver wire immersed in a glass tube containing a 0.1 mol Γ solution of AgN0 in the [C mim][N0 ] ionic liquid separated from the bulk solution by a Vycor plug. All potentials reported are referenced against the Ag(I)/Ag couple. The counter electrode was a platinum coil immersed directly in die bulk solution. The working electrode was a platinum disc (BAS, 1.6 mm diameter or Pine, 5.4 mm diameter) or a glassy carbon disc (BAS, 3.0 mm diameter or Pine, 7.9 mm diameter). Rotating disc electrode experiments were carried out using a Pine Model AFM4RXE Analytical Rotator with MSRX Speed Control. 1

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Results Spectroscopy The electronic absorption spectra of ferrocene obtained in the three ionic liquid systems studied were similar to each other and showed no significant differences with the spectra obtained in conventional solvents (i.e., at ca. 441 nm [ε = 89 1 mol* cm" ] and 321 nm [ε = 571 mol" cm" ] [19]). The solutions of ferrocene in these ionic liquids kept under dinitrogen were stable for weeks. Spectra were also recorded for ferricenium in [C mim][PF ] with = 617 nm [ε = 340 1 mol" cm" ]. Typical spectra for ferrocene and ferricenium in [CîmJIPFd are shown in Figure 1. 1

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Cyclic voltammetry +

The [Fc] /[Fc] couple was found to undergo reversible one-electron transfer in all three ionic liquid systems studied. Cyclic voltammetric experiments were


427 performed at both platinum and glassy carbon (GC) electrodes, over scan rates ( v) rangingfrom0.01 to 0.1 V s" . Typical voltammograms are shown in Figure 2. From data collected at ten different scan rates, the average value of the half-peak potential E and the peak-to-peak separation AE were calculated for each sample. The ratio of the peak currents | z / / ! were calculated by using the Nicholson equation [20] from sets of data collected at 0.05 V s" at ten different switching potentials (Εχ = 0.050-0.275 V). Average values for E , AE and | ÇI i*\ are shown in Table 2. According to the Randles-Sevcik equation (Equation 4) [13] 1

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ip = 2.69 χ 10 n A D C v

(4)

for a reversible diffusion controlled system at 25 °C there is a linear relationship between the peak current and the scan rate, where i is the peak current in A, vis the scan rate in V s' , A is the electrode area in cm , D is the diffusion coefficient in cm s", and C is the bulk concentration in mol cm" . Typical plots of i vs. v are given in Figure 3. Using Equation 4, these plots were used to calculate the diffusion coefficients of ferrocene in each of the ionic liquids. These calculated values are collected in Table 3. p

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Figure 1. UV/VIS spectra of: (a) a solution ofO. 0043 mol Γ [Fc] in [C mim][PF ]; (b) the same solution after 92% conversion to [Fc] by oxidative electrolysis. 4

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Table 2. Cyclic voltammetric data for [Fc] and |Fc] in [C mim]X ionic liquids. 4

Working electrode

[OTf]" [Tf NV 2

Pf GC Pf GC Pt

a

a

e

E /mV [Fcl -0.032 ±0.001 -0.034 ±0.001 -0.011 ±0.001 -0.010 ±0.001 -0.025 ±0.001 [Fc| -0.020 ±0.001 -0.021 ±0.001

AEJmV

m

0.050 ±0.004 0.058 ± 0.004 0.069 ±0.005 0.072 ±0.006 0.058 ±0.004

0.97 ±0.02 1.03 ±0.02 0.95 ±0.02 0.97 ±0.02 0.98 ±0.02

0.058 ±0.005 0.053 ±0.003

1.11 ±0.03 1.05 ±0.02

+C

b

Pt GC"

BAS working electrode. Pine working electrode. [Fc] produced by electrolysis (Method 3).

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Table 3. Diffusion coefficients of [Fc] in [C mim]X calculated from data obtained by cyclic voltammetry (CV), normal pulse voltammetry (NPV) and rotating disk electrode voltammetry (RDE). 4

Solution Electrochemical label technique

Working Slope" R* /cm s' electrode [CîmHPFi]1 0.95 0.9997 8.01 Pt CV 30.39 0.9966 1.11 GC Pt 1.70 1.05 0.9998 A NPV 6.21 0.9908 1.13 GC pb 0.76 0.9770 0.76 RDE 1.04 0.9424 GC 0.66 Pt" 53.71 0.9991 0.80 CV GC 82.41 0.9999 0.83 Pt 10.81 0.79 0.9997 Β NPV GC 15.93 0.9995 0.76 Pt 0.68 0.9575 0.77 RDE GC 0.9541 0.77 1.03 Pt 43.94 0.65 0.9999 CV GC 68.15 0.9996 0.69 Pt 9.82 0.9997 0.79 C NPV 13.62 GC 0.9993 0.67 pb 0.56 0.9446 0.67 RDE GC 0.82 0.9583 0.64 |C miml|OTn Pt 7.29 0.9992 0.65 CV GC 22.29 0.9999 0.49 1 Pt 0.9994 1.56 0.72 NPV GC 4.92 0.58 0.9985 ICîmllNTfj]1 Pt 0.9991 3.04 15.21 CV GC 50.62 0.9995 2.72 G Pf 3.19 0.9997 3.27 NPV GC 10.82 3.04 0.9990 BAS. Ψιηε. Slope from plots of i vs. v* , i(t) vs. f or and i vs. 0.99). Alternatively, the non-zero intercept could be due to the high kinematic viscosities of the ionic liquids, which make the hydrodynamic boundary layers too large. Under such conditions, Equation 6 does not apply [13]. From our data, the average diffusion coefficients calculated at 25 °C for ferrocene in [C mim][PF ], [C mim][NTf ] and [C mim][OTf] (Table 5) were (0.79 ± 0.09), (3.0 ± 0.2) and (0.61 ± 0.09) χ 10" cm s", respectively. According to equation (3a), the estimated radii of ferrocene in [C mim][PF ], [C mim][NTf ] and [C mim][OTfJ would then be of ca 3.0 ± 0.4, 4.3 ± 0.3 and 13 ± 2 Â. Whereas the calculated solvodynamic radii of ferrocene in [C mim][PF ] and [C mim][NTf ] correspond to the crystallographic radius of ferrocene (3.5 Â), the solvodynamic radius of ferrocene in [C mim][OTf] is significantly larger (13 ± 2 Â). The most likely form of interaction between ferrocene and [OTf]" is through hydrogen bonding. Of the three anions studied, [OTf] has the greatest tendency to form hydrogen bonds in the solid state [22]. This type of interaction has also been shown to play a part in the liquid structure of ionic liquids which contain [OTf] [23]. Although the interaction between a neutral molecule, such as ferrocene, and an anion is unexpected, it has been observed in ionic liquids [24]. Interaction between hydrogens on the cyclopentadienylringsand [PF ]" or [NTf ]" would not be probable. However, the nature of the diffusing species in [C mim][OTf| is not clear. Further studies are underway and will be reported in a future paper. From the calculated diffusion coefficient of ferricenium in [C mim][PF ] (0.31 ± 0.04 χ 10' cm s" ), Equation 3a gives a corresponding solvodynamic radius of ca. 7.7 ± 1.0 A. Since the crystallographic radius of ferricenium is not much different from that of ferrocene, the significantly larger solvodynamic radius of ferricenium in the [C mim][PF ] ionic liquid (3.0 ± 0.4 A vs. 7.7 ± 1.0 A) may indicate the presence of strong interactions between ferricenium and [PF ]\ whose crystallographic radius is ca. 3.2 Â [25]. Coulombic interactions of ferricenium with the tetrachloroaluminate anion have also been suggested, since the diffusion coefficient of ferricenium in neutral ΑΓ-butylpyridinium chloride:aluminium(III) chloride was found to be about 25% smaller than that of ferrocene [8b]. This report marks the beginning of the first systematic investigation of the nature of solute-solvent interactions in non-chloroaluminate ionic liquid solutions using electrochemical techniques. As more ionic liquid systems are studied, using a variety of organic, inorganic and organometallic substrates as electrochemical probes, a more certain and detailed picture of these interactions will begin to emerge. Information gathered from crystallographic, spectroscopic and rheological investigations will be invaluable in supporting the theories drawnfromsuch studies. 1

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Acknowledgments The authors thank the industrial members of the Q U I L L Research Centre for their financial support. M . C. L. thanks Fundacion Flores-Vallès, Ministerio de Educacion y Ciencia and The Royal Society for their grants.


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438 [14] Bockris, J. O'M.; Reddy, Α. Κ. N . Modern Electrochemistry, Plenum Press, New York, 1970; p. 377. [15] Seiler, P.; Dunitz, J. D. Acta Crystallogr.Sect.Β 1979, 35, 2020. [16] Robinson, R. Α.; Stokes, R. H . Electrolyte Solutions, 2 ed., Butterworths Scientific Publications, London, 1959; pp. 120-126. [17] Wilkes, J. S.; Levisky, J. Α.; Wilson, R. Α.; Hussey, C. L. Inorg. Chem. 1982 21, 1263. [18] Coates, G. E.; Green, M . L. H.; Wade, K. Organometallic Compounds, Vol. 2, 3 ed, Methuen & Co Ltd., London, 1968; p. 104. [19] Rosemblum, M . Chemistry of the Iron Group Metallocenes: Ferrocene, Ruthenocene, Osmocene; Willey & Sons, New York, 1965; p. 46-47. [20] Nicholson, R. S. Anal. Chem. 1996, 38, 1406. [21] (a) Bonhôte P.; Dias, A.-P.; Papageorgiou, N ; Kalyanasundaram, K; Michael Grätzel, M . Inorg. Chem. 1996, 35, 1168-1178. (b) Seddon, K. R.; Stark Α.; Torres, M.-J. Clean Solvents: Alternative Media for Chemical Reactions and Processing; Abraham, Μ. Α.; Moens, L., Ed.; American Chemical Society, 2002; pp. 34-49. (c) Torres, M.-J. PhD Thesis, The Queen's University of Belfast, 2001. [22] Schaefer, W. P.; Quan, R. W., Bercaw, J. E. Acta Crystallogr. Sect C [Cr. Str. Comm] 1992, 48, 1610-1612. [23] Bradley, A. E.; Hardacre, C.; Holbrey, J. D.; Johnston, S.; McMath, S. E. J.; Nieuwenhuyzen, M . Chem. Mater. 2002, 14, 629-635. [24] Holbrey, J. D.; Nieuwenhuyzen, M . ; Seddon, K. R., unpublished results (1996-2000). [25] Delaplane, R. G.; Lundgren, J.-O.; Olovsson, I. Acta Crystallogr. Sect. Β 1975, 31, 2208. nd


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Solvent-Solute Interactions in Ionic Liquid Media: Electrochemical

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