The Corannulene Reduction Mechanism in Ionic Liquids is

Article pubs.acs.org/JPCC

The Corannulene Reduction Mechanism in Ionic Liquids is Controlled by Ion Pairing Eden E. L. Tanner,† King Yoong Foong,‡ Md. Mokarrom Hossain,‡ Christopher Batchelor-McAuley,† Leigh Aldous,*,‡ and Richard G. Compton*,† †

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom ‡ School of Chemistry, UNSW Australia, Sydney NSW 2052, Australia ABSTRACT: The electroreduction of corannulene (C20H10) has been investigated in a room temperature ionic liquid (RTIL) for the first time. In the RTIL 1-butyl-1methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([Bmpyrr][NTf2]) the resultant voltammetry shows a peak-to-peak separation of 100 mV, and this separation does not vary with scan rate (as predicted by a simple E mechanism). We propose a square scheme that is capable of accurately describing this behavior. Specifically, the use of a square scheme takes into account the effect of ion pairing between the ionic liquid cation and the corannulene anion on the overall reaction mechanism. Importantly, investigation in acetonitrile with a range of conventional electrolytes does not display the trends observed in the RTIL. This result likely provides a general insight into all RTILs as a class of electrolyte, because of the high concentration of ions and the proclivity of RTILs to ion-pair.

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number of electrons transferred) at 25 °C is commonly taken as diagnostic of slow electron transfer kinetics,18 which RTILs are noted for revealing due to their higher viscosity, slowing mass transport rates relative to electron transfer rates thus promoting a switch from electrochemical reversibility to irreversibility.19,20 However, the understanding of the role an RTIL plays purely in terms of slower electron kinetics can lead to misinterpretation of the experimental data, and the impact of ion-pairing on the reaction mechanism needs to be more closely considered. In this paper, we use a square scheme,21 as first proposed by Jacq22 and Laviron,23 to exemplify how coupled homogeneous processes can lead to voltammetry in which the peak-to-peak separation exceeds 57 mV, but the rate at which this separation changes with scan rate does not vary as predicted from a simple one electron process. Under certain kinetic regimes, the electrochemical mechanism may follow a different pathway on the forward and reverse scans. Consequently, the peak position reflects the thermodynamics of bound and unbound redox species and not the electron transfer kinetics. We study the one electron reduction of corannulene to the monoanion in both acetonitrile and the RTIL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpyrr][NTf2]) to demonstrate the importance of properly considering the role ion pairing plays in the reaction mechanism in ionic solvents, as seen in Figure 1, and thus the use of a square scheme to describe the electron transfer dynamics and nonideal voltammetry observed in the RTIL.

INTRODUCTION Interest in the corannulene (C20H10) molecule is high due to its special electronic structure,1,2 its reactivity,3 and the possibility of construction of other carbon nanomaterials using this molecule as a starting point.4 Corannulene can be viewed as being a fragment of a buckministerfullerene,5 and its most stable neutral geometry is bowl-shaped.6 Electrochemically, the reduction of corannulene to its anion, dianion, and trianion species has been documented at low temperature in a number of nonaqueous solvents by Bruno et al.7 The solvent used was found to have a large effect on the appearance of the voltammetry and, therefore, the reaction pathway. Ion pairing effects were shown to influence the formal potential of the C/ C− couple and therefore the ability to electrochemically generate and stabilize the higher anions within the available electrochemical window. Room temperature ionic liquids (RTILs) usually consist of a bulky, asymmetric organic cation and an inorganic anion8 and are liquid below 100 °C.9 RTILs, because of their charged components, are well documented in their proclivity to engage in ion-pairing with the reaction substrate.10−12 RTILs have been known to alter reaction mechanisms,13−15 particularly through their affinity for ion-pairing.12,16,17 Currently, there is no experimental voltammetry of the reduction of corannulene in a RTIL or an exploration of what might occur in an ionic solvent. Complementary to the work of Bruno et al.,7 this work investigates how ion pairing can lead to distorted voltammetric wave shapes as opposed to altered thermodynamics of the reduction process. Voltammetry provides an opportunity to probe changed electron transfer kinetics or chemical reactivity. A peak-to-peak separation of greater than 57 mV (n is the

Received: March 11, 2016 Revised: April 5, 2016 Published: April 5, 2016

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© 2016 American Chemical Society

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Figure 2. Square scheme describing two electron transfers and two chemical steps. Note the directions in which the forward and backward reactions have been set for the chemical steps.

reaction. On the reverse scan, D undergoes an oxidative electron transfer to C, and a chemical step returns C to the starting material, A. However, as will be shown, the details of the above scheme will vary as a function of kf, the forward rate constant of the chemical steps. To reduce the complexity of the system, we assume that the rate of electron transfer is sufficiently fast such that both electron transfers are effectively fully electrochemically reversible (k° = 10 cm s−1). kf and Keq (see Figure 2) dictate the kinetics and the thermodynamics of the chemical steps, respectively. Note that the simulation takes into account kb1 and kb2, the reverse chemical steps, but these values are implicitly determined from the value of kf and Keq, as shown in Figure 2. To reduce the number of variables, we set the square to have symmetric values of kf and Keq, such that kf1 = kf2 = kf and Keq1 = Keq2. The formal potential will be expressed as the difference between Eθf,1 and Eθf,2 as ΔEθf . Energetic constraints require Keq to vary as a function of ΔEθf and as we have set Keq1 = Keq2, for a given ΔEθf only one value of Keq is possible. This thus reduces the problem from eight to two parameters namely ΔEθf and kf. First we consider the situation in which ΔEθf is set at −0.5 V, Keq equals 5.6 × 10−5 and the kf is varied. This scheme has been simulated at 500 mV s−1 and is shown in Figure 3 with five possible kf values ranging from 1 × 10−7 s−1 to 1 × 109 s−1. At very slow values of kf (black line, Figure 3), the voltammetry appears reversible, where the position of the wave reflects the value of Eθf1 with the wave occurring at ∼−0.25

Figure 1. Role of ion pairing on the reduction of corannulene in RTIL, here the 1-butyl-1-methylpyrrolidinium cation is shown.

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THEORY Consider a one electron transfer: A + e− ⇌ B

Efθ

(1)

Eθf

where (V) is the formal electrode potential, and the rate of the electron transfer is governed by a standard electrochemical rate constant, k° (cm s−1). In the reversible limit, the Nernst eq (eq 2) relates the redox potential to the ratio of concentrations of products and reactants present at the electrochemical interface. Voltammetry undertaken under these conditions have peak-to-peak separations (ΔEp−p) of ∼57 mV at 25 °C,24 and the separation is invariant with scan rate.

E = Efθ +

RT [B] ln F [A]

(2)

For systems with slower electron transfer, i.e., for a quasi- or fully irreversible process, the rate of electron transfer may be parametrized by the Butler−Volmer25,26 equation: ⎛ ⎡ −αFη ⎤ ⎞ ⎡ βFη ⎤ I = −FAk°⎜exp⎢ [A] − exp⎢ [B] ⎟ ⎥ ⎥ ⎣ RT ⎦ ° ⎠ ⎝ ⎣ RT ⎦ °

(3)

where I is the current (A), F is the Faraday constant, α and β are the transfer coefficients (where α + β = 1), η is the overpotential (η = E − Eθf ), and [X]o is the concentration of species X at the electrode surface. In contrast to a reversible process, in the irreversible limit the peak-to-peak separation (ΔEp−p) shifts with scan rate where

∂ΔEp − p ∂log10 ν

= 118 mV dec−1. For

situations in which experimental voltammetry exhibits ΔEp−p > 57 mV but the separation is found to be relatively insensitive to the scan rate, a simple E mechanism as given above is unable to accurately describe the experimental voltammetry. Consequently, additional chemical steps must be considered. Jacq22 and Laviron23 introduced the concept of square schemes,27,28 which allow the description of multiple, interconnected, E and C steps (where E indicates and electron transfer and C a chemical step29). Analytical theory has been undertaken for a variety of electrochemical mechnisms.30−33 However, the work considered in this manuscript is predicated upon the use of numerical simulation software. Here we consider the simplest case of a square scheme where the chemical steps are described by a unimolecular reaction, as shown in Figure 2. Moreover, we focus on the thermodynamic situation where for an electrochemical reduction the starting material A is reduced to the product B. B is subsequently converted to D via a unimolecular

Figure 3. Simulated voltammograms at 500 mV−1 of the square scheme shown above, with Eθf,1 being set at −0.25 V, Eθf,2 at 0.25 V, θ θ E f,1 + E f,2

= 0 V , k° at 10 cm s−1, Keq at 5.955 × 10−5, and kf of 1 × 10−7 s (black), 1 × 10−4 s−1 (red), 1 × 10−3 s−1 (blue), 1 s−1 (magenta), and 1 × 109 s−1 (green). −1

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Figure 4. (a) Simulated voltammograms of a one electron process from 10−1000 mV s−1, with k° set at 3.9 × 10−5 cm s −1, Eθf at 0 V. (b) Simulated voltammograms of the sqaure scheme from 10−1000 mV s−1, with kf set at 15 s−1, k° at 10 cm s−1, Keq at 1.175 × 10−5 and ΔEθf at 0.5 V. (c) Ep − log ν plot with peak positions of a one electron, irreversible process, shown as circles, and a square scheme, shown as triangles.

Acetonitrile (anhydrous, 99.8%, Sigma-Aldrich), dichloromethane (anhydrous, 99.8%, Sigma-Aldrich), tetrahexylammonium hexafluorophosphate (≥97%, Sigma-Aldrich), and ferrocene (98%, Sigma-Aldrich) were used as received. Electrochemical Apparatus. Electrochemical experiments (cyclic voltammetry) were conducted using an Autolab PGSTAT101 potentiostat for the experiments in RTIL and a μAutolab II potentiostat for the experiments in acetonitrile (Eco-Chemie, The Netherlands). All experiments were conducted inside a temperature controlled Faraday cage.34 The ionic liquid experiments were conducted in a t-cell which was arranged largely as previously described,35 with an Ag wire as pseudoreference and Pt coil as counter electrode. Around 0.2 mL of solution was placed under high vacuum overnight to ensure the removal of water, oxygen, and other volatile impurities. The working glassy carbon electrode (BASi Analytical, USA), 3 mm diameter, was polished prior to use using diamond sprays (3, 1, 0.1 μm, 5 min on each grade, supplied by Kemet, Kent, UK). For the experiments conducted in acetonitrile, a ca. 10 mL solution of 0.1 M tetrahexylammonium hexafluorophosphate and 2 mM ferrocene was degassed with argon for 15 min prior to recording voltammetry. A water bath was used to maintain the temperature due to the large volume. The solution of corannulene used in the experiments conducted in acetonitrile was prepared by dissolving 2.1 mg of the solid in 10 mL of dichloromethane. The working gold macroelectrode (IJ Cambria Scientific Ltd., UK), 2.4 mm nominal diameter, was polished prior to use using a water-alumina slurry (1, 0.3, 0.05 μm, 5 min on each grade) on soft lapping pads (Buehler, Illinois). A 0.5 mm silver wire was used as a quasi-reference electrode, and a platinum mesh was used as a counter electrode.

V. This arises due to the reaction not progressing further than the first electron transfer in the scheme as the reaction of B to D is too slow to occur over the voltammetric time scale. On the reverse scan, B is converted electrochemically directly back to A. As kf is increased, a peak appears at 0.25 V, fixed at Eθf,2, and the back peak at −0.25 V decreases in magnitude, this reflects the production of D from B (as exemplified by the red line in Figure 3). As kf increases further, both peaks shift inward to once again become reversible, as the overall reaction becomes fully reversible. Under these conditions, the midpoint potential of the wave reflects the mean of the two formal potentials, i.e., θ θ Ef,1 + Ef,2

= 0 V. An interesting situation arises for intermediate values of kf where a large ΔEp−p is observed, but there is no back peak present at −0.25 V. The blue line of Figure 3 exemplifies this case. To set the peak positions for this case, we note that the position of the peaks predominately reflects the thermodynamics of the individual electron transfers, as described by their Eθf . The other parameters are kept constant as above, and simulations from 10−1000 mV s−1 are undertaken, as shown in part (b) of Figure 4. Figure 4a shows exemplar voltammetry of a one electron irreversible process, as described by the Butler− Volmer equation, simulated over the same range of scan rates with k° set to 3.9 × 10−5 cm s−1, and α to 0.5. Part (c) displays the relationship between the peak position, Ep, and the logarithm of the scan rate (Ep − log ν plot). The peak positions of the one-electron irreversible process are shown as circles. This is compared with the square scheme peak positions, shown as triangles. Figure 4 shows an example of a case generated using the square scheme above where the ΔEp−p is relatively insensitive to the scan rate while having peak-to-peak separations larger than ∼57 mV (indicating that the overall reaction is not 2

reversible). In this case,

∂ΔEp − p ∂log10 ν

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= 50 mV dec−1 as compared

with the value predicted for a simple one electron fully irreversible process where

∂ΔEp − p ∂log10 ν

RESULTS AND DISCUSSION

The following sections detail the acquisition of experimental voltammetry of the reduction of corannulene in acetonitrile and [Bmpyrr][NTf2]. The characteristics of this voltammetry are explored and mechanisms are proposed and contrasted for both solvents. Reduction of Corannulene in Acetonitrile. 20 μL of a 0.84 mM solution of corannulene in dichloromethane (DCM) was dropcast onto a gold macroelectrode, where the DCM evaporates leaving solid corannulene on the electrode surface. The modified electrode was then submerged in a 0.1 M solution of tetrahexylammonium hexafluorophosphate and 2 mM ferrocene in acetonitrile, which acts as an internal redox marker. A cyclic voltammogram was recorded by sweeping the

= 118 mV dec−1. The

following sections detail experimental examples of both cases discussed above.

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EXPERIMENTAL SECTION Chemical Reagents. 1-Butyl-3-methypyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpyrr][NTf2], high purity, 99%, IoLiTec, Germany) was used as received. Corannulene (97%) was synthesized by the Siegel group (Tianjin University) and kindly donated by Jason Harper (UNSW Australia). 8407

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Figure 5. (a) Cyclic voltammetry of 20 μL of corannulene dropcast on a macro gold electrode, in 0.1 M tetrahexylammonium tetrafluoroborate and 2 mM ferrocene in acetonitrile. (b) Variable scan rates of the peak attributed to corannulene; 100 (black), 500 (red), 1000 (blue), and 2000 mV s−1 (green). (c) Ep−log ν plot of the positions of the forward and reverse peaks with log(scan rate) of the experimental voltammetry (black) and the simulated voltammetry (red).

Figure 6. (a) Cyclic voltammetry of 10 mM corannulene on a glassy carbon macroelectrode in [Bmpyrr][NTf2]. Variable scan rates; 40 (black), 60 (red), 80 (blue), 100 (magenta), 200 (green), 300 (orange), and 400 mV s−1 (purple). (b) Ep−log ν plot of the positions of the forward and reverse peaks with log(scan rate).

potential from −0.5 to −2.0 V, and up to 1 V on the reverse scan as shown as part (a) of Figure 5. The corannulene peak appears between −1.5 and −2.0 V. A range of scan rates were then recorded, from 100−2000 mV s−1, as seen in part (b) of Figure 5. The peak shape is diffusional rather than surfacebound, as evidenced by the shape of the wave, and the linear relationship of peak current with the square root of scan rate. This suggests that the microcrystals, as previously observed in the dropcast of C60,36,37 on the modified electrode have dissolved into the adjacent solution leading to a diffusional redox response of a one electron reduction. Part (c) of Figure 5 shows a Ep−log ν plot of the peak positions with the logarithm of the scan rate, with the experimental peak positions shown in black. The square markers are taken from a simulation of a one electron reduction modeled using the Butler−Volmer equation, where k° was set to 0.03 cm s−1, and Eθf to −1.665 V vs Ag wire. The shape of the Ep−log ν plot, and the fact that the experimental peak positions show a good fit with the simulated peak positions, indicates that this process is likely to take the form of a quasireversible one electron transfer, with the corannulene reduced to the corannulene radical anion. Reduction of Corannulene in [Bmpyrr][NTf2]. Next, a glassy carbon macroelectrode was submerged in a solution of 10 mM corannulene in [Bmpyrr][NTf2], and a CV was recorded by sweeping from −2.1 to −2.65 V, and back to −2.1 V, over a scan rate range of 40−400 mV s−1, as shown in part (a) of Figure 6. The peak positions of the forward and reverse peaks are plotted against the logarithm of the scan rate in part (b) of Figure 6. In contrast to the voltammetry undertaken in acetonitrile, the experimental voltammetry shows a peak-to-peak separation of

100 mV, and a

∂ΔEp − p ∂log10 ν

of 36 mV dec−1 over the experimental

range. Attempts to fit the experimental voltammetry with a one electron quasireversible process were not successful. Thus, a square scheme, as discussed in the Theory section and shown in Figure 2, must be employed. Ion pairing in RTILs is recognized,38 specifically where a charged product is stabilized by the ionic liquid component of the opposite charge. Although the voltammetry of corannulene has not hitherto been recorded in a pure RTIL, in a RTIL/ benzonitrile solution, the voltammetry of closely related C60 has been reported to shift significantly when undertaken in a solution with a high fraction of RTIL.11 This was attributed to the cations of the ionic liquid ion-pairing in clusters (so-called “nanodomains” of RTIL) with the fullerene anion. Therefore, we can understand the chemical steps in the square scheme to be due to the corannulene anion ion-pairing (and unpairing, with respect to the reaction of C to A) with the [Bmpyrr]+ cation. It is worth noting that the nature of synthesis of RTILs means that the presence of impurities (such as Li+) may contribute to the ion-pairing and thus the voltammetry, but this effect is likely to be small compared with the pairing induced by the RTIL cation which is in far higher concentration. Digisim39,40 was next used to produce voltammetry simulated according the square scheme introduced in the Theory section (Figure 2). It is assumed that this reaction proceeds pseudofirst order due to the vast excess of RTIL cations with respect to the corannulene molecules. The electrochemical steps were again assumed to be fully reversible, and k° set at 10 cm s−1. ΔEθf was optimized to produce ΔEp−p of 100 mV, resulting in a value of 0.08 V. kf was optimized to keep ΔEp−p as small as 8408

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Figure 7. (a) Simulated cyclic voltammetry, with Eθf,1 being set at −2.5 V, Eθf,2 at −2.42 V, k° at 10 cm s−1, Keq at 4.74, and kf of 6 s−1 over a range of scan rates; 40 (black), 60 (red), 80 (blue), 100 (magenta), 200 (green), 300 (orange), and 400 mV s−1 (purple). (b) Ep−log ν plot of the positions of the forward and reverse peaks with log(scan rate).

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possible, and as a result set at 6 s−1. Keq therefore equaled 4.74. The resultant voltammetry, simulated over 40−400 mV s−1 is shown in part (a) of Figure 7. Part (b) of Figure 7 shows a Ep− log ν plot of the peak positions in relation to the logarithm of the scan rate. These simulations, with an ΔEp−p of 100 mV and a

∂ΔEp − p ∂log10 ν

ACKNOWLEDGMENTS The research leading to these results has received partial funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/20072013)/ERC Grant Agreement no. [320403]. E.E.L.T. thanks the Clarendon Fund and St John’s College for funding. The Australian Research Council (ARC DECRA DE130100770) is acknowledged for research funding.

of

27 mV dec−1, demonstrate that a square scheme, as used here, is able to accurately describe the relative invariance of the peak potential of the experimental voltammetry produced in RTIL. In acetonitrile, the mechanism of electron transfer follows a simple quasi-reversible E mechanism. This is evidenced by the ΔEp−p of 80 mV that varies with 66 mV dec−1. In contrast, in a RTIL, an ΔEp−p of 100 mV is observed that varies only by 36 mV dec−1. The use of a square scheme simulation enables the description of the experimental voltammetry in RTIL, whereby the chemical steps describe the ion-pairing of the RTIL cation with the corannulene anion.

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CONCLUSIONS The electroreduction of corannulene has been recorded in both acetonitrile and, for the first time, in a RTIL, [Bmpyrr][NTf2]. In acetonitrile, a simple one electron transfer is sufficient to describe the experimental voltammetry, but in the RTIL, a square scheme is essential. The square scheme, which involves chemical steps that reflect the effect of ion pairing in the IL, is able to accurately describe the voltammetry, in this case, a peakto-peak separation of >100 mV that is relatively invariant with scan rate. This mechanism is likely to be generic in ionic liquids, owing to the role of ion pairing in the scheme and the high concentrations of ions present in RTIL solvents. The RTIL alters not only the wave positioning but also the wave shape. Without due consideration to the role of ion-pairing in electron transfers undertaken in RTILs, the altered experimental voltammetry when compared with molecular solvents can be easily misinterpreted as slow electron transfer kinetics.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +61 (0) 2 9385 4752. *E-mail: [email protected] Phone: +44 (0) 1865 275957. Fax: +44 (0) 1865 275410. Notes

The authors declare no competing financial interest. 8409

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DOI: 10.1021/acs.jpcc.6b02551 J. Phys. Chem. C 2016, 120, 8405−8410