The Observation of Dianions Generated by Electrochemical Reduction

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The Observation of Dianions Generated by Electrochemical Reduction of trans-Stilbenes in Ionic Liquids at Room Temperature Omar Abdul-Rahim,† Alexandr N. Simonov,† Thomas Rüther,*,‡ John F. Boas,§ Angel A. J. Torriero,∥ David J. Collins,† Patrick Perlmutter,† and Alan M. Bond*,† †

School of Chemistry, Monash University, Clayton, Victoria, 3800, Australia CSIRO Energy Transformed Flagship, Clayton, Victoria, 3168, Australia § School of Physics, Monash University, Clayton, Victoria, 3800, Australia ∥ Deakin University, Burwood, Victoria, 3125, Australia ‡

S Supporting Information *

ABSTRACT: Three highly aprotic bis(trifluoromethylsulfonyl)amide (NTf2−) based ionic liquids (ILs) containing the cations trihexyl(tetradecyl)phosphonium (P6,6,6,14+), N-butyl-N-methylpyrrolidinium (Pyrr4,1+), and (trimethylamine)(dimethylethylammine)dihydroborate ((N111)(N112)BH2+) have been examined as media for room temperature voltammetric detection of highly basic stilbene dianions electrochemically generated by the reduction of trans-stilbene (t-Stb) and its derivatives (4-methoxy-, 2-methoxy-, 4,4′-dimethyl-, and 4chloromethyl-). Transient and steady-state data in the ILs were compared with results obtained in the molecular solvent acetonitrile. In all media examined, the t-Stb0/•− process is chemically and electrochemically reversible with a heterogeneous charge transfer rate constant in CH3CN of 1.5 cm s−1, as determined by Fourier transformed AC voltammetry. However, further reduction to the dianion was always irreversible in this molecular but weakly acidic solvent. On the other hand, a substantial level of chemical reversibility for the reduction of t-Stb•− to t-Stb2− on the time scale of cyclic voltammetry is achieved when the concentration of trans-stilbene, [t-Stb], appreciably exceeds the concentration of adventitious water or other proton sources. In particular, these conditions are met when [t-Stb] ≥ 0.1 M in thoroughly dehydrated and purified ILs, while in the presence of CH3CN, t-Stb2− still suffers fast irreversible protonation under these stilbene concentration conditions. The E0/•−0 values (vs Fc0/+) for substituted trans-stilbenes in acetonitrile and (N111)(N112)BH2-NTf2 do not differ substantially, nor do the E0/•−0 and E•−/2−0 differences or other aspects of the voltammetric behavior.

W

redox chemistry of organic molecules. A very negative cathodic potential limit, significant electrochemical stability, along with the high hydrophobicity and hydridic nature of the cation of the (N111)(N112)BH2-NTf2 ionic liquid should favor the registration of processes that require both very negative potentials and stabilization of the resulting products, particularly those that are strong bases and hence readily protonated. A paradigm of this type of process is electroreduction of trans-stilbene, which was chosen by Wawzonek to revolutionize electrochemical studies of organic compounds by pioneering the application of organic solvents such as acetonitrile and dimethylformamide (DMF) as an alternative to aqueous media in 1955.5,6 An apt combination of sufficient polarity, good organic solvating power, and a sufficiently wide electrochemical window has made aprotic molecular solvents containing suitable electrolytes the medium

idely emphasized advantages of room temperature ionic liquids (ILs) as media for electrochemical studies relative to classic aqueous and organic molecular solvents1,2 have fostered continuous advances in their design. A good example of the rapid acceleration in the evolution of ILs is the recent discovery of a new family based on the boronium cation [L1L2BH2]+ which provides a straightforward opportunity to tune the structural and physicochemical properties by variation of substituents L1 and L2.3 Detailed examination of the properties of representatives of this new type of IL revealed reasonable conductivities and wide electrochemical windows that facilitated their application in stable Li metal battery cycling.4 The reported characteristics of the boronium-cation-based class of ILs, in particular, of (trimethylamine)(dimethylethylammine)dihydroborate bis(trifluoromethylsulfonyl)amide ((N 111 )(N 112 )BH 2 -NTf 2 , Chart1), suggest their suitability as electrolytes not only for lithium batteries but also for studies of the mechanisms of the © 2013 American Chemical Society

Received: April 5, 2013 Accepted: May 10, 2013 Published: May 10, 2013 6113

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Analytical Chemistry

Article

Chart 2. Structures of the Substituted trans-Stilbenes Used in This Study

Chart 1. Structures of the (N111)(N112)BH2+, Pyrr4,1+, and P6,6,6,14+ Cation and the NTf2− Anion Components of the Ionic Liquids Used in This Study

electrochemical studies of processes that take place at extreme potentials. As reference media, the electrochemical behavior of trans-stilbene also is reported in the N-butyl-N-methylpyrrolidinium (Pyrr 4,1 + ) and trihexyl(tetradecyl)phosphonium (P6,6,6,14+) bis(trifluoromethylsulfonyl)amide ionic liquids (Chart 1) as well as in the conventional acetonitrile solvent using tetrabutylammonium hexafluorophosphate ((nBu)4NPF6) or P6,6,6,14-NTf2 as supporting electrolytes.

of choice for numerous electrochemical studies for over 50 years. In the case of trans-stilbene, use of an aprotic solvent allowed detection of a fast and reversible one-electron reduction to yield the radical anion (reaction 1). At even more negative potentials, irreversible one-electron reduction of the anion radical may produce a highly basic dianion, which undergoes rapid protonation to form 1,2-diphenylethane (bibenzyl; reactions 2 and 3).7−10 Under some conditions, a chemically irreversible dimerization of the stilbene radical anion can occur, resulting in the formation of the dianion, which upon protonation produces 1,2,3,4-tetraphenylbutane.5 t ‐Stb + e− ⇄ t ‐Stb•−

(1)

t ‐Stb•− + e− ⇄ t ‐Stb2 −

(2)

2−

t ‐Stb

+

+ 2H → bibenzyl

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EXPERIMENTAL SECTION Materials. Acetonitrile and dichloromethane were of Lichrosolv HPLC grade (Merck) and were distilled prior to use over calcium hydride and phosphorus pentoxide, respectively. After distillation, each solvent was transferred using a cannula into an individual sealed flask, which was immediately introduced into a glovebox. All of these operations were performed under a high purity nitrogen atmosphere (99.999%, H2O < 3 ppm, O2 < 2 ppm). Distilled solvents were used within one week after purification. (n-Bu)4NPF6 (98%, Wako) was recrystallized twice from ethanol (96%, Merck Emplura) and dried under a vacuum at 40 °C prior to use. The ultrapure Pyrr4,1-NTf2 IL (>99%; bottled under Ar; Merck Millipore) was unpacked inside the glovebox and used as received. The P6,6,6,14-NTf2 IL (≥98%; Strem Chemicals) was dried in a vacuum at 70−80 °C for at least two days before use, providing a water content in the dehydrated material of 50 ppm (by weight) as determined by the Karl Fisher titration method. For purification, the (N111)(N112)BH2NTf2 IL (95%, Frontier Scientific) was dissolved in dry distilled dichloromethane. The organic phase was then washed with deionized water and filtered through a 0.5 μm membrane filter to remove insoluble impurities. After removal of the solvent on a rotary evaporator and drying under high vacuum conditions at 40−50 °C for two days, the pure material was stored near finely cut lithium metal strips contained in a drying tube and introduced to a nitrogen filled glovebox. The purity of the resulting IL was established by ICP-MS (Agilent 7700 instrument): Li+ < 2 ppm, I− < 2 ppm (by weight). The water content in the purified (N111)(N112)BH2-NTf2 IL was below 50 ppm (by weight) as determined by the Karl Fisher method. After being transferred to the glovebox, the ionic liquids were used within three weeks of purification. During this period of time, no notable changes in their water content were detected. trans-Stilbene (t-Stb; 96%, Sigma-Aldrich), trans-4methoxystilbene (95%, Sigma-Aldrich), trans-2-methoxystilbene (95%, Sigma-Aldrich), trans-4,4′-dimethylstilbene (96%, Sigma-Aldrich), trans-4-chloromethylstilbene (95%, SigmaAldrich), ferrocene (Fc; 98%, Riedel-deHaen), and AgNO3 (99.998%, Sigma-Aldrich) were used as received.

(3)

Extreme affinity of the trans-stilbene dianion for protons has precluded chemical reversibility of the t-Stb•−/2− process being achieved on the time scale of cyclic voltammetry until the introduction of neutral alumina for drying of the DMF solvent11,12 and performing the experiments at very low temperatures in liquid ammonia and alkylamines.13,14 Unfortunately, the authors of the ref 11, who were the first to report a reversible t-Stb•−/2− process, did not provide crucial details of their experimental conditions, thus leaving uncertainty as to the temperature of the measurements and the concentration of tStb. At the same time, Pletcher and co-workers12 also reported a reversible t-Stb•−/2− process, but the method used for assigning the electroreduction of the t-Stb•− species as being both an electrochemically and chemically reversible process in DMF was insufficient, as discussed below in the Results and Discussion section. Nevertheless, the fact that the stability of the trans-stilbene dianion appears to be improved when special attention is paid to dehydration of the electrolyte suggests that adventitious water may be one of the main sources of protons and hence the cause of irreversible conversion of the t-Stb2− species into bibenzyl.11,12 However, highly basic t-Stb2− could still be capable of fast abstraction of a proton from the organic solvents such as acetonitrile or DMF, so the origin of the proton source in this type of medium is still not resolved. The extreme sensitivity of the electroreduction of the transstilbene radical anion to its dianion to the presence of a source of protons, together with the very negative potentials required, persuaded us to choose trans-stilbene and four derivatives thereof (Chart 2) as prototype examples for assessing the use of ionic liquid (N111)(N112)BH2-NTf2 as a suitable solvent for 6114

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Electrochemical Instrumentation and Procedures. Electrochemical experiments were performed in three-electrode cells at ambient temperature (23−25 °C) under glovebox conditions using an Epsilon Electrochemical Workstation (BAS), an Autolab PGSTAT100 potentiostat (Eco Chemie), or a homemade instrument that has the capacity of being used for Fourier Transformed AC Voltammetric (FTACV) analysis.15 Measurements were performed using glassy carbon (GC) disk working electrodes following the standard procedures, which are detailed in the Supporting Information. During and between experiments, a high purity dry N2 atmosphere was maintained continuously in the glovebox. Unless otherwise stated, potentials are reported versus the formal potential of the Fc0/+ couple, as measured in the same solution, and current data are normalized to the electrode surface area. Electron Paramagnetic Resonance (EPR) Detection of the trans-Stilbene Radical Anion. The trans-stilbene radical anion was generated under glovebox conditions by bulk electrolysis of t-Stb in CH3CN (0.1 M (n-Bu)4NPF6) electrolyte, and the resulting solutions were analyzed by EPR at 120 and 250 K with a Bruker ESP380E CW/FT X-band (ca. 9.4 GHz) spectrometer as described in the Supporting Information. Theory. Simulations of the DC and AC cyclic voltammograms were performed using DigiElchProfessional 7.F16 and Monash Electrochemistry Simulator (MECSim) software, respectively.

Figure 1. DC cyclic voltammograms obtained with a GC electrode at v = 0.10 V s−1 for reduction of trans-stilbene at low (a, b) and high (c) concentrations in CH3CN (0.1 M (n-Bu)4NPF6; [t-Stb] = 1 (red line) and 170 mM (red dashed line)), Pyrr4,1-NTf2 (blue line, [t-Stb] = 5 and 150 mM), P6,6,6,14-NTf2 (green line, [t-Stb] = 12 mM), and (N111)(N112)BH2-NTf2 (black line, [t-Stb] = 220 mM). Potentials are referred to E0 for the t-Stb0/•− process. The arrow in (a) shows a voltammogram with a switching potential of −0.2 V vs t-Stb0/•−. Arrows in (c) indicate the relevant ordinate axis for each curve.

■

RESULTS AND DISCUSSION Stability of Electrochemically Generated trans-Stilbene Radical Anions and Dianions at Room Temperature. Cyclic voltammetry of trans-stilbene in dry acetonitrile containing 0.1 M (n-Bu)4NPF6 as the supporting electrolyte allows the fast and essentially chemically and electrochemically reversible t-Stb0/•− process to be detected (Figure 1a) with a formal potential (E0/•−0) of −2.67 V vs Fc0/+, where E0 is assumed to be equal to the midpoint potential calculated from the average of the reduction and oxidation peak potentials. The reversible and diffusion controlled nature of the process in eq 1 was established from the linear dependence of peak height on concentration (1 to 15 mM), the linear dependence of peak height on the square root of the scan rate (0.05 to 1.0 V s−1), the unity ratio of the magnitude of reductive and oxidative peak currents, and by finding ΔEp = EpOx − EpRed to be close to 0.06 V, after taking the Ohmic iRu drop into account. At more negative potentials, a chemically irreversible reduction process (Figure 1a) is detected consistent with coupling of the tStb•−/2− electron transfer and protonation reactions given in eqs 2 and 3 and also with data provided in other reports.5−10 That is, no voltammetric evidence for the dianion is detected under these conditions. Rotating disk electrode (RDE) measurements (Figure S1) confirm that both reduction steps give the same limiting current per unit concentration values and hence involve the transfer of the same number of electrons, which in this case is equal to unity. The latter is testified by the coulometric analysis of data obtained from exhaustive bulk electrolysis of trans-stilbene at −2.82 V, as well as by the classical criteria used for analysis of DC cyclic voltammetric data and the slope of the Tafel plot extracted from the RDE data (0.055 V). The EPR spectra of acetonitrile solutions of trans-stilbene frozen to 120 K after bulk electrolysis at E = −2.82 V exhibited

a resonance of peak-to-peak derivative width of 1.32 mT at g = 2.0028 ± 0.0002. Poorly resolved hyperfine interactions of magnitude around 0.27 mT could be observed. The resonance intensity depended on the duration of electrolysis with the maximum signal intensity being achieved after 20−25 min of electrolysis (Figure S2a). Such behavior is indicative of the limited stability of the radical anion under the employed experimental conditions, despite the careful measures taken for purification of the solvent and maintaining an inert atmosphere in a glovebox. Identification of the resonances as being due to the t-Stb•− radical anion was achieved through the observation of the liquid phase EPR spectrum of the trans-stilbene acetonitrile solution after 23 min of electrolysis. The spectrum at 250 K (Figure S2b) consisted of at least 100 resonances of varying field separations and intensities centered at g = 2.00080 ± 0.00015 and is well matched by a simulation using this g value and the proton hyperfine interaction constants for t-Stb•− listed in refs 17 and 18. Importantly, there was no EPR evidence for the transformation of the trans-stilbene radical anion to the cis isomer, which has a different g value and proton hyperfine interaction constants.17,18 It is on this basis that eq 1 is written with retention of the trans-isomer geometry. Although no proof of the isomeric form of the dianion under the conditions employed is available, retention of the trans configuration is assumed, since reduction back to the starting trans-stilbene form is indicated (Figure 1a). The low temperature cyclic voltammetric studies in alkylamines13 and liquid ammonia14 with macroelectrodes imply that both the t-Stb0/•− and t-Stb•−/2− processes are chemically 6115

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Table 1. E0 Values (V vs Fc0/+) Derived from Reduction of trans-Stilbene in CH3CN and Ionic Liquidsa

reversible under these conditions. At the same time, the only detailed report of reversible behavior for the t-Stb•−/2− process at standard ambient temperature available to date is contained in the paper by Pletcher and co-workers.12 The classic criteria used in this work for evaluation of electrochemical reversibility of voltammetric data obtained under a slow scan rate, near steadystate conditions using a microelectrode is based on estimation of the |E3/4 − E1/4| difference (E3/4 and E1/4 are the potentials at 1/4 and 3/4 of the limiting current values, respectively) and comparison with the theoretically predicted one of 0.057 V at 25 °C. The problem is that chemical reversibility is not readily inferred from this method. Convincing evidence for generation of the stable transstilbene dianion on the time scale of cyclic voltammetry at room temperature is acquired when the electroreduction is carried out in (N111)(N112)BH2-NTf2 and Pyrr4,1-NTf2 ionic liquids at the atypically high concentrations (>0.1 M) of the analyte (Figure 1c). In the case of the Pyrr4,1-NTf 2 ionic liquid, almost complete chemical reversibility is found, while partial reversibility is attained in the boronium IL. Experiments performed in the ILs at concentrations of stilbene below 15 mM provided chemically irreversible voltammograms (Figure 1b) that resemble those obtained in acetonitrile. The origin of the chemical irreversibility is attributed to abstraction of the protons from trace amounts of water or other impurity present in the ionic liquids. The concentration of H2O in the (N111)(N112)BH2-NTf2 and Pyrr4,1-NTf2 samples did not exceed 50 and 30 ppm (mass), respectively. These values correspond to millimolar concentration levels of H2O, which along with other impurities seem to be adequate to completely deplete the t-Stb2− dianion generated upon reduction of the stilbene at usually used concentrations of ≤15 mM. Thus, higher than usual t-Stb concentrations are needed to acquire an excess of electrochemically generated t-Stb2− over adventitious proton sources so that the dominant reaction pathway is provided by reaction 2 rather than by the sequence of reactions (2→3). Evidently, this condition is fully met for the Pyrr4,1NTf2 medium for [t-Stb] > 0.1 M, while the reduction of transstilbene radical anions in (N111)(N112)BH2-NTf2 at this concentration level proceeds via a mixed mechanism where a contribution from the protonation pathway is detected by manifestation of the irreversible reduction process prior to the reversible t-Stb•−/2− process in cyclic voltammograms (Figure 1c). Cyclic voltammograms obtained in the blank boronium IL did not show any significant Faradaic current in the potential range of −3.15 to −3.25 V (Figure S3). Thus, the origin of the peak appearing prior to the reversible t-Stb•−/2− process in (N111)(N112)BH2-NTf2 (Figure 1c) is attributed to partial protonation of the dianions by adventitious proton source. The separation in the reversible E0 values for t-Stb0/•− and tStb•−/2− extracted by simulation of the DC cyclic voltammograms (see below) obtained at a high concentration of transstilbene is 0.37 V in Pyrr4,1-NTf2 and 0.44 V in the boronium IL. Interestingly, the formal potential for t-Stb0/•− of −2.55 V in the pyrrolidinium ionic liquid is approximately 0.10 V more positive than in (N111)(N112)BH2-NTf2 and acetonitrile, when referred to Fc0/+. These differences may be indicative of medium-dependence of the E0 of the Fc0/+ couple or of a stronger interaction between the trans-stilbene radical anion and dianion with the N-butyl-N-methylpyrrolidinium cation. E0 values vs Fc0/+ in the other media were found to be similar (Table 1).

solvent/electrolyte

E0/•−0

E•−/2−0

CH3CN/(n-Bu)4NPF6 CH3CN/P6,6,6,14-NTf2 P6,6,6,14-NTf2 (N111)(N112)BH2-NTf2 Pyrr4,1-NTf2

−2.67 −2.67 −2.67 −2.69 −2.55

≤ −3.1 ≤ −3.0 ≤ −3.1 −3.13 −2.92

a

Determined by simulation of the cyclic voltammetry obtained at a GC electrode at v = 0.1 V s−1.

To support the hypothesis that instability of t-Stb2− in acetonitrile is due to the protonation of the dianion from the solvent, cyclic voltammograms for 0.17 M trans-stilbene were recorded in this medium with a water concentration of