3182
J. Phys. Chem. B 2001, 105, 3182-3186
A Microelectrode Study of the Reduction of Phenyl-Substituted Ethenes in Toluene/Dimethylformamide Mixtures M. D. Geraldo, M. I. Montenegro,* and L. Slevin† Centro de Quı´mica/IBQF, UniVersidade do Minho, Largo do Pac¸ o, 4700-320 Braga, Portugal
D. Pletcher Department of Chemistry, The UniVersity, Southampton SO17 1BJ, England ReceiVed: June 16, 2000
The voltammetry of trans-stilbene, triphenylethene, and tetraphenylethene has been investigated in DMF/ toluene mixtures containing Bu4NBF4 as a function of the electrolyte concentration, solvent composition and substrate concentration. The voltammetric data show that all three ethenes undergo reduction in two 1esteps but the separation of the potentials for the two steps depends substantially on the medium as well as the structure of the ethenes. The behavior can be understood in terms of ion pairing phenomena.
1. Introduction The electrochemical reduction of phenyl-substituted ethenes has been studied previously in dimethylformamide and other aprotic solvents. Early polarographic investigations showed one or two waves depending on the structure of the compound.1-3 Later studies by cyclic voltammetry4-7 have shown that transstilbene (I) and triphenylethene (II) give responses with two reduction peaks while tetraphenylethene (III) displays only one. For I and II, the first electron addition leads to the formation of an anion radical, which is then further reduced to the corresponding dianion at a more negative potential.
A + e- a A•-
(1)
A•- + e- a A2-
(2)
In general, the anion radical is not a strong base and, therefore, stable; the first reduction step is reversible. On the other hand, the dianion is a common powerful base and will protonate rapidly rendering the second step irreversible. In the case of III, a single 2e- wave has been reported to arise because of the close proximity of the formal potentials for steps (1) and (2). Then, in addition to direct 2e- reduction, a mechanism involving the disproportionation of the anion radical in solution
2A•- a A + A2-
(3)
must also be considered. The equilibrium constant for reaction 3 depends directly on the separation of the two formal potentials for reactions 1 and 2.7 While substituent effects on the voltammetry of these ethenes have been thoroughly investigated,8-10 the influence of ion pairing has not been systematically studied. This is in contrast to chemical reductions, where the cation present, as well as the choice of solvent, has been shown to have a strong influence * Author for correspondence. Telephone: 351 253 604370. Fax: 351 253 678467. E-mail:
[email protected]. † Present address: Johnson Matthey Ltd, Blounts Court, Sonning Common, Reading, Berks RG4 9NH, England.
on the thermodynamics and kinetics of disproportionation.11-13 In this paper, the concentration of the ethenes and the electrolyte (Bu4NBF4) as well as the ratio of DMF to toluene in the medium have been varied in order to demonstrate the way ion pairing produces a systematic change in the voltammetric behavior of the ethenes with these parameters. 2. Experimental Section The electrochemical experiments were carried out using a potentiostat, Autolab type PGSTAT 20, Ecochemie. The linear sweep experiments were carried out in a two-electrode cell at 298 K. The working electrodes were gold microdisks of radii 3.5 and 6.0 µm. The electrode radii were checked following a procedure described elsewhere.14 They were initially polished with fine emery paper and then using a moist polishing cloth on a polishing machine (Buehler, Ecomet 3) using alumina (Buehler) with decreasing grades, from 0.3 down to 0.05 µm. Before each experiment, the electrode was repolished with 0.05 µm alumina, rinsed with water and dried. The reference electrode was a Metrohm saturated calomel electrode (catalog number 6.0702.100). The solutions were prepared using solvents that had been carefully dried. Toluene (Fluka, 99.5%, stored over molecular sieves) was transferred, through a cannula under argon, into an Erlenmeyer flask containing 4 Å molecular sieves. N,Ndimethylformamide (DMF) (Lab-Scan, a.r., 99.8%) was first dried with MgSO4 for several hours and then passed through an activated, neutral aluminum oxide column (Merck, for column chromatography, particle size 63-200 µm); finally, it was distilled under vacuum and collected in a Schlenck flask over 4 Å molecular sieves. The supporting electrolyte was Bu4NBF4 (Aldrich, 99%) dried at 353 K under vacuum. The tetraphenylethene (Merck, >98%), triphenylethene (Aldrich, 99%), and trans-stilbene (Aldrich, 96%) were used without further purification. All solutions were deoxygenated with a fast stream of dry argon (CNO, 99.999%) prior to the experiments. During the studies of the effect of the solvent composition, to guarantee the same concentration of substrate over the composition range studied, two solutions, one in DMF and the
10.1021/jp002163f CCC: $20.00 © 2001 American Chemical Society Published on Web 03/31/2001
Reduction of Phenyl-Substituted Ethenes
J. Phys. Chem. B, Vol. 105, No. 16, 2001 3183
Figure 1. Steady-state voltammograms for solutions of 5 mM transstilbene in DMF for several concentrations of Bu4NBF4: (a) 1, (b) 10, (c) 50, (d) 100, (e) 500 mM. Gold microdisk (r ) 6.0 µm). Scan rate 50: mV/s.
other in toluene, were prepared. Each contained the electroactive species and the electrolyte. In the toluene solution a few drops of DMF (0.5 cm3 in 10 cm3) were added to allow complete dissolution of the electrolyte. The different solvent compositions were then obtained by simply mixing appropriate volumes of the two solutions. Solution conductivities were determined with a Philips PW 9527 digital conductivity bridge and a Philips PW 9550/60 cell, with two platinum wire electrodes and a nominal cell constant of 0.88 cm-1. The cell constant was determined from an aqueous 1.00 M KCl solution, made with deionized water (resistivity 18 MΩ cm-1) from a Barnstead E-pure purifier and KCl (Merck >99.5%), previously dried under vacuum at 373 K. The cell containing the solution was placed in a thermostatic bath at 298 K. The tabulated conductivity under this condition is 100 mS cm-1 and the obtained cell constant is 1.033 ( 0.001 cm-1. Cell resistances were calculated from the solution conductivities using a procedure proposed by Pen˜a, Fleischmann, and Garrard15 and used by Oldham16 and were used to correct the data for IR drop. The formal potential for the Cp2Fe/Cp2Fe+ couple was used to estimate corrections for liquid junction potentials between the various solutions, and the wave shape for the oxidation of ferrocene was used to check that the IR corrections of the voltammetric data were reasonable. For quantitative treatment of the data, the experimental approach was to carry out repeated experiments, each at a repolished Au microdisk electrode and the I-E responses were stored in a computer; this allowed rapid data analysis and, in particular, the reproducibility of the voltammograms to be checked. The data in the tables are the mean of at least five experiments and the standard deviation for each set of experiments is reported in brackets. 3. Results 3.1. Influence of the Electrolyte Concentration. Figure 1 shows a set of slow sweep voltammograms recorded at a gold microdisk electrode (radius 6.0 µm) for solutions of 5 mM transstilbene in DMF as a function of the concentration of Bu4NBF4 between 1 and 500 mM and at a gold microdisk electrode. When presented as untreated experimental data, the pattern appears
Figure 2. Normalized steady-state voltammograms for the limiting current of the second reduction wave of 5 mM trans-stilbene in DMF. Same conditions as Figure 1.
complex. In agreement with previously reported voltammetric data, each solution gave a voltammogram with two well-defined waves of approximately equal height. For the largest concentration of Bu4NBF4 used, 500 mM, the first wave appears at a potential of -2130 mV and the second at -2520 mV vs SCE. In this solution, the limiting currents are low but this is only a viscosity effect. Several other interesting features can also be observed. The first reduction wave shows a small shift to more negative potentials (vs SCE) as the electrolyte concentration decreases and it appears to be less steep for the lower concentrations. The second reduction wave shows a much larger shift in half-wave potential with decreasing electrolyte concentration, and the change in slope is also more pronounced. To allow a more detailed interpretation of the data,17 the responses are re-presented in Figure 2 after the following treatment: (a) the y axis has been converted to I/IL2 (where IL2 is the limiting current at the plateau of the second reduction wave) in order to correct for the significant variations in viscosity, especially for the concentrated electrolyte solution, (b) the potentials have been converted to a scale versus the formal potentials for the Cp2Fe/Cp2Fe+ in the DMF/Bu4NBF4 solution under study, and (c) the voltammograms have been corrected for IR drop (in DMF these corrections are small especially for electrolyte concentrations above 1 mM). The responses now show more clearly the trends with electrolyte concentration. Both waves shift positive with increasing electrolyte concentration, but the magnitude of the shift is much larger for the second wave. Data taken from these curves is reported in Table 1 which contains values of E1/2 and |E3/4 E1/4| for both waves and also values of the waves separation, ∆E1/2, ratio of the limiting currents for the second and first waves, IL2/IL1, and diffusion coefficients. In addition to the shifts in the two waves, it can be seen that the first wave corresponds to a reversible 1e- reduction while the second wave is also reversible in more concentrated electrolyte solutions (it appears that the data from the 1 mM electrolyte solution is over-corrected for IR drop as would be expected on theoretical grounds for a solution where [Bu4NBF4]/[I] < 1.018). As expected, the ratio between the two limiting currents is always very close to 2 and the diffusion coefficients, calculated from the limiting currents
TABLE 1: Effect of Bu4NBF4 Concentration on the Voltammetry of trans-Stilbene (5 mM) in DMF, IR Corrected, vs Formal Potential for Cp2Fe/Cp2Fe+ in the Solutions under Investigation wave 1
wave 2
[TBAB]/mM
-E1/2/mV
|E3/4 - E1/4|/mV
-E1/2/mV
|E3/4 - E1/4|/mV
∆E1/2/mV
IL2/IL1
106D/cm2 s-1
1 10 50 100 500
2709 ( 1 2701 ( 1 2673 ( 1 2661 ( 1 2620 ( 1
47 61 ( 2 58 ( 1 62 ( 1 57 ( 1
3263 ( 7 3193 ( 7 3107 ( 6 3098 ( 6 3008 ( 2
56 93 ( 4 70 ( 2 64 ( 3 60 ( 1
554 511 462 437 388
1.9 2.1 2.0 1.9 2.0
9.1 9.3 8.7 8.7 5.3
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Geraldo et al.
Figure 3. Normalized steady-state voltammograms for the limiting current of the second reduction wave of 5 mM triphenylethene in DMF for several concentrations of Bu4NBF4: 1, 10, 50, 100, and 500 mM. Gold microdisk (r ) 3.5 µm). Scan rate: 50 mV/s.
Figure 4. Normalized steady-state voltammograms for the limiting current of 5 mM tetraphenylethene in DMF for concentrations of Bu4NBF4: 1, 10, 50, 100, and 500 mM. Gold microdisk (r ) 3.5 µm). Scan rate: 50 mV/s.
for the first wave,
IL1 ) 4nFDcr
(4)
are close to those expected for DMF solutions. The low value in the 500 mM electrolyte being due to the very viscous nature of this solution. Figure 3 and Table 2 present similar data from normalized and corrected voltammograms for the reduction of 5 mM triphenylethene in DMF as a function of the Bu4NBF4 concentration. The normalized steady-state voltammogram for an electrolyte concentration of 500 mM shows a reversible 1ereduction step with a half-wave potential of -2562 mV and a second 1e- process with a half-wave potential of -2814 mV vs Cp2Fe+/Cp2Fe. With triphenylethene, the two reduction steps are significantly closer than with trans-stilbene. On the other hand, both reduction waves shift positive with increasing electrolyte concentration although the magnitude of the shift is much greater for the second wave. In addition, the second reduction step is slightly irreversible and there is a trend for the kinetics of electron transfer to become slower with decreasing electrolyte concentration. It may also be seen that the addition of an electron is easier to the more substituted ethene and that the diffusion coefficients for the larger molecule, triphenylethene, are lower than for trans-stilbene in the same solutions. The normalized voltammograms for tetraphenylethene in DMF containing various concentrations of Bu4NBF4 are shown in Figure 4. Earlier papers4,9 have reported that tetraphenylethene reduces in a single 2e- wave although the wave was broadened because the wave results from two 1e- processes with very similar formal potentials. A single 2e- reduction wave is, indeed, clearly seen in Figure 4 for the higher concentrations of the electrolyte. Below 10 mM Bu4NBF4, however, the wave is highly distorted in the top half of the wave and in 1 mM Bu4NBF4, two reduction waves are apparent although the second wave remains smaller than the first. Table 3 reports the voltammetric characteristics of this compound but only for the three higher electrolyte concentrations; for lower values it is impossible to estimate half wave potentials and limiting currents
Figure 5. Normalized steady-state voltammograms for the limiting current of the second reduction wave of 5 mM trans-stilbene for three solvent compositions: 100% DMF; 50% DMF/50% toluene; 10% DMF/90% toluene, containing 100 mM of Bu4NBF4. Gold microdisk (r ) 6.0 µm). Scan rate: 50 mV/s.
TABLE 3: Effect of Bu4NBF4 Concentration on the Voltammetry of Tetraphenylethylene (5 mM) in DMF, IR Corrected, vs Formal Potential for Cp2Fe/Cp2Fe+ in the Solutions under Investigation [TBAB]/mM
-E1/2/mV
|E3/4 - E1/4|/mV
106D/cm2 s-1
50 100 500
2577 ( 1 2563 ( 1 2521 ( 1
103 ( 1 87 ( 2 61 ( 2
5.4 5.1 3.2
due to the overlap of the two waves. Clearly, the mechanisms for the reduction of tetraphenylethene in the literature are only applicable to higher concentrations of the Bu4NBF4 electrolyte. 3.2. Influence of the Solvent Medium. To probe further the influence of the medium on the voltammetry of the phenyl substituted ethenes I-III , voltammograms were recorded at a gold microdisk electrode in two DMF/toluene mixtures containing 100 mM Bu4NBF4. Slow sweep rate voltammograms were recorded for each of the three ethenes, in DMF, 50% DMF/ 50% toluene and 10% DMF/90% toluene. They were corrected for the IR drop and presented as log I/IL2 vs E vs SCE. In all three solvent compositions, the voltammograms for trans-stilbene, I, show two waves of equal height and the first 1e- reduction is reversible; see Figure 5. It is very clear, however, that the separation of the two waves decreases as the toluene content of the solution increases. Figure 6 reports the
TABLE 2: Effect of Bu4NBF4 Concentration on the Voltammetry of Triphenylethylene (5 mM) in DMF, IR Corrected, vs Formal Potential for Cp2Fe/Cp2Fe+ in the Solutions under Investigation wave 1
wave 2
[TBAB]/mM
-E1/2/mV
|E3/4 - E1/4|/mV
-E1/2/mV
|E3/4 - E1/4|/mV
∆E1/2/mV
IL2/IL1
106D/cm2 s-1
1 10 50 100 500
2646 ( 2 2626 ( 1 2600 ( 2 2590 ( 1 2562 ( 1
55 58 ( 0 56 ( 3 58 ( 3 57 ( 1
2982 ( 13 2889 ( 9 2854 ( 8 2814 ( 8
103 ( 4 85 ( 3 82 ( 6 70 ( 1
356 ( 12 290 ( 9 264 ( 8 253 ( 9
1.9 1.9 1.9 1.88
7.1 6.5 6.5 6.3 3.3
Reduction of Phenyl-Substituted Ethenes
J. Phys. Chem. B, Vol. 105, No. 16, 2001 3185
Figure 6. Normalized steady-state voltammograms for the limiting current of the second reduction wave of 5 mM triphenylethene for three solvent compositions: 100% DMF; 50% DMF/50% toluene; 10% DMF/90% toluene, containing 100 mM of Bu4NBF4. Gold microdisk (r ) 3.5 µm). Scan rate: 50 mV/s.
TABLE 4: Effect of Substrate Concentration, on the Voltammetry of trans-Stilbene in DMF, Containing 100 mM of Bu4NBF4, IR Corrected, vs Formal Potential for Cp2Fe/ Cp2Fe+ in the Solutions under Investigation [stilbene]/mM
-E11/2/mV
∆E/mV
2 5 10 20
2696 ( 3 2699 ( 2 2698 ( 2 2699 ( 1
416 ( 7 442 ( 2 432 ( 4 456 ( 3
normalized voltammograms for II. Only in 100% DMF does one observe two reduction steps of equal height. As the toluene content of the medium is increased, the two waves increasingly merge and there is a significant increase in height of the first wave at the expense of the second. For III, the voltammograms in all three media show a single 2e- wave and there is no appreciable change in shape. 3.3. Influence of trans-Stilbene Concentration. For one of the ethenes, I, the influence of substrate concentration was investigated. Slow scan voltammograms were recorded at a gold microdisk electrode for four concentrations of I in DMF containing 100 mM Bu4NBF4. All the responses, showed two waves of equal height. Moreover, the limiting current for the first reduction wave varies linearly with the concentration of I, the relation following the equation
IL1 ) 1.97c + 0.2
(5)
(r ) 0.99996), where IL1 is the current in nA and c is the concentration in mM of I. Table 4 reports the values of E1/2 for the first wave and ∆E1/2, after correction for the IR drop and to the potential of the Cp2Fe+/Cp2Fe couple. It can be seen that the half-wave potential of the first wave remains constant over the concentration range studied, but the separation of the two reduction waves increases as the concentration of stilbene increases. 4. Discussion The microdisk voltammetry confirms that the three ethenes, I, II, and III all undergo reduction in two 1e- steps in DMF and DMF/toluene mixtures containing Bu4NBF4. The formal potentials for the first reduction step are relatively insensitive to the structure of the ethene; see Tables 1-3. In contrast, the formal potentials for the A•-/A2- couples are very sensitive to the extent of substitution, and hence the separation of the two voltammetric reduction waves dependes strongly on their structure; the separation increases along the series III < II < I.
It is also evident from the data that the voltammetry of these ethenes can only be understood if ion pairing between the reduced species and the cation of the electrolyte is taken into account. This is the case even for the large tetrabutylammonium ion. Ion pairing leads to the changes in voltammetry with the concentration of the electrolyte, solvent composition and concentration of the substrate. Of course, ion pairing will be expected to become more important with increasing electrolyte concentration and decrease in the dielectric constant of the solvent medium. These are the trends observed. Indeed, we believe that equations (1) and (2) should be written
A + Bu4N+ + e- a Bu4N+A•-
(6)
Bu4N+A•- + Bu4N+ + e- a (Bu4N+)2A2-
(7)
where ion pairing between the tetraalkylammonium ion and the anion radical is relatively weak compared to that between the cation and the dianion. Ion pairing of the products of reduction will make the transfer of electrons easier, shifting the formal potentials to more positive potentials. Hence, the data show that the formal potentials for the A/A-‚ couples do shift positive with increasing Bu4NBF4 concentration although the shift is less than 100 mV for a 500-fold change in the electrolyte concentration. The positive shift in the second reduction waves is much larger, in the case of trans-stilbene reaching almost 300 mV. The overall influence on the voltammetry is considerable, as can be seen from Figures 2-4. A new aspect of the chemistry of III results from extending the voltammetry to low electrolyte levels. In the solution containing only 1 mM Bu4NBF4, the single wave is split into two. This implies that the stability of the anion radical depends strongly on the countercation concentration. Moreover, in voltammetry with “normal” electrolyte concentrations ion pairing is influencing significantly the form of response. This might lead to significantly different conclusions from chemical studies without excess electrolyte and electrochemical investigations with a large excess of electrolyte. We are less confident in the corrections for liquid junction potentials when the solvent composition is varied. For this reason, we prefer to comment only on the overall voltammetric response rather than absolute potentials (and, above, the data are reported versus the experimental reference electrode). Even so, the strong influence of ion pairing is very clear. In the case of I, the two 1e- reduction steps become much closer with increasing toluene content of the medium; in 90% toluene, ion pairing will be much stronger than in DMF. For II, the result of increasing the toluene content is more interesting. In DMF, two clearly separated waves of equal height are observed while in 90% toluene, the two waves overlap strongly and the two waves are far from equal height. Both these observations result from the shift in the formal potential for the second reduction step toward that for the first electron transfer. The increase in the height of the first wave at the expense of the second arises because of the disproportionation reaction
2 Bu4N+A•- a (Bu4N+)2A2- + A
(8)
which occurs as a homogeneous chemical reaction in the diffusion layer close to the electrode surface, followed by reaction 6. The equilibrium constant for disproportionation is directly related to the separation of the two electron-transfer steps (6) and (7).11-13 As the formal potentials approach, the extent of disproportionation increases and this leads to enhancement of the first wave as the neutral ethene is reduced for a
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Figure 7. Variation of Ee,IP with Bu4NBF4 concentration for (b) first and ([) second reduction waves for 5 mM of trans-stilbene in DMF. Gold microdisk (r ) 6.0 µm). Scan rate: 50 mV/s.
second time. Little disproportionation is apparent in DMF while in 90% toluene disproportionation is extensive because the stronger ion pairing of the dianion drives the equilibrium (8) to the right. It is interesting to note that the response for III in DMF/1 mM BuN4BF4 is very similar to that for II in 10% DMF/ 90% toluene/100 mM BuN4BF4, the two extreme conditions studied (with respect to ion pairing). Since III gives a single 2e- reduction wave in DMF, increasing the influence of ion pairing by changing the solvent has little effect on the voltammetric response. In principle, it is possible to treat the ion pairing more quantitatively. Ion pairing is analogous to the complexation of transition metals, and it is possible to write an equation
Ee,IP - Ee )
2.3RT (m log cBu4N+ - log KIP) nF
of I and II can be seen to increase as the electrolyte concentration is decreased. This is not an error in the IR correction and it must be concluded that ion pairing catalyzes the electron transfer. Finally, we would note that Bu4N+ is a large, singly charged cation and would ion pair relatively weakly compared with smaller or multicharged cations. Thus, in systems where the cation is Li+, Na+, or Ba2+,11-13 ion pairing must be expected to have a much stronger, even dominant influence. In chemical studies using solutions of such ions, it is likely that the anion radicals will be much less stable to disproportionation and in voltammetric experiments, the two reduction waves will be much closer. 5. Conclusions The use of microelectrodes has allowed the study of the phenyl-substituted ethenes, I, II, and III, over a much wider range of solution conditions, particularly concentration of the electrolyte and solvent composition, and this has demonstrated that previous studies in the literature7,9 lead to conclusions which are oversimplified because they correspond only to conditions where ion pairing is relatively strong (in contrast to many chemical studies). In reality, the voltammetry for all three compounds shows a significant dependence on the medium. References and Notes
(9)
where Ee,IP and Ee are the formal potential with and without ion pairing, KIP is the equilibrium constant for the ion pairing reaction and m is the number of cations in the ion cluster.19,20 Figure 7 shows plots of Ee,IP versus the concentration of BuN4BF4 for both first and second reduction waves for trans-stilbene in DMF and it can be seen that linear relationships are found. The data analysis cannot, however, be taken further since Ee is not known and, more importantly, the concentration of Bu4N+ cannot be assumed to be the same as the concentration of BuN4BF4. The Bu4N+ may well be ion paired with BF4- especially in the toluene rich medium. The ion pairing must be thought of in terms of a competition between the reduced ethenes and the tetrafluoroborate. Variation of the trans-stilbene concentration (see Table 4) does not lead to large changes in the potentials for reduction. Even so, it can seen that as the ratio [I]/[BuN4BF4] is increased, there is a trend for the two reduction waves to separate, and this implies a weaker influence of ion pairing, particularly on the second electron-transfer step. So far, all the discussion has focused on thermodynamic effects. While the first reduction step is invariably reversible, showing the first electron transfer to be fast, ion pairing does also seem to influence the kinetics of the second electron-transfer step. In Tables 1 and 2, the Tomes slope for the second wave
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