Studies of Potential Inversion in the Electrochemical Reduction of 11

J. Phys. Chem. B 2006, 110, 5155-5160

5155

Studies of Potential Inversion in the Electrochemical Reduction of 11,11,12,12-Tetracyano-9,10-anthraquinodimethane and 2,3,5,6-Tetramethyl-7,7,8,8-tetracyano-1,4-benzoquinodimethane Norma A. Macı´as-Ruvalcaba and Dennis H. Evans* Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721 ReceiVed: December 19, 2005; In Final Form: January 20, 2006

The electrochemical reduction of the title compounds, 2a and 3a, and 7,7,8,8-tetracyanoquinodimethane, 1a, was studied in acetonitrile. The reduction of 1a shows normal ordering of potentials, i.e., the potential for insertion of the first electron, E°1, is more positive than the potential for the second step of reduction, E°2. Thus, E°1 - E°2 > 0. By contrast, 2a and 3a show inversion of potentials where introduction of the second electron occurs with greater ease than the first (E°1 - E°2 < 0). The extent of inversion has been determined by simulation of the cyclic voltammograms obtained at 298 and 257 K. Electron paramagnetic resonance measurements at room temperature of solutions containing equimolar mixtures of the neutral and dianion allow determination of the concentration of anion radicals from which the disproportionation equilibrium constant and E°1 - E°2 can be calculated. The results were in good agreement with the voltammetric determinations. Calculations were conducted to characterize the structural changes accompanying reduction to the anion radical and dianion forms. Fast scan experiments at low temperatures (up to 10 000 V/s at 257 K; 500 V/s at 233 K) were conducted in an attempt to detect intermediates in the reduction, but none was found. Thus, it is not possible to state whether structural change and electron transfer are concerted or occur in discrete steps.

1. Introduction

anthra-, duro-family: In each case that has been studied, the

Normally when a neutral molecule, A, containing two identical electroactive groups is reduced in a nonaqueous medium, the reduction occurs in a stepwise fashion: first an electron is introduced into the neutral form to produce the anion radical, A•-, and, at more negative potentials, introduction of the second electron into the already charged radical produces the dianion, A2-.

A + e- h A•A•- + e- h A2-

E°1, ks,1, R1 E°2, ks,2, R2

(1) (2)

The fact that the potentials required to reduce identical groups are not the same is traced to electrostatic effects whereby it is more difficult to insert an electron into the negatively charged anion radical than into the neutral. This arrangement of potentials has been called “normal ordering”1b and corresponds to E°1 being situated at less negative potentials than E°2, that is, E°1 - E°2 > 0. In some instances, however, it is observed that introduction of the second electron occurs with greater ease than the first. That is, the second potential lies to the positive side of the first and E°1 - E°2 < 0. This situation has been called “potential inversion”.1b In almost all known cases, the causes of potential inversion are significant structural changes that accompany the electron-transfer reactions. Potential inversion has been found to occur for several classes of compounds1 including those based upon the following benzo-, * To whom correspondence should be addressed. E-mail: dhevans@ email.arizona.edu.

benzo derivative shows normal ordering of potentials, whereas the anthra and duro forms show potential inversion due to structural changes that accompany the electron transfers. For 1-3a, the subjects of the present paper, one starts with the neutral compounds and carries out reduction. Potential inversion has been reported for 2a,2a-c and for 3a a single two-electron process has been observed, which often signifies potential inversion.2d,e For 1-3a, one starts with the neutral compounds as a matter of convenience. As will be seen, the potential inversion observed for 2a and 3a can be studied equally well by starting with the dianions and oxidizing them to the neutral forms. Compounds 1-3b have been previously investigated.3 In this case, it is convenient to begin with the neutral compounds and carry out oxidations to observe potential inversion for 2b and 3b. The dications are susceptible to hydrolysis, with formation of the corresponding quinones making it difficult to study solutions of the dications. In the case of 1-3c, one starts with

10.1021/jp0573893 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/15/2006

5156 J. Phys. Chem. B, Vol. 110, No. 10, 2006 the neutral dinitro compound and carries out reduction to the dianions shown above. Potential inversion has been observed and characterized for 2c and 3c.4 Oxidation of 2d has also been shown to occur in a single two-electron process, but the extent of potential inversion has not been determined.5 A key question in the case of potential inversion is whether the structural changes accompanying the reactions are concerted with electron transfer or occur as two-step processes, with structural change as a discrete chemical step before or after electron transfer. In most instances, it has not been possible to discriminate between these two possibilities, but it has been recently shown that, for the case of 2b and 3b, the reactions must be two-step processes as the experimentally determined gas-phase inner reorganization energies are too small to account for the rather small values of the standard rate constants, ks, that were obtained from analysis according to the concerted mechanism.6 In addition to quantitative determination of the extent of potential inversion in 2a and 3a and rationalization of the phenomenon on the basis of theoretical calculation of structures, the present work seeks to discriminate between the concerted and two-step processes based on attempts to detect the intermediates necessarily present if the two-step process was followed. However, no intermediates were detected at scan rates as large as 10 000 V/s (257 K) or 600 V/s (233 K), signifying that such intermediates, if formed, are very short-lived. 2. Experimental Section 2.1. Chemicals and Reagents. The source and treatment of solvents and electrolyte were the same as previously reported.4b 7,7,8,8-Tetracyanoquinodimethane, 1a, was obtained from Aldrich and used as received. 11,11,12,12-Tetracyano-9,10anthraquinodimethane, 2a, and 2,3,5,6-tetramethyl-7,7,8,8tetracyano-1,4-benzoquinodimethane, 3a, were prepared by condensing malononitrile (Aldrich) with the corresponding quinones (Aldrich) in the presence of TiCl4 and pyridine.2e,7 The compounds were purified by column chromatography (7:3 hexane/ethyl acetate), crystallization (acetic acid), and finally by preparative thin-layer chromatography (7:3 hexane/ethyl acetate). 2.2. Electrochemical Cells, Electrodes, and Instrumentation. Except as indicated below, these were the same as previously reported including the polishing of the glassy carbon electrode (0.0814 cm2) and the determination of the uncompensated resistance.4b A circulating water bath was used to maintain the jacketed cell temperature for experiments at 298 and 257 K (50% ethylene glycol/water) while for 233 K, a jacketless cell was immersed in a dry ice/acetone slush. The potential of the silver reference electrode (Ag, 0.01 M AgNO3, 0.10 M Bu4NPF6 in acetonitrile) was periodically measured vs the potential of the ferrocenium/ferrocene couple in the solvent being studied and all potentials are reported vs ferrocene. For cyclic voltammetry at scan rates exceeding 100 V/s, the three-electrode cell was replaced by a two-electrode cell with a 52-µm diameter platinum microelectrode as working electrode and a coil of 1-mm diameter silver wire as combined counter and quasireference electrode. The EG&G Princeton Applied Research (PAR) potentiostat/galvanostat (model 283) was operated without computer control and a PAR model 175 universal programmer was used as external signal source. The output current of the 283 was recorded with a Gould digital oscilloscope, model 6100. In this arrangement, no resistance compensation was applied. The relatively small effects were accounted for by inclusion of the uncompensated resistance in the simulations.

Macı´as-Ruvalcaba and Evans Controlled potential coulometry was conducted with a platinum gauze working electrode as described earlier.8 The potentials for reductive coulometry of 2a and 3a were -1.02 and -0.73 V, and for reoxidation of the dianions they were -0.50 and -0.30 V, respectively. Each electrolysis required 15-30 min to reach completion. 2.3. Electron Paramagnetic Resonance. The determination of the concentration of the anion radicals in equilibrium with equimolar concentrations of the neutral and the dianion was achieved using electron paramagnetic resonance (EPR) spectroscopy. The experiments were performed at room temperature on a Bruker EPR spectrometer, model ESP-300. For each measurement, a thin (0.8 mm ID) capillary was filled with sample solution to a length exceeding the height of the resonator and was inserted in the resonator in such a way that both ends protruded from the resonator. This technique allowed us to avoid measurement errors related to differences in sample volumes. The spectra were recorded at a microwave (mw) frequency of 9.653 GHz, mw power of 200 µW, and modulation amplitude of 1 G. It was confirmed that the EPR signals did not saturate at the mw power used. The spectra were double-integrated, and the unknown radical concentrations were obtained from comparison with standard samples of the anion radical of 1a, whose concentrations were known. 2.4. Calculations. Digital simulations were conducted using DigiElch, version 2.0, a free software package for the digital simulation of common electrochemical experiments (http:// www.digielch.de).9 Complete geometry optimization and frequency calculations were performed according to the density functional theory (DFT) using the B3LYP/6-31G(d,p) level with the Gaussian 03 program.10 For the radical anions, the corresponding unrestricted (UB3LYP) method was used. Structural optimization was followed by frequency calculations to be certain that there were no imaginary frequencies. 3. Results and Discussion 3.1. Voltammetric Investigation of 7,7,8,8-Tetracyanoquinodimethane, 1a. To provide an example of a system showing normal ordering of potentials, 1a was studied. The electrochemistry of 1a was first investigated11 shortly after the original synthesis,12 and it was found to display normal ordering with the value of E°1 - E°2 being a few tenths of a volt, somewhat dependent on the cation of the supporting electrolyte.13 A voltammogram of 1.10 mM 1a in acetonitrile at a glassy carbon working electrode is shown in Figure 1 where the points represent a simulation that has been fit to the experimental voltammogram with E°1 ) -0.193 V and E°2 ) -0.745 V. The resulting value of E°1 - E°2, 0.552 V, is in excellent agreement with earlier studies14 as is the diffusion coefficient of 1a required to fit the data, 1.42 × 10-5 cm2/s.15 Unlike the case where the dianion of 1a was oxidized,15 the disproportionation reaction 3 had no effect on the simulations.

2A•- h A + A2-

(3)

Data were obtained for scan rates from 0.100 to 10.0 V/s, and the same simulation parameters were found to provide very good fits at all scan rates. In particular, the best fits for the standard rate constants were ks,1 ) 0.66 and ks,2 ) 0.33 cm/s. The value for ks,1 is somewhat larger than found for platinum and gold electrodes16 and is consistent with the very small reorganization energy for the neutral/anion radical couple, 7.2 kcal/mol.17 3.2. Electrochemical behavior of 11,11,12,12-Tetracyano9,10-anthraquinodimethane (2a) and 2,3,5,6-Tetramethyl-

Potential Inversion

J. Phys. Chem. B, Vol. 110, No. 10, 2006 5157 TABLE 1: Simulation Parametersa cmpd conc, (temp/°C) mM 1a/25 2a/25 2a/25b 2a/-16 2a2-/25 3a/25 3a/-16 3a2-/25

Figure 1. Cyclic voltammogram of 1.10 mM 7,7,8,8-tetracyanoquinodimethane, 1a, in acetonitrile containing 0.10 M tetrabutylammonium hexafluorophosphate. Glassy carbon working electrode. Full curve: background-corrected voltammogram. Symbols: Simulation using parameters from Table 1. Scan rate: 1.00 V/s. Temperature: 298 K.

Figure 2. Cyclic voltammogram of 1.03 mM 11,11,12,12-tetracyano9,10-anthraquinodimethane, 2a. Other conditions as in Figure 1.

Figure 3. Cyclic voltammogram of 1.03 mM 2,3,5,6-tetramethyl7,7,8,8-tetracyano-1,4-benzoquinodimethane, 3a. Other conditions as in Figure 1.

7,7,8,8-tetracyano-1,4-benzoquinodimethane (3a). Cyclic voltammograms of 2a and 3a under the same conditions as used in Figure 1 are shown in Figures 2 and 3. In marked contrast to the normal ordering seen for 1a, Figures 2 and 3 show only a single reduction peak and a single oxidation peak, which is highly suggestive that potential inversion has occurred. The result for 2a is consistent with an earlier report.2a To demonstrate that the reduction process was an overall twoelectron reaction, exhaustive controlled potential electrolyses were conducted on 15-20 µmol of 2a and 3a. The number of electrons transferred per molecule was 1.98 for each compound.

1.10 1.03 0.83 1.08 1.15 1.03 0.96 1.88

E°1 V

E°2 V

-0.193 -0.845 -0.837 -0.837 -0.832 -0.499 -0.475 -0.501

-0.745 -0.662 -0.635 -0.628 -0.672 -0.421 -0.405 -0.419

ks2 D × 105 E°1 - E°2 ks1 V cm s-1 cm s-1 cm2 s-1 +0.552 -0.183 -0.202 -0.209 -0.160 -0.078 -0.070 -0.082

0.66 0.42 0.21 0.18 0.47 0.30 0.06 0.32

0.33 0.15 0.013 0.025 0.20 0.50 0.10 0.17

1.42 0.78 0.42 0.52 0.65 1.06 0.74 0.80

a The same parameter values provided good fits for scan rates from 0.10 to 10.0 V/s. Diffusion coefficients of all three species (neutral, anion radical, and dianion) were assumed equal. All values of alpha were 0.5 except R1 ) 0.6 for 2a at -16 °C. Total uncompensated resistance was 130 Ω for acetonitrile at 25 °C and 220 Ω for acetonitrile at -16 °C. In most cases, virtually all of this resistance was electronically compensated. In a few cases a minor amount (10 Ω) was not compensated and needed to be included in the simulations. b DMF. Inclusion of disproportionation (reaction 3) with forward rate constant at the diffusion-controlled limit improved the fits. 260 Ω resistance was compensated electronically and 14 Ω was included in the simulations.

Furthermore, oxidation of the solution of dianion formed in the electrolysis recovered 96% of the charge for 2a and 93% for 3a, attesting to the overall reversibility of the process and the stability of the dianions under these conditions. The voltammograms shown in Figures 2 and 3 include simulations based on reactions 1 and 2, which combine electron transfer and any structural change in a single step, i.e., electrontransfer rate constants in the simulations were formulated in the normal way, e.g., kf ) ksexp[(-RF/RT)(E - E°)]. Thus, the values of E°, ks, and R for each step correspond to reactions where the electron transfer and any structural change are concerted. Table 1 gives simulation parameters for each compound. In each case (except for 1a, discussed above) it was necessary to invoke potential inversion to provide adequate fits of the voltammograms, somewhat larger for 2a (-0.183 to -0.202 V) than 3a (-0.078 V). At 298 K, the values of the electron-transfer rate constants were in the range of 0.1 to 0.5 cm/s, consistent with relatively small reorganization energies. Diffusion coefficients decrease in the order 1a > 3a > 2a, consistent with the size of the molecules. Results for 2a in DMF are also given in Table 1. The diffusion coefficient of 2a in DMF is somewhat smaller than in acetonitrile (consistent with the higher viscosity of DMF) and a smaller value of ks2 was found. Included in Table 1 are results for voltammetric studies of solutions of the dianions of 2a and 3a prepared by electrolysis (see above). The voltammograms are simply the inverse of those starting with the neutral compounds (see Figure 4 for 2a2-), and the simulation parameters needed for the dianion oxidations are very similar to those used for neutral reductions. The differences in the extent of inversion between entries 2 and 5 (Table 1), -0.183 for 2a vs. -0.160 V for 2a2-, reflect, we believe, the difficulties in accurately assessing the extent of inversion when it becomes large.4a The smaller inversion seen for 3a is noticeably more reproducible. 3.3. Investigation of the Extent of Disproportionation by EPR. The standard potentials of reactions 1 and 2 are related to the equilibrium constant for disproportionation, reaction 3, by eq 4.

E°1 - E°2 ) -(RT/F)ln Kdisp

(4)

In the EPR experiments, solutions containing equimolar con-

5158 J. Phys. Chem. B, Vol. 110, No. 10, 2006

Macı´as-Ruvalcaba and Evans

Figure 4. Cyclic voltammogram of 1.15 mM dianion of 2a prepared by controlled potential electrolysis. Other conditions as in Figure 1.

TABLE 2: Determination of the Extent of Potential Inversion by EPR compd 2a 3a

E°1 - E°2/V voltammetry

initial conc. A and A2- mM

Kdisp

EPR

1.00 2.66 1.08 2.14

5300 2500 37 51

-0.220 -0.201 -0.093 -0.101

-0.183 -0.078

centrations of the neutral (A) and dianion (A2-) forms were prepared by electrolysis, that is, by electrolyzing A until one electron had been transferred per molecule of A. The concentration of anion radical, A•-, was determined by doubly integrating the EPR signal using as standard a solution of the anion radical of 1a, also prepared by electrolysis. The value of Kdisp was calculated from the concentration of A•- found by EPR and the concentrations of A and A2- corrected for the amount of anion radical formed. Results are summarized in Table 2 where it may be seen that experiments were performed at two different initial concentrations. The corresponding values of E°1 - E°2 from eq 4 are very similar to those found by voltammetry (Table 1), with the largest difference being about 25 mV. 3.4. Structural Changes Accompanying the Reduction of 2a and 3a. X-ray diffraction studies of these two compounds

provide the structures of the neutrals in the solid state.2b,18 Both compounds have a boatlike six-membered ring connecting the two dicyanomethylidene units with resulting folding of the benzoannulands of 2a or the methyl groups of 3a in the opposite direction. Figure 5 illustrates for neutral 2a and 3a the structures calculated by DFT, which are almost identical to the X-ray structures.2b,18 Also shown in Figure 5 are the structures of the anion radical and dianion of the two compounds, also calculated by DFT. In the case of 2a•-, a structure that is similar to that of the neutral was found. The distortion from planarity of the anthracene ring system is smaller than in the neutral 2a, and there is no turning of the dicyanomethylidene groups. However, upon formation of the dianion, 2a2-, the anthracene core becomes completely planar and the two dicyanomethylidene groups are turned out of that plane, in opposite directions, by 52°. These results are in good agreement with earlier calculations.19 For 3a•-, unlike 2a•-, the dicyanomethylidene groups are turned (36°) and the six-membered ring is almost planar. For 3a2-, the turning of the dicyanomethylidene groups increases to a value identical to that in 2a2-, 52°. For the anion radicals we searched for structures of 3a•- that resembled that of 2a•-, that is, C(CN)2 groups that are not turned with respect to the central six-membered ring. However, we did not find any such structures that were true minima. Similarly, for 2a•- we could find no structures that resembled 3a•-. In previous work, it was shown how the predicted changes in structure cause changes in the gas-phase energies that in turn render disproportionation (reaction 3) to be less unfavorable. When the gas-phase energies were combined with empirically derived solvation energies, potential inversion was predicted for 2a and 3a.1b So far we have interpreted the voltammetry in terms of twoelectron transfers, each with structural change being concerted with the electron transfer. Alternatively, the structural change could occur as a discrete chemical reaction either before or after the electron transfer. A simplified scheme for the reduction of 3a is depicted in Figure 6. This is based on the fact that only two types of structure were found in our calculation: those that resembled the folded structure with a boatlike central ring that

Figure 5. Optimized structures of neutral, anion radical and dianion of 2a and 3a.

Potential Inversion

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Figure 6. Hypothetical reaction scheme for reduction of 3a.

is characteristic of the neutral compounds (designated by F) and those that resembled the planar structures (P), with turning of the dicyanomethylene units, characteristic of the dianions. In the mechanism, the folded neutral is supposed to be reduced at E°1F to a form of the anion radical (F•-), not too different from the neutral. This is followed by a rapid change to a P-type structure, P•- (the preferred form of 3a•- found by calculation, Figure 5), with rate constant kf1. This planar form would be easily reduced with minor structural change, at E°2P, to form the final dianionic product. Likewise, in the reduction of 2a the significant structural change would occur after the second electron transfer (cf. structures in Figure 5). If fast enough experiments could be performed, or if kf1 could be reduced in magnitude through lowering the temperature, it might be possible to reduce F•- before it converts to P•-. Since the structural changes associated with the processes at E°1F and E°2F are postulated to be small, these two-electron transfer reactions would be expected to show normal ordering of potentials, i.e., E°1F would be positive of E°2F. Thus, two effects should be noted if kf1 can be outrun: the main reduction peak should decrease from two-electron height toward one-electron height and a new reduction peak, due to F•- + e- f F2- should appear at potentials negative of the first peak. Such behavior was indeed observed for reduction of 1,1′-dimethylbianthrone, 4, at -56 °C and 20 V/s.20 With 4, however, the steric contributions to the isomerization barrier are considerably larger than for 2a and 3a. Also shown in Table 1 are simulation results

for experiments conducted at -16 °C for experiments up to 10 V/s. Neither of the two expected effects was detected: the peak heights were well accounted for by simulations of a two-electron mechanism, and no new peaks were seen in the data. Figure 7 shows a voltammogram for 2a at -16 °C and 10 V/s with a simulation performed with the parameters listed in Table 1. Results for 3a at -16 °C up to 10 V/s were similarly well accounted for by simulations that consider structural change and electron transfer to proceed in a concerted manner. For 3a, microelectrode voltammetry was carried out up to 10 000 V/s. The raw data are dominated by a very large charging current background, but there was no clear indication of the appearance of new peaks. Furthermore, the height of the cathodic peak was well accounted for by simulations using the parameters in Table 1 for microelectrode voltammetry from 100 to 10 000 V/s. This suggests that the postulated F•- f P•- reaction is too fast at -16 °C to be outrun, even at 10 000 V/s. Similarly, 600 V/s at -40 °C revealed no support of the postulated ECE sequence given in Figure 6.

Figure 7. Cyclic voltammogram of 1.08 mM 11,11,12,12-tetracyano9,10-anthraquinodimethane, 2a, at -16 °C and 10 V/s. Other conditions as in Figure 1.

It is usually assumed that the structural change associated, for example, with the first step of reduction of 3a (F + e- f P•-) is too substantial to be concerted with electron transfer, so that the major part of the structural change (F•- f P•-) must follow the electron transfer. If that is indeed true, the rate constant for the structural change must be of the order of 105 s-1 or greater, as shown by simulation, to prevent its measurement in the present work. Acknowledgment. Support of this research by the National Science Foundation, Grant CHE 0347471, is gratefully acknowledged. We thank Dr. Andrei Astashkin, Electron Paramagnetic Resonance Facility, University of Arizona, for the EPR measurements. References and Notes (1) (a) For reviews of earlier work see 1b,c. (b) Evans, D. H.; Hu, K. J. Chem. Soc., Faraday Trans. 1996, 92, 3983-3990. (c) Evans, D. H.; Lehmann, M. W. Acta Chem. Scand. 1999, 53, 765-774. (d) Kraiya, C.; Evans, D. H. J. Electroanal. Chem. 2004, 565, 29-35. (e) Dı´az, M. D.; Illescas, B. M.; Martı´n, N.; Viruela, R.; Viruela, P. M.; Ortı´, E.; Brede, O.; Zilbermann, I.; Guldi, D. M. Chem. Eur. J. 2004, 10, 2067-2077. (f) Nishiumi, T.; Chimoto, Y.; Hagiwara, Y.; Higuchi, M.; Yamamoto, K. Macromolecules 2004, 37, 2661-2664. (g) EÅ azar, Z.; Majcen Le Mare´chal, A.; Lorcy, D. New J. Chem. 2003, 27, 1622-1626. (h) Buerro, M.; Carlier, R.; Boubekeur, K.; Lorcy, D.; Hapiot, P. J. Am. Chem. Soc. 2003, 125, 3159-3167. (i) Lehmann, M. W.; Singh, P.; Evans, D. H. J. Electroanal. Chem. 2003, 549, 137-143. (j) Perepichka, D. F.; Bryce, M. R.; Perepichka, I. G.; Lyubchik, S. B.; Christensen, C. A.; Godbert, N.; Batsanov, A. S.; Levillain, E.; McInnes, E. J. L.; Zhao, J. P. J. Am. Chem. Soc. 2002, 124, 14227-14238. (k) Ludwig, K.; Quintanilla, M. G.; Speiser, B.; Stauss, A. J. Electroanal. Chem. 2002, 531, 9-18. (l) Uhrhammer, D.; Schultz, F. A. J. Phys. Chem. A 2002, 106, 11630-11636. (m) Wolff, J. J.; Zietsch, A.; Nuber, B.; Gredel, F.; Speiser B.; Wu¨rde, M. J. Org. Chem. 2001, 66, 27692777. (n) Carlier, R.; Hapiot, P.; Lorcy, D.; Robert, A.; Tallec, A. Electrochim. Acta 2001, 46, 3269-3277. (o) Saito, G.; Hirate, S.; Nishimura, K.; Yamochi, H. J. Mater. Chem. 2001, 11, 723-735. (p) Guldi, D. M.; Sanchez, L.; Martı´n, N. J. Phys. Chem. B 2001, 105, 7139-7144. (q) Hapiot, P. F.; Kispert, L. D.; Konovalov, V. V.; Save´ant, J. M. J. Am. Chem. Soc. 2001, 123, 6669-6677. (r) Bellec, N.; Boubekeur, K.; Carlier, R.; Hapiot, P.; Lorcy, D.; Tallec, A. J. Phys. Chem. A 2000, 104, 9750-9759. (s) Du¨mmling, S.; Speiser, B.; Kuhn, N.; Weyers, G. Acta Chem. Scand. 1999, 53, 876-886 (t) Speiser, B.; Wu¨rde, M.; Maichle-Mo¨smer, C. Chem. Eur. J. 1998, 4, 222-233. (u) Burkholder, C.; Dolbier, W. R.; Me´debielle, M. J. Org. Chem. 1998, 63, 5385-5394. (v) Felderhoff, M.; Smelkova, N.; Gornastaev, L. M.; Rieker, A. J. Chem. Soc., Perkin Trans. 2 1998, 343348. (w) Martı´n, N.; Sa´nchez, L.; Seoane, C.; Orti, E.; Viruela, P. M.; Viruela, R. J. Org. Chem. 1998, 63, 1268-1279. (x) Capon, J. F.; Kergoat, R.; Le Berre-Cosquer, N.; Pe´ron, S.; Saillard, J. Y.; Talarmin, J. Organometallics 1997, 16, 4645-4656. (y) Kispert, L. D.; Gao, G.; Deng, Y.; Konovalov, V.; Jeevarajan, A. S.; Jeevarajan, J. A.; Hand, E. Acta Chem. Scand. 1997, 51, 572-578. (z) Moore, A. J.; Bryce, M. R. J. Chem. Soc., Perkin Trans. 1 1991, 157-168. (2) (a) Kini, A. M.; Cowan, D. O.; Gerson, F.; Mo¨ckel, R. J. Am. Chem. Soc. 1985, 107, 556-562. (b) Aumu¨ller, A.; Hu¨nig, S. Liebigs Ann. Chem. 1984, 618-621. (c) Schubert, U.; Hu¨nig, S.; Aumu¨ller, A. Liebigs Ann.

5160 J. Phys. Chem. B, Vol. 110, No. 10, 2006 Chem. 1985, 1216-1222. (d) Kini, A.; Mays, M.; Cowan, D. J. Chem. Soc., Chem. Commun. 1985, 286-287. (e) Bryce, M. R.; Davies, S. R.; Grainger, A. M.; Hellberg, J.; Hursthouse, M. B.; Mazid, M.; Bachmann, R.; Gerson, F. J. Org. Chem. 1992, 57, 1690-1696. (3) Hu, K.; Evans, D. H. J. Electroanal. Chem. 1997, 423, 29-35. (4) (a) Kraiya, C.; Evans, D. H. J. Electroanal. Chem. 2004, 565, 2935. (b) Macı´as-Ruvalcaba, N. A.; Evans, D. H. J. Phys. Chem. B 2005, 109, 14642-14647. (c) Geske, D. H.; Ragle, J. L.; Bambenek, M. A.; Balch, A. L. J. Am. Chem. Soc. 1964, 86, 987-1002. (d) Kitagawa, T.; Ichimura, A. Bull. Chem. Soc. Jpn. 1973, 46, 3792-3795. (e) Yildiz, A.; Baumga¨rtel, H. Ber. Bunsen-Ges. Phys. Chem. 1977, 81, 1177-1182. (5) Moore, A. J.; Bryce, M. R. J. Chem. Soc., Perkin Trans. 1 1991, 157-168. (6) Gruhn, N. E.; Macı´as-Ruvalcaba, N. A.; Evans, D. H., submitted to J. Phys. Chem. B. (7) Donkers, R. L.; Workentin, M. S. J. Am. Chem. Soc. 2004, 126, 1688-1698. (8) Macı´as-Ruvalcaba, N. A.; Evans, D. H. J. Electroanal. Chem. 2005, 585, 150-155. (9) (a) Rudolph, M. J. Electroanal. Chem. 2003, 543, 23-29. (b) Rudolph, M. J. Electroanal. Chem. 2004, 571, 289-307. (c) Rudolph, M. J. Electroanal. Chem. 2003, 558, 171-176. (d) Rudolph, M. J. Comput. Chem. 2005, 26, 619-632. (e) Rudolph, M. J. Comput. Chem. 2005, 26, 633-641. (f) Rudolph, M. J. Comput. Chem. 2005, 26, 1193-1204. (10) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,

Macı´as-Ruvalcaba and Evans X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; C. Gonzalez, C.; Pople, J. A., Gaussian 03, Revision B.05; Gaussian, Inc.: Pittsburgh, PA, 2003. (11) Acker, D. S.; Hertler, W. R. J. Am. Chem. Soc. 1962, 84, 33703374. (12) Acker, D. S.; Harder, R. J.; Hertler, W. R.; Mahler, W.; Melby, L. R.; Mochel, W. E. J. Am. Chem. Soc. 1960, 82, 6408-6409. (13) Khoo, S. B.; Foley, J. K.; Pons, S. J. Electroanal. Chem. 1986, 215, 273-285. (14) Lehmann, M. W.; Evans, D. H. J. Phys. Chem. B 1998, 102, 99289933. (15) Zhou, R.; Evans, D. H. J. Electroanal. Chem. 1995, 385, 201207. (16) Sharp, M. J. Electroanal. Chem. 1978, 88, 193-203. (17) Larsen, H.; Pedersen, S. U.; Pedersen, J. A.; Lund, H. J. Electroanal. Chem. 1992, 331, 971-983. (18) Rosenau, B.; Krieger, C.; Staab, H. A. Tetrahedron Lett. 1985, 26, 2081-2084. (19) Orti, E.; Viruela, R.; Viruela, P. M. J. Phys. Chem. 1996, 100, 6138-6146. (20) Evans, D. H.; Busch, R. W. J. Am. Chem. Soc. 1982, 104, 50575062.