Mechanism of Dimerization of 1, 4-Dithiafulvenes into TTF Vinylogues

704, UniVersite´ de Rennes, Campus de Beaulieu, 35042 Rennes, France, and. Laboratoire d'Electrochimie organique, Unite´ Associe´e au CNRS No. 439 ...
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J. Phys. Chem. 1996, 100, 14823-14827

14823

Mechanism of Dimerization of 1,4-Dithiafulvenes into TTF Vinylogues Philippe Hapiot,*,1a Dominique Lorcy,1b Andre´ Tallec,1c Roger Carlier,1c and Albert Robert1b Laboratoire d’Electrochimie Mole´ culaire de l’UniVersite´ Denis Diderot (Paris 7,) Unite´ de Recherche Associe´ e au CNRS No. 438, 2 place Jussieu, 75251 Paris Cedex 05, France, Groupe de Chimie Structurale, Unite´ Associe´ e au CNRS No. 704, UniVersite´ de Rennes, Campus de Beaulieu, 35042 Rennes, France, and Laboratoire d’Electrochimie organique, Unite´ Associe´ e au CNRS No. 439, UniVersite´ de Rennes, Campus de Beaulieu, 35042 Rennes, France ReceiVed: April 8, 1996; In Final Form: June 3, 1996X

Mechanism of the oxidative dimerization of DTF (dithiafulvenes) to form TTF vinylogues (tetrathiafulvalenes) has been investigated by cyclic voltammetry at low and high scan rates for a series of substituted DTF. It involves first the formation of the cation radical which couples to form the protonated dication. This dication slowly deprotonates to give the final TTF (k ) 0.5-1 s-1). The dimerization rate constant was found to be in the range of kdim ) (2-4) × 108 L mol-1 s-1 and not vary much with the nature of the substituent.

Introduction Tetrathiafulvalenes (TTF) and their analogues have been intensively studied in attempt to prepare novel organic materials (for general reviews see ref 2). Modifications on the basic TTF framework such as including a conjugated spacer group between the two dithiole rings have been realized with the aim of discovering new and better π-donors (see for example ref 3 and references therein). Among all the extended TTF reported in the literature, attention has been focused on TTF vinylogues where a vinyl spacer group is located between the dithiole rings. In the literature, oxidative dimerization of substituted 1,4dithiafulvenes has been known for some time to afford the dimeric dication.4,5 Electrochemical synthesis of TTF vinylogues starting from 1,4-dithiafulvenes has been reported as a convenient way for preparing this new kind of extended TTF.5 These compounds are obtained in a one-pot procedure by oxidation-reduction of substituted 1,4-dithiafulvenes. In this dimerization process, several reaction steps must be involved namely the electron transfers, the carbon-carbon bond formation and also the deprotonation steps with the departure of two protons. It is interesting to understand the dimerization mechanism allowing the synthesis of this class of TTF not only to be able to control the reaction process but also for fundamental investigations; they are a nice example of oxidative coupling reactions involving C-C bond formations. For this reason, we present a detailed study of the mechanism of this TTF formation by cyclic voltammetry on classical electrode and ultramicroelectrode. In the text, DTF-R and TTF-R will be used for the 1,4-dithiafulvenes and tetrathiafulvalene vinylogues respectively, with R ) NO2, CN, Cl, H, CH3, OCH3, and N(CH3)2 (Chart 1). Experimental Section Chemicals. Acetonitrile was Uvasol quality (Merck) and was used as received. The supporting electrolyte NEt4BF4 and the 2,6-lutidine (2,6-dimethylpyridine) were from Fluka (puriss). The synthesis of 1,4-dithiafulvenes has been published previously,5 all the molecules are present as two isomers (Z/E) which were not separated. Solution were prepared freshly before experiments and oxygen was removed by bubbling argon. Electrochemical Experiments. All the cyclic voltammetry experiments were carried out at 20 ( 0.1 °C using a cell equipped with a jacket allowing circulation of water from the thermostat. The counter electrode was a Pt wire and the X

Abstract published in AdVance ACS Abstracts, August 1, 1996.

S0022-3654(96)01048-9 CCC: $12.00

CHART 1: General Formula of the Studied 1,4-Dithiafulvene (DTF-R), AH, the Corresponding Dimer (TTF-R), AA, and the Different Intermediates

reference electrode an aqueous saturated calomel electrode (E°/ SCE ) E°/NHE - 0.2412 V) with a salt bridge containing the supporting electrode. The SCE electrode was checked against the ferrocene/ferricinium couple (E° ) +0.405 V/SCE) before and after each experiment. For low scan rate cyclic voltammetry (0.05-500 V s-1), the working electrode was either a glassy carbon disk (0.8 mm diameter Tokai Corp.), a 1 mm diameter gold or platinum disk. They were carefully polished before each set of voltammograms with 1 µm diamond paste and ultrasonically rinsed in absolute ethanol. Electrochemical instrumentation consisted of a PAR Model 175 Universal programer and of home-built potentiostat equipped with a positive feedback compensation device.6 The data were acquired with a 310 Nicolet oscilloscope. For high scan rate cyclic voltammetry, the ultramicroelectrode was a gold or platinum wire (10 µm diameter) sealed in soft glass.7 The signal generator was a Hewlett Packard 3314A, and the curves were recorded with a 4094C Nicolet oscilloscope with a minimum acquisition time of 5 ns per point. Simulations were made with the BAS Digisim simulator 2.0. Results and Discussion Mechanism of Dimer Formation. Investigations were performed by cyclic voltammetry on platinum, gold, or carbon electrodes. The same general results were observed on the three types of electrode, indicating that the nature of electrode does not influence the reaction. To elucidate the dimerization mechanism, we focused first on the oxidation of DTF-CN © 1996 American Chemical Society

14824 J. Phys. Chem., Vol. 100, No. 35, 1996

Hapiot et al.

Figure 2. High scan rate voltammetry of a solution of 10-3 mol L-1 of DTF-CN in acetonitrile (+0.2 mol L-1 of NEt4BF4) on a 10 µm platinum disk electrode. V ) 6000 V s-1.

Figure 1. Cyclic voltammetry of a solution of 10-3 mol L-1 of DTFCN in acetonitrile (+0.2 mol L-1 of NEt4BF4) on a 1 mm platinum disk electrode: (a) without added base; (b) after addition of 8.6 × 10-3 mol L-1 2,6 lutidine (numbers on each curve are the scan rates in V s-1).

derivative, 2; this compound was stable enough to allow several experiments with the same solution. The cyclic voltammetry of the oxidation of the DTF-CN is displayed in Figure 1a at different scan rates on a platinum electrode in acetonitrile (+0.2 mol L-1 of NEt4BF4). Similar results were obtained with the other compounds (except for the NMe2+ derivative which will be discussed separately). At low scan rate (0.2 V s-1) and on the first anodic scan, an irreversible peak is visible (Ep ) 0.74). During the reverse cathodic scan, a reversible wave appears at a less positive potential when the oxidation peak of the dithiafulvene has been scanned beforehand. This new redox system was identified as the corresponding dimer TTF-CN by comparison with the cyclic voltammogram of an authentic sample. Thus, this result shows that during the scan, the oxidation of the DTF-CN led to the formation of the dimer TTFCN which is oxidized at a less positive potential than the DTFCN. It is interesting to notice that the peak currents for the generated TTF-CN are quite small (less than half of the oxidation peak of the monomer) showing that a low quantity of dimer was formed during the scan. Also, a crossover is clearly visible in the middle of the voltammogram recorded at low scan rates (see Figure 1a, 0.2 V s-1). These observations suggest that the overall reaction leading to the formation of the TTF-CN is rather slow, in the order of several seconds. This last point may appear in disagreement with the fact that the voltammogram of the oxidation of the DTF-CN remained irreversible up to scan rates in the order of several hundreds volts per second (highest available scan rate with millimetric electrodes), indicating a lifetime for the first generated intermediate lower than 10 ms. When increasing the

scan rate (see Figure 1a), the oxidation peak potential became more positive and, simultaneously, the relative height of the reversible redox system corresponding to the formation of the TTF-CN decreased and almost disappeared for scan rates higher than 10 V s-1. Upon raising the scan rate above 1000 V s-1 (experiment performed on ultramicroelectrodes), a slight reversibility of the anodic peak became visible, indicating that the lifetime of the electrogenerated cation radical was in the order of the experimental time (see Figure 2). As discussed before, different reaction steps are involved in the dimer formation, namely, one or several electron transfers, carbon-carbon bond formation, and elimination of protons. The first electron transfer leads to the formation of the cation radical which can be observed by high scan rate voltammetry. Thus, the first important question concerns the sequence of the different possible steps, in other words, whether the carboncarbon bond formation involves the initially formed cation radical or the neutral radical resulting from its deprotonation. For answering to this question, we added to the solution large amounts of base (2,6-lutidine) which is a much stronger base than the acetonitrile itself.8 No considerable effects were observed on the reversibility of the fast scan voltammogram in the range of 1000-10 000 V s-1, indicating that the lifetime of the cation radical was not modified by the addition of base. With the same set of solutions, the voltammograms before and after addition of base were recorded at several scan rates and are displayed in Figure 1b. In these conditions (scan rates in the range of 0.2-100 V s-1), large modifications are clearly observed between the voltammograms recorded in pure acetonitrile and after the addition of 8.6 × 10-3 mol L-1 of 2,6lutidine.9 At the lowest scan rates, we can first noticed an increase of the height of the anodic peak and also of the reversible system corresponding to the formation of the TTFCN. At higher scan rates (100 V s-1), the height of the DTFCN oxidation peak did not change much, but the quantity of formed dimer increased a lot. Several conclusions can be drawn from these experiments; first, the fact that the lifetime of the cation radical remained unchanged after the addition of base shows that the first reaction step is not a deprotonation and thus that the neutral radical A• is not an intermediate. However, the simultaneous increase of the quantity of generated dimer confirms that deprotonation step occurs during the formation of dimer certainly after the carbon-carbon bond formation.10 The next problem to solve concerns the nature of the coupling step, i.e., does it involve the reaction between the two electrogenerated cation radicals (radical-radical coupling, R-R, or “DIM1 mechanism”) or by condensation of the cation radical with the starting molecule (substrate-radical coupling R-S,

Dimerization of 1,4-Dithiafulvenes into TTF Vinylogues

Figure 3. Cyclic voltammetry of solutions of DTF-CN in acetonitrile (+0.2 mol L-1 of NEt4BF4) on a 1 mm platinum disk electrode: (a) variation with the logarithm of the scan rates in V s-1, C° ) 10-3 mol L-1; (b) variation with the logarithm of the initial concentrations of DTF-CN in mol L-1, V ) 0.2 V s-1.

“DIM2 mechanism”).11 To answer to this question, we performed a series of voltammetry experiments where the variation of the peak potential was studied as a function of the experimental parameters (scan rates and initial concentration of DTF-CN) as the expected variation for the different kinds of mechanism are different.11 We found that the anodic peak potential varies linearly with the logarithm of the scan rate with a slope, ∂Ep/∂(log(V)) ) 19.9 mV, and with the bulk concentration of DTF-CN ∂Ep/∂(log(c) ) -19.4 mV per 10-fold increase (see Figure 3). These observations only agree with an R-R mechanism (theoretical variation for R-R mechanism: ∂Ep/∂(log(V)) ) 19.4 mV and ∂Ep/∂(log(c) ) -19.4 mV at 20 °C).11,12 This means that the oxidation mechanism involves first a fast electron transfer to form a cation radical AH•+, followed by an irreversible coupling of AH•+ to form the protonated dimer +HAAH+.

AH h AH•+ + eAH•- + AH•- f +HAAH+ We can also notice that the R-R coupling mechanism falls in line with the preceding experiments where no variation of the cation radical lifetime, AH•+, was observed after the addition of base. After having determined the nature of the mechanism reaction, it is now interesting to characterize the different chemical steps by measuring their kinetic or thermodynamic constants. From the experiments performed at high scan rate where partially reversible voltammograms were recorded, we can immediately derive the value of the standard oxidation potential E° (oxidation of DTF-CN to its cation-radical) as the midpoint between the anodic and the cathodic peak potentials. A value of 0.838 ((10 mV) was thus found for the CN derivative. The value of the apparent standard electron-transfer rate constant (uncorrected from the double layer effect), ks, for this couple can be estimated from the separation between the anodic and cathodic peaks at high scan rates by the same simulation procedure as described earlier (taking into account the effect of the residual ohmic

J. Phys. Chem., Vol. 100, No. 35, 1996 14825 drop)7 assuming that R ) 0.5 and taking D ) 10-5 cm2 s-1. We thus found ks ) 2-3 cm s-1. Such value indicates a fast electron transfer in the order of the fastest systems known in electrochemistry for example for the anthracene/anthracene anion radical (ks ) 3-5 cm s-1).7 From the same experiment, we can also determine the dimerization rate constants by comparison of the reversibility of the voltammogram at several scan rates with simulated curves. The simulated voltammograms for the radical-radical coupling mechanism were calculated through the Digisim program assuming that the kinetics of the reactions of deprotonation do not interfere with the dimerization step (which will be justified later). We thus found a value of kdim ) (2-3) × 108 L mol-1 s-1. It is clear from the preceding measurements that the fast kinetics of the coupling step cannot be responsible for the low quantity of dimer formed during the voltammetric scans (the coupling step is done in less than 10-4 s). Thus, the slow formation of AA is certainly related to kinetics limitations during the deprotonations of +HAAH+. This point was confirmed by the effect of the addition of base which made faster the deprotonation reaction of the protonated dimer and thus, increased the quantity of dimer formed during the voltammetry scan (compare Figure 1, a and b). As reported previously and as it is clearly visible on the voltammograms of Figures 1 and 2, the dimer AA is much easier to oxidize than the starting DTFCN, AH. This oxidation was found to be bielectronic and leads to the formation of the corresponding dication.5 It results that the generated dimer is immediately oxidized to the corresponding dication at a potential located at the level of the oxidation of AH. The mechanism can be summarized by the following scheme:

AH H AH•+ + ekdim

AH•+ + AH•+ 98 +HAAH+ +

kH

HAAH+ + base 98 AA AA H +AA+ + 2e-

AA + 2AH•+ H +AA+ + 2AH We will not discuss here the exact nature of the two-electron exchange which can also proceed through homogeneous reactions. But because the deprotonation steps are very slow, the TTF-CN, AA, is formed far from the electrode surface and thus the electron exchange is more likely to occur by the homogeneous pathway than at the electrode surface. Taking into account this reaction scheme, comparison of the data (height of the reversible wave of the TTF-CN) and simulated curves allows an estimation of kH around 0.3 s-1. It is interesting to note that the apparent number of electron per monomer passes from 1 when the rate of the deprotonation of +HAAH+ is slower than the experimental time (high scan rates) to a limiting value of 2 when the reaction is faster (slow scan rate or addition of base). The variation of nelectron with the scan rate gives us another way for checking the validity of our mechanism and a way of estimating the deprotonation rate constant. It is possible to show that the variation of nelectron depends on only one single parameter λ ) kHRT/FV.7b,11b The experimental results for the oxidation of 10-3 mol L-1 of DTF-CN in acetonitrile without added base and after addition of 8.6 mol L-1 of 2,6-lutidine are displayed in Figure 4 together with the expected variation plotted in function of λ for the previously described mechanism. As expected, the addition of 2,6-lutidine increased the value of nelectron and a good agreement is found between the theoretical variation of nelectron and the experimental variations. The fitting

14826 J. Phys. Chem., Vol. 100, No. 35, 1996

Hapiot et al. TABLE 1: Electrochemical Oxidation of DTF-R and TTF-R compd

R

1 2 3 4 5 6 7

-NO2 -CN -Cl -H -CH3 -OCH3 -N(CH3)2

E°DTF-R/ E°TTF-R/ (V/SCE)a (V/SCE)a,b 0.869 0.838 0.767 0.733 0.707 0.637 0.406

0.550 0.528 0.453 0.438 0.395 0.350

kHc/ ∂Ep/∂(log(V))/ kdimc/ (mV (L mol-1 (L mol-1 s-1) s-1) log(V)-1) 19.5 19.8 20.5 19.2 19.3 19.9 17.5

3 × 108 2 × 108 2 × 108 2 ×108 2 × 108 4 × 108 5 × 105

0.3 0.3 0.2 0.3 0.2 1.0

a Error ( 10 mV. b Measured during the backward scan (see text).c Error ( a factor of 2.

Figure 4. Cyclic voltammetry of a 10-3 mol L-1 solution of DTF-CN in acetonitrile (+0.2 mol L-1 of NEt4BF4) on a 1 mm platinum disk electrode. Variation of the apparent number of electrons per monomer with the logarithm of the scan rates in V s-1.. (9) Without added base; (b) after addition of 8.6 mol L-1 of 2,6-lutidine. The lines are the theoretical behavior (see text). Lower scale: logarithm of the scan rate V. Upper scales: logarithm of λ ) kHRT/FV.

of the position of the working curves with the data allows the measurement of kH before and after addition of base. Thus, we found values for kH around 1.0 and 25 s-1 in pure acetonitrile and after addition of lutidine, respectively. The kH constant measured by this method in pure acetonitrile is close to the value determined previously by the height of the peak of the TTF. From the apparent first-order constant, we can derive a secondorder reaction for the reaction of the dication with the 2,6lutidine around 3 × 103 L mol-1 s-1. These low values confirm that the deprotonation of +HAAH+ occurs a long time after the irreversible coupling step (even after addition of base). It also explained the low amount of TTF observed during the voltammogram and also the crossover which is due to the continuous formation of the TTF (see Figure 1). Moreover, it is worth noting that the value of kH does not change much after addition of base if we consider that the 2,6-lutidine is a much stronger base than the acetonitrile itself or the residual water. However, low values for the deprotonation rates are commonly found with C-H+ acids even when the acids are strong and in several cases have already been described in the literature. Such behavior has for example been clearly observed in the rate of deprotonation of several cation radical generated from NADH analogues.8 In the field of electropolymerization (oxidative coupling of pyrroles, thiophenes, ...), recent results have suggested the intervention of slow deprotonation reactions in the kinetics of formation of higher oligomers.13 In our case, it is difficult to say more about the exact nature of the deprotonation steps involved in the DTF-CN oxidation which corresponds to the departure of two protons but which are only visible as one global step leading to the formation of AA. Oxidation of the Substituted Dithiafulvenes DTF-R. As discussed before, the other DTF-R exhibit the same general features upon oxidation (the case of the NMe2 derivative will be discussed separately. We have performed the same experiments concerning the variation of the peak potential with the scan rate, and the measured slopes are recorded in Table 1. For all the studied compounds, slopes close to the theoretical value of 19.4 mV/log(v) were obtained as we found before for the oxidation of DTF-CN which indicates that the same R-R mechanism is involved for the oxidation of all the DTF-R, i.e., a fast electron transfer to form the cation radical which reacts with another cation radical. Using high scan rates voltammetry (V > 1000 V s-1), it was always possible to obtain voltammograms presenting at least a partial reversibility allowing us to determine the dimerization rate constants, kdim. As seen from

Figure 5. Variation of the formal oxidation potentials E° of DTF-R (9) and the corresponding dimers (2) as a function of the Brown coefficients σ+ for the different substituents.

Table 1, the values of kdim do not change significantly with the nature of the substituents, ranging from 2 × 108 to 4 × 108 L mol-1 s-1 (this variation is smaller than error on the estimation). About the deprotonation reactions, the same effects as with 2 were observed when adding lutidine (increase of the quantity of generated TTF-R and of the apparent number of electrons, nelectron, per monomer). Similarly, the values of the deprotonation rate constants in pure acetonitrile were estimated by the comparison between the oxidation peak current of the DTF-R and the height of the reversible wave corresponding to the quantity of formed TTF-R for several experiments performed at different scan rates. Relatively low deprotonation rates were found around 0.2-0.3 s-1, except for the methoxy-substituted compound, 6, which seems slightly higher. All these results also confirm previous published experiments, after electrolysis of a mixture of two DTF-R with different substituents, that the unsymmetrical dimers were only obtained when oxidation was carried out at sufficiently positive potential to allow the formation of the cation radicals.5 The standard potentials, E°, for the AH/AH•+ couples and for the bielectronic oxidation of the dimers AA/AA2+ were measured with the same solution of dithiafulvene respectively from the reversible voltammograms at high scan rate and from the voltammograms at low scan rate during the backward scan. The variations of the standard oxidation potentials, E,° with the brown coefficient σp+ 14 are displayed on Figure 5 . A good correlation is observed for the two series (R ) 0.991 and R ) 0.994) if the value for the NMe2 derivative is not taken into account (see text below). It is worth noting that the two slopes for the monomer and the dimer measured in these conditions are very close (F ) 0.144 and 0.129). Oxidation of the DTF-NMe. The dimethylamino-substituted compound DTF-NMe2 (7) displays a special behavior. The voltammograms for the oxidation of the DTF-NMe2 are presented in Figure 6 for experiments performed at several scan rates. At low scan rates (0.2 V s-1) and when the oxidation scan is limited to 0.75 V, a large anodic peak a is visible which

Dimerization of 1,4-Dithiafulvenes into TTF Vinylogues

J. Phys. Chem., Vol. 100, No. 35, 1996 14827 can occur with basic impurities contained in the solvent but also in a “father-son” pathway where the monomer or the dimer can abstract the proton from the species +HAAH+. Conclusion In summary, the sequence of the different steps involved in the oxidative dimerization leading to the formation of TTF vinylogues has been established as well as the nature of the coupling step. After a fast first electron transfer the generated cation radicals undergo a fast dimerization into a protonated dimer. Then this protonated dication slowly deprotonates into the expected TTF vinylogue, which is oxidized at a lower potential than the starting 1,4-dithiafulvene. Moreover we also explain the special behavior of DTF-NMe2 by its possibility to be oxidized in the monomeric dication. References and Notes

Figure 6. Cyclic voltammetry of a solution of 10-3 mol L-1 of DTFNMe2 in acetonitrile (+0.2 mol L-1 of NEt4BF4) on a 1 mm platinum disk electrode. Numbers on each curve are the scan rates in V s-1.

suggests a partially reversible system. In fact, it was shown previously that the oxidation potentials of the dimer and the monomer were very close.5 Thus, the reduction peak d visible during the reverse scan is not due to the reduction of cation radical but to the electrogenerated dimer which is reduced to its neutral state. When the scan rate is increased, a new reversible system (peaks b, c) appears at a more positive potential. Simultaneously, a new cathodic peak d′ develops as the reduction peak d (of the dimer) decreases. For scan rate around 50 V s-1, the two peaks d and d′ have similar heights but because their potentials are very close, it is difficult to distinguish one from the other and they just look like a single broad peak. When the scan rate is increased above 200 V s-1, only peak d′ remains. For higher scan rates, the height of peak d′ still increases and the ratio between the peaks a/d′ indicates a reversible system. From the preceding results, the system a/d′ observed at high scan rates can be ascribed to the reversible oxidation of DTF-NMe2 to its cation radical and the fully reversible system b/c to a further oxidation of the cation-radical to a dication. As we did for the other DTF-R, we studied the variation of the peak potential a as function of the scan rate between 0.1 and 10 V s-1. A slope around 18 mV/log(V) was found, indicating that the dimerization mechanism for DTFMMe2 was the same as for the other DTF-R. From the passage from peaks d to d′, we can derive the rate constant for the carbon-carbon bond formation between two cation radical, kdim, as 5 × 105 L mol-1 s-1 and from the reversible voltammogram recorded at high scan rate, we can derive the E° for the monomer/monomer cation radical, AH/AH•+, as 0.406 V/SCE. The measured value of kdim is considerably lower (500 times) than the dimerization rate constants measured for the other DTF-R and may be related to a special stabilization of the cation-radical by the NMe2 group. Similarly, the measured oxidation potential E° is clearly below the correlation line in the E°-σ+ variation (see Figure 5). It is also noticeable that the second reversible system b,c is not visible with the other DTF-R and that the dication generated at this potential is more stable than the cation radical and does not dimerize. For what concerns the deprotonation steps the situation is more complicated than before, because tertiary amines R-NMe2 are strong bases in acetonitrile.8 It follows that the deprotonation

(1) (a) Universite´ Denis Diderot. (b) Groupe de Chimie Structurale, Universite´ de Rennes. (c) Laboratoire d’Electrochimie Organique, Universite´ de Rennes. (2) For general reviews see: (a) Narita, M.; Pittman, C. U. Synthesis. 1976, 489. (b) Krief, A. Tetrahedron 1986, 42, 1209. (c) Schukat, G.; Richter, A. M.; Fangha¨nel, E. Sulfur Rep. 1987, 7, 155. (d) Schukat, G.; Fangha¨nel, E. Sulfur Rep. 1993, 14, 245. (3) Ogura, F.; Otsubo, T.; Aso, Y. Sulfur Rep. 1992, 11, 439 and references cited. (b) Hansen, T. K.; Lakshmikantham, M. V.; Cava, M. P.; Niziurski-Man, R. E.; Jensen, F.; Becher, J. J. Am. Chem. Soc. 1992, 114, 5035. (c) Benahmed-Gasmi, A. S.; Fre`re, P.; Garrigues, B.; Gorgues, A.; Jubault, M.; Carlier, R.; Texier, F. Tetrahedron Lett. 1992, 33, 6457. (d) Ohta, A.; Kobayashi, T.; Kato, H. J. Chem. Soc., Chem. Commun. 1993, 431. (e) Yamashita, Y.; Tanaka, S.; Tomura, M. J. Chem. Soc., Chem. Commun. 1993, 652. (f) Roncali, J.; Giffard, M.; Fre`re, P.; Jubault, M.; Gorgues, A. J. Chem. Soc., Chem. Commun. 1993, 689. (g) Takahashi, K.; Nihira, T.; Tomitani, K. J. Chem. Soc., Chem. Commun. 1993, 1617. (h) Lorcy, D.; Robert, A.; Carlier, R.; Tallec, A. Bull. Soc. Chim. Fr. 1994, 131, 774. (4) (a) Kirmse, W.; Horner, L. Liebigs Ann. Chem. 1958, 614, 4. (b) Mayer, K.; Kro¨ber, H. J. Prakt. Chem. 1974, 316, 907. (c) Cava, M. P.; Lakshmikantham, M. V. J. Heterocycl. Chem. 1980, 17, S39. (d) Scho¨berl, U.; Salbeck, J.; Daub, J. AdV. Mater. 1992, 4, 41. (e) Benahmed-Gasmi, A.; Fre`re, P.; Roncali, J.; Elandaloussi, E.; Orduna, J.; Garin, J.; Jubault M.; Gorgues, A. Tetrahedron Lett. 1995, 36, 2983. (f) Ohta, A.; Yamashita, Y. J. Chem. Soc., Chem. Commun. 1995, 1761. (5) Lorcy, D.; Carlier, R.; Robert, A.; Tallec, A.; Le Maguere`s, P.; Ouahab, L. J. Org. Chem. 1995, 60, 2443. (6) Garreau, D.; Save´ant, J.-M., J. Electroanal 1972, 35, 309. (7) (a) Andrieux, C. P.; Garreau, D.; Hapiot, P.; Pinson J.; Save´ant, J.-M. J. Electroanal. Chem. 1988, 321, 243. (b) Andrieux, C. P.; Hapiot P.; Save´ant, J.-M.Chem. ReV. 1990, 90, 723. (8) Anne, A.; Fraoua, S.; Hapiot, P.; Moiroux, J.; Save´ant, J.-M. J. Am. Chem. Soc. 1995, 117, 7412. (9) It is noticeable that the cathodic wave of the dimer-dication is much larger than its corresponding reoxidation wave. This difference is due to the diffusion of the dimer from the electrode to the solution during the voltammetric scan. (10) (a) The protonated dimer +HAAH+ is certainly a stronger acid than the monomeric cation-radical since the cation radical AH.+ can be stabilized by resonance which is not possible with +HAAH+. Similar results and conclusions concerning anions and protonations have been found in studies about the reductive electrodimerization of activated olefines.10b (b) Lamy, E; Nadjo, L.; Save´ant, J.-M. J. Electroanal. Chem. 1974, 50, 141. (11) (a) Andrieux, C. P.; Nadjo, L.; Save´ant, J.-M. J. Electroanal. Chem. 1973, 42, 223. (b) Andrieux, C. P.; Save´ant, J.-M. Electrochemical Reactions. In InVestigation of Rates and Mechanism of Reactions; Bernasconi, C. F., Ed.; Wiley: New York, 1986; Vol. 6, 4/E, Part. 2 pp 305390. (12) The slopes, ∂Ep/∂(log(V)) and ∂Ep/∂(log(c)) for the R-S and the R-R mechanisms are different (theoretical variation for R-S mechanism, when the coupling step is irreversible: ∂Ep/∂(log(V)) ) 29.1 mV and ∂Ep/ ∂(log(c) ) -29.1 mV or when the coupling step is a fast reversible equilibrium: ∂Ep/∂(log(V)) ) 19.4 mV and ∂Ep/∂(log(c) ) -38.8 mV at 20 °C.11 (13) (a) Niziurski-Mann, R. E.; Scordilis-Kelley, C.; Liu, T.-L.; Cava, M. P.; Carlin, R. T. J. Am. Chem. Soc. 1993, 115, 887. (b) Audebert, P.; Catel, J.-M.; Le Coustumer, G.; Duchenet, V.; Hapiot, P. J. Phys. Chem. 1995, 99, 11923. (14) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 195.

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