New Insights into Electrocatalysis and Dissociative Electron Transfer

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J. Phys. Chem. C 2009, 113, 14983–14992

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New Insights into Electrocatalysis and Dissociative Electron Transfer Mechanisms: The Case of Aromatic Bromides Abdirisak A. Isse,† Patrizia R. Mussini,‡ and Armando Gennaro*,† Department of Chemical Sciences, UniVersity of PadoVa, Via Marzolo 1, 35131, PadoVa, Italy, and Department of Physical Chemistry and Electrochemistry, UniVersity of Milano, Via Golgi 19, 20133 Milano, Italy ReceiVed: May 22, 2009

The electrochemical reduction of a series of aromatic bromides including substituted bromobenzenes and polycyclic bromoarenes has been investigated in acetonitrile at silver and glassy carbon (GC) electrodes. Whereas GC behaves as a noncatalytic electrode, Ag exhibits remarkable electrocatalytic activities for the reduction of all compounds. The electrocatalytic effects are strongly influenced by the molecular structure of the aromatic bromide, decreasing with increasing electron-withdrawing ability of the substituents as well as with extension of the polycyclic aromatic system. Dissociative electron transfer (ET) to an organic halide RX may occur either in a single step (concerted mechanism) or in two distinct steps with the formation of an intermediate radical anion (stepwise mechanism). The mechanism of the reduction process was analyzed at both catalytic and noncatalytic electrodes. Electroreduction of all compounds at GC occurs according to a stepwise mechanism, whereas at Ag both mechanisms were observed depending on the structure of the molecule. This study reports unprecedented examples of a passage from one dissociative ET mechanism to the other upon a change of the nature of the electrode material. The process at Ag involves adsorption of reagents, intermediates and products and is catalytic regardless of the mechanism of the dissociative ET. Analysis of the data herein reported together with literature data on the electroreduction of different types of organic halides sheds some light on the relation between catalysis and dissociative ET mechanism. RX•- f R• + X-

Introduction The electrochemical reductive cleavage of carbon-halogen bonds in organic compounds has been the object of intense interest and investigation for many years, both from mechanistic1 and synthetic viewpoints.2 The mechanism at inert electrodes such as glassy carbon is well understood. Overall, the reduction process consumes two electrons, which are transferred in two successive steps (eqs 1, 2), the reduction potential of the intermediate radical R• being in most cases more positive than that of the starting halide RX, especially with chlorides and bromides.

RX + e- f R•+X-

(1)

R• + e- a R-

(2)

An important issue of the reduction process is whether the reductive cleavage occurs in a single step (eq 1), breaking of the bond being concerted with the electron transfer (ET), or in two steps with the transient formation of a radical anion (eqs 3 and 4)

RX + e- a RX•-

(3)

* Corresponding author. Tel: (+39) 049 8275132. Fax: (+39) 049 8275239. E-mail: [email protected]. † University of Padova. ‡ University of Milano.

(4)

The concerted versus stepwise dichotomy of the dissociative ET to RX has been extensively investigated and it has been shown that reduction of alkyl halides follows a concerted mechanism,3 whereas ET to aromatic halides gives transient radical anions.4 Substituted benzyl halides are in a borderline situation;5 it has been shown that reduction of nitrobenzyl halides follows a stepwise mechanism, whereas a concerted mechanism was found with less electron-withdrawing substituents.5a From the synthetic viewpoint, many electrode materials have been considered.2,6,7 Very often, the reductive cleavage of the carbon-halogen bond requires very negative potentials unless the bond is activated by substituents. Therefore, the nature of the electrode material is very important and, if catalytic surfaces are used, both the reduction overpotential and product selectivity may be drastically affected. Because of its catalytic properties, Hg has been the preferred electrode material for a long period,6 but it is now being abandoned for environmental reasons. It has recently been discovered that Ag possesses remarkable electrocatalytic properties toward the electroreduction of organic halides.8,9 In a very recent study on the electrocatalytic properties of a large number of cathode materials for the reduction of several organic halides, silver was found to be one of the best electrocatalytic materials.10 The extraordinary catalytic properties of silver have been applied in several electrosynthetic processes.11,12 The mechanism of the electrocatalytic reduction of RX at Ag is still subject of investigation. So far, various factors such as the type of the halogen atom,8a the molecular structure of RX,8b the surface morphology of the electrode,13 and adsorption/ desorption behavior of the halide ions14 have been identified to

10.1021/jp904797m CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009

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SCHEME 1: Molecular Structures of the Investigated Aryl Bromides

play an important role in the process. We have previously shown for a series of chlorides the existence of a strong linkage between catalysis and mechanism of the dissociative ET to RCl.15 Catalysis was observed only when the reductive cleavage follows a concerted mechanism, regardless of the nature of the electrode material. This means that aromatic chlorides, which undergo stepwise reductive cleavage at inert electrodes, do not show any catalytic effects at silver. More recently, the strong dependence of electrocatalysis at Ag on the concerted nature of dissociative ET has been confirmed in a study on the electrochemical reduction of various halides including chlorides as well as bromides and iodides.16 In that study, however, we observed that also aromatic bromides and iodides are catalytically reduced at Ag electrodes. The aim of this paper is to examine the role played by the Ag surface in the electrocatalytic reduction of aromatic bromides. Since the catalytic effect may be influenced by the molecular structure of the substrate, we selected a series of compounds (Scheme 1) appropriately substituted so as to modulate the kinetics of decomposition of the intermediate radical anion RBr•-. As we shall see, compared to noncatalytic surfaces, Ag offers for all compounds a more favorable reaction pathway, with the possibility, in some cases, of a mechanism change from stepwise to concerted. Results and Discussion Voltammetry at GC. Several different voltammetric patterns, of which representative examples are illustrated in Figure 1, have been observed for the series of bromides selected for this study. Compounds 1-3 show only a single reduction peak without coupled oxidation partner (Figure 1A). A second group of compounds, namely 4 and 7-9, exhibits two reduction peaks. The first reduction peak of each compound has no anodic partner, whereas the second peak is reversible. Examples of the voltammetric pattern of this group of bromides is also included in Figure 1 (curves B and C). The last group of compounds (5, 6, 10, and 11) exhibits three reduction peaks. The first peak in each case is irreversible, whereas the second and third peaks are reversible and irreversible, respectively (Figure 1D). The electrochemistry of most of the compounds of Scheme 1 has been previously investigated at inert electrodes,4,17 and the voltammetric pattern herein described agrees well with the published results. The mechanism of reductive cleavage of aromatic halides is quite complex. Often, transient radical anions are formed and the complexity of the chemistry following the first ET depends on the molecular structure and the stability of RX•-. The aryl radical arising from the reductive cleavage (eqs 3 and 4) is, in general, much more easily reduced than the parent

Figure 1. Cyclic voltammetry of some aryl bromides (0.5-0.8 mM) in CH3CN + 0.1 M Et4NBF4 at GC (full lines) and Ag (dashed lines) at V ) 0.2 V s-1.

molecule and can undergo further reduction either at the electrode surface (eq 2) or in solution (eq 5). The aryl anion formed in reaction 2 or 5 is a strong base and will be readily protonated by any proton donor BH present in solution such as, for instance, the residual water.

R• + RX•- a R- + RX

(5)

R- + BH a RH + B-

(6)

In organic solvents, the aryl radical can abstract a hydrogen atom from the solvent (eq 7) thus generating a solvent radical, which may be reduced to the solvent conjugate base at the electrode (eq 8) or in solution (eq 9).

R• + SH f RH + S•

(7)

S• + e- a S-

(8)

S• + RX•- a S- + RX

(9)

The three competitive mechanisms outlined above, i.e., ECE (eqs 3, 4, and 2), DISP1 (eqs 3-5), and hydrogen atom abstraction (eqs 3, 4, and 7-9), all lead to an overall 2ereduction of the organic halide. The experimental data reported for a large number of aromatic halides is consistent with the above reaction pathways. In a number of cases, however, an electron stoichiometry between one and two has been observed.4b,18 This discrepancy is caused by some reactions involving the aryl and solvent radicals as well as the solvent anion S-. Dimerization of the organic radicals, cross-coupling of R• with S• and nucleophilic substitution involving S- have all been reported.18,19 The neutral compound RH arising from the reduction of RX is usually reduced at potentials more negative than that of the starting halide. Generally, reduction of aromatic compounds occurs in successive two 1e- steps yielding first a stable radical anion and then a dianion, which being a strong base is rapidly protonated (eqs 10-12).20 In cyclic voltammetry, two 1ereduction peaks of which only the first is reversible are observed.

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RH + e- a RH•-

(10)

RH•- + e- a RH2-

(11)

RH2- + BH a RH2-+B-

(12)

Reduction of most of the compounds examined here gives hydrodehalogenated compounds RH that are reducible in the potential window available in the solvent/electrolyte system used. Thus, all compounds give a first irreversible reduction peak involving cleavage of the carbon-bromine bond, followed by voltammetric patterns belonging to the hydrodehalogenated compounds arising from the reductive cleavage process. We shall therefore focus our attention on the first reduction process of each compound. A comparison of the first reduction peak with the second one (when present) clearly shows that the first peak represents an overall 2e- process. This reduction peak corresponds to the ECE reaction sequence shown in eqs 3, 4, and 2. Other mechanisms (DISP1 and H abstraction) involving homogeneous reactions of RBr•- and the aryl radical R• are less likely because cleavage of RBr•- is very fast4 and the aryl radical Q• is much easier to reduce than the starting halide (E Ph /Ph has 21 been reported to be 0.05 V vs SCE ). Owing to the rapidity of these reactions, the aryl radical is formed near the electrode where it is immediately reduced to the corresponding carbanion and this makes negligible both reactions 5 and 7. The voltammetric data pertaining to the first reduction peak were analyzed to establish the mechanism of the dissociative ET to RBr. The stepwise mechanism can be easily evidenced if the transient radical anions have lifetimes longer than the time scale of the cyclic voltammetry experiment. In fact, when such a condition is fulfilled, a reversible (or at least partially reversible) peak couple is observed as in the reduction of the nonhalogenated compounds such as benzonitrile or anthracene (see the second peak of Figure 1B-D). The first reduction peak of all investigated compounds is completely irreversible in the whole range of scan rate (V) values employed (0.05-5 V s-1). The reason for this irreversibility is mainly due to the fast dehalogenation of RBr•-, which has rate constants greater than 105 s-1.4 In such a case, a simple approach in the investigation of the reductive cleavage mechanism consists in the observation of the dependence of Ep on V and of the values of the half peak width, ∆Ep/2 ) Ep/2 - Ep. This kind of analysis allows determination of the transfer coefficient R, which is a helpful parameter in the analysis of dissociative ET mechanisms.22 If the process follows a concerted mechanism, the theoretical values of the slope ∂Ep/∂logV of the Ep versus logV plot and the half peak width at 25 °C are -29.6/R mV and 47.7/R mV, respectively.23 In the stepwise mechanism, the values of ∂Ep/∂logV and Ep/2 - Ep depend on the rate-determining step. At 25 °C the values of the slope and half peak width are -29.6 mV and 47.7 mV, respectively, if the cleavage reaction (eq 4) is the rate-determining step.23 Conversely, if the ET (eq 3) is rate-determining, ∂Ep/∂logV ) -29.6/R mV and Ep/2 - Ep ) 47.7/R mV as in the case of the concerted mechanism. A mixed kinetic regime in which both steps contribute significantly to the kinetic control of the reduction process may also be observed. Under such circumstance, the true value of R cannot be obtained from ∂Ep/∂logV or half peak width. Voltammetric analysis of the peak affords a valuable means of distinguishing between the dissociative ET mechanisms. In particular, the stepwise mechanism can be easily evidenced if the system is under kinetic control of the cleavage reaction or

Figure 2. Potential energy diagrams for a dissociative electron transfer to R-X under kinetic control of (a) the bond cleavage reaction, (b) both bond cleavage and electron transfer reactions and (c) electron transfer. The subscripts e and c stand for the electron transfer and bond rupture reactions, respectively.

under mixed kinetic control. As mentioned above, the transfer coefficient R can be obtained only when ET to RX is rate-determining. We may use, however, the same expressions (eqs 13 and 14) to calculate an apparent value of R, which we denote κ to avoid confusion with the true R uncorrected for double layer effects, which is often known as apparent transfer coefficient, Rapp.

κ)-

κ)

1.15RT/F 0.0296 V )∂Ep /∂logυ ∂Ep /∂logυ

1.857RT 0.0477 V ) F(Ep/2 - Ep) Ep/2 - Ep

at 25 °C (13)

at 25 °C

(14)

The competition between ET and the follow-up reaction is schematically represented in Figure 2. The passage from one kinetic regime to the other may be obtained by changing the rates of both reactions 3 and 4 or more simply by changing the rate constant, kc, of the chemical reaction for a fixed value of the standard rate constant, ks, of the ET; the latter is more likely in the case of the compounds studied here, especially the series of substituted bromobenzenes. If the reaction is kinetically controlled by the cleavage step, κ approximately equals 1. If on the other hand ET is the rate-determining step, κ becomes equal to R. In situations between these limiting conditions, values of κ lying between 0.5 and 1 will be obtained. Therefore, κ is a good indicator of the kinetic regime of the electrode process; it increases and tends toward 1 as the activation barrier of the chemical reaction becomes increasingly greater than that of the ET reaction (Figure 2). The values of the peak potentials measured at V ) 0.2 V s-1 at GC as well as the results of the voltammetric analyses, carried out in the range of V values from 0.05 V s-1 to 5 V s-1, are reported in Table 1. The average values of κ (column 7) are greater than 0.5 and in some cases approach 1. The only exception is with bromobenzene and (4-ethyl)bromobenzene for which values of κ only slightly smaller than 0.5 were obtained.

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TABLE 1: Data for the Electrochemical Reduction of Aryl Bromides in CH3CN + 0.1 M Et4NBF4 at Glassy Carbon RBr

Epa (V vs SCE)

-∂Ep/∂logV (V)

κb

Ep/2 - Epa (V)

κc

average κ

log(kc/s-1)d

1 2 3 4 5 6 7 8 9 10 11

-2.612 -2.622 -2.610 -2.141 -1.595 -1.741 -1.852 -1.924 -2.041 -1.610 -1.960

0.067 0.076 0.069 0.048 0.033 0.031 0.043 0.055 0.046 0.035 0.046

0.44 0.39 0.43 0.62 0.90 0.95 0.69 0.54 0.64 0.85 0.64

0.106 0.087 0.077 0.071 0.048 0.071 0.075 0.094 0.070 0.053 0.067

0.45 0.55 0.62 0.67 0.99 0.67 0.64 0.51 0.68 0.90 0.71

0.45 0.47 0.53 0.65 0.95 0.81 0.67 0.53 0.66 0.87 0.68

g10.3e 10.5f 5.0g 7.5g 8.8e 9.3e 9.0e 5.8i

a Obtained at V ) 0.2 V s-1. b Calculated from κ ) -1.15 RT/F(∂Ep/∂logV) in the 0.05-5 V s-1 range. c Average of the values calculated from κ ) 1.857RT/F(Ep/2 - Ep) in the 0.05-5 V s-1 range. d In DMF unless otherwise stated. e From ref 4c. f In NMP, from ref 24. g From ref 4b. i In CH3CN from ref 17.

Figure 3. Voltammetric data for the reduction of aryl bromides in CH3CN + 0.1 M Et4NBF4 at (9) GC and (b) Ag. Variation of the kinetic regime indicator κ as a function of Ep measured at V ) 0.2 V s-1.

These results, in good agreement with literature reports,4 indicate that dissociative ET to aromatic bromides at inert electrodes follows a stepwise mechanism. The process is kinetically controlled by the ET in the case of compounds 1 and 2, whereas kinetic control is essentially due to the cleavage reaction for the bromides 5, 6, and 10. All other compounds conform to a mixed kinetic regime in the range of scan rates employed. The competition between ET and bond cleavage depends on the reduction potential of RBr. As shown in Figure 3 a plot of the kinetic regime indicator κ as a function of Ep exhibits roughly a linear increase of κ with increasing peak potential. This can be rationalized by considering the dependence of both the standard ET rate constant ks and the cleavage rate constant kc on the molecular structure. In the case of the polycyclic aromatic bromides, ks is expected to increase with the number of aromatic rings owing to the major delocalization of the incoming charge and hence decreasing reorganization energy. Thus, considering 1 and 9-11, ks would increase in the order 1 < 9 < 11 < 10. On the other hand, the stability of the radical anion RBr•- increases with the number of polycyclic aromatic rings. The cleavage rate constants of 1, 9, and 10 are g2 × 1010, 109, and 6 × 105 s-1, respectively (Table 1). The combined effect of these opposing trends is that while for bromobenzene heterogeneous ET is slower than the follow-up cleavage reaction and the process is under kinetic control of the charge transfer step, the opposite kinetic regime prevails for 9-bromoanthracene. A similar reasoning holds for the substituted bromobenzenes. In this case, while ks is probably little affected by the nature of the substituent,25 the cleavage rate constant dramatically varies with the substituent. The data reported in Table 1, taken from the literature, show a decrease of kc exceeding 5 orders of magnitude on passing from unsubstituted bromobenzene to 4-bromobenzophenone.

Voltammetry at Silver. The voltammetric behavior of the aromatic bromides 1-11 at Ag is much similar to that described for GC. Some representative voltammograms are reported in Figure 1. All compounds exhibit an irreversible 2e- cathodic peak, corresponding to the reduction of RBr to RH, followed by the proper voltammetric pattern of the latter. In the case of compounds 1-3, reduction of RH could not be observed in the accessible potential range. For all other compounds at least the first 1e- reduction of RH to its radical anion, RH•-, could be observed, with a reduction potential and an overall voltammetric response identical to those observed at GC. This means that reduction of nonbrominated compounds does not involve specific interaction of neither RH nor its radical anion with Ag. As shown in Figure 1, the shape of the reduction peaks obtained at Ag resembles that of the noncatalytic GC electrode and is typical of diffusion controlled electrode processes. Voltammetric analyses of the first reduction peak of RBr at Ag show linear variations of Ep as a function of logV. In addition, the peak current, Ip, varies linearly with V1/2, which clearly points out a reduction process under diffusion control.23 It is to be stressed, however, that Ag exhibits catalytic effects for all compounds implying that the reduction process involves surface interactions of RBr and/or its reduction intermediates and products. Since the overall process at Ag is diffusion-controlled, adsorption processes involving the electrode surface ought to be fast as compared to diffusion. Thus, even though the overall process at Ag is more complex than at GC, eqs 13 and 14, which are valid for electrode processes under diffusion control can still be applied for the voltammetric analysis at Ag. In fact, Laviron26 has carried out theoretical analyses of cyclic voltammetry of an electron transfer process followed by a chemical reaction (which is the case of the reduction of aromatic bromides) both in the absence and presence of specific adsorption. He has shown that if the adsorption processes are faster than diffusion, the overall voltammetric response is identical to that of an electrode process involving heterogeneous electron transfer and an apparent homogeneous chemical reaction. Therefore, equations previously developed for systems without specific adsorption can be applied for mechanistic investigations as well as for determination of kinetic parameters. The results obtained at Ag pertaining to the first reduction process, which involves the carbon-halogen bond, are summarized in Table 2. The average values of κ calculated from both ∂Ep/∂logV and Ep/2 - Ep are significantly smaller than 0.5 in most cases, clearly indicating that the reduction process is kinetically controlled by the first electron transfer to RBr. In these cases, the observed values of the kinetic regime indicator κ are effectively the true transfer coefficients. Concerning the

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TABLE 2: Data for the Electrochemical Reduction of Aryl Bromides in CH3CN + 0.1 M Et4NBF4 at Ag RBr

Epa (V vs SCE)

-∂Ep/∂logV, (V)

κb

Ep/2 - Epa (V)

κc

average κ

E(Ag) - E(GC)d (V) p p

Q e • ERX/R +X- (V vs SCE)

1 2 3 4 5 6 7 8 9 10 11

-1.765 -1.747 -1.782 -1.654 -1.504 -1.613 -1.534 -1.611 -1.597 -1.401 -1.462

0.118 0.123 0.148 0.087 0.057 0.059 0.054 0.058 0.066 0.070 0.064

0.25 0.24 0.20 0.34 0.52 0.50 0.55 0.51 0.45 0.42 0.46

0.149 0.170 0.191 0.129 0.076 0.092 0.080 0.087 0.111 0.109 0.087

0.32 0.28 0.25 0.37 0.63 0.52 0.60 0.55 0.43 0.44 0.55

0.29 0.26 0.23 0.36 0.58 0.51 0.58 0.53 0.44 0.43 0.51

0.847 0.875 0.828 0.487 0.091 0.128 0.318 0.313 0.444 0.209 0.498

-1.661 -1.652 -1.654 -1.652 -1.644 -1.643 -1.648 -1.643 -1.613 -1.422 -1.574

a Obtained at V ) 0.2 V s-1. b Calculated from κ ) -1.15 RT/F(∂Ep/∂logV) in the 0.05-5 V s-1 range. c Average of the values calculated from κ ) 1.857RT/F(Ep/2 - Ep) in the 0.05-5 V s-1 range. d E(Ag) and E(GC) are the peak potentials measured at Ag and GC cathodes, p p respectively. e Standard potential of the concerted dissociative ET calculated from eq 20.

dependence of Ep on σ- can be fit to a linear equation of the form28

EXp ) EHp + Fσ-

Figure 4. Voltammetric data for the reduction of substituted bromobenzenes 1-8 in CH3CN + 0.1 M Et4NBF4 at V ) 0.2 V s-1. Dependence of Ep measured at (b) GC and (2) Ag, and (9) catalytic effect at Ag on Hammett substituent constants.

mechanism, a slow initial electron transfer is compatible with both concerted and stepwise dissociative electron transfer mechanisms. Thus, the observed values of κ alone do not allow a reliable discrimination between the two mechanisms for most compounds. For some of them (namely, 5, 6, 7, 8, and 11), however, values of κ greater than 0.5 have been found, clearly indicating the occurrence of a stepwise mechanism. In Figure 3, the average values of κ are plotted as a function of Ep. The data show the same trend already discussed for GC; κ increases with increasing electron-withdrawing ability of the substituents as well as with increasing number of aromatic rings. Comparison between GC and Ag. A comparison of the reduction potentials at Ag with the values of Ep measured at GC shows that all aromatic bromides are catalytically reduced at the Ag surface. However, the catalytic effect strongly depends on the molecular structure, especially the electron-withdrawing ability of the substituent. Plots of Ep values measured at both electrodes as well as the catalytic effect at Ag as a function of Hammett substituent constants are reported in Figure 4. Note that since the redox reaction occurs with the evolution of a negative charge, we used σ- instead of σ.27 In addition, only the series of substituted bromobenzenes has been considered for this analysis. As can be seen both electrodes show the same trend, i.e., Ep becomes more positive upon increasing electronwithdrawing power of the substituent. For both electrodes, the

(15)

where X stands for a substituent on the phenyl ring. A good linear correlation is obtained (correlation coefficient, R2 ) 0.96) at GC only if bromobiphenyl 4 and the two brominated ketones 5 and 6 are excluded from the linear regression. Also a good linear correlation of Ep with σ- is obtained at Ag if compounds 4 and 5 are excluded. The Hammett substituent constant of a phenyl group accounts only for its very small polar effect.27 Although the two phenyl rings are not completely coplanar in the starting 4-bromobiphenyl, it appears that the radical anion is somehow stabilized also by conjugation. Indeed, the Ep values measured for this compound are considerably more positive than those predicted by the regression equation, especially in the case of the GC electrode. The bromoketone derivatives deviate from the linear regression probably because ET to these compounds involves the carbonyl moiety. This hypothesis is supported by theoretical calculations on radical anions of aromatic ketones, showing that 38-67% of the charge density is located in the carbonyl group.29 The slope of the linear correlation of Ep with σ- gives the reaction constant F, which is indicative of the sensitivity of the electron transfer to molecular structure. The values of F calculated for the two electrodes are 0.18 and 0.68 V for Ag and GC, respectively. Reduction of the aromatic bromides at GC is ca. 4 times more sensitive to the substituents with respect to Ag. This difference of sensitivity is also well evident if all bromides are considered. As shown in Tables 1 and 2, whereas Ep values measured at GC span in a wide range of ca. 1 V (from -2.612 to -1.595 V), the values at Ag show an overall variation smaller than 0.4 V. When reduction occurs at an inert electrode such as GC, acting as an outer-sphere electron donor, the incoming electron is accommodated in a π* orbital either of the aromatic system or, though much less frequently, of the substituent. Therefore, the presence of a good electronwithdrawing substituent helps delocalization of the incipient charge and hence shifts the reduction potential to more positive values. Likewise, extension of the aromatic system to more polycyclic rings lowers the energy of the LUMO orbital, resulting in more positive values of Ep. Although this trend remains also at the catalytic Ag electrode, the molecular structure has a quite limited effect on Ep. It appears that the presence of the catalytic surface shifts, at least partially, the site of the negative charge in the 1e- reduced species. Probably now the

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charge is mainly located at the bromine atom so that both the electron-withdrawing power of the substituent and the increased conjugation offered by polycyclic rings become less important. Figure 4 also shows the dependence of the catalytic effect, measured as the positive shift of Ep at Ag with respect to GC, on the Hammett substituent constants. The catalytic effect decreases with increasing σ-. A similar trend may be observed for polycyclic aromatic systems; on passing from bromobenzene to 9-bromoanthracene, the catalytic effect decreases from 0.85 to 0.21 V. Overall, the data show a considerable decrease of catalysis as the ability of RBr to delocalize the incoming negative charge increases. As shown in Figure 3, the values of κ obtained at Ag are significantly smaller than the corresponding values at GC. In particular, the process at Ag is kinetically controlled by the first ET and tends toward a mixed kinetic control only in the presence of strongly electron-withdrawing substituents such as CN and carbonyl groups. In contrast, at GC κ is almost always greater than 0.5, with the possibility of observing the whole range of kinetic regimes from rate-determining ET for bromobenzenes not bearing electron-withdrawing groups to rate-determining cleavage reaction in the presence of good electron-withdrawing groups or extended aromatic conjugation. In conclusion, while silver exhibits good electrocatalytic activities for most investigated bromoarenes, it is characterized by lower ET rate constants as compared to GC. This suggests that catalysis is not related merely to the kinetics of dissociative ET but is to be considered as the result of a more profound modification of the mechanism of the reductive cleavage. Mechanism of Electroreduction at Ag. The results described in the previous sections have clearly evidenced that Ag is an electrocatalyst for the reductive cleavage of all investigated bromides. The results also show that the electrocatalysis is related to the presence of the carbon-bromine bond since reduction of the debrominated compounds is not influenced by the nature of the electrode material. The extraordinary electrocatalytic properties of Ag toward reduction of organic halides probably stems from the high affinity of the metal for the halide ions. It is well-known that these ions are strongly adsorbed on Ag surfaces in both protic30 and aprotic31 solvents. The standard Gibbs free energy of adsorption, ∆GQads, of Br- at polycrystalline Ag at the potential of zero charge is reported to be -90 kJ mol-1 in aqueous solutions.30b The value of ∆GQads in acetonitrile has not been measured. It is, however, known that adsorption Q of halide ions on different metal surfaces has -∆Gads values greater in nonaqueous solvents than in aqueous solutions.32 This is due to the significant difference between the solvation energies of X- in water and in organic solvents such as acetonitrile. Since Br- is much better solvated in H2O than in organic solvents,33 Q we may reasonably assume that ∆Gads < -90 kJ mol-1 in acetonitrile. In view of such high adsorption energy, any mechanistic proposal for the reduction process at Ag is deemed inappropriate if the possibility of Br- adsorption is not taken into consideration. In fact, strong adsorption of Br- can significantly affect both the kinetics and thermodynamics of the process, regardless of the intrinsic nature of the dissociative ET to RBr (concerted or stepwise). In addition, interaction of the reagents and products with the electrode surface may play an important role in the electrocatalytic process. Although the results of this study do not show a clear evidence of adsorption phenomena, there are several reports showing the importance of interactions of Ag surface with RX as well as with R• in the catalytic reduction of some alkyl halides.9b,15b,34–37 We have recently shown that reduction of benzyl halides34 and polychlo-

Isse et al. SCHEME 2: Hypothetical Electrocatalytic Reduction Mechanism Involving Surface Assisted Dehalogenation and Adsorption of Bromide Ions

romethanes35 involves adsorption of the halogenated substrates. Electroreduction of alkyl iodides at smooth silver and palladized silver electrodes has recently been reported to involve a prereaction of RI with Ag leading to rupture of the C-I bond and surface modification of the electrode.36 Reductive dehalogenation of alkyl halides at Ag(111) surfaces is a well-known process; the reaction involves dissociative adsorption of RX on the metal surface leading to the formation of adsorbed alkyl radicals.37 As shown by the results of the voltammetric analysis, all investigated compounds undergo a stepwise dissociative electron transfer at the noncatalytic GC electrode. If we assume that neither RBr nor R• is adsorbed, while Br- undergoes strong adsorption, a reduction mechanism similar to that at GC may be proposed for Ag (Scheme 2). The first electron transfer to RBr occurs at the outer Helmholtz plane (OHP), which is the usual reaction site for nonspecifically adsorbed species in ET processes. The radical anion generated in this step moves to the inner Helmholtz plane (IHP) (the site of reaction of specifically adsorbed species), where it dehalogenates with the assistance of the electrode surface to give an aromatic radical R• and an adsorbed Br-. The process is completed by desorption of Br-, which thus liberates the active reaction site and diffuses away from the electrode surface. According to this mechanistic scheme, besides being the source of electrons, the role of the Ag surface would be to accelerate the cleavage reaction thanks to the adsorption of Br-. This means the overall mechanism can be viewed as an EC reaction scheme with an enhanced rate constant for the chemical step as compared to the same process at a noncatalytic electrode. In the case of a reversible ET followed by an irreversible chemical reaction, which is the case of most of the aromatic bromides investigated here, the peak potential is given by23,38

Ep ) E1/2 - 0.780

( )

RT RT RTkc + ln F 2F FV

(16)

where E1/2 is the half-wave potential (E1/2 ≈ EQ), kc is the rate constant of the chemical reaction, V is the scan rate and all other parameters have their usual meanings. Equation 16 has been originally derived for an EC process under pure kinetic conditions, assuming no interactions of reagents, intermediates or products with the electrode surface.23 However, it has been later shown by Laviron26 to be applicable also to EC processes with specific adsorption provided that the overall process is under diffusion control, which is the case of RBr reduction at the Ag electrode. Application of this equation to the peak potentials measured at GC and Ag allows an estimation of the

Electrocatalytic Reduction of Bromoarenes at Ag

J. Phys. Chem. C, Vol. 113, No. 33, 2009 14989

SCHEME 3: Reduction Mechanism of Aromatic Bromides at a Ag Electrode

enhancement of the cleavage rate constant due to adsorption of Br- at Ag. If we denote kc and kcsurf as the respective rate constants of the homogeneous and surface-assisted cleavage reactions, the ratio kcsurf/kc is given by

ksurf c 2F (Ag) ) exp - E(GC) ) (E p kc RT p

[

]

Figure 5. Potential energy diagrams for a stepwise dissociative electron transfer to R-X occurring at an inert electrode (solid curves) and a catalytic electrode (dashed curves); ∆rGQ ) -FEQ is the free energy of the ET step.

(17)

provided that all experimental parameters are kept constant. Using the Ep(Ag) - Ep(GC) values listed in Table 2 (column 8), 3 ksurf c /kc values ranging from 1.2 × 10 for 4-bromobenzophenone to 3.8 × 1029 for (4-ethyl)bromobenzene are obtained. This ratio can be used together with the kc values listed in Table 1 (column predicted by the mechanism of 8) to estimate the values of ksurf c smaller than 1013 s-1, the maximum Scheme 2. Values of ksurf c value of the rate constant of a first-order reaction, have been obtained only for three compounds. These are 5, 6 and 10 for ) 1.2 × 108, 6.7 × 1011, and 7.4 × 1012 s-1, which ksurf c respectively. All other compounds give highly unlikely values in the 1019-1038 s-1 range. The catalytic effects observed for most compounds are too strong to be explained by enhanced cleavage rate of RBr•- due to Br- adsorption at the Ag surface. We are therefore led to discard this mechanistic possibility. The catalytic effect of Ag is not due merely to kinetic modifications of the reductive cleavage of RBr. This has been shown not only by the foregoing kinetic analysis but also by careful examination of the substituent effects. It is possible that interactions of reagents, intermediates and products with the Ag surface play a crucial role in the electrocatalytic process. We propose a reduction mechanism in which, as illustrated in Scheme 3, all reactions occur on the electrode surface. We consider two possible reaction routes both leading to adsorbed R• and Br- for the dissociative ET: stepwise mechanism involving adsorbed radical anions and a concerted mechanism yielding adsorbed products. This dissociative ET is followed by reduction of R•ads to give the corresponding carbanion, which probably is not adsorbed at the negatively charged electrode. Also the bromide ion desorbes and diffuses into the bulk solution, thus regenerating the surface active sites of the electrode. We assume that the reagents are only weakly adsorbed while the bromide ion is strongly adsorbed. Adsorption of the intermediate radical anion RBr•-, presumably through the bromine atom, is also strong. Figure 5 shows potential energy diagrams of a stepwise dissociative ET at both catalytic and noncatalytic electrodes. The potential energy profiles of reagents, intermediates and products at the catalytic surface are lowered with respect to the

Figure 6. Potential energy diagrams for a dissociative electron transfer to R-X at an inert electrode (blue lines) and at a catalytic electrode (red lines) with a mechanism change from stepwise to concerted. The dashed lines represent the energy surfaces of the reagents at Ep, whereas ∆rGQ ) -FEQ is the free energy of the ET step and η ) Ep - EQ is the overpotential at Ep.

noncatalytic electrode. This is mainly due to adsorption, especially in the case of RBr•- and the reduction products R• and Br-, which strongly interact with the Ag surface. Regarding the starting reagents (RBr + e-), their potential energy profile is lowered mainly because now the process occurs at less negative potentials. The result of the strong interaction of RBr•with the electrode surface is a decrease of the Gibbs free energy of the reaction, ∆rGQ and hence an increase of the standard potential. Furthermore, adsorption of R• and Br- may enhance the cleavage rate of the radical anion. The combination of these two effects results in a positive shift of Ep at the catalytic electrode with respect to the noncatalytic one. A similar analysis of the potential energy profiles of a catalytic process involving concerted dissociative ET at Ag is shown in Figure 6. Now there is a change of mechanism from stepwise to concerted on passing from the noncatalytic electrode to the catalytic one. As before, the potential energy profiles of the reagents, intermediates and products of the catalytic process are lowered by surface interactions and by the increase of the reduction potential at the catalytic electrode. It is worth noting that the change of dissociative ET mechanism from stepwise at GC to concerted at Ag causes a drastic change of both the standard free energy, ∆rGQ, and the activation free energy, ∆G‡, of the ET. Regarding ∆rGQ, the following two reactions are to be considered:

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J. Phys. Chem. C, Vol. 113, No. 33, 2009

RBr + e- a RBr•• RBrads + e- f Rads + Brads

Q ERBr/RBr •-

Q E(RBr/R •+Br-)ads

Isse et al.

(18) (19)

Q It is well recognized that E stepwise is considerably more 5,39 Q even without taking into account negative than E concerted, adsorption phenomena, which would shift the latter to more positive values because of the preferential adsorption of the products with respect to the reagents. On the other hand, since the concerted process involves rupture of a chemical bond, its activation free energy is often greater than that of the stepwise process. The potential energy profiles of the reagents at the reduction potential of RBr are shown in Figure 6 (dashed lines) for both stepwise and concerted dissociative ET mechanisms. Catalysis arises from a combination of these kinetic and thermodynamic effects, both related to the ET to RBr. The process at Ag has a remarkable thermodynamic advantage over ET at a noncatalytic electrode, but loses something from the kinetic standpoint. The overall result, however, favors the catalytic surface with a net positive shift of the reduction potential. As shown in Figure 6, the concerted process has a greater overpotential, η, and a higher reduction potential. Let us now try to distinguish between stepwise and concerted mechanisms in the reductive cleavage of aromatic bromides at Ag. A first discrimination can be made on the bases of the kinetic regime indicator κ. Compounds 5-8 and 11 show κ values lying between 0.51 and 0.63, which is a clear indication of a stepwise mechanism. In contrast, κ values smaller than 0.5 have been found for compounds 1-4, 9, and 10 clearly indicating a reductive cleavage process under kinetic control of the ET to RBr. In this case the observed values of κ stand for the true transfer coefficient, R, which is compatible with both stepwise and concerted mechanisms. Thus, other means of discriminating between the dissociative ET mechanisms ought to be sought. Estimation of the standard potential of the concerted dissociative ET (eq 19) may offer some possibility of distinguishing between the mechanisms. Unfortunately, data on the adsorption of neither RBr nor its reduction products in acetonitrile or in any other aprotic solvent are available. What we can estimate is EQRBr/R•+Br- of an ET not involving adsorption Q • and this can be used as a lower limit value of E(RBr/R +Br-)ads. The standard potential of a concerted dissociative ET can be calculated from thermochemical data according to the equation (see the Supporting Information)

Q Q Q FERBr/R + FEBr •+Br- ) -BDE + T∆S •/Br-

(20)

where BDE is the C-Br bond dissociation energy and ∆SQ is the corresponding entropy change. The standard potential of the Br•/Br- couple in acetonitrile is 1.526 V vs SCE.40 As to the C-Br bond dissociation energies, it is widely accepted that BDE of C-Br in aromatic molecules is essentially independent of the molecular structure.4d,41,42 Szwarc and coworkers42 have shown that |∆BDE| of a series of substituted bromobenzenes as well as polycyclic bromoarenes lies in a range of 0-22.2 kJ mol-1. More recently, these results have been confirmed by theoretical computations on several polycyclic aromatic bromides.41 We used the reported ∆BDE values together with the most up-to-date available BDE value for bromobenzene (337 kJ mol-1)43 to calculate the bond dissociation energies of compounds 2-11. The entropic term, T∆SQ,

in eq 20 has been reported to be 28.9 kJ mol-1 (0.3 eV) for various organic halides.3d,39b,44 Q • The values of ERBr/R +Br- computed for all compounds according to eq 20 are listed in Table 2 (last column). We expect these values to be positively shifted by a few hundreds of millivolt if appropriate correction is made for adsorption of the reagents and products, especially Br-. A comparison of the computed EQRBr/R•+Br- values with the measured Ep values shows that EQRBr/R•+Br- e Ep for all compounds except 1-3. Considering that a concerted dissociative ET has a high intrinsic barrier and therefore occurs with a high overpotential; that is, Ep , Q •+ -, this comparison points out the impossibility of a ERBr/R Br concerted mechanism for compounds 4-11. We recall that we arrived at the same conclusion for most of these compounds on the basis of the experimental κ values. These compounds conform to the catalytic stepwise mechanism shown in Figure 5. Since adsorption of RBr•- at Ag is stronger than that of RBr, the standard potential and hence Ep will be positively shifted with respect to a noncatalytic process. It is also very likely that dehalogenation at the electrode is faster than in the homogeneous solution phase and this will further shift Ep to more positive values (eq 16). These compounds show moderate to good catalytic effects with positive peak shifts lying in the 0.1-0.5 V range. The catalytic effect shows a marked dependence on the molecular structure of RBr, decreasing with the increasing ability of the molecule upon decreasing the negative charge density on the bromine atom either through delocalization on an extended aromatic structure or by partial localization on a good electron-withdrawing substituent such as CN, COOR and COR. Q • Compounds 1-3 show calculated ERBr/R +Br- values that are ca. 0.1 V more positive than the corresponding peak potentials. Q Q • If we consider that E(RBr/R +Br-)ads > ERBr/R•+Br- by about a few hundreds of millivolt, the observed Ep values are compatible with a concerted mechanism. The very small transfer coefficient values (