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2007, 111, 2819-2822 Published on Web 02/01/2007
Electrochemical Concerted Proton and Electron Transfers. Further Insights in the Reduction Mechanism of Superoxide Ion in the Presence of Water and Other Weak Acids Jean-Michel Save´ ant* Laboratoire d’Electrochimie Mole´ culaire, UniVersite´ de Paris 7 - Denis Diderot, Case Courrier 7107, 2 place Jussieu, 75251 Paris Cedex 05, France ReceiVed: December 4, 2006
Deciphering of the role of water (or other weak acids) in the reduction of superoxide is based on three observations: (i) very large peak potential shifts (and hence a large increase of the apparent standard rate constant) associated with the reaction O2•----H2O + e- f -O2H---OH- upon addition of water (or methanol, or 2-propanol) to the acetonitrile solution; (ii) increase of the hydrogen/deuterium isotope effect with water concentration; (iii) the fact that the positive shift of the peak potential (or equivalently the increase of the standard reduction rate constant) is larger with methanol than with water. Mere invocation of specific solvation by water (or the other weak acids) of the reduction products cannot explain these facts. They instead reveal that short hydrogen-bonded water chains, involving approximately three water molecules, are involved in the concerted proton-electron transfer process, thus providing a preliminary picture of the mechanisms operating in pure water or other H-bonding and H-bonded media.
Proton-coupled electron transfers (PCET) where proton and electron transfers involve different molecular centers currently attract active attention from a fundamental point of view and because of their involvement in many natural processes.1 Particular emphasis has been laid on the possibility that the two steps are concerted giving rise to a concerted proton and electron transfer (CPET) reaction. Several homogeneous2-4 or electrochemical5-8 systems have been investigated in view of illustrating the occurrence of CPET pathways, rather than the competing stepwise pathways, which involve the transfer of an electron followed by the transfer of a proton and/or the reversed sequence. The first case where the occurrence of an electrochemical concerted proton-electron transfer reaction was unambiguously proved concerns the reduction in acetonitrile or dimethylformamide of superoxide ions hydrogen-bonded by water (Scheme 1).5,9 SCHEME 1
A subsequent more detailed study5b revealed an additional important feature of the effect of water addition on the reduction of O2•-, namely that the reduction potential, as measured, e.g., by the cyclic voltammetric peak, undergoes a considerable positive shift upon addition of a relatively modest amount of water, of the order of 600 mV per decade as shown in Figures 1 and 2. Similar effects were observed with methanol and 2-propanol. * To whom correspondence should be addressed. E-mail: saveant@ paris7.jussieu.fr.
10.1021/jp068322y CCC: $37.00
Figure 1. Voltammograms of air-saturated acetonitrile with added water with 0.10 M Bu4NPF6 at a glassy-carbon electrode. Scan rate: 0.50 V/s. Temperature: 298 K. From Figure 6 in reference 5b.
A previous interpretation5b of the observed variation consisted in applying the Marcus model of outersphere electron transfer (or more exactly the Marcus-Hush-Levich model5a,c,10) replacing the standard potential of the reaction by the equilibrium potential obtained from the application of the Nernst law to the reaction in Scheme 1.5b The driving force of the reaction would thus be a function of the reactant concentrations. In fact, the driving force of the reaction11
-∆GO0 2•-- -H2O F -O2H- -OH- ) E - EO0 2•-- -H2O/-O2H- -OHis independent of the reactant concentrations since the standard potential is referred to the standard states of the reactants © 2007 American Chemical Society
2820 J. Phys. Chem. C, Vol. 111, No. 7, 2007
Figure 2. Peak potential of the second wave as a function of water (circles), methanol (squares), and 2-propanol (diamonds) concentration. Same conditions as in Figure 1. Adapted from Figure 8 in reference 5b. Lines in a: simulation of the peak potential variation with the involvement of two (blue), three (green), and four water molecules in the reaction (see text). Lines in b: simulation of the peak potential variation with the involvement of four methanol or 2-propanol molecules (see text).
Letters from 1.9 to 5.6 between 0.2 and 1 M, and with 2-propanol, from 1.3 to 2.4 between 0.4 and 1.2 M). In contrast, the association of HO2- with water is insensitive to the replacement of hydrogen by deuterium (the same is true for methanol and 2-propanol). The same is expected for association of HO2- and OH-. It follows that the observed effect is a kinetic hydrogen/ deuterium isotope effect that increases with water concentration. A third observation pointing to the same conclusion is that the positive shift of the peak potential (or equivalently the increase of kS) is larger with methanol than with water in spite of the fact that the two constants of successive association of O2•- with one and two molecules are practically the same in both cases. These observations suggest the mechanism sketched in Scheme 2. The first water molecule associated with O2•- allows SCHEME 2 a
E02,1 ) EO0 2•-+H2O F O2H-+OH- ) µO0 2•-- -H2O - µO0 2- -OH(the µ0’s are the standard chemical potentials of the subscript reactants). How then can the large positive shift of the reduction peak potential, Ep, and hence the large increase of the standard rate constant, kS, upon increasing the water concentration be explained? As a first lead, one may think of a specific solvation of the reduction products by water (versus acetonitrile) thanks notably to hydrogen bonding. O2•- is complexed successively by one and two water molecules (Figure 3).5b The one-electron-
Figure 3. (a) Open circles: experimental variation of apparent standard potential of the O2/O2•- couple with water concentration (From Figure ox ox 3 in reference 5b). Blue line: fitting with Kox 1 ) 20, K2 ) 5, K3 ) 0, ox ox ox ox -1 K4 ) 0 M ; red line: fitting with K1 ) 20, K2 ) 4, K3 ) 0.4, Kox 4 ) 0.4 M-1. (b) Distribution of the 1:1 (black), 1:2 (blue), 1:3 (green), and 1:4 (red) O2•-/H2O adducts corresponding to the last set of equilibrium constants.
transfer product, which consists of a set of two negative ions, HO2- and OH- ions, is likely to be sensitive to this specific solvation to an even larger extent, possibly involving more water molecules. A positive shift of the standard potential, and consequently of the peak potential, should result. However, unrealistic values of the ensuing gains in solvation free energy would be required to match the observed peak potential shifts. Also, the same mechanism cannot explain the observed increase of the hydrogen/deuterium isotope with water concentration. The H/D ratio of the standard rate constants, kHS /kDS , passes from 1.7 to 3 between 0.3 and 1 M H2O (with methanol kHS /kDS increases
a The E0 and kS are the standard potentials and standard rate 2 2 constants respectively. All complexation equilibria are assumed to be fast.
the formation of HO2- and OH- by way of a CPET reaction during which a proton moves from the water molecule to O2•-. The second water molecule involved is likely to interact with the other end of O2•-. It results, upon electron transfer, in a moderately stabilizing interaction with HO2-. Additional water molecules, weakly hydrogen-bound to the first one, allow, along a shortwater chain, the concerted formation of an OH- ion at a larger distance from the negative charge borne by the farthest oxygen end of HO2-. Association of a third water molecule is therefore expected to strongly stabilize the product system by decrease of the Coulombic repulsion between the two negative charges between which, solvent (acetonitrile) molecules do not penetrate. The same favorable effect is expected from the association of a fourth water molecule, albeit to an attenuated extent. The resulting strong gain in standard potentials may be
Letters
J. Phys. Chem. C, Vol. 111, No. 7, 2007 2821
counteracted to some extent by an increase of the reorganization energy and a decrease of the pre-exponential factor owing to a more extended proton tunneling distance. These effects are deemed small compared with the acceleration that results from the strong positive shift of the standard potentials. As shown earlier,5b the superoxide ion forms 1-1 and 1-2 water adducts. The association constants (association equilibria are assumed to be fast) were derived from the variation of the apparent standard potential of the O2/O2•- couple (first wave in Figure 1) as recalled in Figure 3, where the black dotted line corresponds to
E0,ap ) E10 + 1
RT 2 ox ox ln(1 + Kox 1 [H2O] + K1 K2 [H2O] ) (1) F
ox -1 with Kox 1 ) 20, K2 ) 5 M . Encounter with a third and fourth water molecules would lead to
E0,ap ) E10 + 1
RT ox ox 2 ln(1 + Kox 1 [H2O] + K1 K2 [H2O] + F 3 4 ox ox ox ox ox ox Kox 1 K2 K3 [H2O] + K1 K2 K3 K4 [H2O] )
If the interaction energy for the binding of the third and fourth ox molecule was negligible, Kox 3 and K4 would be approximately -1 12 0.05 M . Slightly larger values are in fact likely from weak H-bonding of the third water molecule to the first and of the fourth to the third. Figure 3a shows that the experimental data ox ox -1 -1 can be fitted with eq 2 taking Kox 1 ) 20 M , K2 ) 4 M , K3 ox -1 ) K4 ) 0.4 M (red line) as well as with eq 1 with the former values. The last two values are not actually derived from the fitting to the experimental points, but we just note that they are compatible with these data. These values result from a compromise between consistency with the data in Figure 3a and crude estimate of the free energy of a weak hydrogen bond. Thus, assuming that the transfer coefficient, R, is approximately the same (R = 0.2) for all four CPET reactions, the linearized rate law of the electrochemical electron transfer to O2•- writes, when taking into account the contribution of the four O2•- water clusters
{ }
RF I ) kS2,ap exp - (E - E02,1) [O2•-] F RT
[
kS2,ap ) kS2,1
]
[A1] kS2,2 RF 0 + S exp (E - E02,1) [A2] RT 2,2 k2,1 kS2,3 RF 0 + S exp (E - E02,1) [A3] RT 2,3 k 2,1
+
kS2,4
kS2,1
[ [ RF exp[ (E RT
0 2,4
] ] )][A ]
- E02,1
[A3] )
3 4 Kox Kox 3 [H2O] 4 [H2O] , [A4] ) DENOM DENOM
ox ox ox 4 [H2O]3 + Kox 1 K2 K3 K4 [H2O] (5)
(see the graphic representation in Figure 3b). The peak potential is then given by13
(
S RT k2,ap Ep ) Ep,2,1 + ln S RF k2,1
(3)
Ep,2,1 ) E02,1 - 0.78
( x )
RT RT + ln kS2,1 RF RF
RT RFVD
(7)
TABLE 1: Parameters for the Simulations in Figure 2 parameter values -1 Kox 1 (M ) -1) Kox (M 2 -1 Kox 3 (M ) ox K4 (M-1) Ep,2,1 (V vs
kS2,2 kS2,3 kS2,4 kS2,1
4
(6)
Figure 2 shows the simulation of the peak-potential-[H2O] variation, according to eqs 3-7, for the involvement of two, three and four water molecules successively. A satisfactory fit is obtained for the four-[H2O] scheme with the values of the ox parameters listed in Table 1. Once the values of Kox 3 and K4 have been fixed, the values of the acceleration parameters above cannot be varied to a large extent (not more than (10%) without degrading the quality of the fit. Involvement of two and three water molecules is clearly not sufficient to explain the observed peak potential variation with [H2O], as seen in Figure 1. In this connection, no satisfying fitting could be reached with two water molecules by varying the right-hand value in eq 8 or with three water molecules by varying the right-hand values in eqs 8 and 9.
kS2,1
(4)
RT xRFVD )
where Ep,2,1 is the peak potential corresponding to the 1-1 adduct alone, the one observed at the lower end of the [H2O] range
kS2,1
I is the current density and F is the faraday, where the molar fractions of each adduct are given by 2 Kox Kox 1 [H2O] 2 [H2O] [A1] ) , [A2] ) DENOM DENOM
2 ox ox ox ox ox DENOM ) 1 + Kox 1 [H2O] + K1 K2 [H2O] + K1 K2 K3
SCE)
water
methanol
2-propanol
20 4 0.4 0.4 -2.50
30 3 0.4 0.4 -2.50
15 2 0.4 0.4 -2.55
exp
(E [(RF RT )
- E02,1)
]
4
20
2
exp
(E [(RF RT )
- E02,1)
]
20
34
10
exp
(E [(RF RT )
- E02,1)
]
15
4
8
0 2,2
0 2,3
0 2,4
Similar fitting of the methanol and 2-propanol data are shown in Figure 2b, corresponding to the parameters values listed in Table 1. The variation of kHS /kDS with water (or methanol, or 2-propanol) concentration reflects, the increase of the tunneling distance from 1 to 1 to the 1-3 and the 1-4 adducts. The observation that the reaction is faster with methanol than with H2O, whereas the interactions with O2•- are practically the same and can be interpreted by the distance between the two negative charges being larger in the first case than in the second for all four clusters, resulting in a decisive advantage in terms of standard potentials. In summary, the mechanism we propose is based on three observations: (i) very large peak potential shifts (and hence large increase of the apparent standard rate constant) associated with the reaction O2•----H2O + e- f -O2H---OH- upon
2822 J. Phys. Chem. C, Vol. 111, No. 7, 2007 addition of water (or methanol, or 2-propanol) to the acetonitrile solution; (ii) increase of the hydrogen/deuterium isotope effect with water concentration; (iii) the fact that the positive shift of the peak potential (or equivalently the increase of kS) is larger with methanol than with water. Mere invocation of specific solvation by water (or the other weak acids) of the reduction products cannot explain these facts. They instead reveal that short hydrogen-bonded water chains, involving approximately three water molecules, are involved in the concerted proton-electron transfer process, thus providing a preliminary picture of the mechanisms operating in pure water or other H-bonding and H-bonded media. More sophisticated simulations of the observed kinetic behaviors than the ones above may presumably be developed. Our purpose in this preliminary approach was rather to pinpoint the nature of the kinetic processes triggered by water or other weak acids. Acknowledgment. Prof. Dennis H. Evans (University of Arizona, Tucson) and Dr. Cyrille Costentin (Universite´ Paris 7 - Denis Diderot) are thanked for pleasant and helpful discussions on the matter of this manuscript and for several precious suggestions. References and Notes (1) (a) Stubbe, J.; van der Donk, W. A. Chem. ReV. 1998, 98, 705. (b) Stubbe, J.; Nocera, D. G.; Yee, C. S.; Chang, M. C. Y. Chem. ReV. 2003, 103, 2167. (c) Chang, C. J.; Chang, M. C. Y.; Damrauer, N. H.; Nocera, D. G Biochim. Biophys. Acta 2004, 1655, 13. (d) Mayer, J. M.; Rhile, I. J. Biochim. Biophys. Acta 2004, 1655, 51. (f) Renger, G. Biochim. Biophys. Acta 2004, 1655, 195. (g) McEvoy, J. P.; Brudvig, G. W. Phys. Chem. Chem. Phys. 2004, 6, 4754. (h) Meyer, T. J.; Huynh, M.-H. V.; Thorp, H. H. Angew. Chem., Int. Ed. 2006. (2) (a) Seyedsayamdost, M. R.; Yee, C. S.; Reece, S. Y.; Nocera, D. G.; Stubbe, J. J. Am. Chem. Soc. 2006, 128, 1562. (b) Seyedsayamdost, M.
Letters R.; Reece, S. Y.; Nocera, D. G.; Stubbe, J. J. Am. Chem. Soc. 2006, 128, 1569. (3) (a) Binstead, R. A.; Meyer, T. J. J. Am. Chem. Soc. 1987, 109, 3287. (b) Huynh, M. H. V.; Meyer, T. J. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 13138. (c) Fecenko, C. J.; Meyer, T. J.; Thorp, H. H. J. Am. Chem. Soc. 2006, 128, 11020. (d) Biczo´k, L.; Gupta, N.; Linschitz, H. J. Am. Chem. Soc. 1997, 119, 12601. (e) Gupta, N.; Linschitz, H. J. Am. Chem. Soc. 1997, 119, 6384. (f) Shukla, D.; Young, R. H.; Farid, S. J. Phys. Chem. A 2004, 108, 10386. (4) (a) Sjo¨din, M.; Styring, S.; Åkermark, B.; Sun, L.; Hammarstro¨m, L. J. Am. Chem. Soc. 2000, 122, 3932. (b) Sjo¨din, M.; Styring, S.; Åkermark, B.; Sun, L.; Hammarstro¨m, L. Philos. Trans. R. Soc. London, Ser. B 2002, 357, 1471. (c) Sjo¨din, M.; Ghanem, R.; Polivka, T.; Pan, J.; Styring, S.; Sun, L.; Sundstro¨m, V.; Hammarstro¨m, L. Phys. Chem. Chem. Phys. 2004, 6, 4851. (d) Sjo¨din, M.; Styring, S.; Wolpher, H.; Xu, Y.; Sun, L.; Hammarstro¨m, L. J. Am. Chem. Soc. 2005, 127, 3855. (e) Sjo¨din, M.; Irebo, T.; Utas, J. E.; Lind, J.; Merenyi, G.; Åkermark, B.; Hammarstro¨m, L. J. Am. Chem. Soc. 2006, 128, 13076. (5) (a) Costentin, C.; Evans, D. H.; Robert, M.; Save´ant, J.-M.; Singh, P. S. J. Am. Chem. Soc. 2005, 127, 12490. (b) Singh, P. S.; Evans, D. H. J. Phys. Chem. B 2006, 110, 637. (c) Costentin, C.; Robert, M.; Save´ant, J.-M. J. Electroanal. Chem. 2006, 588, 197. (6) (a) Haddox, R. M.; Finklea, H. O. J. Phys. Chem. B 2004, 108, 1694. (b) Madhiri, N.; Finklea, H.O. Langmuir ASAP. (7) (a) Costentin, C.; Robert, M.; Save´ant, J.-M. J. Am. Chem. Soc. 2006, 128, 4552. (b) Costentin, C.; Robert, M.; Save´ant, J.-M. J. Am. Chem. Soc. 2006, 128, 8726. (8) Macias-Ruvalcaba, N. A.; Okumura, N.; Evans, D. H. J. Phys. Chem. B 2006, 110, 22043. (9) (a) It is also an interesting system in view of the possible involvement of CPET in the disproportionation of superoxide ion by superoxide dismutase. (b) Miller, A.-F. Curr. Opin. Chem. Biol. 2004, 8, 162. (10) Save´ant, J.-M. Elements of Molecular and Biomolecular Electrochemistry; Wiley-Interscience: Hoboken, NJ, 2006; pp 39, 40, 368, 369. (11) Energies in eV, potentials in V. (12) Save´ant, J.-M. J. Electroanal. Chem. 2000, 485, 86. (13) ref. 10 p. 53.
