Electroreduction of Dialkyl Peroxides. Activation-Driving Force Relationships and Bond Dissociation Free Energies 1a Sabrina Antonello, 1b Martin Musumeci, 1b,c Danial D. M. Wayner, 1d and Flavio Maran* ,1b Contribution from the Dipartimento di Chimica Fisica, UniVersita ` di PadoVa, Via Loredan 2, 35131 PadoVa, Italy, and the Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex DriVe, Ottawa, Ontario, Canada K1A 0R6 ReceiVed May 5, 1997 X Abstract: The electrochemical reduction of five dialkyl peroxides in DMF was studied by cyclic voltammetry. The electron transfer (ET) to the selected compounds is concerted with the oxygen-oxygen bond cleavage (dissociative ET) and is independent of the electrode material. Such an electrochemical behavior provided the opportunity to study dissociative ETs by using the mercury electrode and therefore to test the dissociative ET theory by using heterogeneous activation-driving force relationships. The convolution voltammetry analysis coupled to the double- layer correction led to reasonable estimates of the standard potential (E°) for the dissociative ET to dialkyl peroxides, as supported, whenever possible, by independent estimates. A thermochemical cycle based on the dissociative ET concept was employed to calculate the bond dissociation free energies (BDFEs) of the five peroxides, using the above E°s together with electrochemical or thermochemical data pertaining to the redox properties of the leaving alkoxide ion. The BDFEs were found to be in the 25-32 kcal/mol range, suggesting a small substituent effect. The dissociative ET E°s were also used together with the experimental quadratic free energy relationships to estimate the heterogeneous reorganization energies. Introduction Radical anion formation by electron transfer (ET) to an organic molecule AB (eq 1) can be followed by an irreversible bond-breaking reaction (eq 2) leading to a radical, A • , and an anion, B - , 2 where the intermediate species AB •- may fragment with rates which can vary over a wide range. However, when the lifetime of the radical anion is so short to be of the same order of magnitude as that of the vibration of the bond which is going to be broken, there is no bound state for AB •- . The ET can no longer be considered as an outer sphere process 3 and is now best described as taking place in the single step (eq 3) and referred to as dissociative ET. The theory for such a concerted mechanism has been described by Save ´ant, who considered the adiabatic aspects of this reaction. 4 More recently a corresponding nonadiabatic treatment has been proposed. 5 In the classical limit, where Morse-like potential energy curves are adopted to describe reagents and products of the ET and the harmonic approximation is used to describe activation by the solvent, the development of the model leads to the formulation of a simple Marcus-like activation-driving force relationship in which the activation free energy ∆G # depends quadratically on the free energy ∆G° of the reaction, according to eq 4: 4 The intrinsic barrier ∆G 0 # , i.e. the activation free energy at zero driving force, is given by three contributions: two of them are the outer or solvent reorganization λ o and the inner reorganization λ i , where the latter term does not include the contribution of the mode corresponding to the breaking A-B bond; the third, very important term is the bond dissociation energy, BDE. Accordingly, Whereas there is a wide literature concerning stepwise reductive cleavages, either thermally induced or photoinitiated, 2 the number of investigations on dissociative ETs is still limited. The dissociative ET mechanism has been studied in dipolar aprotic solvents by using either inert electrodes or aromatic radical anions as electron donors. Specific, relevant examples concern the dissociative reduction of alkyl 6,7 or benzyl halides, 8,9 X Abstract published in AdVance ACS Abstracts, September 1, 1997. (1) (a) Issued as NRCC publication No. 40836. (b) Universita ` di Padova. (c) Permanent address: Chemistry Department, Junior College, University of Malta, Msida, Malta. (d) National Research Council of Canada. (2) (a) Saeva, F. D. In Topics in Current Chemistry; Mattay, J., Ed.; Springer-Verlag: Berlin, 1990; Vol. 156, p 59. (b) Schuster, G. B. In AdVances in Electron Transfer Chemistry; Mariano, P. S., Ed.; JAI Press: Greenwich, 1991; Vol. 1, p 163. (c) Maslak, P. In Topics in Current Chemistry; Mattay, J., Ed.; Springer-Verlag: Berlin, 1993; Vol. 168, p 1. (d) Save ´ant, J.-M. Acc. Chem. Res. 1993, 26, 455. (3) See for example: (a) Newton, M. D.; Sutin, N. Annu. ReV. Phys. Chem. 1984, 35, 437. (b) Marcus, R. A.; Sutin, N. Biochem. Biophys. Acta 1985, 811, 265. (4) (a) Save ´ant, J.-M. J. Am. Chem. Soc. 1987, 109, 6788. (b) Save ´ant, J.-M. In AdVances in Electron Transfer Chemistry; Mariano, P. S., Ed.; JAI Press: Greenwich, 1994; Vol. 4, p 53. (5) German, E. D.; Kuznetsov, A. M. J. Phys. Chem. 1994, 98, 6120. (6) (a) Andrieux, C. P.; Gallardo, I.; Save ´ant, J.-M.; Su, K. B. J. Am. Chem. Soc. 1986, 108, 638. (b) Andrieux, C. P.; Gallardo, I.; Save ´ant, J.- M. J. Am. Chem. Soc. 1989, 111, 1620. (c) Andrieux, C. P.; Ge ´lis, L.; Medebielle, M.; Pinson, J.; Save ´ant, J.-M. J. Am. Chem. Soc. 1990, 112, 3509. (7) German, E. D.; Kuznetsov, A. M.; Tikhomirov, V. A. J. Phys. Chem. 1995, 99, 9095. AB + e a AB •- (1) AB •- f A • + B - (2) AB + e f A • + B - (3) ∆G # ) ∆G 0 # ( 1 + ∆G° 4∆G 0 # 29 2 (4) ∆G 0 # ) λ o + λ i + BDE 4 (5) 9541 J. Am. Chem. Soc. 1997, 119, 9541-9549 S0002-7863(97)01416-9 CCC: $14.00 © 1997 American Chemical Society