Intramolecular, Intermolecular, and Heterogeneous Nonadiabatic Dissociative Electron Transfer to Peresters Sabrina Antonello, † Fernando Formaggio, ‡ Alessandro Moretto, ‡ Claudio Toniolo, ‡ and Flavio Maran* ,† Contribution from the Department of Physical Chemistry, UniVersity of PadoVa, Via Loredan 2, 35131 PadoVa, Italy, and the Biopolymer Research Center, C.N.R., Department of Organic Chemistry, UniVersity of PadoVa, Via Marzolo 1, 35131 PadoVa, Italy ReceiVed March 28, 2001 Abstract: The electron transfer to peresters was studied by electrochemical means in N,N-dimethylformamide. The reduction was carried out by three independent methods: (i) heterogeneously, by using glassy carbon electrodes, (ii) homogeneously, by using electrogenerated radical anions as the donors, and (iii) intramolecularly, by using purposely synthesized donor-spacer-acceptor (D-Sp-A) systems. Convolution analysis of the heterogeneous data led to results in excellent agreement with the dissociative electron transfer theory. The homogeneous redox catalysis data also confirmed the reduction mechanism. The cyclic voltammetries of the D-Sp-A molecules could be simulated, leading to determination of the corresponding intramolecular dissociative rate constants. Analysis of the results showed that, regardless of the way by which the acceptor is reduced, the investigated dissociative electron transfers are strongly nonadiabatic and, particularly, that the experimental rates are several orders of magnitude smaller than the adiabatic limit. A possible mechanism responsible for the observed behavior is discussed. In the last 15 years the study of intramolecular electron transfer (ET) reactions in D-Sp-A molecules, in which a donor (D) and an acceptor (A) are separated by a molecular spacer (Sp), has provided a variety of information on how electrons are transferred through bonds and space. 1 Conversely, almost nothing is known about dissociative electron transfers (DETs), i.e., those reactions in which ET and bond cleavage are concerted. 2-4 Indeed, there is a relatively large amount of studies on the decay of radical anions occurring by fragmentation of a σ bond. 2,4,5 In most cases, however, the mixing of the π* (donor side) and σ* (acceptor side) orbitals is so strong that analysis of the rate data, e.g., in terms of the relevant thermodynamic parameters, may require several approximations. In fact, it has been argued that describing the reductive cleavage of these systems in terms of electron uptake followed by intramolecular transfer from the π* orbital to the σ* orbital is not a realistic model. 6 Generally speaking, the study of intramolecular ET processes, whether dissociative or not, is best performed by using systems in which the D and A functional moieties are spatially separated. Very recently, as a first step in this direction, we reported data on the ΔG° dependence of intramolecular DETs, using systems in which A was C-Br, D was a series of ring-substituted benzoates, and Sp was cyclohexyl. 7 The in- tramolecular rate constants were found to be more sensitive to variation of ∆G° than the corresponding intermolecular data. This was attributed to the substituent-dependent variation of the effective distance between the orbitals involved in the transfer. Concerning the distance effect caused by variation of the spacer’s length, we expect the rate to decrease with distance along similar lines as reported for a variety of nondissociative systems. This is because of reduced electronic coupling between the reactant and product states, leading to nonadiabatic pro- cesses. In fact, there are some interesting results on the dissociative electron attachment to chloronorbornenes in which the efficiency of chloride ion production was related to the electronic coupling between localized CsCl σ* and CdC π* orbitals. 8 Some data on the intramolecular DET in D-Sp-A systems in which Sp is a variable-length alkyl chain have also been reported. 9 The absence of rigidity in the investigated molecules appears to be responsible for the small distance effect on the intramolecular DET rates. The role of orbital symmetry restrictions on the efficiency of π*/σ* coupling has been stressed for the intramolecular reduction of halides. 8a,10 Besides distance or symmetry effects, however, we obtained some evidence * Corresponding author: (tel) +39 (049) 827-5147; (fax) +39 (049) 827- 5135; (e-mail) f.maran@chfi.unipd.it. † Department of Physical Chemistry. ‡ Department of Organic Chemistry. (1) For example, see: (a) Closs, G. L.; Miller, J. R. Science 1988, 240, 440. (b) Paddon-Row, M. N. Acc. Chem. Res. 1994, 27, 18. (c) Electron Transfer-From Isolated Molecules to Biomolecules; Jortner, J., Bixon, M., Eds.; Wiley: New York, 1999; Part 1. (2) (a) Save ´ant, J.-M. In AdVances in Electron-Transfer Chemistry; Mariano, P. S., Ed.; JAI Press: Greenwich, CT, 1994; Vol. 4, p 53. (b) Save ´ant, J.-M. AdV. Phys. Org. Chem. 2000, 35, 117. (3) Eberson, L. Acta Chem. Scand. 1999, 53, 751. (4) Maran, F.; Wayner, D. D. M.; Workentin, M. S. AdV. Phys. Org. Chem. 2001, 36, in press. (5) (a) Maslak, P. In Topics in Current Chemistry; Mattay, J., Ed.; Springer-Verlag: Berlin, 1993; Vol. 168, p 1. (b) Save ´ant, J.-M. J. Phys. Chem. 1994, 98, 3716. (6) Burrow, P. D.; Gallup, G. A.; Fabrikant, I. I.; Jordan, K. D. Austr. J. Phys. 1996, 49, 403. (7) Antonello, S.; Maran, F. J. Am. Chem. Soc. 1998, 120, 5713. (8) (a) Pearl, D. M.; Burrow, P. D.; Nash, J. J.; Morrison, H.; Jordan, K. D. J. Am. Chem. Soc. 1993, 115, 9876. (b) Pearl, D. M.; Burrow, P. D.; Nash, J. J.; Morrison, H.; Nachtigallova, D.; Jordan, K. D. J. Phys. Chem. 1995, 99, 12379. (9) (a) Kimura, N.; Takamuku, S. Bull. Chem. Soc. Jpn. 1991, 64, 2433. (b) Kimura, N.; Takamuku, S. Bull. Chem. Soc. Jpn. 1992, 65, 1668. (c) Kimura, N.; Takamuku, S. J. Am. Chem. Soc. 1995, 116, 4087. (d) Kimura, N.; Takamuku, S. J. Am. Chem. Soc. 1995, 117, 8023. (10) Addock, W.; Andrieux, C. P.; Clark, C. I.; Neudeck, A.; Save ´ant, J.-M.; Tardy, C. J. Am. Chem. Soc. 1995, 117, 8285. 9577 J. Am. Chem. Soc. 2001, 123, 9577-9584 10.1021/ja010799u CCC: $20.00 © 2001 American Chemical Society Published on Web 09/01/2001