Electroinduced and Spontaneous Metal-Halide Bond Dissociation in [Co(η 5 -C 5 H 5 )(η 3 -2-MeC 3 H 4 )I] Maria Gabriela Teixeira, † Francesco Paolucci, ‡ Massimo Marcaccio, ‡ Teresa Aviles, † Carmen Paradisi, ‡ Flavio Maran, § and Sergio Roffia* ,‡ Departamento de Quimica, Universidade Nova de Lisboa, Monte da Caparica, Lisboa, Portugal, Dipartimento di Chimica, Universita ` di Bologna, Via Selmi 2, 40126 Bologna, Italy, and Dipartimento di Chimica Fisica, Universita ` di Padova, Via Loredan 2, 35131 Padova, Italy Received July 28, 1997 The electrochemical behavior of the species [Co(η 5 -C 5 H 5 )(η 3 -2-MeC 3 H 4 )I] and [Co(η 5 -C 5 H 5 )- (η 3 -2-MeC 3 H 4 )(ACN)] + in ACN solutions, at 25 °C, is described. The kinetic analysis of the cyclic voltammetry curves indicates that the introduction of one electron in the former complex is concerted with the dissociation of the Co-I bond. The ensuing radical undergoes fast solvation to yield the solvato complex [Co(η 5 -C 5 H 5 )(η 3 -2-MeC 3 H 4 )(ACN)] • , which then acts as an efficient electron donor toward the starting material with the formation of [Co(η 5 - C 5 H 5 )(η 3 -2-MeC 3 H 4 )(ACN)] + ; finally, the cation is electroreduced at the working potentials to conclude an overall autocatalytic sequence. The solvato complex [Co(η 5 -C 5 H 5 )(η 3 -2- MeC 3 H 4 )(ACN)] • , formed as a product of the above reduction process, can be reversibly reduced to the corresponding anion at more negative potentials. Confirmation of the above mechanism and of the fact that the solvato complex can act as a solution electron donor toward the starting material was obtained by studying the electrochemical behavior of the solvato complex itself and through calculations aimed to better define the dissociative electron-transfer process to [Co(η 5 -C 5 H 5 )(η 3 -2-MeC 3 H 4 )I]. The dissociation of the metal- halide bond in the neutral complex [Co(η 5 -C 5 H 5 )(η 3 -2-MeC 3 H 4 )I], with the formation of [Co(η 5 - C 5 H 5 )(η 3 -2-MeC 3 H 4 )(ACN)] + , was also found to occur spontaneously, in the bulk, through the observation of a progressive change of the cyclic voltammetric pattern. Support for the occurrence of the reaction between the starting complex and the solvent was confirmed by conductivity and spectroscopic measurements, which allowed the rate constant for the homogeneous solvolysis to be determined. Introduction Studies of nucleophilic additions to organotransition metal complexes have led to an increasing interest in the preparation of these species. 1-3 In particular, efforts have been made to synthesize and characterize transi- tion metal complexes able to perform the photo- and/or electrocatalytic activation of carbon dioxide and other small molecules. 4 In this light, many cobalt complexes containing either porphyrinic or non-porphyrinic mac- rocyclic ligands as well as other ligands, such as diimines and phosphines, with interesting catalytic properties have been synthesized in the past two decades. 1 Transition metal complex mediated cycliza- tion reactions have received much attention in organic synthesis. In particular, η 5 -cyclopentadienylCo(I) com- plexes have been intensively studied and developed into a versatile synthetic method. 2,3 η 5 -Cyclopentadienyl Co(III) complexes, also bonded to styrenevinylbenzene copolymers, have been used as hydrogenation and Fischer-Tropsch catalysts. 3 η 5 -CyclopentadienylCo com- plexes often have interesting chemical and physical properties. 3 In particular, they undergo facile electron- transfer reactions accompanied by structural and spec- tral changes. 5 In light of their possible application in catalytic and electrocatalytic cycles, fundamental infor- mation concerning the bonding nature, stereochemistry, reactivity, and electronic structure of such organo- transition metal complexes is needed. In particular, as far as the electrochemical properties are concerned, since a drastic influence on the reactivity of these species derives from the charge localization within the * To whom correspondence should be addressed. Fax: +39-51- 259456. E-mail: roffia@ciam.unibo.it. † Universidade Nova de Lisboa. ‡ Universita ` di Bologna. § Universita ` di Padova. (1) Ojima, I.; Tzamarioudaki, M.; Li, Z.; Donovan, R. J. Chem. Rev. 1996, 96, 635 and references therein. (2) (a) Rybinskaya, M. I. J. Organomet. Chem. 1990, 383, 113. (b) Krivykh, V. V.; Gusev, O. V.; Rybinskaya, M. I. J. Organomet. Chem. 1989, 362, 351. (c) Masotti, H.; Wallet, J. C.; Peiffer, G.; Petit, F.; Mortreux, A.; Buono, G. J. Organomet. Chem. 1986, 308, 241. (3) (a) King, J. A., Jr.; Vollhardt, P. C. J. Organomet. Chem. 1989, 369, 245. (b) Ito, Y.; Inouye, M.; Murakami, M.; Shiro, M. J. Organomet. Chem. 1989, 359, C57. (c) Wakatsuki, Y.; Yoshimura, H.; Yamazaki, H. J. Organomet. Chem. 1989, 366, 215. (d) Hart, W. P.; Rausch, M. D. J. Organomet. Chem. 1988, 355, 455. (e) Kuhn, N.; Bru ¨ ggemann, Winter, M. J.; De Bellis, V. M. J. Organomet. Chem. 1987, 320, 391. (f) Aviles, T.; Barroso, F.; Royo, P. J. Organomet. Chem. 1987, 326, 423 and references therein. (4) (a) Costamagna, J.; Ferraudi, G.; Canales, J.; Vargas Coord. Chem. Rev. 1996, 148, 248. (b) Collin, J. P.; Sauvage, J. P. Coord. Chem. Rev. 1989, 93, 24. (5) Sun, S.; Sweigart, D. A. In Advances in Organometallic Chem- istry; Stone, F. G. A., West, R., Eds.; Academic Press: San Diego, CA, 1996; Vol. 40 and references therein. 1297 Organometallics 1998, 17, 1297-1304 S0276-7333(97)00636-5 CCC: $15.00 © 1998 American Chemical Society Publication on Web 03/03/1998