Reactivity of the r-Agostic Methyl Bridge in the Unsaturated Complex [Mo 2 (η 5 -C 5 H 5 ) 2 (μ-η 1 :η 2 -CH 3 )(μ-PCy 2 )(CO) 2 ]: Migratory Behavior and Methylidyne Derivatives M. Angeles Alvarez, Daniel Garcı ´a-Vivo ´, M. Esther Garcı ´a, M. Eugenia Martı ´nez, Alberto Ramos, and Miguel A. Ruiz* Departamento de Quı ´mica Orga ´nica e Inorga ´nica/IUQOEM, UniVersidad de OViedo, E-33071 OViedo, Spain ReceiVed January 23, 2008 Summary: The bridging methyl ligand present in the title compound can migrate up to the cyclopentadienyl (Cp) site upon reaction with CO to achieVe an oVerall exchange between the methyl and hydrogen (Cp) positions. In the presence of different metal-carbonyl complexes, easy dehydrogenation of the methyl group takes place under photochemical conditions to giVe methylidyne-bridged heterometallic clusters. Methyl- and other alkyl-bridged complexes are species of interest for several reasons, including the fact that they serve as models both for intermediates in alkyl-transfer processes and for adsorbates in several heterogeneously catalyzed reactions such as the Fischer-Tropsch synthesis and also because they are implied as catalysts or precursors of the homogeneous catalysts used in the polymerization of olefins. 1,2 Although a large number of such binuclear complexes have been reported so far, only some of them display metal-metal bonds, these generally exhibiting an asymmetric coordination of the methyl (or alkyl) bridge in which a C-H bond is involved in an R-agostic interaction with one of the metal atoms, the ligand then behaving formally as a three-electron donor (Chart 1). The reactivity reported so far for the methyl bridges in the latter complexes includes the oxidative addition of the agostic C-H bond at trimetal centers, 3 rearrangement of the ligand to a terminal coordination mode, 4 deprotonation, 5 reductive elimina- tion with other ligands, 6 and insertion of CO. 4b,c,7 Among all the alkyl-bridged complexes described so far, however, only a few of them display multiple intermetallic bonding, 8 and their reactivity has not been explored. In this context, our recent preparation of the unsaturated methyl complex [Mo 2 Cp 2 (µ-η 1 : η 2 -CH 3 )(µ-PCy 2 )(CO) 2 ](1; 9 Cp ) η 5 -C 5 H 5 ) gave us the op- portunity to study the chemical behavior of an R-agostic methyl ligand bridging a multiple metal-metal bond. In this paper we report our preliminary results on the reactivity of compound 1, which reveal several unusual features such as the facile migration of the methyl ligand up to the coordinated cyclopen- tadienyl groups and its easy dehydrogenation in the presence of metal-carbonyl fragments, then providing a rational synthetic route to novel heterometallic clusters having methylidyne bridges (Scheme 1). According to recent DFT calculations, 9a the agostic interaction in 1 is rather weak; therefore, the view of the methyl ligand as a 3-electron donor to yield a 32-electron species is somewhat exaggerated yet is a useful formalism to interpret its reactivity. In any case, the presence of this weak agostic interaction along with the multiple intermetallic bond makes this compound quite reactive toward simple donors such as carbon monoxide, isocyanides, and diphosphines at room temperature, to give a variety of products, many of which involve the migration of the methyl ligand. The most remarkable migratory behavior is that observed in the reaction with CO, this giving as major products the acetyl-bridged complex [Mo 2 Cp 2 {µ-η 1 :κ 1 -C(Me)O}- (µ-PCy 2 )(CO) 3 ](2) and the hydrides [Mo 2 Cp(η 5 -C 5 H 4 Me)(µ- H)(µ-PCy 2 )(CO) 4 ](3) and [Mo 2 Cp{η 5 -C 5 H 4 C(O)Me}(µ-H)(µ- PCy 2 )(CO) 4 ](4), the last two complexes having methyl- and acetyl-substituted cyclopentadienyl ligands, respectively. The relative amounts of these products are dependent on the experimental conditions, and separate experiments indicate that toluene solutions of the acetyl complex 2 decompose progres- sively at room temperature to give mainly the methylcyclopen- tadienyl hydride complex 3, along with small amounts of 4 and other products. * To whom correspondence should be addressed. E-mail: mara@uniovi.es. (1) (a) Braunstein, P.; Boag, N. M. Angew. Chem., Int. Ed. 2001, 40, 2427. (b) Marks, T. J. Acc. Chem. Res. 1992, 25, 57. (2) For some recent work on alkyl-bridged complexes see, for example: (a) Bolton, P. D.; Clot, E.; Cowley, A. R.; Mountford, P. J. Am. Chem. Soc. 2006, 128, 15005. (b) Dietrich, H. M.; Grove, H.; To ¨rnroos, K. W.; Anwander, R. J. Am. Chem. Soc. 2006, 128, 1458. (c) Weng, Z.; Teo, S.; Koh, L. L.; Hor, T. S. A. Chem. Commun. 2006, 1319. (3) (a) Calvert, R. B.; Shapley, J. R. J. Am. Chem. Soc. 1977, 99, 5225. (b) Dutta, T. K.; Vites, J. C.; Jacobsen, G. B.; Fehlner, T. P. Organometallics 1987, 6, 842. (4) (a) Wigginton, J. R.; Trepanier, S. J.; McDonald, R.; Ferguson, M. J.; Cowie, M. Organometallics 2005, 24, 6194. (b) Rowsell, B. D.; McDonald, R.; Cowie, M. Organometallics 2004, 23, 3873. (c) Trepanier, S. J.; McDonald, R.; Cowie, M. Organometallics 2003, 22, 2638. (5) (a) Davies, D. L.; Gracey, B. P.; Guerchais, V.; Knox, S. A. R.; Orpen, A. G. J. Chem. Soc., Chem. Commun. 1984, 841. (b) Casey, C. P.; Fagan, P. J.; Miles, W. H. J. Am. Chem. Soc. 1982, 104, 1134. (c) Dawkins, G. M.; Green, M.; Orpen, A. G.; Stone, F. G. A. J. Chem. Soc., Chem. Commun. 1982, 41. (6) (a) Carlucci, L.; Proserpio, D. M.; D’Alfonso, G. Organometallics 1999, 18, 2091. (b) Noh, S. K.; Sendlinger, S. C.; Janiak, C.; Theopold, K. H. J. Am. Chem. Soc. 1989, 111, 9127. (7) Gao, Y.; Jennings, M. C.; Puddephatt, R. J. Organometallics 2001, 20, 1882. (8) Apparently only a few complexes (all dichromium ones) have been reported to have alkyl bridges across multiple metal-metal bonds: (a) Heintz, R. A.; Ostrander, R. L.; Rheingold, A. L.; Theopold, K. H. J. Am. Chem. Soc. 1994, 116, 11387. (b) Morse, P. M.; Spencer, M. D.; Wilson, S. R.; Girolami, G. S. Organometallics 1994, 13, 1646. (c) Andersen, R. A.; Jones, R. A.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1978, 446. (9) (a) Garcı ´a, M. E.; Ramos, A.; Ruiz, M. A.; Lanfranchi, M.; Marchio, L. Organometallics 2007, 26, 6197. (b) Garcı ´a; M. E.; Melo ´n, S.; Ramos, A.; Riera, V.; Ruiz, M. A.; Belletti, D.; Graiff, C.; Tiripicchio, A. Organometallics 2003, 22, 1983. Chart 1 Organometallics 2008, 27, 1973–1975 1973 10.1021/om800061z CCC: $40.75  2008 American Chemical Society Publication on Web 04/03/2008