Acetylene Cyclotrimerization by Early Second-Row Transition Metals in the Gas Phase. A Theoretical Study Mayra Martinez, Maria del Carmen Michelini, Ivan Rivalta, Nino Russo, and Emilia Sicilia* Dipartimento di Chimica and Centro di Calcolo ad Alte Prestazioni per Elaborazioni Parallele e Distribuite-Centro d’Eccellenza MURST, UniVersita ` della Calabria, I-87030 ArcaVacata di Rende, Italy Received July 29, 2005 The acetylene cyclotrimerization reaction mediated by the left-hand-side bare transition metal atoms Y, Zr, Nb, and Mo has been studied theoretically, employing DFT in its B3LYP formulation. The complete reaction mechanism has been analyzed, identifying intermediates and transition states. Both the ground spin state and at least one low-lying excited state have been considered to establish whether possible spin crossings between surfaces of different multiplicity can occur. Our results show that the overall reaction is highly favorable from a thermodynamic point of view and ground state transition states lie always below the energy limit represented by ground state reactants. After the activation of two acetylene molecules and formation of a bis-ligated complex, the reaction proceeds to give a metallacycle intermediate, as the alternative formation of a cyclobutadiene complex is energetically disfavored. All the examined reaction paths involve formation of a metallacycloheptatriene intermediate that in turn generates a metal-benzene adduct from which finally benzene is released. Similarities and differences in the behaviors of the considered four metal atoms have been examined. 1. Introduction Transition metals are involved in a myriad of catalytic processes, and their all-pervading presence finds an explana- tion in their ability to adopt different oxidation states, coordination modes, bonding patterns, etc. One approach that has been revealed to be very precious for chemists in their attempt to better understand these systems was to study model reactions in the gas phase, unencumbered by the effects of ligands and solvent. 1-21 A combination of experi- mental and computational studies of the reactivities of metal atoms and clusters in the gas phase can enhance our understanding of the elementary processes occurring in real catalysts, which is essential for a rational catalyst design. Computational chemistry studies facilitate systematic inves- tigation of reactivities across entire rows of bare transition * To whom correspondence should be addressed. Tel: +39-0984-492048. Fax: +39-0984-492044. E-mail: siciliae@unical.it. (1) Allison, J.; Freas, R. B.; Ridge, D. P. J. Am. Chem. Soc. 1979, 101, 1332. (2) Gas-Phase Inorganic Chemistry; Russel, D. H., Ed.; Plenum: New York, 1989; p 412. (3) Armentrout, P. B.; Beauchamp, J. L. Acc. Chem. Res. 1993, 26, 213. (4) Armentrout, P. B. In SelectiVe Hydrocarbons ActiVation: Principles and Progress; Davies, J. A., Watson, P. L., Greenberg, A., Liebman, J. F., Eds.; VCH: New York, 1990. (5) Armentrout, P. B. Annu. ReV. Phys. Chem. 1990, 41, 313. (6) Eller, K.; Schwarz, H. Chem. ReV. 1991,91, 1121. (7) (a) Weisshaar, J. C. AdV. Chem. Phys. 1992,82, 213. (b) Weisshaar, J. C. Acc. Chem. Res. 1993, 26, 213. (8) Armentrout, P. B.; Kickel, B. L. In Organometallic Ion Chemistry; Freiser, B. S., Ed.; Kluwer: Dordrecht, 1996. (9) Armentrout, P. B. in Topics in Organometallic Chemistry; Brown, J. M., Hofmann, P., Eds.; Springer-Verlag: Berlin, 1999. (10) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 2nd ed.; John Wiley and Sons: New York, 1994. (11) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley and Sons: New York, 1994. (12) (a) Siegbahn, P. E. M.; Blomberg, M. R. A. In Theoretical Aspects of Homogeneous Catalysis; van Leeuwen, P. W. N. M., Morokuma, K., van Lenthe, J. H., Eds.; Kluwer Academic Publishers: Dordrecht, 1995; p 15-63. (b) Wittborn, A. M. C.; Costas, M.; Blomberg, M. R. A.; Siegbahn, P. E. M. J. Chem. Phys. 1997, 107, 4318. (c) Blomberg, M. R. A.; Siegbahn, P. E. M.; Svensson, M. J. Am. Chem. Soc. 1992, 114, 6095. (13) Musaev, D. G.; Morokuma, K. J. Chem. Phys. 1994, 101, 10697. (14) Irigoras, A.; Fowler, J. E.; Ugalde, J. M. J. Phys. Chem. 1998, 102, 293. (15) Irigoras, A.; Fowler, J. E.; Ugalde, J. M. J. Am. Chem. Soc. 1999, 121, 574; 8549. (16) Irigoras, A.; Elizalde, O.; Silanes, I.; Fowler, J. E.; Ugalde, J. M. J. Am. Chem. Soc. 2000, 122, 114. (17) Russo, N.; Sicilia, E. J. Am. Chem. Soc. 2001, 123, 2588. (18) Russo, N.; Sicilia, E. J. Am. Chem. Soc. 2002, 124, 1471. (19) Michelini, M. C.; Russo, N.; Sicilia, E. J. Phys. Chem. 2002, 106, 8937. (20) Michelini, M. C.; Russo, N.; Sicilia, E. Inorg. Chem. 2004, 43, 4944. (21) Chiodo, S.; Kondakova, O.; Irigoras, A.; Michelini, M. C.; Russo, N.; Sicilia, E.; Ugalde, J. M. J. Phys. Chem. A 2004, 108, 1069. Inorg. Chem. 2005, 44, 9807-9816 10.1021/ic051281k CCC: $30.25 © 2005 American Chemical Society Inorganic Chemistry, Vol. 44, No. 26, 2005 9807 Published on Web 11/12/2005