1. Three C-H  -Bonds Activated in Propane by the CpW(NO)(=CH 2 ) Carbene Complex Yubo Fan and Michael B. Hall Department of Chemistry, Texas A&M University, College Station, TX 77843-3255 B3LYP Optimized Structures (Unit for bond length is Å) Abstract: The mechanism for the reaction between CpW(NO)(=CH 2 ) and propane to generate CpW(NO)(H)(Allyl) and release methane was studied by B3LYP DFT calculations. The calculations indicate that an agostic species is formed at the beginning of the reaction. A direct hydrogen transfer over a low energy barrier forms CpW(NO)(Me)(n-Pr), with a  -H (on n-Pr) agostic structure. The agostic hydrogen in this intermediate moves to methyl to form the third agostic species CpW(NO)(CH 4 )(CH 3 CH=CH 2 ), which has an agostic bond between methane and tungsten and a  dative bond between propene and the metal. Releasing methane is favored entropically. Lastly, one hydrogen on the methyl of propene transfers to tungsten to produce CpW(NO)(H)(Allyl). This C-H bond activation reaction is fairly rapid with an overall energy barrier of ~18 kcal/mol. Introduction Carbene Cp*W(NO)(=CH-t-Bu) is an active intermediate and has been used to activate various C-H bonds. 1 In alkanes or silanes without  -H, only single dehydrogenation (single C-H bond activation) occurs for the methyl groups; a double activation occurs for alkanes with  -H and a triple one for those with  -H (excluding steric effects). Under the same reaction conditions (70°C, 40 h) for this series of activations, the generation of this carbene is the rate- determining step, because it is apparently highly energetic. Based on DFT calculations, the energy barrier for the generation of CpW(NO)(=CH 2 ) from CpW(NO)(CH 3 ) 2 is over 35 kcal/mol. 2 For the first type of C-H bond activation, one H atom directly transfers from methyl in alkane or silane to the C atom connected to W. For the second type, a  -H transfers to the same C atom to form a leaving alkane. For the third, a  -H transfers to W. Computational Details  Cp* is simplified and modeled by Cp, neo-pentyl by methyl and methylcyclohexane (or ethylcyclohexane) by propane.  All calculations have been carried out by Gaussian 98 quantum chemistry software package. 3  B3LYP Density Functional Theory (DFT) used to fully optimize all structures. 4  Basis Sets: W – LanL2DZ ECP and modified LanL2DZ (341/341/21) basis set with the replacement of the two outermost p functions by a (41) split ; 5 C, N, O and H on Cyclopentadienyl (Cp) – 6-31G*; 6 H on Me and n-Pr (or correspondent groups or moleclues): 6-31G**. 6  Frequency calculated at the same level to examine all minima and transition states.  Thermodynamic functions calculated for 298.15 K and 1 atm. Results and Discussion Carbene (1) is a highly energetic and active species. Because the open side has a very large LUMO lobe and the orbital energy is quite low (only -0.1109 Hartree), 1 readily reacts with Lewis bases, such as ammonia, phosphines, etc. The LUMO of 1 interacts not only with lone pair of electrons in Lewis bases strongly, but also with bonding orbitals in alkanes. LUMO of CpW(NO)(=CH 2 ) Conclusions  The generation of Carbene is the rate-determining step.  The dialkyl intermediate is stable enough that no further reaction occurs without involvement of  -H.  The  dative bonding intermediate is formed in the process of the reaction, but is considerably unstable and reacts further to form allyl.  The allyl complex is quite stable and is easily produced without steric effects between W and  -H. Acknowledgment We would like to thank the National Science Foundation (Grant No. CHE 9800184) and The Welch Foundation (Grant No. A-648) for their generous support. 1.Tran, E.; Legzdins, P. J. Am. Chem. Soc., 1997, 119, 5071; (b) Adams, C. S.; Legzdins, P.; Tran, E. J. Am. Chem. Soc., 2001, 123, 612. 2.Poli, R.; Smith, K. M. Organometallics, 2000, 19, 2858; (b) Fan, Y.; Hall, M. B. J. Chem. Soc., Dalton Trans., 2002, 713. 3.Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, A. C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.6 and A.7; Gaussian, Inc.: Pittsburgh, PA, 1998. 4.Becke, A. D. J. Chem. Phys., 1993, 98, 5648; (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B, 1988, 37, 785. 5.Hay, P. J.; Wadt, W. R. J. Chem. Phys., 1985, 82, 270; (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys., 1985, 82, 284. (c) Couty, M.; Hall, M. B. J. Comp. Chem., 1996, 17, 1359. 6.(a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys., 1971, 54, 724; (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys., 1972, 56, 2257; (c) Hariharan, P. C., Pople, J. A. Mol. Phys., 1974, 27, 209; (d) Gordon, M. S. Chem. Phys. Lett., 1980, 76, 163; (e) Hariharan, P. C.; Pople, J. A. Theo. Chim. Acta., 1973, 28, 213. Relative Energies (  E o ) and Relative Gibbs Free Energies (  G) for the Species in the Whole Reaction 1 associates with propane to form agostic species 2. By a H-transfer process, 4, CpW(NO)(Me)(n-Pr), is formed via transition state 3-TS. Then, there are two paths for 4 to react further. The first path is similar to a reverse process from 1 plus propane to 4. Via 5-TS, another agostic species is easily formed and dissociates to 7 and methane thermodynamically. The second path is for the triple dehydrogenation. W interacts with one  -H to form an intermolecular agostic species 9 through 8-TS. After this agostic-bonded H transfers to methyl via 10-TS, agostic species 11 is formed; 11 has a  dative bond between W and propene. 12 is produced by methane leaving. Finally, one of H atoms on the methyl of propene  –H transfers (though 13-TS) to W to produce CpW(NO)(H)(Allyl) 14.