Three C-H -Bonds Activated in Propane by the CpW(NO)(=CH2) 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)(=CH2) 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)(CH4)

(CH3CH=CH2), 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.

ONH

ONH

ON+

70 C, 40 h

ONH

ONH

ON+

70 C, 40 h

~91% ~9%

~90% ~10%

ON

PMe3

ON

70 C, 40 h

ON

Si

ON

Si

CD3

D3C CD3

D

DD

70 C

, 40 h70

C, 4

0 h

Me 4

Si

Me

4 Si-d12

PMe3

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)(=CH2) from CpW(NO)(CH3)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)(=CH2)

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.

1+Propane

2

3-TS

4

5-TS

6

7+CH4

8-TS

9

10-TS

11

12+CH4

13-TS+CH4

14+CH4

0.00-1.01

8.13

-25.71

3.39

-4.24

-3.56

-22.33

-23.21

-7.57

-14.39

-18.48

-16.20

-40.40

G -

kca

l/mo

l

1+Propane

2

3-TS

4

5-TS

6

7+CH4

8-TS

9

10-TS

11

12+CH4

13-TS+CH4

14+CH4

0.00

-11.50

-3.30

-36.47

-7.68

-14.79

-5.45

-34.46-35.20

-19.84

-25.47

-20.99

-43.34

-19.32

E0 -

kca

l/mol

Relative Energies (Eo) 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.