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
Abstract: The mechanism for the reaction between CpW(NO)(=CH2) and propane to generate
Results and Discussion
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.
Introduction
0
70
C
,4
e4
M
h
Si
ON
ON
D
D
Si
D3C
70
PMe3
C
,4
0
h
Si
LUMO of CpW(NO)(=CH2)
ON
D
PMe3
CD3
CD3
B3LYP Optimized Structures (Unit for bond length is Å)
1+Propane
-3.30
3-TS
0.00
ON
+
ON
ON
H
H
70 C, 40 h
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.
~90%
ON
ON
ON
H
H
70 C, 40 h
2
-11.50
~10%
+
-5.45
7+CH4
-7.68
5-TS
6
-14.79
E0 - kcal/mol
Under the same reaction conditions
(70°C, 40 h) for this series of activations,
the generation of this carbene is the ratedetermining 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
ON
Me4Si-d12
70 C, 40 h
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).
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.
11
-25.47
8-TS
-34.46
~91%
4
-36.47
~9%
8.13
3-TS
 All calculations have been carried out by Gaussian 98 quantum chemistry software
 B3LYP Density Functional Theory (DFT) used to fully optimize all
3.39
5-TS
package.3
structures.4
 Basis Sets:
-3.56
7+CH4
• 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.
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.
 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.
-23.21
8-TS
4
-25.71
9
-22.33
11
-14.39
-16.20
13-TS+CH4
12+CH4
-18.48
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.
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• 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
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.
Conclusions
6 10-TS
-4.24
-7.57
G - kcal/mol
 Cp* is simplified and modeled by Cp, neo-pentyl by methyl and methylcyclohexane (or ethylcyclohexane)
by propane.
9
-35.20
14+CH4
-43.34
1+Propane
0.00
2
-1.01
Computational Details
-19.32
-20.99 13-TS+CH4
12+CH4
-19.84
10-TS
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.
14+CH4
-40.40
Relative Energies (DEo) and Relative Gibbs Free Energies
(DG) for the Species in the Whole Reaction
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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,
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