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

1
Three C-H ?-Bonds Activated in Propane by the
CpW(NO)(CH2) Carbene Complex Yubo Fan and
Michael B. Hall Department of Chemistry, Texas
AM University, College Station, TX 77843-3255
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)(CH3CHCH2), 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.
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.
Introduction Carbene CpW(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
(70C, 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.
B3LYP Optimized Structures (Unit for bond length
is Å)

• 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 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. Th
e 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.

• 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) 6-31G6
• 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.

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Relative Energies (DEo) and Relative Gibbs Free
Energies (DG) for the Species in the Whole
Reaction