Transcript Profiling an optimal Pt(II) based methane activation

Profiling an Optimal Pt(II) Based
Methane Activation Catalyst by
Density Functional Theory
Hongjuan Zhu, Tom Ziegler
Department of Chemistry
University of Calgary
May 28, 2008
Methane Activation
Energy-intensive, two-step process
[Ni]
CH4(g) + H2O(g)
CO(g) + 3H2(g)
[Cu/ZnO]
CO(g) + 2H2(g)
CH3OH(g)
Direct oxidation process
CH4(g) + 1/2O2
CH3OH(l)
S. S. Stahl, J. A. Labinger, J. E. Bercaw, Angew. Chem. Int. Ed. 1998, 37, 2180
Shilov Reactions
PtCl42RH + D+
RCH3 +
PtCl62- +
H2O
(Cl-)
PtCl42H2O
RD + H+
RCH2OH (RCH2Cl) + PtCl4 2-+2HCl
Improve catalyst
efficiency?
Aleksandre Evgen’yevich Shilov
N. F. Gol’dshleger, M. B. Tyabin, A. E. Shilov, A. A. Shteinman, Zh. Fiz. Khim. (Engl. Transl.) 1969, 43, 1222
N. F. Gol’dshleger, V. V. Es’kova, A. E. Shilov, A. A. Shteinman, Zh. Fiz. Khim. (Engl. Transl.) 1972, 46, 785
PtTX2L
CH4 + D+
X1
T
II
-L
X1
T
Pt
X2
L
X1
T
IV
II
+ CH4
Pt
X2
CH3D + H+
CH3
H
Pt
X2
H
CH3
- H+ + D+
X1
T
II
+ CH3D
Pt
X2
L
+L
X1
T
II
Pt
X2
D
X1
T
IV
CH3
Pt
X2
D
CH3
N. F. Gol’dshleger, M. B. Tyabin, A. E. Shilov, A. A. Shteinman, Zh. Fiz. Khim. (Engl. Transl.) 1969,
43, 1222
Outline

Determining factor for the energy barrier of
methane coordination

Determining factor for the energy barrier of CH bond cleavage

How to balance the two energy barriers to
obtain optimal Pt(II) catalyst
Methane Coordination
T
F
Cl
CH4
G12#
(kcal/mol)
Cl
T
Cl
23.4
22.0
20.9
CN
24.9
H. Zhu, T. Ziegler, Organometallics 2007, 26, 2277
H. Zhu, T. Ziegler, J. Organomet. Chem. 2006, 691, 4486
H2O
TS(1-2)
31.3
23.3
Br
I
NO2
Pt
#
G12
T
Pt
Cl
CH4
Cl
2
T
Cl
Pt
Cl
H2O + CH4
1
+ H2O
Methane Coordination
T
Pt-C (Å) Pt-O (Å)
F
2.88
2.88
Cl
2.97
2.89
Br
2.99
2.89
I
3.01
2.97
NO2
3.00
3.02
CN
3.00
3.03
CH4
Cl
T
Cl
Pt
OH2
Geometry of TS(1-2)
Methane Coordination
CH4
trans-directing ability of L
Cl
T
F < Cl < Br < I < NO2 < CN
weak
Pt
Cl
strong
H2O
TS(1-2)
Pt-CH4 bond strength trans to L
#
G12
F > Cl > Br > I > NO2 > CN
strong
weak
T
energy barrier of methane coordination
Pt
Cl
CH4
Cl
2
F > Cl > Br > I > NO2 > CN
high
low
T
Cl
Pt
Cl
H2O + CH4
1
+ H2O
C-H Bond Cleavage
T
G23#
(kcal/mol)
F
0
Cl
Br
I
NO2
1.1
2.1
3.6
16.6
CN
28.6
T
Cl
Cl
Pt
H + H2O
CH3
TS(2-3)
#
G23
Cl
T
Cl
Pt
CH4 + H2O
2
H
T
Pt
Cl
CH3 + H2O
Cl
3
H. Zhu, T. Ziegler, Organometallics 2007, 26, 2277
H. Zhu, T. Ziegler, Organometallics 2008, in press
C-H Bond Cleavage
Cl
T
Pt-C (Å)
Intermediate
F
2.27
Pt-H (Å)
TS
-
Intermediate
1.63
TS
-
H
T
Pt
CH3
Cl
2.36
2.14
1.72
1.59
Br
2.38
2.13
1.74
1.60
Cl
Pt-methane intermediate
Cl
H
I
NO2
CN
2.41
2.05
1.77
1.63
2.53
1.93
1.85
1.63
2.53
2.15
1.98
1.61
T
Pt
CH3
Cl
Transition State of C-H
bond cleavage
C-H Bond Cleavage
trans-directing ability
F < Cl < Br < I < NO2 < CN
weak
T
Cl
strong
Cl
Pt
H + H2O
CH3
TS(2-3)
Pt-C bond length in Pt-methane intermediate
F < Cl < Br < I < NO2 < CN
short
#
G23
long
Pt-C bond length in Transition State
F ~ Cl ~ Br ~ I ~ NO2 ~ CN
T
Cl
similar
energy barrier of C-H bond cleavage
F < Cl < Br < I < NO2 < CN
low
Cl
high
Pt
CH4 + H2O
2
H
T
Pt
Cl
CH3 + H2O
Cl
3
Trans Effect
Facilitation of the replacement or substitution of a
second ligand trans to the first by an external ligand
trans-directing ability
F < Cl < Br < I < NO2 < CN
weak
strong
Cl
Cl
Pt
T
+ L (CH4)
L (CH4)
Cl
Cl
electronegativity
of T
Pt
T
ligand orbital
energy
d-composition in
PtCl2T- orbitals
overlap between
PtCl2T- and L (or CH4)
R. H. Crabtree, The Organometallic Chemistry of the Transition Metals; Wiley: New York (1998)
Optimal Pt(II) catalyst
H
T
•Trans ligand T affects the two
energy barriers in opposite
directions
Cl
+ CH4
H2O
- H2O
Pt
Cl
T
Cl
CH4
Pt
Pt
Cl
Cl
1
T
Cl
CH3
3
2
TS(1-2)
•Too weak or too strong trans
ligand T would make one
energy barrier dominate.
TS(2-3)
#
G12
•Optimal Pt(II) catalyst should
have medium trans ligand T
because medium T would
reduce the first energy barrier
while still keep the second
one relatively lower
#
G23
T
Cl
T
Cl
Cl
Pt
H2O
+ CH4
1
Cl
CH4
Pt
+ H2O
2
H
Cl
CH3
T
Pt
Cl
+ H2O
3
Conclusions
• The determining factor for the energy barriers of
both methane coordination and C-H bond activation
is the trans ligand T, which changes the two energy
barriers in opposite directions
• Optimal Pt(II) catalyst for the Shilov reaction can
be obtained by using ligand T with medium trans
directing ability