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

CH4(g) + H2O(g) CO(g) + 3H2(g)[Ni]

CO(g) + 2H2(g) CH3OH(g)[Cu/ZnO]

Energy-intensive, two-step process

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

Aleksandre Evgen’yevich Shilov

PtCl42-

RH + D+ RD + H+

PtCl42-

H2ORCH2OH (RCH2Cl) + PtCl4

2-+2HClRCH3 + PtCl62- + H2O (Cl-)

Improve catalyst efficiency?

N. F. Gol’dshleger, M. B. Tyabin, A. E. Shilov, A. A. Shteinman, Zh. Fiz. Khim. (Engl. Transl.) 1969, 43, 1222N. F. Gol’dshleger, V. V. Es’kova, A. E. Shilov, A. A. Shteinman, Zh. Fiz. Khim. (Engl. Transl.) 1972, 46, 785

PtTX2LCH4 + D+ CH3D + H+

X1

X1

CH3

IV

H

Pt

T

T

X2

CH3

X1

X1

II

II

H

Pt

Pt

T

T

X2

PtCH3X2

IV

DX2

D

CH3

X1

X1

L

L

II

II

Pt

Pt

T

T

X2

X2

+ CH4

- L

+ D+

+ CH3D+ L

- H+

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 C-H bond cleavage

How to balance the two energy barriers to obtain optimal Pt(II) catalyst

Methane Coordination

Pt

Pt

Pt CH4

TS(1-2)

+ CH4

+ H2O

1

2

ΔG12#

Cl

Cl

Cl

Cl

Cl

Cl

T

T

CH4

H2O

H2O

T

T ΔG12#

(kcal/mol)

F 31.3

Cl 23.3

Br 23.4

I 22.0

NO2 20.9

CN 24.9

H. Zhu, T. Ziegler, Organometallics 2007, 26, 2277H. Zhu, T. Ziegler, J. Organomet. Chem. 2006, 691, 4486

Methane Coordination

Pt

Cl

ClT

CH4

OH2

Geometry of TS(1-2)

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

Methane Coordination

Pt

Pt

Pt CH4

TS(1-2)

+ CH4

+ H2O

1

2

ΔG12#

Cl

Cl

Cl

Cl

Cl

Cl

T

T

CH4

H2O

H2O

T

F < Cl < Br < I < NO2 < CN

trans-directing ability of L

weak strong

F > Cl > Br > I > NO2 > CN

energy barrier of methane coordination

lowhigh

F > Cl > Br > I > NO2 > CN

Pt-CH4 bond strength trans to L

weakstrong

C-H Bond Cleavage

Pt CH4

PtCH3

Pt CH3

TS(2-3)

+ H2O

2

3

ΔG23#

Cl

Cl

T

T H

Cl

Cl

+ H2O

Cl

ClT

H

+ H2O

T ΔG23#

(kcal/mol)

F 0

Cl 1.1

Br 2.1

I 3.6

NO2 16.6

CN 28.6

H. Zhu, T. Ziegler, Organometallics 2007, 26, 2277

H. Zhu, T. Ziegler, Organometallics 2008, in press

C-H Bond Cleavage

Pt

Cl

Cl

CH3

H

T

Pt

Cl

Cl

CH3

H

T

Pt-methane intermediate

Transition State of C-H bond cleavage

T Pt-C (Å) Pt-H (Å)

Intermediate TS Intermediate TS

F 2.27 - 1.63 -

Cl 2.36 2.14 1.72 1.59

Br 2.38 2.13 1.74 1.60

I 2.41 2.05 1.77 1.63

NO2 2.53 1.93 1.85 1.63

CN 2.53 2.15 1.98 1.61

C-H Bond Cleavage

Pt CH4

PtCH3

Pt CH3

TS(2-3)

+ H2O

2

3

ΔG23#

Cl

Cl

T

T H

Cl

Cl

+ H2O

Cl

ClT

H

+ H2O

trans-directing ability

F < Cl < Br < I < NO2 < CN

weak strong

Pt-C bond length in Pt-methane intermediate

F < Cl < Br < I < NO2 < CN

short long

Pt-C bond length in Transition State

F ~ Cl ~ Br ~ I ~ NO2 ~ CN

similar

energy barrier of C-H bond cleavage

F < Cl < Br < I < NO2 < CN

low high

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

Trans Effect

Pt

Cl

Cl

TPt

Cl

Cl

T + L (CH4) L (CH4)

electronegativity of 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

Pt

Pt CH4

Pt CH3

TS(1-2)

TS(2-3)

+ CH4

1

2

3

ΔG12#

ΔG23#

T H2O

Cl

Cl

T

Cl

Cl

+ H2OH

TCl

Cl + H2O

Pt Pt CH4 Pt CH3

+ CH4

1 2 3

T H2O

Cl

ClT

Cl

ClH

TCl

Cl- H2O•Trans ligand T affects the two energy barriers in opposite directions

•Too weak or too strong trans ligand T would make one energy barrier dominate.

•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

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