Natural Gas: An Alternative to Petroleum?
 Natural gas reserves: ~ 60 years
Petroleum reserves: ~ 40 years
 Combustion of natural gas releases more
energy per gram than that of petroleum
 Combustion of natural gas releases more
energy per CO2 molecule than that of petroleum
 Approximately twice the amount of natural gas
produced for consumption is vented or burned at its source
 Pressurization and refrigeration required for liquefaction (bp -164 °C)
 Largest reserves located in remote regions of the world
Crabtree, R. H. Chem. Rev. 1995, 95, 987-1007
American Methanol Institute, 2000
Natural Gas is a Source of Methane
H
C
H
H
H
Limitations for the Practical Use of Methane
Physical
pressurization and refrigeration required for liquification
boiling point = -164 °C
Chemical
strong carbon-hydrogen bond
CH4
CH3 + H
439 kJ/mole
very weakly acidic
CH4
CH3- + H+
pKa = 48
high ionization potential
CH4
CH4+ + e-
1255 kJ/mole
low proton affinity
CH4 + H+
CH5+
443 kJ/mole
Crabtree, R. H. Chem. Rev. 1995, 95, 987-1007
Methanol: a Fuel and a Chemical Feedstock
10% acetic acid
polyethylene terephthalate (PET)
41% methyl t-butylether
oxygenated fuels
fuel cells
25% formaldehyde
resins, urethane plastics,
Spandex
I K EA
1995 U.S. Production
2.2 billion gallons
27% other
cleaning fluid, solvents,
refrigerants,
chlorine-free bleaches
www.methanex.com
Direct Conversion of Methane to Methanol
CH4(g) + 1/2 O2(g)
CH3OH(l) HO = -130 kJ
thermodynamically favored but the high temperature required to activate the strong C-H bond (439 kJ/mol)
leads to overoxidation, i.e. CO2 and H2O
CH4 + O2
1 : 20
450 o C
50 atm
CH3OH
8 % conversion
81 % selectivity
Methane
Monooxygenase
CH4 + O2 + NAD(P)H + H+
CH3OH + NAD(P)+ + H2O
Crabtree, R. H. Chem. Rev. 1995, 95, 987-1007
Periana, R. A. et al. Science 1993, 259, 340-343
Conversion of Methane to Methanol
via Heterogeneous Catalysis
Steam Reforming
CH4(g) + H2O(g)
Nickel
Catalyst
700-1000 oC
10-20 atm
CO(g) + 3H2(g)
synthesis gas
ZnO, Cu, Alumina
CO(g) + 2H2(g)
o
250 C
H° = + 205 kJ
CH3OH(g)
H° = - 90 kJ
50-100 atm
Substantial capital investment required to implement
Crabtree, R. H. Chem. Rev. 1995, 95, 987-1007
Industrial Hydrogen Production
CH4(g) + H2O(g)
CO(g) + 3 H2(g)
H = 206 kJ
CO(g) + H2O(g)
CO2(g) + H2(g)
H = -41 kJ
water gas shift reaction
H = -519 kJ
CH4(g) + 3/2 O2(g)
CO(g) + 2 H2O(g)
2CH4(g) + 3/2 O2(g)
CO2(g) + CO(g) + 4 H2(g)
H = -354 kJ
Methane to Methanol Catalyzed by Soluble Pt(II) Salts
PtCl42-
CH4 + PtCl62- + H2O
PtII
+
120 °C
CH3OH + CH3Cl + PtCl42+ H+
PtII
CH4
CH3
CH3OH
PtIV
CH3Cl
H2O
Cl-
PtII
PtIV
CH3
Gol'dshleger, N. F.; Es'kova, V. V.; Shilov, A. E.; Shteinman, A. A. Zh. Fiz. Khim. (Engl. Transl.) 1972, 46, 785-786
Alkane C-H Bond Activation Using Electron
Rich Transition Metal Complexes
Oxidative Addition
h
Ir
Me 3P
- H2
RT
H
H
Me 3P
Ir(III)
Ir(III)

H
R
- RH
Ir(I)
H
R
Ir(III)
Ir(I)
L
Reductive Elimination
Ir
Ir
Me 3P
Ir
Me 3P
Ir(I)
Ir
Me 3P
RH
Ir
Me 3P
L
Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1982, 104, 352-354
C-H Bond Activation by an Electron Rich
Metal Center
oxidative addition
Mn+2
Mn + RH
reductive elimination
R = alkyl or aryl
M = Rh, Ir, Pt
R
H
C-H Bond Activation by an Electron Rich
Metal Center
Oxidative Addition
has occurred
C-H Bond Activation Selectivity
Me
H
Me
H
H
>
Me
>
Me
Me
tertiary
Oxidative Addition
by Late Transition Metal
Complexes
H
Me
Radical Process
H
H
secondary
primary
>
H2C CH2
> CH4 >
the stronger C-H bond is favored
>
>
H
A Remarkably Stable Pt(IV) Methyl Hydride
K
H
N N
Pt
N
N
B
H
N
CH3
HCl
N N
CH3
N
THF
RT
N
H
Pt
CH3
CH3
B N N
N
Tp’PtMe2H in the solid state begins to decompose at 140 °C
O'Reilly, S. A.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc. 1996, 118, 5684
Lewis Acid Generates a Vacant Site at Pt(II)
bu
bu
N
t
bu
+
t
t
N
Pt
CH3
B(C6F5)3
CH3
N
t
N
Pt
CH3
CH3B(C6F5)3-
bu
N
L
t
bu
+
t
CH3B(C6F5)3-
N
Pt
CH3
L
bu
Hill, G. S.; Rendina, L. M.; Puddephatt, R. J. J. Chem. Soc., Dalton Trans. 1996, 1809
K
Would
N N
Pt
N
N
B
H
N
N
CH3
CH3
react similarly?
C-H Activation at Pt(II)
K
H
N N
Pt
N
N
B
H
N
CH3
CH3
B(C6F5)3
N N
N
RH
25-60 oC
N
H
Pt
CH3
R
+ K[CH3B(C6F5)3]
B N N
N
R = Ph, C5H9, C6H11
the first stable Pt(IV) alkyl hydride formed by alkane oxidative addition to Pt(II)
Wick, D. D.; Goldberg, K. I. J. Am. Chem. Soc. 1997, 119, 10235
Proposed Mechanism of C-H Activation
K
N N
Pt
N
N
B
H
N
CH3
CH3
B(C6F5)3
-K[CH3B(C6F5)3]
N
CH3
RH
N
H
H
N N
Pt
N
N
B N N
H
N N
Pt
N
N
B
H
N
CH3
R
N N
Pt
N
N
B
H
N
N
CH3
R
N N
Pt
N
N
B
H
N
N
CH3
H
R
C-H Bond Activation by an Electron Rich
Metal Center
Arrested State
An Alkane Complex
Oxidative Addition
has occurred
Mechanism of Reductive Elimination Involves Alkane
Complexes
H
B
N N N
Ir
Me3P
Me3P
H
Rh
CH2CH3
H2N
Rh
(0.7)*
H
H
N N N
NC
(0.5)*
H
H2N
CH3
CH3
Pt
CH3
Cl
(0.62)*
(0.29)*
H
H
[M]
W
H
+
[M]
[M] + CH4
CH2
CH3
H
N
N
Rh
CH3
Me3P
Re
H
CH3
(0.8)*
H
CH3
+
(0.75)*
N
W
H
CH3
(0.77)*
(0.74)
Pt(IV) Dimethyl Hydride Reacts with Oxygen
H
O
H
N N
Pt
N
N
N
B
N
H
O
CH3
+
CH3
O2
1 atm
C6D6
RT
2 days
N N
Pt
N
N
N
B
N
H
O2
Tp'PtMe2D
C6D6
Tp'PtMe 2(OOD)
86% D
Wick, D. D.; Goldberg, K. I. J. Am. Chem. Soc. 1999, 121, 11900
CH3
CH3
A Pt(IV) Dialkyl Hydroxide
H
O
OH
O
N N
Pt
N
N
N
B
N
H
CH3
CH3
heat
C6D6
N N
Pt
N
N
N
B
N
H
CH3
CH3
Hydroxide is thermally stable
Catalytic Functionalization of Methane by Pt(II)
(bpym)PtCl2
CH3OSO3H + 2H2O + SO2
CH4 + 2H2SO4
220 °C
N
N
PtII
X
+
N
X
XPtII
X
N
HX
CH4
N
N
PtII
X
CH3
SO3 + 2HX
X = OSO3H
N
N =
N
N
N
N
CH3X
SO2 + H2O
X
N
PtIV
N
Periana, R. A. et al. Science 1998, 280, 560-564
X
X
CH3
Acknowledgements
University of Washington
The Goldberg Research Group
Funding
The National Science Foundation
The Union Carbide Innovation Program
The Dupont Educational Aid Program
The University of Washington
Synthesis of Dichloride Precursor
H
B
RhTp'(Cl)2CH3CN
CNCH2CMe3
N N N
C6H6, reflux
N N N
Rh
NC
Cl
Cl
80 % yield
1H-NMR
7
6
5
4
3
2
1
ppm
Structures of Isopropyl and
Cyclopropyl Complexes
Distribution
Distribution of
of Species
Species
100
90
80
% distribution
70
[Rh] Cl

60
k1 =
k2 =
k3 =
k4 =
50
40
30
k1
[Rh] H
[Zr]H2

1.0
3.8
1.8
5.7
-3
k2
[Rh] H

k3
k4
-1
X 10 s
-4 -1
X 10 s
-4 -1
X 10 s
-4 -1
X 10 s
[Rh] D

d5
20
10
0
0
50
100
150
tim e (m in)
200
250
300
Methyl Hydride Rearrangement
H
B
H
B
N N N
Keq = 6(1)
N N N
N N N
C6H6
Rh
NC
N N N
Rh
22 oC
CH3
D
NC
H
CH2D
d, 1.225 ppm
JRhH = 2 Hz
d, 1.236 ppm
JRhH = 2 Hz
1.28
1.28
1.30
1H{2H}-NMR
1.28
1.26
1.24
1.22
t=0
1.20
1.18
1.26
ppm
1.24
1.22
t=1h
1.20
ppm
1.26
1.24
1.22
t=3h
1.20
ppm
Reductive Elimination of Methane
H
B
H
B
N N N
C6D6
N N N
o
Rh
CH2D(H)
H(D)
NC
1H
d, -14.818 ppm
JRhH = 24 Hz
-14.2
-14.4
D
C6D5
-NMR
t, 0.134 ppm
*
0.20
-14.0
+ CH3D
N N N
22 C
16 h
Rh
NC
N N N
-14.6
-14.8
-15.0
-15.2
-15.4
ppm
0.18
0.16
0.14
0.12
0.10
0.08
0.06
ppm
Loss of Methane Shows Isotope Effects

C6D6
[Rh](CD3)(D)
C6H6
 [Rh](CH3)(H)

C6D6
[Rh](CH3)(H)
-4
-1
[Rh](C6D5)(D) + CD4
kobs = 2.48(17) × 10 s
[Rh](C6H5)(H) + CH4
kobs = 1.63(4) × 10 s
[Rh](C6D5)(D) + CH4
kobs = 1.52(4) × 10 s
-4
-1
-4
-1
ln(methyl hydride integration/total hydride integration)
0
[Rh](CH3)(H) in C6D6
-0.5
[Rh](CH3)(H) in C6H6
-1
kH/kD = 0.62(7)
Solvent kH/kD = 1.07(6)
-1.5
-2
[Rh](CD3)(D) in C6D6
-2.5
0
2000
4000
6000
8000
time (sec)
10000
12000
14000
16000
Loss of Methane is Dependent on
Benzene Concentration
[Rh](CH 3)(H)
C6D6 / C6F6
[Rh](C 6D5)(D) + CH 4
ln(methyl hydride integration/total hydride
integration)
0.1
[C6D6]
 2.82

5.64
 8.47

11.29
-0.4
-0.9
-1.4
-1.9
-2.4
0
5000
10000
15000
time (sec)
20000
25000
[C6D6]
kobs (× 10-4 s-1)
2.82
0.661(2)
5.64
1.04(3)
8.47
11.29
1.34(4)
1.52(5)
Double Reciprocal Plot
16000
0.0003
asymptote = 2.73 e-4
14000
0.00025
12000
1/kobs (sec)
kobs (sec-1)
0.0002
0.00015
10000
8000
6000
0.0001
4000
0.00005
2000
0
0
0
10
20
30
40
50
benzene concentration ([C6D6]) (M)
Plot is consistent with saturation behavior,
i.e. a reversible Keq followed by the rate
determining step
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
1/benzene concentration (1/[C6D6]) (1/M)
Plot of 1/kobs vs. 1/[C6D6] is linear
Kinetic Data are Consistent with an Alkane Complex
N
H
B
N
N
H
k1
CH3
k-1
H B
Rh
Rh
N
CNR
N
H
N
CH3
CNR
A
B
k2
N
H B
[C6D6]
N
fast
CNR
N
H B
CNR
N
Rh
Rh
N
H
N
CH3
d6
d6
fast
H
B
N
N
D
Rh
Ph-d5
N
CNR
Kinetic Scheme
Reductive Elimination from Pt(IV)
H
N
N
PtII
CH3
HCl
CH3
CD2Cl2
-78 °C
N
N
N
PtIV
N
CH3
CH3
below
RT
+
H
N
N
PtIV
ClCH3
CH3
- CH4
N
N
Cl
= tmeda, tbu2bpy
a 5-coordinate intermediate is required for
both reductive elimination and oxidative addition
Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 5961
Hill, G. S.; Rendina, L. M.; Puddephatt, R. J. Organometallics 1995, 14, 4966
PtII
CH3
Cl
Mechanism of Shilov Type C-H Bond Activation
H
PtII
+ RH
B-
PtIV
PtII
+ BH
R
R
Oxidative Addition followed by Deprotonation of a Pt(IV) Alkyl Hydride
BPtII
+ RH
PtII
H
R
Deprotonation of a Pt(II) Alkane Complex
PtII
+ BH
R
C-H Activation at Pt(II)
+
N
II
Pt
N
NC5F5
CH3
N
[BArf]-
+
30 atm 13CH4
N
NC5F5
N
85°C
PtII
NC5F5
13
[BArf]-
+ CH4
CH3
= tmeda
N
+
N
N
PtII
+ 13CH4
CH3
H
N
13
PtIV
N
CH3
+
+
N
CH3
PtII
N
NC5F5
CH3
H313C
- CH4
H
oxidative addition
N
PtII
13
+
CH3
sigma bond metathesis
N
Holtcamp, M. W.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1997, 119, 848
Effect of Radical Initiator/Inhibitor
Tp’PtMe2H
O2, 1 atm
Tp’PtMe2(OOH)
C 6D 6
Reaction
Conditions
Time (hr)
% Conversion of
PtTp'Me2H
% Yield of
PtTp'Me2(OOH)
50 C
Dark
1
4
100
50 C
17 mole % AIBN
Dark
1
31
100
Ambient
Temperature and
Light
48
100
98
Ambient
Temperature and
Light
40 mole % 1,4cyclohexadiene
48
46
94
Reaction of Pt(IV) Dialkyl Hydride
with Oxygen is Promoted by Light
Tp’PtMe2H
O2, 1 atm
C6D6/RT
Tp’PtMe2(OOH)
Reaction
Conditions
Time (hr)
% Conversion of
PtTp'Me2H
% Yield of
PtTp'Me2(OOH)
Ambient Light
48
100
98
Dark
48
14
100
High Intensity
Light
 > 345 nm
1
75
90
1
NR
NR
High Intensity
Light
 > 345 nm
No O2
Proposed Radical Mechanism
Initiation
In-In
2 In
[Pt]-H
[Pt]
+ In
+ H-In
Propagation
[Pt]
+ O2
[Pt]-OO
+
[Pt-OO]
[Pt]-H
[Pt]-OOH + [Pt]
Termination
[Pt]
+ [Pt]-OO
[Pt]-OO-[Pt]