Lecture 22 February 25, 2011
Metal Oxide Catalysis
Nature of the Chemical Bond
with applications to catalysis, materials
science, nanotechnology, surface science,
bioinorganic chemistry, and energy
William A. Goddard, III, wag@wag.caltech.edu
316 Beckman Institute, x3093
Charles and Mary Ferkel Professor of Chemistry,
Materials Science, and Applied Physics,
California Institute of Technology
Teaching Assistants: Wei-Guang Liu <wgliu@wag.caltech.edu>
Caitlin Scott <cescott@caltech.edu>
Ch120a-Goddard-L22
© copyright 2011 William A. Goddard III, all rights reserved
1
Last time
Ch120a-Goddard-L22
© copyright 2011 William A. Goddard III, all rights reserved
2
Extremely important for these systems (pH from -10 to +30) in very highly polar
solvents: accuracy of predicting Solvation effects in QM
The Poisson-Boltzmann Continuum Model in Jaguar/Schrödinger is extremely accurate
Calculate Solvent Accessible Surface of the solute by rolling a
sphere of radius Rsolv over the surface formed by the vdW
radii of the atoms.
Calculate electrostatic field of the solute based on electron
density from the orbitals
Calculate the polarization in the solvent due to the
electrostatic field of the solute (need dielectric constant )
This leads to Reaction Field that acts back on solute atoms,
which in turn changes the orbitals. Iterated until selfconsistent.
Solvent:  = 99
Calculate solvent forces on solute atoms
Rsolv= 2.205 A
Use these forces to determine optimum geometry of solute in
Implementation in Jaguar
solution.
(Schrodinger Inc):
Can treat solvent stabilized zwitterions
pK organics to ~0.2 units,
Difficult to describe weakly bound solvent molecules
solvation to ~1 kcal/mol
interacting with solute (low frequency, many local minima)
(pH from -20 to +20)
Short cut: Optimize structure in the gas phase and do single
3
pointCh120a-Goddard-L22
solvation calculation. Some
calculations
done
this way
© copyright
2011 William
A. Goddard
III, all rights reserved
Comparison of Jaguar pK with experiment
pKa: Jaguar (experiment)
E_sol: zero (H+)
6.9 (6.7)
-3.89 (-52.35)
5.8 (5.8)
4.96 (-49.64)
6.1 (6.0)
-3.98 (-55.11)
5.3 (5.3)
-3.90 (-57.94)
Ch120a-Goddard-L22
5.0 (4.9)
4.80 (-51.84)
© copyright 2011 William A. Goddard III, all rights reserved
4
Jaguar predictions of Metal-aquo pKa’s
Protonated Complex
Experimental pKa Calculated (B3LYP) pKa(MAD: 1.1)
(diethylenetriamine)Pt(OH2)2+
6.3
5.5
PtCl3(OH2)17.1
4.1
Pt(NH3)2(OH2)22+
5.5
5.2
Pt(NH3)2(OH)(OH2)1+
7.4
6.5
cis-(bpy)2Os(OH)(H2O)1+
11.0
11.3
Calculated (M06//B3LYP) pKa
Experimental pKa
(MAD: 1.6)
2+
cis-(bpy)2Os(H2O)2
7.9
9.1
1+
cis-(bpy)2Os(OH)(H2O)
11.0
8.8
2+
trans-(bpy)2Os(H2O)2
8.2
6.2
1+
trans-(bpy)2Os(OH)(H2O)
10.2
10.9
2+
cis-(bpy)2Ru(H2O)2
8.9
13.0
1+
cis-(bpy)2Ru(OH)(H2O)
>11.0
15.2
2+
trans-(bpy)2Ru(H2O)2
9.2
11.0
1+
trans-(bpy)2Ru(OH)(H2O)
>11.5
13.9
2+
(tpy)Os(H2O)3
6.0
5.6
1+
(tpy)Os(OH)(H2O)2
8.0
6.3
(tpy)Os(OH)2(H2O)
11.0
10.9
5
Ch120a-Goddard-L22
© copyright 2011 William A. Goddard III, all rights reserved
Use theory to predict optimal pH for each catalyst
G (kcal/mol)
Predict the relative free energies of possible
catalyst resting states as a function of pH.
50
40
30
20
10
0
-10
-20
-30
-40
LnOsII(OH2)3+2
LnOsII(OH2)2(OH)+
LnOsII(OH2)(OH)2
LnOsII(OH)3LnOsII(OH2)3+2 LnOsII(OH2)(OH)2
is stable
is stable
0
Ch120a-Goddard-L22
5
10
LnOsII(OH)3is stable
LnOsII(OH2)2(OH)+
never most stable
15
20
pH
© copyright 2011 William A. Goddard III, all rights reserved
6
Use theory to predict optimal pH for each catalyst
G (kcal/mol)
pH-dependent free energies of formation for
transition states are added to determine the
effective activation barrier as a function of pH.
50
40
30
20
10
0
-10
-20
-30
-40
Insertion
transition states
Resting states
Optimum pH region
0
Ch120a-Goddard-L22
5
10
15
20
pH
© copyright 2011 William A. Goddard III, all rights reserved
7
Use theory to predict optimal pH for each catalyst
G (kcal/mol)
we determine the pH at which an elementary step’s rate
is maximized.
50
40
30
20
10
0
-10
-20
-30
-40
Insertion
transition states
34.6
40.0
34.6
32.6
37.9
Resting states
Best, 2 kcal/mol better than pH 14
0
Ch120a-Goddard-L22
5
10
15
20
pH
© copyright 2011 William A. Goddard III, all rights reserved
8
Predicted Pourbaix Diagram for Trans(bpy)2Ru(OH)2
• Black  experimental data
from Meyer,
• Red is from QM calculation (no
fitting) using M06 functional,
no explicit solvent
• Maximum errors:
– 200 meV, 2pH units
Experiment:
Dobson and Meyer, Inorg.
Chem. Vol.
No.19,
Ch120a-Goddard-L22
© copyright
201127,
William
A.1988.
Goddard III, all rights reserved
9
Experimental discovery: Periana
HN
Cl
et al., Science, 1998 Pt
N
3
H3N
H3N
Cl
NCl
Cl
Pt
N
N
Pt
H3N
N
Cl
Cl
N
Pt
Cl
(NH3)2PtCl2
TOF: 1x10-2 s-1
t½ = 15 min
Rate ok, but decompose far
too fast. Why?
Cl
N
(bpim)PtCl
N
2
-3
TOF: 1x10 s-1
t½ = >200 hours
Not decompose but rate 10 times too slow
Also poisoned by H2O product
How improve rate and eliminate poisoning
Two Platinum compounds (out of laaarge number examined) catalyze
conversion of methane to methylbisulfate in fuming sulphuric acid (102%)
CH4 + H2SO4 + SO3  CH3OSO3H + H2O + SO2
CH3OSO3H + H2O  CH3OH + H2SO4
SO2 + ½O2
 SO3
Catalytica: Many $$$ trying to solve these problems experimentally, failed, cancelled
project.
Periana
came to USC, teamed
up with2011
Goddard,
Chevron
funded.
Ch120a-Goddard-L22
© copyright
William A.
Goddard III,
all rightsFound
reservedsuccess
10
al/mol
N
N
Cl
H
Pt
3 K)N=
H
al/mol
H
N
N
N
OSO3H
N
Cl
Is H+ on the Catalytica Pt catalyst in fuming H2SO4 (pH~-10)?
Pt
H
N
First Step: Nature of (Bpym)PtCl2 catalyst
OSO3H
8
2HSO4- (at
)
14.1
2+
Cl
H N
(16.5)
Pt
H
N
Cl
2+
8
Pt
H N
Cl
H N
H(0 K) =
(0 K) =2HSO - (at )
OSO3H H
H
SO
HCl
2
4
4N
N
N
N
Cl
OSO
H
Cl
+6.3
kcal/mol
+6.1kcal/mol
H 3
OSO
H
3
Pt
11.6
Pt
H(0 K)(+4.9)
=
HN2HSO4 Cl
OSO3H
N = NH
OSO3H G(453NK) =
(10.6)
453 K)
Cl
N
N 8.6
H2SO4
N
N +6.3 kcal/mol H
+5.2 kcal/molCl Pt+
kcal/mol
Pt
H NH
(10.8)
+
N
Cl
H
OSO
H
H
OSO3H G(453 K) =
3
Pt
N
N
N
N
Pt
- +5.2 kcal/mol
N
Cl
2HSO
)
HSO4 (at )
4 (at
N
OSO3H
H
HCl
2SO4
H
H2SO4 (at )
OSO
H
-7.6
3 kcal/mol
OSO
H

2HSO
(at
)
HSOH
(at
)
3
4
4
-5.4
(G kcal/mol)
(-7.8)
H(0
K)
=
H(0
K)
=
HN2SO4 (at
)
H
SO
N
N
N
(-8.7)
2N
4 N
6.4
C
Cl
Cl
+6.3 kcal/mol H H2SO4
-8.9 kcal/mol
Pt
Pt
Pt (11.7)
H(0 K) =
Cl
H(0 K) = N
N H N Cl
N
N
N
N Pt H N
Cl
Cl
O
Cl
OSO3H
G(453
K)
= Pt N
OSO
H
G(453
K)
=
+6.3
kcal/mol
-8.9
kcal/mol
3
N
H
N
N
N
N
Pt
Pt
N
Cl Pt
+5.2
kcal/mol
-8.8 kcal/mol
N
OSO3H H
H
SO
HCl
2
4
H
2
H
SO
H
OSO3H
2 OSO
4 3H G(453 K) =
OSO3H G(453 K) =
N
N
N
N
N
-1.2N
2
H
2SO4
-1.6
+5.2
kcal/mol
-8.8 kcal/mol
2+

2HSO4 (
(15.5)
2H2SO4 (at )

HSO
(at
)
bpymH
4
2
In acidic media (bpym)PtCl
one proton112+
(15.8)
2 has
Ch120a-Goddard-L22
© copyright 2011 William A. Goddard III, all rights
reserved
bpymH
8
8
8
8
8
8
8
8
8
Mechanisms for CH activation
To discuss kinetics of C-H activation for
(NH3)2Pt Cl2 and (bpym)PtCl2
Need to consider the mechanism
H X
L
Pt
X
Pt
C
L
C
L
X
oxidative addition
C
H
L
X
X
H
Oxidative addition Form 2 new bonds in TS
L
X
L
L
Pt
Pt
L
X
X
C
L
X
H
X
L
X
Pt
C
L
H
L
Pt
C
L
X
Pt
L
C
X
X
 metathesis
Sigma metathesis (2s + 2s) Concerted, keep 2 bonds in TS
L
X
L
X
Pt
C
L
Pt
C
L
H
X
Ch120a-Goddard-L22
H
X
electrophilic
Electrophilic
addition
Orb. in TS12
© copyright
2011substitution
William
A. GoddardStabilize
III, all rightsOccupied
reserved
Use QM to determine mechanism: C-H
activation step. Requires high accuracy (big
basis, good DFT)
Theory led to new
mechanism, formation
of ion pair
intermediate, proved
with D/H exchange
1. Form Ion-Pair
intermediate
2. Rate determining
step is CH4 ligand
association NOT CH
activation!
(bpym)PtCl2
Start
0.0; A1
Ch120a-Goddard-L22
Oxidative
addition
45.6, TS2a
40.5, TS2b
38.0, TS1
-bond metathesis
Electrophilic
addition
36.0, TS2c
32.1; B
27.0; C1
22.1; C2
CH4
complex
20.9; C3
H(sol, 0K)
kcal/mol
3. Electrophilic
CH3 complex13
Addition
wins
© copyright 2011 William A. Goddard III, all rights reserved
C-H Activation Step for (bpymH+)Pt(Cl)(OSO3H)
Solution Phase QM (Jaguar)
N
RDS is CH4
ligand association
NOT CH
activation!
N
Cl
H
Pt
H
N
N HO3SO
N
N
Cl
CH3
Pt
T2b
H
+35.4
N HO3SO
N
H
CH3
+33.1
+32.4
+27.4
T1
N
N
B
Cl
Pt
H
H
N
H
HO3SO
N
(
kcal/mol
+10.2
N
Cl
Pt
0.0
C
H
H
N
A
N
H
CH2
H2SO4
OSO3H
N
N
Cl
Pt
H
N
N
Start
Ch120a-Goddard-L22
addition
ElectrophilicElectrophilic
substitution
Substitution
CH2
N
Oxidative
Oxidative
Addition
T2
OSO3H
CH4
CH4 complex
Form Ion-Pair
intermediate
N
N
Cl
Pt
H
N
N
Differential of
33.1-32.4=0.7
kcal/mol
confirmed with
detailed H/D
exchange
experiments
CH3
CHIII,3 complex
© copyright 2011 William A. Goddard
all rights reserved
14
Theory based mechanism: Catalytic Cycle
N
Start
here
N
L2Pt
Cl
Cl
1st
Pt
N
Adding CH4 leads to ion pair
with displaced anion
Cl
turnover
Cl
N
-CH4
After first turnover, the catalyst
is (bpym) PtCl(OSO3H) not
(bpym)PtCl2
+CH4
Catalytic step
Cl
+CH4
L2Pt
OSO3H
Cl
+
L2Pt
-CH4
-
X
CH4
H2SO 4
met hane complex
CH3OSO3H
C -H acti vati on
fun cti on al iz ati on
-
HX + OSO3H
X = Cl, OSO3H
OSO3H
Cl
Cl
oxi dation
L2Pt
CH3
OSO3H
P t(IV) complex SO 2 + H2O
Ch120a-Goddard-L22
L2Pt
CH3
P t(II)-CH
3 complex
SO 3 + 2H2SO 4
© copyright 2011 William A. Goddard III, all rights reserved
15
N
N
H
+35.4
N HO3SO
N
N HO3SO
N
N
N
N
N
CH3
L2PtCl2 – Water Inhibition
H
Pt
2b
H
Cl
T2b
CH3
H
+35.4
+33.1
+32.4
.4
+27.4
T1
Oxidative
Addition
2
Oxidative
Addition
Experimental Observation:
Reaction
strongly inhibitedT2
N
N
Cl
B
Pt goes
by water, shuts off as solvent
from 102% toElectrophilic
96%
H
H
Substitution
Is this because of interaction
ofH CH
water with reactant,
N
N
SO
catalysis, transition state orHO product?
ilic
on
2
3
+10.2
N
(a)
(
kcal/mol
C
H2SO4
N
N
N
Cl
N
H
N
N
CH3
N
N
)
N
H
CH2
OSO3H
N
N
Cl
N
H
H
N
HSO4- (at
CH4
N
OH2
N
Cl
Pt
Pt
Cl
8
H2O (at
C
H
H2SO4
H(0 K) =Pt
H
OSO3H
-6.8
N kcal/mol
N
8
N
N
A
N
OSO3H
N
Barrier
(b)
33.1 kcal/mol
Pt
H
Cl
H
+10.2
Cl
0.0
N
Pt
Pt
N
N
CH3
)
Barrier
39.9 kcal/mol
N
N
Cl
Pt
N
Cl
Pt
8
8
Theory:HComplexation
of water to activatedH catalyst is 7Cl kcal/mol
OSO3H
H(0 K) =
N
N
N
N
exothermic,
making
barrier -6.1
7 kcal/mol
higher.
Product
formation  0
kcal/mol
Thus inhibition is a ground state effect
H SO4 (at make
)
(at )
Challenge: sinceHHCl
the
2O is a product of the reaction, 2must
catalyst
(c) less attractive to H2O but still attractive to CH4
16
Summary
less positive Pt leads to easier CH4 oxidation addition
activation
more positive Pt makes electrophilic substitution easier.
L
Cl
Lower oxidation state,
Pt
easier oxidation step
L
A strong Pt-L bond
Lower oxidation state,
less water inhibition
Cl
A weak Pt-Cl bond
facilitates
prevents precipitation
Ch120a-Goddard-L22
electrophilic substitution
© copyright 2011 William A. Goddard III, all rights reserved
17
A catalyst that can activate CH4 should even more easily
activate CH3OH.
CH bond CH4 is 105 kcal/mol
CH bond of CH3OH is 94 kcal/mol
How can the Periana Catalyst
work?
Product Protection, the Key to Developing High
Performance Methane Selective Oxidation Catalysts,
M. Ahlquist, RJ Neilsen, RA Periana, and wag
Marten
Ahlquist
Ch120a-Goddard-L22
JACS, just published
© copyright 2011 William A. Goddard III, all rights reserved
18
Recall mechanism (1 mM of CH4 in solution)
Assuming a 1 mM of CH4 in
solution, reaction barrier
for methane coordination
27.5 kcal/mol,
Followed by insertion of Pt
into CH bond and
Reductive deprotonation to
give the platinum(II) methyl
intermediate
Add
CH4
Pt-CH
deprotonation
Mechanism for the C-H activation of methane by the Periana-Catalytica catalyst.
19
Free energies (kcal/mol)
500William
K including
by H2SO4.
Ch120a-Goddard-L22
© copyrightat2011
A. Goddardsolvation
III, all rights reserved
Next step: Oxidation of the PtII-Me intermediate by
sulfuric acid
Get CH3OSO3H + SO2 products
CH3-O-SO3H
Free energies (kcal/mol) at 500 K including solvation by H2SO4.
Ch120a-Goddard-L22
© copyright 2011 William A. Goddard III, all rights reserved
SO2
20
reaction path for C-H activation of methyl bisulfate
by the Periana-Catalytica catalyst.
41.5 kcal/mol Barrier react with CH3-O-SO3H
27.5 kcal/mol Barrier react with CH4
27.2 kcal/mol Barrier react with CH3OH
Get product
protection
Free energies (kcal/mol) at 500 K including solvation by H SO
.
2
4
21
Proposed pathway for oxidation of
activated CH3-O-SO3H
The rate limiting step in the oxidation of
methyl bisulfate is C-H cleavage (41.5) rather
than oxidation (35.3)
For methane the activation barrier is (27.5)
while the oxidation barrier is 32.4
Ch120a-Goddard-L22
© copyright 2011 William A. Goddard III, all rights reserved
22
Activation of CH3OH by the Periana Catalyst
include the energy
for formation of
free methanol from
methyl bisulfate,
Assuming free
methanol,
Ch120a-Goddard-L22
Free energies (kcal/mol) at 500 K
including
solvation
© copyright 2011 William A. Goddard
III, all rights
reservedby H2SO4. 23
Quantum Mechanics Rapid Prototyping (QM-RP)
With an understanding of basic mechanistic steps, use QM
to quickly test other ligands and metals computationally
Other metals (Ir, Rh, Pd?)
Other activating
Ligands X
Other
stabilizing
ligands L
Identify leads for further
theory
For best cases do
experiment synthesis,
characterization
Other solvents
Ch120a-Goddard-L22
© copyright 2010 William A. Goddard III, all rights reserved
24
Switch from IrIII NCN to IrIII NNC
Eliminate trans-effect by
switching ligand central C to N
Get some water inhibition, but
low ligand lability
Continue
40
30
N
HN
Ir
CH
HO
20
20.6
N
HN
N
10
Ir
HO
HN
Ir
HO
CH
OH
OH 2
CH
-OH-
OH
8.0
-H2O
0
0.0
-10 Ch120a-Goddard-L22
Solvated (H2O)
© copyright 2010 William A. Goddard III, all rights reserved
25
Further examine IrIII NNC
40
30
CH4 activation by Sigma
bond metathesis
- Neutral species Kinetically accessible with
total barrier of 28.9 kcal/mol
N
HN
H3C
28.9
CH
Ir
OH
H O
H
20
N
N
10
HN
Ir
HO
0
OH
OH 2
HN
Ir
HO
CH
-H2O
CH
OH
N
8.0
HN
Ir
H3C
0.0
-10
-20
CH
OH
OH 2
-9.0
Passes Test
Ch120a-Goddard-L22
Solvated (H2O)
© copyright 2010 William A. Goddard III, all rights reserved
26
Oxidize with N2O prior to Functionalization
IrIII - NNC
Passes Test
N
30
HN
H3C
Ir
CH
OH 2
O
24.5
20
10
+N2O
N
0
HN
Ir
H3C
-10
N2
N
CH
HN
OH
OH 2
-9.0
Ir
H3C
-OH-
CH
-N2
OH 2
N
-7.4
HN
Ir
H3C
-20
Solvated (H2O)
O
CH
OH 2
-19.8
-30
Ch120a-Goddard-L22
Oxidation by N2O
Kinetically
accessible
© copyright 2010
William A. Goddard
III, all rights reserved
27
Re-examine Functionalization for IrIII NNC
Passes Test
N
20
H3C
10
HN
H3C
0
Ir
CH
-2.1
-10
8.3
O H
O H
-60
O
H
-11.2
N
-30
-50
OH
-19.8
-20
-40
CH
Ir
HN
N
HN
N
HN
Ir
H3C
H3C
CH
OH 2
O
OH
Solvated (H2O)
-70
Ch120a-Goddard-L22
Ir
CH
OH
OH
HN
Thus reductive
elimination from IrV
Is kinetically
-65.9
accessible
© copyright 2010
William A. Goddard III, all rights reserved
N
Ir
CH
OH
OHCH3
28
CH activation
28.9
CH4  CH3OH
A solution
IrIII – NNC
N
N
HN
HN
Ir
HO
Ir
CH
CH
OH
OH 2
+CH4
8.0
-H2O
N
OH
HO
HN
H3C
0.0
Oxidation
N
Ir
CH
HN
OH
Ir
H3C
H O
H
OH
OH 2
-9.0
24.5
+N2O
N
-N2
N
N
HN
Ir
H3C
CH
HN
OH
OH 2
Ir
-7.4
-OH-
-9.0
CH
OH 2
H3C
HN
H3C
Ir
CH
OH 2
N
O
HN
N2
20
Ir
H3C
CH
OH 2
O
-19.8
10
To avoid H2O
CH
poisoning, work in
strong base instead of
strong acid.
Use lower oxidation
states, e.g. IrIII and IrI
QM optimum ligands
(Goddard) 2003
Tested experimentally
(Periana) 2009 It works
Functionalization
8.3
0
Experimental ligand
-2.1
-10
-19.8
-20
N
N
-30
HN
H3C
N
-40 HN
N
OH
Ir
Ir
HO
OH
OH
CH
HN
HO
CH
HN
N
Ch120a-Goddard-L22
-50
-11.2
Ir
H3C
O
CH
Ir
O H
O H
HN
N
CH HN
Ir
H3C
OH
OH
CH
Ir
H3C
CH
OH
O
H
OH 2
Predicted: Muller, Philipp, Goddard
©Topics
copyright
William A.
Goddard
in2010
Catalysis
2003,
23,III,81all rights reserved
N
HN
Ir
CH
OH
OHCH3
-60
-65.9
29
New material
Ch120a-Goddard-L22
© copyright 2011 William A. Goddard III, all rights reserved
30
Catalytic cycle: Au in H2SO4/H2SeO4
Product.
AuI to III
Act. CH4
Act. CH4
I
AuI to III
Cycle: oxidation →
CH activation →
SN2 attack
Problem: Inhibited
by water
Accessibility of both AuI and AuIII oxidation states
Jones, Periana, Goddard, et al.,
prevents deactivation due to oxidization of catalyst
Angew. Chem. Int Ed. 2004, 43,
1. CH activation by electrophilic substitution.
4626.
-1
2. Functionalization
attack
byA. HSO
180°C,
bar CH4, TOF 10-3 s31
Ch120a-Goddard-L22 by nucleophilic
© copyright 2011
William
Goddard
rights 27
reserved
4 .III, all
Consider AuIII in H2SO4/H2SeO4: CH activation by AuIII
O O
S
Enthalpy / kcal mol-1
CH4
O
O
HSO4-/H2SO4
AuIII
O
O
6.8
2
S
O O
O OH
1
H = 0.0
S
O
CH4
Au
O
H2SO4
Start with
O
OH
O S O
O
H
CH3
Au
O
O
SO2
HSO4-/H2SO4
O
SO2
AuIII
28.1
3
Protonated
AuIII
complex
Add CH4 to
AuIII complex
H extracted by
bound HSO4Assisted by
solvent H2SO4
CH3
HO3SO
Au
H2SO4
O
O
SO2
4
-13.6
Form
Au-CH3
bond to
AuIII
complex
HO3SO
CH3
Au
OSO3H
HO3SO
5
-18.6
Equilibrium
Complex
with
Au-CH3
CH activation relies on solvent,
Jones, Periana, Goddard, et al., Angew.
H2SO
32
Chem.III,
IntallEd.
2004,
43, 4626.
4, or conjugate base.
Ch120a-Goddard-L22
© copyright 2011 William A. Goddard
rights
reserved
AuIII in H2SO4/H2SeO4: Functionalization
O O
28.1
S
S
3
CH4O
O
O
S OH H O OH CH3OSO3H
O H
HSO4-/HO
AuIII
2SO4
C O S O
H
Au
H
O
O
O
CH3O
6.8
CH3 S
CH3
HO
HO3SO
O O
Au
S
2
Au
O
OHO
O SO
HO S
O
Au
CH
3
3 O OH
HO3SO
O
H
SO
O
2
4
O
O
1
O
Au
O
SO2 Au
S
Au
SO2 H = 0.0
6
SO
O
O
CH
OSO
H
OSO3H
HO3SO
3
32
HO3SO
-7.8
4
O4
product
HSO
/H
SO
CH
4
2
Au
5
4
6
O
5
S OH
-13.6
O
O
HSO4 solvent
Separate by
H2SO4 -18.6
O
-18.
SO2
7
adding H2O
SN2 attack on
-37.0
O
Enthalpy / kcal mol-1
O
HO
Au-CH3 bond
Functionalization relies on solvent,
H2SO4, or conjugate base.
Jones,
Periana, Goddard, et al.,
Angew. 2011
Chem.
Int Ed.
2004, 43,
4626.
Ch120a-Goddard-L22
© copyright
William
A. Goddard
III, all
rights reserved
33
General strategy to developing new catalysts
CH4
LnM-X
Y
½ O2
reoxidation
YO
functionalization
CH3OH
Identify and elucidate
elementary mechanistic steps
for activation,
functionalization/oxidation
and reoxidation that connect
to provide a complete,
electronically consistent
cycle.
CH Activation
LnM-CH3
+ HX
Ch120a-Goddard-L22
© copyright 2011 William A. Goddard III, all rights reserved
34
Mo Tc Ru Rh Pd Ag Cd
W
Re Os Ir Pt Au Hg
Early successes in methane
functionalization used the electrophilic
paradigm:
Electronegative Metals Pt, Au, Hg, Pd:
∙ good selectivity, rates, and stability
∙ product protection by esterification
-but∙ inhibited by water and methanol
∙ require strong oxidants
Consequently we shifted to the nucleophilic
paradigm, which can coordinate CH4 under
milder acid or concentrated base conditions.
Ch120a-Goddard-L22
(NH3)2PtCl2
TOF: 1x10-2 s-1
t½ = 15 min
(bpim)PtCl2
TOF: 1x10-3 s-1
t½ = >200 hours
Pt: Periana et al., Science, 1998
Au: Periana, wag; Angew. Chem. 2004
Hg: Periana et al., Science, 1993
© copyright 2011 William A. Goddard III, all rights reserved
35
Progress towards CH4 + ½O2→ CH3OH
• PtCl4= (Shilov)
(not commercial, requires strong oxidant)
• Au,Hg/H2SO4
(not commercial, inhibited by water,
Au requires strong oxidant)
• (bpym)PtCl2/H2SO4
(impressive, but not commercial, inhibited
by
water)
– 70% one pass yield
– 95% selectivity for CH3OSO3H
– TOF ~ 10-3 s-1, TON > 1000
• PdII/H2SO4
(modest selectivity for CH3COOH)
• (NNC)IrIII(OH)2
(requires strong oxidant)
Progress, but major problems
Need new strategy
Ch120a-Goddard-L22
© copyright 2011 William A. Goddard III, all rights reserved
36
Mo Tc Ru Rh Pd Ag Cd
W
Re Os Ir Pt Au Hg
Nucleophilic
pH = 14
K+/Na+ OH-
Electrophilic
Solvent pH
1M OHH 2O
1M H+
pH < 0
H2SO4
H2SO4
H2SeO4
Oxidant
(H2O)
DMSO
H2SeO3
Product protection
CH3O-
CH3OH
CH3OH2+
Ru, Re, Os, Ir are good nucleophilic metals for base or weak acid
Ch120a-Goddard-L22
© copyright 2011 William A. Goddard III, all rights reserved
37
We have identified 3 Mechanistic pathways
CH3X
LnM-X
CH4
Insertion
New mechanisms for
nucleophilic metals
Electrophilic
Nucleophilic
Base-assisted
Substitution
CH Activation
Functionalization
LnM-CH3
We are discovering new and manipulating old mechanistic steps that will be
38
active for less electrophilic
metals
operating
aqueous
solution.
Ch120a-Goddard-L22
© copyright 2011
William
A. Goddard in
III, all
rights reserved
Functionalization by nucleophilic attack (SN2)
(bpy)IrIII(pyr)(OH)2(CH3)
(trpy)OsIV(OH)2(CH3)
SN2 barriers (reductive functionalization) very high for earlier
(electron-rich)
Ch120a-Goddard-L22
© copyright 2011metals.
William A. Goddard III, all rights reserved
39
Switch to less electronegative metals, e.g. Os
Functionalize (acac)2OsIV(CH3)(OH) using (acac)2OsVI(=O)(=O)
3+2
VI
IV
Migratory
Insertion
[Oxidant]
3+2
Backside attack
Ch120a-Goddard-L22
G298K, pH = 14
Barriers are pH dependent.
40
This
oxidant,
OsVIreserved
(O)2], is privileged.
© copyright
2011
William A. [cis-(acac)
Goddard III, all2rights
Functionalization of (acac)2OsIV(CH3)(OH)
Reactant M-CH3 bond
[Oxidant]
Oxidant LUMO accepting
2 electrons and CH3 in TS
Electrophilic attack on methyl by the more stable [trans-(acac)2OsVI(O)2] is exciting.
Oxidation is consistently 2-electron in the backside attack mechanism, regardless of
Mn-CH
III, IV). 2011 William A. Goddard III, all rights reserved
Ch120a-Goddard-L22
© copyright
3 oxidation state (n = II,
41
Functionalization using transfer of CH3 to Se
SN2 process
Ch120a-Goddard-L22
© copyright 2011 William A. Goddard III, all rights reserved
42
Full cycle
O
Re(CO)
n 5-OH
LM -OH
Se
H3C
CH4
OH
+ H2O
YO
CH Activation
Functionalization
O
Y
H2O
n
LM -CH3
Se
HO
Oxidation
OH
1/2 O2
+ CH3OH
Re(CO)5-CH3
Net Reaction: CH4 + 1/2 O2
CH3OH
Catalytic Oxy-Functionalization of a Low Valent Metal Carbon Bond with Se(IV)
William J. Tenn, III, Brian L. Conley, Mårten Ahlquist, Robert J. Nielsen, ‡
Jonas
Oxgaard, William A. Goddard,
and
RoyA.A.Goddard
Periana
Ch120a-Goddard-L22
© copyright III
2011
William
III, all rights reserved
43
Homogeneous CH4 functionalization: how to best choose new metals
Our QM mechanistic studies for a variety of complexes from AuIII to
ReI show the continuum of charge transfer to methane
Charge transfer
Electron-rich
methyl groups
HX +
Electron-poor
methyl groups
44
CH activation and functionalization by nucleophilic d6 metals
M-CH3 polarization based on C1s chemical shift
The carbon 1s orbital energy is
an excellent measure of the
electron density on the
methyl carbon.
This illustrates the extremes of
the polarity scale, which
require very different
functionalization
mechanisms.
45
Ongoing Work in Homogeneous CH4 Functionalization
Insertion
Substitution
We modeled bipyridine complexes of RuII, OsII and
ReI to determine the dependence of ground states
(protonation), H3C-H activation barriers
(substitution and insertion) and functionalization
barriers on metal and p-donating ligand
substituents.
Going forward, we are considering the kinetics of
these steps using d5 and d4 metals and new
coordinated bases (i.e. –NH2).
46
Ongoing Work in Homogeneous
CH4 Functionalization
substitution
insertion
metal
substituent
RuII
X=H
=NH2
OsII
X=H
=NH2
ReI
X=H
=NH2
-1.1
0.0
5.2
31.1
12.7
n.a.
n.a.
-7.5
0.0
-1.7
23.6
9.4
n.a.
n.a.
7.8
0.0
11.1
41.8
21.7
31.7
30.7
1.5
0.0
3.1
34.5
17.1
20.5
17.0
4.8
0.0
4.0
41.9
26.5
26.9
18.5
0.1
0.0
-0.6
36.6
23.6
19.4
7.1
(bpy)2Ru(OH)2 complexes do not participate in insertion mechanisms (i.e., the products
are not a minimum on the potential energy surface), only in the substitution path.
(bpy)2Os(OH)2 complexes allow both pathways (each are identifiable saddle points).
However, the insertion pathway is preferred.
Electron-donating substituents labilize hydroxide, creating vacancies.
Insertion barriers decrease with the electron-donating ability of the substituent.
The catalyst’s susceptibility to oxidation also increases with the C-H activation rate.
After the resting state switches to Ru(OH)(OH2), the substituents weakly effect substitution barriers.
Insertion barriers can be tuned over an extreme range by varying the ligand and metal.
Substitution barriers cannot be similarly tuned.
47
CH4 functionalization with homogeneous catalysts
Reductive functionalization mechanisms (red. elim., SN2) well known for late metals
(M-CH3d+).
With Periana we have sought complimentary mechanisms appropriate for electron
rich metals:
Barrier
Baeyer-Villiger
Nucleophilic attack
Electrophilic attack
HX +
Periodic table
Reductive elimination
Transalkylation
• Going forward: Determine what combinations of Group 9 and 10 metals, ligands
and nucleophiles will allow SN2 functionalization with thermally accessible barriers.
48
Going forward in homogeneous CH4 functionalization
We explore functionalization mechanisms in which the oxidant is a
higher oxidation state of the hypothetical CH activation catalyst:
OsVI + OsIV
L = (acac)2
OsVI + OsIII
L = terpyridine
49
Catalytic cycle: Au in H2SO4/H2SeO4
Product.
AuI to III
Act. CH4
Act. CH4
I
AuI to III
Accessibility of both AuI and AuIII oxidation states
prevents deactivation due to oxidization of catalyst
1. CH activation by electrophilic substitution.
2. Functionalization by nucleophilic attack by HSO4-.
Cycle: oxidation →
CH activation →
SN2 attack
Problem: Inhibited
by water
Jones, Periana, Goddard, et al.,
Angew. Chem. Int Ed. 2004, 43,
4626.
180°C, 27 bar CH4, TOF 10-350
s-1
Plan for bringing to pilot new CH4 to liquids catalysts
Mo Tc Ru Rh Pd Ag Cd
W
Re Os Ir Pt Au Hg
Middle Transition Metals
Late Transition Metals
Now couple our new functionalization
Mechanistic steps sufficient to get
mechanisms with our proven CH
through a complete cycle, with
activation mechanisms using either
mechanisms for protection, are proven
nucleophilic substitution or insertion
and understood.
mechanisms with product protection by
Plan: Use theory to address the likely
acid or base.
performance-limiting aspect of each
Plan Use theory to identify and study
metal, then design the ligand, pH, and
scope of new functionalization
oxidant around the rate-limiting step.
mechanisms, and to study the effect of
high pH on CH activation of CH4 and
OCHCh120a-Goddard-L22
© copyright 2011 William A. Goddard III, all rights reserved
3.
51
Industrially Important Catalytic Processes
Many important organic chemicals are produced by catalytic
processes
 85% of industrial organic chemicals are produced from petroleum
and natural gas
 21% are produced by heterogeneous catalysis: allylic oxidation
(acrolein, H2C=CHCHO) and ammoxidation (acrylonitirle,
H2C=CHCN), epoxidation, aromatic oxidation
52

Reaction Mechanism Study for
Propane Oxidative Dehydrogenation
on the Cubic V4O10 cluster
Manufacturing processes for Olefins
Title
1. Thermal Dehydrogenation of Alkanes
C3H8
C3H6 + H2 E= 38.0 kcal/mol
Disadvantage: high temperature (>900 K); Low selectivity!!
2. Oxidative Dehydrogenation (ODH) of Alkanes
C3H8 + 1/2 O2
Catalyst
C3H6 + H2O E= -13.0 kcal/mol
Advantage: very thermodynamically favorable
Difficulty: the catalyst is still not understood
53
Unsupported Vanadium oxide
Selectivity: 31-18 %
(bulk V2O5)
Supported Vanadium Oxide (VOx/Al2O3, VOx/ZrO2)
Advantages: (a) higher mechanical strength
Selectivity: 90-29 %
(b) better thermal stability
(c) larger surface area
(d) highly dispersed VO4 unit
54
How model V=O chemistry for VV?
Want V=O with 3 V-O single bonds
Also want each singly bonded O to be bonded to two V
Do not want edges with different chemistry
Solution: periodicially infinite, but this is
computationally complex
How do finite system?
V4O10 cluster model
55
Cluster Model
V2O5(001) surface
RV-O(1)=1.58 Å
RV-O(2)=1.78 Å
RV-O(3)=1.88 Å
RV-O(3’)=2.02 Å (D-A bond)
V4O10 cluster model
RV-O(1)=1.57 Å
RV-O(2)=1.79 Å
56
Compare CH bonds propane
H3C—H 105 kcal/mol
H2MeC—H 100 kcal/mol
HMe2C—H 97 kcal/mol
Me3C—H 93 kcal/mol
Thus
H3C—CH2—CH3
100
97
Expect to extract the H
from the central C
57
Which O of V4O10?
Probe by bond H to them
DH = - xx kcal/mol
DH = - xx kcal/mol
Thus much better to attack
V=O bond
58
1.57
C-H Activation (propane)
1.79
1.28
1.29
1.68
+
Ts-1,2
23.9
0.0
RDS
1.86
1.01
1.73
2S
18.0
1.00
1.88
1.73
2T
18.1
59
1.00
1.24
1.44
1.73
2.38
1.86
1.41
1.88
1.90
1.21
2.46
1.48
1.64
B,4B
.2
4
1.88
Functionalization: form
propene + H2O
2T
18.1
Ts-3B,4C
11.8
IV-OH
V=O
 V2
Path
VIII-OH2
Path 3
PropylO-iPr
VIV-OH extract
H from Propyl
1.79
1.79
+
4.3
+ Propene product
2.05
2.03
2.03
1.49
1.49
1.79
2.03
+
4C
3B
3B
-19.0
-19.0
Path 3
3B
60
8.3
-19.0
Functionalization
1.41
1.22
45
1.00
1.88
1.78
2.38
1.73
1.86
1.41
1.21
1.48
1.64
A
Ts-3B,4B
12.2
2T
18.1
1.79
IV-OH
Path
V=O
 V2
Path 3
2 VIV-OH products
PropylO-iPr
2nd VV=O
extract H from
1.76
Propyl
2.03
+ Propene product
+
.4
1.74
1.79
1.84
1.49
+
2.03
1
1.76
3B
-19.0
4B
-4.3
3B
61
-19.0
Get propene product plus
either One VIII-OH2 Eact = 11.8 kcal/mol
or two VIV-OH Eact = 12.2 kcal/mol
Ultimately must get OH2 to desorb the
product H2O
But H2O bond to VIII very strong (xx
kcal/mol)
62
Instead, react O2 with VIII site
O2 has S=1 and VIII has S=1,
thus combining them leads to S=0, 1, 2
Form ground state cyclic peroxide singlet
Goes through V-O-O biradical with unpaired spin
on V and on outer O
This is initially triplet but crosses with singlet and
then forms cyclic peroxide
Now H2O desorbs easily (xx kcal/mol)
Cyclic VOO can also react with propane to form
new V-OOH plus i-Pr
63
Regeneration
of the Reduced Surface
1.41
1.85
1.41
1.41
1.77
1.77
1.81
1.85
2.29
1.77
2.29
5
6
5
1.03 1.22
1.43
1.79
1.46
1.83
1.82
1.35
1.99
2.05
7 Ts-6,7
1.46
1.03
1.46
1.79
1.70
2.35
1.821.76
2.05
7
8
64
VVOOH
Cyclic VOO react with propane to form new VOOH plus i-Pr
The V--OOHiPr then goes to HO—V--O-iPr
Which rearranges to position 2nd H above OH
Then transer this H to form H2O and V=O plus
propene
65
6
1.41
1.43 1.22
Regeneration
of the Reduced Surface
1.41
1.77
1.77
1.43 1.22
1.81
1.03
1.46
1.81
1.79
1.35
1.99
1.83
1.82
2.05
6
Ts-6,7
6
1.70
1.76
Ts-6,7
7
2.35
1.22
1.22
1.70
2.35
1.42
1.42
1.46
1.46
1
1.99
1.83
2.1
2.17
1.46
1.46
1.6
1.62
1.89
1.89
66
8
8
Ts-8,1
Ts-8,1
Potential Energy Surface
Methylene C-H bond of propane is activated
in the rate-determining step with the barrier
of 27.0 kcal/mol. (kinetic isotopic effects)
Theory says 29.6 kcal/mol
67
Propene,
H2O out
propane in
H
H
O
O
V
O
+ H2O
Complete Catalytic Cycle on isolated V2O5
O
O
O V
O V
O V
O
O
23.9
29.6
O
propane + O2  propene
V
O
O
O
30.0
24.2
CH3
H3C
Start here
O V
O V
O V
O
O
O
OH
CH3
H3C
J. Phys. Chem. C,
CH
111: 5115 (2007)
H
3
O
HO
V
H
O
O
O
O V
O V O V
O
O
CH3
Exper RDS 27.0
kcal/mol. (KIE)
Theory gets 29.6
kcal/mol
O
H3C
V
O
O
O
O V
O V
O V
O
O
30.8
26.9
propene out
H
CH3
H
V
O
O
O V
O V
O V
O
O
O
O
O
O
16.5
22.2
O
O
V
O
O
O V
O V
O V
O
O
V
O
O
O
O
H
O
O V
O V
O V
O
O
O
H2O
H2O out
H
O
O
O
V
O V
O V
O
O
O2
O
propane in
V
O
Net: 2 propane+O22 propene + 2 H2O
O
H
O
H
O
O
O
H2O product
cannot desorb
from V3+ site
O2 in
promotes
desorption of H2O
while reactivating
catalyst
68
stop
69