1© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Nature of the Chemical Bond with applications to catalysis, materials
science, nanotechnology, surface science, bioinorganic chemistry, and energy
Lecture 21 February 23, 2011
CH4 CH3OH catalysis
William A. Goddard, III, [email protected] Beckman Institute, x3093
Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics,
California Institute of Technology
Teaching Assistants: Wei-Guang Liu <[email protected]>Caitlin Scott <[email protected]>
2© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Last time
3© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Mechanism: actual catalyst is a metal-alkylidene
R1 R1 R2 R2+
R1 R22
M
R2
R1 R3
M
R2
R1 R3
M
R2
R1 R3
Catalytically make and break double bonds!
OLEFIN METATHESIS
4© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Ru Olefin Metathesis BasicsRu Olefin Metathesis Basics
5© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Well-defined metathesis catalysts
Ru
PCy3
Ph
Cl
ClNN MesMes
Ru
PCy3
Ph
Cl
ClNN MesMes
R R
R=H, Ph, or -CH2-(CH2)2-CH2-
R R
R=H, Cl
NMo
PhCH3
CH3(F3C)2MeCO
(F3C)2MeCO
iPr iPrRuPCy3
PCy3
Ph
Cl
Cl
1 2 3 4Schrock 1991alkoxy imido molybdenum complexa
Bazan, G. C.; Oskam, J. H.; Cho, H. N.; Park, L. Y.; Schrock, R. R. J. Am. Chem. Soc. 1991, 113, 6899-6907
Grubbs 1991 ruthenium
benzylidene complexb
Grubbs 19991,3-dimesityl-imidazole-2-ylidenes
P(Cy)3 mixed ligand system”c
Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H. Tetrahedron Lett. 1999, 40, 2247-2250.
Wagener, K. B.; Boncella, J. M.; Nel, J. G. Macromolecules 1991, 24, 2649-2657
6© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Examples 2nd Generation Grubbs Metathesis Catalysts
General mechanism of Metathesis
7© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Ru-Methylidene Double Bond
Ru dx2 - C sp2 Ru-C Sigma bond
CH2 is triplet state with singly occupied and orbitals get spin pairing bond to Ru dx2 and bond to Ruxz
z
x
Ru dxz-C pzRu-C Pi bond
3B1 CH2
Ruxz
Ru2xx-yy-zz
Cz=Cp
C
8© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Ru-Methylidene Double Bond
Ru-C Sigma bond (covalent)
Ru dx2 - C sp2
Ru-C Pi bond (covalent)Ru dxz - C pz
CH2 is triplet state with singly occupied and orbitals get spin pairing bond to Ru dx2 and bond to Ruxz
z
x
Bond dist. Theory ExperimentRu-CH2 1.813 1.841Ru-Carbene 2.109 2.069
9© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Carbene sp2-Ru dz2 Don-Accep Bond
Ru-Carbene Sigma donor bond (Lewis base-Lewis acid)C sp2 Ru dz2
Singlet Carbene (Casey Carbene or Fisher carbene stablized by donation of N lone pairs, leads to LUMO
Planar N with 3 bonds and 2 e in pp orbital
Planar N with 3 bonds and 2 e in pp orbital
Singlet methylene or carbene with 2 bonds to C and 2 electrons in C lone pair but empty p orbital
Bond dist. Theory ExperimentRu-CH2 1.813 1.841Ru-Carbene 2.109 2.069
10© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Carbene sp2-Ru dz2 Don-Accep Bond
Ru-Carbene Sigma donor bond (Lewis base-Lewis acid)C sp2 Ru dz2
Carbene p- LUMO)Antibonding to N lone pairs
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Ru-dyz - Carbene py Don-Accep Bond
Carbene p- LUMO)Antibonding to N lone pairs
Ru dyz Lone Pair (Lewis base-Lewis acid)
Ru dyz Carbene py LUMO
Ru dyz Lewis Base
to Carbene py pi acid stabilizes the RuCH2
in the xy plane
This aligns RuCH2 to overlap incoming olefin
12© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Ru-CH2 * (antibonding) LUMO Acceptor for olefin bond Orients Olefin Perpendicular to plane
Ru dxy Lone Pair Want perpendicular to C-Ru-C planeAvoid overlap with NCN bondsOrients Methylidene Perpendicular to Plane
Because RuCH2 is perpendicular to plane, the emptyantibonding orbital overlaps the bonding pi orbital of the incoming olefin
Ru LP and Ru-CH2 Acceptor Orbitals
13© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Ru(CH2)Cl2(phosphine)(carbene)
Ru-Cl bonds partially ionic (50% charge transfer),
consider as RuII (Cl-)2
RuII: (dxz)1(dx2)1 (dxy)2(dyz)2(dz2)0
Ru (dx2)1 covalent sigma bond to
singly-occupied sp2 orbital of CH2
Ru (dxz)1 covalent pi bond to
singly-occupied pz orbital of CH2
( the CH2 is in the triplet or methylidene form)
Ru (dxy)2 nonbonding
Ru (dyz)2 overlaps empty carbene y orbital stabilizing RuCH2 in xy plane
Ru (dz2)0 stabilizes the carbene and phosphine donor orbitals
RuCH2 * (antibonding) LUMO overlaps the bonding orbital of incoming olefin stabilizing it in the confirmation required for metallacycle formation.
Ru Electronic Configuration
14© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
L
Cl2Ru
R
L
Cl2Ru
R
L
Cl2RuR
L
RuCl2
R
L
RuCl2
R
A
TSAB
B
TSBC
C
Generally Accepted Mechanism Generally Accepted Mechanism
E or Z olefin products
RN N
Cl2RuR
+ +
Olefin + [Ru]=CR [Ru]=CR' + Olefin
N N
Cl2Ru
15© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Metal [2+2] cycloaddition is thermally allowed
All-carbon [2+2]cycloaddition isforbidden
H
H
H
HHH
HOMO
LUMO
d orbital has differentphase overlaps; otherorbitals available
(more details to follow inupcoming lectures!)
Woodward-Hoffman rules still apply, but d-orbitals now participate
16© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Ph
O
O
O
O
N N
Ru ClCl
Ph
O
O
O
O
N N
RuCl
Cl
Ph
O
O
O
O
N N
RuCl
Cl
Product-Substrate exchange is rate determining step
B3LYPB3LYP
17© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
New material
18© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Catalyst Challenges for the Selective Chemistry needed for Sustainable Development
Enormous experimental efforts have been invested in solving these problems but better solutions are needed more quickly
I claim that Theory and Modeling are poised to provide guidance to achieve these goals much more quickly
Challenge: improved catalysts for industrial applications including
•Low temperature conversion of methane to fuels and organic feedstocks
•High selectivity and activity for converting alkanes to organic feedstocks
•Fuel cell cathode catalysts for the oxygen reduction reaction (ORR) with decreased overpotential, much less Pt, and insensitive to deactivation
•Fuel cell anode catalysts capable of operating with a variety of fuels but insensitive to CO and to deactivation
•A methane fuel cell (CH4 + H2O CO2 + power [8 (H+ and e-)]
•Efficient catalysts for photovoltaic production of energy and H2
•Efficient catalysts for storing and recovering hydrogen
•Catalysts for high performance Li ion and F ion batteries
19© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
•Propane ammoxidation - structure of new phases in Mixed Metal Oxide (Mitsubishi, BP) catalysts: MoVNbTaTeOx TUESDAY• butane MA over VOPO and ODH over V2O5
•Fuel Cell cathode electrocatalysis: nonPt and CoPt,NiPt alloys•Direct methanol fuel cell: PtRu-RuOHy at anode•CuSix catalysis of MeCl to Si(Me)2Cl2 and role additives•Organometallic Catalysts CH4 to liquids: Pt, Ir, Os, Re, Rh, Ru TODAY•Pd-mediated activation of molecular oxygen •Mechanism of the Wacker reaction in aqueous solution •Single Site Polymerization catalysts for polar monomers
Projects in Catalysis: First establish mechanism then use mechanism to
design improved catalyst
Ni
O
Al
O
Al
NN Cl
Cl
Cl
Cl
ClCl
Cl
Ni
O
Al
O
Al
NN
Cl
Cl
Cl
Cl
Cl
Ni
O
Al
O
Al+NN
ClCl
Cl
ClClCl
A
B
C
0.0+1.0
-13.6
E (kcal/mol)
20© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Role of Theory in Developing Catalysts
N N
N N
Pt
Cl
Cl1. Establish Mechanism of current catalysts:Use QM to predict all plausible reaction paths, Determine transition states (TS) and stable reaction intermediates (RI)Calculate vibrational frequencies (vf) to prove TS (one negative curvature) and RI Use frequencies to calculate entropy, Cp. Use QM and Poisson-Boltzmann to get free solvation energy. Get free energy at reaction temperatures
G = Eelec + ZPE + Hvib(T) + Hlib(T) –TSvib – TSlib + Gsolv Use to estimate ratesThis provides the conceptual framework to interpret experiments
2. Validation: Predict new experiments to test mechanism3. Lead discovery: Combinatorial Computational Rapid PrototypingIn silico search for new lead candidates for Ligands, Metals, Solvents
4. Experiments: optimize best predicted ligands and reaction conditions. Continue theory and simulation in collaboration with experimentsCritical to new role of theory: accuracy and reliability for novel systemsMust trust the theory well enough to do only 1 to 10% of the systemsFocus experiments on these 1% to 10% predicted to be best
0.0
+27.9
A
A'
kcal/mol
O Ir
N
N
CH4
HN
O
NHIrN
O
NIr
=
Cl
+34.6
C+26.7
B T2
+6.2
D
O Ir
N
N
CH4
HCl
O Ir
N
NH
Cl
CH2
H
H
O Ir
N
NH
Cl
CH3
H
O Ir
N
NH
Cl
CH3
H
O Ir
N
N
HCl
CH3
O Ir
N
NH
Cl
OH2
+0.7
W
+49.4
E
O Ir
N
NH
Cl
CH2
H
H
21© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Has theory ever contributed to catalysis development?
Case study:
New catalysts for low temperature activation of CH4 and functionalization
to form liquids (CH3OH)
Over last 30 years quantum mechanics (QM) theory has played an increased role in analyzing and
interpreting experimental results on catalytic systems
But has QM led to new catalysts before experiment and can we count on the results from theory to
focus experiments on only a few systems?
22© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
(NH3)2PtCl2TOF: 1x10-2 s-1
t½ = 15 minRate ok, but decompose far too fast. Why?
(NH3)2PtCl2TOF: 1x10-2 s-1
t½ = 15 minRate ok, but decompose far too fast. Why?
(bpim)PtCl2TOF: 1x10-3 s-1
t½ = >200 hoursNot decompose but rate 10 times too slowAlso poisoned by H2O productHow improve rate and eliminate poisoning
(bpim)PtCl2TOF: 1x10-3 s-1
t½ = >200 hoursNot decompose but rate 10 times too slowAlso poisoned by H2O productHow improve rate and eliminate poisoning
Experimental discovery: Periana et al., Science, 1998
Catalytica: Many $$$ trying to solve these problems experimentally, failed, cancelled project. Periana came to USC, teamed up with Goddard, Chevron funded. Found success
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
23© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
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 self-consistent. Calculate solvent forces on solute atomsUse these forces to determine optimum geometry of solute in solution.Can treat solvent stabilized zwitterionsDifficult to describe weakly bound solvent molecules interacting with solute (low frequency, many local minima)Short cut: Optimize structure in the gas phase and do single point solvation calculation. Some calculations done this way
Extremely important for these systems (pH from -10 to +30) in very highly polar solvents: accuracy of predicting Solvation effects in QM
Solvent: = 99 Rsolv= 2.205 A
Implementation in Jaguar (Schrodinger Inc): pK organics to ~0.2 units, solvation to ~1 kcal/mol(pH from -20 to +20)
The Poisson-Boltzmann Continuum Model in Jaguar/Schrödinger is extremely accurate
24© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
6.9 (6.7) -3.89 (-52.35)
6.1 (6.0) -3.98 (-55.11)
5.8 (5.8) -4.96 (-49.64)
5.3 (5.3) -3.90 (-57.94)
5.0 (4.9) -4.80 (-51.84)
pKa: Jaguar (experiment)
E_sol: zero (H+)
Comparison of Jaguar pK with experiment
25© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Protonated Complex(diethylenetriamine)Pt(OH2)2+
PtCl3(OH2)1-
Pt(NH3)2(OH2)22+
Pt(NH3)2(OH)(OH2)1+ cis-(bpy)2Os(OH)(H2O)1+
Calculated (B3LYP) pKa(MAD: 1.1)5.54.15.26.5
11.3
Experimental pKa
6.37.15.57.4
11.0
cis-(bpy)2Os(H2O)2 2+
cis-(bpy)2Os(OH)(H2O)1+
trans-(bpy)2Os(H2O)2 2+
trans-(bpy)2Os(OH)(H2O)1+
cis-(bpy)2Ru(H2O)22+
cis-(bpy)2Ru(OH)(H2O)1+
trans-(bpy)2Ru(H2O)2 2+
trans-(bpy)2Ru(OH)(H2O)1+
(tpy)Os(H2O)32+
(tpy)Os(OH)(H2O)21+
(tpy)Os(OH)2(H2O)
Calculated (M06//B3LYP) pKa
(MAD: 1.6)9.18.86.2
10.913.015.211.013.95.66.3
10.9
Experimental pKa
7.911.08.2
10.28.9
>11.09.2
>11.56.08.0
11.0
Jaguar predictions of Metal-aquo pKa’s
26© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Use theory to predict optimal pH for each catalyst
-40
-30
-20
-10
0
10
20
30
40
50
0 5 10 15 20pH
G (
kcal
/mol
)
LnOsII(OH2)(OH)2
LnOsII(OH)3-
LnOsII(OH2)2(OH)+
Predict the relative free energies of possible catalyst resting states as a function of pH.
Os
OH
OHN
N
NOH
LnOsII(OH2)3+2
LnOsII(OH2)(OH)2 is stable
LnOsII(OH)3-
is stable LnOsII(OH2)3
+2 is stable
LnOsII(OH2)2(OH)+ never most stable
27© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
-40
-30
-20
-10
0
10
20
30
40
50
0 5 10 15 20pH
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.
LnOsII
OH2
H3C
OH
H
LnOsII
OH
H3C
OH
H
Resting states
Insertiontransition states
Use theory to predict optimal pH for each catalyst
Optimum pH region
28© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
-40
-30
-20
-10
0
10
20
30
40
50
0 5 10 15 20pH
G (
kcal
/mol
)
32.6
34.6 40.0
37.9
34.6
we determine the pH at which an elementary step’s rate is maximized.
Resting states
Insertiontransition states
Best, 2 kcal/mol better than pH 14
Use theory to predict optimal pH for each catalyst
29© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
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. 27, No.19, 1988.
30© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Pipes and Meyer, Inorg. Chem. 1986, 25, 4042.Meyer, et al. Inorg. Chem. 1984, 23, 1845.
OsV
OsIV OsIII
OsII
(trpy)Os(OHn)3
1 Volt
0.5 V
Evaluating multi-oxidation state cycles for nucleophilic metals
Oxidation states VI→II are present within ~0.5 V window.Aqua ligands stabilize many oxidation states. Odd-electron oxidations are common.Ligands,anions influence the redox properties over a very wide range.
(trpy)(bpy)Os(OHn)
31© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
H(0 K) = -8.9 kcal/mol
2H2SO4 (at )8
N N
N N
Pt
Cl
OSO3HH
N N
N N
Pt
Cl
OSO3HHG(453 K) = -8.8 kcal/mol
H(0 K) = +6.3 kcal/mol
G(453 K) = +5.2 kcal/mol
N N
N N
Pt
Cl
OSO3H
2HSO4- (at )8
H2SO4 (at )8
HSO4- (at )8
H
H(0 K) = -8.9 kcal/mol
2H2SO4 (at )8
N N
N N
Pt
Cl
OSO3HH
N N
N N
Pt
Cl
OSO3HHG(453 K) = -8.8 kcal/mol
H(0 K) = +6.3 kcal/mol
G(453 K) = +5.2 kcal/mol
N N
N N
Pt
Cl
OSO3H
2HSO4- (at )8
H2SO4 (at )8
HSO4- (at )8
H
First Step: Nature of (Bpym)PtCl2 catalyst
H(0 K) = -8.9 kcal/mol
2H2SO4 (at )8
N N
N N
Pt
Cl
OSO3HH
N N
N N
Pt
Cl
OSO3HHG(453 K) = -8.8 kcal/mol
H(0 K) = +6.3 kcal/mol
G(453 K) = +5.2 kcal/mol
N N
N N
Pt
Cl
OSO3H
2HSO4- (at )8
H2SO4 (at )8
HSO4- (at )8
H
H(0 K) = -8.9 kcal/mol
2H2SO4 (at )8
N N
N N
Pt
Cl
OSO3HH
N N
N N
Pt
Cl
OSO3HHG(453 K) = -8.8 kcal/mol
H(0 K) = +6.3 kcal/mol
G(453 K) = +5.2 kcal/mol
N N
N N
Pt
Cl
OSO3H
2HSO4- (at )8
H2SO4 (at )8
HSO4- (at )8
H
H(0 K) = -8.9 kcal/mol
2H2SO4 (at )8
N N
N N
Pt
Cl
OSO3HH
N N
N N
Pt
Cl
OSO3HHG(453 K) = -8.8 kcal/mol
H(0 K) = +6.3 kcal/mol
G(453 K) = +5.2 kcal/mol
N N
N N
Pt
Cl
OSO3H
2HSO4- (at )8
H2SO4 (at )8
HSO4- (at )8
H
H(0 K) = -8.9 kcal/mol
2H2SO4 (at )8
N N
N N
Pt
Cl
OSO3HH
N N
N N
Pt
Cl
OSO3HHG(453 K) = -8.8 kcal/mol
H(0 K) = +6.3 kcal/mol
G(453 K) = +5.2 kcal/mol
N N
N N
Pt
Cl
OSO3H
2HSO4- (at )8
H2SO4 (at )8
HSO4- (at )8
H
Is H+ on the Catalytica Pt catalyst in fuming H2SO4 (pH~-10)?
In acidic media (bpym)PtCl2 has one protonIn acidic media (bpym)PtCl2 has one proton
H kcal/molG kcal/mol)
32© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
To discuss kinetics of C-H activation for (NH3)2Pt Cl2 and (bpym)PtCl2
Need to consider the mechanism
Mechanisms for CH activation
Electrophilic addition
Sigma metathesis (2s + 2s)
Oxidative addition Form 2 new bonds in TS
Concerted, keep 2 bonds in TS
Stabilize Occupied Orb. in TS
33© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
H(sol, 0K)kcal/mol
Electrophilic addition
Oxidative addition
Start
CH4 complex
CH3 complex
-bond metathesis
Use QM to determine mechanism: C-H activation step. Requires high accuracy (big
basis, good DFT)
3. Electrophilic Addition wins
(bpym)PtCl2
2. Rate determining step is CH4 ligand
association NOT CH activation!
1. Form Ion-Pair intermediate
Theory led to new mechanism, formation
of ion pair intermediate, proved with D/H exchange
34© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
N N
N N
Pt
Cl
OSO3H
CH4
N N
N N
Pt
Cl
CH2
H
H
N N
N N
Pt
Cl
CH2
H
H
N N
N N
Pt
Cl
H2SO4
CH3
+33.1
+27.4+32.4
+10.2
+35.4
A
C
B
T1OxidativeAddition
T2
T2b
kcal/mol
OSO3H
HO3SO
N N
N N
0.0
Pt
Cl
CH3HO3SO
HN N
N N
Pt
Cl
CH3HO3SO
HH
H
H
H
H
H
ElectrophilicSubstitution
C-H Activation Step for (bpymH+)Pt(Cl)(OSO3H) Solution Phase QM (Jaguar)
Oxidative addition
Start
CH4 complexForm Ion-Pair intermediate
CH3 complex
Electrophilic substitution
RDS is CH4 ligand association
NOT CH activation!
Differential of 33.1-32.4=0.7
kcal/mol confirmed with
detailed H/D exchange
experiments
35© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
L2Pt
Cl
Cl
L2Pt
CH3
ClL2Pt
CH3
Cl
OSO3H
OSO3H
C-H activation
oxidation
HX + OSO3H-
SO3 + 2H2SO4SO2 + H2O
CH3OSO3H functionalization
H2SO4
L2Pt
OSO3H
Cl
L2Pt
CH4
Cl+
X-
X = Cl, OSO3H
+CH4-CH4
+CH4
-CH4
methane complex
Pt(II)-CH3 complex
Pt(IV) complex
Theory based mechanism: Catalytic Cycle
Adding CH4 leads to ion pair with displaced anion
After first turnover, the catalyst is (bpym) PtCl(OSO3H) not
(bpym)PtCl2
Start here
1st turnover
Catalytic step
36
L2PtCl2 – Water Inhibition
Theory: Complexation of water to activated catalyst is 7 kcal/mol exothermic, making barrier 7 kcal/mol higher. Product formation 0Thus inhibition is a ground state effectChallenge: since H2O is a product of the reaction, must make the catalyst less attractive to H2O but still attractive to CH4
Theory: Complexation of water to activated catalyst is 7 kcal/mol exothermic, making barrier 7 kcal/mol higher. Product formation 0Thus inhibition is a ground state effectChallenge: since H2O is a product of the reaction, must make the catalyst less attractive to H2O but still attractive to CH4
Experimental Observation: Reaction strongly inhibited by water, shuts off as solvent goes from 102% to 96%Is this because of interaction of water with reactant, catalysis, transition state or product?
Experimental Observation: Reaction strongly inhibited by water, shuts off as solvent goes from 102% to 96%Is this because of interaction of water with reactant, catalysis, transition state or product?
Barrier 33.1 kcal/mol
Barrier 39.9 kcal/mol
37© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Pt
Cl
Cl
L
L
A weak Pt-Cl bond
facilitates
electrophilic substitution
less positive Pt leads to easier CH4 oxidation addition activation
more positive Pt makes electrophilic substitution easier.
A strong Pt-L bond
prevents precipitation
Lower oxidation state,
easier oxidation step
Lower oxidation state,
less water inhibition
Summary
38© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
A catalyst that can activate CH4 should even more easily activate CH3OH.
Marten Ahlquist
CH bond CH4 is 105 kcal/mol
CH bond of CH3OH is 94 kcal/mol
Product Protection, the Key to Developing High Performance Methane Selective Oxidation Catalysts,
M. Ahlquist, RJ Neilsen, RA Periana, and wag
JACS, just published
How can the Periana Catalyst work?
39© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Recall mechanism (1 mM of CH4 in solution)
N N
N N
PtIIH OSO3H
Cl
+
1
N N
N N
PtIIH OSO3H2
Cl
2+
2
N N
N N
PtH
OSO3H2
Cl
2+
3ts
CH4
27.5
N N
N N
PtIIH CH4
Cl
2+
4
N N
N N
PtCH3
Cl
5ts
H
H
2+
N N
N N
PtIVCH3
Cl
2+
6
H
H
18.1
N N
N N
PtCH3
Cl
7ts
H
H
27.2
OSO3H+
N N
N N
PtIIH CH3
Cl
2+
8
H
17.515.9
0.80.0 kcal mol-1
23.9
Mechanism for the C‑H activation of methane by the Periana-Catalytica catalyst. Free energies (kcal/mol) at 500 K including solvation by H2SO4.
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
40© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Next step: Oxidation of the PtII‑Me intermediate by sulfuric acid
N N
N N
PtIIH CH3
Cl
2+
8
H
17.5
N N
N N
PtIVCH3
Cl
2+
9
H
SO O
OH
11.8
N N
N N
Pt CH3Cl
2+
10ts
H
SO O
OH
N N
N N
PtIVS
Cl
2+
11
H
O O
OH
CH3
N N
N N
PtS
Cl
2+
12ts
H
O OH
OH
CH3
OSO3H
21.8
7.7
32.4
N N
N N
PtIIS
Cl
2+
H
O OH
OH
N N
N N
PtIIS
Cl
2+
H
O O
OH2
-3.6
3.7
N N
N N
Pt
S
Cl
2+
15ts
H
O O
OH2
17.6
N N
N N
PtIIOH2
Cl
2+
H
-18.9
13
14
16
Free energies (kcal/mol) at 500 K including solvation by H2SO4.
CH3-O-SO3H
SO2
Get CH3OSO3H + SO2 products
41
reaction path for C‑H activation of methyl bisulfate by the Periana-Catalytica catalyst.
N N
N N
PtH
O
Cl
2+
19ts
CH3
12.3
N N
N N
PtIIH O
Cl
2+
18
N N
N N
PtH
Cl
21ts
H2C
H
N N
N N
PtIIH
Cl
2+
20N N
N N
PtIVH
Cl
2+
22
H
2+
34.3
28.1 29.8
41.5
S
OOH
OCH3
SO
OHO
OS
H
O OHO
OS
O
OHO
OS
O
O
HOCH2
N N
N N
PtIIH OSO3H
Cl
+
1
N N
N N
PtIIH OSO3H2
Cl
2+
2
N N
N N
PtH
OSO3H2
Cl
2+
17ts
OSO3CH3
20.1
0.80.0
Free energies (kcal/mol) at 500 K including solvation by H2SO4.
41.5 kcal/mol Barrier react with CH3-O-SO3H27.5 kcal/mol Barrier react with CH4
27.2 kcal/mol Barrier react with CH3OH
Get product protection
42© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Proposed pathway for oxidation ofactivated CH3-O-SO3H
N N
N N
PtIVH
Cl
2+
22
H
29.8
OS
O
O
HO
N N
N N
Pt HCl
2+
23ts
H
31.6
OS O
OOH
N N
N N
PtIV
Cl
2+
24
H
24.3
OSO3H
H
N N
N N
PtCl
2+
25ts
HO
H SOOH
O
25.1
16.6
N N
N N
PtII
Cl
2+
26
HOSO3HH
N N
N N
PtIV
Cl
2+
27
H
17.0
OSO3H
SO O
OH
N N
N N
PtCl
2+
28ts
H
SO O
OH
29.7
OSO3H
N N
N N
PtIVS
Cl
2+
29
H
O O
OH
N N
N N
PtS
Cl
2+
30ts
H
O OH
OH
H2C
OSO3H
15.6
35.3
N N
N N
PtIIS
Cl
2+
H
O OH
OH
-0.7
13
OSO3H
OSO3HThe 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
43© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Activation of CH3OH by the Periana Catalyst
N N
N N
PtH
O
Cl
2+
32ts
H
-1.9
N N
N N
PtIIH O
Cl
2+
31
N N
N N
PtH
Cl
34ts
H
N N
N N
PtIIH
Cl
2+
33
N N
N N
PtIIH
Cl
2+
35
H
2+
27.2
21.2
14.9
25.2
CH2
CH2H
CH3
H
H
OH
C
OH
HH
H
(12.3)
(41.4)
(35.4)
(29.1)
(39.4)
N N
N N
PtIIH OSO3H
Cl
+
1
0.0
Free energies (kcal/mol) at 500 K including solvation by H2SO4.
include the energy for formation of free methanol from methyl
bisulfate,
Assuming free methanol,
44
CH4
k1 k2KP
k3
CH3OH CH3P
CO2CO2 kox = k2/(1+KP) + k3KP/(1+KP)
[prod](t) = [CH3OH] + [CH3P] = (k1PCH4/kox)[1-exp(-koxt)]
S(t) = (1 - exp(-koxt)) / koxt
0%10%20%30%40%50%60%70%80%90%
100%
1.E-08 1.E-06 1.E-04 1.E-02 1.E+00 1.E+02Product concentration [prod] (M)
Se
lect
ivity
KP = 0 KP →∞
"99%"
KP = 2x106
k1PCH4= 3.5x10-5s-1
t →
"100%"
KP = 2x107
k1PCH4= 3.7x10-4s-1
A (bpym)PtCl2 reaction in 102% sulfuric acid has a best selectivity of
80%, which is why we need dry sulfuric acid (large KP) and a large ratio
of k1:k3.
Begin with 100% selectivity, no product.
Meeting these requirements will be a challenge for less
electrophilic metals.
A simple kinetic model can be used to illustrate the dependence of selectivity and product concentration over the course of a batch
reaction:
45© copyright 2010 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Other metals (Ir, Rh, Pd?)
Other stabilizing ligands L
With an understanding of basic mechanistic steps, use QM to quickly test other ligands and metals computationally
Quantum Mechanics Rapid Prototyping (QM-RP)
Other activating Ligands X
Other solvents
Identify leads for further theory
For best cases do experiment synthesis,
characterization
46© copyright 2010 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Quantum Mechanical Rapid Prototyping
• QMRP: computational analogue of combinatorial chemistry• Three criteria for CH4 activation:
1. Thermodynamic Criterion: Energy cost for formation of R-CH3 must be less than 10 kcal mol-1. Fast to calculate because need only minimize stable M-CH3 Reaction Intermediate
2. Poisoning Criterion: Species must be resistant to poisoning from water (i.e. water complexation is endothermic) Fast to calculate because minimize only M-H2O intermediate.
3. Kinetic Criterion: Barrier to product formation must be less than 35 kcal mol-1. Test for minimized M-(CH4). Barrier only a few kcal/mol higher. Slower to calculate because of weakly bound anion and CH4, but minimize only intermediate.
4. Do real barriers only when 3 is less than 35 kcal/mol
Small set systems for lab experiment
Muller, Philipp, Goddard Topics in Catalysis 2003, 23, 81
Many cases of Metal, ligand,
solvent
1 2 3 4 experpilot
47© copyright 2010 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Tri-site ligands
However we considered alternate ligands in which the 3 coordination sites [(N,N,N) in this case] are be replaced by various other ligands such as C, O, P, S
Fe(II), sd6, S=0 (singlet, low-spin), cationB3LYP/LACVP* (LACVP** for )
FeN
FeN
NN
NN
-24.5
A
+3.8
CB
C
0.0
-3.4
B(1)
+
=
-0.6
N
MN
N
H
N
MN
NH
B(2)
N
MN
NH
N
MN
N
H
N
MN
NH
+9.1
w/o bulkysidegroups
with bulkysidegroups
-26.3
-8.6-13.5
-17.1
+0.4
EWe simplify the ligands to include the parts that affect the chemistry but not the modifications (ligands on the outer N such as mesityl, the embedding the middle N into an aromatic ring) used to protect and stabilize the catalyst under experimental conditions (but which are expected to have only a modest effect on controlling rates). We validated the accuracy of the simplified ligands by doing the Brookhart catalysts both ways.
We also consider various metals and oxidation states.
We considered first a class of tri-site ligands analogous to those studied by Brookhart in Fe and Cr based catalysts for olefin polymerization.
48© copyright 2010 William A. Goddard III, all rights reservedCh120a-Goddard-L21-10
0
10
20
30
40
N
CHHN Ir
OH2
HO OH
0.0
20.6
8.0-H2O
N
CHHN IrHO OH
N
CHHN IrHO
-OH-
Solvated (H2O)
Eliminate trans-effect by switching ligand central C to NGet some water inhibition, but
low ligand labilityContinue
Eliminate trans-effect by switching ligand central C to NGet some water inhibition, but
low ligand labilityContinue
Switch from IrIII NCN to IrIII NNC
49© copyright 2010 William A. Goddard III, all rights reservedCh120a-Goddard-L21
-20
-10
0
10
20
30
40
N
CHHN Ir
OH2
HO OH
0.0
28.9
8.0
N
CHHN Ir
OOHH3C
HH
N
CHHN IrHO OH
-H2O
N
CHHN IrH3C OH
OH2
-9.0
CH4 activation by Sigma bond metathesis
- Neutral species -Kinetically accessible with
total barrier of 28.9 kcal/mol
CH4 activation by Sigma bond metathesis
- Neutral species -Kinetically accessible with
total barrier of 28.9 kcal/mol
Solvated (H2O)
Further examine IrIII NNC
Passes Test
50© copyright 2010 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Oxidize with N2O prior to Functionalization
IrIII - NNC
-30
-20
-10
0
10
20
30
-9.0
24.5
-7.4
N
CHHN IrH3C OH2
O
N2
N
CHHN IrH3C OH
OH2
-19.8
N
CHHN IrH3C OH2
-OH-
+N2O
N
CHHN IrH3C OH2
O
-N2
Solvated (H2O)
Passes Test
Oxidation by N2OKinetically accessible
Oxidation by N2OKinetically accessible
51© copyright 2010 William A. Goddard III, all rights reservedCh120a-Goddard-L21-70
-60
-50
-40
-30
-20
-10
0
10
20
8.3
-2.1 -11.2
N
CHHN IrH3C O
O HH
-19.8
N
CHHN Ir
OHCH3
OH
N
CHHN IrH3C OH
OH
N
CHHN Ir
H3C OHO
H
-65.9
Thus reductive elimination from IrV
Is kinetically accessible
Thus reductive elimination from IrV
Is kinetically accessible
Solvated (H2O)
Re-examine Functionalization for IrIII NNC
Passes Test
N
CHHN IrH3C OH2
O
52© copyright 2010 William A. Goddard III, all rights reservedCh120a-Goddard-L21
A solutionIrIII – NNC
0.0
28.9
8.0
N
CHHN Ir
OOHH3C
HH
N
CHHN IrHO OH
-H2O
N
CHHN IrH3C OH
OH2
-9.0
N
CHHN Ir
OH2
HO OH+CH4
-9.0
24.5
-7.4
N
CHHN IrH3C OH2
O
N2
N
CHHN IrH3C OH
OH2
-19.8
N
CHHN IrH3C OH2
-OH-
+N2O
N
CHHN IrH3C OH2
O
-N2
8.3
-2.1 -11.2
N
CHHN IrH3C O
O HH
-19.8
N
CHHN IrH3C OH2
O
N
CHHN IrH3C OH
OH
N
CHHN Ir
H3C OHO
H
-65.9
CH activation
Oxidation
Functionalization
CH4 CH3OH
N
CHHN IrHO OH
N
CH
HN
Ir HOOH
OH
N
CHHN IrHO OH
N
CH
HN
Ir HOOH N
CHHN IrHO OH
N
CHHN IrHO OH
N
CH
HN
Ir HOOH
OH
N
CHHN Ir
OHCH3
OH
To avoid H2O poisoning, work in strong base instead of strong acid.Use lower oxidation states, e.g. IrIII and IrI
QM optimum ligands (Goddard) 2003Tested experimentally (Periana) 2009 It works
Experimental ligand
Predicted: Muller, Philipp, Goddard Topics in Catalysis 2003, 23, 81
53© copyright 2010 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Xray of IrIII NNC
Thermal ellipsoid plot of 1-TFA with 50% probability. Hydrogens, and benzene co-solvent removed for clarity. bond lengths (Å): bond angles (deg):
bond lengths (Å): Ir(1)-N(2) 2.017(6), Ir(1)-C(16) 2.078(8), Ir(1)-C(27) 2.174(9), Ir(1)-N(1) 2.164(6), Ir(1)-C(29) 2.081(11), Ir(1)-O(1) 2.207(6).
bond angles (deg): N(2)-Ir(1)-C(16) 78.7(3), N(2)-Ir(1)-C(27) 161.0(3), N(2)-Ir(1)-N(1) 76.8(2), C(16)-Ir(1)-N(1) 155.4(3), C(27)-Ir(1)-N(1) 84.2(3), C(29)-Ir(1)-O(1)
171.1(5).
Experimental realization of catalytic CH4 hydroxylation
predicted for an iridium NNC pincer complex, demonstrating
thermal, protic, and oxidant stability; Young, KJH;
Oxgaard, J; Ess, DH; Meier SK, Stewart T, Goddard WA, Periana RA; Chem. Comm.,
(22): 3270-3272 (2009)
54© copyright 2010 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Final step: QM for Experimental Ligand
enthalpy solvent corrections in kcal mol-1 (453K) for HTFA ( = 8.42 radius = 2.479 Å).
Chem. Comm., (22): 3270-3272 (2009)
Message: it took 2 years of experiments to synthesize the desired ligand and incorporate
the Ir in the correct ox. state. Periana persisted only because he was confident it
would work. Not practical to do this for the 1000’s of cases examined in QMRP
55© copyright 2010 William A. Goddard III, all rights reservedCh120a-Goddard-L21
From here on I would like to summarize each of the systems we understand, with orbitals and discussion
electronic structure
• Hg based
• MTO oxidation
56© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Electronic effects in CH activation by OsII-IV:
OsII: Already shows low barriers to C-H activation (Oxidative Addition)Liable to be oxidized by even weak oxidants or protonated
OsIII: (acac)2OsIII-Ph complex shows a low C-H activation barrierMore stable to oxidation (than OsII) and disproportionation (than OsIV)
OsIV: Has not shown low barriers in C-H activationProne to disproportionationWithin 2 electrons of OsVI, an oxidant useful for functionalization.
Os
O
O
O
OPhOH
OH-C6H6
q
Os
O
O
O
O
Ph
C6H6
q
Os
O
O
O
O
Ph
Ph
q
H Os
O
O
O
O
C6H6
Ph
q
Let’s consider this CH activation step as a function of oxidation state.
57© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
(acac)2OsII(Ph) in benzene:
• Anionic OsII: Highly nucleophilic, wants to get rid of electron density
• The d-orbital used to form the Os-H bond drops in energy (27 kcal/mol lower in 7-coordinate intermediate).
• The mechanism is Oxidative Addition (a stable Os-H intermediate appears) with almost no barrier.
Singlet OsII
0.0 kcal/mol
Inser tion TSSinglet OsII
2.3
OsO
O
O
O
OsO
O
OO
H
H
Os-H: 1.63 A
2.26
C=C: 1.44
2.25
2.06 2.09
1.47
OsIV intermediateSinglet OsII
1.8
OsO
O
OO
H
Os-H: 1.59 A
2.11
77o
2.13
C-H: 1.95
58© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
(acac)2OsIII(Ph) in benzene:
• The mechanism is a concerted oxidative hydrogen migration with a 14.1 kcal/mol barrier.• The d-orbital used to form the Os-H bond stays at the same energy during the reaction.
• Singly occupied orbital is ‘delta’ with respect to Os-H bond, since a doubly occupied d-orbital is used to bond to the hydrogen.
• 2.42 A Os-C bonds suggest moderate backbonding from Os d-orbitals to benzene.
OsO
O
O
O
OsO
O
OO
H
OsO
O
OO
H
H
Doublet OsIII
0.0 kcal/molDoublet OsIII
14.1 kcal/mol
Os-H: 1.59 A
2.42
C=C: 1.42
2.42
2.08 2.12
1.83
Spin density in transition state
59© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
(acac)2OsIV(Ph) in benzene:
• Cationic OsIV: not strongly nucleophilic (OsII) nor electrophilic (PtII) relative to CH4.
• The d-orbital used to form the Os-H bond rises in energy in the TS (16 kcal/mol), as it is pulled from the metal.
• Os d-orbitals do little back-bonding to benzene.
OsO
O
O
O
OsO
O
OO
H
OsO
O
OO
H
H
Triplet OsIV
0.0 kcal/molTriplet OsIV
24.7 kcal/mol
Os-H: 1.61 A
2.46
C=C: 1.42
2.86
2.03 2.10
1.79
Triplet OsIV
0.0 kcal/mol
60© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
CH activation by OsIII hydroxides:We’d like to work in an inert medium, like water.The higher oxidation state (e.g. OsIII vs OsII) instantly extends the catalyst’s stability another ~0.5V, but the CH activation barriers are much higher than for OsII-OH and OsIII-Ph:
Through several ligand/anion combinations, no OsIII-OH has yielded a calculated H-CH3 activation barrier <35 kcal/mol.
(acac)2OsIII(OH)2 in water:
OsIII
O
O
O
OOHOH
2A0.0 kcal/mol
OH-CH4
Os
O
O
O
OCH4
OH
2B31.8
Os
O
O
O
O
CH3
OH2
2D28.5
Os
O
O
O
O
CH3
OHH
2C54.9
Weaker base than OsII-OH
Bound more strongly than in OsII-OH
61© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
(acac)2OsIII(OH)2 in water (1M OH-):
• OH- is bound more strongly to OsIII (than to OsII) due to the decrease in d-p repulsion, so the coordination step is endothermic.
• The singly-occupied orbital “follows” the remaining hydroxide lone-pair, making the hydroxide less basic. Since the hydroxide lone-pair accepts the hydrogen from methane, the cleavage barrier is also high. (This contrasts the Os-Ph case, where a doubly occupied d-orbital was oriented to stabilize the hydrogen in the TS.)
• C-H activation mechanism becomes Electrophilic Substitution (-OH, not Os, accepts H).
• Net energy change (52→54) is still thermoneutral.
• Why does OsIII(OH) give a higher C-H activation barrier than OsIII(C6H5) and OsII(OH)?
OsIII
O
O
O
OOHOH
2A0.0 kcal/mol
OH-CH4
Os
O
O
O
OCH4
OH
2B31.8
Os
O
O
O
O
CH3
OH2
2D28.5
Os
O
O
O
O
CH3
OHH
2C54.9
Spin densities
62© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
stopped
63© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Catalytic cycle: Au in H2SO4/H2SeO4
Y=O
HX
CH4
Y
M CH3
MIII
X
XMI X
HX
CH4
MIII
X
CH3Y=O Y
X-
CH3X
X
X
Jones, Periana, Goddard, et al., Angew. Chem. Int Ed. 2004, 43, 4626.180°C, 27 bar CH4, TOF 10-3 s-1
Cycle: oxidation → CH activation →
SN2 attack
Accessibility of both AuI and AuIII oxidation states prevents deactivation due to oxidization of catalyst1. CH activation by electrophilic substitution. 2. Functionalization by nucleophilic attack by HSO4
-.
Problem: Inhibited by water
I
AuI to III
Act. CH4Act. CH4
AuI to III
Product.
64© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Consider AuIII in H2SO4/H2SeO4: CH activation by AuIII
Jones, Periana, Goddard, et al., Angew. Chem. Int Ed. 2004, 43, 4626.
Start with AuIII
Protonated AuIII
complex
Add CH4 to AuIII complex
H extracted by bound HSO4-
Assisted by solvent H2SO4
Form Au-CH3 bond to
AuIII complex
Equilibrium Complex
with Au-CH3
CH activation relies on solvent, H2SO4, or conjugate base.
65© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
AuIII in H2SO4/H2SeO4: Functionalization
Jones, Periana, Goddard, et al., Angew. Chem. Int Ed. 2004, 43, 4626.
Functionalization relies on solvent, H2SO4, or conjugate base.
HSO4- solvent
SN2 attack on Au-CH3 bond
CH3OSO3H product
Separate by adding H2O
66© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
General strategy to developing new catalysts
LnM-X
CH3OH
LnM-CH3
Identify and elucidate elementary mechanistic steps
for activation, functionalization/oxidation and
reoxidation that connect to provide a complete,
electronically consistent cycle.
+ HX
YO
CH4
½ O2
Y
CH Activation
func
tiona
lizat
ion
reox
idat
ion
67© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Electronegative Metals Pt, Au, Hg, Pd: ∙ good selectivity, rates, and stability∙ product protection by esterification -but-∙ inhibited by water and methanol∙ require strong oxidantsConsequently we shifted to the nucleophilic paradigm, which can coordinate CH4 under milder acid or concentrated base conditions.
Early successes in methane functionalization used the electrophilic
paradigm:N
N
Pt
Cl
Cl
N
NH3N
H3N
Pt
Cl
Cl
(NH3)2PtCl2TOF: 1x10-2 s-1
t½ = 15 min
(NH3)2PtCl2TOF: 1x10-2 s-1
t½ = 15 min
(bpim)PtCl2TOF: 1x10-3 s-1
t½ = >200 hours
(bpim)PtCl2TOF: 1x10-3 s-1
t½ = >200 hours
Pt: Periana et al., Science, 1998Au: Periana, wag; Angew. Chem. 2004Hg: Periana et al., Science, 1993
Pt AuIr Hg Os ReW
Pd AgRh Cd Ru Tc Mo
68© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
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 problemsNeed new strategy
69© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Pt AuIr Hg Os ReW
Pd AgRh Cd Ru Tc Mo
K+/Na+ OH- 1M OH- H2O 1M H+ H2SO4
(H2O) DMSO H2SeO3 H2SO4 H2SeO4
CH3O- CH3OH CH3OH2+
Electrophilic Nucleophilic
Solvent pH pH < 0pH = 14
Oxidant
Product protection
Ru, Re, Os, Ir are good nucleophilic metals for base or weak acid
70© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
CH4
We have identified 3 Mechanistic pathways
LnM-X
CH3X
LnM-CH3
CH3
HM
CH3
HMInsertion
Base-assistedSubstitutionM CH3
HX
M CH3
HX
New mechanisms for nucleophilic metals
NucleophilicElectrophilic
We are discovering new and manipulating old mechanistic steps that will be active for less electrophilic metals operating in aqueous solution.
CH ActivationFunctionalization
71© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Functionalization by nucleophilic attack (SN2)
(trpy)OsIV(OH)2(CH3)
SN2 barriers (reductive functionalization) very high for earlier (electron-rich) metals.
(bpy)IrIII
CH3
OHN
OH(bpy)Ir
H3C
pyr
OH
OH-
(bpy)IrI
pyr
OH
3.3a0.0 kcal/mol
3.3b49.5
3.3c12.4
HOCH3pyr
(trpy)OsIV
OH
OH
CH3(trpy)Os
OH
OH
H2C
OHSO2O
33.4a0.0 kcal/mol
13.4b67.8
(bpy)IrIII(pyr)(OH)2(CH3)
72© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Os
O
O
O
O
CH3
OH
OsO
OOO
OHO
Os
O
O OO
OH
CH3
Os
O
O
O
O
O
O
OsHO
CH3
O
Os
O
Os
O
O
O
O
O
O Os(acac)2
CH3Os
O
O
O
O
O
OOs(acac)2
CH3
Os
O
O
O
O
O
OOs(acac)2
CH3
Os
O
O
O
O
O
O
Os(acac)2
CH3
Os
O
O
O
O
O
O
Os(acac)2
CH3
OH-
0.0 kcal/mol
-27.923.0
8.3 37.0
41.0
33.6 -23.7
G298K, pH = 14Barriers are pH dependent.
This oxidant, [cis-(acac)2OsVI(O)2], is privileged.
Backside attack
MigratoryInsertion
3+2
3+2
Switch to less electronegative metals, e.g. Os
[Oxidant]
Functionalize (acac)2OsIV(CH3)(OH) using (acac)2OsVI(=O)(=O)
IVVI
73© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Os
O
O
O
O
CH3
OH
Os
O
O
O
O
O
O Os(acac)2
CH3
Os
O
O
O
O
O
O
Os(acac)2
CH3
Os
O
O
O
O
O
O
Os(acac)2
CH3
OH-
0.0 kcal/mol
31.9
8.8
46.1OsO O
OO
O
O
Os
HO CH3
OsO
OO
OO
O
Os
O
O
O
O
O
O
(acac)2Os
CH3
Os
O
O
O
O
O
OOs(acac)2
CH3
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-CH3 oxidation state (n = II, III, IV).
Functionalization of (acac)2OsIV(CH3)(OH)
[Oxidant]
Reactant M-CH3 bond
Oxidant LUMO accepting 2 electrons and CH3 in TS
74© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Functionalization using transfer of CH3 to Se
SN2 process
75© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
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, III and Roy A. Periana
CH Activation Functionalization
LMn-OH
LMn-CH3
CH4
H2O Y
YO
+ H2O
+ CH3OH
Net Reaction: CH4 + 1/2 O2 CH3OH
Oxidation
1/2 O2
Se
O
OHH3C
Se
O
OHHORe(CO)5-CH3
Re(CO)5-OH
Full cycle
76
Homogeneous CH4 functionalization: how to best choose new metals
HX + Electron-poor
methyl groups
Electron-rich methyl groups
Charge transfer
Our QM mechanistic studies for a variety of complexes from AuIII to ReI show the continuum of charge transfer to methane
77
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.
Pt
H
N
ClN
CH3
N
HN
ReO CH3
O
O
-46.4 kcal/mol
CH4
31.0
25.2
22.721.3
21.3
17.416.5
0.0
M OH2NN
CH3N
N
H2N
H2N
H2N
H2N
q+
2+
M OH2NN
CH3N
N
q+M = Re Ru Os
Re = MRuOs
CH activation and functionalization by nucleophilic d6 metalsM-CH3 polarization based on C1s chemical shift
78
Ongoing Work in Homogeneous CH4 Functionalization
M OHNN
OHN
N+ CH4
MOH
NN
HN
N
+1
CH3
-OH-X
X
X
X
X = NO2, H, NH2, O-
M = RuII, OsII, ReI
MOH
NN
H
N
N
+1
H3C
X
X
X
X
X
X
X
X
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 -donating ligand substituents.
MOH
NN
CH3N
N
X
X
X
X
MOH
NN
CH3N
N
X
X
X
X
O
Os(acac)2
O
Going forward, we are considering the kinetics of these steps using d5 and d4 metals and new coordinated bases (i.e. –NH2).
79
metal substituent
RuII X=H -1.1 0.0 5.2 31.1 12.7 n.a. n.a.
=NH2 -7.5 0.0 -1.7 23.6 9.4 n.a. n.a.
OsII X=H 7.8 0.0 11.1 41.8 21.7 31.7 30.7
=NH2 1.5 0.0 3.1 34.5 17.1 20.5 17.0
ReI X=H 4.8 0.0 4.0 41.9 26.5 26.9 18.5
=NH2 0.1 0.0 -0.6 36.6 23.6 19.4 7.1
Ongoing Work in Homogeneous CH4 Functionalization
(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.
OH2
L4M OH
OH
L4M OH L4M OH
H3C
L4M OH
HCH3
L4M OH2
CH3
L4M OH
H CH3
L4M OH
H
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.
insertionsubstitution
Insertion barriers can be tuned over an extreme range by varying the ligand and metal. Substitution barriers cannot be similarly tuned.
80
CH4 functionalization with homogeneous catalysts
• Going forward: Determine what combinations of Group 9 and 10 metals, ligands and nucleophiles will allow SN2 functionalization with thermally accessible barriers.
Nucleophilic attack
MCH3
SeOO
Se OHHO
O
H2O H3C Se OH
O
HX +
CH3
M
XCH3X
M CH3
O
Y
OCH3M
Y
H2O2
IO4-
PhIO
M CH3 O
M'
CH3O
M'
O
M'
Transalkylation
Baeyer-Villiger
Reductive elimination
Electrophilic attack
CH3M X- CH3X
Periodic tableBarr
ier
Reductive functionalization mechanisms (red. elim., SN2) well known for late metals (M-CH3
+).With Periana we have sought complimentary mechanisms appropriate for electron rich metals:
81
We explore functionalization mechanisms in which the oxidant is a higher oxidation state of the hypothetical CH activation catalyst:
CH3
G 27.6 kcal/mol
OsIIIHO
OH
OsVI
O
O
HO(trpy)OsIII(OH)2(CH3)
(trpy)OsVI(O)2(OH)+
+OsIV
O
O
O
OCH3
OHOsVI
O
O
O
O
O
O
Os
O
O
O
O
O
O Os(acac)2
CH3
G 23.0 kcal/mol
G 37.0 kcal/mol
+
OsIV O
O
O
O
CH3OH
OsVIO
O
O
O
O O
OsVI + OsIV
L = (acac)2 OsVI + OsIII
L = terpyridine
Going forward in homogeneous CH4 functionalization
82
Catalytic cycle: Au in H2SO4/H2SeO4
Y=O
HX
CH4
Y
M CH3
MIII
X
XMI X
HX
CH4
MIII
X
CH3Y=O Y
X-
CH3X
X
X
Jones, Periana, Goddard, et al., Angew. Chem. Int Ed. 2004, 43, 4626.180°C, 27 bar CH4, TOF 10-3 s-1
Cycle: oxidation → CH activation →
SN2 attack
Accessibility of both AuI and AuIII oxidation states prevents deactivation due to oxidization of catalyst1. CH activation by electrophilic substitution. 2. Functionalization by nucleophilic attack by HSO4
-.
Problem: Inhibited by water
I
AuI to III
Act. CH4Act. CH4
AuI to III
Product.
83© copyright 2011 William A. Goddard III, all rights reservedCh120a-Goddard-L21
Pt AuIr Hg Os ReW
Pd AgRh Cd Ru Tc Mo
Late Transition MetalsMechanistic steps sufficient to get through a complete cycle, with mechanisms for protection, are proven and understood.Plan: Use theory to address the likely performance-limiting aspect of each metal, then design the ligand, pH, and oxidant around the rate-limiting step.
Middle Transition MetalsNow couple our new functionalization mechanisms with our proven CH activation mechanisms using either nucleophilic substitution or insertion mechanisms with product protection by acid or base. Plan Use theory to identify and study scope of new functionalization mechanisms, and to study the effect of high pH on CH activation of CH4 and OCH3
-.
Plan for bringing to pilot new CH4 to liquids catalysts
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