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Transcript of © copyright 2009 William A. Goddard III, all rights reservedEEWS-90.502-Goddard-L15 1 Nature of the...
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EEWS-90.502-Goddard-L15 1© copyright 2009 William A. Goddard III, all rights reserved
Nature of the Chemical Bond with applications to catalysis, materials
science, nanotechnology, surface science, bioinorganic chemistry, and energy
William A. Goddard, III, [email protected] Professor at EEWS-KAIST and
Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics,
California Institute of Technology
Course number: KAIST EEWS 80.502 Room E11-101Hours: 0900-1030 Tuesday and Thursday
Senior Assistant: Dr. Hyungjun Kim: [email protected] of Center for Materials Simulation and Design (CMSD)
Teaching Assistant: Ms. Ga In Lee: [email protected] assistant: Tod Pascal:[email protected]
Lecture 22, November 24, 2009
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EEWS-90.502-Goddard-L15 2© copyright 2009 William A. Goddard III, all rights reserved
Schedule changesNov. 24, Tuesday, 9am, L22, as scheduled
Nov. 26, Thursday, 9am, L23, as scheduled
Dec. 1, Tuesday, 9am, L24, as scheduled
Dec. 2, Wednesday, 3pm, L25, additional lecture, room 101
Dec. 3, Thursday, 9am, L26, as scheduled
Dec. 7-10 wag lecturing Seattle and Pasadena; no lectures,
Dec. 11, Friday, 2pm, L27, additional lecture, room 101
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EEWS-90.502-Goddard-L15 3© copyright 2009 William A. Goddard III, all rights reserved
Last time
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EEWS-90.502-Goddard-L17 4© copyright 2009 William A. Goddard III, all rights reserved
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EEWS-90.502-Goddard-L17 5© copyright 2009 William A. Goddard III, all rights reserved
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EEWS-90.502-Goddard-L17 6© copyright 2009 William A. Goddard III, all rights reserved
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EEWS-90.502-Goddard-L17 7© copyright 2009 William A. Goddard III, all rights reserved
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EEWS-90.502-Goddard-L17 8© copyright 2009 William A. Goddard III, all rights reserved
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EEWS-90.502-Goddard-L17 9© copyright 2009 William A. Goddard III, all rights reserved
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EEWS-90.502-Goddard-L17 10© copyright 2009 William A. Goddard III, all rights reserved
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EEWS-90.502-Goddard-L17 11© copyright 2009 William A. Goddard III, all rights reserved
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EEWS-90.502-Goddard-L17 12© copyright 2009 William A. Goddard III, all rights reserved
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EEWS-90.502-Goddard-L17 13© copyright 2009 William A. Goddard III, all rights reserved
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EEWS-90.502-Goddard-L17 14© copyright 2009 William A. Goddard III, all rights reserved
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EEWS-90.502-Goddard-L17 15© copyright 2009 William A. Goddard III, all rights reserved
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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?
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(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
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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
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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
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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)
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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
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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
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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
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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 CycleAdding 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
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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
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New material
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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
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HN NHPt
O
HN NHPt
O
O OPt
HN NHIr
O
HN NHIr
O
O OIr
HN NHOs
O
HN NHOs
O
O OOs
HN
Au
NH O
Au
O
N
HN NHPt
N
HN NHIr
N
HN NHOs
N N
HN NHPt
N N
HN NHIr
N N
HN NHOs
HN
Pt
NH HN
Ir
NHNHHN
Ir
A few of the prototype ligand/metal sets evaluated by QMRP
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O
Ir NHHN
X
O
Ir NH
NH
X
O
Ir
XNH
NH
S
Ir
XNH
NH
S
Ir NH
NH
X
S
Ir NHHN
X
O
Ir OO
X
O
Ir OO
X
O
Ir
XOO
O
Ir NHHN
X
NHHN
O
N
N N
N
IrNH
NH
X
O
HN
HN NH
HN
Ir
X
HN NH
O
Ir NHHN
X
F3C CF3
O
Ir NHHN
X
F
F F
FO
Ir NH
NH
X
F
F
F F
F
F
More exotic ligand/metal sets evaluated by QMRP. Since calculations are fast, a couple of hours, can try wild guesses
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0.0
+51.3*
A
Ckcal/mol
HN NHPt
N
C
NPt
=
C Pt
N
N
Cl
CH4
*H(0)=49.0,G(298)=49.9
C Pt
N
N
CH3
H
Cl0.0
+34.6
A
C
kcal/mol
HN
N
NHPt
N
N
NPt
=
N Pt
N
N
Cl
CH4
N Pt
N
N
CH3
H
Cl
(NCN)Pt(II) (NCN)Pt(II) (NNN)Pt(II)+(NNN)Pt(II)+
E(A-C) too high for both complexesE(A-C) too high for both complexes
QMRP: PtII NCN and NNN ligands, reject
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0.0
+32.7
A
C
kcal/mol
HN NHOs
N
C
NOs
=
C Os
N
N
Cl
H3C C Os
N
NCH3
HCl
H 0.0
+39.1
A
C(2)
kcal/mol
C Pt
N
NCl
CH4
HN NHPt
N
C
NIr
=
Cl
C Pt
N
NCH3
HClCl
Cl
Cl
+8.0
C(1)
C Pt
N
N
CH3
HClCl
Cl
QMRP: OsII NCN and PtIV NNN ligands, rejectQMRP: OsII NCN and PtIV NNN ligands, reject
(NCN)Pt(IV)(NCN)Pt(IV)
E(A-C) too high for (NCN)Os(II), but acceptable
for (NCN)Pt(IV)
E(A-C) too high for (NCN)Os(II), but acceptable
for (NCN)Pt(IV)
(NCN)Os(II)
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0.0
-12.4
A
A'
kcal/mol
C Ir
N
NCl
CH4
HN
N N
NHIr
N
C
NIr
=
Cl
C Pt
N
NCH3
HCl
Cl
Cl
Cl
+17.7
C'
+14.6
B
+33.7
T2
+15.6
C''
C Ir
N
NCH3
Cl
Cl
H
C Ir
N
NCl
Cl
CH4
C Ir
N
N
Cl
Cl
H3CH
C Ir
N
N
Cl
Cl
H
H3C
+
+
+
+
C
+20.2
C Pt
N
NCH3
H
Cl
Cl
H
+
H
-23.0
W
C Ir
N
NCl
Cl
+
OH2
(NCN)Ir(III) system passes QM-RP tests 1 and 3,
but is not resistant to water (test 2)
QMRP: IrIII NCN, passes 1,3, fails 2, reject
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0.00.5
W A'
kcal/mol
N Ir
N
N
CH4
HN
N
NHIr
N
N
NIr
=
OH2
C'+5.4
B
+31.8
T2
+2.4
N Ir
N
NCH3
Cl
H
N Ir
N
N
Cl CH4
N Ir
N
N
Cl
H3CH
N Ir
N
N
Cl
H
H3C
(NNN)Ir(I) picked as focal point for more detailed
studies
(NNN)Ir(I) picked as focal point for more detailed
studies
(NNN)Ir(I) system passes all three tests!
QMRP: IrI NNN, passes 1,2,3 examine further(NNN)Ir(I) (NNN)Ir(I)
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34
N
NHHN Ir
Cl
N
NHHN Ir
ClH
+H3O+
-H2O
N
NHHN Ir
ClOH2
+ H2O
N
NHHN Ir
ClHH2O
+ H2O
0.0 -32.7-28.7
4.6
Ir(I) not compatible with acidic media – protonation to Ir(III) predicted to be
rapid and irreversible.
Ir(I) not compatible with acidic media – protonation to Ir(III) predicted to be
rapid and irreversible.
QMRP: further examination of IrI NNN.Not stable in acid media, reject
Oxidation state of IrI too lowmove to IrIII
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35
-10
0
10
20
30
40
NHHN Ir
OH2
HO OH
0.0
18.6
2.0-H2O
NHHN IrHO OH
NHHN IrHO
-OH-
Solvated in (H2O)
Even though (NCN)IrICl failed QC-RP tests, could (NCN)IrIII(OH)2 be viable?
Even though (NCN)IrICl failed QC-RP tests, could (NCN)IrIII(OH)2 be viable?
Very slight water inhibition, low ligand lability,
Both good
Very slight water inhibition, low ligand lability,
Both good
QMRP: IrIII NCN
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36
0
10
20
30
40
50
60
NHHN Ir
CH3
HO H
20.1
10.6
?
+CH4
NHHN IrHO
H3CH
NHHN IrHO
46.1
NHHN Ir
H3CHOH
?NHHN Ir
OCH3
HH
Unfavorable to have covalent Ir-CH3 bond trans to Ir-Ph bond
Oxidative addition Thermodynamically
Inaccessible. Thus reject
Unfavorable to have covalent Ir-CH3 bond trans to Ir-Ph bond
Oxidative addition Thermodynamically
Inaccessible. Thus reject Solvated (H2O)
QMRP: Further examine IrIII - NCN
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37-10
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
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38
-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
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39
-10
0
10
20
30
40
50
-9.0
44.3
-7.0
N
CHHN Ir
OOH2
CH3H
N
CHHN IrH3C OH
OH2-1.3
N
CHHN Ir
H3CHOOH2
N
CHHN IrH3C
OHOH2
Reductive Elimination to form CH3OHKinetically inaccessible
Reductive Elimination to form CH3OHKinetically inaccessible
Maybe problem is that IrIII -> IrI unfavorableNeed to Oxidize to IrV prior to functionalization?
Maybe problem is that IrIII -> IrI unfavorableNeed to Oxidize to IrV prior to functionalization?
Solvated (H2O)
Examine Functionalization for IrIII - NNC
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40
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
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41-70
-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
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42
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
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43
Experimental Synthesis of IrIII NNC system
N
N
t-Bu
t-Bu1. [Ir(C2H4)2Cl]2
C2H4CH2Cl2-50C
2. 25C 16h
N
N
t-Bu
t-Bu
IrClEt
1-Cl
AgTFACH2Cl2
48h
N
N
t-Bu
t-Bu
IrTFAEt
1-TFA
HTFA N
N
t-Bu
t-Bu
IrTFATFA
1-TFA2
HTFA/DTFA
N
N
t-Bu
t-Bu
IrTFATFA
1A
OHF3C
O
N
N
t-Bu
t-Bu
IrTFA
1B
OCF3
O
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)
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44
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)
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45
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
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46
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.
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47
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.
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48
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
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49
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
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50
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
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51
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
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52
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
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53
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
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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)
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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
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56
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
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57
Functionalization using transfer of CH3 to Se
SN2 process
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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
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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
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-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
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-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
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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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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?
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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
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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
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Proposed 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-SO3H
27.5 kcal/mol Barrier react with CH4
Get product protection
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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
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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,
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Simple kinetic model to determine overall selectivity
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
Kinetic model relating product protection and selectivity for the Periana-Catalytica catalyst
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Effect of product protection on selectivity and product concentration for the Periana
catalyst.
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
Selectivity and product conc. Catalytica reaction starting at 102% H2SO4
KP=0 no protection; KP=10∞ maximum protection. protection drops significantly already at 99%. 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
Commercial success if get here
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M06 leads to slightly better relative free energies (G298) (by 2 to 3 kcal/mol) and relative abundances of isomers of 5 in CH2Cl2 at 298K than B3LYP
Method Comparison in the Prediction of StableIsomers of Ru Olefin Metathesis Catalysts in Solution
RuCl
N N
Me Me
Cl
H
H
HRu
Cl
N N
Me
Me
Cl
H
H
HRu
Cl
N N
Me Me
ClH
H
H RuCl
N N
Me
Me
ClH
H
H
5a 5d5b 5c
Geometry B3LYP B3LYP M06-L B3LYP B3LYP M06-LExperiment
SP Energy B3LYP M06 M06 B3LYP M06 M06Structure Relative Energy (kcal mol−1) Relative Abundance 1H-NMR
5a 0.0 0.0 0.0 9.8 15.9 95.9 10
5b 0.36 0.44 2.21 5.4 7.6 2.3 4
5c 0.29 0.78 2.82 6.0 4.3 0.8 2
5d 1.35 1.64 2.70 1.0 1.0 1.0 1
5e 0.25 0.02 4.88 6.5 15.4 0.0 N.O.
5f 1.67 1.98 5.61 0.6 0.6 0.0 N.O.
5g 1.70 2.57 7.76 0.6 0.2 0.0 N.O.
Stewart, Benitez, O'Leary, Tkatchouk, Day, Goddard, Grubbs, J. Am. Chem. Soc., 2009, 131, 1931–1938.
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Geometry B3LYP B3LYP M06-L B3LYP B3LYP M06-LExperiment
SP Energy B3LYP M06 M06 B3LYP M06 M06
Structure Relative Energy (kcal mol−1) Relative Abundance 1H-NMR
3a 0.13 0.0 0.02.9 1.2 7.0 6.7 (syn)
3c 0.0 0.37 0.45
3b 0.75 0.66 1.151 1 1 1 (anti)
3d 0.40 0.04 0.95
ClRu
Cl
O
N NMe Me
ClRu
Cl
O
N NMe
Me
ClRu
Cl
O
N N
Me Me
ClRu
Cl
O
N N
Me
Me
3c 3d3a 3b
Benitez, Tkatchouk, Goddard Organometallics 2009, 28, 2643–2645.
Method Comparison in the Prediction of StableIsomers of Ru Olefin Metathesis Catalysts in Solution
M06 leads to slightly better (0.5 kcal/mol) relative free energies (G298) and relative abundances of isomers in CH2Cl2 at 298K than B3LYP
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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
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Ring closing metathesis (RCM)
Ring opening metathesis polymerization (ROMP)
Acyclic diene metathesis (ADMET)
M
R
M
R
M
R
M M M
n
n
Applications of olefin metathesis
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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 4
Schrock 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
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History of Olefin Metathesis Catalysts
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Examples 2nd Generation Grubbs Metathesis Catalysts
General mechanism of Metathesis
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Structure Grubbs Carbene Catalyst
CH2
P(iPr)3
Ru-CH2 1.813
Ru-Carbene 2.109
CH2-Ru-Carb 100.5 ºCl(1)-Ru-Cl(2) 174.5º
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Compare QM and (Xray)
• Bond Lengths (Å)• Ru-CH2 1.813 (1.841) Ru-P 2.506 (2.419)• Ru-Carbene 2.109 (2.069) Ru-Cl(2) 2.471 (2.383)• Ru-Cl(1) 2.467 (2.393) C(1)-N(1) 1.370 (1.366)• Carb-N(2) 1.370 (1.354) C(2)-C(3) 1.351 (1.296)• Bond Angles (deg)• CH2-Ru-Carb 100.5 (99.2) CH2-Ru-Cl(2) 90.0 (87.1)• Carb-Ru-Cl(2) 87.8 (86.9) CH2-Ru-Cl(1) 94.3 (104.3)• Cl(1)-Ru-Cl(2) 174.5 (168.6) CH2-Ru-P 93.9 (97.1)• Carb-Ru-P 165.6 (163.2) Cl(1)-Ru-P 89.4 (89.9)• Carb-N(1)-C(2) 111.2 (112.1) N(1)-C(1)-N(2) 104.0 (101.0)• Important Torsion Angles (deg)• Cl(1)-Ru-CH2-H 177.3 N(1)-Carb-Ru-Cl 75.7• Carb-Ru-CH2-H 88.6 N(1)-Carb-Ru-CH2 169.7
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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
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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
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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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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
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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
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Ru-CH2 * (antibonding) LUMO Acceptor for olefin bond
Ru dxy Lone Pair No special role
the empty RuCH2
antibonding orbital overlaps the bonding pi orbital of the incoming olefin IF it is perpendicular to plane
Ru LP and Ru-CH2 Acceptor Orbitals
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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
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2 plausible intermediates for Ruthenium Metathesis
Trans Cis
Trans is direct product of initiation. All previous mechanistic studies have assumed Trans.Either could explain propagation
Trans
Cis
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Previous mechanisms have assumed that the Ru-Cl bonds remain trans throughout the reaction “trans” products
To probe the mechanism Grubbs designed a ligand that could go into either cis or trans Cl structure
For this constrained ligand, cis is more stable than trans by 0.8 kcal/mol
But cis initiates more rapidly than trans
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Use DFT QM to determine Structures and Energetics for Isomerization between cis and trans
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Validation of DFT calculations
Ru
L Cl
Cl N
Ru
L Cl
ClN
Ru
L Cl
ClN
Ru
L Cl
N Cl
4 4d 5d 5
0 14.95 23.03 6.78
0 14.64 22.07 8.17
0 13.55 18.83 -1.120 11.67 17.62 -0.70
Gas phase
631G**6311G**++
Solvent phase
631G**6311G**++
G (kcal/mol)
Theory: polar solvent (ε>20) leads to 100% cis Thus can tune stereochemistry of product by solvent polarityNot tested experimentally
Experiment: K=3.5 ΔG = -0.78 kcal/molTheory: ΔG = -0.70 kcal/mol
CH2Cl2: ε=9.1,
R0=2.4A
Experiment: benzene solvent only observe trans ΔG > 2 kcal/molTheory: ΔG = 2.2 kcal/mol (ε=2.3, R0=2.6A)
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Analysis of results
The strong dependence on solvent polarity results from the enormous difference in the dipole moment from the wavefunctions of the complexes (in methylene chloride)
1.5 Debye for trans and 12.4 Debye for cisThis difference arises from the polarity in the Ru-Cl bonds, which cancel in the trans geometry. This marked difference in polarity translates to very different solvation energies calculated
14.8 kcal for trans and 22.7 kcal for cis, which dramatically increases the relative stability of the cis chloride structure.
Analysis of the cis-trans Chloride Isomerization Mechanism
Ru
NNMes MesCl
Cl NRu
NNMes MesCl
N Cl
is a much faster initiator than
RuL Cl
Cl N
RuL Cl
ClN
RuL Cl
ClN
RuL Cl
N Cl
0 15 23 7
0 14 19 -1
Gas phase
PBF/Dichloromethane = 9.1, solvent radius = 2.4A
14 KcalInitiation Energy 20 Kcal
Trans Cis
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Analysis of cis-trans Cl isomerization Rates of metathesis initiation
experimentally
Thus expect cis initiation should be much slower than trans: agrees with experiment
Analysis of the cis-trans Chloride Isomerization Mechanism
Ru
NNMes MesCl
Cl NRu
NNMes MesCl
N Cl
is a much faster initiator than
RuL Cl
Cl N
RuL Cl
ClN
RuL Cl
ClN
RuL Cl
N Cl
0 15 23 7
0 14 19 -1
Gas phase
PBF/Dichloromethane = 9.1, solvent radius = 2.4A
14 KcalInitiation Energy 20 Kcal
initiates much slower than
TransCis
0 11.7 17.7 -0.7 kcal/mol
Trans 11.7 barrier Cis 18.4 barrier
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