© copyright 2009 william a. goddard iii, all rights reservedeews-90.502-goddard-l15 1 nature of the...

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


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


Last time

16

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?

17

(NH3)2PtCl2TOF: 1x10-2 s-1

t½ = 15 minRate ok, but decompose far too fast. Why?


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


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

18

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

19

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

20

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


2H2SO4 (at )8

N N

N N

Pt

Cl

OSO3HH

N N

N N

Pt

Cl



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


2H2SO4 (at )8

N N

N N

Pt

Cl

OSO3HH

N N

N N

Pt

Cl



G(453 K) = +5.2 kcal/mol

N N

N N

Pt

Cl

OSO3H

2HSO4- (at )8

H2SO4 (at )8

HSO4- (at )8

H


2H2SO4 (at )8

N N

N N

Pt

Cl

OSO3HH

N N

N N

Pt

Cl



G(453 K) = +5.2 kcal/mol

N N

N N

Pt

Cl

OSO3H

2HSO4- (at )8

H2SO4 (at )8

HSO4- (at )8

H


2H2SO4 (at )8

N N

N N

Pt

Cl

OSO3HH

N N

N N

Pt

Cl



G(453 K) = +5.2 kcal/mol

N N

N N

Pt

Cl

OSO3H

2HSO4- (at )8

H2SO4 (at )8

HSO4- (at )8

H


2H2SO4 (at )8

N N

N N

Pt

Cl

OSO3HH

N N

N N

Pt

Cl



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)

21

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

22

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

23

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

24

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

25

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

26

New material

27

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

28

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

29

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

30

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

31

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)

32

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

33

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)

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

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

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

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

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

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

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

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

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

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)

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)

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

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.

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.

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

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

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


t½ = 15 min


t½ = 15 min


t½ = >200 hours


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

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

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

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

54

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)

55

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

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

57

Functionalization using transfer of CH3 to Se

SN2 process

58

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

59

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

60

-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


Optimum pH region

61

-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


62

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

63

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?

64

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

65

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

66

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


41.5 kcal/mol Barrier react with CH3-O-SO3H

27.5 kcal/mol Barrier react with CH4

Get product protection

67

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

68

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


include the energy for formation of free methanol from methyl

bisulfate,

Assuming free methanol,

69

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

70

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

71

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.

72

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

73

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

74

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

75

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

76

History of Olefin Metathesis Catalysts

77

Examples 2nd Generation Grubbs Metathesis Catalysts

General mechanism of Metathesis

78

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

79

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

80

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

81

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

82

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

83

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

84

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

85

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

86

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

87

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

88

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

89

Use DFT QM to determine Structures and Energetics for Isomerization between cis and trans

90

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)

91

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

92

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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