Towards More Realistic Molecular Modeling of Homogenous Catalysis:

Combined QM/MM and ab initio Molecular Dynamics Investigations

N

Ni+N

RRiPr

iPriPr

iPr

R

Tom K. Woo, Peter M. Margl, Liqun Deng, Luigi Cavallo, Tom Ziegler

Department of Chemistry, University of Calgary,

Calgary, Alberta, CANADA

Brookhart Polymerization CatalystC&EN Feb. 5, 1996:

“Polymer Catalyst System:Dupont Eyes New Polyolefin Business”

Brookhartcatalyst

highly linear to moderately branched

Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 2343.

• high MWs

• good activities

N

Ni+N

RRiPr

iPriPr

iPr

R

Brookhart Polymerization CatalystC&EN Feb. 5, 1996:

“Polymer Catalyst System:Dupont Eyes New Polyolefin Business”

Brookhartcatalyst

highly linear to moderately branched

Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 2343.

N

Ni+N

RRiPr

iPriPr

iPr

R

temperature: Temp branching

monomer pressure: [Et] branching

bulk of substituents: bulk branching MW

Polymerization Mechanisms

isomerizationisomerization

terminationtermination

insertioninsertion

(branched)(branched)

resting state

Ni

P

HNi

P

H

Ni

P

H

Ni

P

H

Ni

H2CP

H

Ni

CH2

H

P

NiCH3

P

Ni

CH3P

Polymerization Mechanisms

isomerizationisomerization

terminationtermination

insertioninsertion

(branched)(branched)

resting state

barriers: barriers: insertion < isomerization < termination

Ni

P

HNi

P

H

Ni

P

H

Ni

P

H

Ni

H2CP

H

Ni

CH2

H

P

NiCH3

P

Ni

CH3P

“REAL” systemPURE QM

model system

Pure QM Calculations

N

Ni+N

RRiPr

iPriPr

iPr

R

N

NiN

HH

HH

ADF Density Functional Code: Baerends and Ziegler

Becke 88 - Perdew 86 energies and gradients

static, gas phase simulation of model system

Ni

Pure QM Results - Pure QM Results - Calculated BarriersCalculated Barriers

insertion:

isomerization:

termination:

17.5 kcal/mol

12.8 kcal/mol

10.8 kcal/mol

Ni

Ni

Adding the Bulky LigandsAdding the Bulky Ligands

termination

Transition States

insertion

Combined QM/MM - Overview

QMregion

MM region

QM region (active site)

MO calculation

MM region

MM force field

• regions are treated simultaneously• QM - MM regions interact via vdW & Coulomb

• QM electronic system capped

“real” system QM system

NiH H

Ni

N

NiN

RRiPr

iPriPr

iPr

Maseras, F.; Morokuma, K. J. Comp. Chem. 1995, 16, 1170.

Insertion Profiles (kcal/mol)

14.7 13.2

10.0

19.4 16.8

6.7

TS

PURE QM

hybridQM/MM

Ni

P

H

Ni

P

H Ni

P

H

Orientation of the Aryl Rings

N

NiN

RRiPr

iPriPr

iPr

resting state &termination TS

insertion TS

Isomerization

15.3

-0.8

12.8

-1.8

PURE QM

hybridQM/MM

Ni

P

HNi

H2CP

H

NiCH3

P

Termination Profiles (kcal/mol)

TS

19.4

9.7

PURE QM

14.7 18.6hybridQM/MM

Ni

P

H Ni

P

H

Ni

P

H

Comparison with Experiment

Pure QM: 16.8QM/MM: 13.2

EXP: 10 - 11 kcal/mol

QM then MM: 18.5

insertion barrier

QM/MMExp.

insertion < isomerization < termination

insertion < isomerization < termination

Order of Barrier Heights

Comparison with Experiment

13.2 15.3 18.6

Pure QM: 16.8QM/MM: 13.2

EXP: 10 - 11 kcal/mol

QM then MM: 18.5

insertion barrier

Relative to Insertion

Comparison with Experiment

Pure QM (∆∆Hel)

QM/MM (∆∆Hel)

Exp. (∆∆G)

insertion isomerization termination

0.0

0.0

0.0

-4.0 -7.1

2.1

1.3* 5.6

5.4

*monomer capture may be rate limiting for branching process

Primary Effects of Bulky Ligands

• destabilizes resting state

• N-C(aryl) torsion important

• bulkier the groups, the higher the activity

•diimine methyl important in chain termination

N

NiN

CH3CH3iPr

iPriPr

iPr

Insertion:

• bulkier the groups, the higher the MW

termination:

• monomer capture may dominate

isomerization:

Combined QM/MM ab initio MDCoupled AMBER with Car-Parrinello AIMD code

PAW code - Peter Blöchl of IBM Zürich

Becke 88 - Perdew 86 QM potential

300 K simulation temperature

slow growth simulation

2 weeks on an IBM 3CT workstation with 64 MB ram

Ni

P

HNi

P

H

N

NiN

CH3CH3iPr

iPriPr

iPr

H

Ni

P

H

40 000 time step simulations

Reaction Barriers with AIMD•thermodynamic integration free energy barriers

•reaction coordinate constrained during dynamics

•RC slowly varied, leading system over barrier

•relative free energies along RC calculated by:

ΔF =∂E

∂λ λ ,Tλ∫ dλ

Termination BarrierExp: 15-16 kcal/mol

Brookhart Termination Movie

QuickTime™ and aAnimation decompressor

are needed to see this picture

Conclusions

implemented QM/MM ab initio molecular dynamics method

determined free energy barrier with QM/MM AIMD

methods show good promise

QM/MM boundary is dubious

successfully applied QM/MM method to transition metal based homogenous catalysis

Future Work

• multiple time step AIMD

• electrostatic coupling in AIMD for solvent simulations

• validation and calibration of QM/MM method

• simulation of monomer trapping in Brookhart catalyst

Acknowledgments• NSERC of Canada• Novacor Chemicals of Calgary

Alberta Heritage Scholarship Fund

Killam Memorial Foundation

NSERC• TKW

Visit our Home Page at:http://www.chem.ucalgary.ca/groups/ziegler

• Maurice Brookhart

• Peter Blöchl at IBM Zürich

• PMMAustrian Fonds zur Förderung deWissenschaftlichen Forschung