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A Combined Density Functional Theory and Molecular Mechanics Study of Iron(II)- and

Cobalt(II)- Based Catalysts for the Polymerization of Ethylene

Liqun Deng, Peter Margl, and Tom Ziegler*

Department of Chemistry, University of Calgary

Calgary, Alberta, Canada T2N 1N4

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AbstractThe new family of olefin polymerization catalysts

based on iron and cobalt bisimino pyridine complexes discovered by Brookhart’s and Gibson’s groups have been investigated by means of theoretical model calculations. A density functional theory/molecular mechanical coupling was used to expose the differences between the original catalysts {[2,6- (ArN=C(Me))2C5H3N]MC3H7}+ and their generic pendants {[2,6-(HN=CH)2C5H3N]MC3H7}+ (Ar = 2,6-C6H3(i-Pr)2; M = Fe, Co). It has been shown that the activity of the original catalysts are inhibited by steric crowding that imposes barriers on olefin capture and internal rearrangements, while at the same time lowering the insertion barrier and increasing the chain termination barriers.

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V.C. Gibson M. Brookhart

- Robust - Low-cost - Simple to make

- High activities- High selectivity

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Experimental Observations

1 R = R’ = i-Pr2 R = R’ = Me3 R = t-Bu, R’=H4 R = i-Pr, R’ = H5 R = Et, R’ = H6 R = Me, R’ = H

Fe/Co catalysthighly linear high density

Gibson, V. C. et al. Chem. Commun., 1998, 849.Small, B.L.; Brookhart, M.; Bennett, A.M.A. J. Am. Chem. Soc. 1998, 120, 4049.Small, B.L.; Brookhart, M.; J. Am. Chem. Soc. 1998, 120, 7143.

N

NFe

MeMe

NR

R'R'

R

• monomer pressure: [Et] activities: Fe Co no change

• bulk of substituents: bulk MW no branching

• Metals : activities of Fe complexes > activities of analogous Co ones

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P

Hβ

P

Hβ

P

Hβ

P

Hβ

π-complex

Fe +

P

Hβ

Fe +

N

N

N

Fe +

Fe +

Fe +

2

3TS[2-3]

TS[2-7] 7BHT leads chain termination/branching

Propagation by ethylene insertion

N

N

N

N

N

N

N

N

N

N

N

N

H2C P

Hβ

Hβ

H2CP

Hβ

CH 2

P

Fe +

Fe +

Fe +

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BHE leads chain brainching/terminationcationic alkyl complex

N

N

N

N

N

N N

N

N

H

HβFe +

N

N

N

H P

Polymerization Mechanisms

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Computational Models

‘Real’ System Generic System

Pure DFT Calculations

N

NNH H

HH

M

N

NiPr

iPr

MeMe

NiPr

iPr

M

M = Fe, Co

• DFT part -the generic system• MM part - Me and 2,6-iPr(C6H4)

Combined DFT/MM Approach:

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Computational Methods

• BP86 for DFT levels of theory• Basis sets: double- plus polarization function

for non-metal atoms and triple- for Fe and Co

• Relative energies includes the relativistic contribution first-order perturbation

The DFT method

• AMBER95 force field for MM potential with van der Waals parameter of Fe and Co being replaced by Rappè’s universal force field

The MM method

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-9.02-8.95-8.79

-5.93

-6.19

-9.31-9.29-9.11-9.00

-6.14

-9.31-9.29-9.11-9.00

-6.14

-9.18-904

-8.66

-5.54

-6.73

HOMOHOMO

0.0 kcal/mol (singlet)6.6 kcal/mol (triplet) 11.5 kcal/mol (singlet)

Fe-Generic System: Structures of the Activated Catalyst

N

N

N

Fe

Me

HHN

N

N

Fe

MeH

H H Me

H

N

N

N

Fe

- Singlet ground state- The Cα( ) alkyl prefers to situate on the axial position− to destory it costs energy and leads the system to cross the triplet potential energy surface

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C NC C

C

C

C FeN

C

CCNC

C NCC

C

C

FeNC

C

C

C

CCNC

C N

C

CC

C

C

FeNC

C

C

CCNC

6.1

5.9

29.7

23.0

C

C

C2H4 Backside Attack

C2H4 Frontside Attack on alkyl-complex 1a

1a

2a

TS[2a-2b]

TS[1a-2b]

FS-π complex

BS-π complex 2b

Iron Generic System:π-complex formation

• The most stable -complex is formed by ethylene FS attacking on the alkyl-complex 1a without energy barrier.

• The less stable -complex 2b is the insertion precursor which formed by ethylene BS attacking on the alkyl complex with a barrier of 6.1 kcal/mol.

• 2a and 2b are separate by a barrier of 23 kcal/mol.

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Capture1a

2a

2b

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Summary of Fe Generic system• At monomer concentrations normally for

polymerization, the activated catalyst, alkyl-complex 1a, readily bind ethylene to form the stable π-complex 2a.

• 2a is inert towards insertion owing to a high barrier (23 kcal/mol) for it to convert to the insertion precursor 2b; The barrier originates in bring the Cα of 1a from axial position to equatorial position, and the TS lies on the triplet potential energy surface.

• 2a is also inert towards chain termination because the β-H transfer product is as stable as 2a.

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Fe ‘Real’ System: Steric Modification by Bulky of the Aryl Rings

CC

C

C

C

CCN

C

C

C

C

C

N N

C

Fe

C

C

C

C

C

C

C

CC

C

C

C

C

C

C

C

C

CC

C

C

C

C2H4 FS attack

C 2H4 B

S attac

k• FS attack of C2H4 is retarded by the i-Pr groups • BS attack of C2H4 is induced by van der Waal’s attraction

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Summary of Fe ‘Real’ system• The only available chain termination channel is

bimolecular BHT path. The unimolecular BHE termination chain is unfeasible due to highly endothermic for the ejection of the transferred polymer chain.

• Monomer capture is the rate determining step for both chain propagation and termination

• The chain termination chain is retarded by the i-Pr groups both kinetically and thermodynamically.

• Chain propagation chain becomes favorable due to openness of the alkyl-complex for the BS uptake of ethylene to form the insertion precursor.

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L

+

CoH

CoH

L

HL Co

HL Co

HL Co

H

L

+

Co

+

H

L Co6.7 kcal/mol

16.0 kcal/mol

13.1 kcal/mol9.5 kcal/mol

0.0

3.8

The Generic Co-System is a Catalyst for ehtylene oligomerization

β- H Transfer

β- H Elimination

Chain Propagation

Co-Generic System

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HL Co

HL Co

HL Co

Co

H

L

H

L Co

L +H

Co

HL +Co

7 (-0.7)

6a (3.3)

6a-6b (13.6)

6a-7 (11.6) 3a-6a (8.4)

3b (-9.3)

2a-3b (16.9)

4a (-24.6) 4b (-24.1)4c (-28.0)

4a-4c (-19.7)4b-4a /4b-4c (-15.8)

2a-3c (6.0) 3c-4b (6.0)

2b-5a (12.4)

5a (10.5)

5a-5b (22.7)

8 (43.7)

3c (-3.3)

L CoH

HL Co

Chain Transfer via BHT Chain Transfer via BHE

Chain Propagation

HCo+

L

L+

CoH

3b-4a (-3.6)

3b-3c (-1.2)

L +

H

Co

N

N

N

Me

Me

Ar

L =

Ar

Ar = 2,6-iPr2C6H3

2a (0.0)

Co 'Real' System: Energy (kcal/mol) Profile

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Conclusions

Electronic Properties:• Fe - no polymerization ability and activity• Co - good oligomerization ability and activity

Steric Modifications:• Fe - screen the termination entrance - eliminate the insertion barrier - suppress the isomerization channel •Co - Screen both termination and propagation entrances - diminish insertion barrier - block termination and isomerization channels

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Acknowledgment

This investigation has been supported by the National Sciences and Engineering Research Council of Canada (NSERC), and by the donors of the Petroleum Research Fund, administered by the American Chemical Society (ACS-PRF No. 31205-AC3), as well as by Novacor Research and Technology Corporation (NRTC) of Calgary.