1. 1 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

2. 2 Abstract The 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)) 2 C 5 H 3 N]MC 3 H 7 } + and their generic pendants {[2,6-(HN=CH) 2 C 5 H 3 N]MC 3 H 7 } + (Ar = 2,6-C 6 H 3 (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.

3. 3 V.C. GibsonM. Brookhart - Robust - Low-cost - Simple to make - High activities - High selectivity

4. 4 Experimental Observations 1 R = R’ = i-Pr 2 R = R’ = Me 3 R = t-Bu, R’=H 4 R = i-Pr, R’ = H 5 R = Et, R’ = H 6 R = Me, R’ = H Fe/Co catalyst highly 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. 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

5. 5 Polymerization Mechanisms

6. 6 ‘Real’ SystemGeneric System N NN HH HH M N N i Pr i Pr Me Me N i Pr i Pr M M = Fe, Co DFT part -the generic system MM part - Me and 2,6- i Pr(C 6 H 4 ) Combined DFT/MM Approach:

7. 7 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

8. 8 -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 HOMO

9. 9 6.1 5.9 29.7 23.0 C 2 H 4 Backside Attack C 2 H 4 Frontside Attack on alkyl-complex 1a 1a 2a TS[2a-2b] TS[1a-2b] FS-  complex BS-  complex 2b Iron Generic System:  -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.

10. 10 Capture 1a 2a 2b

11. 11 Summary of Fe Generic system At monomer concentrations normally for polymerization, the activated catalyst, alkyl- complex 1a, readily bind ethylene to form the stable  -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 C  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  -H transfer product is as stable as 2a.

12. 12 Fe ‘Real’ System: Steric Modification by Bulky of the Aryl Rings C 2 H 4 FS attack C 2 H 4 BS attack 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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14. 14 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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17. 17 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

18. 18 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.