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Polymerization-Catalysts with d n -Electrons (n = 1 – 4): A possible promising Cr-d 2 Catalyst

Investigating the potential of Cr-d2 Catalyst for polymerization, emphasizing the crucial factors of Olefin binding energy, insertion barrier, and termination reactions. The study explores the influence of d-electrons on catalyst reactivity and proposes promising ligand designs for effective polymer chain growth.

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Polymerization-Catalysts with d n -Electrons (n = 1 – 4): A possible promising Cr-d 2 Catalyst

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  1. Polymerization-Catalysts with dn-Electrons (n = 1 – 4):A possible promising Cr-d2 Catalyst Rochus Schmid and Tom Ziegler University of Calgary, Department of Chemistry, 2500 University Drive NW Calgary, Alberta, Canada T2N 1N4

  2. ? Brookhart et al. McConville et al. Sc V Cr Mn Fe Co Ni Ti Y Nb Mo Tc Ru Rh Pd Zr La Ta W Re Os Ir Pt Hf The Quest:Polymerization-Catalysts with dn-Electrons (n = 1 – 4)

  3. Possible Polymerization Catalysts M = Ti, V, Cr, Mn L L = NH3, NH2- M R 'L R = Me, Et • First row transition metals • Cationic high-spin complexes • Two nitrogen ligands • Me or Et as model for the growing polymer chain

  4. Elementary Steps of Ethylene Polymerization Chain Propagation H 2 CH CH L L L C CH 2 3 H C CH + 3 2 2 M M M CH CH 2 3 CH 2 CH 'L 'L 'L 2 C H C H 2 2 # IN OC H C 2 L CH H L 2 Chain Termination M M H CH 'L 'L 2 CH 2 H C H C 2 2 # BHE BHT

  5. Prerequisites for Active Catalysts • Olefin Binding EnergyMust be sufficiently high to compensate for the entropic barrier of the bimolecular reaction. • Olefin Insertion BarrierBarrier of chain propagation must be low. • Termination BarrierTermination barriers must be higher than the insertion barrier.

  6. d1 d2 d3 d4 Olefin Binding Energy • Olefin binding energy correlates with the number of d-electrons. • d3 and d4 systems have lowest binding energy because of destabilized the acceptor orbital for the -d-interaction. Olefin binding energy for R = Me

  7. Orbital Interactions during the Olefin Insertion  a.b. M R M R d-levels R M  b. for example: a d1 system M R R M 3 sp R M SOMO becomes significantly destabilized during the insertion.  b. M M R R IN OC b. = bonding; a.b. = antibonding

  8. Olefin Insertion Barrier (R = Me) • All insertion barriers are below 20 kcal/mol. • The insertion barriers correlate well with the destabilization of the lowest SOMO.

  9. H C 2 CH 2 L L H M H M 'L 'L CH 2 CH 2 C H C 2 H 2 OC BHT Termination Reactions • BHE reaction is in most cases less facile than the BHT reaction. • BHT reaction coordinate involves a shift of the olefin in the BHT plane similar to the insertion reaction. • The major contribution for BHT barrier stems from the breaking of the C-H bond.

  10. BHT Termination Barrier (R = Et) • BHT termination barrier is in general higher than the insertion barrier. • Due to similar a destabilization of the lowest SOMO in both the BHT and IN transition state, the corresponding barriers follow the same trend.

  11. Summary for Model Systems • Olefin binding energy: decreases with increasing number of d-electrons because of the destabilization of the acceptor orbital of the -d-interaction • Olefin insertion barrier: mainly due to loss of the d-*-back donation, which stabilizes the OC.All barriers are significantly below 20 kcal/mol and do not depend directly on the number of d-electrons. • Termination:dominant process for most systems is the BHT mechanism. Its barrier is generally higher and follows the same trends as the insertion barrier.

  12. ? Brookhart et al. McConville et al. Sc V Cr Mn Fe Co Ni Ti Y Nb Mo Tc Ru Rh Pd Zr La Ta W Re Os Ir Pt Hf The Quest:Polymerization-Catalysts with dn-Electrons (n = 1 – 4)

  13. Sc V Cr Mn Fe Co Ni Ti Y Nb Mo Tc Ru Rh Pd Zr La Ta W Re Os Ir Pt Hf The Quest:Polymerization-Catalysts with dn-Electrons (n = 1 – 4) A possible Answer:A Cr(IV) d2-Catalyst

  14. How could it look like? Use a ligand known for M(IV) systems: Cr R’ = Pr R = H; 2,5-iPr-C6H3

  15. Disappointing Results UPT INS BHT -18.3 6.2 11.4 -16.8 13.2 14.8 -13.0 10.8 15.1 [CrR’(NH2)2]+ R = H R = 2,5-iPr-C6H3 (Energies in kcal/mol)

  16. Ligand Design: The rotational position of the amides UPT INS BHT free -18.3 6.2 11.4 90/90 -17.5 5.2 10.6 0/180 -15.9 11.3 12.4 (Energies in kcal/mol)

  17. Ligand Design: Real size non-chelating ligands Cr

  18. Ligand Design: Real size non-chelating ligands Cr

  19. Ligand Design: Promising Results UPT INS BHT NH2 -18.3 6.2 11.4  HN-(CH2)3-NH -16.8 13.2 14.8  NMe2-14.7 11.9 18.6  N(SiH3)2-10.4 9.6 20.2  (Energies in kcal/mol)

  20. Preliminary Summary for “Real Size” Systems • Higher oxidation state systems are interesting candidates. • In addition to steric effects of the auxiliary ligands, which are dominant for d0-systems, electronic interactions must be considered in the ligand design. • The promising Cr(IV) d2-system can be turned into a potential catalyst even with simple ligand systems. • Ligands serving the “electronic needs” of a particular system can be constructed.

  21. Nobel-Price 1998 in Chemistry for “The Theory” W. Kohn (DFT) and J. Pople (ab initio) Theory as a valuable tool in chemical research

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