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

Polymerization-Catalysts with d n -Electrons (n = 1 – 4): A possible promising Cr-d 2 Catalyst. Rochus Schmid and Tom Ziegler University of Calgary, Department of Chemistry, 2500 University Drive NW Calgary, Alberta, Canada T2N 1N4. ?. Brookhart et al. McConville et al. Sc. V. Cr. Mn.

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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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