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.

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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

The Quest: The Quest:Polymerization-Catalysts with d n -Electrons (n = 1 – 4) Sc Ti VCrMnFeCoNi Y Zr NbMoTcRuRhPd La Hf TaWReOsIrPt ? McConville et al. Brookhart et al.

M L 'L R M = Ti, V, Cr, Mn L = NH 3, NH 2 - R = Me, Et Possible Polymerization Catalysts First row transition metals Cationic high-spin complexes Two nitrogen ligands Me or Et as model for the growing polymer chain

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

Prerequisites for Active Catalysts sufficiently high Olefin Binding Energy Must be sufficiently high to compensate for the entropic barrier of the bimolecular reaction. low Olefin Insertion Barrier Barrier of chain propagation must be low. higher than the insertion barrier Termination Barrier Termination barriers must be higher than the insertion barrier.

Olefin Binding Energy d1d1 d2d2 d3d3 d4d4 Olefin binding energy for R = Me Olefin binding energy correlates with the number of d-electrons. d 3 and d 4 systems have lowest binding energy because of destabilized the acceptor orbital for the  -d-interaction.

M MR M M R R R M R MR MR MR d-levels a.b. b. sp 3 OC IN   Orbital Interactions during the Olefin Insertion for example: a d 1 system SOMO becomes significantly destabilized during the insertion. b. = bonding; a.b. = antibonding

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.

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. M C H 2 CH 2 'L L OC BHT H M H 2 C CH 2 'L L H CH 2 H 2 C

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.

Summary for Model Systems Olefin binding energy: 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: 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: Termination: dominant process for most systems is the BHT mechanism. Its barrier is generally higher and follows the same trends as the insertion barrier.

The Quest: The Quest:Polymerization-Catalysts with d n -Electrons (n = 1 – 4) Sc Ti VCrMnFeCoNi Y Zr NbMoTcRuRhPd La Hf TaWReOsIrPt ? McConville et al. Brookhart et al.

The Quest: The Quest:Polymerization-Catalysts with d n -Electrons (n = 1 – 4) Sc Ti V Cr MnFeCoNi Y Zr NbMoTcRuRhPd La Hf TaWReOsIrPt A possible Answer: A possible Answer: A Cr(IV) d 2 -Catalyst

Cr How could it look like? Use a ligand known for M(IV) systems: R’= Pr R = H; 2,5-iPr-C 6 H 3

Disappointing Results UPTINSBHT (Energies in kcal/mol) [CrR’(NH 2 ) 2 ] + R = H R = 2,5-iPr-C 6 H 3

Ligand Design: The rotational position of the amides UPTINSBHT free / / (Energies in kcal/mol)

Ligand Design: Real size non-chelating ligands Cr

Ligand Design: Real size non-chelating ligands Cr

Ligand Design: Promising Results UPTINSBHT NH  HN-(CH 2 ) 3 -NH  NMe  N(SiH 3 )  (Energies in kcal/mol)

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 d 0 -systems, electronic interactions must be considered in the ligand design. The promising Cr(IV) d 2 -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.

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