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

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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

The Quest:The Quest: Polymerization-Catalysts with dn-Electrons (n = 1 – 4)

Sc Ti V Cr Mn Fe Co Ni

Y Zr Nb Mo Tc Ru Rh Pd

La Hf Ta W Re Os Ir Pt

NMCl2NRR

M = Ti, Zr,HfNMCl2NRR

M = Ni, Pd, Pt??McConville et al. Brookhart et al.

M = Ti, V, Cr, Mn

L = NH3, NH2

R = Me, Et

Possible Polymerization CatalystsPossible Polymerization Catalysts

First row transition metals Cationic high-spin complexes Two nitrogen ligands Me or Et as model for the growing

polymer chain

CH2CH3 M

L CH2CH3

H2C CH2

H2CCH2

Chain Propagation

Chain Termination

BHE BHT#

OC IN#

Elementary Steps of Ethylene PolymerizationElementary Steps of Ethylene Polymerization

Prerequisites for Active CatalystsPrerequisites for Active Catalysts

Olefin Binding EnergyMust be sufficiently highsufficiently high to compensate for the entropic barrier of the bimolecular reaction.

Olefin Insertion BarrierBarrier of chain propagation must be lowlow.

Termination BarrierTermination barriers must be higher than the higher than the insertion barrierinsertion barrier.

Olefin Binding EnergyOlefin Binding Energy

d1 d2 d3 d4

Olefin binding energy for R = Me

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.

M RM R

d-levels

Orbital Orbital Interactions Interactions during the during the Olefin Olefin Insertion Insertion

for example:a d1 system

SOMO becomes significantly destabilizedduring the insertion.

b. = bonding; a.b. = antibonding

Ti1 V1 V2 Cr2 V3 Cr3 Mn3 Cr4 Mn4

[kcal/mol]

Insertin Barrier

SOMO(OC)-SOMO(IN)

Olefin Insertion Barrier (R = Me)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 ReactionsTermination 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.

CH 2'L

OC BHT

H 2CCH 2

Ti1 V1 V2 Cr2 V3 Cr3 Mn3 Cr4 Mn4

Insertion BarrierBHT Barrier

BHT Termination Barrier (R = Et)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 SystemsSummary 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.

NMCl2NRR

M = Ti, Zr,HfNMCl2NRR

M = Ni, Pd, Pt??McConville et al. Brookhart et al.

A possible Answer:A possible Answer:

A Cr(IV) d2-Catalyst

How could it look like?How could it look like?

Use a ligand known for M(IV) systems:

CrNNRRR'

R’ = PrR = H; 2,5-iPr-C6H3

Disappointing ResultsDisappointing Results

UPT INS

-18.3 6.211.4

-16.8 13.214.8

-13.0 10.815.1

(Energies in kcal/mol)

[CrR’(NH2)2]+

CrNNRRR'

R = 2,5-iPr-C6H3

Ligand Design:Ligand Design:The rotational position of the amidesThe 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)

HHH90/90CrN

HHH0/180

Ligand Design: Ligand Design: Real size non-chelating ligandsReal size non-chelating ligands

MeMeMe

Ligand Design: Ligand Design: Real size non-chelating ligandsReal size non-chelating ligands

H3SiH3SiH3Si

N(SiH3)2

Ligand Design: Ligand Design: Promising ResultsPromising 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)

Preliminary SummaryPreliminary Summaryfor “Real Size” Systemsfor “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.

Nobel-Price 1998 in ChemistryNobel-Price 1998 in Chemistryfor “The Theory”for “The Theory”

W. Kohn (DFT) and J. Pople (ab initio)

Theory as a valuable tool in chemical research

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

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