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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
L
'L
R
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
H2C
CH2
+M
'L
L
CH2CH3 M
'L
L CH2CH3
M
'L
LH2C
M
'L
L H
H2C
CH2
CH3
CH2
CH2
H2C CH2
M
'L
L
H2CCH2
H2CCH2
H
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
M R
M
M
R
R
R
M R
M R
M RM R
d-levels
a.b.
b.
b.
sp3
OC IN
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
0.0
5.0
10.0
15.0
20.0
Ti1 V1 V2 Cr2 V3 Cr3 Mn3 Cr4 Mn4
[kcal/mol]
-0.4
0.0
0.4
0.8
1.2
[eV]
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.
M
CH2
CH 2'L
L
OC BHT
H M
H 2CCH 2
'L
LH
CH 2
H2C
0
5
10
15
20
25
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.
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.
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
A possible Answer:A possible Answer:
A Cr(IV) d2-Catalyst
Cr
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
BHT
-18.3 6.211.4
-16.8 13.214.8
-13.0 10.815.1
(Energies in kcal/mol)
[CrR’(NH2)2]+
CrNNRRR'
R = H
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)
CrNN
RH
HHH90/90CrN
NRH
HHH0/180
Ligand Design: Ligand Design: Real size non-chelating ligandsReal size non-chelating ligands
CrNN
RMe
MeMeMe
NMe2
Cr
Ligand Design: Ligand Design: Real size non-chelating ligandsReal size non-chelating ligands
CrNN
RH3Si
H3SiH3SiH3Si
N(SiH3)2
Cr
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