Axial donating ligands: A new strategy for late transition metal olefin polymerization catalysis

Article abstract of DOI:10.1021/ja8017847

An α-diimine ligand (1) containing an axial donating pyridine group is developed for late metal polymerization catalysis. Despite having no substitution on the bottom face of the ligand, the nickel and palladium complexes of 1 are highly active for ethyle

Full text of DOI:10.1021/ja8017847

Published on Web 05/22/2008
Axial Donating Ligands: A New Strategy for Late Transition Metal Olefin
Polymerization Catalysis
Dennis H. Leung, Joseph W. Ziller, and Zhibin Guan*
Department of Chemistry, UniVersity of California, 1102 Natural Sciences 2, IrVine, California 92697-2025
Received March 13, 2008; E-mail: zguan@uci.edu
Late transition metal polymerization catalysts are under intense
investigation because they offer unique features that early metal
systems do not have.1 A key insight from Brookhart and co-workers
is that the incorporation of sterically bulky axial substituents in
the R-diimine ligand is essential in the generation of high molecular
weight polymer.2 On the basis of Brookhart’s mechanistic model,
prior to monomer insertion, the catalytically active 14 e- cationic
complex containing the growing polymer chain can undergo
reversible ꢀ-hydride elimination to form a square planar 16 e- olefin
hydride intermediate. This olefin complex is susceptible to associa-
tive ligand exchange by coordination of a new olefin at the axial
site, leading to premature chain termination.1a,2 Catalysts with axial
steric bulk can effectively prevent this from occurring. Many ligand
modifications have been explored, including steric and electronic
tuning, changing the backbone structure, and varying the chelating
groups.1 Despite these studies, bulky substituents are still required
for the preparation of high molecular weight polymers. Herein, we
report a fundamentally different approach: olefin polymerization
catalysts that bear axial donating ligands.
Figure 1. Axial donating ligand and metal complexes.
We initially envisioned that a donating group at the axial site
could interact with the metal through the empty p orbital.3 This
would prevent interception of the olefin hydride intermediate by
axial coordination of a new monomer but still allow equatorial
insertion of new monomers in the active cationic species. Signifi-
cantly, unlike other R-diimine-based late metal catalysts that require
steric bulk at the top and bottom axial sites,1,2b our complexes would
leave the bottom face open, yet produce polyethylene (PE) with
high molecular weight. To the best of our knowledge, this represents
the first example of R-diimine ligand-based late transition metal
catalysts that yield high molecular weight polymers without the
need for bulky substituents.
We prepared ligand 1 that incorporates a pyridine donor (Figure
1). Ligand 1 was characterized by X-ray diffraction (see Supporting
Information). In order to dissect axial interaction from steric effects,
we also prepared ligand 2, containing a nondonating phenyl group
in place of the pyridine. Complexation to nickel and palladium
precursors afforded the corresponding metal complexes (Figure 1).
The crystal structure of 5 revealed that the PdII center is chelated
by the R-diimine nitrogens in a square planar fashion with the
pyridine donor situated above the metal center (Figure 2). While
in the crystal form of the neutral complex the pyridine nitrogen is
beyond bonding distance to the metal, the flexible alkyl tethers may
allow for metal coordination in the activated cationic species in
solution.
Figure 2. ORTEP drawing of Pd complex 5 (50% probability). Hydrogen
atoms are omitted for clarity. Selected bond lengths: N(3)-Pd(1) 3.814 Å,
N(1)-Pd(1) 2.054 Å, N(2)-Pd(1) 2.195 Å.
Table 1. Ni Ethylene Polymerization Reactions and Resultsa
f
entry catalyst
activator
tempb TOFc
Mnd
PDI branchese Tm
1
2
3
4
5
6
7
8
9
3
3
3
3
3
3
3
3
4
4
4
MAO
AlMe3
35
35
5
<5g n.d.
n.d.
n.d.
n.d.
n.d.
1.4
3.4
3.6
11.3
1.9
5.0
11.3
11.6
12.9
n.d.
n.d.
137
134
131
125
137
133
126
123
122
<5g n.d.
AlMe2Cl
AlMe2Cl
AlMe2Cl
AlMe2Cl
AlEt2Cl
Al(iBu)2Cl
MAO
187 109224 2.1
15 535 39337
35 588 56464
55 428 17587
5
5
35 561 5648
35 294 6994
35 348 6931
3.2
2.2
3.9
2.6
2.9
7.2
6.6
5.6
104 91782
35 61628
10
11
AlMe3
AlMe2Cl
a Polymerization conditions: 2 µmol catalyst, 6 mmol activator, 100
mL of toluene, 200 psig ethylene, 10 min. b Reaction temperature in °C.
c
×10-3/h. d Determined by GPC with polystyrene standards.
e Determined by 1H NMR and is the number of Me’s per 1000 carbons.
f Determined by DSC in °C. g Only trace amounts of polyethylene
formed.
temperature is raised. This may be due to low thermal stability of
the active catalytic species or disruption of the pyridine interaction
by higher thermal energy.
We first investigated the ethylene polymerization behavior of
nickel complex 3 with a number of aluminum activators (Table 1).
While only negligible yields were obtained with MAO or AlMe3,
the addition of AlMe2Cl resulted in a highly active polymerization
catalyst. Despite lacking steric bulk at the bottom face, catalyst 3
produces PE with high molecular weight and relatively narrow
polydispersities (PDI). Similar to other R-diimine-based catalysts,
both the activity and molecular weight of PE formed fall as
In sharp contrast, nickel catalyst 4, containing a nondonating
phenyl group, only afforded low molecular weight oligomers with
broad PDI when using MAO, AlMe3, or AlMe2Cl as activators.
This result is consistent with our initial proposal that the axial
donating pyridine group in catalyst 3 can prevent chain transfer.
The lack of such axial interactions in catalyst 4 results in high
susceptibility to chain transfer from the open bottom face, leading
to low molecular weight oligomers.
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7538 J. AM. CHEM. SOC. 2008, 130, 7538–7539
10.1021/ja8017847 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Pd Ethylene Polymerization Reactions and Resultsa
specific interaction with the pyridine group (Table 1, entries 7 and
8). Although having similar Lewis acidity, larger aluminum reagents
may encounter unfavorable steric interactions with the catalyst,
leading to decreased activity. Furthermore, no polymerization
activity was observed when BF3•Et2O was added as a competitive
Lewis acid additive for the pyridine donor site, suggesting that
pyridine interaction with AlMe2Cl plays an active role in the
polymerization process.
d
entry
catalyst
P (atm)
TON
Mnb
PDI
branchesc
Tm
1
2
3
4
5e
5e
6
1
6
1
6
535
1872
1248
2941
880379
730999
1025
1.9
2.2
1.5
1.6
5.1
2.5
104.5
105.8
133
134
n/a
6
1121
n/a
a Polymerization conditions: 10 µmol catalyst, 20 mL of toluene, 25
°C, 24 h. b Determined by GPC using polystyrene standards.
c Determined by 1H NMR and is the number of Me’s per 1000 carbons.
d Determined by DSC in °C. e With 1 mmol AlMe2Cl added.
In conclusion, we have demonstrated a new strategy for the
design of late transition metal olefin polymerization catalysts that
utilize axial donor ligands. In this study, a pyridine group was
introduced to an R-diimine ligand to interact with the metal through
the axial site. The pyridine, facilitated by AlMe2Cl, has been shown
to play an active role in inhibiting chain transfer and producing
high molecular weight PE. This interaction also results in significant
changes in the polymer microstructures for both the nickel and
palladium systems, yielding highly linear polymer. We have
demonstrated that the use of axial donating ligands is a powerful
approach for controlling catalyst activity and polymer properties.
Studies at expanding the scope of this concept and exploring the
nature of the interaction between the axial donor group and AlMe2Cl
are currently underway.
In addition to the dramatic increase in molecular weight when
using the pyridine-based catalysts, DSC and 1H NMR spectroscopic
analyses revealed that the PE formed from catalysts 3 and 4 have
different microstructures. Under identical polymerization conditions
(Table 1, entries 5 and 11), catalyst 3 produces more linear PE
with significantly higher melting temperatures (Tm) than catalyst
4. This suggests that the axial pyridine donor not only inhibits
associative chain transfer but also suppresses ꢀ-hydride elimination
and catalyst chain walking,4 resulting in highly linear polymer.
We next investigated the ethylene polymerization activity of the
corresponding palladium catalysts (Table 2). While direct activation
of phenyl catalyst 6 with NaBArf in the presence of ethylene resulted
in the formation of low molecular weight PE oils, surprisingly no
activity was observed with pyridine catalyst 5 under the same
conditions. However, addition of AlMe2Cl to the mixture of 5 and
NaBArf resulted in formation of highly linear PE. Remarkably, the
PE formed by catalyst 5 is of extremely high molecular weight. In
addition, the PEs obtained from catalyst 5 have remarkably low
branching densities and high Tm’s. While linear PE has recently
been obtained by phosphine-sulfonate palladium catalysts,5c–g most
palladium catalysts, especially other PdII-R-diimine systems, gener-
ate highly branched PE due to catalyst chain walking.4a,5 To our
knowledge, this is the first example of a PdII-R-diimine catalyst
which produces highly linear PE with high molecular weight.
The polymerization results suggest that a modification of our
initial mechanistic hypothesis is necessary to account for the peculiar
role of AlMe2Cl. Instead of a direct axial coordination between
the pyridine donor group and the metal center, AlMe2Cl appears
to play a specific and active role in generating the active species
for the pyridine-based complexes. While phenyl-based Ni complex
4 shows no selectivity between MAO, AlMe3, and AlMe2Cl
activators, only AlMe2Cl is effective in activating pyridine complex
3. Similarly for pyridine-based Pd complex 5, addition of AlMe2Cl
is critical to generate the active catalyst for polymerization. In
contrast, addition of AlMe2Cl to phenyl-based Pd complex 6
resulted in decreased activity and formation of short oligomers.
Previous studies have shown that AlR2Cl reagents are effective
activators for nickel catalysts,6 and there is some evidence for the
formation of bimetallic Ni-Al complexes bridged by the halide.6d
Specific reactivity with AlR2Cl reagents has been seen for lanthanide
polymerization catalysts which may also involve a bimetallic
species.7 Whereas the exact mechanism is still elusive at this
moment,8 we propose that the pyridine nitrogen in our system may
be interacting directly with AlMe2Cl, bringing it close to the metal
center to form an active bimetallic species bridged by the chlorine
atom. Presumably, this axial coordination suppresses ꢀ-hydride
elimination process and chain transfer, hence resulting high
molecular weight, linear polymer structures.
Acknowledgment. We thank the National Science Foundation
(CHE-0456719 & DMR-0703988) for financial support. Z.G.
acknowledges a Camille Dreyfus Teacher-Scholar Award. We thank
Chris Popeney, Chris Levins, and Dr. Fiona Lin for helpful
discussion, and Brycelyn Boardman and Prof. Guillermo Bazan at
UC Santa Barbara for GPC assistance.
Supporting Information Available: Experimental details for the
synthesis and characterization of compounds, polymerization data, NMR
experiments, and X-ray crystal structures. This material is available
free of charge via the Internet at http://pubs.acs.org.
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interaction of AlMe2Cl with the pyridine-based catalysts has been
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