Nickel α-Keto-β-diimine initiators for olefin polymerization

Article abstract of DOI:10.1002/anie.200804661

(Figure Presented) A new ligand-metal combination (see picture, Ni green, Br brown, N blue, O red, C gray) was designed to be cationic and to benefit from removal of electron density by the action of a Lewis acid on the ligand framework. In the presence o

Full text of DOI:10.1002/anie.200804661

Angewandte
Chemie
DOI: 10.1002/anie.200804661
Homogeneous Catalysis
Nickel a-Keto-b-Diimine Initiators for Olefin Polymerization**
Jason D. Azoulay, Rene S. Rojas, Abigail V. Serrano, Hisashi Ohtaki, Griselda B. Galland,
Guang Wu, and Guillermo C. Bazan*
Late transition metal initiators for olefin polymerization and
oligomerization are of interest owing to their low oxophilicity
and high functional group tolerance.[1] Nickel- and palladium-
based initiators provide several benefits. These include
control over molecular weight characterisitics,[2] backbone
stereochemistry,[3] and comonomer incorporation.[1,4] “Chain
walking” reactions, in which the metal center migrates along
the growing polymer chain through a series of b-hydride
eliminations and reinsertions, allow for the generation of a
wide variety of novel materials.[5] New ligand structures
continue to appear and offer further optimization of reactivity
and polymer properties.[1d,e,4b,5c,6]
Scheme 1 shows representative cationic and zwitterionic
nickel active sites. Example 1a (charge compensating anion is
not shown) corresponds to the broadly studied class of a-
diimine complexes, for which the resting state is a cationic
alkyl olefin species.[7] These species can be generated from a
dihalide precursor in the presence of olefin and an appro-
priate activator, such as methylaluminoxane (MAO).[7] Also
shown are two prototypical zwitterionic initiators (1b and 1c;
important resonance contributions are not shown).[8]
A
Scheme 1. Examples of cationic and zwitterionic polymerization/oligo-
merization active sites. R=alkyl.
relevant perspective for understanding the reactivity of
zwitterionic species is that Lewis acid complexation onto a
basic ligand site reduces the electron density of the metal
center in a neutral complex.[4c,8,9] This concept is more easily
appreciated by examination of 1c. Bulky substituents that
occupy axial sites are important structural components for
minimizing chain-growth termination in both cationic and
zwitterionic systems: for example, 1b produces high-molec-
ular-weight polyethylene (PE), whereas 1c yields a distribu-
tion of 1-alkenes.[8]
be cationic and also to benefit from removal of electron
density from the metal center by the action of a Lewis acid on
the ligand framework. We targeted a nickel complex con-
taining a bulky a-keto-b-diimine ligand with 2,6-diisopropyl-
phenyl substituents (2, Scheme 2). The exocyclic carbonyl
functionality was anticipated to provide an electronically
delocalized conduit extending from the potentially cationic
metal center to a site of Lewis acid interaction through the
oxygen lone pairs. The previously reported b-diimine complex
Herein, we report on the synthesis, characterization, and
reactivity of a ligand–metal combination that was designed to
[*] J. D. Azoulay, H. Ohtaki, Dr. G. Wu, Prof. Dr. G. C. Bazan
Mitsubishi Chemical Center for Advanced Materials
Center for Polymers and Organic Solids
Departments of Chemistry & Biochemistry and Materials
The University of California, Santa Barbara, CA 93106 (USA)
Fax: (+1)805-893-4120
E-mail: bazan@chem.ucsb.edu
Prof. Dr. R. S. Rojas, A. V. Serrano
Departmento de Quꢀmica Inorgꢁnica
Pontificia Universidad Catꢂlica de Chile, Santiago (Chile)
Prof. Dr. G. B. Galland
Instituto de Quꢀmica
Universidade Federal do Rio Grande do Sul, Porto Alegre (Brazil)
[**] This work was funded through MC-CAM. R.S.R. and A.V.S. thank
FONDECYT: 11060384.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200804661.
Scheme 2. Complexes 2 and 3 and the proposed active species derived
from activation of 2. LA=Lewis acid, R=alkyl.
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3[10] (Scheme 2) is a close structural analogue of 2, differing
only with respect to the presence of the carbonyl group at the
a-site, and will be used for comparative purposes. Complex 3
is a precursor for ethylene polymerization that differs from
the analogous a-diimine counterpart 1a in two significant
ways: lower activities are observed, and the PE produced is
more crystalline upon activation with modified methylalumi-
noxane (MMAO).[10,11]
Scheme 3 shows the synthesis of 2. The preparation of the
ligand,
2,4-bis(2,6-diisopropylphenylimino)pentan-3-one,
proceeds in good overall yield starting from 4-(2,6-diisopro-
Figure 1. ORTEP drawing of 2. Thermal ellipsoids are set at the 50%
probability level. Hydrogen atoms are omitted for clarity.
(Figure 1) is consistent with a neutral N,N-bound 2,4-
bis(2,6-diisopropylphenylimino)pentan-3-one ligand. Bond
lengths within the six-membered chelate ring are consistent
À
À
with the proposed structure (C3 N2: 1.292(10), C2 C3:
À
À
À
1.518(12), C1 N1: 1.287(11), C1 C2: 1.511(12), and C2 O1:
1.223(10) ꢀ).[13] The Ni atom displays a pseudo-tetrahedral
coordination geometry , and the chelate ring adopts a boat-
like conformation similar to that observed in 3.[10] Bond
lengths for 2 and 3 are in close agreement; the main structural
difference stems from the presence of the exocyclic carbonyl
functionality on C2 in 2.
A series of ethylene polymerizations were carried out
using 2 under various reaction conditions and in the presence
of different activators (Table 1). Polymerizations were ini-
tiated by the introduction of a stock solution of 2 into a
stirring solution of activator in a specified volume of toluene
in the presence of ethylene. In all cases, the formation of high-
molecular-weight polyethylene (PE) is observed. Table 1,
entry 1 illustrates that AlMe3 can be used as an activator to
Scheme 3. Synthesis of 2. dme=1,2-dimethoxyethane.
pylphenyl)iminopent-2-en-2-(2,6-diisopropylaniline)amine
4.[10,12] Addition of 4 to a stirring solution of Cu(O2CCH3)2 in
CH3OH/CH2Cl2 (1:1) at room temperature led to the
formation of 5 in 88% yield. X-ray crystallographic studies
(see the Supporting Information)
Table 1: Selected ethylene polymerization reactions.
confirmed that 5 is a mononuclear
complex with a distorted square-
planar geometry around the copper
center. Treatment of 5 with oxygen
in methanol led to the nearly quan-
titative formation of complex 6, in
which copper exhibits N,N’,O,O’
Entry[a]
Conditions[b]
TOFꢃ10À3 [hÀ1
]
Mn ꢃ10À3[c]
PDI
1.7
Branches[d] Tm [8C]
[e]
1/5
250 AlMe3,
64
956
28
19
47
30
25
23
32
45
53
T
T
m1 =95
m2 =110
117
328C, 300 psig, 10 min
250 MAO,
108C, 300 psig, 10 min
250 MAO,
328C, 100 psig, 10 min
250 MAO,
328C, 300 psig, 10 min
250 MAO,
328C, 500 psig, 10 min
250 MAO,
328C, 800 psig, 10 min
500 MMAO,
328C, 300 psig, 30 min
500 MMAO,
508C, 450 psig, 10 min
500 MMAO,
2/5
9.6
260
1.1
1.3
1.4
1.3
1.8
5.7[f]
2.2
2.6
3/5
62
1112
1121
1190
1306
1188
625
T
T
T
T
m1 =72
m2 =114
m1 =88
m2 =114
116
coordination within
a
square-
4/2.5
5/2.5
6/2.5
7/10
8/5
125
125
127
207
813
157
planar environment (see the Sup-
porting Information). Treatment of
6 with aqueous ammonia gave 2,4-
bis(2,6-diisopropylphenylimino)-
pentan-3-one 7 in nearly quantita-
tive yield.
117
90
Compound 2 is obtained in 49%
yield by the addition of 7 to a
stirring suspension of [(dme)NiBr2]
in CH2Cl2 at room temperature
(Scheme 3). Single crystals suitable
for X-ray crystallographic examina-
tion were obtained from a concen-
trated CH2Cl2 solution at À358C.
85
9/5
183
73
758C, 300 psig, 30 min
[a] Entry/mmol 2. [b] Entries 1–6 were carried out in a 300 mL autoclave reactor in 100 mL toluene,
entries 7–9 were carried out in a 2000 mL autoclave reactor in 1000 mL toluene. [c] Mn [g molÀ1
]
determined by gel permeation chromatography (GPC) versus polystyrene standards. [d] Number of
branches per 1000 carbon atoms determined by 13C NMR spectroscopy. [e] Determined by DSC. [f] The
The
molecular
connectivity broad PDI may be due to the rapid formation of high molecular weight PE and long reaction time.
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Table 2: Propene and 1-hexene polymerization reactions.
obtain ethylene polymerization with activities on the order of
1800 kgPE(molNi)À1 hÀ1 at 328C and 300 psig ethylene
pressure (PC H ). MAO can also serve as an activator
Entry[a] Conditions[b]
TOF
Mn ꢃ10À3[c] PDI Tg[d]
[hÀ1
]
[8C]
2
4
(Table 1, entries 2–6). Table 1, entry 2 shows a PE with low
polydispersity (PDI = 1.1) when the reaction is carried out at
108C. Comparison of Table 1, entries 2 and 4 shows an
increase in polymerization activity from 269 to 3500 kgPE
(molNi)À1 hÀ1 upon increasing the temperature from 10 to
328C. There is no substantial increase in activity when PC H is
1/30
2/30
250 MAO, 08C,
5 mL propene, 60 min
250 MAO, 258C,
150 psig
propene, 30 min
250 MAO, 358C,
150 psig
14
138
316
1.1 À23
1780
2330
1.5 À25
3/5
191
2.1 À32
2
4
increased from 300 to 800 psig, consistent with rate-limiting
migratory insertion (Table 1, entries 4–6).[14] The results of
13C NMR spectroscopy (see the Supporting Information)
indicate that the branching length and frequency increase at
lower pressures and at elevated temperatures, as in the case of
a-diimine-based nickel initiators.[7] Differential scanning
calorimetry (DSC) analysis shows that the polymer products
display a range of melting temperatures; those with more
linear backbones display higher melting transitions.
propene, 30 min
250 MAO, 08C,
10 mL 1-hexene, 60 min
250 MAO, 258C,
10 mL 1-hexene, 60 min
4/10
5/10
89
157
120
1.2 À55
2.0 À56
356
[a] Entry/mmol 2. [b] Entries 1–3 carried out in a 300 mL autoclave
reactor in 100 mL toluene; entries 4 and 5 carried out in a Schlenk flask
in 10 mL toluene. [c] Mn [g molÀ1] determined by GPC versus polystyrene
standards. [d] Determined by DSC.
Table 1 also contains the results obtained using MMAO.[11]
This activator was found to yield higher activities, thus larger
reactors were used for the polymerizations in Table 1,
entries 7–9. A larger excess of MMAO was also used
(500 equivalents) to serve as a scrubbing agent. At 328C
with PC H = 300 psig, an activity of 5800 kgPE(molNi)À1 hÀ1 is
entry 1) and 1-hexene (Table 2, entry 4) at 08C leads to
polymers with narrow molecular weight distributions (PDI =
1.1 and 1.2 respectively). Microstructural analysis (see the
Supporting Information) by 13C NMR spectroscopy revealed
that the polypropylene and poly-1-hexene products are
atactic; signatures arising from 2,1-insertions were not
detected.[16]
In short, we report on a new type of highly active late
transition metal catalyst precursor that can be used to
polymerize olefins to high molecular weight polymers in the
presence of various activators. Comparison with the reactivity
observed with the b-diimine complex 3 illustrates that the
presence of the carbonyl functionality in 2 leads to an increase
in activity by approximately two orders of magnitude for
ethylene polymerization under analogous reaction conditions.
Furthermore, a-olefins can be polymerized to high molecular
weights by 2.
On the basis of the considerations above, we propose that
the increase in reactivity with 2 versus 3 is a result of the
attachment of a Lewis acid to the exocyclic oxygen site on the
propagating cationic species. The general concept is illus-
trated in Scheme 2, which shows one of several possible
resonance structure interpretations and is not meant to serve
as a complete description of the electronic structure. MAO
and related activators are known to be Lewis acidic.[17] It is
straightforward to anticipate reduction of the electron density
at the nickel center by inductive effects and formation of a
partial positive charge on the nitrogen donors, which leads to
the delocalized structure illustrated in Scheme 2. A detailed
picture requires additional mechanistic and theoretical
examination; however, it is helpful to recall the widely used
activation of organic substrates with carbonyl functionalities
in the presence of aluminum species.[18] Further optimization
of reaction conditions, types of activators, and variations in a-
keto-b-diimine frameworks will enable improvements in
polymerization control. Our current efforts are aimed at
delineating the nature of the propagating species.
2
4
observed (Table 1, entry 7). The fact that ethylene consump-
tion does not decrease over the course of the reaction, as
measured by an ethylene flow rate meter, demonstrates that
the catalytic species is long-lived under these conditions.
Increasing the reaction temperature to 508C and PC H to
2
4
450 psig leads to an activity of 22800 kgPE(molNi)À1 hÀ1, that
is, an apparent turnover frequency (TOF) of 8.1 ꢁ 105 hÀ1
(Table 1, entry 8). Activities of this magnitude are compara-
ble to the a-diimines,[7] salicylaldimines,[1c] and “classical”
metallocenes.[15] Table 1, entry 9 illustrates that the catalytic
species is stable over the course of 30 min at 758C; however,
the high viscosity of the reaction medium under these
conditions may lead to an underestimation of the polymer-
ization rate.
It is informative at this point to compare the ethylene
polymerization activities obtained with 2 and 3. Complex 3 in
the presence of MMAO leads to an activity of approximately
14.5 kgPE(molNi)À1 hÀ1 (308C and 280 psig), an apparent
TOF of 514hÀ1.[10] Under the conditions in Table 1, entry 7 but
using complex 3 (i.e. 3/MMAO, 328C, 300 psig, 30 min), we
obtained an activity of approximately 8 kgPE(molNi)À1 hÀ1.
The higher activity attained with 2/MMAO (5800 kgPE
(molNi)À1 hÀ1, corresponding to an apparent TOF of 2.07 ꢁ
105 hÀ1), illustrates that complex 2 is a precursor to a
significantly more active catalyst.
Propene and 1-hexene can also be polymerized using 2 to
yield high-molecular-weight polymers (Table 2). Using 2/
MAO at a propene pressure of 150 psig and 358C leads to an
activity of 98 kgpolymer(molNi)À1 hÀ1, that is, an apparent
TOF of 2330 hÀ1 (Table 2, entry 3). Table 2, entries 1–3 show
an increase in activity with increasing reaction temperature
similar to the trend observed with ethylene. Table 2, entry 5
illustrates that 1-hexene can be polymerized at 258C to high-
molecular-weight product with an activity of 31 kgpolymer
(molNi)À1 hÀ1. The polymerization of propene (Table 2,
Received: September 23, 2008
Published online: December 9, 2008
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K. Itigaki, G. Wu, G. C. Bazan, Organometallics 2008, 27, 2273 –
2280.
Keywords: homogeneous catalysis · nickel · polymerization ·
substituent effects
.
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