Well-defined cationic methallyl α-Keto-β-diimine complexes of nickel

Article abstract of DOI:10.1002/anie.201003125

Complex considerations: The synthesis and characterization of a discrete cationic nickel-methallyl complex (see structure: Ni green, N blue, O red) provided insight into the propagating sites generated by using α-keto-β-diimine-nickel olefin-polymerization catalysts. The addition of Al(C₆F₅)₃ led to carbonyl coordination to aluminum, further depletion of the electron density at the nickel center, and a substantial increase in reactivity towards ethylene.

Full text of DOI:10.1002/anie.201003125

Communications
DOI: 10.1002/anie.201003125
Discrete Cationic Complexes
Well-Defined Cationic Methallyl a-Keto-b-Diimine
Complexes of Nickel**
Jason D. Azoulay, Zachary A. Koretz, Guang Wu, and Guillermo C. Bazan*
Angewandte
Chemie
7890
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 7890 –7894

Angewandte
Chemie
Polyolefins are a range of commodity materials with major
economic implications.[1] Despite a significant historical
record, the requirement of efficient methods for their
preparation and the search for new macromolecular archi-
tectures continue to drive the development and design of
homogeneous transition-metal complexes in both academic
and industrial laboratories. Late-transition-metal systems are
less oxophilic than their early-transition-metal counterparts.[1]
They have been shown to participate in “chain-walking”
reactions,[2] exhibit living behavior,[3] incorporate and tolerate
polar functionalities,[1,4] and have been used in water.[5] Such
flexibility in reactivity is relevant for the development of new
materials.[1g,6]
Cationic catalysts and/or initiators are typically generated
by electrophilic abstraction reactions from neutral complexes
by using coactivators, such as methylaluminoxane
(MAO).[2b,7] The activated complex is in most cases an ion-
paired species consisting of an electrophilic transition-metal
cation and a bulky noncoordinating counterion. Other
methods of activation have also been reported. More
specifically, complexes containing a-iminocarboxamidato
and related ligands can be activated by the binding of Lewis
acids to lone pairs of electrons on functional groups within the
ligand framework.[6b,8] Such a process leads to the formation
of zwitterionic complexes, enables activation at a site
removed from the monomer-insertion trajectory, and results
in a net withdrawal of electron density from the metal atom.[8]
The molecular requirements for the two activation
processes described above led to the design of compound 1
(Scheme 1), in which a nickel center is coordinated by a bulky
a-keto-b-diimine ligand. Active sites are generated by using
trimethylaluminum (TMA), MAO, and modified MAO
(MMAO) coactivators.[9,10] The presence of the carbonyl
functionality on the ligand framework leads to an increase in
the rate of polyethylene (PE) production by two orders of
magnitude relative to that observed with a nearly isostruc-
tural b-diimine analogue.[11] This increased activity has been
attributed to the formation of a cationic species in which the
metal center is further depleted of electron density by the
attachment of a Lewis acid at the carbonyl site, as in 1A.
Living-polymerization characteristics can be attained by
reducing the TMA content in MAO through the removal of
volatile species.[3c]
Scheme 1. Proposed active site upon activation with MAO. R=alkyl,
Aꢀ =charge-compensating counterion, Ar=2,6-diisopropylphenyl.
ratios of alkyl aluminum species.[12] Furthermore, the excess
aluminum (Al/Ni 250:1) required to activate 1 complicates
the in situ observation and characterization of initiating/
propagating sites. In response to these challenges, we report
the synthesis and characterization of a discrete, cationic nickel
analogue of 1: [2,4-bis(2,6-diisopropylphenylimino)pentan-3-
one-k2N,N’]Ni(h3-C4H7)+BAr’4ꢀ (2, Ar’ = 3,5-(CF3)2C6H3). We
found evidence for increased polymerization rates upon the
binding of a Lewis acid to the carbonyl functionality. Addi-
tionally, we disclose new reactivity with alkyl aluminum
compounds that is relevant for understanding the experimen-
tal conditions required to achieve living polymerization.
Complex 2 was synthesized in 78% yield by the reaction
of 2,4-bis(2,6-diisopropylphenylimino)pentan-3-one with
ꢀ
[(C4H7)NiCl]2 in the presence of NaBAr’4 (Scheme 2). In
contrast to the paramagnetic analogue 1, complex 2 is
Scheme 2. Synthesis of 2. Ar=2,6-diisopropylphenyl, Ar’=3,5-
(CF3)2C6H3.
1
diamagnetic. Two resonances were observed in the H NMR
spectrum of 2 in CD2Cl2 for the syn and anti allylic hydrogen
atoms: singlets at d = 2.30 (Hanti) and 2.20 ppm (Hsyn). The
presence of these resonances indicates the structural rigidity
of the p-allyl ligand on the NMR time scale.[13] In the
13C NMR spectrum, the CH2 carbon atoms appear as a singlet
at d = 66.0 ppm, the imine carbon atoms at d = 175.5 ppm,
and the carbonyl carbon atom at d = 183.6 ppm; these signals
indicate that the ligand is bound in an N,N’ fashion. The 11B
and 19F NMR spectra show characteristic singlets for the
BAr’4ꢀ counterion at d = ꢀ6.7 and ꢀ64.2 ppm, respectively. A
single-crystal X-ray diffraction study confirmed the identity
of the complex (Figure 1). A distorted square-planar geom-
etry around the nickel center was observed, whereby the six-
membered chelate adopts a boatlike conformation similar to
that in 1.[9] The 2,6-diisopropyl groups project toward the axial
sites of the cation in a configuration that was anticipated to
retard the rate of chain transfer and lead to the formation of
high-molecular-weight polymers.[7a]
The formation of 1A and the responsibility of this species
for promoting the polymerization reactions have not been
fully verified. Commercially available MAO and MMAO are
complex mixtures of oligomers containing various weight
[*] J. D. Azoulay, Z. A. Koretz, 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
[**] We are grateful to the Department of Energy BES and the Mitsubishi
Chemical Center for Advanced Materials for support of this
research.
Interactions with different Lewis acidic aluminum com-
pounds were first probed by examination of the reaction of 2
with Al(C6F5)3·(C7H8)0.5 (1 equiv).[14] A new organometallic
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003125.
Angew. Chem. Int. Ed. 2010, 49, 7890 –7894
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7891

Communications
Scheme 4. Reaction of 2 with TMA. Reaction conditions: AlMe3
(1 equiv), toluene, ꢀ358C!RT. Ar=2,6-diisopropylphenyl, Ar’=3,5-
(CF3)2C6H3.
crystallography were obtained at ꢀ358C from a toluene
solution. The results of the structural characterization of 4 are
shown in Figure 2. The addition of TMA in toluene to 2
resulted in alkylation of the exocyclic carbonyl group and
rearrangement of the ligand framework. This reaction led to a
Figure 1. ꢀORTEP drawing of the cation in 2.[19] Hydrogen atoms and
the BAr’4 counterion have been omitted for clarity. Thermal ellipsoids
are drawn at 35% probability. Selected bond lengths [ꢀ] and angles [8]:
C(2)–N(2) 1.279(6), C(2)–C(3) 1.515(7), C(1)–N(1) 1.272(6), C(1)–
C(3) 1.527(7), C(3)–O(1) 1.203(6), Ni(1)–N(1) 1.947(4), Ni(1)–N(2)
1.946(4), Ni(1)–C(5) 1.995(6); N(1)-C(1)-C(3) 118.6(5), C(1)-C(3)-C(2)
122.4(5), N(2)-C(2)-C(3) 119.6(5), C(2)-N(2)-Ni(1) 123.0(4), N(1)-
Ni(1)-N(2) 96.37(18), Ni(1)-N(1)-C(1) 123.9(4).
1
product formed within minutes, as determined by H NMR
spectroscopy in C6D6 or CD2Cl2 (in which the compound is
unstable after 1 h). For example, in CD2Cl2, the resonances
for the allylic protons appeared at d = 2.38 (Hanti) and
2.30 ppm (Hsyn). The downfield shift of these signals relative
to those of 2 is indicative of a more electrophilic metal center.
Characteristic resonances observed in the 19F NMR spectrum
(C6D6) at dF = ꢀ139 (6F, ortho), ꢀ154 (3F, para), and
ꢀ162 ppm (6F, meta) are consistent with carbonyl coordina-
tion to aluminum.[15] A downfield shift in the 13C NMR
Figure 2.ꢀORTEP drawing of the cation in 4.[19] Hydrogen atoms and
the BAr’4 counterion have been omitted for clarity. Thermal ellipsoids
are drawn at 35% probability. Selected bond lengths [ꢀ] and angles [8]:
Ni(1)–N(2) 1.904(6), N(2)–C(2) 1.267(8), C(2)–C(3) 1.546(9), C(3)–
O(1) 1.407(7), O(1)–Ni(1) 1.904(5), C(3)–C(1) 1.538(9), O(1)–Al(1)
1.806(5), Al(1)–N(1) 1.972(6), N(1)–C(1) 1.278(7); Ni(1)-N(2)-C(2)
118.1(5), N(2)-C(2)-C(3) 112.3(6), C(2)-C(3)-O(1) 107.9(5), C(3)-O(1)-
Ni(1) 112.8(4), O(1)-Ni(1)-N(2) 104.90, C(3)-O(1)-Al(1) 118.66, O(1)-
Al(1)-N(1) 82.7(2), Al(1)-N(1)-C(1) 114.3(5), N(1)-C(1)-C(3) 115.1(6),
C(1)-C(3)-O(1) 107.7(5).
=
spectrum in C6D6 indicated a more polarized C O bond in the
product relative to that in 2 (DdCO = 16.1 ppm; see the
Supporting Information). These observations are consistent
with the formation of 3 (Scheme 3). Single crystals of 3 were
not obtained despite repeated attempts owing to the lack of
crystallinity in this complex. The carbonyl stretching fre-
quency in the IR (KBr) spectrum of 3 occurs at a lower
frequency in comparison to that observed for 2 (DnCO
=
ꢀ1
=
ꢀ52 cm ), as would be expected for the decreased C O
bond order upon activation.
binuclear organometallic species in which nickel and alumi-
num are ligated in an N,O,N’ fashion with the oxygen atom
bridging the aluminum and nickel centers. The nickel atom in
this complex adopts a distorted square-planar geometry with
a planar five-membered N(2),O chelate. The 2,6-diisopropyl
substituents on the aryl group bound to N(2) project toward
the axial positions of the nickel coordination plane. Alumi-
num is N,O-bound with a tetrahedral arrangement of ligands.
The reaction of 2 with triethylaluminum (TEA) yielded a
distinct organometallic product. Specifically, the addition of
TEA (1 equiv) to 2 resulted in the quantitative formation of
The reaction of 2 with TMA (1 equiv) resulted in the
formation of 4 (Scheme 4). The 1H and 13C NMR spectra of 4
are consistent with the formation of a mixture of diastereo-
mers in a 1.4:1 ratio. Four peaks corresponding to the methyl
groups attached to aluminum were observed between d =
ꢀ0.50 and ꢀ0.72 ppm. Single crystals of 4 suitable for X-ray
complex
5 (Scheme 5). The molecular structure of 5
(Figure 3) was determined by single-crystal studies. The
characterization of 5 by NMR spectroscopy in solution was
consistent with this structure (see the Supporting Informa-
tion). Interestingly, although compounds 4 and 5 have
structural similarities, and alkylation reactions with TMA
Scheme 3. Reaction of 2 with Al(C6F5)3 to form 3. Ar=2,6-diisopropyl-
phenyl, Ar’=3,5-(CF3)2C6H3.
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ular-weight PE (Tm = 1288C) in low yield (Table 1, entry 1).
The addition of ethylene to a solution of 2 in CD2Cl2 in an
NMR tube resulted in the consumption of the monomer and
ꢀ
ꢀ
the appearance of a singlet corresponding to (CH2)n at d =
1.28 ppm along with the formation of a white precipitate. That
there is no large difference in the integration of signals
attributed to 2 suggests that initiation is inefficient and that
only a few sites rapidly propagate, in agreement with reports
of other initiator systems containing p-allyl ligands.[18]
Scheme 5. Reaction of 2 with TEA. Reaction conditions: AlEt3
(1 equiv), toluene, ꢀ358C!RT. Ar=2,6-diisopropylphenyl, Ar’=3,5-
(CF3)2C6H3.
A significant increase in reactivity was observed in the
presence of Al(C6F5)3 (Table 1, entries 3–5). The productivity
was two orders of magnitude larger with 5 equivalents of the
coactivator than with 2 alone. The higher activity observed
with 5 equivalents of Al(C6F5)3 than with 1 equivalent may be
attributed to the scrubbing of impurities within the reaction
medium and/or a shift of the equilibrium towards the adduct.
The addition of Al(C6F5)3 also led to a large increase in the
molecular weight of the product (from Mw ꢁ 50 ꢀ 103 gmolꢀ1
with 2 to Mw ꢁ 1 ꢀ 106 gmolꢀ1 with 2/Al(C6F5)3; Table 1,
entries 2 and 5). Comparison of the molecular weight of the
products and productivities indicate that 2/Al(C6F5)3 does not
initiate the reaction completely at the beginning of the
polymerization. An increase in the amount of the coactivator
from 1 to 5 equivalents resulted in an almost threefold
increase in the activity (Table 1, entries 3 and 4). An increase
in the reaction temperature to 758C under these conditions
resulted in a large increase in ethylene consumption and an
average productivity of 4800 kg of PE per mol of Ni per hour
(Table 1, entry 5), which corresponds to an apparent turnover
frequency (TOF) of 1.7 ꢀ 105 hꢀ1. The molecular-weight dis-
tribution of the polymer in entry 5 of Table 1 is broadened.
We attribute this effect to inefficient initiation, limitation to
short reaction times, and gelation of the reaction medium as a
result of the rapid production of high-molecular-weight PE.
However, the activity observed for the reaction in entry 5 of
Table 1 is comparable to that observed with the 1/MAO
combination.
Figure 3. ꢀORTEP drawing of the cation in 5.[19] Hydrogen atoms and
the BAr’4 counterion have been omitted for clarity. Thermal ellipsoids
are drawn at 35% probability. Selected bond lengths [ꢀ] and angles [8]:
Ni(1)–N(2) 1.924(3), N(2)–C(2) 1.282(5), C(2)–C(3) 1.536(5), C(3)–
O(1) 1.422(5), O(1)–Ni(1) 1.918(3), C(3)–C(1) 1.513(6), O(1)–Al(1)
1.829(3), Al(1)–N(1) 1.999(4), N(1)–C(1) 1.296(5); Ni(1)-N(2)-C(2)
116.8(3), N(2)-C(2)-C(3) 114.0(4), C(2)-C(3)-O(1) 108.5(3), C(3)-O(1)-
Ni(1) 112.7(2), O(1)-Ni(1)-N(2) 83.05, C(3)-O(1)-Al(1) 118.12, O(1)-
Al(1)-N(1) 83.0(4), Al(1)-N(1)-C(1) 113.7(3), N(1)-C(1)-C(3) 115.7(4),
C(1)-C(3)-O(1) 108.4(3).
The reactivity of 4 and 5 was lower than that observed
with 2/Al(C6F5)3 (Table 1, entries 6 and 7). These observa-
tions may bear implications for why it is important to remove
TMA from MAO to achieve living-polymerization conditions.
Specifically, the formation of analogues of 4 by reaction with
TMA would increase and modify the diversity of propagating
are well-established,[16] the reaction with TEA and subse-
quent formation of complex 5 proceeds with the elimination
of ethylene, as evidenced by the 1H NMR spectrum (d =
5.4 ppm in CD2Cl2 at 210 K), probably through direct b-
hydrogen transfer to the coordinated carbonyl group.[16,17]
Comparison of the structural fea-
tures of 4 and 5 shows that the
Table 1: Selected ethylene-polymerization reactions.[a]
bulkier ethyl groups in 5 result in a
more crowded coordination plane
at the nickel center (Figures 2 and
3).
We carried out a series of
ethylene-polymerization test reac-
tions with 2–5 under various con-
Activity[b]
MW
MW/Mn
Tm
[d]
Entry
Initiator
Coactivator
(equiv)
t
T
[8C]
[min]
[103 gmolꢀ1 [c]
]
[8C][e]
1
2
3
4
5
6
7
2
2
2
2
2
4
4
–
–
60
20
20
20
10
20
20
50
75
50
50
75
50
50
20
73
300
800
4800
15
47
51
1.9
6.6
4.3
3.5
128
118
125
121
118
131
124
Al(C6F5)3 (1)
Al(C6F5)3 (5)
Al(C6F5)3 (5)
–
–
1088
836
1050
180
295
bimodal
2.1
ditions (Table 1). Compound
2
could initiate ethylene polymeri-
zation in the absence of a coacti-
vator (Table 1, entries 1 and 2).
Thus, the exposure of 2 to ethylene
(500 psig) at 508C resulted in the
formation of crystalline low-molec-
70
2.6
[a] Polymerizations were carried out in a 300 mL autoclave reactor in toluene (50 mL) with 10 mmol of
the complex at an ethylene pressure of 500 psig. [b] The activity is given in kg of PE per mol of Ni per
hour. [c] The weight-average molecular weight of the product was determined by gel-permeation
chromatography versus polystyrene standards. [d] Mn =number-average molecular weight. [e] The
melting temperature of the product was determined by differential scanning calorimetry.
Angew. Chem. Int. Ed. 2010, 49, 7890 –7894
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Communications
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over the PE structure. The addition of up to 5 equivalents of
TMA (for 4) or TEA (for 5) did not lead to appreciable
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In conclusion, we have described the synthesis and
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which offers insight into the propagating sites generated by
using 1 in the presence of various coactivators. Multinuclear
NMR spectroscopy is consistent with the addition of Al-
(C6F5)3 to the carbonyl site in 2 to form the cationic complex
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TEA proceeds by a more complex mechanism, but the final
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a-keto-b-diimine not only serves as a conduit for modification
of the electron density at the metal, but can also participate in
deactivation reactions that must be carefully mitigated to
ensure maximum control over the product properties.
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Received: May 24, 2010
Revised: July 14, 2010
Published online: August 25, 2010
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Keywords: cationic complexes · homogeneous catalysis · nickel ·
polymerization · polyolefins
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[19] Crystal data for 2: C131.5H121B2C13F48N4Ni2O2, Mr = 2946.71,
ꢀ
triclinic, space group P1, a = 18.798(5), b = 18.977(6), c =
20.160(6) ꢁ, a = 78.460(5), b = 80.121(5), d = 72.287(6)8, V=
6653(3) ꢁ3, Z = 2, red, crystal size 0.3 ꢀ 0.25 ꢀ 0.2 mm3, MoKa
,
l = 0.71073 ꢁ, T= 150(2) K, R = 7.51, wR = 16.69 for 26345
unique reflections with I > 2s(I). Crystal data for 4:
C71.5H69AlBF48N2NiO, Mr = 1524.79, monoclinic, space group
P21/c, a = 18.788(3), b = 19.309(4), c = 21.682(4) ꢁ, b =
103.596(3)8, V= 7645(2) ꢁ3, Z = 4, yellow, crystal size 0.35 ꢀ
0.3 ꢀ 0.08 mm3, MoKa
, l = 0.71073 ꢁ, T= 150(2) K, R = 8.18,
wR = 20.70 for 26345 unique reflections with I > 2s(I). Crystal
data for 5: C69H69AlBF24N2NiO, Mr = 1494.76, triclinic, space
ꢀ
group P1, a = 12.801(4), b = 14.565(5), c = 20.084(7) ꢁ, a =
84.322(5), b = 72.372(5), d = 87.243(6)8, V= 3551(2) ꢁ3, Z = 2,
yellow, crystal size 0.3 ꢀ 0.25 ꢀ 0.25 mm3, MoKa, l = 0.71073 ꢁ,
T= 150(2) K, R = 7.86, wR = 16.72 for 13875 unique reflections
with I > 2s(I). CCDC 777724 (2), 777723 (4), and 777722 (5)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[3] a) C. M. Killian, D. J. Tempel, L. K. Johnson, M. Brookhart, J.
Am. Chem. Soc. 1996, 118, 11664 – 11665; b) A. C. Gottfried, M.
Brookhart, Macromolecules 2001, 34, 1140 – 1142; c) J. D. Azou-
lay, Y. Schneider, G. B. Galland, G. C. Bazan, Chem. Commun.
2009, 6177 – 6179.
7894 www.angewandte.org
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Angew. Chem. Int. Ed. 2010, 49, 7890 –7894