Bimetallic effects on ethylene polymerization in the presence of amines: Inhibition of the deactivation by Lewis bases

Article abstract of DOI:10.1021/ja210990t

Dinickel complexes supported by terphenyl ligands appended with phenoxy and imine donors were synthesized. Full substitution of the central arene blocks rotation around the aryl-aryl bond and allows for the isolation of atropisomers. The reported complexes perform ethylene polymerization in the presence of amines. The inhibiting effect of polar additives is up to 250 times lower for the syn isomer than the anti isomer. Comparisons with mononuclear systems indicate that the proximity of the metal centers leads to the observed inhibitory effect on the deactivation of the catalysts.

Full text of DOI:10.1021/ja210990t

Communication
pubs.acs.org/JACS
Bimetallic Effects on Ethylene Polymerization in the Presence of
Amines: Inhibition of the Deactivation by Lewis Bases
Madalyn R. Radlauer, Michael W. Day, and Theodor Agapie*
Department of Chemistry and Chemical Engineering, California Institute of Technology, 1200 East California Boulevard, MC 127-72,
Pasadena, California 91125, United States
*
S Supporting Information
work, salicylaldimine motifs were used as metal-binding sites on
the outer aryl rings. For comparison, both bimetallic (1-s and 1-
a) and monometallic (2-s and 2-a) species were prepared
ABSTRACT: Dinickel complexes supported by terphenyl
ligands appended with phenoxy and imine donors were
synthesized. Full substitution of the central arene blocks
rotation around the aryl−aryl bond and allows for the
isolation of atropisomers. The reported complexes perform
ethylene polymerization in the presence of amines. The
inhibiting effect of polar additives is up to 250 times lower
for the syn isomer than the anti isomer. Comparisons with
mononuclear systems indicate that the proximity of the
metal centers leads to the observed inhibitory effect on the
deactivation of the catalysts.
(
Figure 1). Separation and purification of the atropisomers
n recent years, a bioinspired strategy has been used for the
design of multimetallic olefin polymerization catalysts in
I
which the proximity of the active nuclei is intended to facilitate
1
,2
catalysis similar to the effects seen in many metalloproteins.
A wide variety of multimetallic olefin polymerization catalysts
have been reported, with a broad range of distances between
the metal centers and varying degrees of flexibility of the
3
−9
ancillary ligand.
In comparison with their monometallic
counterparts, some bimetallic early transition metal catalysts
have been reported to incorporate more comonomer and
9
−14
Figure 1. Mono- and bimetallic catalysts for the polymerization of
ethylene.
bulkier olefins in copolymerizations with ethylene.
1
,2
Enhanced stability and activity have been reported as well.
Bimetallic catalysts based on late metals have been shown to
increase the incorporation of olefins displaying polar moieties
were achieved during the ligand synthesis. The nickel
complexes were synthesized by reacting the phenols with
1
5
in copolymers with ethylene. Although the nature of
monomer interactions with bimetallic catalysts has been
investigated in a few cases, studies of the effect of ligand
rigidity and metal−metal distance on the polymerization
outcome have been hindered by the scarcity of architectures
in which these parameters can be controlled. Furthermore, the
development of olefin polymerization catalysts that are not
significantly affected by the presence of polar groups or that can
incorporate polar monomers is of interest. Herein we report a
series of bi- and monometallic nickel polymerization catalysts
with rigid geometries and restricted intermetal distances. The
dinickel catalyst with the metal centers found in proximity
shows less inhibition of catalysis by amines, a favorable
consequence of the bimetallic effect.
NiMe (tmeda) (tmeda = tetramethylethylenediamine) in the
2
presence of pyridine. Structural assignment of each atropisomer
was accomplished on the basis of single-crystal X-ray diffraction
1
1
studies for the bimetallic systems (Figure 2) and H− H
nuclear Overhauser effect NMR spectroscopy (NOESY)
16
experiments for the monometallic systems.
The solid-state structures of 1-s and 1-a revealed that the
distance between the two metal centers is 7.1 Å in the syn
isomer (averaged over the two molecules in the asymmetric
unit) and 11.1 Å in the anti isomer (Figure 2). The
coordination environment around each metal is similar to
that in previously reported nickel−phenoxyiminato com-
3
−9,17
plexes.
A slight distortion of the square-planar geometry
In the design of a ligand for bimetallic catalysts, a 1,4-
is notable in 1-s; the pyridine ligands extend toward the other
terphenyl moiety bearing four methyl substitutents on the
central ring and one ortho oxygen substitutent on each
peripheral aryl ring was chosen as a suitably rigid backbone with
restricted rotation around the aryl−aryl bonds. In the present
metal center and bend away from each other as a result of steric
Received: November 22, 2011
Published: January 6, 2012
©
2012 American Chemical Society
1478
dx.doi.org/10.1021/ja210990t | J. Am. Chem.Soc. 2012, 134, 1478−1481

Journal of the American Chemical Society
Communication
anti isomers of either the di- or mononickel complexes was
observed over 13 h at 50 °C.
Ethylene polymerization trials were performed with the
isolated nickel complexes in toluene at 25 °C (Table 1). The
present catalysts performed ethylene polymerization with
activities similar to those of previously reported pyridine-
1
7,18
ligated nickel−phenoxyiminato systems.
These experiments
generated polyethylene with methyl branches (4−20 branches
per 1000 C atoms). Of the studied complexes, catalysis with
-s was the slowest by a factor of 5, likely because of the
19
1
increased steric bulk at the active site in comparison with the
other systems. The neutral ligands coordinated to the nickel
centers in 1-s reach toward the other metal and hinder
coordination of the olefin. This proposal is supported by the
distortion observed in the solid-state structure of 1-s. Although
the methoxy substitutent is located syn with respect to nickel,
the steric bulk in 2-s is likely not as large as that caused by the
pyridine ligand bound to the second metal in the bimetallic
system.
Ethylene polymerization trials in the presence of excess
primary, secondary, and tertiary amines showed distinct
inhibition trends (Tables 1 and 2). Complexes 1-a, 2-a, and
2
-s were inhibited by 2 orders of magnitude upon the addition
of N,N-dimethylbutylamine, although the difference between 1-
a and 2-a is not well understood in light of the similar steric
environments (Table 2). This deactivation effect in the
presence of added amines is similar to that reported previously
20
for related mononickel systems. In contrast, 1-s was inhibited
by only 1 order of magnitude. Consequently, in some cases
(
Table 1, entries 7, 8, and 11−17), addition of a tertiary amine
afforded a syn catalyst that was more productive than the anti
analogue. The inhibition of the deactivation by amines
observed only with 1-s is hereafter termed the bimetallic effect.
The ratio of the deactivations for 1-a versus 1-s (R, Table 1)
provides a quantitative measure of this effect. Relative to 1-s,
catalyst 1-a was inhibited 10−25 times more by triethylamine,
N-methyldipropylamine, and N,N-dimethylbutylamine and up
to 270 times more by tripropylamine. Differential inhibition by
triethylamine at a shorter polymerization time that resulted in
lower polymer yields for both 1-s and 1-a was also observed,
indicating that the calculated R is not due to decomposition of
the catalysts at different rates (Table 1, entries 7 and 8). The
use of secondary or primary amines resulted in greater
inhibition than the use of tertiary amines and, in all cases
that yielded polymer, also displayed greater inhibition of 1-a
than 1-s (Table 1, entries 23 and 25−28). Relative to 1-s,
catalyst 1-a was inhibited ca. 10 times more with diisopropyl-
amine and 70−140 times more with 1,1-dimethylpropylamine
and 1,1,3,3-tetramethylbutylamine (Table 1, entries 23 and 25−
2
8).
The effect of amines on 1-s and 1-a was studied by H NMR
1
spectroscopy. New Ni−CH3 peaks were observed upon
addition of 1 equiv of 1,1-dimethylpropylamine or of a large
excess (≥100 equiv) of N,N-dimethylbutylamine or N,N-
dimethylethylamine to 1-a and 1-s, indicating competitive
substitution of pyridine. N,N-Dimethylbenzylamine did not
displace pyridine even upon addition of 100 equiv. All of the
investigated amines displaced more pyridine from 1-a than
from 1-s. Qualitatively, the binding ability was found to vary in
the following order: pyridine ≈ 1,1-dimethylpropylamine ≫
N,N-dimethylbutylamine > N,N-dimethylethylamine ≫ N,N-
dimethylbenzylamine (additional analysis is included in the
Supporting Information). This trend mirrors the degree of
Figure 2. Solid-state structures of (top) 1-s, (center) 1-a, and
(
bottom) 1-a-(1,1-dimethylpropylamine) . Solvent molecules and H
2
atoms have been omitted for clarity.
repulsion. The structure of 1-a forbids cooperative reactivity
because the two nickel centers are on opposite faces of the
central arene. The monometallic analogues 2-s and 2-a emulate
the steric effect of the terphenyl backbone without the presence
of a second metal center. No interconversion of the syn and
1
479
dx.doi.org/10.1021/ja210990t | J. Am. Chem.Soc. 2012, 134, 1478−1481

Journal of the American Chemical Society
Communication
a
Table 1. Ethylene Polymerization Trials with 1-s and 1-a and Polar Additives
b
yield (g)
TOF
c
entry
additive
none
equiv
1-s
1-a
1-s
1-a
R
1
2
3
4
5
6
7
8
9
n/a
n/a
500
500
5000
500
500
500
225
500
500
500
500
500
500
500
500
500
500
20
0.574
0.894
0.150
0.148
0.068
0.128
0.039
0.010
0.058
0.062
0.036
0.070
0.055
0.047
0.066
0.012
0.003
0.619
0.252
3.415
1.893
1.440
1.032
0.103
0.181
0.016
0.006
0.071
0.111
0.025
0.019
0.001
0.019
0.028
0.009
341
531
89
2029
2250
856
613
61
−
−
d
d
none
NMe Et
0.5
0.7
3.3
3.5
12.2
8.0
4.0
2.7
7.2
18.4
269
12.1
11.6
6.3
−
2
NMe Et
88
2
NMe Et
41
2
NMeEt2
NEt3
76
108
9
23
d
d
d
d
NEt3
12
7
1
e
,
f
NMe2R
103
36
21
41
33
25
126
66
15
11
1
1
e
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
NMe R
2
n
2
NMe Pr
n
NMe Pr
2
n
N Pr
3
n
2
NMe Bu
10
n
2
i
i
i
i
NMe Bu
39
17
n
NMe Bu
7
5
2
n
j
j
N Bu
−
2
−
3
NMe Ph
2.867
367
150
1703
1.1
0.9
−
2
NMe Bz
1.330
790
2
n
j
j
j
j
HN Pr
−
−
−
−
−
−
−
−
2
n
j
j
j
j
HNMe Bu
20
−
−
−
−
n
j
j
j
j
HN Bu
20
−
−
2
i
HN Pr
20
0.299
j
0.149
178
88
9.9
2
n
j
j
j
H N Bu
5
−
−
−
−
−
−
−
2
2
2
2
3
g
g
g
h
j
j
H NR
50
0.011
0.022
0.080
0.086
−
7
−
2
j
j
H NR
2
20
−
13
48
51
−
H NR
2
5
0.003
2
136
69.4
−
H NR
2
5
0.006
4
j
j
j
j
pyridine
10
−
−
−
−
a
All polymerizations were run for 3 h at 25 °C under ethylene at 100 psig in 25 mL of toluene with 10 μmol of dinickel complex. The number of
b
−1 −1
c
equivalents of base listed is the number of equivalents per nickel. TOF = turnover frequency in units of (mol of C H ) (mol of Ni) h . R =
2
4
d
[
(TOF for 1-a with no additive)/(TOF for 1-a with additive)]/[(TOF for 1-s with no additive)/(TOF for 1-s with additive)]. Polymerization was
e f g h i
1 2 3
run for 1.5 h. R = allyl. Polymerization was run for 1 h. R = 1,1-dimethylpropyl. R = 1,1,3,3-tetramethylbutyl. Different catalyst batches than in
entry 14. Insufficient product for accurate determination of the mass (<1 mg).
j
Table 2. Ethylene Polymerization Trials with 500 equiv of
N,N-Dimethylbutylamine per Ni
decrease the overall polymerization rate and polymer
a
23−26
yield.
While steric bulk from the ligand framework could
cause a decrease in deactivation by hindering the binding of
amine, the studied complexes showed similar inhibition profiles
for 1-a, 2-a, and 2-s in contrast to 1-s. The proximal
arrangement of the two metal centers in compound 1-s is
proposed to cause the difference in deactivation of 1-s relative
to 1-a, 2-a, or 2-s (Figure 3). Simultaneous binding of a bulky
base to each nickel center of 1-s is expected to be sterically
disfavored relative to binding of bases to all of the metal centers
of 1-a, 2-a, or 2-s. Hence, for 1-s, ethylene may compete
successfully with the amine for coordination to nickel. This has
the net effect of inhibiting deactivation by the base for 1-s
relative to 1-a, 2-a, or 2-s. Intriguingly, the proposed
mechanism might also be relevant to the polymerization of
olefins with binuclear cationic early transition metal catalysts,
with the couteranions acting as inhibiting bases instead of
b
c
entry
complex
yield (g)
TOF
Ra
1
2
3
4
5
6
7
8
1-s
1-s
1-a
1-a
2-s
2-s
2-a
2-a
0.047
0.066
0.019
0.028
0.012
0.010
0.053
0.048
28
39
11
17
7
15
11
190
130
105
121
57
6
31
29
62
a
All polymerizations were run for 3 h at 25 °C under ethylene at 100
b
psig in 25 mL of toluene with 20 μmol of nickel. TOF = turnover
−1
−1
c
frequency in units of (mol of C H ) (mol of Ni) h . R = (TOF
2
4
a
with no additive)/(TOF with additive).
inhibition recorded in ethylene polymerizations (Table 1,
entries 4, 14, 19, 27, and 29). The correlation suggests that
stronger amine binding to nickel increases the bimetallic effect.
The observed catalytic behavior suggests a bimetallic effect
on the extent of inhibition by added base. Polymer formation is
dependent on coordination of olefin and turnover-limiting
11−14
amines.
In agreement with the above mechanistic proposal, the extent
of inhibition was found to be dependent on the nature of the
amine. The smallest amines induced a smaller difference
between 1-s and 1-a. Binding of a smaller amine to one of the
nickel centers of 1-s leaves space to bind a second amine to the
other nickel center, thereby effecting inhibition similar to that
2
1,22
olefin insertion into the metal−polymeryl bond.
bases compete with olefin for coordination to the metal and
Lewis
1
480
dx.doi.org/10.1021/ja210990t | J. Am. Chem.Soc. 2012, 134, 1478−1481

Journal of the American Chemical Society
Communication
ASSOCIATED CONTENT
Supporting Information
Experimental procedures, characterization data, and crystallo-
■
*
S
graphic details for 1-s, 1-a, and 1-a-(1,1-dimethylpropylamine)2
(
CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
agapie@caltech.edu
■
ACKNOWLEDGMENTS
■
We thank Lawrence M. Henling for crystallographic assistance.
We are grateful to Caltech and Dow Chemical for funding. The
Bruker KAPPA APEXII X-ray diffractometer was purchased via
an NSF CRIF:MU award to Caltech (CHE-0639094). The 400
MHz NMR spectrometer was purchased via NIH Award
RR027690.
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dx.doi.org/10.1021/ja210990t | J. Am. Chem.Soc. 2012, 134, 1478−1481