Thermally Robust Heterobimetallic Palladium-Alkali Catalysts for Ethylene and Alkyl Acrylate Copolymerization

Article abstract of DOI:10.1021/acs.organomet.8b00561

We report on the synthesis and characterization of a new family of palladium-alkali olefin polymerization catalysts using site-differentiated dinucleating platforms. Metal binding studies indicate that our palladium phosphine phosphonate polyethylene glycol (PEG) complexes form 1:1 adducts with alkali cations in solution, with relative affinities in the order Na⁺ ≈ K⁺ > Li⁺. We observed that these palladium-alkali complexes are more active for ethylene homopolymerization and ethylene/alkyl acrylate copolymerization in comparison to their monopalladium counterparts, although their effect on polymer branching and molecular weight is relatively modest. In some cases, the addition of external sodium salts to conventional palladium complexes (i.e., those that do not have pendant PEG chains) also led to remarkable catalyst enhancements, presumably also due to the formation of palladium-sodium species. The unique features of our heterobimetallic systems are that they display long catalyst lifetimes at 100 °C and can even operate at temperatures as high as 140 °C. Although we have shown that secondary cations clearly increase the electrophilicity of the metal catalyst, their precise role in these polymerization reactions is still under investigation.

Full text of DOI:10.1021/acs.organomet.8b00561

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Thermally Robust Heterobimetallic Palladium−Alkali Catalysts for
Ethylene and Alkyl Acrylate Copolymerization
Zhongzheng Cai and Loi H. Do*
Department of Chemistry, University of Houston, 4800 Calhoun Road, Houston, Texas 77004, United States
S
* Supporting Information
ABSTRACT: We report on the synthesis and characterization of a new family of
palladium−alkali olefin polymerization catalysts using site-differentiated dinucleating
platforms. Metal binding studies indicate that our palladium phosphine phosphonate
polyethylene glycol (PEG) complexes form 1:1 adducts with alkali cations in
solution, with relative affinities in the order Na+ ≈ K+ > Li+. We observed that these
palladium−alkali complexes are more active for ethylene homopolymerization and
ethylene/alkyl acrylate copolymerization in comparison to their monopalladium
counterparts, although their effect on polymer branching and molecular weight is
relatively modest. In some cases, the addition of external sodium salts to conventional palladium complexes (i.e., those that do
not have pendant PEG chains) also led to remarkable catalyst enhancements, presumably also due to the formation of
palladium−sodium species. The unique features of our heterobimetallic systems are that they display long catalyst lifetimes at
100 °C and can even operate at temperatures as high as 140 °C. Although we have shown that secondary cations clearly increase
the electrophilicity of the metal catalyst, their precise role in these polymerization reactions is still under investigation.
Chart 1. Representative Examples of Late Transition Metal
Catalysts for Olefin Homo- and Copolymerization
INTRODUCTION
■
Greater than 70% of the world’s ethylene-based polymers is
manufactured using transition-metal-catalyzed processes.1
Because the intrinsic properties of polyolefins are determined
by how their building blocks are assembled,2 having a broad
assortment of metal catalysts to synthesize polyolefins is
required to meet the diverse needs of the global market. To be
industrially useful, metal catalysts must also be highly active,
tolerant of trace impurities, and compatible at high reactor
temperatures (e.g., >80 °C).3,4
The discovery that cationic late-transition-metal diimine
complexes are capable of copolymerizing ethylene with methyl
acrylate5 inspired many researchers to search for new families
of polar group compatible molecular catalysts (Chart 1).6−13
For example, the development of neutral palladium phosphine
sulfonate complexes by Drent et al.14 has enabled the
copolymerization of ethylene with difficult to incorporate
polar monomers such as vinyl acetate,15 vinyl halide,16,17
acrylonitrile,18 and acrylamide.19 Most recently, the research
groups of Nozaki20,21 and Jordan22,23 disclosed that cationic
palladium compounds ligated by P,O-ligands also gave active
copolymerization catalysts.24−26 A major drawback in many of
these metal-based systems, however, is that the presence of
polar monomers causes a significant decrease in catalytic
activity in comparison to that in the presence of just ethylene.9
Furthermore, the molecular weights of the copolymers also
tend to be low (Mn < 105). In addition to other economic and
environmental considerations (e.g., cost, time, waste, etc.),
these shortcomings have thus prevented late transition metal
complexes from being viable catalysts for commercial
polyolefin synthesis.
Our laboratory has been exploring new strategies to
overcome the deficiencies of conventional olefin polymer-
ization catalysts. We were inspired by several elegant examples
in which Lewis acids were used successfully to promote diverse
molecular transformations,27 including carbon dioxide hydro-
genation,28,29 carbon monoxide insertion,30,31 dihydrogen
activation,32,33 dioxygen activation,34,35 and C−H bond
oxidation.36 In previous studies, we discovered that the
incorporation of pendant Lewis acids into nickel phenox-
Received: August 4, 2018
© XXXX American Chemical Society
A
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yimine37 polyethylene glycol (PEG) complexes led to a
dramatic enhancement in their catalyst activity and changes
to the microstructures of their polymer products.38−40 An
intriguing question that arose from this work was whether the
performance-boosting effects of secondary cations is a general
catalyst design feature that could be exploited in other
molecular polymerization systems.41 The underlying hypoth-
esis in these studies is that catalysts bearing two functionally
distinct metal centers (i.e., heterobimetallics) could work
synergistically during polymerization to access reactivity
patterns that could not be achieved using either monometallic
or homobimetallic catalysts.42−44 Specifically, secondary Lewis
acids could modulate the electrophilicity of the active catalyst
or assist in discrete steps during polymerization, such as
monomer binding or chain propagation. Although we38,39 and
others45 have shown that outer-sphere interactions with Lewis
acidic metal cations could facilitate ethylene homopolymeriza-
tion, their applications in ethylene and polar vinyl olefin
copolymerization have not been explored extensively.46
diarylphosphine, followed by reaction with methyldiglycol
chlorophosphate (2), provided ligands 3a (Ar = phenyl) and
3b (Ar = 2-methoxyphenyl) in moderate yields. Metalation of
these ligands by treatment with Pd(COD)(Me)(Cl) (COD =
1,5-cyclooctadiene) gave complexes 4a and 4b, respectively.
Chloride abstraction from 4a/4b using AgSbF6 and reaction
with pyridine furnished complexes 5a/5b. Single crystals of
compounds 4b and 5b were analyzed by X-ray crystallography
(Figure 1A and Figure S60, respectively). In both structures,
Herein, we demonstrate that the pairing of alkali ions with
palladium phosphine phosphonate ester complexes22 imparts
greater reactivity and thermal stability to the parent catalysts in
both ethylene homo- and copolymerization. We show that the
key to success is the installation of two PEG chains to the
ligand’s phosphonate group, which provides a well-defined
binding site for alkali ions to yield discrete heterobimetallic
species even in the presence of excess polar monomers.
RESULTS AND DISCUSSION
■
Synthesis of P,O-Ligands and Their Palladium
Complexes. Our palladium phosphine phosphonate ester
complexes were readily obtained through the synthetic
sequence shown in Scheme 1. Lithiation of (2-bromophenyl)-
Scheme 1. Synthesis of Ligands 3 and Palladium Complexes
4 and 5
Figure 1. X-ray structures of the monopalladium 4b (A) and
heterobimetallic 4b-Na (B) (ORTEP view, displacement ellipsoids
drawn at 50% probability). Hydrogen atoms and borate anions are
omitted for clarity.
the palladium center is square planar and the methyl group is
trans to the phosphonate moiety. The bond metrics at the
palladium core in 5b are similar to those in the analogous
diethylphosphonate variant reported by Jordan and co-
workers.22
Secondary Alkali Ion Complexation. To investigate the
interactions of our PEGylated palladium compounds with
alkali ions, solution studies were carried out in chloroform.
Using the method of continuous variation,47,48 the NMR
spectra of 4a in the presence of various amounts of MBArF
4
salts (M = Li+, Na+, K+; BArF = tetrakis(3,5-
4
trifluoromethylphenyl)borate) were recorded (Tables S1−
S3). We observed that the Ha1 and Ha2 methylene hydrogen
atoms in the PEG chains of 4a (∼4.1 ppm) showed greater
chemical shift separations when larger amounts of M+ were
present (Figure 2 and Figures S1−S3).49 This change in Δδ
most likely reflects the different local environments of Ha1 vs
Ha2 in the more rigid heterobimetallic structures in comparison
to those in their monopalladium counterparts. The changes in
the chemical shifts of Ha1 were used to construct Job plots
(Figure 3; ΧPd is defined as [4a]/([4a] + [M+]). The peak
B
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Scheme 2. Reaction of Complex 6b with NaBArF to Give
4
a
6b-Na
a
The Na+ cation is proposed to bind to the PO donor of 6b, but
interactions with the other oxygen atoms of the phosphonate ester are
also possible.
1
Figure 2. H NMR spectra (CDCl3, 600 MHz) of complex 4a and
LiBArF mixed in different ratios. The mole fraction XPd is defined as
4
[4a]/([4a] + [Li+]). The peaks labeled as Ha1 and Ha2 are assigned to
the methylene hydrogen atoms attached to carbon 1 of the PEG
chains in 4a.
chloroform rather than in toluene, which is the solvent used
in our polymerization studies below, we expect that the
optimal 1:1 palladium−alkali binding stoichiometry should still
be maintained in the latter.
To probe the electronic effect of M+ on the palladium
complexes, cyclic voltammetry measurements were conducted
(Figure 4). Due to solubility reasons and the highly negative
Figure 3. Job plots for complex 4a with MBArF (M = Li+ (green
4
circles), Na+ (blue squares), K+ (red triangles)) in CDCl3. The total
concentration of 4a/MBArF is 6 mM for all data points.
4
Figure 4. Cyclic voltammograms of complexes 4a, 4a-Li, 4a-Na, and
4a-K (1 mM, bottom to top) recorded in THF at 0.05 V/s in THF
with 0.1 M NBu4PF6 supporting electrolyte. The data were acquired
using a glassy-carbon working electrode, platinum counter electrode,
and silver reference electrode. The reduction potentials measured are
as follows: 4a, −2.5 V; 4a-Li, −2.2 and −2.5 V; 4a-Na, −2.3 and −2.5
V; 4a-K, −2.4 and −2.5 V (difficult to deconvolute due to peak
broadness) (vs Fc/Fc+). The peaks at −2.5 V are assigned to the
presence of 4a rather than from the heterobimetallic palladium−alkali
species.
maxima of the Job plots for 4a/M+ all occur at ΧPd = 0.5,
which indicates that a 1:1 stoichiometry is optimal between
complex 4a and alkali ions. The slopes of the three plots
suggest that the alkali ion affinity of 4a follows the order Na+ ≈
K+ > Li+. The dinuclear structure of the palladium−sodium
complex 4b-Na was confirmed by X-ray crystallographic
analysis (Figure 1B). The structure revealed that the palladium
center in 4b-Na has the expected square-planar geometry, with
slightly modified bond distances in comparison to those in 4b
(Pd−O and Pd−Cl are longer, whereas Pd−P and Pd−C are
shorter). The sodium ion is six-coordinate due to ligation by
five oxygen donors and a bridging chloride. The Pd−Na bond
distance is ∼3.45 Å.
For complex 5b, which contains a pyridine donor instead of
chloride as in 4b, similar alkali ion binding behavior was
observed (Figures S4 and S5). Complexation of Na+ to 5b led
to a downfield shift of the phosphine oxide signal at 22 ppm to
23 ppm in the 31P NMR spectrum, which we have attributed to
the electron-withdrawing effects of sodium. Complex 6b ,on
the other hand (Scheme 2), showed more minor changes in its
NMR spectra when sodium salt was added in comparison to
5b (Figures S6 and S7). These data suggest that, although 6b
can form adducts with Na+, it does so more weakly than 5b,
which is expected since it lacks metal chelating PEG chains.
Although our metal binding studies were performed in
reduction potential of our compounds, THF was used as the
electrochemical solvent. In the absence of M+, 4a displayed a
cathodic peak at −2.5 V (vs ferrocene/ferrocenium), which we
have tentatively assigned to reduction of the Pd(II) center.
The cyclic voltammograms of 4a-Li, 4a-Na, and 4a-K showed
additional broad irreversible waves at approximately −2.2,
−2.3, and −2.4 V, respectively,50,51 and were ascribed to Pd-
centered reduction processes in the heterobimetallic species.
This trend is consistent with the electrophilicity of the alkali
ions,52 which would be expected to cause a decrease in the
electron density at the palladium core through electronic
induction.53 We believe that,in THF, an appreciable amount of
both monopalladium 4a and heterobimetallic 4a-M species are
present in equilibrium due to the lower alkali ion binding
affinity of 4a in coordinating solvents in comparison to that in
noncoordinating solvents (e.g., chloroform).
C
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a
Table 1. Ethylene Homopolymerization Data for 5a,b and 6b
b
c
c
entry
complex
salt
polymer yield (g)
activity (kg/(mol·h))
branches/1000 C
Mn(×103)
Mw/Mn
1
2
3
4
5
6
7
8
5a
5a
5b
5b
5b
5b
6b
6b
none
Na+
none
Li+
Na+
K+
trace
trace
2.33
6.15
6.75
4.67
3.95
8.95
0
0
233
615
675
467
395
895
7
11
10
8
9
10
2.17
2.15
2.90
2.90
1.66
1.82
1.27
1.65
1.46
1.50
1.48
1.70
none
Na+
a
Polymerization conditions: Pd catalyst (5 μmol), MBArF (5 μmol, if any), ethylene (400 psi), 2 mL of DCM, 48 mL of toluene, 2 h at 80 °C.
4
b
c
The total number of branches per 1000 carbons was determined by 1H NMR spectroscopy. Determined by GPC in trichlorobenzene at 150 °C.
Ethylene Homopolymerization. Our palladium phos-
to catalyst stability, we observed that the palladium−sodium
6b-Na complex is less active at high temperatures in
comparison to PEG-ligated 5b-Na compounds (vide infra).
Temperature Study. To determine the optimal reaction
temperature, we screened several catalysts in ethylene
polymerization from 80 to 140 °C in mesitylene/dichloro-
methane (24/1) for 1 h (Figure 5). For the compounds 5b,
phine phosphonate PEG complexes were tested as catalysts for
ethylene homopolymerization (Table 1). We found that, at 80
°C in toluene under 400 psi of ethylene, complex 5a was
completely inactive (entry 1) whereas complex 5b displayed
moderate activity (entry 3, activity = 233 kg/(mol·h)). The
poor reactivity of 5a is most likely due to the insufficient steric
protection of the palladium center, which is typically required
to help promote chain growth over chain termination. In
comparison, the Jordan-type catalyst 6b yielded polyethylene
with an activity of 395 kg/(mol·h) under the same reaction
conditions (entry 7).22 In all cases, the polymers produced
were relatively linear (10 branches or less per 1000 carbons)
and their molecular weights are low (Mn ≈ (1.66−2.17) ×
103), which is consistent with other reported Pd(P,O-ligand)
systems.21 The slower reaction rate of 5b in comparison to that
of 6b suggests that the free PEG chains in the former might be
self-inhibiting, perhaps by competing with ethylene for
coordination to the palladium catalyst.
Figure 5. Comparison of the activities of catalysts 5b, 5b-Na, 6b, and
6b-Na in ethylene homopolymerization at various temperatures. See
Table S4 for more details.
Next, we proceeded to evaluate the effects of alkali salts on
the catalyst’s reactivity toward ethylene (Table 1). Using the
same polymerization conditions as above, we observed that the
reaction of 5b and MBArF4 (1:1) with ethylene led to catalytic
rate enhancements of about 2.6×, 2.9×, and 2.0× for Li+ (entry
4), Na+ (entry 5), and K+ (entry 6), respectively, in
comparison to 5b alone. The polymer molecular weight and
polydispersity remained relatively constant in both the
presence and absence of alkali ions. The activity trend
observed, Na+ > Li+ > K+, was somewhat surprising because
we noted previously that potassium ions had a more beneficial
effect on nickel phenoxyimine-PEG catalysts than lithium
ions.38,39 In the current system, we hypothesize that a
combination of at least two different factors account for the
heterobimetallic effectthe electrophilicity (i.e., Li+ > Na+ >
K+)52 and the association constant (i.e., Na+ ≈ K+ > Li+) of the
secondary metals.
5b-Na, 6b, and 6b-Na, reactions at 100 °C afforded the highest
activity and their relative rates were generally observed in the
order 5b-Na > 6b-Na > 6b > 5b. The only exception to this
trend is that 6b-Na is about 1.5× more active than 5b-Na at 80
°C (Table S4, entry 5 vs 13). At elevated temperatures,
however, the opposite was observed. For example, at 140 °C,
5b-Na (1065 kg/(mol·h)) was about 1.8× faster than 6b-Na
(585 kg/(mol·h)). In this temperature regime, both
palladium−sodium complexes were far more active than their
corresponding parent 5b and 6b complexes (activity = 85 and
32 kg/(mol·h), respectively). Our temperature studies revealed
two notable features of this catalyst system. First, for
polymerizations conducted at 80 °C or below, the addition
of stoichiometric amounts of alkali salts to standard palladium
phosphine phosphonate ester complexes (e.g., those that are
not functionalized with PEG chains) is sufficient to boost
catalyst activity. Second, to maximize outer-sphere interactions
at high temperatures, the catalyst supporting ligands should
have suitably strong secondary metal chelators to prevent
dissociation of the heterobimetallic structure.
Interestingly, the reaction of 6b and NaBArF (1:1) with
4
ethylene led to activity enhancements of about 2.3× in
comparison to 6b alone (Table 1, entry 7 vs 8). Because 6b
can associate weakly with Na+, presumably via metal chelation
by the phosphonate ester group (Figures S6 and S7), this
result is consistent with our proposed heterobimetallic model.
However, we have not discounted other possibilities, such as
changes to the solvent medium due to the addition of salts.
Since NaBArF4 does not dissolve in neat toluene without either
binding to the palladium catalyst first or mixing with
surfactants, it is not likely to contribute to the ionic strength
of the reaction solution. To support our claim that the
introduction of PEG chains to the ligand platform is beneficial
Time Study. To examine the catalyst lifetimes, we carried
out time-dependent polymerizations for both complexes 5b-Na
and 6b at 100 °C (Table S5). During the period from 15 to 60
min, both catalysts maintained their catalytic performance.
However, 5b-Na showed about an ∼1.9× greater activity
(average = 2605 kg/(mol·h)) than 6b (average = 1343 kg/
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a
Table 2. Ethylene and Alkyl Acrylate Copolymerization Data for 5b and 6b
comonomer (concn,
M)
salt
(equiv)
temp
(°C)
polymer yield
(g)
activity
(kg/(mol·h))
branches/
b
c
d
d
entry
cat.
1000 C
inc (%) Mn(×103) Mw/Mn
1
2
3
4
5
6
7
MA (1.5)
MA (1.5)
MA (1.5)
MA (1.5)
MA (1.5)
MA (1.5)
MA (1.5)
BA (1.0)
BA (1.5)
BA (1.0)
BA (1.0)
BA (1.5)
BA (1.0)
BA (1.0)
BA (1.0)
BA (1.5)
BA (1.5)
5b
5b
5b
5b
5b
6b
6b
5b
5b
5b
5b
5b
5b
5b
6b
6b
6b
none
80
80
80
80
80
0.68
1.54
1.26
1.79
1.22
1.12
1.28
1.41
0.86
trace
1.67
1.37
1.44
1.25
1.57
0.97
1.13
34
76
63
90
61
56
64
70
43
0
84
69
72
63
78
48
56
7
5
5
5
5
5
5
8
8
1.4
1.5
1.5
1.4
1.3
1.5
1.5
1.2
1.6
1.15
1.63
1.73
3.15
1.99
0.90
0.98
1.04
1.43
1.67
2.00
2.17
1.24
2.00
2.54
1.83
1.64
1.28
Li+ (1.0)
Na+ (1.0)
Na+ (5.0)
K+ (1.0)
none
80
80
Na+ (1.0)
none
none
none
Na+ (1.0)
Na+ (1.0)
Na+ (5.0)
Na+ (1.0)
none
e
8
9
100
100
120
100
100
100
120
100
100
100
e
e
10
11
12
13
14
15
16
17
e
6
6
6
7
5
8
8
1.2
1.9
1.7
1.3
1.0
1.7
1.6
2.30
2.48
2.14
1.73
0.99
1.03
1.14
1.47
1.56
1.41
1.55
2.16
1.93
2.07
e
e
e
e
e
none
Na+ (1.0)
e
a
Polymerization conditions unless specified otherwise: Pd catalyst (10 μmol), MBArF4 (10 μmol, if any), ethylene (400 psi), 50 mL total solution
b
c
1
1
volume in toluene, 2 h. The total number of branches per 1000 carbons was determined by H NMR spectroscopy. Determined by H NMR
spectroscopy. Determined by GPC in trichlorobenzene at 150 °C. These reactions were performed in mesitylene/dichloromethane (24/1) in a
total volume of 50 mL.
d
e
(mol·h)). The thermal robustness of the 5b-Na complex is
exciting from an industrial standpoint because most large-scale
commercial solution polymerizations are conducted at 140 °C
or above.3 In fact, homogeneous late-transition-metal catalysts
that can operate within this temperature regime are relatively
uncommon.1,54−58
spectra of 5b-Na remained unchanged when up to 100 equiv of
MA was added. An important point to keep in mind, however,
is that under actual catalytic conditions the amount of MA
present is greater than 7000× that of the palladium catalyst.
Thus, is it entirely possible that in our polymerization
reactions, the dissociation of Na+ from 5b-Na can occur
much more readily.
To investigate the copolymerization behavior of our catalysts
at high temperatures, we replaced MA with tert-butyl acrylate
(BA) as the polar monomer (Table 2). In general, we observed
that increasing the starting BA concentration enhanced the
percentage of BA incorporation into the copolymer. Although
the addition of sodium salts to 5b led to modest increases in
catalyst activity and molecular weight under certain conditions
(e.g., entry 9 vs entry 12), its most pronounced effect was on
the catalyst’s thermal stability. For example, at 120 °C, the
activity of 5b-Na was 63 kg/(mol·h) (entry 14), whereas that
of 5b was nearly zero (entry 10). For benchmarking, the
addition of Na+ to 6b led to a 1.2× improvement in reaction
yield (entry 16 vs entry 17) at 100 °C. Importantly, all of the
poly(ethylene-co-tert-butyl acrylate) obtained showed relatively
narrow Mw/Mn values (1.28−2.23), which suggest that our
heterobimetallic catalysts are single-site species.
Ethylene and Alkyl Acrylate Copolymerization.
Finally, our palladium catalysts were tested in ethylene and
methyl acrylate (MA) copolymerization (Table 2). The
reaction of 5b with ethylene/MA at 80 °C afforded linear
poly(ethylene-co-methyl acrylate) containing ∼1.4 mol % of in-
chain polar groups (entry 1). Under similar polymerization
conditions, the 5b-M complexes also furnished copolymers
with low molecular weight and modest MA ratio. Consistent
with our ethylene homopolymerization studies above, the
heterobimetallic complexes were more active than their
monopalladium counterparts. The highest activity was
achieved using 5 equiv of NaBArF relative to 5b, which gave
4
about a 2.6× improvement over that of the monopalladium
complex itself (compare entry 1 vs entry 4). A slight increase in
the copolymer Mn was also obtained in the presence of sodium
ions (Mn = 3.15 × 103 for 5b-Na vs 1.15 × 103 for 5b). The
polycrystalline copolymers displayed Tm values ranging from
102 to 113 °C (Table S7). Comparison of the polymerization
results between 6b (entry 6) vs 6b-Na (entry 7) showed that
the two catalysts behaved quite similarly, although 6b-Na (64
kg/(mol·h)) is slightly more active than 6b (56 kg/(mol·h)).
Interestingly, the main differences between the Jordon-type
catalyst 6b and the heterobimetallic 5b-M catalysts are that the
latter consistently gave copolymers with somewhat higher
activity and molecular weight. Although these heterobimetallic
effects are relatively small, they do suggest that the addition of
alkali salts is beneficial to the copolymerization reactions.
To determine whether methyl acrylate can displace the alkali
ions from the heterobimetallic complexes, we measured the
1H/31P NMR spectra of 5b-Na in the presence of MA (Figures
S58 and S59). Interestingly, our data showed that the NMR
CONCLUSION
■
In summary, we have provided a simple and versatile strategy
to site selectively incorporate secondary metals into conven-
tional palladium catalyst platforms. We have shown that the
pairing of alkali ions with late-transition-metal complexes could
lead to significant catalyst enhancements, including improve-
ments in their reaction rates and thermal stability. A surprising
result in this study was that the addition of external alkali salts
to simple palladium phosphine diethylphosphonate complexes
was sufficient to generate palladium−alkali species that are
more active than their parent complexes. However, we found
that the use of PEGylated palladium analogues could better
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dissolved in DCM (5 mL) at room temperature. The reaction mixture
was stirred at room temperature for 1 h and then filtered through a
pipet plug. The filtrate was dried under vacuum. Pentane was added
to the residue and the mixture stirred until a white solid formed (125
support secondary metals, which was most clearly demon-
strated in high-temperature polymerization reactions.
This work raises several intriguing questions that still need
to be addressed. First, it is unclear what the precise role of the
secondary metals during polymerization is. As indicated by
cyclic voltammetry measurements, the alkali ions increase the
electrophilicity of the palladium complexes. At the same time,
however, binding of the alkali metals also leads to bulking up of
the catalyst structure. Thus, to what extent do steric vs
electronic factors contribute to the heterobimetallic effect? The
precise effects of steric or electronic variations on catalyst
reactivity are sometimes difficult to rationalize, since other
factors such as catalyst stability and solubility could also play a
role.59−61 Second, in our copolymerization studies, the
heterobimetallic catalysts seemed to have little influence on
the comonomer incorporation ratio in comparison to the
monometallic catalyts. We have preliminary data suggesting
that ethylene binding and insertion into palladium−alkyl
acrylate intermediates is faster when secondary cations are
present. However, how does this change affect the relative rates
of other elementary steps in the polymerization process?
Future studies will focus on investigating these and other
questions, which we anticipate will lead to a better under-
standing of the use of outer-sphere Lewis acids in catalyst
design. Ultimately, these studies will allow us to create novel
catalyst constructs to access a wider range of ethylene-based
polymers for both common and specialized applications.
1
mg, 0.18 mmol, 83%). H NMR (CDCl3, 600 MHz): δ (ppm) 7.98
(dd, JPH = 5.2 Hz, JHH = 2.0 Hz), 7.55−7.38 (m, 12H), 7.12 (m, 1H),
4.17−4.10 (m, 4H), 3.55−3.44 (m, 12H), 3.31 (s, 6H), 0.69 (d, JPH
=
2.4 Hz, 3H). 13C NMR (CDCl3, 100 MHz): δ (ppm) 136.62 (m),
134.83 (m), 134.45 (d, JPC = 12.7 Hz), 133.16 (m), 132.79 (m),
131.21, 130.76 (d, JPC = 13.2 Hz), 129.63, 129.13, 128.88 (d, JPC
=
10.7 Hz), 71.87, 70.35, 69.82 (d, JPC = 6.8 Hz), 66.82 (d, JPC = 5.8
Hz), 59.13, 0.93 (note: the signals in the aromatic region could not be
assigned due to overlapping peaks). 31P NMR (CDCl3, 162 MHz): δ
(ppm) 32.15 (d, JPP = 13 Hz), 20.76 (d, JPP = 13 Hz). Anal. Calcd for
C29H39ClO7P2Pd: C, 49.52; H, 5.59. Found: C,49.34; H, 5.55.
Preparation of 4b. Inside the drybox, compound 3b (165 mg, 0.27
mmol, 1.0 equiv) and Pd(COD)(Me)(Cl) (72 mg, 0.27 mmol, 1.0
equiv) were combined in a small vial and then dissolved in DCM (5
mL) at room temperature. The reaction mixture was stirred at room
temperature for 1 h and then filtered through a pipet plug. The filtrate
was dried under vacuum and washed with Et2O to form a white solid
1
(185 mg, 0.24 mmol, 89%). H NMR (CDCl3, 600 MHz): δ (ppm)
7.88 (m, 1H), 7.51−7.46 (m, 3H), 7.41 (t, JHH = 7.2 Hz, 1H), 7.34
(br s, 1H), 7.29−7.25 (m, 2H), 6.95 (t, JHH = 7.2 Hz, 2H), 6.91 (dd,
JHH = 8.4 Hz, JPH = 4.8 Hz, 2H), 4.22−4.07 (m, 4H), 3.64 (s, 6H),
3.61−3.46 (m, 12H), 3.34 (s, 6H), 0.57 (d, JPH = 3 Hz, 3H). 13C
NMR (CDCl3, 125 MHz): δ (ppm) 160.72 (d, JPC = 3.8 Hz), 136.52,
136.50 (dd, JPC = 34.9, 10.7 Hz), 134.50 (d, JPC = 15.9 Hz), 134.10 (t,
J
PC = 6.4 Hz), 133.30, 131.45 (dd, JPC = 6.1, 2.5 Hz), 130.26 (dd, JPC
= 185.8, 17.1 Hz), 129.82 (d, JPC = 13.5 Hz), 120.92 (d, JPC = 9.8
Hz), 116.14 (d, JPC = 51.4 Hz), 111.23 (d, JPC = 5.0 Hz), 71.87, 70.35,
70.03 (d, JPC = 7.4 Hz), 66.54 (d, JPC = 6.1 Hz), 59.11, 55.47, −0.22.
31P NMR (CDCl3, 243 MHz): δ (ppm) 23.39 (d, JPP = 8.3 Hz), 21.51
(d, JPP = 8.2 Hz). Anal. Calcd for C31H43ClO9P2Pd: C, 48.77; H, 5.68.
Found: C, 48.75; H, 5.85.
Preparation of 5a. Inside the drybox, 4a (76 mg, 0.11 mmol, 1.0
equiv) and AgSbF6 (37 mg, 0.11 mmol, 1.0 equiv) were combined in
a small vial. A solution of DCM (5 mL) and pyridine (0.1 mL) was
added at room temperature, and the reaction mixture was stirred for 1
h. The mixture was then filtered through a pipet plug and the filtrate
was dried under vacuum. A solution of Et2O was added to wash the
residue to give a sticky oil (86 mg, 0.09 mmol, 81%). Trace amounts
of residual solvent could not be removed completely by vacuum
drying. 1H NMR (CDCl3, 400 MHz): δ (ppm) 8.71 (d, JHH = 4.8 Hz,
2H), 8.13 (m, 1H), 7.93 (m, 1H), 7.67−7.47 (m, 14H), 7.15 (m,
EXPERIMENTAL SECTION
■
General Procedures. Commercial reagents were used as received.
All air- and water-sensitive manipulations were performed using
standard Schlenk techniques or under a nitrogen atmosphere using a
glovebox. Anhydrous solvents were obtained from an Innovative
Technology solvent drying system saturated with argon. High-purity
polymer grade ethylene was obtained from Matheson TriGas without
further purification. The compounds (2-bromophenyl)-
diphenylphosphine62 and Pd(COD)(Me)(Cl)63 were prepared
according to literature procedures.
NMR spectra were acquired using JEOL spectrometers (ECA-400,
-500, and -600) and referenced using residual solvent peaks. All 13C
NMR spectra were proton decoupled. 31P NMR spectra were
referenced to phosphoric acid. For polymer characterization, we
1H), 4.07 (m, 4H), 3.54−3.39 (m, 12H), 3.32 (s, 6H), 0.58 (d, JPH
=
1
3.2 Hz, 3H). 13C NMR (CDCl3, 100 MHz): δ (ppm) 150.11, 139.26,
135.44 (m), 134.99 (m), 134.35 (d, JPC = 9.9 Hz), 133.41 (m),
131.97, 131.60, 131.49 (m), 129.39 (d, JPC = 8.8 Hz), 128.29, 127.86,
125.86, 71.76, 70.25, 69.34 (d, JPC = 5.9 Hz), 67.39 (d, JPC = 5.8 Hz),
59.01, 3.95 (note: the signals in the aromatic region could not be assigned
due to overlapping peaks). 31P NMR (CDCl3, 243 MHz): δ (ppm)
33.11 (d, JPP = 19.4 Hz), 21.01 (d, JPP = 19.4 Hz).
used the following techniques. H NMR spectroscopy: each NMR
sample contained ∼20 mg of polymer in 0.5 mL of 1,1,2,2-
tetrachloroethane-d2 (TCE-d2) and was recorded on a 500 MHz
spectrometer using standard acquisition parameters at 120 °C.64 13C
NMR spectroscopy: each NMR sample contained ∼50 mg of polymer
and 50 mM (8.7 mg) chromium acetylacetonate Cr(acac)3 in 0.5 mL
of TCE-d2 and was recorded at 120 °C (125 MHz). The samples were
acquired using a 90° pulse of 11.7 μs, a relaxation delay of 4 s, an
acquisition time of 0.81 s, and inverse gated decoupling. The samples
were preheated for 30 min prior to data acquisition. The carbon
spectra were assigned on the basis of the chemical shift values
reported in the literature.12 High-resolution mass spectra were
obtained from the mass spectral facility at the University of Houston.
Elemental analyses were performed by Atlantic Microlab.
Gel permeation chromatography (GPC) data were obtained using
a Malvern high temperature GPC instrument equipped with refractive
index, viscometer, and light scattering detectors at 150 °C with 1,2,4-
trichlorobenzene (stabilized with 125 ppm BHT) as the mobile phase.
A calibration curve was established using polystyrene standards in
triple detection mode. All molecular weights reported are based on
triple detection.
Preparation of 5b. Inside the drybox, 4b (155 mg, 0.20 mmol, 1.0
equiv) and AgSbF6 (70 mg, 0.20 mmol, 1.0 equiv) were combined in
a small vial. A solution of DCM (10 mL) and pyridine (0.1 mL) was
added at room temperature, and the reaction mixture was stirred for 1
h. The mixture was then filtered through a pipet plug and the filtrate
was dried under vacuum. A solution of Et2O was added to wash the
residue to give a sticky oil (201 mg, 0.19 mmol, 95%). Trace amounts
of residual solvent could not be removed completely by vacuum
drying. 1H NMR (CDCl3, 600 MHz): δ (ppm) 8.67 (d, JHH = 3.6 Hz,
2H), 7.97 (m, 1H), 7.92 (t, JHH = 7.2 Hz, 1H), 7.58−7.56 (m, 5H),
7.51 (t, 7.8 Hz, 1H), 7.43−7.31 (m, 3H), 7.05 (t, JHH = 7.8 Hz, 2H),
6.99 (dd, JHH = 8.4 Hz, JHH = 4.8 Hz, 2H), 4.00−3.87 (m, 4H), 3.68
(s, 6H), 3.54−3.42 (m, 12H), 3.32 (s, 6H), 0.39 (d, JPH = 3.6 Hz,
3H). 13C NMR (CDCl3, 100 MHz): δ (ppm) 160.70 (d, JPC = 2.9
Hz), 150.15, 139.02, 136.80 (d, JPC = 11.7 Hz), 135.03 (d, JPC = 16.5
Hz), 134.62 (dd, JPC = 26.2, 3.6 Hz), 134.36, 134.05 (t, JPC = 8.3 Hz),
Synthesis. Preparation of 4a. Inside the drybox, compound 3a
(117 mg, 0.21 mmol, 1.0 equiv) and Pd(COD)(Me)(Cl) (57 mg,
0.21 mmol, 1.0 equiv) were combined in a small vial and then
132.15 (d, JPC = 16 Hz), 130.52 (d, JPC = 14.5 Hz), 128.95 (dd, JPC
=
F
DOI: 10.1021/acs.organomet.8b00561
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
187.6, 17.6 Hz), 125.70, 121.27 (d, JPC = 11.6 Hz), 114.49 (d, JPC
56 Hz), 111.73, 71.77, 70.28, 69.42 (d, JPC = 5.8 Hz), 66.84 (d, JPC
=
=
increased to 400 psi of ethylene and the contents were stirred
vigorously for 2 h. To stop the polymerization, the autoclave was
vented and cooled in an ice bath. A solution of MeOH (100−200
mL) was added to precipitate the polymer. The polymer was collected
by vacuum filtration, rinsed with MeOH, and dried under vacuum at
80 °C overnight. The reported yields are average values of triplicate
runs and typically have less than 5% deviation by weight.
6.8 Hz), 59.02, 55.56, 3.19. 31P NMR (CDCl3, 243 MHz): δ (ppm)
23.93 (d, JPP = 14.3 Hz), 21.54 (d, JPP = 14.3 Hz).
Preparation of 6b. Inside the drybox, the palladium phosphine
diethylphosphonate chloride complex22 (115 mg, 0.19 mmol, 1.0
equiv) and AgSbF6 (65 mg, 0.19 mmol, 1.0 equiv) were combined in
a small vial. A solution of DCM (10 mL) and pyridine (0.1 mL) was
added at room temperature, and the reaction mixture was stirred for 1
h. The mixture was then filtered through a pipet plug and the filtrate
was dried under vacuum. A solution of Et2O was added to wash the
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00561.
■
S
1
residue to give a white solid (149 mg, 0.17 mmol, 89%). H NMR
(CDCl3, 600 MHz): δ (ppm) 8.66 (d, JHH = 4.9 Hz, 2H), 7.93 (t, 7.8
Hz, 1H), 7.83 (m, 1H), 7.62−7.53 (m, 6H), 7.38−7.34 (m, 3H), 7.05
(t, JHH = 7.5 Hz, 2H), 6.99 (dd, JHH = 8.4 Hz, JHH = 4.8 Hz, 2H),
3.87(m, 4H), 3.68 (s, 6H), 1.10 (t, JHH = 7.0 Hz, 6H), 0.38 (d, JPH
3.0 Hz, 3H). 13C NMR (CDCl3, 100 MHz): δ (ppm) 160.66 (d, JPC
2.2 Hz), 150.11, 139.14, 136.81 (d, JPC = 11.3 Hz), 135.24 (d, JPC
14.1 Hz), 134.80 (dd. JPC = 46.7, 11.4 Hz) 134.36, 133.42 (dd, JPC
Experimental procedures, characterization of com-
pounds and polymers, NMR spectra, and crystallo-
graphic data (PDF)
=
=
=
=
Accession Codes
9.3, 8.2 Hz), 132.11 (d, JPC = 7.0 Hz), 130.63 (d, JPC = 12.2 Hz),
129.18 (dd, JPC = 184.7, 17.5 Hz) 125.74, 121.24 (d, JPC = 11.3 Hz),
114.49 (d, JPC = 56 Hz), 111.73 (d, JPC = 4.4 Hz), 64.44 (d, JPC = 6.4
Hz), 55.58, 15.89 (d, JPC = 6.5 Hz), 3.12. 31P NMR (CDCl3, 243
MHz): δ (ppm) 24.11 (d, JPP = 14.1 Hz), 21.08 (d, JPP = 14.1 Hz).
Metal Binding Studies. The method of continuous variation
(Job plot analysis) was used to determine the binding stoichiometry
of our palladium complexes with alkali ions.48 To perform these
CCDC 1848561−1848563 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
experiments, stock solutions of 4a (6 mM, 6 mL) and MBArF (6
AUTHOR INFORMATION
Corresponding Author
*E-mail for L.H.D.: loido@uh.edu.
4
■
mM, 15 equiv of Et2O to solubilize the salts, 6 mL, M = Li+, Na+, K+)
were prepared separately in CDCl3. Various amounts of each stock
solution were placed in an NMR tube so that a total volume of 1 mL
was obtained. Ten different NMR samples were prepared, each
containing a different 4a:M ratio. The samples were recorded at room
ORCID
Loi H. Do: 0000-0002-8859-141X
Notes
The authors declare no competing financial interest.
1
temperature by H NMR spectroscopy. As shown in Figures S1−S3,
the two hydrogen resonances centered at ∼4.1 ppm corresponding to
the C1 methylene unit of the PEG chains in 4a shift in the presence of
alkali ions. The chemical shift separation between Ha1 and Ha2
ACKNOWLEDGMENTS
1
■
increases as the Pd mole fraction decreases. The changes in the H
We are grateful to the Welch Foundation (Grant No. E-1894),
the ACS Petroleum Research Fund (Grant No. 54834-DNI3),
and the National Science Foundation (Grant No. CHE-
1750411) for their generous support of this research. We also
thank Po-Ni Lai for help with cyclic voltammetry measure-
ments and Prof. Tom Teets for use of a potentiostat.
NMR signals of Ha as a function of the mole fraction of 4a are
provided in Tables S1−S3.
Ethylene Polymerization. Inside the drybox, the palladium
complexes (5 μmol) and alkali salts (5 μmol) were dissolved in 10 mL
of toluene/DCM (8/2) and stirred for 10 min. By visual inspection,
the resulting palladium−alkali complexes appeared to be soluble in
the reaction mixture. The mixture was sealed inside a vial using a
rubber septum and brought outside of the drybox. Under an
atmosphere of N2, the catalyst solution was loaded into a syringe. To
prepare the polymerization reactor, 40 mL of dry toluene was placed
in an empty autoclave and preheated to the desired temperature. The
autoclave was purged with ethylene (20 psi) for 1 min, and then the
catalyst solution was injected into the autoclave via syringe. The
reactor pressure was increased to 400 psi of ethylene, and the contents
were stirred vigorously for 2 h. To stop the polymerization, the
autoclave was vented and cooled in an ice bath. A solution of MeOH
(100−200 mL) was added to precipitate the polymer. The polymer
was collected by vacuum filtration, rinsed with MeOH, and dried
under vacuum at 80 °C overnight. The reported yields are average
values of triplicate runs and typically have less than 5% deviation by
weight.
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DOI: 10.1021/acs.organomet.8b00561
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