Substituent effects of the backbone in α-diimine palladium catalysts on homo- and copolymerization of ethylene with methyl acrylate

Article abstract of DOI:10.1021/om300380b

On the basis of different approaches for modifying α-diimine palladium catalysts, a series of methyl chloride palladium complexes with various α-diimine ligand backbones were synthesized and characterized. The corresponding cationic palladium complex chelating esters were further obtained by treatment of methyl chloride palladium complexes with methyl acrylate (MA). It was observed that decomposition of a cationic palladium complex chelating ester can occur to produce a new cationic palladium complex chelating two ligands and two counteranions, which provides a new pathway for deactivation of palladium catalysts and formation of palladium black by a fragmentation pattern with ester loss. These α-diimine palladium catalysts were employed in the homopolymerization of ethylene and copolymerization of ethylene and MA to evaluate substituent effects of the ligand backbone. A bulky camphyl α-diimine palladium catalyst was found to show better thermal stability and afford high-molecular-weight copolymer with higher incorporation of polar monomer. Longstanding living polymerization of ethylene was also achieved within 12 h using a bulky camphyl α-diimine palladium catalyst.

Full text of DOI:10.1021/om300380b

Article
pubs.acs.org/Organometallics
Substituent Effects of the Backbone in α‑Diimine Palladium Catalysts
on Homo- and Copolymerization of Ethylene with Methyl Acrylate
Lihua Guo,†,‡ Haiyang Gao,*,† Qirui Guan,† Haibin Hu,† Juean Deng,† Jun Liu,† Fengshou Liu,†
and Qing Wu*,†
†DSAPM Laboratory, PCFM Laboratory, Institute of Polymer Science, School of Chemistry and Chemical Engineering, Sun Yat-Sen
University, Guangzhou 510275, People's Republic of China
‡The Key Laboratory of Life-Organic Analysis, School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu,
Shandong 273165, People's Republic of China
S
* Supporting Information
ABSTRACT: On the basis of different approaches for modifying α-diimine palladium
catalysts, a series of methyl chloride palladium complexes with various α-diimine
ligand backbones were synthesized and characterized. The corresponding cationic
palladium complex chelating esters were further obtained by treatment of methyl
chloride palladium complexes with methyl acrylate (MA). It was observed that
decomposition of a cationic palladium complex chelating ester can occur to produce a
new cationic palladium complex chelating two ligands and two counteranions, which
provides a new pathway for deactivation of palladium catalysts and formation of
palladium black by a fragmentation pattern with ester loss. These α-diimine palladium
catalysts were employed in the homopolymerization of ethylene and copolymerization
of ethylene and MA to evaluate substituent effects of the ligand backbone. A bulky
camphyl α-diimine palladium catalyst was found to show better thermal stability and
afford high-molecular-weight copolymer with higher incorporation of polar monomer.
Longstanding living polymerization of ethylene was also achieved within 12 h using a bulky camphyl α-diimine palladium catalyst.
in this process. Increasing the steric bulk of the ligand is
therefore expected to make this process less favorable, thus
enhancing the stability of the palladium catalyst. Numerous
modifications to the α-diimine ligand have been reported,
especially to the aniline moiety of the ligand.9,12,27−30 For
instance, cyclophane-based α-diimine palladium catalysts
designed and synthesized by Guan27 show better thermal
stability for olefin polymerization and higher incorporation of
methyl acrylate (MA) for copolymerization of ethylene and MA
than the acyclic analogues.28 The electronic effect of aniline
moieties on the homo- and copolymerization of ethylene with
MA has also been studied by Guan in detail.29,30 In comparison
with modification of the aniline moiety, modification of the
backbone in α-diimine palladium catalysts has been rarely
studied. To date, mainly three types of α-diimine ligand
backbones (ArNC(R)(R)CNAr, R = H, Me, acenaph-
thene) have been used to synthesize palladium complexes.9,19,31
Current literature has shown that changing the backbone
substituent from hydrogen to a methyl group results in a
significant increase in stability of the palladium catalyst and a
decrease in chain transfer occurring via associative exchange
from a palladium olefin hydride intermediate.26 Increasing the
bulk of the backbone substituent is thereby anticipated to
INTRODUCTION
■
Since Brookhart’s initial reports, α-diimine palladium catalysts
for olefin polymerization have received much attention because
they can produce polyolefins with various types of branches and
have good polar group tolerance.1−19 One attractive feature of
α-diimine palladium catalysts is their ability to precisely control
polymer topology (linear, hyperbranched, and dendritic
polyethylene) by varying ethylene pressure and polymerization
temperature.5−9 The reduced oxophilicity of the palladium
center also gives the catalyst the ability to produce copolymers
with a variety of polar-functionalized olefins.2,3,9,15,19 Further-
more, a tandem polymerization strategy combining chain-
walking polymerization using α-diimine palladium catalysts
with controlled radical polymerizations such as atom transfer
radical polymerization (ATRP)20−24 or reversible addition−
fragmentation chain transfer (RAFT) polymerization25 has also
been developed to synthesize functional copolymers.
Despite these striking features, α-diimine palladium catalysts
are prone to deactivation under comparatively mild conditions
via pathways that are still not completely clear. Palladium black
in the polymerizing mixture is usually observed, strongly
indicating the occurrence of catalyst decomposition during the
polymerization process.19 Brookhart and co-workers have
shown that deactivation of α-diimine palladium catalysts can
take place by C−H activation with alkyl groups on the α-
diimine ligand to form palladacyclic intermediates.26 Rotation
of the ligand alkyl groups to the metal square plane is necessary
Received: May 7, 2012
Published: August 14, 2012
© 2012 American Chemical Society
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improve the stability and catalytic activity of the palladium
catalyst.
group. All of the cationic palladium complexes were
characterized by elemental analysis and NMR (see the
Experimental Section). Brookhart previously reported that the
five-membered chelate isomer was found in an α-diimine
palladium complex with a methyl backbone (C5a), though the
six-membered chelate is always predominant (>80%).2,3 In
contrast, 13C NMR spectra of the obtained cationic palladium
complexes confirm that only one six-membered chelate isomer
exists in solution under the determined conditions. Only one
resonance assigned to carbonyl (CO) in the range 182.7−
188.3 ppm was observed for each cationic palladium complex,
suggesting the absence of the five-membered chelate isomer.
Crystal Structures. Single crystals of palladium complexes
2a, 3a, and C1a suitable for X-ray analysis were grown in
hexane/CH2Cl2 solutions. ORTEP diagrams are given in
Figures 1−3, respectively, along with selected bond lengths
Recently, we have addressed backbone effects of α-diimine
nickel catalysts on the reactivity of ethylene polymerization and
successfully explored a type of thermostable α-diimine nickel
catalyst with a bulky camphyl backbone.32 A bulky α-diimine
nickel catalyst with a camphyl backbone is capable of
polymerizing ethylene at 80 °C and also exhibits remarkable
chain-walking ability. Introduction of camphyl on the ligand
backbone can achieve polymerization performance similar to
that of a cyclophane-based α-diimine ligand,28 while camphyl α-
diimine ligands are more conveniently accessible.31,32 Inspired
by the results of ethylene polymerization using a bulky camphyl
α-diimine nickel catalyst,32 we herein synthesize and character-
ize α-diimine palladium catalysts with various backbone
structures such as camphyl, phenyl, 4-fluorophenyl, 4-
methylphenyl, and methyl and investigate backbone substituent
effects on the homo- and copolymerization of ethylene with
MA in detail. A thermostable camphyl α-diimine palladium
catalyst has been successfully discovered for living polymer-
ization of ethylene, and it also is more tolerant to polar
comonomers and affords copolymers with higher incorporation
of MA. Additionally, we also present a new deactivation
pathway of α-diimine palladium catalysts and the formation
mechanism of palladium black.
RESULTS AND DISCUSSION
■
Synthesis of α-Diimine Palladium Complexes. The
synthetic route for α-diimine palladium complexes is shown in
Scheme 1. Four α-diimine ligands with phenyl and camphyl
Scheme 1. Synthetic Route of Cationic α-Diimine Palladium
Complexes
Figure 1. Molecular structure of complex 2a. Hydrogen atoms are
omitted for clarity. Selected bond distances (Å) and angles (deg):
Pd(1)−C(1), 2.050(4); Pd(1)−N(2), 2.055(3); Pd(1)−N(1),
2.157(3); Pd(1)−Cl(1), 2.277(15); C(1)−Pd(1)−N(2), 98.53(15);
C(1)−Pd(1)−N(1), 175.98(15); N(2)−Pd(1)−N(1), 77.45(13);
C(1)−Pd(1)−Cl(1), 84.76(13); N(2)−Pd(1)−Cl(1), 176.60(10);
N(1)−Pd(1)−Cl(1), 99.26(10); Pd(1)−N(2)−C(14), 116.0(3);
Pd(1)−N(1)−C(21), 113.8(3).
and bond angles. The palladium complexes (α-diimine)-
PdMeCl (2a and 3a) feature a square-planar coordination of
the central metal. The bond angles and distances in the
palladium coordination plane are within the standard range for
these types of complexes.26 Two aniline moieties of 2a and 3a
are nearly perpendicular to the five-membered coordination
plane (dihedral angles of 89.6 and 89.4°), showing no obviously
repulsive interactions between the aniline moiety and backbone
substituents. The cationic palladium complex C1a also adopts a
square-planar coordination geometry with the α-diimine ligand
and 2,1-inserted MA.33 The bond angles and distances in the
palladium coordination plane are very close to the values for a
cationic palladium complex with a methyl backbone reported
by Ye.24 It is significant to note that the gem-dimethyl
substituents (C(25)H3 and C(26)H3) are oriented toward
the axial position, indicating steric effects on the palladium
metal center. In addition, the methyl (C(27)H3) on the rigid
camphyl lies out of one side, which may effectively suppress the
potential rotation or fluctuation of the CAr−N bond, thus
prohibiting C−H activation.26,30
backbones were synthesized according to our previous report.32
Two bulky α-diimine ligands with 4-fluorophenyl and 4-
methylphenyl backbones (with two 2,6-diisopropylphenyl
moieties) were also prepared to study the electronic effects of
the ligand backbone. Complexation of α-diimine ligands and
Pd(COD)MeCl (COD = 1,5-cyclooctadiene) afforded methyl
chloride palladium complexes.1−3 These methyl chloride
palladium complexes were further treated with MA and sodium
tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (NaBAF) to
yield the cationic palladium complexes [(α-diimine)Pd-
(CH2)3C(O)OMe]+BAF−. However, an attempt to obtain the
cationic palladium complex C3a with a 4-fluorophenyl
backbone was unsuccessful under the same reaction conditions,
which may result from the electron-withdrawing effect of the F
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by a reduction process from Pd(II) to Pd(0), though a detailed
reduction process is still unclear.
It is interesting to note that the obtained palladium complex
chelating esters [(α-diimine)Pd(CH2)3C(O)OMe]+BAF− ex-
hibit different stabilities. A clear trend is that palladium
complexes bearing bulky ligands are more stable in solution. It
is observed that the solution color of cationic palladium
complex C2b slowly changed from orange to red and palladium
black precipitated, suggesting decomposition of C2b and the
appearance of a new palladium complex. A single crystal was
obtained by careful treatment of the red solution and slow
evaporation. Single-crystal X-ray diffraction data confirm that
the new palladium complex (L2b)2Pd(BAF)2 is composed of
two L2b chelate ligands, palladium metal, and two BAF
counteranions (Figure 4). (L2b)2Pd(BAF)2 adopts a distorted
Figure 2. Molecular structure of complex 3a. Hydrogen atoms are
omitted for clarity. Selected bond distances (Å) and angles (deg):
Pd(1)−C(1), 2.056(5); Pd(1)−N(2), 2.072(4); Pd(1)−N(1),
2.144(4); Pd(1)−Cl(1), 2.2802(13); C(1)−Pd(1)−N(2), 99.01(18);
C(1)−Pd(1)−N(1), 175.60(18); N(2)−Pd(1)−N(1), 76.85(15);
C(1)−Pd(1)−Cl(1), 86.38(15); N(2)−Pd(1)−Cl(1), 172.53(11);
N(1)−Pd(1)−Cl(1), 97.88(11); Pd(1)−N(2)−C(2), 119.2(3);
Pd(1)−N(1)−C(28), 121.7(3).
Figure 4. Molecular structure of complex (L2b)2Pd(BAF)2. The
hydrogen, two BAF− anions, and solvent molecules are omitted for
clarity.
plane around the palladium atom, where the two α-diimine
ligands act as bidentate N,N-chelators. Scheme 2 depicts the
Figure 3. Molecular structure of complex C1a. The hydrogen and
BAF− anion are omitted for clarity. Selected bond distances (Å) and
angles (deg): Pd(1)−C(1), 1.994(8); Pd(1)−O(1), 2.049(5); Pd(1)−
N(1), 2.046(6); Pd(1)−N(2), 2.172(6); C(1)−Pd(1)−O(1), 90.6(3);
C(1)−Pd(1)−N(1), 97.1(3); O(1)−Pd(1)−N(1), 172.1(2); C(1)−
Pd(1)−N(2), 176.1(3); O(1)−Pd(1)−N(2), 92.2(2); N(1)−Pd(1)−
N(2), 80.2(2).
Scheme 2. Deactivation Pathway of C2b and Formation of
Palladium Black
Palladium Catalyst Stability. Generally, α-diimine palla-
dium catalysts with ether auxiliary ligands for ethylene
polymerization are prone to deactivation. On the basis of
observation of low-temperature NMR spectra of α-diimine
palladium with ether, Brookhart has previously presented a C−
H activation mechanism.19,26 Formed by the addition of MA,
the stability of the cationic ester chelate palladium complex [(α-
diimine)Pd(CH2)3C(O)OMe]+BAF− can be improved through
prohibition of C−H activation.26 Nevertheless, decay of
polymerization activity and precipitation of palladium black
were often observed during ethylene polymerization using
palladium catalysts with a chelating six-membered ester. This
strongly suggests that a different deactivation pathway occurs
pathway of decomposition of C2b and formation of (L2b)2Pd-
(BAF)2 and palladium black. A molecule of C2b can abstract
the ligand of another molecule to produce a molecule of
(L2b)2Pd(BAF)2 and a molecule of palladium black. GC-MS
analysis of the organic fraction confirmed the existence of ester
compounds, including methyl butyrate and methyl but-3-
enoate, further supporting this deactivation pathway. This is the
first direct identification of a detailed reduction pathway of a α-
diimine palladium(II) catalyst to palladium black. This new
deactivation pathway provides access to a better understanding
of how to optimize and design thermostable α-diimine
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palladium catalysts. Therefore, increasing the steric hindrance
not only can retard chain transfer by associative exchange but
also can inhibit deactivation of α-diimine palladium catalysts by
a fragmentation pattern with ester loss.
Ethylene Polymerization. All of the palladium catalysts
were screened for ethylene polymerization to evaluate
substituent effects at 35 °C, and the detailed polymerization
results are summarized in Table 1 (entries 1−6). Ethylene
C1a with 2,6-diisopropyl groups on the aniline moiety afforded
high-molecular-weight polymer (44.6 kg/mol) with a TOF of
2132 h−1 (entry 1), while camphyl C1b with 2,6-dimethyl
groups afforded low-molecular-weight products (14.6 kg/mol)
with a TOF of 467 h−1 (entry 5). In addition, reducing the
steric hindrance of the aniline moiety by substituting o-methyl
groups for o-isopropyl groups also results in a reduced
polymerization rate. C1a showed a rather stable ethylene
consumption flow, as monitored by an ethylene flow rate meter
over a period of 6 h, while C2a exhibited slightly reduced
ethylene consumption. It was found that the ethylene
consumption flow with C2b decreased constantly during the
polymerization process over 6 h, and only slight consumption
of ethylene was observed after 6 h. Simultaneously, a large
quantity of palladium black precipitated, suggesting deactiva-
tion of palladium complex C2b. This observation can be well
explained by the decomposition pathway of C2b presented in
Scheme 2. On prolonging the reaction time, two molecules of
C2b gradually decompose to (L2b)2Pd(BAF)2 and a molecule
of palladium black. (L2b)2Pd(BAF)2 is inactive for ethylene
coordination polymerization because of the absence of a Pd−C
bond. Thus, reducing the palladium active center in the
catalytic system leads to reduced ethylene consumption. Also
note that decomposition of palladium complex C2b was
accelerated in the presence of ethylene in comparison to that
for a C2b solution in CH2Cl2, which may be attributed to the
fact that ethylene makes ester chelate opening more favorable.3
Considering the easy modification of the backbone phenyl,
we further designed and synthesized two other palladium
analogues, 3a and C4a, and studied the electronic effects of the
para substituent on the backbone phenyl. A comparison of
entries 2 and 3 demonstrates that introduction of F on the para
position of the aryl leads to a decrease in TOF and molecular
weight of the polyethylene. When methyl is used instead of F,
increasing the TOF and molecular weight of the polyethylene
can be observed (entries 2 and 4). Introduction of an electron-
donating group on the α-diimine ligand backbone leads to the
formation of high-molecular-weight polyethylene with an
enhanced activity, which is in accord with that of a para
substituent on the aniline moiety.29 In addition, a catalyst with
an electron-donating methyl group, C4a, was found to afford
polymer with high branching density, which is in contrast with
the electronic effect of the aniline moiety.29 This result shows
that there are different electronic effects of the aniline moiety
and backbone on the chain-walking ability of the palladium
center.
Table 1. Ethylene Polymerizations with Different α-Diimine
a
Palladium Catalysts
b
c
c
c
d
entry
cat.
Tp (°C)
TOF
Mn
Mw
PDI
BD
86
1
2
3
4
5
6
7
C1a
C2a
35
35
35
35
35
35
5
2132
458
185
491
467
442
193
208
958
872
638
0
28.8
25.9
22.4
41.8
9.5
44.6
42.2
33.8
69.8
14.3
4.7
1.55
1.63
1.51
1.67
1.50
1.51
1.15
1.06
1.47
1.57
1.79
85
84
e
3a
C4a
C1b
C2b
C1a
C1a
C1a
C1a
C1a
C2a
C5a
93
107
104
89
3.1
9.5
10.9
11.6
39.4
30.1
26.7
f
8
5
10.9
26.8
19.3
15.1
85
9
20
50
70
70
70
86
10
11
12
13
89
91
g
trace
a
Reaction conditions: 10 μmol of catalyst, ethylene pressure 400 psi,
solvent 50 mL of toluene, reaction time 2 h. Polymerizations were
b
quenched by triethylsilane. Turnover frequency, which is calculated
as the moles of monomer per mole of palladium per hour, in units of
c
h−1. Determined by GPC calibration using polystyrene standards.
d
1
Branching density, branches per 1000 carbons, determined by H
e
f
NMR. In situ activated by 2 equiv of NaBAF. The solvent is
chlorobenzene. Not determined.
g
polymerizations were directly initiated by addition of ethylene
of 400 psi and reaction for 2 h. Because the cationic palladium
catalyst C3a could not be obtained, ethylene polymerizations
were carried out by an in situ activation of 3a with 2 equiv of
NaBAF.
The two new palladium catalysts C1a with camphyl and C2a
with phenyl were first employed in the homopolymerization of
ethylene to evaluate the substituent effects of the ligand
backbone. A comparison of entries 1 and 2 in Table 1
demonstrates that C1a with a camphyl backbone shows a
higher turnover frequency (TOF), up to 2132 h−1, and yielded
a higher molecular weight of polyethylene (28.8 kg/mol) than
did C2a with a phenyl backbone. When phenyl groups were
substituted on the backbone of the α-diimine instead of a
camphyl group, a massive drop of 79% in the TOF was
observed. This same effect of the backbone framework of α-
diimine palladium catalyst as for a nickel catalyst may be
explained as the electron-withdrawing and conjugate effect of
phenyl.32
It was reported that a bulky aniline moiety is required for α-
diimine palladium catalysts to achieve high TOF and high-
molecular-weight polymer.1,2 An attempt to optimize the steric
effects of α-diimine ligand framework by the increasing steric
bulk of the backbone and reducing the steric hindrance of
aniline moiety is impractical. The steric bulk of the aniline
moiety is more important than the steric bulk of the backbone.
Decreasing the steric hindrance of the aniline moiety with a
fixed backbone results in a drop in TOF and molecular weight
of the PE (C1a vs C1b, C2a vs C2b). For example, camphyl
It is known that the Brookhart-type palladium catalyst C5a
has poor thermal stability, and the reported ethylene polymer-
izations were generally carried out below 35 °C.19,26,34,35
Herein, catalyst C5a only afforded a trace of polymer at 70 °C
(entry 13), while catalyst C2a afforded no polymer (entry 12).
Catalyst C1a, containing a camphyl backbone, showed higher
thermal stability for ethylene polymerization than its α-diimine
palladium analogues. Table 1 also gives the results of ethylene
polymerizations with C1a in the temperature range 5−70 °C
(entries 1 and 7−11). The observed basic trend is that the TOF
of C1a reaches a maximum value at 35 °C and then decreases
with elevated temperature. Even at 70 °C, a good TOF of 638
h−1 and a molecular weight of 15.1 kg/mol can be achieved. We
attribute the enhanced thermal stability of catalyst C1a to its
rigid and bulky bicyclic-substituted backbone, which can
prohibit C−H activation by free CAr−N bond rotation or
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fluctuation32 and deactivation by the fragmentation pattern
presented in Scheme 2.
Note that molecular weight distributions (PDIs) of the
polymers become narrow with decreasing temperature. PDI
values of 1.15 can be achieved at 5 °C using C1a in toluene
(entry 7). When chlorobenzene was used instead of toluene, a
narrower polydispersity of 1.06 was observed (entry 8).
Therefore, living polymerizations of ethylene were performed
at 5 °C with a pressure of 400 psi in chlorobenzene.
Polymerizations were quenched with Et3SiH prior to polymer
isolation to avoid the occurrence of chain coupling. Figure 5b
1
Figure 6. H NMR spectrum of E-MA copolymer catalyzed by C1a.
Table 2. Copolymerization of Ethylene and MA with α-
a
Diimine Palladium Catalysts
MA incorp
b
c
d
d
d
e
entry
cat.
TOF
(mol %)
Mn
Mw
PDI
BD
84
14
15
16
17
18
19
20
C1a
C2a
11
0
1.2
12000
19100
1.59
f
3a
0
■
▲
Figure 5. (a) Plots of Mn ( ) and Mw/Mn (PDI) ( ) as a function of
polymerization time using C1a at 5 °C (polymerization conditions:
400 psi, 10 μmol of Pd, 50 mL of chlorobenzene, reactions quenched
by triethylsilane). (b) GPC traces at different times.
C4a
C1b
C2b
C5a
15
13
0.32
0.50
0.30
0.52
5600
1500
2000
12100
8700
3300
4200
19000
1.57
2.20
2.09
1.57
89
95
93
99
trace
66
a
Reaction conditions: 30 μmol of catalyst, reaction temperature 35 °C,
monomer concentration 0.5 M, ethylene pressure 15 psi, solvent
CH2Cl2, total volume 100 mL, reaction time 36 h. Polymerizations
shows monomodal GPC traces of the polymers obtained using
C1a at different polymerization times, which shift to the higher
molecular weight region with longer polymerization times.
Plots of number-average molecular weight (Mn) and Mw/Mn
(PDI) as a function of polymerization time (Figure 5a) also
illustrate that Mn grows linearly with the polymerization time,
and the PDI values are below 1.10. PE with an Mn value of ∼34
is formed after 12 h using C1a, and its molecular weight is still
precisely controlled (PDI = 1.09). No obvious palladium black
was found in an aliquot sample of 12 h polymerization. This
result supports the notion that the palladium catalyst is stable
and long-lived under the adopted conditions. Therefore,
ethylene polymerizations using C1a proceed in a living manner
over 12 h. To the best of our knowledge, camphyl palladium
catalyst C1a is one of the rare late-transition-metal catalytic
systems for living polymerization of ethylene.36−40 Living
polymerization of ethylene using camphyl palladium catalyst
C1a provides a viable access to the precise synthesis of
monodisperse PE and corresponding block copolymers.
b
were quenched by triethylsilane. Turnover frequency, in units of h−1.
Because of the low yield and incorporation of MA, the TOF of MA is
c
d
1
omitted. Determined by H NMR. Determined by GPC calibration
e
using polystyrene standards. Branching density, branches per 1000
carbons, determined by H NMR. In situ activated by 2 equiv of
NaBAF.
f
1
observation is in contrast to the electronic effects of
substituents of the aniline moiety reported by Guan,29 also
suggesting different electronic influences on the incorporation
of MA between the backbone and aniline moiety. It is
interesting to note that the camphyl palladium catalyst shows
the highest incorporation of MA among these catalysts (1.2 mol
%) (entry 14), though the Brookhart-type palladium catalyst
C5a exhibits the highest TOF (entry 20). The copolymers
obtained by C1a and C5a also have nearly the same molecular
weights. Presumably, binding of the electron-deficient olefin
(MA) relative to ethylene is increased by enhancement of the
steric bulk of the backbone substituents. The unique gem-
dimethyl structure of the camphyl backbone can reduce the rate
of comonomer exchange, thus enhancing the incorporation of
olefins.28 As for ethylene homopolymerization, reducing the
steric bulk of the aniline moiety of the palladium catalyst results
in a drop in molecular weight of the copolymerization product.
Camphyl palladium complex C1b with 2,6-dimethylphenyl only
can afford a copolymer with an Mn value of 1500. Therefore,
the steric bulk of the aniline moiety has more significant
influence on copolymerization than that of the backbone.
Copolymerization of Ethylene with MA. One striking
feature of palladium catalysts is their good copolymerization
ability for ethylene with a polar monomer. The electronic
effects of substituents of the aniline moiety on the
copolymerization of ethylene with MA were previously
reported by Guan.29,30 We herein investigated the effects of
backbone substituents on the copolymerization of ethylene
with MA. MA incorporation in the copolymer can be clearly
1
identified by H NMR of ethylene−MA copolymer (E-MA)
(Figure 6) and can be calculated by eq 1. Results in Table 2
show that the backbone substituent has an important influence
on the incorporation of MA. A clear trend is that a backbone
containing an aryl group leads to a great drop in TOF and
incorporation of MA. C2a and 3a hardly show copolymeriza-
tion TOF (entries 15 and 16), while C4a with a 4-
methylphenyl backbone can afford low-molecular-weight
copolymer with low incorporation of MA (entry 17). This
2(ICH
)
3
BD = 1000 ×
3(ICH +CH + ICH
)
(1)
2
3
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ethylene and vented three times. The autoclave was then charged with
48 mL of solvent (toluene or chlorobenzene) under 200 psi of
ethylene at the initialization temperature. The system was maintained
by continuous stirring for 5 min, and then 2 mL of a solution of the
palladium catalyst (3a was activated by 2 equiv of NaBAF) was
charged into the autoclave under 200 psi of ethylene. The ethylene
pressure was raised to the specified value (400 psi), and the reaction
was carried out for a certain time. Polymerization was terminated by
addition of 1.0 mL of triethylsilane after releasing the ethylene
pressure. The solvents were removed on a rotary evaporator. When
chlorobenzene was used as the polymerization medium, any residual
chlorobenzene was removed under vacuum overnight. The obtained
polymer was dissolved in hexane and filtered through a plug of silica
gel to remove palladium black before precipitating in methanol. The
resulting precipitated polymers were collected and treated by filtering,
washing with methanol several times, and drying under vacuum at 40
°C to a constant weight.
CONCLUSIONS
■
A series of PdII(α-diimine)MeCl complexes and cationic
[PdII(α-diimine)(CH2)3C(O)OMe]+BAF− complexes with var-
ious backbone frameworks were synthesized and characterized.
A new pathway of decomposition of cationic palladium
complexes and formation mechanism of palladium black by a
fragmentation pattern with ester loss was presented and
confirmed. Overall, the substituent effects of the backbone in
α-diimine palladium catalysts are usually different from the
substituent effects of the aniline moiety and are less significant
for homopolymerization of ethylene and copolymerization of
ethylene with MA. The camphyl α-diimine palladium catalyst
was found to show better thermal stability for ethylene
homopolymerization and afford high-molecular-weight E-MA
copolymer with high incorporation of polar monomer, though
the Brookhart-type α-diimine palladium catalyst with a methyl
backbone exhibited the highest TOFs for copolymerization of
ethylene with MA. This provides a different approach for
enhancing the stability and tolerance to polar comonomer of α-
diimine palladium, which complements the previous strategy of
modifying the aniline moiety of α-diimine palladium. By
optimizing the reaction conditions, longstanding living
polymerization of ethylene can be also achieved using a bulky
camphyl α-diimine palladium catalyst.
Copolymerization of Ethylene and MA. In a typical procedure,
a 250 mL round-bottom Schlenk flask with a stirring bar was heated
for 3 h to 150 °C under vacuum and then cooled to room temperature.
The flask was pressurized to 15 psi of ethylene and vented three times.
The appropriate CH2Cl2 solvent and MA were introduced into the
glass reactor under an ethylene atmosphere at 35 °C. The system was
maintained by continuous stirring for 10 min, and then a 10 mL
solution of the palladium catalyst (30 μmol) (3a was activated by 2
equiv of NaBAF) in CH2Cl2 was syringed into the well-stirred solution
and the total reaction volume was kept at 100 mL. The ethylene
pressure was kept at a constant value of 15 psi by continuous feeding
of gaseous ethylene throughout the reaction. The polymerizations
were terminated by the addition of a large amount of methanol after
continuous stirring for 36 h. Then the methanol was decanted off, and
the sticky polymer was redissolved in petroleum ether. The polymer
solution was filtered through alumina and silica to remove catalyst
residues. The resulting precipitated polymers were collected and
treated by concentration and drying under vacuum at 40 °C to a
constant weight. The MA incorporation (mol %) was calculated from
1H NMR analysis, as was done before in previous studies of MA
EXPERIMENTAL SECTION
■
General Considerations. All manipulations involving air- and
moisture-sensitive compounds were carried out under an atmosphere
of dried and purified nitrogen with standard vacuum-line, Schlenk, or
glovebox techniques.
Materials. 2,6-Dimethylaniline and 2,6-diisopropylaniline were
purchased from Aldrich Chemical and were distilled under reduced
pressure before being used. TMA (1 M, hexane) was purchased from
Aldrich Chemical. Benzil, 4,4′-dimethylbenzil, 4,4′-difluorobenzil, and
L-camphorquinone were purchased from Alfa Aesar Chemical and used
as received. Toluene, hexane, and diethyl ether were refluxed over
metallic sodium for 24 h before being used. Dichloromethane and
chlorobenzene were dried over phosphorus pentoxide for 8 h and
distilled under a nitrogen atmosphere. Methyl acrylate was dried over
CaH2 prior to use in polymerization. CDCl3 was dried over CaH2 prior
to use in NMR for palladium complexes. Et3SiH (98%) was purchased
from Alfa Aesar and used as received. (COD)PdMeCl10 and the
complexes [(ArNC(Me)C(Me)NAr)PdII(CH2)3C(O)-
OMe]+BAF− (C5a) were prepared according to literature methods.1−3
Characterization. Elemental analyses were performed with a Vario
EL series elemental analyzer from Elementar. The NMR data of
ligands and polymer samples were obtained on a Varian Mercury-Plus
300 MHz spectrometer at ambient temperature, using CDCl3 as
solvent, and referenced versus TMS as standard. The molecular weight
and the molecular weight distribution (PDI) of the polyethylenes were
determined on a Waters Breeze instrument at 40 °C, and THF was
employed as the eluent at a flow rate of 1.0 mL/min. The GC-MS data
were recorded with a Finnigan Voyager GC-8000 TOP series GC-MS
system with a DB-5MS GC column.
copolymers.2
Synthesis of Ligands. α-Diimine ligands were prepared according
to our previously reported method32 and fully characterized by NMR
and elemental analysis.
Synthesis of 2,6-(iPr)2C6H3NC(camphyl)C(camphyl)N-2,6-
(iPr)2C6H3 (L1a). Under a nitrogen atmosphere, 2,6-diisopropylpheny-
laniline (2.124 g, 12 mmol) in toluene (20 mL) solution was injected
into a Schlenk flask, trimethylaluminum (12 mL, 1.0 M in hexane) was
added slowly by a syringe at room temperature, and then the reaction
mixture was heated to reflux for 2 h. After the solution was cooled to
ambient temperature, L-camphorquinone (0.83 g, 5 mmol) was added.
The mixture was stirred for another 6 h at reflux temperature. After the
solution was cooled to 0 °C, the reaction mixture was carefully
hydrolyzed with 5% aqueous NaOH solution. The desired product was
obtained by extraction with ethyl acetate and evaporation. The crude
product was further purified by column chromatography on silica gel
using petroleum ether/ethyl acetate as eluent and was crystallized from
ethanol as yellow crystals in 38.0% yield. A mixture of two geometrical
isomers was determined by NMR, which is consistent with our
previous report.32 1H NMR (300 MHz, CDCl3; δ (ppm) (an isomer
ratio of 1.8:1])): major isomer, 7.06−6.81 (m, 6H, Ar-H), 2.88 (m,
4H, CH(CH3)2), 2.36 (m, 1H, tertiary hydrogen at camphyl), 1.86 (m,
4H, CH2 at camphyl), 1.24 (d, 24H, CH(CH3)2), 0.96 (s, 6H, CH3 at
camphyl), 0.77 (s, 3H, CH3 at camphyl); minor isomer, 7.06−6.81 (m,
6H, Ar-H), 2.69 (m, 4H, CH(CH3)2), 2.36 (m, 1H, tertiary hydrogen
at camphyl), 1.86 (m, 4H, CH2 at camphyl), 1.12 (d, 24H,
CH(CH3)2), 0.94 (s, 6H, CH3 at camphyl), 0.77 (s, 3H, CH3 at
camphyl). 13C NMR (75 MHz, CDCl3; δ (ppm)): major isomer,
168.57 (CN), 144.89 (C-N), 136.10 (CAr-iPr), 122.53, 121.78,
55.92, 50.68, 45.43, 32.27, 28.41, 24.64, 22.44, 17.97, 11.30; minor
isomer, 168.57 (CN), 144.89 (C-N), 134.71 (CAr-iPr), 123.42,
121.46, 55.92, 50.68, 45.43, 32.27, 28.70, 24.73, 22.96, 17.97, 11.30.
Crystal Structure Determination. The X-ray diffraction data of
single crystals were obtained with the ω−2θ scan mode on a Bruker
SMART 1000 CCD diffractometer with graphite-monochromated Mo
Kα radiation (λ = 0.710 73). The structure was solved using direct
methods, and further refinement with full-matrix least-squares on F2
was obtained with the SHELXTL program package. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were introduced
in calculated positions with the displacement factors of the host carbon
atoms.
Ethylene Polymerization. A mechanically stirred 100 mL Parr
reactor was heated to 150 °C for 2 h under vacuum and then cooled to
room temperature. The autoclave was pressurized to 100 psi of
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Anal. Calcd for C34H48N2: C, 84.24; H, 9.98; N, 5.78. Found: C, 84.13;
H, 9.87; N, 5.69.
Synthesis of Palladium Complexes. Synthesis of (2,6-
(iPr)2C6H3NC(camphyl)C(camphyl)N-2,6-(iPr)2C6H3)PdMeCl
(1a). To a solution of L1a (0.30 g, 0.62 mmol) in dry CH2Cl2 (20 mL)
was added 0.16 g (0.61 mmol) of (COD)PdMeCl. After the mixture
was stirred for 12 h at room temperature, the solution was filtered
through Celite, and the solvent of the filtrate was evaporated to give a
yellow residue. The residue was washed with 3 × 10 mL hexane and
dried under vacuum. The product was isolated as a yellow solid in 41%
yield. In CDCl3, trans and cis geometrical isomers were observed in a
Synthesis of 2,6-(Me)2C6H3NC(camphyl)C(camphyl)N-2,6-
(Me)2C6H3 (L1b). Following the above procedure, L1b was isolated
as yellow crystals in 84.7% yield. 1H NMR (300 MHz, CDCl3; δ
(ppm) (an isomer ratio of 1.2:1)): major isomer, 6.86−6.70 (m, 6H,
Ar-H), 2.07 (s, 12H, CAr-CH3), 1.79 (m, 4H, CH2 at camphyl), 1.39
(m, 1H, tertiary hydrogen at camphyl), 1.26 (s, 3H, CH3 at camphyl),
1.07 (s, 6H, CH3 at camphyl); minor isomer, 6.86−6.70 (m, 6H, Ar-
H), 2.04 (s, 12H, CAr-CH3), 1.86 (m, 4H, CH2 at camphyl), 1.42 (m,
1H, tertiary hydrogen at camphyl), 1.26 (s, 3H, CH3 at camphyl), 0.93
(s, 6H, CH3 at camphyl). 13C NMR (75 MHz, CDCl3; δ (ppm)):
major isomer, 170.87 (CN), 149.37 (C-N), 127.53 (CAr-CH3),
123.96, 122.71, 55.30, 51.18, 45.46, 32.59, 23.38, 21.68, 18.42, 11.20;
minor isomer, 168.56 (CN), 148.11 (C-N), 127.07 (CAr-CH3),
124.88, 123.17, 55.30, 51.18, 45.46, 32.59, 23.38, 21.68, 18.26, 11.20.
Anal. Calcd for C26H32N2: C, 83.82; H, 8.66; N, 7.52. Found: C, 83.70;
H, 8.74; N, 7.46.
1
2.5:1 ratio on the basis of integration. H NMR (300 MHz, CDCl3; δ
(ppm)): major isomer, 7.22−7.13 (6H, m, Ar-H), 3.18−2.09 (m, 4H,
CH(CH3)2), 1.56−1.37 (m, 5H, camphyl-H), 1.36−1.28 (24H, m,
CH(CH3)2), 1.16 (3H, s, camphyl-CH3), 0.91 (3H, s, camphyl-CH3),
0.58 (s, 3H, Pd-CH3); minor isomer, 7.22−7.13 (6H, m, Ar-H), 2.97−
2.91 (m, 4H, CH(CH3)2), 1.56−1.37 (m, 5H, camphyl-H), 1.36−
1.28(24H, m, CH(CH3)2), 1.16 (3H, s, camphyl-CH3), 0.62 (3H, s,
camphyl-CH3), 0.53 (s, 3H, Pd-CH3). 13C NMR (75 MHz, CDCl3; δ
(ppm)): major isomer, 184.20, 178.70, 140.53, 140.08, 139.53, 138.96,
138.65, 137.87, 128.00, 127.14, 124.30, 123.78, 123.21, 58.07, 51.82,
31.77, 29.16, 28.4, 25.60, 24.88, 24.43, 23.03, 18.10, 11.6, 2.32; minor
isomer, 183.31, 177.78, 140.21, 139.98, 139.37, 138.84, 138.54, 127.82,
126.87, 123.93, 123.56, 122.92, 57.04, 50.67, 31.23, 28.67, 28.4, 25.36,
24.66, 24.16, 21.98, 11.61, 4.27. Anal. Calcd for C34H48N2PdMeCl: C,
65.51; H, 8.01; N, 4.37. Found: C, 65.34; H, 7.89; N, 4.32.
Synthesis of 2,6-(iPr)2C6H3NC(Ph)C(Ph)N-2,6-(iPr)2C6H3 (L2a).
Following the above procedure, L2a was isolated as orange crystals in
53.7% yield. 1H NMR (300 MHz, CDCl3; δ (ppm) (an isomer ratio of
2.5:1)): major isomer, 7.95−7.00 (m, 16H, Ar-H), 2.92 (m, 4H,
CH(CH3)2), 1.08 (d, 24H, CH(CH3)2); minor isomer, 7.95−7.00 (m,
16H, Ar-H), 3.10 (m, 4H, CH(CH3)2), 0.94 (d, 24H, CH(CH3)2). 13
C
Synthesis of (2,6-(CH3)2C6H3NC(camphyl)C(camphyl)N-2,6-
(CH3)2C6H3)PdMeCl (1b). Following the above procedure, 1b was
isolated in 83% yield. In CDCl3, trans and cis geometrical isomers
NMR (75 MHz, CDCl3; δ (ppm)): major isomer, 166.36 (CN),
145.51 (C-N), 137.2, 134.5, 130.7, 128.7, 125.6, 123.6, 122.1, 29.07
(CH), 24.62 (Me); minor isomer, 162.57 (CN), 145.17 (C-N),
137.6, 135.5, 130.1, 128.2, 125.3, 123.6, 122.1, 28.31 (CH), 22.89
(Me). Anal. Calcd for C38H44N2: C, 86.31; H, 8.39; N, 5.30. Found: C,
86.33; H, 8.40; N, 5.32.
1
were observed in a 1.5:1 ratio on the basis of integration. H NMR
(300 MHz, CDCl3; δ (ppm)): major isomer, 7.10−6.99 (6H, m, Ar-
H), 2.37−2.28 (12H, q, Ar-CH3), 2.01−1.51 (5H, m, camphyl-H),
1.15 (3H, s, camphyl-CH3), 0.87 (3H, s, camphyl-CH3), 0.56 (3H, d,
camphyl-CH3), 0.46 (3H, d, Pd-CH3); minor isomer, 7.10−6.99 (6H,
m, Ar-H), 2.37−2.28 (12H, q, Ar-CH3), 2.01−1.51 (5H, m, camphyl-
H), 1.15 (3H, s, camphyl-CH3), 0.87 (3H, s, camphyl-CH3), 0.51 (3H,
d, camphyl-CH3), 0.38 (3H, d, Pd-CH3). 13C NMR (75 MHz, CDCl3;
δ (ppm)): major isomer, 184.61, 179.03, 143.34, 142.48, 129.47,
128.75, 128.42, 127.63, 127.18, 126.03, 57.81, 51.89, 32.08, 23.30,
19.59, 19.00, 18.56, 17.82, 10.92, 0.18; minor isomer, 183.85, 178.14,
142.59, 141.98, 129.24, 128.57, 128,13, 127.50, 127.00, 126.26, 56.62,
50.60, 31.53, 22.77, 19.14, 18.73, 17.82, 10.74, 1.95. Anal. Calcd for
C26H32N2PdMeCl: C, 61.25; H, 6.66; N, 5.29. Found: C, 60.93; H,
6.54; N, 5.31.
Synthesis of (2,6-(i-Pr)2C6H3NC(Ph)C(Ph)N-2,6-(i-Pr)2C6H3)-
PdMeCl (2a). Following the above procedure, 2a was isolated in
32% yield. 1H NMR (300 MHz, CDCl3; δ (ppm)): 7.01−7.04, 6.77−
6.98 (m, 16H, Ar-H), 3.12−3.24 (m, 4H, CH(CH3)2), 0.94−0.96 (d,
12H, CH(CH3)2), 0.72 (s, 3H, Pd-CH3). 13C NMR (75 MHz, CDCl3;
δ (ppm)): 174.15, 169.73, 142.05, 138.75, 138.17, 133.54, 133.32,
130.15, 129.03, 128.91, 128.04, 124.06, 123.29, 29.54, 29.09, 23.43,
23.15, 6.06. Anal. Calcd for C38H44N2PdMeCl: C, 68.31; H, 6.91; N,
4.09. Found: C, 68.08; H, 6.83; N, 3.83.
Synthesis of (2,6-(CH3)2C6H3NC(Ph)C(Ph)N-2,6-(CH3)2C6H3)-
PdMeCl (2b). Following the above procedure, 2b was isolated in 92%
yield. 1H NMR (300 MHz, CDCl3; δ (ppm)): 6.76−6.70 (m, 6H, Ar-
H), 6.79−6.97 (m, 10H, Ar-H), 2.33−2.34 (d, 12H, Ar−CH3), 0.61 (s,
3H, Pd−CH3). 13C NMR (75 MHz, CDCl3; δ (ppm)): 174.4, 169.6,
144.42, 133.54, 133.32, 130.34, 128.80, 127.99, 127.37, 126.92, 126.32,
19.33, 18.84, 4.25. Anal. Calcd for C30H28N2PdMeCl: C, 64.93; H,
5.45; N, 4.88. Found: C, 64.95; H, 5.67; N, 4.83.
Synthesis of (2,6-(i-Pr)2C6H3NC(4′-F-Ph)C(4′-F-Ph)N-2,6-(i-
Pr)2C6H3)PdMeCl (3a). Following the above procedure, 3a was
isolated in 47% yield. 1H NMR (300 MHz, CDCl3; δ (ppm)):
6.77−6.81, 6.98−7.01, 7.10−7.14 (m, 14H, Ar-H), 3.17−3.25 (m, 4H,
CH(CH3)2), 0.94−0.98 (d, 12H, CH(CH3)2), 0.73 (s, 3H, Pd−CH3).
13C NMR (75 MHz, CDCl3; δ (ppm)): 172.72, 168.44, 164.57,
Synthesis of 2,6-(Me)2C6H3NC(Ph)C(Ph)N-2,6-(Me)2C6H3
(L2b). Following the above procedure, L2b was isolated as yellow
1
crystals in 69.2% yield. H NMR (300 MHz, CDCl3; δ (ppm) (an
isomer ratio of 2.1:1)): major isomer, 7.96−6.51 (m, 16H, Ar-H), 1.33
(s, 12H, CH3); minor isomer, many peaks are obscured by major
isomer, observed 7.96−6.51 (m, 16H, Ar-H), 1.60 (s, 12H, CH3). 13
C
NMR (75 MHz, CDCl3; δ (ppm)): major isomer, 164.92 (CN),
147.03 (C-N), 137.73 (CCN), 134.72 (CAr-CH3), 129.55, 128.34,
127.74, 127.25, 126.36, 123.04, 18.57 (Me); minor isomer, 165.19
(CN), 147.44 (C-N), 137.73 (CCN), 134.72 (CAr-CH3), 130.68,
128.42, 127.91, 127.33, 126.23, 123.15, 18.85 (Me). Anal. Calcd for
C30H28N2: C, 86.50; H, 6.78; N, 6.72. Found: C, 86.33; H, 6.79; N,
6.51.
Synthesis of 2,6-(iPr)2C6H3NC(4-F-Ph)C(4-F-Ph)N-2,6-
(iPr)2C6H3 (L3a). Following the above procedure, L3a was isolated as
1
orange crystals in 46.8% yield. H NMR (300 MHz, CDCl3; δ (ppm)
(an isomer ratio of 1.5:1)): major isomer, 7.91−6.41 (m, 14H, Ar-H),
2.84 (m, 4H, CH(CH3)2), 1.09 (d, 24H, CH(CH3)2); minor isomer,
7.91−6.41 (m, 14H, Ar-H), 3.06 (m, 4H, CH(CH3)2), 0.97 (d, 24H,
CH(CH3)2). 13C NMR (75 MHz, CDCl3; δ (ppm)): major isomer,
164.90 (C-F), 160.98 (CN), 145.04 (C-N), 135.14, 131.47, 131.44,
124.30, 115.54, 29.02 (CH), 24.00 (Me); minor isomer, 161.56 (C-F),
160.76 (CN), 145.01(C-N), 136.09, 131.44, 123.87, 122.66, 115.27,
28.27 (CH), 22.69 (Me). Anal. Calcd for C38H42F2N2: C, 80.82; H,
7.50; N, 4.96. Found: C, 80.41; H, 7.49; N, 5.10.
Synthesis of 2,6-(iPr)2C6H3NC(4-Me-Ph)C(4-Me-Ph)N-2,6-
(iPr)2C6H3 (L4a). Following the above procedure, L4a was isolated as
1
yellow crystals in 63.4% yield. H NMR (300 MHz, CDCl3; δ (ppm)
(an isomer ratio of 1.5:1)): major isomer, 8.03−6.87 (m, 14H, Ar-H),
2.78 (m, 4H, CH(CH3)2), 1.77 (s, 6H, CH3), 1.07 (d, 24H,
CH(CH3)2); minor isomer, 8.12−6.82 (m, 14H, Ar-H), 2.76 (m,
4H, CH(CH3)2), 1.78 (s, 6H, CH3), 1.09 (d, 24H, CH(CH3)2). 13C
NMR (75 MHz, CDCl3; δ (ppm)): major isomer, 165.73 (CN),
149.45 (C-N), 143.81 (CCN), 130.78 (CAr-Me), 129.37, 128.51,
127.25, 125.73, 122.66, 28.53, 24.42, 18.69; minor isomer, 165.08
(CN), 148.53 (C-N), 141.59 (CCN), 131.65 (CAr-Me), 129.23,
127.96, 127.83, 125.31, 122.78, 27.42, 24.30, 18.47. Anal. Calcd for
C40H48N2: C, 86.28; H, 8.69; N, 5.03. Found: C, 86.36; H, 8.56; N,
5.25.
161.15, 141.87, 141.77, 138.54, 138.00, 131.40, 128.18, 127.49, 124.07,
115.85, 115.56, 29.49, 29.05, 23.43, 23.14, 6.26. Anal. Calcd for
C38H42F2N2PdMeCl: C, 64.91; H, 6.29; N, 3.88. Found: C, 64.58; H,
6.31; N, 3.85.
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Article
Table 3. Crystal Data and Structure Refinement Details for Complexes 2a, 3a, C1a, and (L2b)2Pd(BAF)2
2a
3a
C1a
(L2b)2Pd(BAF)2·0.2H2O·0.33(Et2O,C6H14)
empirical formula
formula wt
cryst syst
space group
a (Å)
C39H47ClN2Pd
685.64
C39H45ClF2N2Pd
721.62
C71H69BF24N2O2Pd
1555.49
C127.33H88B2F48N4O0.53Pd
2726.03
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/n
triclinic
P1
̅
16.721
21.262(4)
9.4135(17)
17.715(3)
90
10.0707(10)
40.419(4)
18.8682(15)
90
28.532(3)
14.8505(15)
35.569(4)
90
b (Å)
17.548
c (Å)
24.927
α (deg)
74.65
β (deg)
92.592(3)
90
111.797(4)
90
106.844(2)
90
87.55
γ (deg)
V (Å3)
76.33
3541.9(11)
4
7131.2(11)
8
14 424(3)
8
6851.9
Z
2
D(calcd) (g/cm3)
1.286
1.344
1.433
1.321
F(000)
1432
2992
6336
2748
cryst size (mm)
θ range (deg)
index ranges
0.20 × 0.15 × 0.10
1.92−28.41
−28 ≤ h ≤ 15
−9 ≤ k ≤ 12
−23 ≤ l ≤ 23
0.30 × 0.30 × 0.20
1.54−27.08
−11 ≤ h ≤ 12
−51≤ k ≤ 51
−21 ≤ l ≤ 24
0.42 × 0.41 × 0.22
1.49−27.11
−35 ≤ h ≤ 36
−19 ≤ k≤ 10
−45≤ l ≤ 45
0.43 × 0.24 × 0.22
1.30−26.00
−20 ≤ h ≤ 20
−21 ≤ k ≤ 21
−30 ≤ l ≤ 30
51 806/26 407 (0.0620)
97.9
no. of rflns collected/unique (Rint) 18 864/8545 (0.0476) 41 962/15 461 (0.0326) 85 249/31 620 (0.0766)
data completeness (%)
no. of data/restraints/params
goodness of fit on F2
95.9
98.6
99.2
8545/0/388
1.038
15 461/0/649
1.226
31 620/85/1915
1.099
26 407/215/1825
1.100
final R indices (I > 2σ(I))
R1 = 0.0587
wR2 = 0.1393
R1 = 0.1154
wR2 = 0.1673
1.535, −1.021
R1 = 0.0684
wR2 = 0.1777
R1 = 0.0902
wR2 = 0.1914
2.180, −3.039
R1 = 0.1149
wR2 = 0.3035
R1 = 0.2120
wR2 = 0.3725
6.391, −1.663
R1 = 0.0984
wR2 = 0.2778
R1 = 0.1726
wR2 = 0.3422
2.731, −0.961
R indices (all data)
largest diff peak and hole (e Å−3
)
Synthesis of (2,6-(i-Pr)2C6H3NC(4′-CH3-Ph)C(4′-CH3-Ph)N-
2,6-(i-Pr)2C6H3)PdMeCl (4a). Following the above procedure, 4a was
isolated in 64% yield. H NMR (300 MHz, CDCl3; δ (ppm)): 6.66−
7.13 (m, 14H, Ar-H), 3.12−3.19 (m, 4H, CH(CH3)2), 2.46 (d, 6H,
CH3), 0.91−0.97 (d, 12H, CH(CH3)2), 0.67 (s, 3H, Pd-CH3). 13C
NMR (75 MHz, CDCl3; δ (ppm)): 169.69, 166.37, 145.61, 145.53,
142.32, 140.56, 138.79, 138.19, 135.22, 130.71, 129.10, 128.85, 124.06,
123.88, 123.35, 29.00, 28.93, 24.76, 23.94, 23.88, 5.74. Anal. Calcd for
C40H48N2PdMeCl: C, 69.00; H, 7.20; N, 3.92. Found: C, 69.28; H,
7.31; N, 3.87.
[(ArNC(R)C(R)NAr)PdII(CH2)3C(O)OMe]+BAF− (C1b; Ar = 2,6-
(CH3)2C6H3, R = Camphyl). C1b was synthesized according to the
above general procedure using 1b (288 mg, 0.54 mmol), NaBAF (482
mg, 0.54 mmol), and MA (53.8 μL). The resulting yellow powder was
1
1
isolated in 80% yield. H NMR (300 MHz, CDCl3; δ (ppm)): 7.19−
7.04 (m, 6H, Ar-H), 2.07 (m, 1H, tertiary hydrogen on camphor), 1.92
(m, 4H, CH2 at camphor), 2.42, 2.35, and 2.28 (m, 12H, (Ar-CH3)),
0.92 (s, 6H, CH3 on camphor), 0.68 (s, 3H, CH3 on camphor), 3.49
(s, 3H, OMe), 2.63 (t, 2H, CH2C(O)), 1.34(t, 2H, PdCH2), 0.54
(pentet, 2H, PdCH2CH2CH2C(O)). 13C NMR (75 MHz, CDCl3;
chemical shift of characteristic peaks; δ (ppm)): 188.27 (CO),
183.13 (CN); upfield region, 56.75, 54.32, 52.09, 50.63, 35.50,
31.01, 23.17, 22.66, 22.41, 18.16 (Ar-CH3), 17.00, 9.99. Anal. Calcd for
C63H53BF24N2O2Pd: C, 52.43; H, 3.70; N, 1.94. Found: C, 52.09; H,
3.93; N, 1.74.
General Procedure for the Synthesis of Cationic Palladium
Catalysts [(ArNC(R)C(R)NAr)Pd(CH2)3C(O)OMe]+BAF−. A
glass reactor was used to add 1.1 equiv of MA to a mixture of 1
equiv of NaBAF and 1 equiv of (ArNC(R)C(R)NAr)PdMeCl
suspended in 25 mL of Et2O, and the reaction mixture was stirred for
24 h at room temperature. Sodium chloride was removed from the
reaction mixture via filtration, yielding a clear orange-yellow solution.
The Et2O was removed under vacuum, and the product was washed
with hexane and dried under vacuum.
[(ArNC(R)C(R)NAr)PdII(CH2)3C(O)OMe]+BAF− (C2a; Ar = 2,6-
(i-Pr)2C6H3, R = Ph). C2a was synthesized according to the above
general procedure using 2a (213 mg, 0.31 mmol), NaBAF (275 mg,
0.31 mmol), and MA (30.8 μL). The resulting orange powder was
1
isolated in 30% yield. H NMR (300 MHz, CDCl3; δ (ppm)): 7.21−
[(ArNC(R)C(R)NAr)PdII(CH2)3C(O)OMe]+BAF− (C1a; Ar = 2,6-
(i-Pr)2C6H3, R = Camphyl). C1a was synthesized according to the
above general procedure using 1a (140 mg, 0.22 mmol), NaBAF (195
mg, 0.22 mmol), and MA (22 μL). The resulting yellow powder was
6.79 (m, 16H, Ar-H), 2.99 (m, 4H, CH(CH3)2), 1.41, 1.34, and 1.26
(m, 24H, CH(CH3)2), 3.47 (s, 3H, OMe), 2.63 (t, 2H, CH2C(O)),
1.54 (t, 2H, PdCH2), 0.66 (pentet, 2H, PdCH2CH2CH2C(O)). 13C
NMR (75 MHz, CDCl3; chemical shift of characteristic peaks; δ
(ppm)): 182.71 (CO), 177.65 (CN); upfield region, 54.54, 35.29,
32.84, 29.23 (CH(CH3)2), 23.69 (CH(CH3)2), 22.23. Anal. Calcd for
C75H65BF24N2O2Pd: C, 56.32; H, 4.10; N, 1.75. Found: C, 55.95; H,
4.13; N, 1.57.
1
isolated in 83% yield. H NMR (300 MHz, CDCl3; δ (ppm)): 7.18−
6.90 (m, 6H, Ar-H), 3.50 (s, 3H, OMe), 3.00 (m, 4H, CH(CH3)2),
2.61(t, 2H, CH2C(O)), 2.14 (m, 1H, tertiary hydrogen on camphor),
1.92 (m, 4H, CH2 on camphor), 1.47 (t, 2H, PdCH2), 1.34, 1.25, and
1.21 (m, 24H, CH(CH3)2), 0.95 (s, 6H, CH3 on camphor), 0.73 (s,
[(ArNC(R)C(R)NAr)PdII(CH2)3C(O)OMe]+BAF− (C2b; Ar = 2,6-
(CH3)2C6H3, R = Ph). C2b was synthesized according to the above
general procedure using 2b (226 mg, 0.40 mmol), NaBAF (351 mg,
0.40 mmol), and MA (39.0 μL). The resulting orange powder was
3H, CH3 on camphor), 0.54 (pentet, 2H, PdCH2CH2CH2C(O)). 13
C
NMR (75 MHz, CDCl3; chemical shift of characteristic peaks; δ
(ppm)): 187.90 (CO), 185.72 (CN); upfield region, 57.31, 54.41,
51.64, 50.85, 35.28, 30.89, 28.95 (CH(CH3)2), 23.97 (CH(CH3)2),
23.38, 22.51, 21.67, 17.30, 10.67. Anal. Calcd for C71H69BF24N2O2Pd:
C, 54.82; H, 4.47; N, 1.80. Found: C, 54.95; H, 4.46; N, 1.72.
1
isolated in 59% yield. H NMR (300 MHz, CDCl3; δ (ppm)): 7.19−
6.76 (m, 16H, Ar-H), 2.38 and 2.29 (m, 12H, (Ar-CH3)), 3.68 (s, 3H,
OMe), 2.66 (t, 2H, CH2C(O)), 1.44 (t, 2H, PdCH2), 0.69 (pentet,
6061
dx.doi.org/10.1021/om300380b | Organometallics 2012, 31, 6054−6062

Organometallics
Article
2H, PdCH2CH2CH2C(O)). 13C NMR (75 MHz, CDCl3; chemical
shift of characteristic peaks; δ (ppm)): 183.33 (CO), 178.38 (C
N); upfield region, 53.52, 35.59, 31.32, 23.41, 18.15 (Ar−CH3). Anal.
Calcd for C67H49BF24N2O2Pd: C, 54.11; H, 3.32; N, 1.88. Found: C,
53.86; H, 3.41; N, 1.69.
(15) Anselment, T. M. J.; Vagin, S.-I.; Rieger, B. Dalton Trans. 2008,
4537.
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212, 2029.
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(18) Ye, Z.; Zhu, S. Macromolecules 2003, 36, 2194.
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Springer: Berlin, Heidelberg, 2009.
(20) Chen, G.; Ma, X. S.; Guan, Z. J. Am. Chem. Soc. 2003, 125, 6697.
(21) Chen, G.; Guan, Z. J. Am. Chem. Soc. 2004, 126, 2662.
(22) Chen, G.; Huynh, D.; Felgner, P. L.; Guan, Z. J. Am. Chem. Soc.
2006, 128, 4298.
(23) Zhang, K.; Wang, J.; Subramanian, R.; Ye, Z.; Lu, J.; Yu, Q.
Macromol. Rapid Commun. 2007, 28, 2185.
(24) Zhang, K.; Ye, Z.; Subramanian, R. Macromolecules 2008, 41,
640.
(25) Shi, X.; Zhao, Y.; Gao, H.; Zhang, L.; Zhu, F.; Wu, Q. Macromol.
Rapid Commun. 2012, 33, 374.
(26) Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.;
Brookhart, M. J. Am. Chem. Soc. 2000, 122, 6686.
(27) Camacho, D. H.; Salo, E. V.; Guan, Z. Angew. Chem., Int. Ed.
2004, 43, 1821.
[(ArNC(R)C(R)NAr)PdII(CH2)3C(O)OMe]+BAF− (C4a; Ar = 2,6-
(i-Pr)2C6H3, R = 4-Me-Ph). C4a was synthesized according to the
above general procedure using 4a (296 mg, 0.41 mmol), NaBAF (367
mg, 0.41 mmol), and MA (40.7 μL). The resulting orange powder was
1
isolated in 42% yield. H NMR (300 MHz, CDCl3; δ (ppm)): 7.19−
6.73 (m, 14H, Ar-H), 2.93 (m, 4H, CH(CH3)2), 1.86 (s, 6H, CH3 on
phenyl backbone), 1.45−1.29 (m, 24H, CH(CH3)2), 3.52 (s, 3H,
OMe), 2.61 (t, 2H, CH2C(O)), 1.57 (t, 2H, PdCH2), 0.63 (pentet,
2H, PdCH2CH2CH2C(O)). 13C NMR (75 MHz, CDCl3; chemical
shift of characteristic peaks; δ (ppm)): 183.68 (CO), 178.41 (C
N); upfield region, 57.86, 36.28, 34.69, 32.84, 29.56 (CH(CH3)2),
27.99 (CH(CH3)2), 22.78. Anal. Calcd for C77H69BF24N2O2Pd: C,
56.82; H, 4.27; N, 1.72. Found: C, 56.63; H, 4.25; N, 1.86.
The NMR data indicate the sole presence of a single isomer, the six-
membered chelate. All single crystals of palladium complexes were
grown from a mixed dichloromethane or ethyl ether/hexane solution
by slow evaporation. Crystal data and structure refinement details for
palladium complexes 2a, 3a, C1a, and (L2b)2Pd(BAF)2 are shown in
Table 3.
(28) Popeney, C. S.; Camacho, D. H.; Guan, Z. J. Am. Chem. Soc.
2007, 129, 10062.
(29) Popeney, C. S.; Guan, Z. Organometallics 2005, 24, 1145.
(30) Popeney, C. S.; Levins, C. M.; Guan, Z. Organometallics 2011,
30, 2432.
ASSOCIATED CONTENT
* Supporting Information
CIF files giving detailed crystallographic data of palladium
complexes. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
(31) Schleis, T.; Heinemann, J.; Spaniol, T. P.; Mulhaupt, R.; Okuda,
̈
J. Inorg. Chem. Commun. 1998, 1, 431.
(32) Liu, F.; Hu, H.; Xu, Y.; Guo, L.; Zai, S.; Song, K.; Gao, H.;
Zhang, L.; Zhu, F.; Wu, Q. Macromolecules 2009, 42, 7789.
(33) Borkar, S.; Yennawar, H.; Sen, A. Organometallics 2007, 26,
4711.
(34) Camacho, D. H.; Salo, E. V.; Guan, Z.; Ziller, J. W.
Organometallics 2005, 24, 4933.
AUTHOR INFORMATION
Corresponding Author
*E-mail: gaohy@mail.sysu.edu.cn (H.G.); ceswuq@mail.sysu.
edu.cn (Q.W.). Tel: 86-20-84113250. Fax: 86-20-84114033.
Notes
■
(35) Schmid, M.; Eberhardt, R.; Klinga, M.; Leskela, M.; Rieger, B.
̈
Organometallics 2001, 20, 2321.
(36) Gottfried, A. C.; Brookhart, M. Macromolecules 2001, 34, 1140.
(37) Azoulay, J. D.; Schneider, Y.; Galland, G. B.; Bazan, G. C. Chem.
Commun. 2009, 45, 6177.
(38) Zai, S.; Liu, F.; Gao, H.; Li, C.; Zhou, G.; Guo, L.; Zhang, L.;
Zhu, F.; Wu, Q. Chem. Commun. 2010, 46, 4321.
(39) Gao, H.; Hu, H.; Zhu, F.; Wu, Q. Chem. Commun. 2012, 48,
3312.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by the NSFC (Projects 20974125, 51173209,
and 21174164), CNPC Innovation Foundation, and the
Fundamental Research Funds for the Central Universities
(Project 10lgpy10) are gratefully acknowledged.
(40) Zai, S.; Gao, H.; Huang, Z.; Hu, H.; Wu, H.; Wu, Q. ACS Catal.
2012, 2, 433.
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