Synthesis and structure of imine-N-heterocyclic carbene palladium complexes and their catalytic behavior in norbornene polymerization

Article abstract of DOI:10.1021/om400268y

On the basis of the steric effects of ligand, a series of imine-N-heterocyclic carbene (NHC) ligands and their corresponding five-membered palladium complexes with bulky substituents on both the imine and the NHC moieties were synthesized and characterized. Transpalladation of silver carbene complexes with (COD)PdCl₂ and (COD)PdMeCl afforded the palladium dichloride and methylpalladium complexes, respectively. Bulky cationic palladium complexes were further obtained by treatment of the methylpalladium complexes with sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (NaBAF) in CH ₃CN. Well-defined cationic palladium complexes were confirmed by X-ray crystal diffraction to have trans forms. Palladium dichloride complexes and methylpalladium complexes after activation with MMAO show high activity for norbornene polymerization, whereas cationic palladium complexes can polymerize norbornene alone without any cocatalysts and exhibit a high thermostability. Norbornene polymerization with the cationic palladium catalyst was proven to proceed through a coordination-insertion mechanism by NMR studies. Analysis of oligomers obtained by polymerizing the monomer in the presence of H₂ reveals the existence of a C7 linkage in the polynorbornene (PNB) by σ-bond metathesis, which may be the reason for the insolublity of polynorbornenes obtained by palladium catalysts.

Full text of DOI:10.1021/om400268y

Article
pubs.acs.org/Organometallics
Synthesis and Structure of Imine−N-Heterocyclic Carbene Palladium
Complexes and Their Catalytic Behavior in Norbornene
Polymerization
Juean Deng, Haiyang Gao,* Fangming Zhu, 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
S
* Supporting Information
ABSTRACT: On the basis of the steric effects of ligand, a
series of imine−N-heterocyclic carbene (NHC) ligands and
their corresponding five-membered palladium complexes with
bulky substituents on both the imine and the NHC moieties
were synthesized and characterized. Transpalladation of silver
carbene complexes with (COD)PdCl2 and (COD)PdMeCl
afforded the palladium dichloride and methylpalladium
complexes, respectively. Bulky cationic palladium complexes
were further obtained by treatment of the methylpalladium
complexes with sodium tetrakis(3,5-bis(trifluoromethyl)-
phenyl)borate (NaBAF) in CH3CN. Well-defined cationic
palladium complexes were confirmed by X-ray crystal
diffraction to have trans forms. Palladium dichloride complexes and methylpalladium complexes after activation with MMAO
show high activity for norbornene polymerization, whereas cationic palladium complexes can polymerize norbornene alone
without any cocatalysts and exhibit a high thermostability. Norbornene polymerization with the cationic palladium catalyst was
proven to proceed through a coordination−insertion mechanism by NMR studies. Analysis of oligomers obtained by
polymerizing the monomer in the presence of H2 reveals the existence of a C7 linkage in the polynorbornene (PNB) by σ-bond
metathesis, which may be the reason for the insolublity of polynorbornenes obtained by palladium catalysts.
INTRODUCTION
■
Over the past decade, the chemistry of N-heterocyclic carbenes
(NHCs) and their metal complexes have attracted considerable
attention in the field of organometallics and catalysis since the
precursory work of Arduengo.1 The strong σ-donor ability of
NHC leads to complexes that are more thermodynamically
robust than those of the ubiquitous phosphine ligands.2 All of the
Figure 1. α-Diimine palladium complex (A), six-membered imine−
NHC palladium complex (B), and five-membered imine−NHC
palladium complex (C).
transition metals from group 7 to group 11 in the periodic table
can nearly be coordinated with the NHC ligand.3 Although
transition-metal complexes containing NHCs are now widely
under comparatively mild conditions.13f,16 In view of the
used in the field of C−C cross-coupling reactions,4 olefin
metathesis,5 hydrosilylation,6 [3 + 2] cycloaddition reactions,7
improved thermal stability of the complexes and superior
atom transfer radical polymerization,8 Wacker oxidation,9 and
donor ability of NHC, our interests lie in the design and
CO/ethylene copolymerization,10 successful applications of
NHC-derived complexes for olefin polymerizations are rather
rare so far.11
synthesis of new imine−N-heterocyclic carbene ligands and the
corresponding cationic palladium complexes, which are seem-
ingly similar to the α-diimine analogues. Although cationic
methylpalladium species are well-defined active initiators for a
number of polymerization and catalysis reactions, the literature
reports showed that alkylpalladium−NHC complexes are prone
to deactivation by reductive elimination of 2-alkylimidazolium
salts; such a mode of decomposition is often more facile with
cationic complexes.17 Increasing the steric bulk of the NHC can
Recently, late-transition-metal catalysts for olefin polymer-
ization have been extensively developed.12,13 Brookhart reported
that nickel and palladium complexes bearing a bulky α-diimine
ligand exhibited high catalytic activity for olefin polymerization.14
Specifically, well-defined cationic α-diimine palladium catalysts
(A in Figure 1) can afford polyethylenes with different topologies
by a chain-walking approach and functional polyethylenes.15
Despite the early promising results, cationic α-diimine palladium
catalysts are often thermally unstable and prone to deactivation
Received: March 31, 2013
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of Imine-NHC Ligands and Palladium Complexes 3a−e
Figure 2. ORTEP drawing of the molecular structures of 3a−c,e. All thermal ellipsoids are at the 50% probability level, and hydrogen atoms and
cocrystallized solvent molecules are omitted for clarity.
prevent this decomposition pathway, and a few cationic
methylpalladium−NHC complexes with six-membered [N,C]-
metal chelates (B in Figure 1) have been synthesized in this
way.18 In addition, NHCs with a directly N-bonded imine group
are also unstable because of a [1,2] rearrangement of the imidoyl
moiety.19 Therefore, the synthesis of cationic methylpalladium−
NHC complexes with a directly N bonded imine group still
remains a challenge.
Typically, palladium complexes are highly efficient catalyst
precursors for the vinyl polymerization of norbornene (NB).20
Although numerous palladium catalysts have been developed for
the polymerization of norbornene, current studies are mostly
limited to various ligand or substituent effects of palladium
catalysts on polymerization activity by employment of empirical
methods. This is largely due to a lack of understanding of the
activation mechanism and the nature of the active species and a
lack of understanding of the activity trends as well as the
microstructure and origin of the insolubility of the poly-
norbornene (PNB).21 Studies on the activation process still
remain a great challenge due to the ill-defined structure of MAO
used as cocatalyst, palladium(II) reduction, and the formation of
insoluble gel-like products.22 Exceptions are cationic palladium-
(II) complexes such as [Pd(NCCH3)4](BF4)2 and [Pd-
(PPh3)3(NCCH3)](BF4)2, which require no cocatalyst for their
activation toward norbornene polymerization, but a “classical”
coordination−insertion mechanism cannot be applied to explain
this polymerization because of the lack of a Pd−C bond.23
In this paper, we synthesized and characterized a series of
imine−N-heterocyclic carbene ligands and their corresponding
palladium complexes by the introduction of bulky substituents
on both the imine and the NHC moieties (C in Figure 1). The
first sample of a cationic five-membered imine-functionalized
methylpalladium complex was synthesized and exhibited good
stability. These imine−N-heterocyclic carbene palladium com-
plexes and well-defined cationic NHC−palladium complexes
were also used as catalyst precursors for norbornene polymer-
ization. A preliminary study on the activation mechanism and
structure of the polymer was also performed by NMR analysis.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of the Complexes. The
synthetic routes to imine−NHC ligands and palladium
complexes are shown in Scheme 1. A series of imine-
functionalized NHC ligands with different substituents on both
the imine moiety and the imidazole moiety was synthesized by a
coupling reaction of imidoyl chlorides with N-substituted
imidazole. Imidazolium salts (C∧imine·HCl) (1a−e) containing
different substituents were obtained in high yield. Imine−NHC
silver complexes were usually used as effective transfer reagents
to obtain other transition-metal complexes. Stirring 1a−e with
Ag2O in dichloromethane at room temperature for 6 h afforded
the silver N-heterocyclic complexes 2a−e. Lavoie and co-
workers once reported that C∧imine·HCl (1d) showed no
reactivity toward Ag2O under various experimental conditions,
B
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Table 1. Selected Bond Lengths (Å) and Angles (deg) for Palladium Dichloride Complexes 3a−c,e
3a
3b
3c
3e
Pd(1)−C(1)
Pd(1)−N(1)
Pd(1)−Cl(1)
Pd(1)−Cl(2)
C(1)−Pd−N(1)
C(1)−Pd−Cl(2)
C(1)−Pd−Cl(1)
N(1)−Pd−Cl(1)
1.962(3)
2.039(2)
2.3437(7)
2.2926(7)
79.33(9)
96.47(8)
170.96(8)
93.19(6)
1.959(6)
1.957(4)
1.958(4)
2.056(5)
2.049(4)
2.048(3)
2.3409(18)
2.3012(18)
79.2(2)
2.3418(11)
2.2888(12)
79.49(16)
97.00(13)
173.71(11)
93.73(10)
2.3423(10)
2.2812(10)
79.92(14)
95.72(11))
173.08(11)
93.23(9)
96.94(19)
172.36(19)
93.21(15)
Scheme 2. Synthesis of Methylpalladium Complexes 4c,e and Cationic Palladium Complexes 5c,e
Figure 3. ORTEP drawings of complexes 4c,e. All thermal ellipsoids are at the 50% probability level, and hydrogen atoms and cocrystallized solvent
molecule CH2Cl2 are omitted for clarity.
and Ag2CO3 was thereby used instead of Ag2O.24 However, the
imine−NHC silver complex 2d was herein readily prepared by
reaction of the imidazolium salt 1d with Ag2O under mild
reaction conditions in a good yield of 83%. Elemental analyses
(EA) of the obtained silver complexes show C/H/N ratios
consistent with the corresponding (NHC)AgCl complex. ESI-
MS mass spectra display a molecular ion of the silver bis(NHC)
monocation, which is a result of an occurrence of conversion
between [Ag−(NHC)Cl] and [Ag(NHC)2]+ under the adopted
conditions.25 No interaction between the imine nitrogen atoms
and the Ag center is observed on the basis of the presence of the
νCN absorption at 1678 cm−1 (see the Experimental Section).
The formation of the carbene Ag complexes can be further
established by the absence of the resonance for the 2H-
imidazolium proton in 1H NMR spectra.18
values for the corresponding Ag carbene complexes (δ 1.18, 1.11
ppm for 2c and δ 1.20, 1.16 ppm for 2e). The greater separation
between the two methyl doublets on isopropyl groups is
indicative of coordination between imine and palladium metal.18
The chelated structure was further confirmed by the X-ray
crystal determination. Crystals of 3a−c,e suitable for X-ray
diffraction studies were obtained by layering a saturated CH2Cl2
solution with n-hexane at room temperature. ORTEP drawings
of the palladium complexes are depicted in Figure 2, and selected
bond lengths (Å) and angles (deg) are given in Table 1. All
palladium dichloride complexes display a distorted-square-planar
coordination around the palladium center. Pd−C(carbene) bond
lengths are around 1.96 Å, while Pd−N(imine) bond lengths are
around 2.04 Å.
Transpalladation of silver carbene complexes 2 with a
(COD)PdMeCl precursor instead of (COD)PdCl2 is expected
to afford the methylpalladium complexes (Scheme 2).11b,c Our
experimental results show that transpalladation of silver carbene
complexes 2a,b,d with a (COD)PdMeCl precursor did not afford
the corresponding methylpalladium complexes. Reaction of
2a,b,d with (COD)PdMeCl even at −20 °C afforded a trace of
white methylpalladium complexes. The bulk of generated
products was black insoluble substances, presumably palladium
black (Pd(0)). ESI-MS analysis of organic compounds in
solution also showed the presence of 2-methylimidazolium
salts. Methylpalladium complexes with ligands 1a,b,d were not
obtained because the methylpalladium complexes were prone to
deactivation by reductive elimination.17 The reaction between 2c
The N-heterocyclic carbene Ag complexes can be smoothly
converted to the corresponding palladium dichloride complexes
3a−e by transpalladation with (COD)PdCl2(COD = 1,5-
cyclooctadiene). All of the palladium dichloride complexes
bearing imine−-NHC ligands were fully characterized by EA,
1
ESI-MS, H NMR, and 13C NMR analyses. For instance, the
carbene-C resonances of 3c,e are located at around 165 ppm in
the 13C NMR, and this shift in comparison to the Ag complexes
(183.3 ppm for 2c and 151.4 ppm for 2e) indicates the formation
1
of Pd−C(carbene) bonding. In the H NMR spectra of 3c,e,
separations of the doublets (δ 1.36, 1.12 ppm for 3c and δ 1.28,
1.13 ppm for 3e) from the diastereotopic methyl on isopropyl
groups are over 0.15 ppm, which is somewhat greater than the
C
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and (COD)PdMeCl at above 0 °C afforded only a small amount
of white methylpalladium complex 4c in a very low yield. When
the reaction temperature was set below −20 °C, only 4c was
obtained in 50% yield. However, the reaction of the more bulky
2e with (COD)PdMeCl could smoothly proceed at room
temperature to produce white 4e in 75% yield. Introduction of
bulky substituent groups on imine−NHC ligands can improve
the stability of five-membered methylpalladium complexes and
suppress reductive elimination which may cause alkylpalladium−
NHC complex decomposition.26 Methylpalladium complexes
4c,e have better solubility in organic solvents than palladium
dichloride complexes, and purifications of methylpalladium
complexes are thereby facile.
which is a result of the strong structural trans effect of the methyl
group.
Treatment of complexes 4c,e with sodium tetrakis(3,5-
bis(trifluoromethyl)phenyl)borate (NaBAF) in CH3CN readily
yields the corresponding cationic palladium complexes 5c,e as
white solids. Pure cationic complexes can be almost
quantitatively obtained. The cationic complex 5c,e solids are
stable to air and moisture and remained unchanged for over 3
days in open air. Also, the cationic palladium complexes show the
best solubility in toluene and dichloromethane among the three
types of palladium complexes.
For the cationic palladium complexes 5c,e, methyl bonded to
palladium metal can be still observed at around 0 ppm in the 1H
NMR spectra, while a new methyl signal of the coordinated
CH3CN is also identified on the basis of the resonance around
1.6 ppm. Like the neutral methylpalladium complex, only one
isomer of the cationic palladium complex is present in solution
on the basis of a single set of methyl resonances in 1H NMR and
13C NMR spectra (see the Experimental Section). Crystals of
5c,e suitable for X-ray diffraction analysis were obtained by slow
diffusion of n-hexane into palladium complex solutions in
CH2Cl2. ORTEP drawings of 5c,e are given in Figure 4, and
selected bond distances and angles are given in Table 3. The
The structures of these two methylpalladium complexes have
1
been proven by EA, ESI-MS, H NMR, and 13C NMR. The
1
signals at 0.83 and 0.27 ppm in the H NMR spectra can be
assigned to the palladium-bound methyl groups of 4c,e,
respectively. The 13C NMR resonances of carbene-C of 4c,e lie
at −5.01 and −5.97 ppm, respectively, which shift to low field in
comparison to the chemical shift of a methylpalladium complex
with a pyridine−NHC ligand.11 The single set of resonances
observed for 4c,e indicates the presence of a single isomer.
Single-crystal X-ray diffraction analysis of 4c,e (Figure 3) further
confirmed that methylpalladium complexes are present in the
trans form and feature the methyl group trans to the imine group.
Bond lengths and angles around the palladium center are highly
similar to those in the corresponding palladium dichloride
complexes 3c,e except for the Pd−N(imine) bond length (Table
2). The Pd−N(imine) bond lengths of methylpalladium
Table 3. Selected Bond Lengths (Å) and Angles (deg) for
Cationic Palladium Complexes 5c,e
5c
5e
Pd(1)−C(1)
Pd(1)−C(2)
Pd(1)−N(1)
Pd(1)−N(4)
C(1)−Pd(1)−C(2)
C(1)−Pd(1)−N(4)
C(2)−Pd(1)−N(4)
C(1)−Pd(1)−N(1)
N(4)−Pd(1)−N(1)
C(2)−Pd(1)−N(1)
1.957(5)
2.028(5)
2.136(4)
2.041(4)
98.0(2)
1.965(7)
2.031(8)
2.160(6)
2.053(6)
97.1(3)
171.8(3)
91.1(3)
79.2(3)
92.6(2)
176.4(3)
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Methylpalladium Complexes 4c,e
4c
4e
170.69(18)
91.0(2)
Pd(1)−C(1)
Pd(1)−C(2)
Pd(1)−N(1)
Pd(1)−Cl(1)
C(1)−Pd(1)−C(2)
C(1)−Pd(1)−Cl(1)
C(2)−Pd(1)−Cl(1)
C(1)−Pd(1)−N(1)
Cl(1)−Pd(1)−N(1)
C(2)−Pd(1)−N(1)
1.967(3)
2.053(2)
2.172(2)
2.3587(7)
96.03(11)
171.71(8)
90.75(7)
78.89(9)
94.49(5)
174.50(9)
1.964(3)
2.041(3)
2.176(3)
2.3436(8)
96.22(13)
173.77(9)
89.98(10)
78.69(11)
95.10(7)
174.77(13)
78.99(16)
92.17(15)
175.5(2)
molecular structures of the cationic complexes support the
connectivity pattern deduced from NMR analyses in solution.
The cationic palladium complexes also feature the methyl group
trans to the imine and the coordinated CH3CN trans to the
NHC. Note that the coordination of CH3CN is not linear (Pd−
N4−C3 is 166.9° for 5c and 163.5° for 5e) and is also bent out of
the metal coordination plane.
complexes are substantially longer (2.051(5) Å in 3c vs
2.172(2) Å in 4c, 2.049(4) Å in 3e vs 2.176(3) Å in 4e),
Norbornene Polymerization. Experimental results of
norbornene polymerization with palladium complexes 3a−e,
Figure 4. ORTEP drawing of the molecular structures of 5c,e. All thermal ellipsoids are at the 50% probability level, and hydrogen atoms, cocrystallized
solvent molecule CH2Cl2, and counterion BAF− are omitted for clarity.
D
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4c,e, and 5c,e are summarized in Table 4. Palladium dichloride
complexes 3a−e and methylpalladium complexes 4c,e do not
1, 50/1) were monitored by 1H NMR spectroscopy (Figure 5).
For 5e solution without NB monomer, the integral ratio of two
Table 4. Results of Norbornene Polymerization with
a
Palladium Catalysts
b
entry
catalyst
temp (°C) yield (mg) conversion (%) activity
1
3a/MMAO
3b/MMAO
3c/MMAO
3d/MMAO
3e/MMAO
4c/MMAO
4e/MMAO
5c
30
30
30
30
30
30
30
30
60
80
110
60
60
858
876
847
785
778
650
636
164
210
192
80
71.5
73.0
70.5
65.4
64.8
54.1
53.0
13.6
17.5
16.0
6.7
34.3
35.0
33.8
31.4
31.1
26.0
25.4
2
3
4
5
6
7
8
6.56
9
5c
8.40
7.68
3.20
4.40
10
11
12
13
5c
5c
1
Figure 5. H NMR of the cationic catalyst 5e in combination with
5e
110
9.2
c
different norbornene concentrations at 5 min: (1) 5e, Ia/Ib = 3/8; (2)
NB/5e = 10/1, Ia/Ib = 2.05/8; (3) NB/5e = 20/1, Ia/Ib = 1.44/8; (4)
NB/5e = 50/1, Ia/Ib = 0.48/8.
α-diimine Pd
a
Polymerization conditions: palladium complex, 5 μmol; monomer,
1.2 g; time, 30 min; total volume, 10 mL; solvent, o-dichlorobenzene;
b
Al(MMAO)/Pd ratio, 3000. In units of 104 g of PNB/((mol of Pd)
h). The cationic α-diimine−Pd catalyst structure is given as A in
Figure 1.
peaks at 0.08 and 7.68 ppm assigned to Pd−CH3 protons and
2,6-H of the BAF group is quite consistent with the theoretical
value of 3/8. When norbornene monomer was added, these two
characteristic peaks were still observed, but the integral ratio of
two peaks obviously changed. An integral ratio reduction from 3/
8 to 0.48/8 was observed with an increase in NB/Pd ratio from 0
to 50, suggesting a consumption of the Pd−CH3 bond because of
norbornene insertion into the Pd−CH3 bond. At the same time,
new signals for the PNB sequence appear at 0.9−2.5 ppm.28b
More importantly, a set of resonances at 0.3−0.8 ppm is clearly
observed, which can be assigned to the end group CH3 of the
growing chain (Me−(NB)n−Pd). The NMR results undoubt-
edly prove that norbornene polymerization with well-defined
cationic palladium catalysts proceeds through a coordination−
insertion mechanism.22c,28
c
afford any polymer in the absence of cocatalyst. After activation
with MMAO, all of these palladium complexes can catalyze
norbornene polymerization with high activity. Increasing the
steric hindrance of the NHC moiety leads to a slight decrease in
catalytic activity. It may be that the bulky mesityl substituent
retards the coordination and insertion of the norbornene
monomer.27 Palladium dichloride complexes 3c,e show higher
activities in comparison to methylpalladium analogues 4c,e,
indicating that palladium dichloride complexes are more easily
activated by MMAO than the corresponding methylpalladium
complexes.
Differently from the neutral palladium complexes, cationic
palladium complexes 5c,e can polymerize norbornene alone
without any cocatalysts. However, a “classical” Brookhart-type
cationic α-diimine palladium analogue (A in Figure 1) was found
to be basically inactive for norbornene polymerization (entry
13). Although the cationic palladium complexes exhibit lower
activities than the corresponding 3c,e, the experimental data
(entries 8−11) demonstrate that cationic palladium complexes
5c are thermally stable for norbornene polymerization because of
the strong σ-donor ability of NHC ligand. Even at 80 °C, it still
maintains a stable activity, and at 110 °C, the activity is
significantly reduced. As was previously reported in the literature
for other palladium catalyst systems,21 the PNBs obtained herein
with these palladium catalysts bearing NHC ligands are also
insoluble in organic solvents such as cyclohexane, toluene,
chlorobenzene, dichlorobenzene, and trichlorobenzene even at
elevated temperature.
Currently, two crucial questions, the nature of the active
species for norbornene polymerization and the insolubility origin
of the PNB obtained by palladium catalyst, remain unanswered.
Well-defined cationic palladium complexes 5c,e herein used to
catalyze norbornene polymerization without any cocatalysts are
suitable for research on the polymerization mechanism.
The norbornene polymerization processes in 0.75 mL of
CDCl3 solution containing 0.5 μmol of 5e and different
concentrations of norbornene monomer (NB/Pd = 10/1, 20/
The norbornene polymerization at low norbornene concen-
tration (NB/Pd = 10/1) was further monitored by H NMR
1
spectroscopy at different times (see Figure S9 in the Supporting
Information). Generally, norbornene polymerization was slowly
initiated on the basis of the full disappearance of Pd−Me at ∼7 h
because of coordination completion between norbornene and
CH3CN. When the polymerization time wa prolonged,
norbornene monomer was gradually consumed on the basis of
decreasing integral ratio (Ic/Ib), and ∼60% of monomer was
consumed at 7 h. No obvious decomposition and elimination of
catalyst were observed in the test time scale, suggesting that the
palladium catalyst is stable under the tested conditions.
PNBs obtained with palladium catalysts bearing NHC ligands
are insoluble in organic solvents, even at elevated temperature.
The IR spectra of the PNBs revealed the missing absorption of a
double bond at 1600−1700 cm−1 and the existence of vibration
bands of bicyclics of norbornene at 941 cm−1, indicating that the
PNBs are not ROMP polymers. The wide-angle X-ray diffraction
analysis of the obtained PNBs showed two major broad peaks
(2θ = 9−11 and 17−19°), which implies the obtained PNBs are
noncrystalline and cannot provide more information on the
microstructure.29
In order to obtain a soluble product and gain an insight into
the microstructure of polynorbornene obtained by the palladium
catalyst, a hydrogen-regulated polymerization of norbornene was
performed with 5e under the conditions of 1 atm of H2 pressure,
E
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50 °C, and low norbornene monomer concentration (NB/Pd =
100). A small amount of a toluene-soluble fraction was extracted
from the polymerization product. The soluble fraction as an
oligomer with number-average molecular weight (Mn) of 1500
determined by GPC analysis allowed us to its their micro-
structure and probe the origin of the insolubility of PNB obtained
broad resonances are observed at 20−60 ppm, and the peaks at
very low field (52−57 ppm) are a result of C7 linkage. Moreover,
two characteristic peaks of the norbornene oligomer in Figure 7
are also overlapped in 13C-CP/MAS spectroscopy. These
agreements support the existence of a C7 linkage in the insoluble
PNB obtained by the cationic palladium catalyst.32 Intra-
molecular σ-bond metathesis leads to a rigid helical structure
of the PNB obtained by palladium catalysts, which strongly
affects its solubility. It must be acknowledged that such reactions
may occur much more frequently with palladium catalysts than
with zirconium catalysts on the basis of the frequent existence of
C−H activation reactions for palladium catalysts.33 Therefore,
intermolecular σ-bond metathesis cannot be excluded, and
insoluble PNB with cross-linking structure may be formed.
1
by palladium catalysts. As shown in Figure 6, the H NMR
CONCLUSIONS
■
In summary, a series of new palladium complexes bearing an
NHC ligand with a directly N-bonded imine group, including
palladium dichloride, methylpalladium, and cationic palladium
complexes, has been successfully synthesized and fully
characterized. The reductive elimination of methylpalladium
complexes containing an NHC donor is effectively suppressed by
introduction of a bulky substituent group on both the imine and
the NHC moiety. Neutral palladium complexes activated by
MMAO are highly active toward norbornene polymerization,
whereas well-defined cationic palladium complexes can polymer-
ize norbornene alone without any cocatalysts and show a high
thermostability. Norbornene polymerization with the cationic
palladium catalyst 5e proceeds through a coordination−insertion
mechanism. Analysis of the oligomer from a hydrogen-regulated
polymerization of norbornene reveals the existence of a
palladium-catalyzed C7 linkage in the polynorbornene (PNB)
by σ-bond metathesis. The origin of the insolubility of PNB
obtained by palladium catalysts may come from a rigid helical
structure or cross-linking structure. Our study provides deep
insight into Pd-catalyzed norbornene polymerization and the
microstructure of the PNB obtained by a palladium catalyst.
Figure 6. 1H NMR spectrum of the oligomer of norbornene.
spectrum of the soluble fraction is in a range from 0.6 to 2.5 ppm,
and no traces of any double bond at 5−7 ppm are observed. The
absence of a double bond proves the formation of a saturated
oligomer.
The 13C NMR spectrum of the norbornene oligomer would
provide a basis for a model of the microstructure of the PNBs
obtained by these palladium catalysts. Figure 7 shows a few sets
Figure 7. 13C NMR spectrum of the oligomers of norbornene.
of sharp resonances, which differ distinctly from the spectra of
2,3-exo-disyndiotactic and 2,3-exo-diheterotactic PNBs obtained
by TiCl4/Al(Et)2Cl30a,b and CrCl2(dppa)/MAO30c catalytic
systems. On the basis of the assignments in previous work, the
resonances of methenes and methines appear at 29.3−32.5 ppm
for C5 and C6, ∼35.8 ppm for C7, 36.3−40.4 ppm for C1 and
C4, 46.4 ppm for C3, and 49.1 ppm for C2.31 Two significant
differences are the occurrence of a sharp peak at 41.6 ppm and a
broad peak at low field of 53.8 ppm, which can be assigned to a 7-
linked norbornene unit as a result of σ-bond metathesis.32 The
microstructure of the insoluble PNB was also determined and
analyzed by a 13C cross-polarization (CP)/magic-angle-spinning
(MAS) spectrum (13C-CP/MAS). The 13C-CP/MAS spectrum
shown in Figure 8 is similar to the reported spectrum of insoluble
PNB with a C7-linkage structure obtained by a Zr catalyst.32 Six
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
techniques.
Materials. 2, -Dimethylaniline, 2,4,6-trimethyaniline, and 2,6-
diisopropylaniline were purchased from Aldrich Chemical and were
distilled under reduced pressure before being used. Modified
methylaluminoxane (MMAO, 2.0 M in hexane) was purchased from
Akzo Nobel Corp. Toluene, n-hexane, and diethyl ether were refluxed
over metallic sodium for 24 h before being used. Dichloromethane and
dichlorobenzene were dried over phosphorus pentoxide for 8 h and
distilled under a nitrogen atmosphere. Norbornene was dried over
potassium metal prior to use in polymerization. Methylimidazole was
purchased from Alfa Aesar and used as received. The intermediate
products mesityl-substituted imidazole, imidoyl chlorides, (COD)-
PdCl2, and (COD)PdMeCl were prepared according to literature
methods.12a The cationic α-diimine palladium compound was
synthesized according to the reported methods.12a
Characterizations. Elemental analyses were performed with a Vario
EL series elemental analyzer from Elementar. ESI-MS spectra were
obtained with a Shimadzu LCMS-2010A instrument. IR spectra were
recorded on a Nicolet NEXUS-670 spectrometer. The NMR data of
ligands and complexes were obtained on a Varian Mercury-Plus 300
MHz spectrometer at ambient temperature, using CDCl3, DMSO-d6,
CD3CN, or dichlorobenzene-d4 as solvent and referenced versus TMS
as standard. The molecular weight of the norbornene oligomer were
Figure 8. 13C-CP/MAS spectrum of insoluble PNB.
F
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Article
determined on an Agilent Technologies PL-GPC220 instrument at 150
°C with polystyrene as standard, and 1,2,4-trichlorobenzene was
employed as the eluent. 13C-CP/MAS measurements were performed
on a Bruker AVANCE 400 spectrometer using adamantine as standard.
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 diffractiometer with graphite-monochromated Mo
Kα radiation (λ = 0.71073 Å). 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.
Norbornene Polymerization. In a typical procedure, 1.20 g of
norbornene in 5.5 mL of o-dichlorobenzene and 2.5 mL of MMAO (2
M) (5c,e without MMAO) were added into a flask (100 mL) with
stirring under an N2 atmosphere. After the mixture was kept at the
desired temperature for 2 min, 5 μmol of the palladium complex in 1 mL
of dichloromethane was injected into the flask via syringe, and the total
reaction volume kept at 10 mL by variation of the added o-
dichlorobenzene when necessary. The reaction mixture was continu-
ously stirred at polymerization temperature, which was controlled with
an external oil bath in polymerization experiments. After 30 min, the
polymerization was terminated by addition of acidic ethanol (ethanol/
HCl, 95/5). The precipitated polymer was washed with ethanol and
dried at 60 °C under vacuum to a constant weight.
Hydrogen-Regulated Polymerization of Norbornene. Eight
milliliters of a norbornene solution in toluene (100 mg of NB) was
added into a flask (100 mL) under an H2 pressure of 1 atm. After the
mixture was stirred at 50 °C for 2 min, 1 mL of a 5e (10 μmol) solution
in dichloromethane was injected into the flask via syringe. After the
reaction mixture was continuously stirred at 50 °C for 30 min, the
reaction was terminated by addition of 5 mL of ethanol. The obtained
solid products were fractionated by extraction with toluene, and the
small mass of soluble fraction was concentrated to low volume. Then
addition of ethanol to the solution afforded a white solid. The
precipitated solid was dried and used for characterization by NMR and
GPC.
H), 6.92 (m, 6H, C6H3, C6H2), 2.55 (s, 3H, imine-CH3), 2.25 (s, 3H, p-
Ar-CH3), 1.86 (d, 12H, o-Ar-CH3). 13C NMR (75 MHz, CDCl3): δ
158.3, 148.7, 135.7, 134.5, 129.8, 128.4, 127.5, 127.9, 126.9, 126.1, 117.1,
116.7, 21.3, 18.2, 17.9, 17.6.
Synthesis of [3-(2,4,6-trimethylphenyl)-1-(2,6-diisopropylphenyl)-
iminyl-C3H3N2]+Cl− (1e). Following the above procedure, 1e was
isolated in 93% yield (1.6 g, 3.7 mmol). 1H NMR (300 MHz, CDCl3): δ
12.1 (s, 1H, NCHN), 7.28 (s, 1H, imidazole-H), 8.51 (s, 1H, imidazole-
i
H), 7.12 (m, 5H, Ar-H), 2.78 (s, 3H, imine-CH3), 2.67 (m, 2H, Pr-
CH3), 2.34 (s, 3H, p-Ar-CH3), 2.25 (s, 6H, o-Ar-CH3), 1.16 (dd, 12 H,
iPr-CH3). 13C NMR (75 MHz, CDCl3): δ 140.9, 137.2, 134.9, 130.4,
130.1, 126.0, 124.4, 124.1, 123.9, 118.8, 29.0, 23.7, 21.5, 18.2, 18.4, 17.7.
Synthesis of Ag(I) Complexes. Synthesis of [3-Me-1-(2,6-
dimethylphenyl)iminyl-C3H2N2]AgCl (2a). The dimethylphenylimida-
zolium salt 1a (0.789 g, 3 mmol) was dissolved in 30.0 mL of dry
dichloromethane and the solution added, under nitrogen, to a Schlenk
containing activated powdered 4 A molecular sieves. Ag2O (0.450 g, 2
mmol) was then added and the reaction mixture stirred in the absence of
light for 6 h. The reaction mixture was then filtered in the dark and the
volume of the solution reduced to 5.0 mL. Pentane was added to afford
the product 2a as an off-white solid in 63% yield (694 mg, 1.9 mmol).
Anal. Calcd for C14H17N3AgCl: C, 45.37; H, 4.62; N, 11.34. Found: C,
45.04; H, 4.57; N, 10.87. ESI-MS (m/z): 563, [Ag(ligand)2]+ 563. 1H
NMR (300 MHz, CDCl3): δ 7.88 (s, 1H, imidazole-H), 7.18 (s, 1H,
imidazole-H), 6.88 (m, 3H, Ar-H), 3.86 (s, 3H, N-CH3), 2.34 (s, 3H,
imine-CH3), 1.98 (d, 6H, o-Ar-CH3). 13C NMR (75 MHz, CDCl3): δ
181.4, 152.2, 144.6, 128.3, 126.4, 124.2, 123.1, 119.7, 40.3, 20.3, 18.5.
Synthesis of [3-Me-1-(2,4,6-trimethylphenyl)iminyl-C3H2N2]AgCl
(2b). Following the above procedure, 2b was isolated in 70% yield (804
mg, 2.1 mmol). Anal. Calcd for C15H19N3AgCl: C, 46.84; H, 4.98; N,
10.92. Found: C, 46.94; H, 4.72; N, 10.83. ESI-MS (m/z): 626,
[Ag(ligand)2]+ 626. 1H NMR (300 MHz, CDCl3): δ 8.34 (s, H,
imidazole-H), 7.88 (s, H, imidazole-H), 7.62 (m, 2H, Ar-H), 4.08 (s, 3H,
N-CH3), 2.31 (s, 3H, imine-CH3), 2.31 (d, 6H, o-Ar-CH3), 2.012 (s, 3H,
p-Ar-CH3). 13C NMR (75 MHz, CDCl3): δ 178.9, 152.7, 147.2, 132.3,
128.3, 126.8, 121.1, 116.3, 38.4, 30.6, 21.3, 18.7.
Synthesis of [3-Me-1-(2,6-diisopropylphenyl)iminyl-C3H2N2]AgCl
(2c). Following the above procedure, 2c was isolated in 75% yield (956
mg, 2.2 mmol). Anal. Calcd for C18H25N3AgCl: C, 50.66; H, 5.90; N,
9.85. Found: C, 50.37; H, 5.78; N, 9.76. ESI-MS (m/z): 675,
Synthesis of Ligands. Synthesis of [3-Me-1-(2,6-
dimethylphenyl)iminyl-C3H3N2]+Cl− (1a). Methylimidazole (0.33 g,
4.0 mmol) was added dropwise to a solution of the dimethylphenyli-
minyl chloride (0.71 g, 3.90 mmol) in dry THF (20 mL) over a period of
5 min. The mixture was stirred at ambient temperature for 20 h, and the
product slowly precipitated as a white solid. The crude product was
obtained by filtration, washed with dry THF (3 × 15 mL), and dried
under reduced pressure. 1a was isolated as a white solid in 98% yield
1
[Ag(ligand)2]+ 675. H NMR (300 MHz, CDCl3): δ 8.05 (s, 1H,
imidazole-H), 7.23 (s, 1H, imidazole-H), 7.13 (m, 3H, Ar-H), 3.97 (s,
3H, N-CH3), 2.68 (m, 2H, iPr-CH), 2.50 (s, 3H, imine-CH3), 1.14 (dd,
i
12H, Pr-CH3). 13C NMR (75 MHz, CDCl3): δ 183.3, 151.6, 142.1,
136.7, 124.9, 123.5, 119.7, 40.45, 28.8, 23.6, 18.4. IR: νCN 1678 cm−1.
Synthesis of [3-(2,4,6-trimethylphenyl)-1-(2,6-dimethylphenyl)-
iminyl-C3H2N2]AgCl (2d). Following the above procedure, 2d was
isolated in 83% yield (1.2 g, 2.5 mmol). Anal. Calcd for C22H25N3AgCl:
C, 55.65; H, 5.31; N, 8.85. Found: C, 55.27; H, 5.04; N, 8.61. ESI-MS
(m/z): 769, [Ag(ligand)2]+ 769. 1H NMR (300 MHz, CDCl3): δ 8.307
(s, 1H, imidazole-H), 7.11 (s, 1H, imidazole-H), 6.97 (m, 5H, C6H3,
C6H2), 2.60 (s, 3H, imine-CH3), 2.36 (s, 3H, p-Ar-CH3), 2.09 (d, 12H,
o-Ar-CH3). 13C NMR (75 MHz, CDCl3): δ 152.3, 144.6, 139.9, 134.9,
129.7, 128.3, 126.3, 124.3, 123.0, 119.7, 21.4, 18.6, 18.2.
1
(1.03 g, 3.92 mmol). H NMR (300 MHz, CDCl3): δ 10.7 (s, 1H,
NCHN), 7.93 (s, H, imidazole-H), 7.74 (s, H, imidazole-H), 6.73 (m,
3H, Ar-H), 3.99 (s, 3H, N-CH3), 2.18 (s, 3H, imine CH3), 1.73 (d, 6H,
o-Ar-CH3). 13C NMR (75 MHz, CDCl3): δ 150.2, 145.3, 136.2, 130.5,
129.7, 127.4, 123.5, 118.1, 36.1, 17.3, 16.2.
Synthesis of [3-Me-1-(2,4,6-trimethylphenyl)iminyl-C3H3N2]+Cl−
(1b). Following the above procedure, 1b was isolated in 96% yield
(1.1 g, 3.8 mmol). 1H NMR (300 MHz, CDCl3): δ 9.88 (s, 1H, NCHN),
8.33 (s, H, imidazole-H), 7.86 (s, H, imidazole-H), 7.17 (m, 2H, Ar-H),
3.94 (s, 3H, N-CH3), 2.28 (s, 3H, p-Ar-CH3), 2.27 (s, 3H, imine-CH3),
1.95 (s, 6H, o-Ar-CH3). 13C NMR (75 MHz, CDCl3): δ 152.2, 145.7,
137.3, 132.5, 128.4, 125.5, 121.9, 119.6, 37.5, 23.1, 19.5, 16.8.
Synthesis of [3-(2,4,6-trimethylphenyl)-1-(2,6-diisopropylphenyl)-
iminyl-C3H2N2]AgCl (2e). Following the above procedure, 2e was
isolated in 80% yield (1.3 g, 2.4 mmol). Anal. Calcd for C26H33N3AgCl:
C, 58.82; H, 6.27; N, 7.92. Found: C, 58.70; H, 6.07; N, 7.65. ESI-MS
(m/z): 883, [Ag(ligand)2]+ 883. 1H NMR (300 MHz, CDCl3): δ 8.31
(s, 1H, imidazole-H), 7.20 (s, 1H, imidazole-H), 7.10 (m, 6H, C6H3,
C6H2), 2.71 (m, 2H, iPr-CH), 2.62 (s, 3H, imine-CH3), 2.09 (m, 9H, p-
Synthesis of [3-Me-1-(2,6-diisopropylphenyl)iminyl-C3H3N2]+Cl−
(1c). Following the above procedure, 1c was isolated in 90% yield
(1.2 g, 3.6 mmol). 1H NMR (300 MHz, CDCl3): δ 11.9 (s, 1H, NCHN),
8.19 (d, H, imidazole-HN), 7.55 (d, H, imidazole-H), 7.16 (m, 3H, Ar-
i
Ar-CH3, o-Ar-CH3), 1.16 (m, 12H, Pr-CH3). 13C NMR (75 MHz,
i
CDCl3): δ 151.4, 142.6, 140.4, 136.7, 135.8, 134.9, 130.8, 125.1, 123.6,
119.4, 28.7, 23.5, 21.3, 18.8.
H), 4.32 (s, 3H, N-CH3), 2.59 (m, 2H, Pr-CH), 2.53 (s, 3H, imine-
CH3), 1.13 (m, 12H, iPr-CH3). 13C NMR (75 MHz, CDCl3): δ 148.4,
140.7, 138.4, 136.5, 125.6, 124.7, 123.6, 117.7, 37.57, 28.8, 23.5, 17.1. IR:
νCN 1678 cm−1, νimidazole‑H 3495 cm−1.
Synthesis of Pd(II) Complexes. Synthesis of [3-Me-1-(2,6-
dimethylphenyl)iminyl-C3H2N2]PdCl2 (3a). A cooled solution (−20
°C) of the silver complex 2a (100 mg, 0.230 mmol) in dry
dichloromethane (10 mL) was slowly transferred to a similarly cooled
solution of (COD)PdCl2 (0.061 mg, 0.21 mmol) in dichloromethane
(15 mL). The mixture was slowly warmed to ambient temperature and
Synthesis of [3-(2,4,6-trimethylphenyl)-1-(2,6-dimethylphenyl)-
iminyl-C3H3N2]+Cl− (1d). Following the above procedure, 1d was
isolated in 94% yield (1.4 g, 3.7 mmol). 1H NMR (300 MHz, CDCl3): δ
11.2 (s, 1H, NCHN), 8.15 (d, H, imidazole-H), 7.84 (d, H, imidazole-
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stirred overnight before removal of the volatiles under vacuum. The
crude product was dissolved in acetonitrile (15 mL) and filtered to
remove precipitated silver chloride. The solvent was removed under
vacuum, and the crude product was recrystallized from a pentane/
dichloromethane mixture. 3a was isolated in 85% yield (79 mg, 1.9
mmol). ESI-MS (m/z): 403, MW 403. Anal. Calcd for C14H18N3PdCl2:
(m, 3 H, p-Ar-CH3), 2.13 (s, 6H, o-Ar-CH3), 1.34 (d, 6H, iPr-CH3), 1.14
(d, 6H, iPr-CH3), 0.27 (s, 3H, Pd-CH3). 13C NMR (75 MHz, CD3CN):
δ 179.1, 164.1, 145.8, 140.1, 134.3, 131.7, 129.3, 128.5, 124.1, 33.8, 28.8,
28.6, 25.9, 22.6, 20.9, −5.9.
Synthesis of [(3-Me-1-(2,6-diisopropylphenyl)iminyl-C3H2N2)-
PdMeCH3CN]+BAF− (5c). NaBAF (443 mg, 0.5 mmol) was added to a
solution of 4c (220 mg, 0.5 mmol) in MeCN (10 mL) and stirred at
room temperature for 16 h with exclusion from light. The reaction
mixture was then filtered through Celite, and the volatiles were removed
under reduced pressure. The resulting off-white solid was dissolved in
CH2Cl2 (5 mL), and the solution was stirred with activated charcoal for
30 min. The mixture was then filtered through Celite and concentrated
to 2 mL. An off-white solid was precipitated by addition of excess n-
hexane, filtered, and dried under vacuum. 5c was isolated in 95% yield
(640 mg, 0.48 mmol). ESI-MS (m/z): 405, [NHC-Pd-CH3]+ 405. Anal.
Calcd for C53H44N4BF24Pd: C, 48.44; H, 3.68; N, 4.26. Found: C, 48.33;
H, 3.56; N, 4.24. 1H NMR (300 MHz, CD3Cl): 7.68 (s, 12H, BAF Ar-
H), 7.51 (m, 3H, Ar-H), 7.11 (s, 1H, imidazole-H), 6.71 (s, 1H,
imidazole-H), 3.73 (s, 3H, N−CH3), 2.85 (m, 2H, iPr-CH), 2.22 (s, 3H,
imine-CH3), 1.59 (s, 3H, CH3CN), 1.23 (d, 6H, iPr-CH3), 1.13 (d, 6H,
iPr-CH3), 0.87 (s, 3H, Pd-CH3). 13C NMR (75 MHz, CD3CN): δ 173.5,
1
C, 41.45; H, 4.47; N, 10.36. Found: C, 41.64; H, 4.51; N, 10.28. H
NMR (300 MHz, DMSO-d6): δ = 8.05 (d, 1H, imidazole-H), 7.52 (d,
1H, imidazole-H), 7.07 (m, 3H, Ar-H), 4.11 (s, 3H, N-CH3), 2.25 (s,
3H, imine-CH3), 2.15 (s, 6H, o-Ar-CH3). 13C NMR (75 MHz, DMSO-
d6): δ 165.5, 157.2, 142.1, 131.8, 128.4, 125.6, 119.4, 38.0, 18.9, 15.4.
Synthesis of [3-Me-1-(2,4,6-trimethylphenyl)iminyl-C3H2N2]PdCl2
(3b). Following the above procedure, 3b was isolated in 74% yield (71.1
mg, 0.2 mmol). ESI-MS (m/z): 418, MW 418. Anal. Calcd for
C15H20N3PdCl2: C, 42.93; H, 4.80; N, 10.01. Found: C, 42.99; H, 4.72;
N, 10.12. 1H NMR (300 MHz, DMSO-d6): δ 8.03 (d, 2H, imidazole-H),
7.51 (d, 2H, imidazole-H), 6.87 (m, 2H, Ar-H), 4.10 (s, 3H, N-CH3),
2.22 (s, 6H, o-Ar-CH3), 2.11 (m, 3H, imine-CH3), 2.09 (m, 3H, p-Ar-
CH3). 13C NMR (75 MHz, DMSO-d6): δ 157.6, 150.6, 146.1, 139.8,
138.3, 135.4, 131.5, 129.0, 125.6, 117.5, 38.3, 21.4, 18.9, 15.4.
Synthesis of [3-Me-1-(2,6-diisopropylphenyl)iminyl-C3H2N2]PdCl2
(3c). Following the above procedure, 3c was isolated in 77% yield (81.4
mg, 0.2 mmol). ESI-MS (m/z): 458, MW 458. Anal. Calcd for
C18H26N3PdCl2: C, 46.82; H, 5.68; N, 9.10. Found: C, 46.98; H, 5.75; N,
8.98. 1H NMR (300 MHz, DMSO-d6): δ 7.73 (s, 1H, imidazole-H), 7.33
(s, 1H, imidazole-H), 7.10 (m, 3H, Ar-H), 4.26 (s, 3H, N-CH3), 3.08
168.0, 167.4, 166.7, 166.4, 145.5, 143.9, 140.1, 134.6, 134.2, 131.7, 129.2,
128.1, 43.3, 33.8, 28.5, 28.4, 20.6, −6.3.
Synthesis of [(3-(2, 4, 6-trimethylphenyl)-1-(2, 6-
diisopropylphenyl)iminyl-C3H2N2)PdMeCH3CN]+BAF− (5e). Following
the above procedure, 5e was isolated in 98% yield (704 mg, 0.49 mmol).
ESI-MS (m/z): 509, [NHC-Pd-CH3]+ 509. Anal. Calcd for
C61H52N4BF24Pd: C, 51.66; H, 3.98; N, 3.95. Found: C, 51.47; H,
3.88; N, 3.85. 1H NMR (300 MHz, CD3Cl): δ 7.67 (s, 8H, BAF Ar-H),
7.50 (m, 4H, BAF Ar-H), 7.32 (m, 3H, Ar-H), 6.98 (s, 3H, imidazole-H,
(m, 2H, iPr-CH), 2.31 (s, 3H, imine-CH3), 1.25 (d, 12H, iPr-CH3). 13
C
NMR (75 MHz, DMSO-d6): δ 165.3, 157.1, 142.6, 139.1, 136.4, 129.0,
125.7, 123.9, 120.7, 119.5, 38.4, 28.3, 24.5, 16.3.
Synthesis of [3-(2,4,6-trimethylphenyl)-1-(2,6-dimethylphenyl)-
iminyl-C3H2N2]PdCl2 (3d). Following the above procedure, 3d was
isolated in 80% yield (94 mg, 0.18 mmol). ESI-MS (m/z): 506, MW 506.
Anal. Calcd for C22H26N3PdCl2: C, 51.83; H, 5.14; N, 8.24. Found: C,
51.92; H, 5.22; N, 8.13. 1H NMR (300 MHz, DMSO-d6): δ 8.31 (s, 1H,
imidazole-H), 7.51 (s, 1H, imidazole-H), 7.08 (m, 5H, C6H3, C6H2),
2.29 (s, 3H, imine-CH3), 2.19 (s, 3H, p-Ar-CH3), 2.06 (m, 12H, o-Ar-
CH3). 13C NMR (75 MHz, DMSO-d6): δ 165.1, 159.7, 142.0, 139.0,
134.4, 131.8, 128.4, 125.3, 123.9, 21.5, 18.9, 18.4, 15.7.
Synthesis of [3-(2,4,6-trimethylphenyl)-1-(2,6-diisopropylphenyl)-
iminyl-C3H2N2]PdCl2 (3e). Following the above procedure, 3e was
isolated in 85% yield (110 mg, 0.20 mmol). ESI-MS (m/z): 562, MW
562. Anal. Calcd for C26H34N3PdCl2: C, 55.18; H, 6.06; N, 7.43. Found:
C, 55.25; H, 6.13; N, 7.32.1H NMR (300 MHz, DMSO-d6): δ 8.24 (d,
1H, imidazole-H), 7.53 (d, 1H, imidazole-H), 7.18 (m, 5H, C6H3,
C6H2), 3.08 (m, 2H, iPr-CH), 2.39 (s, 3H, imine-CH3), 2.07 (s, 6H, o-
Ar-CH3), 1.28 (d, 12H,iPr-CH3). 13C NMR (75 MHz, DMSO-d6): δ
165.4, 159.9, 145.2, 141.7, 139.1, 134.3, 129.1, 125.3, 124.0, 120.4, 109.9,
28.8, 24.5, 24.1, 21.5, 18.4, 16.4.
i
Ar-H), 6.84 (s, 1H, imidazole-H), 2.85 (m, 2H, Pr-CH), 2.36 (s, 3H,
imine-CH3), 2.28 (m, 3H, p-Ar-CH3), 1.64 (s, 3H, CH3CN), 1.23 (d,
6H, iPr-CH3), 1.18 (d, 6H, iPr-CH3), 0.07 (s, 3H, Pd-CH3). 13C NMR
(75 MHz, CD3CN): δ 175.4, 168.1, 167.4, 166.7, 166.1, 165.9, 145.5,
140.2, 135.1, 134.7, 134.5, 134.3, 132.8, 131.8, 130.0, 129.3, 128.2, 125.4,
33.8, 28.5, 25.7, 22.3, 20.6, −5.8.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, tables, and CIF files giving detailed NMR sepctra,
crystallographic data of palladium complexes 3a−c,e, 4c,e, and
5c,e, 1H NMR spectroscopy of 5e with norbornene at different
times, and IR and WAXD of the PNB. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: H.G., gaohy@mail.sysu.edu.cn; Q.W., ceswuq@mail.
sysu.edu.cn.
■
Synthesis of [3-Me-1-(2,6-diisopropylphenyl)iminyl-C3H2N2]-
PdMeCl (4c). A solution of 2c (425 mg, 1 mmol) in CH2Cl2 (40 mL)
was added dropwise to a solution of (COD)PdMeCl (263 mg, 1 mmol)
in CH2Cl2 (40 mL) and stirred at −30 °C temperature for 15 h with
exclusion from light. The reaction mixture was then filtered through
Celite, and volatiles were removed under reduced pressure. The
resulting light white solid was washed with n-hexane and dried under
vacuum. 4c was isolated in 50% yield (214 mg, 0.5 mmol). ESI-MS (m/
z): 440, MW 440. Anal. Calcd for C19H29N3PdCl: C, 51.71; H, 6.62; N,
9.52. Found: C, 51.62; H, 6.69; N, 9.47. 1H NMR (300 MHz, CD3CN):
7.85 (d, 1H, imidazole-H), 7.46 (d, 1H, imidazole-H), 7.12 (m, 3H,
C6H3), 3.75 (s, 3H, NCH3), 2.89 (m, 2H, iPr-CH), 2.19 (s, 3H, imine-
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by the NSFC (Projects 21174164, 51173209,
and 21274167), and CNPC Innovation Foundation is gratefully
acknowledged.
REFERENCES
i
i
CH3), 1.18 (d, 6H, Pr-CH3), 1.04 (d, 6H, Pr-CH3), 0.83 (s, 3H, Pd-
CH3). 13C NMR (75 MHz, CD3CN): δ 176.1, 164.8, 145.8, 131.7,
130.1, 128.9, 128.5, 43.1, 33.8, 28.7, 28.6, 20.7, −5.4.
■
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dx.doi.org/10.1021/om400268y | Organometallics XXXX, XXX, XXX−XXX