Synthesis and Reactivity of Methylpalladium Complexes Bearing a Partially Saturated IzQO Ligand

Article abstract of DOI:10.1021/acs.organomet.8b00263

A saturated N-heterocyclic carbene-phenolate bidentate ligand, 3,3a,4,5-tetrahydroimidazo[1,5-a]quinolin-9-olate-1-ylidene (SIzQO), was synthesized and characterized. The SIzQO ligand was then treated with PdClMe(pyridine)₂ and [Pd(μ-Cl)Me(2,6-lutidine)]₂, which afforded C,C-cis-(SIzQO)PdMe(pyridine) and the thermodynamically unstable trans isomer C,C-trans-(SIzQO)PdMe(2,6-lutidine), respectively. The latter isomerizes at 40 °C into the corresponding cis isomer via a dissociative mechanism. These palladium/SIzQO complexes catalyze the polymerization of ethylene at 100-120 °C, although the catalytic activity is lower than that of a previously reported palladium/imidazo[1,5-a]quinolin-9-olate-1-ylidene (IzQO) system.

Full text of DOI:10.1021/acs.organomet.8b00263

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis and Reactivity of Methylpalladium Complexes Bearing a
Partially Saturated IzQO Ligand
Shumpei Akita, Ryo Nakano, Shingo Ito, and Kyoko Nozaki*
Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan
S
* Supporting Information
ABSTRACT: A saturated N-heterocyclic carbene−phenolate bidentate ligand,
3,3a,4,5-tetrahydroimidazo[1,5-a]quinolin-9-olate-1-ylidene (SIzQO), was syn-
thesized and characterized. The SIzQO ligand was then treated with
PdClMe(pyridine)2 and [Pd(μ-Cl)Me(2,6-lutidine)]2, which afforded C,C-cis-
(SIzQO)PdMe(pyridine) and the thermodynamically unstable trans isomer C,C-
trans-(SIzQO)PdMe(2,6-lutidine), respectively. The latter isomerizes at 40 °C
into the corresponding cis isomer via a dissociative mechanism. These palladium/
SIzQO complexes catalyze the polymerization of ethylene at 100−120 °C,
although the catalytic activity is lower than that of a previously reported
palladium/imidazo[1,5-a]quinolin-9-olate-1-ylidene (IzQO) system.
have been attributed to the use of bidentate ligands that bear a
strong σ-donor (e.g., phosphine) and a weak σ-donor (e.g.,
sulfonate), which enables the formation of linear polyethylenes
with high molecular weight by suppression of β-hydride
elimination as side reactions.4 This design is also effective for
the development of many other types of bidentate phosphine
ligands bearing a weak σ-donor.5−7
INTRODUCTION
■
In the last two decades, coordination−insertion copolymeriza-
tion of olefins with polar monomers has been investigated as a
means to introduce functional groups into polyolefins such as
polyethylene and polypropylene.1 For that purpose, group 10
metals, which tend to exhibit relatively high tolerance toward
polar functional groups, are the catalysts of choice owing to
their “soft” nature: i.e., their low oxophilicity.1 The most
representative catalysts for such copolymerizations are
palladium/phosphine−sulfonate catalysts (1; Figure 1),
which were initially reported by Drent et al. in 20022 and
since then have been intensively studied by many other
researchers.1b,d,3 In these studies, successful copolymerizations
Apart from phosphines, N-heterocyclic carbene (NHC)
ligands have been applied to group 10 metal catalyzed
copolymerization reactions as strong σ-donors (Figure 1).8
Although the initially reported catalysts, including 28c and 3,8a
exhibited low catalytic activity, presumably owing to catalyst
decomposition,8 we have recently developed an imidazo[1,5-
a]quinolin-9-olate-1-ylidene (IzQO) ligand (4), whose palla-
dium complexes were successfully applied to ethylene and
propylene/polar monomer copolymerization reactions.9 The
role of the rigid backbone of the IzQO ligand lies in preventing
the rotation of the NHC plane toward the palladium plane so
that reductive elimination leading to catalyst decomposition is
suppressed effectively. In this context, we hypothesized that the
introduction of a saturated NHC group, which should have σ-
donating properties stronger than those of an unsaturated
NHC,10 should further enhance the activity toward polar
monomer copolymerization via destabilization of the σ-
coordination of polar monomers on the metal center. Here,
we report the synthesis and characterization of palladium
complexes 5, which bear a partially saturated IzQO ligand, i.e.,
3,3a,4,5-tetrahydroimidazo[1,5-a]quinolin-9-olate-1-ylidene
(SIzQO), as well as their activity in the polymerization of
Figure 1. Palladium complexes with unsymmetric bidentate ligands
used for the polymerization of olefins.
Received: April 25, 2018
© XXXX American Chemical Society
A
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ethylene. During our investigations, we were able to isolate a
thermodynamically unstable C,C-trans isomer, which revealed
an unprecedented trans/cis isomerization behavior.
trans-5-Lut is shown in Scheme 2. In general, ligand
substitution occurs more easily at a position trans to a ligand
Scheme 2. Possible Mechanism for the Formation of trans-
5-Lut from the Reaction of Silver/SIzQO and [Pd(μ-
Cl)Me(2,6-lutidine)]2
RESULTS AND DISCUSSION
■
Preparation of the SIzQO Ligand and Its Palladium
Complexes. The synthetic route to ligand precursor 8 is
shown in Scheme 1. Starting from 6,11 the imine moiety was
Scheme 1. Synthesis of SIzQO Precursor 8 and Its
Palladium Complexes 5
with a stronger trans effect. The trans effect of carbenes and
methyl groups is stronger than that of 2,6-lutidine, pyridine,
and phenolate.10 When [Pd(μ-Cl)Me(2,6-lutidine)]2 is used as
the methylpalladium precursor, the NHC initially coordinates
to the palladium center via dissociation of the chlorido group
trans to the methyl group.12 Subsequently, the other chlorido
group is replaced by the aryloxide moiety of the SIzQO ligand,
which affords trans-5-Lut. If the aryloxide moiety coordinates
to palladium more quickly than NHC, it would provide cis-5-
Lut (see Section 5 in the Supporting Information for the
detailed mechanistic explanation). However, 1H NMR analysis
revealed no contamination of cis-5-Lut in the crude product.
X-ray Diffraction Analysis of Pd/SIzQO Complexes.
The solid-state structures of cis-5-Py, cis-5-Lut, and trans-5-
Lut were confirmed by single-crystal X-ray diffraction analyses
(Figure 2 and Table 1). A comparison of the bond lengths of
Pd/SIzQO complexes 5 and Pd/IzQO complex 4 revealed that
the Pd−N3 bond length of cis-5-Lut (2.136(4) Å) is
significantly longer than that in cis-4-Lut (2.095(2) Å). This
discrepancy may be attributed to the fact that the σ-donating
properties of saturated NHCs are stronger than those of
unsaturated NHCs.10 The Pd−N3 bond length is significantly
shorter in cis-5-Py (2.095(2) Å) than in cis-5-Lut (2.136 Å),
which may be due to the steric congestion involving the methyl
groups of 2,6-lutidine. A similar trend has been observed in
bis(o-methoxyphenyl)phosphine−sulfonate-supported palladi-
um complexes (2.108(3) Å vs 2.134(2) Å).13 Subsequently, we
examined the lengths of the Pd−Me bonds for the three C,C-
cis Pd/SIzQO complexes, which were almost identical (cis-5-
Py, 2.050(3) Å; cis-5-Lut, 2.055(5) Å; cis-4-Lut, 2.060(6) Å).
We also observed that the C2−Pd1−N3 angle in trans-5-Lut
(100.85(14)°) is larger than the C2−Pd1−C1 angle in cis-5-
Py (94.47(12)°). This difference should be due to the steric
repulsion between the Dip substituent and 2,6-lutidine in
trans-5-Lut. The same trend has been reported for the cis/
trans isomers of 3 (89.1(4) and 100.14(12) Å, respectively).8d
In a previous paper on Pd/IzQO catalysts, we proposed that
the mean dihedral angle between the NHC plane and the
palladium coordination plane should be correlated to the
stability of the metal catalysts during polymerization
reactions.9a The calculated dihedral angle is 34.2° in cis-5-
Lut and 34.6° in trans-5-Lut.8f The dihedral angles of the Pd/
SIzQO complexes 5-Lut are higher than that of the Pd/IzQO
complex cis-4-Lut (22.3°, see Section 3 in the Supporting
Information for comparison) and comparable to those of
ethylene polymerization catalysts such as a Ni/IzQO complex
(33.1°)9b and a Pd/carbene−phosphine oxide complex
(32.5°).8f
reduced with BH3·THF and, subsequently, the quinoline
backbone was partially hydrogenated in the presence of Al-Ni
alloy to afford diimine 7 in two-step 50% yield. Diamine 7 was
protonated with hydrogen chloride and treated with excess
triethyl orthoformate to afford imidazolinium salt 8. After
recrystallization from dichloromethane/hexane, pure 8 was
obtained in 73% yield in two steps from 7.
In order to prepare Pd/SIzQO complexes, 8 was
deprotonated with NaHMDS and treated with AgOTf. The
obtained Ag/SIzQO intermediate was then treated with
PdClMe(pyridine)2 or [Pd(μ-Cl)Me(2,6-lutidine)]2, which
afforded the corresponding Pd/SIzQO complexes. Notably,
the obtained cis/trans structure depended on the palladium
precursor: when trans-PdClMe(pyridine)2 was used, C,C-cis
complex cis-5-Py, in which the carbene ligand and methyl
group are located cis to each other, was obtained in 62% yield.
In contrast, a reaction of the Ag/SIzQO intermediate with
[Pd(μ-Cl)Me(2,6-lutidine)]2 furnished C,C-trans complex
trans-5-Lut in 56% yield. The C,C-trans isomer should be
thermodynamically less stable than the cis isomer, as the
carbene ligand and the methyl group, both of which exhibit a
strong trans influence, are located trans to each other. Complex
cis-5-Lut was synthesized by a ligand exchange reaction from
cis-5-Py in 48% yield.
Possible Mechanism for the Formation of the C,C-
trans Isomer. A possible mechanism for the formation of
Polymerization of Ethylene. Ethylene polymerization
reactions were performed with cis-5-Lut at 100 °C (Table 2,
B
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Figure 2. Single-crystal X-ray structures of (a) cis-5-Py, (b) cis-5-Lut, and (c) trans-5-Lut with thermal ellipsoids set to 50% probability. Hydrogen
atoms and solvent molecules are omitted for clarity, and only selected atoms have been labeled.
obtained from using cis-5-Lut accounted for 33.3 per 1000
carbons, a value which is higher than that using cis-4-Lut
(11.6). These results suggest that the rate of the β-hydride
elimination and the subsequent chain transfer or branch
formation becomes relatively fast in comparison to that with
Pd/IzQO complex 4. We also attempted the polymerization of
ethylene using cis-5-Py in order to investigate the effect of the
fourth ligand; however, the results obtained were comparable
(entry 2). In the case of trans-5-Lut (entries 3−6), the
observed polymerization activity was also comparable (entry
3). The reaction after 2.0 h (entry 4) afforded an increased
amount of polyethylene in comparison to that after 1.0 h
(entry 3). We also probed the effect of polymerization
temperature (entries 5 and 6). While trans-5-Lut did not
afford any polyethylene at 80 °C, the formation of a black
precipitate on the polymer was observed after polymerization
at 120 °C (entry 6). The observed polymerization activities of
catalysts 5 (3.6−5.3 kg mol−1 h−1) are lower than those of
typical group 10 metal catalysts such as palladium/α-diimine
(20−8800 kg mol−1 h−1)1h,14 and palladium/phosphine−
sulfonate (120−5330 kg mol−1 h−1)4a catalysts. The above
experimental results suggested lower stabilities of 5 in
Table 1. Representative Bond Lengths (Å) and Angles (deg)
in Palladium Complexes 4 and 5
cis-5-Py
cis-5-Lut
trans-5-Lut cis-4-Lut9a
Pd1−C1
Pd1−C2
Pd1−N3
Pd1−O1
C2−Pd1−O1
C1−Pd1−C2
C1−Pd1−N3
C2−Pd1−N3
2.050(3)
1.961(3)
2.095(2)
2.085(2)
88.63(10)
94.47(12)
90.39(11)
175.01(10)
29.5
2.055(5)
1.953(5)
2.136(4)
2.096(3)
87.49(16)
92.68(18)
90.23(17)
176.52(17)
34.2
2.060(4)
2.091(4)
2.046(3)
2.037(3)
84.74(13)
173.13(16)
85.52(16)
100.85(14)
34.6
2.060(6)
1.965(7)
2.095(6)
2.062(4)
89.7(2)
97.0(3)
89.3(3)
173.2(3)
22.3
dihedral angle
(NHC/Pd)
dihedral angle
(Pd/pyridine)
49.5
77.3
75.2
82.7
entry 1), which resulted in an activity of 3.6 kg mol−1 h−1 and
the formation of polyethylene with a number-averaged
molecular weight (Mn) of 1.8 × 103. The polymerization
activity of cis-5-Lut is more than 100 times lower than that of
cis-4-Lut (entries 1 and 6), and the Mn value of the resulting
polyethylene is more than 10 times lower than that obtained
with cis-4-Lut. The methyl branches in the polymer chains
a
Table 2. Polymerization of Ethylene using Pd/SIzQO Catalysts 4 and 5
b
c
entry
cat. (amt (μmol))
temp (°C)
time (h)
activity (kg mol−1 h−1
)
103Mn
Mw/Mn
branches/1000C
d
1
cis-5-Lut (10)
cis-5-Py (10)
trans-5-Lut (10)
100
100
100
100
80
1.0
1.0
1.0
2.0
1.0
1.0
1.0
3.6
5.2
5.3
1.8
2.5
7.0
3.6
3.6
2.0
1.7
3.1
33.3
28.9
28.2
27.6
d
2
d
3
d
4
4.1
d
5
trace
4.7
691
d
6
120
100
3.9
76.0
2.4
2.1
34.2
11.6
d
7
cis-4-Lut (1.0)
a
A mixture of the catalyst (10 μmol) and ethylene (4.0 MPa at 20 °C) in toluene (20 mL) was stirred for 1.0 h at the indicated temperature.
b
c
Number-average molecular weight measured by SEC using internal polystyrene standards and corrected by universal calibration. Number of
d
methyl branches determined by quantitative 13C NMR analysis. Data from ref 9a.
C
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a
Scheme 3. Energy Profile of the Ethylene-Insertion and β-Hydride-Elimination Pathways Using Pd/SIzQO Complex 5
a
Values in parentheses refer to Pd/IzQO complex 4.
comparison to 4, which prompted us to investigate the
mechanism by the density functional theory (DFT) method.
DFT Calculations on the Ethylene Polymerization
Mechanism. In order to clarify the reasons behind the low
activity of the SIzQO systems, DFT calculations were
performed with the B3LYP functional15 with empirical
dispersion correction, DFT-D3,16 the Lanl2DZ basis set17
and effective core potential for palladium, and the 6-31G(d)
basis sets18 for the light atoms. To include solvation effects
(with toluene as the solvent), the SMD solvation model19 was
employed.
Initially, we investigated the pathway for the insertion of
ethylene (Scheme 3). The insertion of ethylene using group 10
metal catalysts with an unsymmetrical bidentate ligand
proceeds via ethylene coordination, cis/trans isomerization,
and the insertion of ethylene (blue). Considering intermediate
10 as the starting complex, the transition states of the Pd/
SIzQO systems for the cis/trans isomerization TS(10−10′)
were calculated to be at 14.6 and 20.9 kcal/mol, respectively,
depending on the rotational direction. It should be noted that
the SIzQO ligand contains chiral centers in its backbone;
therefore, two diastereotopic transition states should be
considered in some cases. The difference in ΔG values for
TS(10−10′) of the Pd/SIzQO system is 6.3 kcal/mol. The
transition states for the insertion of ethylene TS(10′−11′)
were calculated to be 17.3 and 17.6 kcal/mol, which are
slightly higher than those of the Pd/IzQO catalyst (cis/trans
isomerization, 16.3 kcal/mol; insertion of ethylene, 16.7 kcal/
mol).
mol) is lower than that of TS(9−12), which originates from
cis-alkyl species 9 (17.7 kcal/mol); therefore, the latter is not
considered in the following discussion. Of the possible
pathways to generate intermediate 9′, we considered the
dissociation of ethylene from 10′ via either the three-
coordinated intermediate 9′NA (15.2 kcal/mol) or five-
coordinated intermediate TS(9′−10′) (20.5 kcal/mol). The
difference between 9′NA and TS(9′−10′) by 5.3 kcal/mol
may not be significant considering the presence of a high
excess amount of ethylene under the polymerization
conditions.4a In the literature, the mole fraction of ethylene
in toluene at 90 °C under 3.8 MPa of ethylene was reported to
be 0.24, meaning that 60 mmol of ethylene is present in 190
mmol (=20 mL at 20 °C) of toluene under the conditions.20
Since we charged 4.0 MPa of ethylene at 20 °C (and heated
the closed system to 80−120 °C) for polymerization, it can be
reasonably assumed that >6000 equiv of ethylene to the
palladium catalyst 5 (10 μmol) exists in the liquid phase under
the reaction conditions. The pathway 9 → TS(9−9′) → 9′
was ruled out because TS(9−9′) is much higher in energy than
9′NA and TS(9′−10′). Overall, the β-hydride-elimination
route 10′ → 9′NA → 9′ → TS(9′−12′) → 12′ (15.2 kcal/
mol) is ≥2.1 kcal/mol lower in energy than the ethylene-
insertion pathway 10′ → TS(10′−11′) → 11′ (17.3/17.6
kcal/mol). In the case of Pd/IzQO complex 4, the ΔG value of
9′NA (16.7 kcal/mol) is identical with that of TS(10′−11′)
(16.7 kcal/mol). These results are thus consistent with the
experimental data, in which Pd/SIzQO complexes 5 afforded
polymers with lower molecular weights and higher numbers of
branches in comparison to those obtained from Pd/IzQO
complex 4.
The differences in molecular weight of the polymers
obtained from 4 and 5 may be explained by the differences
in the β-hydride-elimination pathway (red). In the Pd/SIzQO
system, the transition state for the β-hydride elimination
TS(9′−12′) obtained from trans-alkyl species 9′ (10.3 kcal/
We also elucidated the fate of the hydride complex 12′.
Chain termination may occur via reductive elimination of C
and H (brown) or chain transfer reactions (green). The
D
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contribution of the former reductive elimination pathway may
be limited due to its high activation energy (23.1/25.5 kcal/
mol) (see Section 6.1 in the Supporting Information for the
details). For the chain transfer reactions, the dissociation
pathway 12′ → 13′NA → 13′ (12.9 kcal/mol) is energetically
more favorable than the association pathway 12′ → TS(12′−
13′) → 13′ (17.4 kcal/mol). These values are comparable to
those obtained using a previously reported Pd/IzQO catalyst
(12.5 and 16.3 kcal/mol, respectively).
Scheme 4. Proposed Mechanisms for the Isomerization of
Group 10 Metal Complexes with a Bidentate Ligand
On the basis of the DFT study on complexes 4 and 5, the
lower stability and higher branch-forming rate of 5 in
comparison to those of 4 during ethylene polymerization are
reasoned as follows. The rate of the key steps for ethylene
polymerization, namely cis/trans isomerization and ethylene
insertion, were found to be comparable between the two
catalysts. However, the ethylene dissociation steps, 9′NA and
TS(9′−10′), leading to β-hydride elimination are faster in the
case of complex 5. This could be attributed to the stronger
electron donation provided by the saturated NHC moiety of 5,
which stabilizes low-coordinated electron-poor species such as
9′ and 9′NA. Clearly, the branch formation and catalyst
decomposition happen after β-hydride elimination. It should
also be noted that the catalyst decomposition via C−H
reductive elimination, TS(12′−RE), is faster in complex 5 than
in complex 4. Structurally, this can be explained by the greater
twisting of the saturated NHC plane from the metal square
(Table 1), increasing the crucial orbital overlapping for C−H
reductive elimination between the vacant p-orbital on the
carbene carbon and the σ-orbital of the cis-Pd−hydride bond.
Electronically, the saturated NHCs are better π-acceptor than
the unsaturated NHCs, implying more susceptibility for
nucleophilic attack of the hydride onto the vacant p-orbital
of the carbene carbon (see Section 6.2 in the Supporting
Information for the details).21
Isomerization Reaction of trans-5-Lut. One of the key
steps in the catalytic polymerization of ethylene using group 10
metal complexes with unsymmetric bidentate ligands is cis/
trans isomerization. It is therefore not surprising that the cis/
trans isomerization of square-planar d8-metal complexes of e.g.
Pd(II) has been studied intensely.22 Our mechanistic studies
on palladium/phosphine−sulfonate systems by DFT calcu-
lations revealed that the cis/trans isomerization of palladium/
phosphine−sulfonate complexes proceeds via a transition state
which is similar to that in Berry’s pseudorotation mechanism
(Scheme 4a).4a Jordan and co-workers have reported in their
studies on cis/trans isomerization of palladium/phosphine−
sulfonate complexes that the Berry’s pseudorotation mecha-
nism is assisted by either the sulfonate group or an additional
lutidine molecule (Scheme 4b).23 For carbene−sulfonate
complex 3, the isomerization was proposed to proceed via a
dissociation of the sulfonate group (Scheme 4c),8d which was
based on the fact that the isomerization was not affected by the
addition of lutidine but rather was influenced by the addition
of a Lewis acid such as B(C6F5)3. Mapolie and co-workers have
experimentally confirmed that platinum/imine−phenolate
complex 6 isomerizes via a tetrahedral intermediate (Scheme
4d), although DFT calculations suggested that a dissociative
pathway should be energetically comparable.24
Scheme 5. cis/trans Isomerization of trans-5-Lut
12 h, 40% of trans-5-Lut had been consumed and three new
singlet resonances, attributable to the methyl groups, appeared
1
in the negative region of the H NMR spectrum (−0.35,
−0.44, and −0.82 ppm). The species at −0.82 ppm was
assigned to cis-5-Lut by comparison with an authentic
sample.25 In order to confirm the structures of the other two
species, a solution of the crude reaction mixture was layered
with pentane and stored at −35 °C to obtain colorless single
Against this background, we investigated the isomerization
behavior of trans-5-Lut into cis isomer cis-5-Lut. A solution of
trans-5-Lut (10 μmol) in CH2Cl2 (0.50 mL) was heated to 40
1
1
°C and monitored by H NMR spectroscopy using 1,2,4,5-
crystals. The H NMR spectrum of the crystals in CD2Cl2
tetrabromobenzene as the internal standard (Scheme 5). After
showed a singlet resonance at −0.44 ppm, and an X-ray
E
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diffraction analysis revealed the crystals to be rac-(cis-5)2
(Figure 3). Consequently, the signal at −0.35 ppm was
cis-5-Lut can be considered (Scheme 6). One corresponds to
the isomerization via a tetrahedrally coordinated intermediate
Scheme 6. Possible Mechanisms for the Isomerization of
Pd/SIzQO Complexes
Figure 3. Single-crystal X-ray structure of rac-(cis-5)2 with thermal
ellipsoids set to 50% probability. Hydrogen atoms are omitted for
clarity, and only selected atoms have been labeled. Selected bond
lengths (Å) and angles (deg): Pd1−O1 = 2.155(5), Pd1−O2 =
2.105(5), Pd2−O1 = 2.132(5), Pd2−O2 = 2.170(5), Pd1−Pd2 =
3.1818(14); C2−Pd1−O1 = 88.3(3), O1−Pd1−O2 = 88.3(3).
associated with meso-(cis-5)2. Note that cis-5-Lut, rac-(cis-5)2,
and meso-(cis-5)2 are confirmed to be in equilibrium on the
1
basis of the following observations. H NMR analysis of the
isolated cis-5-Lut revealed the presence of cis-5-Lut and rac-
and meso-(cis-5)2 in CD2Cl2 solution, and the addition of extra
2,6-lutidine cleanly afforded a mixture of pure cis-5-Lut and
free 2,6-lutidine.
The time course of the isomerization (Figure 4) revealed
that the conversion of trans-5-Lut stopped after 3 h. This
through coordination of either 2,6-lutidine (pathway I) or
solvent (CH2Cl2) (pathway II); however, pathway I can be
dismissed, as it would not be affected by the addition of 2,6-
lutidine. Another possible mechanism is the isomerization via
Berry’s pseudorotation (pathway III), which would be
accelerated upon addition of 2,6-lutidine. Thus, pathway III
is not probable in this case either. Our experimental results
that the presence of additional lutidine suppressed the cis/trans
isomerization suggest that only pathways II and IV are possible
(Table 3).
Table 3. Summary of Experimental and Computational
Studies on the Mechanism of cis/trans Isomerization of
Complex 5-Lut
pathway
I
II
III
IV
experiment
calculation
×
√
√
×
×
×
√
√
activation energy(kcal/mol)
32.8
46.3
not found
29.4
Figure 4. Time course plot of the isomerization reaction of trans-5-
Lut at 40 °C.
Next, DFT calculations were carried out (see Sections 6.3
and 6.4 in the Supporting Information for the details). We
could identify the transition states for pathways I, II, and IV,
while a five-coordinated TS in pathway III could not be found.
The estimated activation energy of dissociative pathway IV
(29.4 kcal/mol) and that of pathway I via the tetrahedral
transition state (32.8 kcal/mol) are much lower than that of
pathway II involving solvent coordination (46.3 kcal/mol).
Considering the experimental and theoretical results in their
entirety, we conclude that the dissociative pathway (pathway
IV) is operative in this reaction (Table 3).
phenomenon could be rationalized in terms of an equilibrium
between trans-5 and cis-5; however, this possibility was
dismissed, as the attempted isomerization of cis-5-Lut did not
proceed and as DFT calculations suggested that trans-5-Lut
should be 8.2 kcal/mol more stable than cis-5-Lut. Another
possibility is that a byproduct generated during the isomer-
ization could suppress the isomerization. Assuming that 2,6-
lutidine quenches the isomerization, we investigated the
reaction of trans-5-Lut in the presence of 1.5 equiv of 2,6-
lutidine. Indeed, we observed that the isomerization was
suppressed and that trans-5-Lut decomposed instead (see
Section 5 in the Supporting Information for the details).
On the basis of the aforementioned background, four
possible mechanisms for the isomerization of trans-5-Lut into
CONCLUSION
■
In summary, Pd/SIzQO complexes 5 were synthesized,
characterized, and employed as catalysts in the polymerization
F
DOI: 10.1021/acs.organomet.8b00263
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
spectra were recorded in the presence of additional pyridine in order
not to form (cis-5)2.
of ethylene. The catalytic activity of 5 was more than 100 times
lower than that of the corresponding Pd/IzQO catalyst (4).
During the investigation of the Pd/SIzQO system, an unstable
C,C-trans isomer, trans-5-Lut, was isolated and characterized.
Complex trans-5-Lut isomerized at 40 °C into cis isomers cis-
5-Lut and (cis-5)2. The isomerization proceeds via the
dissociation of 2,6-lutidine, which is supported by the results
of experimental and theoretical investigations.
1H NMR (500 MHz, CD2Cl2): δ ca. 8.6 (2H, H19, overlapped
with those of free pyridine), 7.76 (t, 3JHH = 7.6 Hz, 1H, H21), 7.37 (t,
3
3JHH = 7.8 Hz, 1H, H16), 7.33 (t, JHH = 6.9 Hz, 2H, H20), ca. 7.30
3
(1H, H15, overlapped with those of free pyridine), 7.17 (dd, JHH
=
4
3
7.9 Hz, JHH = 1.5 Hz, 1H, H15), 6.83 (dd, JHH = 7.6, 7.6 Hz, 1H,
H9), 6.57 (dd, 3JHH = 7.9 Hz, 4JHH = 0.9 Hz 1H, H8), 6.28 (dd, 3JHH
= 7.3 Hz, 4JHH = 0.9 Hz, 1H, H10), 4.26−4.20 (m, 1H, H4), 3.92 (dd,
2JHH = 10.8 Hz, JHH = 10.8 Hz, 1H, H3), 3.76 (dd, JHH = 10.7 Hz,
3
2
3JHH = 6.1 Hz, 1H, H3), 3.49 (1H, sept, JHH = 6.7 Hz, 1H, H17),
3
EXPERIMENTAL SECTION
■
3.18 (sept, 3JHH = 6.7 Hz, 1H, H17), 3.06 (m, 1H, H6), 2.96 (dd, 2JHH
= 17.1 Hz, 3JHH = 5.5 Hz, 1H, H6), 2.18−2.05 (m, 2H, H5), 1.75 (d,
General Procedures of Ethylene Polymerization. A 50 mL
stainless steel autoclave was dried in an oven at 120 °C, sealed, and
evacuated under vacuum until it cooled to room temperature. In the
autoclave charged with argon were placed catalyst (10 μmol) and
anhydrous toluene (20 mL). Then, the autoclave was pressurized with
ethylene (4.0 MPa) and the contents were stirred in an isothermal
heating block for 1.0 h at given temperature. After redundant ethylene
was vented, the reaction was quenched by addition of methanol (ca.
50 mL). The formed precipitates were collected by filtration, washed
with methanol, and dried under high vacuum at 100 °C for at least for
5 h to afford polyethylene. The molecular weights and their
distribution were determined by size exclusion chromatography.
The extent of branching in the polymer backbone was determined by
quantitative 13C NMR spectroscopy.
3JHH = 6.7 Hz, 3H, H18), 1.35 (d, JHH = 7.0 Hz, 3H, H18), 1.28 (t,
3
3JHH = 6.1 Hz, 3H, H18), 1.21 (d, 3JHH = 6.7 Hz, 3H, H18), −0.59 (s,
3H, H1). 13C{1H} NMR (126 MHz, CD2Cl2): δ 192.1 (1C, C2),
161.1 (1C, C11), 150.5 (2C, C19), 147.0 (1C, C14), 146.5 (1C,
C14), 137.4 (1C, C21), 136.7 (1C, C13), 129.5 (1C, C12), 128.5
(1C, C16), 127.4 (1C, C7), 125.1 (1C, C9), 124.7 (2C, C20), 124.4
(1C, C15), 123.7 (1C, C15), 116.1 (1C, C8), 112.5 (1C, C10), 59.8
(1C, C3), 57.8 (1C, C4), 30.1 (1C, C5), 28.5 (1C, C17), 28.4 (1C,
C17), 26.8 (1C, C6), 25.8 (1C, C18), 25.7 (1C, C18), 24.3 (1C,
C18), 23.4 (1C, C18), −10.0 (1C, C1). HRMS (ESI): m/z calcd for
C29H36N3OPd+ ([M + H]+) 548.1893, found 548.1897.
Preparation of cis-5-Lut: (SP-4-4)-[2-(2,6-Diisopropylphen-
yl)-3,3a,4,5-tetrahydroimidazo[1,5-a]quinolin-9-olato-κO-1-
ylidene-κC1](2,6-lutidine)(methyl)palladium.
General Procedures of cis/trans Isomerization of trans-5-
Lut in the Presence and Absence of 2,6-Lutidine. An NMR tube
was charged with trans-5-Lut (5.8 mg, 10 μmol) and 1,2,4,5-
tetrabromobenzene (1.0 mg) under an argon atmosphere. To the
mixture was added CH2Cl2 (0.50 mL) or a solution of 2,6-lutidine in
CH2Cl2 (30 mM, 0.50 mL). The NMR tube was kept at a given
temperature and monitored by 1H NMR spectroscopy. The
conversion of trans-5-Lut and the amounts of products were
determined by integration of Pd−CH3 resonances relative to
1,2,4,5-tetrabromobenzene (δ 7.90 ppm in CD2Cl2, 7.89 ppm in
CH2Cl2) as an internal standard. Since the signals of Pd−CH3 in rac-
(cis-5)2 and trans-5-Lut were overlapped, the two signals were
separated by wave separation method (see Section 5 in the
Supporting Information for the details). A crystal of rac-(cis-5)2
appropriate for X-ray crystallographic analysis was prepared as follows.
After the isomerization, the solution was layered with an excess
amount of hexane. After storage at −35 °C overnight, rac-(cis-5)2 was
obtained as white crystals.
To a mixture of 8-hydroxy-2-(2,6-diisopropylphenyl)imidazolino[1,5-
a]quinolinium chloride (8; 76.9 mg, 0.20 mmol) and sodium
bis(trimethylsilyl)amide (NaHMDS; 73.3 mg, 0.20 mmol) in a 15
mL vial was added benzene (10.0 mL) After the mixture was stirred
for 15 min at room temperature, silver triflate (56.5 mg, 0.22 mmol)
was added. After the mixture was stirred for 30 min, PdClMe-
(pyridine)2 (63.0 mg, 0.20 mmol) was added. The mixture was stirred
for 1.0 h at room temperature. The mixture was filtered through a pad
of Celite, and the filtrate was evaporated to dryness. The crude
product of the complex cis-5-Py was dissolved in THF (ca. 5.0 mL),
and to the solution was added 2,6-lutidine (ca. 1.0 mL). The solution
was shaken for several minutes, and the volatile materials were
removed under vacuum. This procedure was repeated twice to
remove residual pyridine. The obtained solid was dissolved in a
minimum amount of THF with 1 drop of lutidine and layered with an
excess amount of pentane. After storage at −35 °C overnight, the title
compound was obtained as yellow crystals (55.0 mg, 48% yield).
Preparation of cis-5-Py: (SP-4-4)-[2-(2,6-Diisopropylphenyl)-
3,3a,4,5-tetrahydroimidazo[1,5-a]quinolin-9-olato-κO-1-yli-
dene-κC1](methyl)(pyridine)palladium.
1
Note: H NMR spectroscopic analysis revealed that cis-5-Lut is in
equilibrium with the dimeric species (cis-5)2. 1H and 13C NMR
spectra were recorded in the presence of additional 2,6-lutidine in
order not to form (cis-5)2.
To a mixture of 8-hydroxy-2-(2,6-diisopropylphenyl)imidazolino[1,5-
a]quinolinium chloride (8; 38.5 mg, 0.10 mmol) and sodium
bis(trimethylsilyl)amide (NaHMDS; 36.8 mg, 0.10 mmol) in a 15
mL vial was added benzene (5.0 mL). After the mixture was stirred for
15 min at room temperature, silver triflate (28.2 mg, 0.11 mmol) was
added. After the mixture was stirred for 30 min, PdClMe(pyridine)2
(31.5 mg, 0.10 mmol) was added. The mixture was stirred for 1.0 h at
room temperature. The mixture was filtered through a pad of Celite,
and the filtrate was evaporated to dryness. The formed solid was
dissolved in a minimum amount of THF with 1 drop of pyridine and
layered with an excess amount of pentane. After storage at −35 °C
overnight, the title compound was obtained as yellow crystals (34.0
mg, 62% yield).
1H NMR (500 MHz, CD2Cl2): δ 7.50 (t, 3JHH = 7.7 Hz, 1H, H21),
7.35 (t, 3JHH = 7.7 Hz, 1H, H16), 7.28 (dd, 3JHH = 7.8 Hz, 4JHH = 1.4
Hz, 1H, H15), 7.16 (dd, 3JHH = 7.6 Hz, 4JHH = 1.5 Hz, 1H, H15), 7.02
3
3
(d, JHH = 7.6 Hz, 2H, H20), 6.74 (dd, JHH = 7.8, 7.8 Hz, 1H, H9),
3
3
6.33 (d, JHH = 8.1 Hz, 1H, H10), 6.23 (d, JHH = 7.0 Hz, 1H, H8),
2
3
4.26−4.19 (m, 1H, H4), 4.02 (dd, JHH = 10.8 Hz, JHH = 10.8 Hz,
1H, H3), 3.79 (sept, 3JHH = 6.8 Hz, 1H, H17), 3.71 (dd, 2JHH = 10.7
3
3
Hz, JHH = 7.3 Hz, 1H, H3), 3.15 (sept, JHH = 6.9 Hz, 1H, H17),
3.09−2.95 (m, 2H, H6), 2.91 (s, 3H, H22), 2.84 (s, 3H, H22), 2.20−
2.17 (m, 2H, H5), 1.61 (d, 3JHH = 6.9 Hz, 3H, H18), 1.31 (d, 3JHH
6.7 Hz, 3H, H18), 1.27 (d, 3JHH = 6.7 Hz, 3H, H18), 1.20 (d, 3JHH
=
=
1
Note: H NMR spectroscopic analysis revealed that cis-5-Py is in
6.9 Hz, 3H, H18), −0.82 (s, 3H, H1). 13C{1H} NMR (126 MHz,
CD2Cl2): δ 190.6 (1C, C2), 161.8 (1C, C11), 159.8 (1C, C19), 159.0
equilibrium with the dimeric species (cis-5)2. 1H and 13C NMR
G
DOI: 10.1021/acs.organomet.8b00263
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(1C, C19), 147.2 (1C, C14), 147.1 (1C, C14), 137.7 (1C, C21),
137.6 (1C, C13), 129.9 (1C, C12), 128.9 (1C, C16), 127.4 (1C, C7),
125.3 (1C, C9), 124.5 (1C, C15), 124.3 (1C, C15), 122.4 (1C, C20),
122.1 (1C, C20), 116.6 (1C, C8), 112.6 (1C, C10), 60.5 (1C, C3),
57.9 (1C, C4), 30.8 (1C, C5), 28.9 (1C, C17), 28.7 (1C, C17), 27.3
(1C, C6), 27.2 (1C, C22), 26.3 (1C, C18), 26.2 (1C, C22), 26.0 (1C,
C18), 23.7 (1C, C18), 23.6 (1C, C18), −14.8 (1C, C1). HRMS
(ESI): m/z calcd for C31H40N3OPd+ ([M + H]+) 576.2201, found
576.2233.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00263.
■
S
Experimental procedures and characterization data of
ligands, palladium complexes, and polymers, X-ray
crystallographic data for cis-5-Py, cis-5-Lut, trans-5-
Lut, and (cis-5)2, and details of the calculations (PDF)
Computed Cartesian coordinates of all of the molecules
reported in this study (XYZ)
Preparation of trans-5-Lut: (SP-4-2)-[2-(2,6-Diisopropyl-
phenyl)-3,3a,4,5-tetrahydroimidazo[1,5-a]quinolin-9-olato-
κO-1-ylidene-κC1](2,6-lutidine)(methyl)palladium.
Accession Codes
CCDC 1833823−1833826 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
To a mixture of 8-hydroxy-2-(2,6-diisopropylphenyl)imidazolino[1,5-
a]quinolinium chloride (8; 76.9 mg, 0.20 mmol) and sodium
bis(trimethylsilyl)amide (73.3 mg, 0.40 mmol) in a 15 mL vial was
added benzene (10 mL). After the mixture was stirred for 30 min at
room temperature, silver triflate (56.5 mg, 0.22 mmol) was added.
After the mixture was stirred for 30 min, [Pd(μ-Cl)Me(2,6-lutidine)]2
(52.8 mg, 0.10 mmol) was added. The mixture was stirred for 1.0 h.
The mixture was filtered through a pad of Celite, and the filtrate was
evaporated to dryness. The formed solid was dissolved in a minimum
amount of THF and layered with an excess amount of pentane. After
storage at −35 °C overnight, the title compound was obtained as
bright yellow crystals (64.3 mg, 56% yield). A crystal appropriate for
X-ray crystallographic analysis was prepared by recrystallization from
CH2Cl2 (containing 0.5% of methanol)/hexane bilayering at −35 °C.
AUTHOR INFORMATION
Corresponding Author
*E-mail for K.N.: nozaki@chembio.t.u-tokyo.ac.jp.
■
ORCID
Shingo Ito: 0000-0003-1776-4608
Kyoko Nozaki: 0000-0002-0321-5299
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by JST CREST Grant Number
JPMJCR1323 (Japan). A part of this work was conducted at
the Research Hub for Advanced Nano Characterization, The
University of Tokyo, supported by MEXT of Japan. The
theoretical calculations were performed using computational
resources provided by Research Center for Computational
Science, National Institutes of Natural Sciences, Okazaki,
Japan. S.A. thanks the Program for Leading Graduate Schools
(MERIT) from the JSPS.
1H NMR (500 MHz, CD2Cl2, room temperature): δ 7.29 (t, 3JHH
7.6 Hz, 1H, H21), 7.26 (d, 3JHH = 5.8 Hz, 1H, H15), 7.26 (d, 3JHH
3.4 Hz, 1H, H15) 6.91 (d, 3JHH = 7.6 Hz, 1H, H20), 6.82 (dd, 3JHH
=
=
=
5.8, 3.4 Hz, 1H, H16), 6.78 (t, 3JHH = 7.8 Hz, 1H, H9), 6.72 (d, 3JHH
3
4
= 7.6 Hz, 1H, H20), 6.60 (dd, JHH = 7.9 Hz, JHH = 1.5 Hz, 1H,
3
4
H10), 6.35 (dd, JHH = 7.5 Hz, JHH = 1.4 Hz, 1H, H8), 4.28−4.21
(m, 1H, H4), 3.81 (dd, 2JHH = 10.7 Hz, 3JHH = 10.7 Hz, 1H, H3), 3.66
2
3
3
(dd, JHH = 10.7 Hz, JHH = 8.2 Hz, 1H, H3), 3.18 (sept, JHH = 6.9
Hz, 1H, H17), 3.08−2.95 (m, 2H, H6), 2.82 (s, 3H, H22), 2.62 (sept,
3JHH = 6.9 Hz, 1H, H17), 2.28−2.24 (m, 1H, H5), 2.21 (s, 3H, H22),
REFERENCES
■
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7.0 Hz, 3H, H18), 1.03 (d, 3JHH = 6.7 Hz, 3H, H18), 0.40 (d, 3JHH
=
=
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reacted with 2,6-lutidine to form cis-5-Lut. For details, see Section
2 in the Supporting Information.
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