Dinickel bisphenoxyiminato complexes for the polymerization of ethylene and α-olefins

Article abstract of DOI:10.1021/om2011694

Dinuclear nickel phenoxyiminato olefin polymerization catalysts based on rigid p-terphenyl frameworks are reported. Permethylation of the central arene of the terphenyl unit and oxygen substitution of the peripheral rings ortho to the aryl-aryl linkages blocks rotation around these linkages, allowing atropisomers of the ligand to be isolated. The corresponding syn and anti dinickel complexes (25-s and 25-a) were synthesized and characterized by single-crystal X-ray diffraction. These frameworks limit the relative movement of the metal centers, restricting the metal-metal distance. Kinetics studies of isomerization of a ligand precursor (7-a) allowed the calculation of the activation parameters for the isomerization process (ΔH = 28.0 ± 0.4 kcal × mol^-1 and ΔS = -12.3 ± 0.4 cal mol ^-1 K^-1). The reported nickel complexes are active for ethylene polymerization (TOF up to 3700 (mol C₂H₄) (mol Ni)^-1 h^-1) and ethylene/α-olefin copolymerization. Only methyl branches are observed in the polymerization of ethylene, while α-olefins are incorporated without apparent chain walking. These catalysts are active in the presence of polar additives and in neat tetrahydrofuran. The syn and anti isomers differ in polymerization activity, polymer branching, and polymer molecular weight. For comparison, a series of mononuclear nickel complexes (26, 27-s, 27-a, 28, 30) was prepared and studied. The effects of structure and catalyst nuclearity on reactivity are discussed.

Full text of DOI:10.1021/om2011694

Article
pubs.acs.org/Organometallics
Dinickel Bisphenoxyiminato Complexes for the Polymerization of
Ethylene and α-Olefins
Madalyn R. Radlauer, Michael W. Day, and Theodor Agapie*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
S
* Supporting Information
ABSTRACT: Dinuclear nickel phenoxyiminato olefin poly-
merization catalysts based on rigid p-terphenyl frameworks are
reported. Permethylation of the central arene of the terphenyl
unit and oxygen substitution of the peripheral rings ortho to
the aryl−aryl linkages blocks rotation around these linkages,
allowing atropisomers of the ligand to be isolated. The
corresponding syn and anti dinickel complexes (25-s and 25-
a) were synthesized and characterized by single-crystal X-ray
diffraction. These frameworks limit the relative movement of
the metal centers, restricting the metal−metal distance.
Kinetics studies of isomerization of a ligand precursor (7-a)
allowed the calculation of the activation parameters for the
isomerization process (ΔH⧧ = 28.0 0.4 kcal × mol−1 and ΔS⧧ = −12.3 0.4 cal mol−1 K−1). The reported nickel complexes
are active for ethylene polymerization (TOF up to 3700 (mol C2H4) (mol Ni)−1 h−1) and ethylene/α-olefin copolymerization.
Only methyl branches are observed in the polymerization of ethylene, while α-olefins are incorporated without apparent chain
walking. These catalysts are active in the presence of polar additives and in neat tetrahydrofuran. The syn and anti isomers differ
in polymerization activity, polymer branching, and polymer molecular weight. For comparison, a series of mononuclear nickel
complexes (26, 27-s, 27-a, 28, 30) was prepared and studied. The effects of structure and catalyst nuclearity on reactivity are
discussed.
alkyl chains to unsaturated olefins, arenes, biphenyls, and rigid
ring systems. The type and position of the linker controls the
relative orientation of the metals and the dynamics of the
catalyst.
The nickel phenoxyiminato system represents a well-studied
catalyst family.12 These catalysts are highly active for ethylene
polymerization, and the incorporation of other monomers has
been reported. Notably, polymerizations can be performed in
the presence of polar additives such as ethers and amines.13
Addition of such bases poisons classical early-metal catalysts.
Examples of ethylene polymerizations performed in aqueous
emulsions are common with nickel phenoxyiminato cata-
lysts.14−17
Partly due to the catalytic versatility and ease of synthesis of
nickel phenoxyiminato systems, bimetallic versions have been
developed (Chart 1).7,11,17−24 In these systems, the two nickel
centers are linked via either the imine donor (a, b, e−g), the
phenoxide donor (d, h), or both (c). The measured or
proposed Ni−Ni distance varies from 3.1 Å (h) to between 7.5
and 8.5 Å (c−f).11,18,19,21−24 Analysis of the solid-state
structures highlights the challenge in orienting the metal
centers such that they would react cooperatively with
substrates. For some of the reported systems the metals are
INTRODUCTION
■
In recent years, a variety of multinuclear polymerization
catalysts have been developed.1 These systems are conceptually
inspired by enzyme active sites, which often contain a
multimetallic core. For example, in hydrolases and lyases
Lewis acidic metal centers cooperatively activate substrates to
facilitate catalysis.2−4 In search of catalysts with enhanced olefin
polymerization abilities, complexes incorporating two potential
polymerization sites have been synthesized and investigated. In
comparison to monometallic analogues, many of these
bimetallic systems have been reported to incorporate higher
levels of bulky comonomers in copolymerizations with
ethylene.5−10 For dinuclear catalysts based on late metals,
increased incorporation of polar monomers has been observed.1
However, these favorable properties are not general and some
systems are reported to have decreased polymerization
activities.11 Further studies are necessary to gain a detailed
understanding of how bimetallic cooperativity can be achieved
and controlled to generate desirable polymers.
Multinucleating ligands are commonly used to organize
metal centers near each other. Cyclopentadienyl, phenoxide,
amide, imine, pyridine, and other neutral and anionic donors
have been utilized as part of such multinucleating ligand
architectures.1 Typically, two identical moieties known to
support olefin polymerization catalysis are connected via a
linker. The nature of the linker varies from flexible, saturated
Received: November 22, 2011
Published: March 8, 2012
© 2012 American Chemical Society
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Chart 1. Previously Reported Dinuclear Nickel Phenoxyiminato Complexes
Scheme 1. Synthesis of Bis-Salicylaldimine Framework
found in geometries that make intramolecular cooperativity
unlikely. For example, system f places the metal centers anti
with respect to the aromatic linker.22 In system d, the
coordination planes of the nickel centers are oriented at
dihedral angles slightly higher than 90°, and the polymerization
sites of each metal are directed away from the other metal.24 In
c, the nickel coordination planes are almost parallel, which may
be a favorable orientation for cooperative binding of substrate,
but the two sites involved face in opposite directions.
The dinuclear complexes in Chart 1 differ from their
mononuclear counterparts in some aspects of catalytic
behavior.7,11,17−24 With systems a and b, ethylene polymer-
ization activities are similar to mononuclear controls, but
enhanced comonomer incorporation and activity occur in the
copolymerization of ethylene with functionalized norbornene
derivatives.19,20 When it is activated with MAO, system e
produces polymer with higher Mw than some previously
reported mononuclear systems.21 Complex f (R = Ph) shows
increased ethylene polymerization activity in comparison to a
mononuclear system studied under the same conditions and
produces polymers with higher Mw but broader molecular
weight distributions.22 The differences between the mono- and
dinuclear systems were attributed to potential electronic
communication of the nickel centers through the ligand bridge
in the bimetallic complex.22 Further investigation of variants of
f indicates similar tolerance for functional groups in comparison
to mononuclear counterparts.23 System g displays higher
ethylene polymerization activity and produces polymers with
increased Mw relative to those for the mononuclear systems
studied.17 Increased incorporation of comonomers and, most
notably, of polar olefins occurs with system h relative to
mononuclear analogues.11 The bimetallic effects in systems a, b,
and h were proposed to involve coordination of the same
monomer to both metal centers due to their spatial
proximity.11,20
The ligand frameworks above highlight a strategy for
synthesizing complexes with a fixed metal−metal distance
the linker must be rigid and the coordination sites involved in
polymerization accessible from the same direction for both
metals. To access a family of dinickel complexes that would
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Scheme 2. Synthesis of Monucleating Biphenyl-Based Salicylaldimine Framework
Scheme 3. Synthesis of Monucleating Terphenyl-Based Salicylaldimine Framework with Methoxy Substitution
allow for studies of the effect of the Ni−Ni distance and
orientation of the metal coordination plane, we developed a
ligand framework based on a terphenyl moiety, with blocked
rotation around aryl−aryl bonds due to ring substitution. The
synthesis of dinickel and mononickel complexes supported by
this ligand architecture is reported herein along with polymer-
ization and copolymerization studies with a variety of olefins.
Bromination of 3 with Br2 ortho to the methoxy groups
generated dibromide 4. Column chromatography was used to
separate the two atropisomers, which were then carried forward
to the final ligand precursors, 7-s and 7-a, by the same synthetic
procedures. Lithium−halogen exchange followed by addition of
excess N,N-dimethylformamide (DMF) provided the diformyl
species 5 upon aqueous workup. Removal of the methyl
protecting groups was accomplished with excess BBr3 to afford
compounds 6. Condensation with aniline generated the
binucleating ligand precursors 7. The syntheses were high
yielding overall: approximately 40% yield for the anti analogue
(7-a) and 25% yield for the syn analogue (7-s).
Synthesis of Mononucleating Salicylaldimine Ligands
Based on Biphenyl and Terphenyl Frameworks. For
comparison with the dinuclear systems, mononucleating ligands
were also prepared. Several aspects of the terphenyl framework
were investigated. The steric effect close to the metal center
was tested by targeting catalysts based on a salicylaldimine
RESULTS AND DISCUSSION
■
Synthesis of Binucleating Salicylaldimine Ligands.
The synthesis of the binucleating ligands is based on well-
documented procedures. 2-Bromo-4-tert-butylanisole (1) and
1,4-dibromo-2,3,5,6-tetramethylbenzene (2) starting materials
were made using published syntheses.25−27 Lithium−halogen
exchange of 1, followed by treatment with ZnCl2, afforded an
arylzinc reagent suitable for a double Negishi cross-coupling
with 2 (Scheme 1). The palladium-catalyzed coupling reaction
led to a mixture of two atropisomers, syn (3-s) and anti (3-a).
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Scheme 4. Synthesis of Monucleating Terphenyl-Based Salicylaldimine Framework without Methoxy Substitution
substituted with pentamethylphenyl ortho to the oxygen (13).
A previously reported variant (29)28 of this ligand includes a
phenyl group instead of a pentamethylphenyl group and was
studied as a more sterically open version of 13. Dinucleating
ligand precursors 7-s and 7-a bear steric bulk on both
peripheral rings of the terphenyl unit. Three mononucleating
terphenyl ligands were prepared to mimic the remote steric
environment of 7-s and 7-a. All are fully substituted on the
central ring. Two have oxygen substitution on both peripheral
aryls in the position ortho to the central ring. This substitution
pattern blocks the aryl−aryl rotation and leads to syn and anti
isomers (19-s and 19-a, respectively). The third mononucleat-
ing terphenyl ligand (24) has 3,5-di-tert-butyl substitution on
the second peripheral ring.
The synthesis of salicylaldimine 13 was accomplished in five
steps (Scheme 2). Negishi cross-coupling of 1 and pentam-
ethylbromobenzene29 afforded biphenyl species 9. Subsequent
steps are similar to the synthesis of ligands 7-s and 7-a.
Bromination, followed by lithium−halogen exchange and DMF
treatment, installed the formyl moiety to give 11. Deprotection
of the ether group and condensation with 2,6-diisopropylaniline
provided 13 in 14% overall yield.
The synthesis of the mononucleating terphenyl ligand
analogues was accomplished via a modification of the
procedure in Scheme 1. Negishi cross-coupling of 2 with 2.2
equiv of zinc reagent stemming from methoxymethyl (MOM)-
protected 2-bromo-4-tert-butylphenol afforded both the ex-
pected terphenyl species as well as the bromo-substituted
biphenyl species 15 (Scheme 3). The isolation of the mono-
cross-coupled product (15) was instrumental to the preparation
of asymmetric terphenyl ligands. A second cross-coupling, with
orthogonally protected 2-bromo-4-tert-butylanisole (1), af-
forded the syn and anti atropisomers of terphenyl species 16
in a ratio of 1:1. Deprotonation directed by the MOM-
protected ether using n-butyllithium and N,N,N′,N′-tetramethy-
lethylenediamine (tmeda) led, upon reaction with DMF and
aqueous workup, to the installation of a single formyl group.
Acid-catalyzed removal of the MOM group followed by
condensation with 2,6-diisopropylaniline afforded 19-a and
19-s in 35 and 37% yields, respectively, starting from
compounds 16. Separation of the two atropisomers was
accomplished by column chromatography after the second
Negishi cross-coupling (compounds 16). The third mono-
nucleating terphenyl ligand was synthesized starting from the
Negishi cross-coupling of 15 with the arylzinc reagent derived
from 3,5-di-tert-butylbromobenzene to yield the asymmetric
terphenyl 21 (Scheme 4). On adaptation of the protocols from
the synthesis of 19, 21 was converted to monophenol 24 in
34% overall yield from 15. A single isomer is expected because
of the lack of substitution ortho to the central ring and due to
the symmetrical substitution pattern on the peripheral aryl.
Studies of the Interconversion of Atropisomers. In the
context of preserving the steric environment and the metal−
metal separation in complexes supported by ligands with
restricted rotation around aryl−aryl bonds, it is of interest to
determine the kinetic and thermodynamic behavior of the
atropisomers. Because the nickel complexes decompose before
isomer interconversion (vide infra), kinetics studies of the
interconversion of ligand precursor 7-a to 7-s were performed
in [D0]-1-bromonaphthalene at 140, 150, 160, and 170 °C and
1
were monitored by H NMR spectroscopy. With either 7-s or
7-a as the starting material, equilibrium was reached over 20 h
at 140 °C, 8 h at 150 °C, 3.5 h at 160 °C, and 1.75 h at 170 °C.
At these temperatures, the equilibrium constant is Keq = [7-s]/
[7-a] = 0.61 (eq 1). The studied processes fit the integrated
rate expression for approach to equilibrium of first-order
kinetics (eq 2; Xe = concentration at equilibrium; X =
concentration at time t) (see the Supporting Information).30
An Eyring plot using the determined rate constants provided
activation energy parameters: ΔH⧧ = 28.0 0.4 kcal mol−1 and
ΔS⧧ = −12.3 0.4 cal mol−1 K−1 (Figure 1). As expected, the
calculated free energy barrier to rotation for 7-a (ΔG⧧ = 32 kcal
mol−1 at 298 K) is significantly higher than for a recently
reported terphenyl system without permethylation of the
central arene (14.6 kcal mol−1).31 Although the entropy of
Figure 1. Eyring plot for the isomerization of 7-a to 7-s.
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Scheme 5. Synthesis of Nickel Complexes
1
activation for conformational dynamic processes is typically
close to 0, the larger absolute value determined here is still in
the range reported for related fluxional processes: for example,
rotation around the C−NMe2 bond of a N,N-dimethylthiour-
isolated cleanly. The H NMR spectra of the isolated nickel
complexes each display a single peak around −0.5 ppm,
diagnostic for the Ni−CH3 moiety. The atropisomers were
assigned by 1H−1H NOESY and ROESY NMR studies.
Through-space cross peaks are observed between the meta
proton of the Ni-bound pyridine and the proton ortho to the
aryl−aryl linkage for only one of the isomers (see the
Supporting Information). This isomer was assigned as the
anti atropisomer (25-a and 27-a).
ethane (ΔS⧧ = −8
2 cal mol−1 K−1).32 The significantly
negative value suggests a relatively ordered transition state
likely corresponding to the geometry with two aryl rings
coplanar. This geometry may require significant distortions of
the ring substituents. The barrier for isomerization for 7-a is
comparable to the reported value for the restricted rotation in
hexaarylbenzenes (ca. 33 kcal mol−1 at 419 K).33 Extrapolating
to 25 °C (the temperature at which most of the polymer-
izations discussed herein were run), the rate constant for the
interconversion of 7-a and 7-s is approximately 10−11 s−1,
indicating that virtually no isomerization takes place over the
course of the polymerization experiment.
NMR spectra of the nickel complexes are each indicative of a
single ligand environment, suggesting that for complexes with
atropisomers no isomerization occurs during synthesis. Heating
solutions of 25-s and 25-a in benzene at 50 °C for 13 h did not
cause isomerization of 25-s to 25-a or of 25-a to 25-s,
respectively (1H NMR spectroscopy). No decomposition was
observed for 25-a, though 70% decomposition of 25-s was
observed, on the basis of the disappearance of the Ni−CH3
peak in the 1H NMR spectrum. Heating of 25-s and 25-a at 70
°C for 8 h led to 100% and 10% decomposition, respectively,
but no isomerization. Further heating of 25-a at 90 °C for 12 h
caused significant decomposition, but no isomerization to 25-s.
Analogous results were seen when heating 27-a and 27-s to 90
°C for 6 h, resulting in about 60% decomposition of 27-s and
80% decomposition of 27-a. These studies indicate that the
energetic barrier is too high for isomerization to occur at any
appreciable rate at 25 °C, consistent with the kinetics studies
completed with the bis-salicylaldimines 7-a and 7-s (vide
supra).
k
as
7‐a XoooY 7‐s
k
(1)
(2)
sa
ln(X − X) = −(k + k )t
e
as
sa
Synthesis of Nickel Complexes. Nickel complexes were
prepared via alkane elimination. Reaction of phenols with a
10% excess of NiMe2(tmeda) in diethyl ether in the presence of
excess pyridine allowed for the isolation of the nickel−methyl
species supported by the corresponding phenoxyiminato
ligands with a bound pyridine (Scheme 5). When acetonitrile
or tertiary amine (N,N-dimethylbutylamine or N,N-dimethyle-
thylamine) were utilized instead of pyridine or if no additional
labile ligand was added, the desired nickel complexes were not
Structure of Dinickel Complexes. X-ray-quality single
crystals were obtained from a concentrated pentane solution
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Figure 2. Solid-state structures of 25-s (left) and 25-a (right) with thermal ellipsoids at the 50% probability level. For clarity, hydrogen atoms and
solvent molecules are omitted. Reprinted with permission from ref 48. Copyright 2012 by the American Chemical Society.
cooled to −35 °C for 25-s and by vapor diffusion of hexanes
into tetrahydrofuran at room temperature for 25-a. X-ray
diffraction studies provided structural confirmation of the
identity of the isomers (Figure 2), as assigned by NMR
spectroscopy above. The methyl groups are located trans to the
phenoxide and the pyridine trans to the imine, as reported for
similar coordination environments.34 The Ni−Ni distance is 7.1
Å (average for the two molecules in the asymmetric unit) for
the syn isomer (25-s). A slight distortion from square-planar
geometry is observed, probably due to the pyridine ligands that
extend toward each other and must tilt to avoid steric
interaction. The planes of the two pyridines are about 3.86 Å
apart, possibly indicative of a weak π interaction.35 The
direction of binding of the pyridine ligands indicates that
appropriate substrates may reach both metal centers for
cooperative interaction. Conversely, for the anti atropisomer
(25-a) intramolecular cooperativity is not possible because of
the large metal−metal distance (11.1 Å), and because the nickel
centers are on opposite faces of the central arene ring. The
N(1)−Ni(1)−N(2) and N(3)−Ni(2)−N(4) angles in the syn
isomer of 173 and 166°, respectively (average for the two
molecules in the asymmetric unit), and the N(1)−Ni(1)−N(2)
angle of 177° in the anti isomer are nearly linear. Ni−N, Ni−O,
and Ni−Me distances are similar to those for known
complexes.19,22,28,34,36,37
Ethylene Polymerization. Ethylene homopolymerization
trials were performed to determine the effects of reaction scale,
reaction time, catalyst loading, and solvent (Table 1). Duplicate
polymerization trials show changes in turnover frequencies
(TOF) of less than 50% in the majority of cases. Increased
reaction time led to increased polymer yield, indicating that the
catalyst remains active over extended periods (e.g., entries 3−5,
Table 1). Ethylene polymerizations in 25 mL of toluene with
25-a and mononuclear counterparts 26, 27-a, 28, and 30
resulted in similar catalytic activities (TOFs 1200−3700 (mol
C2H4) (mol Ni)−1 h−1) (e.g., entries 3, 18, 23, 33, and 37, Table
1). This level of activity is similar to or lower than that seen
with nickel salicylaldimines that have a phosphine or nitrile
ligand in place of the pyridine, which may be due in part to the
stability of the pyridine-bound complex.13 The highest TOFs
were observed using 25-a and 26 (entries 3 and 18, Table 1).
25-s exhibits catalytic activity 1 order of magnitude less than
25-a (entries 10−13, Table 1) and is generally less active than
the other investigated catalysts. Similarly, 27-s has activity 3-
fold lower than 27-a (entries 28 and 29, Table 1). The
observed difference in TOFs between 25-s and 25-a may be
due to the effect of crowding of the catalytic pocket by the
second nickel center. Similarly, steric bulk on the remote aryl of
the terphenyl unit may be responsible for the difference
between 27-s and 27-a.
Decreasing the scale of the polymerization reaction by 5
times (5 mL of toluene) caused a significant drop in activity
(e.g., entry 4 versus entry 7, Table 1). The concentration of
nickel complex was doubled in order to collect enough polymer
for analysis when running polymerizations at this scale. These
changes in scale and concentration resulted in a reduction of
TOF by 2−10-fold (entries 4, 7; 12−15; 18−20; 23−25; 28−
30; 33, 34; 37, 38, 40; Table 1). This effect is not well
understood but may be caused by changes in mixing of the
solution and mass transfer problems, which could lower the
effective concentration of ethylene in solution. To test the
effect of mixing, a polymerization with 25-a was run with
stirring at one-third the rate used for all other polymerizations
(entry 6, Table 1). The TOF in this polymerization was
reduced by 2-fold from that of an identical trial with the higher
stirring rate (entries 4 and 6, Table 1), supporting the
hypothesis that insufficient mixing in the smaller scale
polymerizations could contribute to the drop in activity.
Changing the solvent from toluene to tetrahydrofuran (THF)
did not significantly affect the activity of 25−28 but decreased
the activity of 30 by 4-fold (entries 7−9; 14−17; 20, 21; 30−
32; 34−36; 40−42; Table 1). This drop in activity for 30 is
similar to the 3−5-fold drop in TOF reported for polymer-
izations with phosphine-ligated nickel salicylaldimine com-
plexes in the presence of excess ethers.11,13,23 The notable lack
of inhibition by THF of catalysts 25−28 may be due to the
steric bulk of the fully substituted aryl group ortho to oxygen
disfavoring ether coordination.
1
Polymer characterization by H and 13C NMR spectroscopy
showed only methyl branch formation with peaks in the d2-
tetrachloroethane 13C NMR spectrum at δ 20.1, 27.5, 30.4,
33.4, and 37.6 ppm assigned to the methyl branch carbon, the β
carbon, the γ carbon, the methyne carbon, and the α carbon,
respectively.38 The variation in polymer branching level
1
(determined by H NMR spectroscopy) was less than 35%
for repeated trials, indicating good reproducibility.39,40
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a
Table 1. Ethylene Homopolymerization Trials
b
c
entry
complex
amt of Ni (mmol)
solvent
vol (mL)
time (h)
yield (g)
TOF
branching
1
2
3
4
5
25-a
25-a
25-a
25-a
25-a
25-a
25-a
25-a
25-a
25-s
25-s
25-s
25-s
25-s
25-s
25-s
25-s
26
0.0126
0.0126
0.0200
0.0200
0.0200
0.0200
0.0080
0.0080
0.0080
0.0126
0.0126
0.0200
0.0200
0.0080
0.0080
0.0080
0.0080
0.0200
0.0200
0.0080
0.0080
0.0080
0.0200
0.0200
0.0080
0.0080
0.0080
0.0200
0.0200
0.0080
0.0080
0.0080
0.0200
0.0080
0.0080
0.0080
0.0200
0.0200
0.0200
0.0080
0.0080
0.0080
toluene
toluene
toluene
toluene
toluene
toluene
toluene
THF
25
25
25
25
25
25
5
1
1
1
3
1.5
3
3
3
3
1
1
3
3
3
3
3
3
3
3
3
3
1
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
1
3
3
3
0.680
0.940
2.079
3.415
1.893
1.875
0.118
0.101
0.224
0.150
0.110
0.574
0.894
0.047
0.036
0.041
0.043
5.532
4.791
0.549
0.675
0.172
3.113
2.901
0.083
0.122
0.170
1.205
1.301
0.047
0.037
0.041
3.107
0.100
0.099
0.081
1.975
2.879
0.720
0.416
0.152
0.076
1924
2660
3705
2029
2250
1114
280
150
333
424
311
341
531
69
3.4
7.5
6.0
d
6
7
8
5
18.8
17.3
25.5
27.0
19.6
16.5
9
THF
5
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
toluene
toluene
toluene
toluene
toluene
toluene
THF
25
25
25
25
5
5
53
5
60
70.3
67.5
THF
5
64
toluene
toluene
toluene
THF
25
25
5
3287
2846
815
1003
766
1850
1724
123
182
253
716
773
70
26
26
26
5
7.2
8.4
26
THF
5
27-a
27-a
27-a
27-a
27-a
27-s
27-s
27-s
27-s
27-s
28
toluene
toluene
toluene
THF
25
25
5
3.8
5.1
12.3
20.8
15.7
3.8
5
THF
5
toluene
toluene
toluene
THF
25
25
5
4.3
9.6
5
54
26.8
27.2
THF
5
61
toluene
toluene
THF
25
5
1846
148
147
121
1174
1710
1284
618
225
113
28
10.8
19.6
19.6
28
5
28
THF
5
30
toluene
toluene
toluene
toluene
THF
25
25
25
5
30
30
30
30
5
37.8
40.5
30
THF
5
a
b
All polymerizations were run in a glass reactor under 100 psig of ethylene at 25 °C. TOF = turnover frequency in (mol C2H4) (mol Ni)−1 h−1.
c
d
Branching was determined from 1H NMR spectroscopy and is reported as the number of branches per 1000 carbons. In this polymerization, the
stirring was reduced to one-third of the rate used for all other polymerizations.
Polymers resulting from 25-s have the highest level of
branching by at least 2-fold compared to products from other
catalysts under the same catalytic conditions (up to 70
branches/1000 C, entry 16, Table 1). An increase in polymer
branching was also observed upon the combination of scale
reduction, catalyst concentration increase, and the solvent
change to THF (e.g., compare entries 12 and 16, Table 1).
Polymer branching is caused by chain walking processes that
are dependent on relative rates of olefin insertion and β-H
elimination/isomerization.41−44 Increased ethylene concentra-
tion allows for faster olefin insertion compared to isomerization
and leads to lower levels of branching. Higher branch density in
the small-scale experiments is consistent with lower concen-
tration of monomer due to inefficient mixing (as proposed for
the decreased yield) and with the lower solubility of ethylene in
THF.
The selectivity for methyl branches is notable. Previously
reported dinuclear nickel polymerization catalysts based on
system h (Chart 1) also generate polyethylene with only methyl
branches, and there are a few additional accounts of dinuclear
nickel systems producing polyethylene with predominantly
methyl branches.7,11 This contrasts with previously reported
mononickel systems that show longer branches as well.19,21−24
Catalysts 25−28 and 30 generate polyethylene with only
methyl branches (path A, Scheme 6), suggesting that the
proximal ligand environments hinder the formation of ethyl (or
longer) branches regardless of the contributions from a second
metal center. Bulky ligands can disfavor path B in Scheme 6,
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despite similar TOFs compared with 25-s (entries 16, 17, 31,
and 32, Table 1); this behavior suggests that the simple ligand
sterics explanation is not fully satisfactory. However, a direct
bimetallic interaction of pendant C−H bonds in the chain
walking intermediates, as proposed for h, seems unlikely given
the significant metal−metal distance.
Scheme 6. Insertion and Chain Walking Processes during
Polymerization
Ethylene/1-Hexene Copolymerization. Ethylene/1-hex-
ene copolymerization trials were also performed to determine
the effects of reaction scale, comonomer concentration,
reaction time, reaction temperature and solvent on the resultant
copolymers (Table 2). As with the ethylene homopolymeriza-
tions, polymerizations with 25-s and 27-s produced the least
polymer, and polymers synthesized using 25-s display the
largest amount of branching (e.g., entries 6, 16, 18, 21, 24, 27,
and 29, Table 2). The change in activity from homopolyme-
rizations of ethylene observed in experiments performed on a
25 mL scale (Table 1, entry 1, versus Table 2, entry 1) was 1
order of magnitude, matching previous reports of 1 order of
magnitude decrease in activity from ethylene homopolymeriza-
tion upon addition of an α-olefin comonomer in large excess.36
The drop observed on a 5 mL scale, however, was not as
significant (only up to 4.4 times). The decrease in activity was
previously explained by a slower insertion rate of the α-
olefins.16,36 As expected, lower comonomer concentration led
to higher TOF (Table 2, entries 4 and 5). Extension of the
reaction time from 3 to 12 h resulted in a lowered TOF,
which involves species with nickel bound to a secondary carbon
substituted with an ethyl group and the polymeryl chain.
Similar to system h, the dinuclear syn isomer 25-s generates
increased branch density in comparison to the mononuclear
analogues. One explanation invokes slower propagation kinetics
for 25-s compared to 25-a and the mononuclear systems,
allowing for more extensive chain walking with 25-s. In THF,
compound 27-s produced polymers with lower branching
a
Table 2. Ethylene/1-Hexene Copolymerization Trials
b
c
d
entry complex amt of Ni (mmol) amt of hexene (equiv)
solvent
vol (mL) temp (°C) time (h) yield (g) TOF
branching branch type
1
25-a
25-a
25-a
25-a
25-a
25-a
25-s
25-s
25-s
25-s
25-s
25-s
25-s
25-s
25-s
25-s
26
0.0200
0.0040
0.0080
0.0080
0.0080
0.0080
0.0040
0.0040
0.0040
0.0040
0.0040
0.0040
0.0080
0.0080
0.0080
0.0080
0.0080
0.0080
0.0080
0.0080
0.0080
0.0080
0.0080
0.0080
0.0080
0.0080
0.0080
0.0080
0.0080
8000
8000
4000
4000
3200
3200
8000
8000
8000
8000
8000
8000
4000
4000
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
3200
toluene
toluene
toluene
THF
25
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
25
25
25
25
25
25
25
25
25
25
40
40
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
1
3
0.107
0.030
0.043
0.051
0.112
0.080
e
191
89
36.3
49.1
33.0
34.7
31.1
31.6
92.2
76.6
65.2
78.4
m + b
m + b
m + b
m + b
m + b
m + b
2
3
3
64
4
3
76
5
THF
3
167
118
e
6
THF
3
7
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
THF
1
8
3
0.020
0.016
0.040
0.012
0.044
0.023
0.017
0.018
0.016
0.193
0.078
0.132
0.092
0.066
0.017
0.028
0.028
0.030
0.063
0.108
0.094
0.047
59
m + b
m + b
m + b
9
12
12
3
12
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
36
12
3
32
34
54.0
m + b
3
25
3
26
63.7
62.9
32.8
33.1
31.4
34.8
33.9
33.3
38.9
39.3
53.4
38.5
38.8
44.6
48.4
m + b
m + b
m + b
m + b
m + b
m + b
m + b
m + b
m + b
m + b
m + b
m + b
m + b
m + b
m + b
THF
3
23
THF
3
287
116
195
136
99
26
THF
3
27-a
27-a
27-a
27-s
27-s
27-s
28
toluene
THF
3
3
THF
3
toluene
THF
3
26
3
42
THF
3
42
toluene
THF
3
44
28
3
94
28
THF
3
160
140
70
30
THF
3
30
THF
5
3
a
b
All polymerizations were run in a glass reactor under 100 psig of ethylene. TOF = turnover frequency in (mol C2H4) (mol Ni)−1 h−1. This value is
c
1
not adjusted for the amount of 1-hexene incorporated. Branching was determined from H NMR spectroscopy and is reported as the number of
d
e
branches per 1000 carbons. Determined from 13C NMR spectroscopy: m = methyl, b = butyl. Too little polymer to accurately determine.
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a
Table 3. Ethylene/α-Olefin Copolymerization Trials with 25-a and 25-s
b
c
c
d
e
e
f
f
f
entry complex
comonomer
yield (g) branching
branch type
branch ratio
inc (%)
TOF e
TOF co
Mw
Mn
PDI
1
25-a
25-a
25-s
25-s
25-a
25-a
25-s
25-s
25-a
25-a
25-s
25-s
25-a
25-a
25-s
25-s
25-a
25-a
25-s
25-s
25-a
25-s
0.101
0.224
0.041
0.043
0.087
0.086
0.044
0.028
0.112
0.080
0.018
0.016
0.053
0.045
0.022
0.006
0.017
0.017
0.012
0.009
0.037
0.002
17.3
18.8
70.3
67.5
33.4
31.3
70.2
76.1
31.1
31.6
63.7
62.9
36.0
40.7
68.0
61.3
49.4
49.5
61.0
51.1
m
150
333
60
2
m
47591
6309
7.54
3
m
4
m
64
8114
2697
4271
3.01
3.57
5
1-pentene
1-pentene
1-pentene
1-pentene
1-hexene
1-hexene
1-hexene
1-hexene
1-heptene
1-heptene
1-heptene
1-heptene
1-octene
1-octene
1-octene
1-octene
m + p
m + p
m + p
m + p
m + b
m + b
m + b
m + b
m + pn
m + pn
m + pn
1:1.2
1:1.3
1:0.3
1:0.3
1:2.1
1:2.3
1:0.4
1:0.3
1:2.7
1:3.2
1:0.5
3.9
3.7
3.4
3.7
4.6
4.8
3.9
3.1
6.0
7.3
5.1
124
123
64
2.0
1.9
0.9
0.6
2.6
1.9
0.3
0.2
1.4
1.4
0.5
15238
6
7
7707
2583
3712
2.98
3.80
8
39
9
159
112
25
14088
10
11
12
13
14
15
16
17
18
19
20
21
22
22
2759
9097
893
3.09
3.00
74
3037
61
31
3619
4472
2030
1196
1068
559
3.03
4.19
3.63
m + h
m + h
m + h
1:4.1
1:6.2
1:0.6
10.4
11.5
5.3
22
22
17
0.7
0.7
0.2
g
C13H24O2
2.7
1.6
45
1.3
g
C13H24O2
1.7
0.03
a
All polymerizations were run for 3 h in a glass reactor with 0.0080 mmol of nickel in THF under 100 psig of ethylene with 3200 equiv of
b
comonomer at 25 °C. The total reaction volume was 5 mL. Branching was determined from 1H NMR spectroscopy and is reported as the number
c
d
of branches per 1000 carbons. Determined from 13C NMR spectroscopy: m = methyl, p = propyl, b = butyl, pn = pentyl, h = hexyl. Percent
e
incorporation was calculated from the overall branching and the branch ratio. TOF = turnover frequency in (mol monomer) (mol Ni)−1 h−1; e =
f
g
ethylene, co = comonomer. Calculated from the yield and the percent incoporation of comonomer. Calculated from GPC results. Ethyl
undecylenate; used 2500 equiv (4.8 mL) with 0.2 mL of THF for a total volume of 5 mL.
presumably due to catalyst decomposition over time. Increasing
the temperature resulted in a less than 2-fold decrease in
activity in 3 h polymerization reactions and approximately no
change in activity in 12 h polymerization runs (Table 2, entries
8 and 11 and entries 10 and 12). A change of solvent also had
negligible effect on either the yield or the branching of the
resultant polymers. Overall, the behavior of catalysts 25-a, 25-s,
26, 27-a, 27-s, 28, and 30 is comparable to that of previously
reported monometallic systems.16,36 A noteworthy trend is the
higher branching with the syn catalysts 25-s and 27-s; this may
also be a consequence of the bulkier environment, which slows
propagation in comparison to chain walking. Additionally, all of
the polymers characterized by 13C NMR spectroscopy
displayed only methyl and butyl branches, which is a unique
microstructure. Further study of this phenomenon was
accomplished by polymerization trials with other α-olefins.
Ethylene/α-Olefin Copolymerization. Ethylene/α-olefin
copolymerization trials were performed in duplicate with 25-a
and 25-s and 1-pentene, 1-hexene, 1-heptene, and 1-octene to
evaluate the effects of nickel−nickel proximity on branching,
comonomer incorporation, TOF, molecular weight, and
molecular weight distribution (Table 3). Again, significantly
more branching was observed in polymers produced with 25-s
than in polymers produced with 25-a, but the percent
incorporations of 1-pentene and 1-hexene were similar. This
behavior suggests that the difference in the extent of branching
was due to the presence of additional methyl branches from
chain walking rather than to the incorporation of additional
comonomer. With the longer α-olefins, 1-heptene and 1-
octene, a greater degree of comonomer incorporation was seen
in polymers generated by 25-a than by 25-s, likely due to
increased steric hindrance in 25-s.
In all of the ethylene/α-olefin copolymers examined by 13C
NMR spectroscopy (Tables 2 and 3), only isolated methyl
branches and branches the length of the comonomer chain
were present. These data suggest that chain walking along the
polyethylene chain to methyl branches occurs but that after the
insertion of a comonomer, no chain isomerization takes place
before the coordination and insertion of the next ethylene
monomer (paths C and D, Schemes 6 and 7). To the best of
Scheme 7. Copolymerizations of Ethylene and α-Olefins
Leads to Only Two Types of Branches
our knowledge, this type of polymer microstructure has not
been previously reported for ethylene−α-olefin copolymeriza-
tion; it formally corresponds to an ethylene−propylene−α-
olefin copolymer, without chain walking.16,36 Mecking et al.
specifically report a variety of branch lengths including methyl,
ethyl, and butyl branches in the copolymerization of ethylene
and 1-butene, which are attributed to various modes of
insertion and subsequent chain walking.16 Assuming 1,2-
insertions are favored (paths D and F, Scheme 6), the
difference in polymer microstructure achieved in polymer-
izations with the current systems may arise from the steric
hindrance caused by the supporting ligand, disfavoring path F,
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in which nickel migrates to a tertiary from a primary
carbon.15,45,46
with nickel salicylaldiminato catalysts. Complexes 25−28 retain
polymerization activity in the presence of an excess of polar
additives such as THF, in contrast to the decreased activity of
complex 30. Because no nickel−nickel cooperativity is expected
in 25-a or the mononuclear complexes, this tolerance is
attributed to the steric environment of the permethylated arene
that is not present in 30. While no polymer was produced in
attempted copolymerizations of ethylene with methyl acrylate,
copolymerizations of ethylene with an olefin possessing a distal
polar moiety is observed, indicating some functional monomer
tolerance for 25-s and 25-a. Generally, the syn catalysts were
found to be less active and to generate lower molecular weight
polymers than the anti analogues. More branching is observed
for the syn catalysts. These effects are explained in terms of
increased steric bulk. This steric effect is currently exploited for
polymerizations in the presence of strong Lewis bases that
would significantly deactivate the nickel catalysts in the absence
of a compensating bimetallic effect.48 Although large differences
between the syn and anti dinuclear catalysts reported here have
not been observed, the present systems provide a robust
framework amenable for further studies of dinuclear catalysts
for olefin polymerization. Future investigations include
changing the relative position of the two metal centers on
the terphenyl moiety, the nature of the metals, and the donor
sets.
GPC analysis was performed on several of the ethylene−α-
olefin copolymers (Table 3). In all cases, the molecular weights
of polymers produced with 25-a were higher than those of
polymers produced with 25-s. The molecular weights for
polymers produced with both 25-a and 25-s generally
decreased with increasing comonomer size. The PDI values
were between 3 and 4 except for the homopolymerization of
ethylene with 25-a (PDI = 7.5). Generally, lower PDI values
were observed for 25-s compared to 25-a. The observed
molecular weights and PDIs are in the range previously
reported for mono- and dinickel catalysts.7,13,19,20,34 Notably,
high PDIs (5−8) were reported previously for bimetallic
catalysts (c and f, Chart 1).19,22 The difference in polymer
molecular weight is indicative of the relative rates of
propagation vs chain termination, which depend on the rates
of olefin insertion and β-H elimination, respectively.47 The
lower molecular weights for 25-s vs those for 25-a contrast with
previous reports of a bimetallic catalyst leading to an increase in
Mw vs the monometallic version22 but are consistent with the
trends in TOF and branching level. In comparison to 25-a,
complex 25-s displays lower TOF and higher branching
consistent with lower olefin insertion rates and higher β-H
elimination rates, which is in agreement with the observed
lower molecular weight polymers. Similar agreement between
trends of Mw vs TOF and polymer branching (for 25-a) were
observed upon variation of the comonomer. The larger
comonomers may lead to lower insertion rates due to steric
reasons and result in lower Mw polymers.16,36
EXPERIMENTAL SECTION
■
General Considerations. All air- and/or water-sensitive com-
pounds were manipulated using standard vacuum or Schlenk line
techniques or in an inert-atmosphere glovebox. The solvents for air-
and moisture-sensitive reactions were dried over sodium benzophe-
none ketyl or calcium hydride or by the method of Grubbs.49 All NMR
solvents were purchased from Cambridge Isotopes Laboratories, Inc.
Benzene-d6 was dried over sodium benzophenone ketyl and vacuum-
transferred prior to use. 1-Bromonaphthalene, pyridine, 1-pentene, 1-
hexene, 1-heptene, 1-octene, ethyl undecylenoate, N,N-dimethylallyl-
amine, and methyl acrylate were dried over calcium hydride and
vacuum-transferred prior to use. Ethylene was purchased from
Matheson and equipped with a PUR-Gas in-line trap to remove
oxygen and moisture before use. All 1H, 13C, and 2D NMR spectra of
small organic and organometallic compounds were recorded on Varian
Mercury 300 MHz, Varian 400 MHz, and Varian INOVA-500 or 600
MHz spectrometers at room temperature. All 1H and 13C NMR
spectra of polymers were recorded on the Varian INOVA-500 MHz
spectrometer at 130 °C. Chemical shifts are reported with respect to
residual internal protio solvent. 2-Bromo-4-tert-butylphenol,25 2,27 8,29
chloromethyl methyl ether,50 Ni(acac)2(tmeda),51 NiMe2(tmeda),52
and 3-phenylsalicylaldehyde53 were synthesized according to literature
procedures. 25-a, 25-s, 19-a, and 19-s were synthesized according to
literature procedures.48
Copolymerizations of ethylene and polar monomers were
also attempted. Using a large excess of a comonomer with a
distal polar moiety, ethyl undecylenate (Table 3, entries 21 and
22; 2500 equiv per nickel), led to a modest yield of polymer
and incorporation within the range of previous reports for
related catalysts.36 Copolymerization attempts with 225 equiv
of N,N-dimethylallylamine per nickel resulted in polyethylene
with no polar comonomer incorporation observed by 1H or 13
C
NMR spectroscopy but larger inhibitory effects for 25-a
compared to 25-s.48 In contrast, copolymerization attempts
with 225 equiv of methyl acrylate per nickel resulted in no
observable polymer. These data indicate that 25-a and 25-s
tolerate some polar monomers. Additional investigations of the
copolymerization of ethylene and polar monomers with these
complexes are ongoing.
CONCLUSIONS
■
The syn and anti atropisomers of a dinuclear neutral nickel
bisphenoxyiminato complex (25) were synthesized and
characterized. Kinetic studies of the bis-salicylaldimine
precursors (7) indicate that virtually no isomerization between
the syn and anti atropisomers occurs at 25 °C, making these
terphenyl complexes suitable for the systematic study of the
effects of metal−metal cooperativity. The terphenyl dinuclear
complexes polymerize ethylene similarly to previously reported
mononickel salicylaldimine complexes such as 30. Ethylene
polymerization leads to polyethylene with only methyl
branches. Copolymerizations of ethylene and α-olefins with
complexes 25−30 produce polyethylene with methyl branches
and branches the length of the comonomer side chain (three to
six carbons depending on the comonomer). This polymer
microstructure has not been previously reported, to our
knowledge, for the copolymerization of ethylene and α-olefins
Compound 1. 2-Bromo-4-tert-butylmethoxybenzene was synthe-
sized from 2-bromo-4-tert-butylphenol according to an analogous
synthesis.26 The 1H NMR spectrum matched literature assignments.54
HRMS (EI+): calcd for C11H15OBr 242.0306, found 242.0305.
Compounds 3. Synthesis of these terphenyl compounds was
accomplished via the Negishi coupling of 1,4-dibromo-2,3,5,6-
tetramethylbenzene (2) with 2 equiv of 2-bromo-4-tert-butylmethox-
ybenzene (1).55 In the glovebox, 1 (25.44 g, 104 mmol, 1 equiv) and
250 mL of THF were combined in a large Schlenk tube and frozen in
t
the cold well. BuLi (1.7 M solution in pentane, 129 mL, 219 mmol,
2.1 equiv) was added to the thawing solution and stirred for 1 h while
it was warmed to room temperature. The resultant yellow-orange
solution was refrozen in the cold well. Concurrently, a suspension of
ZnCl2 (9.98 g, 73 mmol, 0.7 equiv) in THF (100 mL) was frozen in
the cold well. The thawing ZnCl2 suspension was added to the thawing
reaction mixture and stirred for 1 h, resulting in a colorless cloudy
solution. 2 (13.75 g, 47 mmol, 0.45 equiv), Pd(PPh3)4 (1.21 g, 1.1
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Organometallics
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mmol, 0.01 equiv), and THF (100 mL) were added to the reaction
mixture at room temperature. The sealed Schlenk tube was brought
out of the glovebox and heated to 75 °C for 5 days. Water was added
to quench the reaction. The solution was filtered over silica gel, and
the silica gel was washed with dichloromethane (DCM). The two
atropisomers of the terphenyl compound were coprecipitated from
methanol as a colorless solid (15.4 g, 71% yield). 1H NMR (300 MHz,
CDCl3): δ 7.36 (2dd, 2H, ArH), 7.19 (2d, 2H, ArH), 6.95 (2d, 2H,
ArH), 3.78 (2s, 6H, OCH3), 1.99 (2s, 12H, ArCH3), 1.34 (2s, 18H,
C(CH3)3) ppm. 13C NMR (75 MHz, CDCl3): δ 154.70 (Ar), 143.25
(Ar), 137.83 (Ar), 137.57 (Ar), 132.22 (Ar), 132.10 (Ar), 130.75 (Ar),
129.31 (Ar), 128.97 (Ar), 124.29 (Ar), 124.21 (Ar), 110.19 (Ar),
110.09 (Ar), 55.81 (OCH3), 55.59 (OCH3), 34.29 (ArC(CH3)3),
31.75 (ArC(CH3)3), 18.07 (ArCH3), 18.00 (ArCH3) ppm. HRMS (EI
+): calcd for C32H42O2 458.3185, found 458.3184.
g). 1H NMR (400 MHz, CDCl3): δ 10.46 (s, 2H, CHO), 7.89 (d, J =
2.6, 2H, ArH), 7.45 (d, J = 2.6, 2H, ArH), 3.45 (s, 6H, OCH3), 2.05 (s,
12H, ArCH3), 1.35 (s, 18H, C(CH3)3) ppm. 13C NMR (101 MHz,
CDCl3): δ 190.99 (ArCHO), 158.71 (Ar), 147.32 (Ar), 137.34 (Ar),
135.88 (Ar), 135.49 (Ar), 132.78 (Ar), 128.73 (Ar), 123.76 (Ar),
61.40 (OCH3), 34.79 (ArC(CH3)3), 31.43 (ArC(CH3)3), 18.29
(ArCH3) ppm. HRMS (EI+): calcd for C34H42O4 514.3083, found
514.3089.
Compound 6-a. BBr3 (7.74 mL, 81.6 mmol, 10 equiv) was
syringed into a Schlenk flask containing a solution of 5-a (4.20 g, 8.16
mmol, 1 equiv) in 200 mL of DCM under a nitrogen atmosphere. The
solution turned from yellow to dark red and was stirred for 1.5 h
before the reaction was stopped by the gradual addition of water and a
color change to dark greenish brown was observed. The desired
product was extracted into DCM. The organic phase was dried over
MgSO4 and filtered, and volatile materials were removed under
vacuum to give a greenish brown solid. Trituration with methanol
followed by filtration yielded 2.55 g (64% yield) of 6-a as an olive
Compounds 4. Compound 3 (10.00 g, 21.8 mmol, 1 equiv), iron
powder (0.0786 g, 1.41 mmol, 0.06 equiv), and 35 mL of DCM were
combined in a 100 mL round-bottom flask equipped with an addition
funnel. The flask was covered with foil. Bromine (2.3 mL, 44.7 mmol,
2.05 equiv) and 5 mL of DCM were added to the addition funnel and
dripped into the flask over 5 min. The reaction mixture was stirred for
an additional 3 h at room temperature. The reaction was quenched
with aqueous sodium hydrosulfite and sodium carbonate. The desired
product was extracted into DCM. The organics were washed with
water, dried with MgSO4, and filtered, and the volatiles were removed
under vacuum. The two atropisomers were separated by column
chromatography (2/1 hexanes/DCM). An 8.49 g amount of white
solid was collected of the anti isomer, and 4.73 g of white solid was
collected of the syn isomer (overall yield of 98%). Data for 4-a are as
1
green solid. H NMR (400 MHz, CDCl3): δ 11.12 (s, 2H, OH), 9.98
(s, 2H, CHO), 7.54 (2s, 4H, ArH), 3.48 (s, 6H, OCH3), 1.97 (s, 12H,
ArCH3), 1.35 (s, 18H, C(CH3)3) ppm. 13C NMR (101 MHz, CDCl3):
δ 197.20 (ArCHO), 156.69 (Ar), 143.04 (Ar), 137.15 (Ar), 135.92
(Ar), 132.59 (Ar), 131.01 (Ar), 128.78 (Ar), 120.10 (Ar), 34.41
(ArC(CH3)3), 31.44 (ArC(CH3)3), 17.93 (ArCH3) ppm. HRMS (EI
+): calcd for C32H38O4 486.2770, found 486.2784.
Compound 6-s. The deprotection of 5-s was accomplished via the
same procedure as for the anti isomer. The desired product was
isolated as a greenish solid in 93% yield (2.95 g). 1H NMR (300 MHz,
CDCl3): δ 10.91 (bs, 2H, OH), 9.98 (s, 2H, CHO), 7.55 (d, J = 2.5,
2H, ArH), 7.45 (d, J = 2.5, 2H, ArH), 3.50 (s, 6H, OCH3), 1.98 (s,
12H, ArCH3), 1.35 (s, 18H, C(CH3)3) ppm. 13C NMR (101 MHz,
CDCl3): δ 196.96 (ArCHO), 142.65 (Ar), 136.80 (Ar), 136.22 (Ar),
132.69 (Ar), 130.95 (Ar), 128.79 (Ar), 120.22 (Ar), 34.39
(ArC(CH3)3), 31.47 (ArC(CH3)3), 17.95 (ArCH3) ppm. HRMS (EI
+): calcd for C32H38O4 486.2770, found 486.2785.
1
follows. H NMR (300 MHz, CDCl3): δ 7.56 (d, J = 2.4, 2H, ArH),
7.03 (d, J = 2.4, 2H, ArH), 3.49 (s, 6H, OCH3), 1.97 (s, 12H, ArCH3),
1.33 (s, 18H, C(CH3)3) ppm. 13C NMR (75 MHz, CDCl3): δ 152.09
(Ar), 148.45 (Ar), 137.61 (Ar), 136.84 (Ar), 131.96 (Ar), 129.12 (Ar),
128.40 (Ar), 117.22 (Ar), 60.35 (OCH ), 34.66 (ArC(CH3)3), 31.54
3
(ArC(CH3)3), 18.31 (Ar CH3) ppm. HRMS (EI+): calcd for
C32H40O2Br81Br 616.1374, found 616.1376. Data for 4-s are as
Compound 7-a. The anti-bis-salicylaldimine compound was
synthesized by mixing 6-a (1.5 g, 3.08 mmol, 1 equiv), p-
toluenesulfonic acid (0.059 g, 0.31 mmol, 0.1 equiv), 2,6-diisopropyl-
amine (1.28 g, 6.78 mmol, 2.2 equiv), and methanol (150 mL) in a
round-bottom flask equipped with a reflux condenser. A color change
from green to orange was observed with the addition of aniline. The
mixture was stirred at reflux for 4 h and then cooled to room
temperature. A pale orange solid was collected from the red solution
via filtration. The precipitate was further purified by column
chromatography (7/1 hexanes/DCM), and a 1.7 g (68% yield) of
1
follows. H NMR (300 MHz, CDCl3): δ 7.56 (d, J = 2.4, 2H, ArH),
7.12 (d, J = 2.4, 2H, ArH), 3.38 (s, 6H, OCH3), 2.00 (s, 12H, ArCH3),
1.32 (s, 18H, C(CH3)3) ppm. 13C NMR (75 MHz, CDCl3): δ 151.98
(Ar), 148.49 (Ar), 137.66 (Ar), 136.45 (Ar), 132.41 (Ar), 129.09 (Ar),
128.18 (Ar), 117.33 (Ar), 59.74 (OCH3), 34.69 (ArC(CH3)3), 31.50
(ArC( CH3)3), 18.22 (ArCH 3) ppm. HRMS (EI+): calcd for C32H40O
2Br81Br 616.1374, found 616.1402.
Compound 5-a. Compound 4-a (5.10 g, 8.28 mmol, 1 equiv) was
dissolved in 200 mL of THF in a 500 mL Schlenk flask in the
glovebox, and the solution was frozen in the cold well. tBuLi (1.7 M in
pentane, 20.44 mL, 34.8 mmol, 4.2 equiv) was added in four portions
to the cold solution of 4-a. The reaction mixture turned yellow upon
1
pale yellow solid was obtained. H NMR (400 MHz, C6D6): δ 13.45
(s, 2H, OH), 8.05 (s, 2H, NCH), 7.42 (d, 2H, ArH), 7.28 (d, 2H,
ArH), 7.11 (bs, 6H, N−ArH), 3.06 (septet, J = 6.8, 4H, CH(CH3)2),
2.24 (s, 12H, ArCH3), 1.29 (s,18H, C(CH3)3), 1.06 (d, J = 6.8, 24H,
CH(CH3)2) ppm. 13C NMR (101 MHz, C6D6): δ 168.05 (ArCHN),
157.53 (Ar), 147.32 (Ar), 141.85 (Ar), 138.96 (Ar), 137.61 (Ar),
133.75 (Ar), 132.81 (Ar), 131.98 (Ar), 125.75 (Ar), 123.54 (Ar),
118.49 (Ar), 34.26 (ArC(CH3)3), 31.60 (ArC(CH3)3), 28.63 (ArCH-
(CH3)2), 23.42 (ArCH(CH3)2), 18.44 (ArCH3) ppm. HRMS (FAB+):
calcd for C56H73O2N2 805.5672, found 805.5693.
Compound 7-s. The imine condensation to form the syn-bis-
salicylaldimine compound from 6-s was accomplished via the same
procedure as the anti isomer. The desired product was isolated as a
pale yellow solid in 43% yield (1.06 g). 1H NMR (400 MHz, C6D6): δ
13.45 (s, 2H, OH), 8.06 (s, 2H, NCH), 7.45 (d, 2H, ArH), 7.29 (d,
2H, ArH), 7.13 (bs, 6H, N−ArH), 3.12 (septet, J = 6.8, 4H,
CH(CH3)2), 2.22 (s, 12H, ArCH3), 1.27 (s, 18H, C(CH3)3), 1.11 (d, J
= 6.8, 24H, CH(CH3)2) ppm. 13C NMR (101 MHz, C6D6): δ 168.04
(ArCHN), 157.58 (Ar), 147.39 (Ar), 141.68 (Ar), 138.95 (Ar), 137.76
(Ar), 133.43 (Ar), 132.92 (Ar), 131.98 (Ar), 127.65 (Ar), 125.77 (Ar),
123.54 (Ar), 118.62 (Ar), 34.19 (ArC(CH3)3), 31.57 (ArC(CH3)3),
28.67 (ArCH(CH3)2), 23.46 (ArCH(CH3)2), 18.47 (ArCH3) ppm.
HRMS (FAB+): calcd for C56H73O2N2 805.5672, found 805.5688.
Compound 28. Metalation of 24 was achieved by a method
analogous to that used for the dinuclear nickel complexes. A solution
t
addition of BuLi and was warmed to room temperature as it was
stirred for 1 h. The reaction mixture was refrozen in the cold well. A
solution of DMF (3.84 mL, 49.6 mmol, 6 equiv) in 10 mL of THF was
also frozen in the cold well before it was added to the reaction mixture
while thawing. The resulting colorless solution was warmed to room
temperature and stirred for 2 h. The flask was brought out of the box,
and the reaction was quenched with 100 mL of water. Volatiles were
removed under vacuum, and the desired product was extracted into
DCM and washed with brine and water. The organic phase was dried
over MgSO4 and filtered, and volatile materials were removed under
vacuum to give an orange solid. Precipitation from methanol yielded
4.2 g (99% yield) of pale yellow solid. 1H NMR (400 MHz, CDCl3): δ
10.45 (s, 2H, CHO), 7.87 (d, J = 2.6, 2H, ArH), 7.35 (d, J = 2.6, 2H,
ArH), 3.51 (s, 6H, OCH3), 2.00 (s, 12H, ArCH3), 1.34 (s, 18H,
C(CH3)3) ppm. 13C NMR (101 MHz, CDCl3): δ 191.17 (ArCHO),
158.82 (Ar), 147.23 (Ar), 137.22 (Ar), 136.06 (Ar), 135.75 (Ar),
132.34 (Ar), 128.56 (Ar), 123.84 (Ar), 62.16 (OCH3), 34.72
(ArC(CH3)3), 31.41 (ArC(CH3)3), 18.30 (ArCH3) ppm. HRMS (EI
+): calcd for C34H42O4 514.3083, found 514.3084.
Compound 5-s. The lithium−halogen exchange and formylation
of 4-s was accomplished via the same procedure as for the anti isomer.
The desired product was isolated as a colorless solid in 85% yield (3.4
2241
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Organometallics
Article
of 24 (0.09 g, 0.14 mmol, 1 equiv) in 4 mL of diethyl ether and a
solution of NiMe2(tmeda) (0.031 g, 0.15 mmol, 1.1 equiv) in 3 mL of
diethyl ether were cooled in the glovebox freezer to about −35 °C.
The solution of the ligand was added to the solution of nickel
precursor, resulting in a red-orange suspension, which homogenized
after about 5 min. Pyridine (0.12 mL, 1.52 mmol, 11 equiv) was
syringed into the mixture, causing a color change to red. The mixture
became gradually darker over 6 h of stirring at room temperature, at
which point volatiles were removed under vacuum. The red-brown
suspension was washed over Celite with hexanes. Black precipitate was
left on the Celite. Volatiles were removed from the filtrate under
vacuum, and the desired product was precipitated from pentane.
Filtration yielded 0.045 g of pure product as an orange solid in the first
The three values were averaged to determine the concentration for
that time point. Time points were recorded until the reactions reached
equilibrium. Keq values did not change between the four samples, so
that the final ratio of 7-s and 7-a was 0.61. The four plots of ln(Xe −
X) versus time are included in the Supporting Information.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, tables, and CIF files giving NMR data, kinetic plots,
and crystallographic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
1
crop of precipitate (40% yield). H NMR (400 MHz, C6D6): δ 8.42
ACKNOWLEDGMENTS
■
(dd, J = 6.5, 1.5, 2H, PyH), 7.72 (s, 1H, ArH), 7.60 (t, J = 1.8 , 1H,
ArH), 7.47 (d, J = 2.7, 1H, ArH), 7.27 (m, 1H, ArH), 7.21 (m, 1H,
ArH), 7.14 (d, J = 2.6, 1H, ArH), 6.90 (tt, J = 7.6, 1.6, 1H, PyH), 6.32
(m, 2H, PyH), 4.35 (septet, J = 6.9, 2H, CH(CH3)2), 2.12 (s, 6H,
ArCH3), 2.01 (s, 6H, ArCH3), 1.54 (d, J = 6.9 , 6H, CH(CH3)2), 1.46
(s, 9H, C(CH3)3), 1.36 (s, 9H, C(CH3)3), 1.34 (s, 9H, C(CH3)3), 1.13
(d, J = 6.8, 6H, CH(CH3)2), −0.67 (s, 3H, NiCH3) ppm. 13C NMR
(101 MHz, C6D6): δ 166.66 (ArCHN), 163.87 (Ar), 152.22 (Ar),
151.23 (Ar), 150.60 (Ar), 150.57 (Ar), 143.47 (Ar), 141.57 (Ar),
141.39 (Ar), 140.15 (Ar), 135.75 (Ar), 135.21 (Ar), 135.07 (Ar),
133.68 (Ar), 133.14 (Ar), 131.05 (Ar), 126.47 (Ar), 124.76 (Ar),
124.10 (Ar), 123.68 (Ar), 122.62 (Ar), 119.86 (Ar), 119.20 (Ar),
35.06 (ArC(CH3)3), 33.97 (Ar C(CH3)3), 31.87 (ArC(CH3)3), 31.75
(ArC(CH3)3), 31.68 (ArC(CH3)3), 28.67 (ArCH(CH3)2), 25.11
(ArCH(CH3)2), 23.37 (ArCH(CH3)2), 18.60 (ArCH3), 18.55
(ArCH3), −7.22 (NiCH3) ppm. Anal. Calcd for C53H70N2NiO: C,
78.61; H, 8.71; N, 3.46. Found: C, 78.43; H, 8.57; N, 3.18.
General Polymerization Procedures. A 3 oz Andrews glass
pressure reaction vessel equipped with Swagelok valves and a gauge
was used for all high-pressure polymerizations. All polymerizations
involving ethylene were carried out under the same conditions. The
high-pressure setup was brought into the glovebox with a magnetic
stirbar and charged with the desired amounts of solvent and
comonomer. A syringe was loaded with a solution of nickel complex,
and the needle was sealed with a rubber septum. The syringe and setup
were brought out of the box, and the setup was clamped firmly over a
hot plate with a mineral oil bath previously regulated to 25 °C (or the
desired temperature). The solution was stirred vigorously (1200 rpm).
A nylon core hose equipped with quick connect adaptors was purged
with ethylene for 1 min, and the pressure was set to 15 psi. The hose
was connected to the setup, and the setup was filled with ethylene. A
bleed needle was inserted into a Teflon septum at the top of the high-
pressure setup and flushed with ethylene. The solution of nickel
complex was added via syringe, and the top of the setup was closed.
The pressure was increased to 100 psi. After the desired time
(generally 1 or 3 h), the ethylene hose was disconnected, the setup was
vented, and the reaction mixture was quenched with acidified
methanol (3 times the reaction volume) to precipitate the polymer,
which was collected as a white or pale yellow solid by filtration over a
fine frit. If only a small amount of polymer was precipitated, the entire
mixture was collected and volatile materials were removed under
vacuum. Sample 1H and 13C NMR spectra are included in the
Supporting Information.
We thank Lawrence M. Henling (Caltech) for assistance with
collection of crystallographic data and Jerzy Klosin (Dow
Chemical) for the collection of GPC data. We are grateful to
Dow Chemical and Caltech for funding. The Bruker KAPPA
APEXII X-ray diffractometer was purchased via an NSF
CRIF:MU Award to Caltech (Grant No. CHE-0639094).
The 400 MHz NMR spectrometer was purchased via an NIH
award (No. RR027690).
REFERENCES
(1) Delferro, M.; Marks, T. J. Chem. Rev. 2011, 111, 2450.
(2) Wilcox, D. E. Chem. Rev. 1996, 96, 2435.
■
(3) Lipscomb, W. N.; Strater, N. Chem. Rev. 1996, 96, 2375.
̈
(4) Cowan, J. A. Chem. Rev. 1998, 98, 1067.
(5) Li, H. B.; Li, L. T.; Schwartz, D. J.; Metz, M. V.; Marks, T. J.;
Liable-Sands, L.; Rheingold, A. L. J. Am. Chem. Soc. 2005, 127, 14756.
(6) Li, H. B.; Marks, T. J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103,
15295.
(7) Rodriguez, B. A.; Delferro, M.; Marks, T. J. Organometallics 2008,
27, 2166.
(8) Salata, M. R.; Marks, T. J. J. Am. Chem. Soc. 2007, 130, 12.
(9) Guo, N.; Li, L.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 6542.
(10) Guo, N.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 2008, 130,
2246.
(11) Rodriguez, B. A.; Delferro, M.; Marks, T. J. J. Am. Chem. Soc.
2009, 131, 5902.
(12) Makio, H.; Terao, H.; Iwashita, A.; Fujita, T. Chem. Rev. 2011,
111, 2363.
(13) Younkin, T. R.; Conner, E. F.; Henderson, J. I.; Friedrich, S. K.;
Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460.
(14) Bauers, F. M.; Mecking, S. Macromolecules 2001, 34, 1165.
(15) Wehrmann, P.; Mecking, S. Macromolecules 2006, 39, 5963.
(16) Wehrmann, P.; Zuideveld, M.; Thomann, R.; Mecking, S.
Macromolecules 2006, 39, 5995.
(17) Wehrmann, P.; Mecking, S. Organometallics 2008, 27, 1399.
(18) Hu, T.; Li, Y. G.; Li, Y. S.; Hu, N. H. J. Mol. Catal. A: Chem.
2006, 253, 155.
(19) Na, S. J.; Joe, D. J.; Sujith, S.; Han, W. S.; Kang, S. O.; Lee, B. Y.
J. Organomet. Chem. 2006, 691, 611.
(20) Sujith, S.; Joe, D. J.; Na, S. J.; Park, Y. W.; Chow, C. H.; Lee, B.
Y. Macromolecules 2005, 38, 10027.
(21) Wang, W. H.; Jin, G. X. Inorg. Chem. Commun. 2006, 9, 548.
(22) Zhang, D.; Jin, G. X. Inorg. Chem. Commun. 2006, 9, 1322.
(23) Chen, Q.; Yu, J.; Huang, J. Organometallics 2007, 26, 617.
(24) Hu, T.; Tang, L.-M.; Li, X.-F.; Li, Y.-S.; Hu, N.-H.
Organometallics 2005, 24, 2628.
(25) Kiesewetter, E. T.; Randoll, S.; Radlauer, M.; Waymouth, R. M.
J. Am. Chem. Soc. 2010, 132, 5566.
(26) Yan, Y.; Qin, B.; Shu, Y. L.; Chen, X. Y.; Yip, Y. K.; Zhang, D.
W.; Su, H. B.; Zeng, H. Q. Org. Lett. 2009, 11, 1201.
(27) Kaur, I.; Jazdzyk, M.; Stein, N. N.; Prusevich, P.; Miller, G. P. J.
Am. Chem. Soc. 2010, 132, 1261.
Kinetic Studies of the Isomerization of 7-a. The four samples
were prepared in the glovebox by dissolving 0.044 g of 7-a in 3.2 mL
of 1-bromonaphthalene. A 0.8 mL amount of the solution was
transferred to each of four J. Young tubes and sealed. These samples
were brought out of the box and placed in preheated silicone oil baths
at 140, 150, 160, and 170 °C. The samples at 140 and 150 °C were
monitored at 30 min intervals and the samples at 160 and 170 °C were
monitored at 15 min intervals by removing the sample from the bath,
cooling to room temperature, and recording the d0 1H NMR spectrum.
The isomerization process was observed by comparing integration of
the phenol peak, the isopropyl methyne peak and the isopropyl methyl
1
peaks of the two isomers, which were distinguishable in the H NMR
(28) Wang, C. M.; Friedrich, S.; Younkin, T. R.; Li, R. T.; Grubbs, R.
spectrum (see the Supporting Information for a sample spectrum).
H.; Bansleben, D. A.; Day, M. W. Organometallics 1998, 17, 3149.
2242
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Organometallics
Article
(29) Zysman-Colman, E.; Arias, K.; Siegel, J. S. Can. J. Chem. 2009,
87, 440.
(30) Moore, J. W.; Pearson, R. G. Kinetics and Mechanism, 3rd ed.;
Wiley: New York, 1981.
(31) Lunazzi, L.; Mazzanti, A.; Minzoni, M.; Anderson, J. E. Org. Lett.
2005, 7, 1291.
(32) Hoogasian, S.; Bushweller, C. H.; Anderson, W. G.; Kingsley, G.
J. Phys. Chem. 1976, 80, 643.
(33) Gust, D. J. Am. Chem. Soc. 1977, 99, 6980.
(34) Zuideveld, M. A.; Wehrmann, P.; Rohr, C.; Mecking, S. Angew.
Chem., Int. Ed. 2004, 43, 869.
(35) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885.
(36) Connor, E. F.; Younkin, T. R.; Henderson, J. I.; Hwang, S. J.;
Grubbs, R. H.; Roberts, W. P.; Litzau, J. J. J. Polym. Sci., Part A: Polym.
Chem. 2002, 40, 2842.
(37) Gottker-Schnetmann, I.; Wehrmann, P.; Rohr, C.; Mecking, S.
Organometallics 2007, 26, 2348.
(38) Crompton, T. R. Analysis of Polymers: An Intoduction; Pergamon
Press: Oxford, U.K., 1989.
1
(39) The extent of polymer branching was determined by H NMR
spectroscopy.
(40) Gates, D. P.; Svejda, S. K.; Onate, E.; Killian, C. M.; Johnson, L.
̃
K.; White, P. S.; Brookhart, M. Macromolecules 2000, 33, 2320.
(41) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc.
1995, 117, 6414.
(42) Gates, D. P.; Svejda, S. A.; Onate, E.; Killian, C. M.; Johnson, L.
̃
K.; White, P. S.; Brookhart, M. Macromolecules 2000, 33, 2320.
(43) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am.
Chem. Soc. 1998, 120, 888.
(44) Jenkins, J. C.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 5827.
(45) Schubbe, R.; Angermund, K.; Fink, G.; Goddard, R. Macromol.
Chem. Phys. 1995, 196, 467.
(46) Mecking reports both 1,2- and 2,1-insertions in α-olefin
homopolymerizations but indicates that this is in contrast with a report
by Fink et al., wherein they report only 1,2-insertions for a
nonsalicylaldiminato nickel system. Mecking suggests that the reason
for this difference is steric crowding of the Ni center disfavoring 2,1
insertions.
(47) Klabunde, U.; Mulhaupt, R.; Herskovitz, T.; Janowicz, A. H.;
Calabrese, J.; Ittel, S. D. J. Polym. Sci., Part A: Polym. Chem. 1987, 25,
1989.
(48) Radlauer, M. R.; Day, M. W.; Agapie, T. J. Am. Chem. Soc. 2012,
134, 1478−1481.
(49) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(50) Berliner, M. A.; Belecki, K. J. Org. Chem. 2005, 70, 9618.
(51) Kaschube, W.; Porschke, K. R.; Wilke, G. J. Organomet. Chem.
1988, 355, 525.
(52) Connor, E. F.; Younkin, T. R.; Henderson, J. I.; Waltman, A. W.;
Grubbs, R. H. Chem. Commun. 2003, 2272.
(53) Gorl, C.; Alt, H. G. J. Organomet. Chem. 2007, 692, 5727.
(54) Lodeiro, S.; Xiong, Q. B.; Wilson, W. K.; Ivanova, Y.; Smith, M.
L.; May, G. S.; Matsuda, S. P. T Org. Lett. 2009, 11, 1241.
(55) Agapie, T.; Bercaw, J. E. Organometallics 2007, 26, 2957.
(56) Akama, T.; Ishida, H.; Shida, Y.; Kimura, U.; Gomi, K.; Saito,
H.; Fuse, E.; Kobayashi, S.; Yoda, N.; Kasai, M. J. Med. Chem. 1997, 40,
1894.
2243
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