α-Diamine Nickel Catalysts with Nonplanar Chelate Rings for Ethylene Polymerization

Article abstract of DOI:10.1002/chem.201602467

A series of novel α-diamine nickel complexes, (ArNH-C(Me)-(Me)C-NHAr)NiBr₂, 1: Ar=2,6-diisopropylphenyl, 2: Ar=2,6-dimethylphenyl, 3: Ar=phenyl), have been synthesized and characterized. X-ray crystallographic analysis showed that the coordination geometry of the α-diamine nickel complexes is markedly different from conventional α-diimine nickel complexes, and that the chelate ring (N-C-C-N-Ni) of the α-diamine nickel complex is significantly distorted. The α-diamine nickel catalysts also display different steric effects on ethylene polymerization in comparison to the α-diimine nickel catalyst. Increasing the steric hindrance of the α-diamine ligand by substitution of the o-methyl groups with o-isopropyl groups leads to decreased polymerization activity and molecular weight; however, catalyst thermal stability is significantly enhanced. Living polymerizations of ethylene can be successfully achieved using 1/Et₂AlCl at 35 °C or 2/Et₂AlCl at 0 °C. The bulky α-diamine nickel catalyst 1 with isopropyl substituents can additionally be used to control the branching topology of the obtained polyethylene at the same level of branching density by tuning the reaction temperature and ethylene pressure.

Full text of DOI:10.1002/chem.201602467

DOI: 10.1002/chem.201602467
Full Paper
&
Homogeneous Catalysis
a-Diamine Nickel Catalysts with Nonplanar Chelate Rings for
Ethylene Polymerization
Heng Liao,[a, b] Liu Zhong,[b] Zefan Xiao,[b] Ting Zheng,[b] Haiyang Gao,*[a] and Qing Wu[a]
Abstract: A series of novel a-diamine nickel complexes,
(ArNH-C(Me)-(Me)C-NHAr)NiBr2, 1: Ar=2,6-diisopropylphenyl,
2: Ar=2,6-dimethylphenyl, 3: Ar=phenyl), have been syn-
thesized and characterized. X-ray crystallographic analysis
showed that the coordination geometry of the a-diamine
nickel complexes is markedly different from conventional a-
diimine nickel complexes, and that the chelate ring (N-C-C-
N-Ni) of the a-diamine nickel complex is significantly distort-
ed. The a-diamine nickel catalysts also display different
steric effects on ethylene polymerization in comparison to
the a-diimine nickel catalyst. Increasing the steric hindrance
of the a-diamine ligand by substitution of the o-methyl
groups with o-isopropyl groups leads to decreased polymeri-
zation activity and molecular weight; however, catalyst ther-
mal stability is significantly enhanced. Living polymerizations
of ethylene can be successfully achieved using 1/Et2AlCl at
358C or 2/Et2AlCl at 08C. The bulky a-diamine nickel catalyst
1 with isopropyl substituents can additionally be used to
control the branching topology of the obtained polyeth-
ylene at the same level of branching density by tuning the
reaction temperature and ethylene pressure.
Introduction
rings with respect to the coordination plane, this can signifi-
cantly slow down chain-transfer reactions by associative dis-
placement or chain transfer to bound monomer.[2] On the basis
of blocking the axial sites above and below the chelate plane,
late transition-metal catalysts with 5- or 6-membered planar
chelate rings have been extensively developed, such as a-di-
imine,[1–3] b-diketiminato,[4] anilido–imino,[5] salicylaldimino,[6]
and bis(imino)pyridine ligands.[7] Numerous studies have also
established that the aryl substituents closely effect the steric
environment of the nickel metal, thus greatly influencing poly-
merization reactivity and polymeric microstructure.[8] Despite
these contributions, none of the aforementioned studies have
elucidated how the steric effect operates for ethylene polymer-
ization if the chelate ring is nonplanar and largely distorted.
Indeed, planar chelating rings are usually formed by conju-
gating effects, and few nickel catalysts with largely distorted
chelate rings for olefin polymerization have been reported to
date.[1–6] Our previous investigations have addressed the role
that the N–Ni combination plays in the coordination geometry
of nickel and polymerization reactivity.[9] a-Diimine nickel cata-
lysts with two sp2 hybrid nitrogen donor atoms confer a coordi-
nation plane which is nonliving systems for ethylene polymeri-
zation.[2d,3a] However, hemilabile amine–imine nickel catalysts
with sp3 and sp2 hybrid nitrogen donor atoms adopt a slightly
distorted planar chelating ring system (Figure 1B), which can
polymerize ethylene in a living fashion at elevated tempera-
tures.[9a–d] This highlights the powerful influence of the steric
environment around the nickel metal originating from alterna-
tion of N–Ni combination on ethylene polymerization reac-
tivity. We can envision that an a-diamine nickel catalyst chelat-
ing two sp3 hybrid nitrogen donor atoms with tetrahedral con-
figuration possesses a largely distorted chelate ring (Fig-
Late transition-metal catalysts have received much attention
since Brookhart and co-workers discovered bulky a-diimine
nickel and palladium catalysts for olefin polymerization.[1]
A
key feature for producing high-molecular-weight polymers lies
in suppression of chain transfer by the steric bulk of the o-aryl
substituents (Figure 1A). The o-aryl substituents are located in
an axial position above and below the coordination plane; as
a consequence of the perpendicular arrangement of the aryl
Figure 1. Coordination geometries of nickel catalysts bearing a-diimine,
amine-imine, and a-diamine ligands.
[a] Dr. H. Liao, Prof. H. Gao, Prof. Q. Wu
School of Materials Science and Engineering, PCFM Laboratory
Sun Yat-sen University, Guangzhou 510275 (China)
E-mail: gaohy@mail.sysu.edu.cn
[b] Dr. H. Liao, Dr. L. Zhong, Dr. Z. Xiao, Dr. T. Zheng
School of Chemistry and Chemical Engineering, GD HPPC Laboratory
Sun Yat-sen University, Guangzhou 510275 (China)
Supporting information for this article can be found under
http://dx.doi.org/10.1002/chem.201602467.
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ure 1C), which can be used as a nickel catalyst model to study
the influences of chelate ring geometry and the N–Ni combina-
tion on ethylene polymerization. Herein, we report the success-
ful synthesis of novel a-diamine nickel complexes with five-
membered nonplanar chelate rings and their living characteris-
tics for ethylene polymerization. We also demonstrate that the
steric bulk of the o-aryl substituent of the a-diamine ligand
has a critical influence on the coordination and insertion of
ethylene, chain transfer, and chain walking, thereby greatly in-
fluencing the microstructure of the resulted polymer.
Figure 3. Molecular structure of ligand rac-L2. Hydrogen atoms are omitted
for clarity; ellipsoids are set at 30% probability.
Results and Discussion
The a-diamine ligands (ArNHÀC(Me)À(Me)CÀNHAr, L1: Ar=2,6-
diisopropylphenyl, L2: Ar=2,6-dimethylphenyl, L3: Ar=
phenyl) containing various o-aryl substituents were readily pre-
pared in high yield by a reduction reaction of the correspond-
ing a-diimine compound with excessive LiAlH4 (Scheme 1). All
Nickel complexes 1–3 were obtained by addition of the li-
gands to a suspension of (DME)NiBr2 in CH2Cl2 at room temper-
ature (Scheme 1). Single crystals of nickel complexes 1 and 2
suitable for X-ray diffraction analysis were obtained by slow
evaporation of the nickel complex solution in CH2Cl2. Complex
1 adopts a four-coordinate geometry for the nickel center
(Figure 4), whilst the dimerization of complex 2 gives a five-co-
ordinate geometry (Figure 5). Despite the same atom connec-
Scheme 1. Synthesis route of a-diamine nickel complexes.
of the reported ligands were fully characterized by 1H NMR,
13C NMR, ESI-MS, and elemental analysis. NMR analyses show
that bulky ligand L1 is a mixture of meso and rac diastereo-
mers with a ratio of 1:2.4 whilst ligands L2 and L3 exist as
a rac individual diastereomer (Supporting Information, Fig-
ures S1–S10). Two diastereomers of L1 were readily separated
by column chromatography on silica gel using n-hexane/di-
chloromethane (3:1) as the eluent, and were recrystallized
from ethanol as white solids. The rac diastereomer of ligand L1
was also found to be preferred 2:1 as determined by X-ray
crystallographic analysis of the major product (Figure 2), while
a single crystal of rac-L2 was merely obtained (Figure 3).
Figure 4. Crystal structure of a-diamine nickel complex 1. Hydrogen atoms
are omitted for clarity; ellipsoids are set at 30% probability.
Figure 5. Crystal structure of a-diamine nickel complex 2. Hydrogen atoms
and two CH2Cl2 molecules are omitted for clarity; ellipsoids are set at 30%
probability.
Figure 2. Molecular structure of ligand meso- and rac-L1. Hydrogen atoms
are omitted for clarity; ellipsoids are set at 30% probability.
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tivity, a-diamine nickel complex 1 showed different bond
lengths to the a-diimine nickel analogue. The observed NÀNi
(2.06 ꢁ) and CÀN (1.51 ꢁ) bond lengths of a-diamine nickel
1 are longer than a-diimine nickel 4 ((ArN=C(Me)À(Me)C=NAr)-
NiBr2, Ar=2,6-diisopropylphenyl; NÀNi 2.00 ꢁ, CÀN 1.28 ꢁ).[10]
The longer NiÀN distance presumably causes a reduction in
the ethylene polymerization activity of the nickel catalyst by
using a-diamine instead of a-diimine ligand.[9] Because the two
sp3 hybrid carbon atoms adopt tetrahedral configurations, the
chelated ring (N-C-C-N-Ni) is significantly distorted. The two
methyl groups on the backbone and the aniline moieties
swing to two opposite sides, showing axially steric effects on
the nickel metal. Besides, the bite angle of N-Ni-N of a-diamine
nickel complex 1 is 82.078, which is larger than those of
amine–imine nickel (81.208) and a-diimine nickel 4 (80.588).
This observation strongly suggests that there is an increased
steric environment, including both axial and equatorial sites
around the Ni metal, by changing the N–Ni combination from
N(imine)–Ni to N(amine)–Ni. In contrast to the a-diimine nickel
analogue, the a-diamine nickel complex 1 has a nonplanar
chelate ring, twisted aniline moieties, and a large angle of N-
Ni-N, which may cause distinctive steric effects on ethylene
polymerization.
was firstly investigated. Polymerization results in Table 1 (en-
tries 2, 3, 4 versus 9, 10, 11) show that reducing the steric hin-
drance of the a-diamine ligand by substituting the o-isopropyl
groups with the o-methyl groups results in an increased poly-
merization activity. Catalyst 1 with isopropyl groups showed
the highest activity of 25.9 kgPE(molNi)À1 hÀ1 at 358C, whereas
catalyst 2 with methyl groups exhibited about 7 times the ac-
tivity at the optimized temperature of 08C (177.0 kgPE(mol
Ni)À1 hÀ1). This steric effect of o-aryl substituents on ethylene
polymerization activity conflicts with previous observations
made using a-diimine nickel catalysts.[2d] Generally, a-diamine
nickel catalysts are less active than a-diimine nickel catalysts
and afford lower molecular weight PE,[2d,9c] partly because the
a-diimine ligand is a stronger p-acceptor (planar chelate ring)
than an a-diamine ligand (largely distorded chelate ring).[9g]
A different steric effect of the a-diamine nickel catalyst on
polymer molecular weight is also observed, and a circa sixfold
increase in polymer molecular weight can be observed by sub-
stitution of o-methyl groups for o-isopropyl groups. Generally,
reducing steric hindrance can decrease the blocking of the
axial position for a-diimine and amine–imine nickel catalysts
with a planar chelate ring, thus leading to a drop in molecular
weight because of the acceleration of chain transfer. Herein,
both high catalyst activity and high polymer molecular weight
can be successfully achieved by the a-diamine nickel catalyst 2
with o-methyl groups on the aryl substituents. This significant
difference in steric effect is closely related to the steric environ-
ment around the nickel atom. Because the a-diamine nickel
catalyst has a nonplanar chelate ring, the bulky isopropyl
groups may block equatorial coordination sites, thus slowing
down the trapping of ethylene. Higher molecular weight poly-
mer obtained by the less bulky a-diamine nickel catalyst 2 can
be attributed to a rapid ethylene insertion rate and axial steric
effect of the two methyl groups on sp3 carbon atoms (see the
X-ray part of the Experimental Section) for suppressing chain
transfer. A lack of the substitutents on N-aryl moiety decreases
steric hindrance and leads to a remarkable drop in activity and
molecular weight of the obtained polyethylene (PE), whereas
molecular weight of the obtained PE is much higher than that
of the PE produced by a-diimine nickel analogue (ArN=C(Me)-
(Me)C=NAr)NiBr2, Ar=phenyl (C4 and C6 oligomers).[11]
Ethylene polymerizations were carried out using nickel com-
plexes 1, 2, and 3 activated by Et2AlCl at various polymeri-
zation temperatures (Table 1). Steric effect of o-aryl substitu-
ents on polymerization activity and polymer molecular weight
Table 1. Ethylene polymerization results using a-diamine nickel catalysts
1–3/Et2AlCl.[a]
[c]
[e]
[e]
Entry Ni
P
T
Yield Act.[b] Mn
[psig] [8C] [g]
PDI[c] Br[d] Tm
Tg
[8C]
[8C]
[h]
1
2
3
4
5
6
7
8
1
1
1
1
1
1
2
2
2
2
2
3
3
3
3
1
1
2
2
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
À20 0.076
7.6 11.1 1.02 140
0.133 13.3 20.1 1.01 148
–
À66
À66
À65
À67
À66
À69
–
0
–
–
–
–
–
20 0.207 20.7 29.9 1.02 139
35 0.259 25.9 37.6 1.05 146
50 0.200 20.0 24.8 1.29 137
65 0.075
7.5
8.3 1.85 136
À40 0.074
7.4 17.5 1.12
20 128
44 102
À20 0.623 62.3 128.2 1.13
–
9
0
1.770 177.0 218.7 1.07 105 À3
À56
À65
À68
–
–
–
The polymerization results clearly demonstrate that steric ef-
fects influence the thermal stability of a-diamine nickel cata-
lyst. Increasing steric hindrance of o-aryl substituents signifi-
cantly enhances the thermal stability of the a-diamine nickel
catalysts. Catalyst 1 is active in a wide range of temperatures
from À20 to 658C, while catalyst 3 only displays apparent ac-
tivity below 08C. Moreover, a huge drop of 96% activity is ob-
served for catalyst 2 when the temperature is increased from 0
to 358C, whilst an increased activity is observed for catalyst 1.
This observation can be attributed to the protection and stabi-
lization of the nickel metal center provided by bulky substitu-
ents. It is known that the classic a-diimine nickel catalyst 4 un-
dergoes catalyst decomposition above 508C whereas a-di-
amine nickel catalyst 1 was found to maintain good catalytic
activity at 508C although the coordinating ability of the imine
is stronger than the amine group.[2d] A possible interpretation
10
11
12
13
14
15
16[f]
17[f]
18[g]
19[g]
20 0.373 37.3 76.1 1.63 137
35 0.074
–
–
7.4 17.7 2.01 144
3.4 15.5 1.74
À40 0.034
19 129
À20 0.122 12.2 11.5 1.94
39 107, 87
87 À8
À10 0.228 22.8
0.180 18.0
5.6 1.61
0
8.3 1.94 106 À31
–
75
150
75
150
35 0.532
35 0.762
213 212.2 1.01 121
305 298.0 1.02 119
867 246.5 1.03
–
–
À63
À60
–
0
0
0.289
62 87
32 111
0.493 1479 427.4 1.04
–
[a] Polymerization conditions: 10 mmol of nickel, Al/Ni=200, 60 min,
29 mL toluene and 1 mL CH2Cl2. [b] Activity: kg PE (mol Ni)À1 hÀ1. [c] Mn
[kgmolÀ1] and PDI were determined by gel permeation chromatography
(GPC) in 1,2,4-trichlorobenzene at 1508C using a light-scattering detector.
[d] Branching density; branches per 1000 carbon atoms determined by
1H NMR spectroscopy. [e] Determined by DSC. [f] 5 mmol of nickel,
30 min, 59 mL toluene. [g] 2 mmol of nickel, 10 min, 59 mL toluene.
[h] Not determined.
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is that CÀH activation occurs by forming an equatorial co-
plane between the nickel center and the isopropyl substituents
because of NÀC(aryl) rotations for a-diimine nickel catalyst,
whereas, in case of a-diamine nickel catalyst 1 with two twist-
ed aniline moieties, it hardly proceeds because of the abstrac-
tion of an equatorial co-plane.[2d,3c,d,8b]
to the steric effect of o-aryl substituents. The same trend is ob-
served in a-diimine nickel catalysts in that increasing steric hin-
drance leads to an increase in branching density, especially at
low temperatures.[2d] Differently, steric effects of o-aryl substitu-
ents have a dramatic influence on dependence of branching
density on polymerization conditions such as temperature and
ethylene pressure. The branching density of PE obtained by
catalyst 2 with methyl substituents increases with increased
temperature and reduced ethylene pressure (Figure 7), which
It is noteworthy that narrowly dispersed polyethylene (PDI
<1.1) can be formed using catalyst 1 below 358C or catalyst 2
below 08C, suggesting the reaction occurs in a living polymeri-
zation fashion. Figure 6 shows symmetric GPC traces of the
polymers obtained at different polymerization times, which
Figure 7. The dependency of branching density of PE obtained by 1 and 2/
Et2AlCl on polymerization temperature and ethylene pressure.
is similar to previous observations regarding a-diimine nickel
and amine–imine nickel catalysts.[2d,9c] However, the bulky a-di-
amine catalyst 1 with isopropyl groups shows a similar obser-
vation to the a-diimine palladium catalyst in that the branch-
ing density of the PE is nearly independent to polymerization
temperature and ethylene pressure (Figure 7). Even at very low
polymerization temperatures (À40, À608C), the obtained PEs
have high branching densities of about 140/1000C. Despite
the mild change of total branching density within the tested
ethylene pressure and temperature range for 1/Et2AlCl, the ap-
parent morphology of the obtained PE changes from viscous
oil at 3 psig to a white and elastic solid at 150 psig (Figure 8).
~
&
Figure 6. Plots of Mn ( ) and Mw/Mn (PDI) ( ) as a function of polymerization
time using 1/Et2AlCl at 358C (top) and 2/Et2AlCl at 08C (down). Polymeri-
zation conditions: 3 psig, 10 mmol 1, and 2 mmol 2, Al/Ni=200). Right: GPC
traces at different times using a light scattering detector.
shift to the higher molecular weight region with prolonged
polymerization time. Plots of number-average molecular
weights (Mn) as a function of polymerization time also illustrate
that Mn grows linearly with the polymerization time, and Mw/
Mn (PDI) values are below 1.10. Undoubtedly, the living poly-
merization of ethylene can be successfully achieved using 1/
Et2AlCl at 358C or 2/Et2AlCl at 08C. MMAO compound can also
replace Et2AlCl as the activator for the living polymerization of
ethylene using 1 at 358C, with polyethylene of a higher molec-
ular weight being formed (Supporting Information, Table S9
and Figure S28). This kind of a-diamine nickel catalyst is one
example of a rare late transition-metal catalytic system for the
living polymerization of ethylene,[9a–d,12] whereas a-diimine
nickel catalyst 4 is known to polymerize ethylene in a nonliving
fashion.[2d,3a]
Chain walking is one of the most distinguishing features for
nickel and palladium catalysts and is responsible for the gener-
ation of branched polyethylene.[1a,b] 1H NMR analysis showed
that the branching density of the obtained PE is closely related
Figure 8. The dependency of intrinsic viscosity on molecular weight of the
PEs obtained by 1/Et2AlCl at various ethylene pressures. Inset: Images of PE
samples.
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Branching distributions of the obtained PEs determined by
13C NMR spectroscopy also revealed that the contents of long
branches and the branch-on-branch structures generated
through tertiary carbons (sec-butyl group; 19.44 and
11.72 ppm) increased gradually with reducing ethylene pres-
sure and increasing temperature (Supporting Information,
Table S6), suggesting a change of branching topology with var-
iation in temperature and pressure.
sure and temperature, which is similar to the a-diimine palladi-
um catalyst.[14] The resting state of the less bulky catalyst 2 can
predominantly be the b-agostic complex (B), and branching
density thereby decreases with increasing pressure and reduc-
ing temperature.[14] Various branches originate from the chain-
walking process involving b-hydride elimination and reinser-
tion. Branching density and topology is a result of the compe-
tition between ethylene insertion and chain walking. For cata-
lyst 1, the trapping of ethylene is slow and the catalyst may
walk multiple carbons before being trapped, leading to the
formation of a hyperbranched topology, especially at low eth-
ylene pressure. The ethylene polymerization results using 1/
Et2AlCl over the range of Al/Ni ratio from 100 to 800 (Support-
ing Information, Table S10) show that there is no substantial
change in catalytic activity, molecular weight, and polydispers-
ity of the obtained polymers, strongly indicating no occurrence
of chain transfer to the aluminum co-catalyst. Therefore, chain-
transfer reactions should occur though associative displace-
ment or chain transfer to bound monomer at high tempera-
tures.[1,2]
Dilute solution properties also enable us to examine the
branching topology of the PE obtained at different ethylene
pressures. The dependency of intrinsic viscosity on polymer
molecular weight detected by high-temperature gel perme-
ation chromatography with a viscosity detector can directly re-
flect the branching topology of the PE. As shown in Figure 8,
the intrinsic viscosity of the obtained PE increases with increas-
ing polymerization pressure under the same molecular weight.
These results strongly indicate that the PE topology changes,
and linear PE with moderate branching is formed at a high
pressure of 150 psig, whereas hyperbranched PE (HBPE) is pro-
duced at a low pressure of 3 psig using the 1/Et2AlCl system.
Generally, the bulky nickel catalyst 1 showed a different chain-
walking feature to the reported nickel catalyst in that it can
tune the branching topology while maintaining the same level
of branching density. In contrast to previous work on control-
ling the branching topology of PE by noble a-diimine palladi-
um catalysts, the nickel-based catalyst 1 is cheaper, and the
difficulty in separation and purification of hyperbranched PE is
significantly decreased because palladium black formation can
be avoided.[6,7]
Conclusions
In summary, we have successfully developed conveniently ac-
cessible novel a-diamine nickel catalysts. Reducing the steric
hindrance of the a-diamine ligand by substituting the o-iso-
propyl groups with the o-methyl groups results in increased
polymerization activity and molecular weight at the cost of
thermal stability. Living polymerization of ethylene can be
achieved using a-diamine nickel catalysts 1 or 2 by the sup-
pression of chain-transfer reactions. The bulky a-diamine nickel
catalyst 1 with isopropyl substituents can also control the
branching topology of PE at the same level of branching densi-
ty by tuning reaction temperature and ethylene pressure. This
kind of a-diamine nickel catalyst is a promising candidate for
a deep and fundamental understanding of late transition-
metal-catalyzed olefin polymerizations from a coordination ge-
ometry point of view. Further optimization of a-diamine frame-
work and the a-olefin polymerization reactions are ongoing.
Considering the fact that the polymerization activity of cata-
lyst 1 is nearly insensitive to ethylene pressure but that of cata-
lyst 2 is significantly sensitive, a general mechanism for ethyl-
ene polymerization with a-diamine nickel catalysts is proposed
in combination with chain walking for branch formation
(Scheme 2).[13] Cationic nickel species for ethylene polymeri-
zation are generated by the activation of nickel dihalide com-
plexes with alkylaluminum compounds. On the basis of single-
crystal X-ray diffraction analysis, a-diamine nickel catalysts
adopt a distorted ring system because of the two sp3 hybrid
nitrogen donor atoms. The resting state of bulky catalyst 1 is
solely an alkyl olefin species A because bulky isopropyl groups
may suppress the formation of the b-agostic complex (B).
Therefore, branching density is independent to ethylene pres-
Experimental Section
All manipulations involving air- and moisture sensitive
compounds were carried out under an atmosphere of
dried and purified nitrogen using standard vacuum-line,
Schlenk, or glovebox techniques. Dichloromethane was
dried over phosphorus pentoxide for 8 h, and distilled
under nitrogen atmosphere. Toluene, hexane, and tetra-
hydrofuran (THF) were refluxed over from Na/K alloy.
Butane-2,3-dione, aniline, 2,6-dimethylaniline, 2,6-diiso-
propylaniline, and (DME)NiBr2 were purchased from Al-
drich and used as received. MMAO (7 wt% Al in hep-
tane) was purchased from Akzo-Nobel and used as re-
ceived. Diethylaluminium chloride (Et2AlCl, 1.0m in
hexane) and lithium aluminum hydride (LiAlH4, 95%,
powder) were purchased from Acros. Ethylene (99.99%)
Scheme 2. Proposed mechanism for a-diamine nickel-catalyzed ethylene polymerization.
Chem. Eur. J. 2016, 22, 1 – 9 www.chemeurj.org
was purified by passing through Agilent moisture and
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oxygen traps. Other commercially available reagents were pur-
chased and used without purification.
with methanol several times, and dried under vacuum at 408C to
a constant weight.
High-pressure polymerization of ethylene
Measurements
A mechanically stirred 100 mL Parr reactor was heated to 1508C
for 2 h under vacuum and then cooled to room temperature. The
autoclave was pressurized to 100 psig of ethylene and vented
three times. The autoclave was then charged with solution of
Et2AlCl in toluene under 3 psig of ethylene at initialization temper-
ature. The system was maintained by continuously stirring for
5 min, and then a 1 mL solution of the nickel complex in CH2Cl2
was charged into the autoclave under 3 psig of ethylene. The eth-
ylene pressure was raised to the specified value. The reaction tem-
perature was controlled by means of a water bath and found to be
Æ28C as monitored by an internal thermocouple. The reaction was
carried out for a certain time. Polymerization was terminated by
addition of acidic methanol after releasing ethylene pressure. The
resulting precipitated polymers were collected and treated by fil-
tration, washed with methanol several times, and dried under
vacuum at 408C to a constant weight.
Elemental analyses were performed on a Vario EL microanalyzer.
Mass spectra were obtained using electrospray ionization (ESI)
LCMS-2010 A (for ligands) or fast atom bombardment (FAB) LCQ
DECA XP (for nickel complexes). NMR spectra of organic com-
pounds were carried out on a Bruker 400 MHz instrument in CDCl3
using TMS as a reference. 13C NMR spectra of polymers were car-
ried out on a Bruker 500 MHz at 1208C. Sample solutions of the
polymer were prepared in o-C6H4Cl2/o-C6D4Cl2 (50% v/v) in
a 10 mm sample tube. The spectra of the quantitative 13C NMR
were taken with a 748 flip angle, an acquisition time of 1.5 s, and
a delay of 4.0 s. The chemical shifts were referenced to main chain
of PE (À(CH2)nÀ) (30 ppm). DSC analyses were conducted with a Per-
kinElmer DSC-7 system. The DSC curves were recorded as second
heating curves from À100 to 1408C at
a heating rate of
108CminÀ1 and a cooling rate of 108CminÀ1. GPC analysis of the
molecular weights and molecular weight distributions (PDI=Mw/
Mn) of the polymers at 1508C were performed on a PL-GPC 220
high-temperature chromatograph equipped with a triple-detection
array, including a differential refractive-index detector, a two-angle
light-scattering detector, and a four-bridge capillary viscometer.
The detection angles of the LS detector were 15 and 908, and the
laser wavelength was 658 nm. 1,2,4-Trichlorobenzene (TCB) was
Synthesis of a-diimine compounds
a-Diimine compounds were prepared according to the reported
procedure. A solution of substituted anilines (2 equiv) in ethanol in
the presence of formic acid was added to a solution of butane-2,3-
dione (1.72 g, 20 mmol) in ethanol (50 mL). The solution was al-
lowed to stir for 5 h at 508C, and cooled to À208C overnight. The
yellow crystalline solids were collected by filtration and washed
with cold ethanol to provide the isolated product in good yield.
used as the eluent at a flow rate of 1.0 mLminÀ1
.
Crystal structure determination
The crystals were mounted on a glass fiber and transferred to
a Bruker CCD platform diffractometer. Data obtained with the w-2q
scan mode was collected on a Bruker SMART 1000 CCD diffractom-
eter with graphite-monochromated Cu or Mo. The structures were
solved using direct methods, while further refinement with full-
matrix least square on F2 was obtained with the SHELXTL program
package. All non-hydrogen atoms were refined anisotropically. Hy-
drogen atoms were introduced in calculated positions with the dis-
placement factors of the host carbon atoms. Crystal data and struc-
tural refinement for the four structures are collated in Table S1.
CCDC 1453215 (1), 1453216 (2), 1453217 (L1), and 1453218 (L2)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
Synthesis of a-diamine ligands
ArÀNHÀC(Me)À(Me)CÀNHÀAr (Ar=2,6-diisopropylphenyl) (L1):
A solution of a-diimine compound (ArÀN=C(Me)À(Me)C=NÀAr
(Ar=2,6-diisopropylphenyl)) (0.81 g, 2 mmol) in THF (30 mL) was
slowly added to a suspension of LiAlH4 (0.31 g, 8.2 mmol) in THF
(40 mL) under a nitrogen atmosphere over a period of 5 min at
08C. The mixture was then heated to 508C overnight. After cooling
to 08C using ice/water bath, the reaction mixture was carefully hy-
drolyzed with a 10% aqueous NaOH solution. The organic layer
was separated and washed with water three times, and dried over
MgSO4. The solvent was then evaporated off, and the resulting
pale yellow oil was dissolved in hot ethanol and the solution
cooled to room temperature to give a white solid. Yield: 0.72 g,
88%. Diastereomers were separated by column chromatography
on silica gel using n-hexane/dichloromethane (75:25) as the eluent,
and were recrystallized from ethanol as white solids with a ratio of
2.4:1 determined by 1H NMR spectroscopy. 1H NMR (CDCl3,
400 MHz): meso-diastereomer: d=7.10 (d, 4H, Ar-H), 7.03 (t, 2H,
Ar-H), 3.60 (s, 2H, NH), 3.52–3.45 (m, 2H, CH(CH3)), 3.40 (sep, 4H,
CH(CH3)2), 1.22 (dd, 24H, CH(CH3)2), 1.01 ppm (d, 6H, CH(CH3)); rac-
diastereomer: 7.10–6.99 (m, 6H, Ar-H), 3.37–3.17 (m, 8H, NH+
CH(CH3)2 +NCH (CH)(CH3)), 1.19 (dd, 24H, CH(CH3)2), 1.11 ppm (dd,
6H, CH(CH3)); 13C NMR (100 MHz, CDCl3): rac-diastereomer: d=
142.63, 141.35, 123.48, 123.35, 59.66, 27.78, 24.35, 23.92,
15.68 ppm; meso-diastereomer: 141.80, 141.64, 123.49, 122.73,
58.86, 27.72, 24.18, 23.91, 15.50 ppm; ESI-MS (m/z): 411, 410, 409
[M++1]; elemental analysis calcd for C28H44N2: C 82.99, H 10.85,
N 6.85; found: C 82.75, H 11.10, N 6.88.
Atmospheric-pressure ethylene polymerization
A round-bottom Schlenk flask with stirring bar was heated for 3 h
at 1508C under vacuum and then cooled to room temperature.
The flask was pressurized to 15 psig of ethylene and vented three
times. The appropriate alkyl aluminum compound as co-catalyst
and toluene were added into the glass reactor under 3 psig of eth-
ylene. The system was continuously stirred for 5 min, and then tol-
uene and 1 mL of a solution of nickel complex in CH2Cl2 were
added sequentially by syringe to the well-stirred solution, and the
total reaction volume was kept at 30 mL. The ethylene pressure
was kept constant at 3 psig by the continuous feeding of gaseous
ethylene throughout the reaction. The other reaction temperatures
were controlled with an external oil bath or a cooler in polymeri-
zation experiments. The polymerizations were terminated by the
addition of 200 mL of acidic methanol (95:5 ethanol/HCl) after con-
tinuously stirring for an appropriate period. The resulting precipi-
tated polymers were collected and treated by filtration, washed
ArÀNHÀC(Me)À(Me)CÀNHÀAr (Ar=2,6-dimethylphenyl) (L2):
Following the above procedure, ligand L2 ArÀNHÀC(Me)À(Me)CÀ
NHÀAr (Ar=2,6-dimethylphenyl) was isolated as white solid in
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1
91% yield. H NMR (CDCl3, 400 MHz): d=6.99 (d, 4H, Ar-H), 6.80 (t,
Keywords: branching topology · homogeneous catalysis ·
living polymerization · N ligands · nickel
2H, Ar-H), 3.63 (s, 4H, NH, CHCH3), 2.30 (s, 12H, CH3), 1.02 ppm (d,
6H, CHCH3). 13C NMR (100 MHz, CDCl3): d=144.61, 129.06, 129.01,
121.22, 55.51, 19.40, 16.10 ppm; ESI-MS (m/z): 299, 298, 297 [M+
+1]; elemental analysis calcd for C20H28N2: C 81.03, H 9.52, N 9.45;
found: C 81.23, H 9.56, N 9.53.
[1] a) L. K. Johnson, C. M. Killian, M. Brookhart, J. Am. Chem. Soc. 1995, 117,
6414–6415; b) L. K. Johnson, S. Mecking, M. Brookhart, J. Am. Chem.
Soc. 1996, 118, 267–268; c) S. D. Ittel, L. K. Johnson, M. Brookhart,
Chem. Rev. 2000, 100, 1169–1203; d) S. Mecking, Angew. Chem. Int. Ed.
2001, 40, 534–540; Angew. Chem. 2001, 113, 550–557; e) G. W. Coates,
P. D. Hustad, S. Reinartz, Angew. Chem. Int. Ed. 2002, 41, 2236–2257;
Angew. Chem. 2002, 114, 2340–2361; f) E. Y. X. Chen, T. J. Marks, Chem.
Rev. 2000, 100, 1391–1434; g) T. M. J. Anselment, S. I. Vagin, B. Rieger,
Dalton Trans. 2008, 4537–4548; h) J. D. Azoulay, R. S. Rojas, A. V. Serra-
no, H. Ohtaki, G. B. Galland, G. C. Bazan, Angew. Chem. Int. Ed. 2009, 48,
1089–1092; Angew. Chem. 2009, 121, 1109–1112; i) C. Chen, S. Luo, R. F.
Jordan, J. Am. Chem. Soc. 2010, 132, 5273–5284.
ArÀNHÀC(Me)À(Me)CÀNHÀAr (Ar=phenyl) (L3): Following the
above procedure, ligand L3 ArÀNÀC(Me)À(Me)CÀNÀAr (Ar=
1
phenyl) was isolated as colorless oil in 93% yield. H NMR (CDCl3,
400 MHz): d=7.20(m, 4H, Ar-H), 6.73 (m, 6H, Ar-H), 3.74 (s, 2H,
NH), 3.63 (m, 2H, -CH-CH3), 1.22 ppm (d, 6H, CH3); 13C NMR
(100 MHz, CDCl3): d=147.7, 129.5, 129.4, 117.7, 117.6, 113.8, 113.6,
52.6, 52.3, 17.1, 16.35 ppm; elemental analysis calcd for C16H20N2:
C 79.96, H 8.39, N 11.66; found: C 79.92, H 8.37, N 11.63; ESI-MS
(m/z): 243, 242, 241 [M++1].
[2] a) D. H. Camachoa, Z. Guan, Chem. Commun. 2010, 46, 7879–7893; b) S.
Mecking, L. K. Johnson, L. Wang, M. Brookhart, J. Am. Chem. Soc. 1998,
120, 888–899; c) S. A. Svejda, L. K. Johnson, M. Brookhart, J. Am. Chem.
Soc. 1999, 121, 10634–10635; d) D. P. Gates, S. A. Svejda, E. Onate, C. M.
Killian, L. K. Johnson, P. S. White, M. Brookhart, Macromolecules 2000,
33, 2320–2334.
[3] a) F. Liu, H. Hu, Y. Xu, L. Guo, S. Zai, K. Song, H. Gao, L. Zhang, F. Zhu, Q.
Wu, Macromolecules 2009, 42, 7789–7796; b) L. Guo, H. Gao, Q. Guan,
H. Hu, J. Deng, J. Liu, F. Liu, Q. Wu, Organometallics 2012, 31, 6054–
6062; c) J. L. Rhinehart, L. A. Brown, B. K. Long, J. Am. Chem. Soc. 2013,
135, 16316–16319; d) J. L. Rhinehart, N. E. Mitchell, B. K. Long, ACS
Catal. 2014, 4, 2501–2504; e) S. Dai, X. Sui, C. Chen, Angew. Chem. Int.
Ed. 2015, 54, 9948–9953; Angew. Chem. 2015, 127, 10086–10091; f) L.
Guo, S. Dai, X. Sui, C. Chen, ACS Catal. 2016, 6, 428–441.
[4] a) N. A. Eckert, E. M. Bones, R. J. Lachicotte, P. L. Holland, Inorg. Chem.
2003, 42, 1720–1725; b) J. Zhang, H. Gao, Z. Ke, F. Bao, F. Zhu, Q. Wu, J.
Mol. Catal. A 2005, 231, 27–34.
[5] a) H. Gao, W. Guo, F. Bao, G. Gui, J. Zhang, F. Zhu, Q. Wu, Organometal-
lics 2004, 23, 6273–6280; b) H. Gao, J. Zhang, Y. Chen, F. Zhu, Q. Wu, J.
Mol. Catal. A 2005, 240, 178–185; c) H. Gao, Y. Chen, F. Zhu, Q. Wu, J.
Polym. Sci. Part A 2006, 44, 5237–5246; d) H. Gao, Z. Ke, L. Pei, K. Song,
Q. Wu, Polymer 2007, 48, 7249–7254; e) H. Gao, L. Pei, K. Song, Q. Wu,
Eur. Polym. J. 2007, 43, 908–914.
[6] a) T. R. Younkin, E. F. Connor, J. I. Henderson, S. K. Friedrich, R. H. Grubbs,
D. A. Bansleben, Sciences 2000, 287, 460–462; b) C. Wang, S. Friedrish,
T. R. Younkin, R. H. Grubbs, Organometallics 1998, 17, 3149–3151.
[7] a) B. L. Small, M. Brookhart, A. M. A. Bennett, J. Am. Chem. Soc. 1998,
120, 4049–4050; b) G. J. P. Britovsek, V. C. Gibson, B. S. Kimberley, P. J.
Maddox, S. J. McTavish, G. A. Solan, A. J. P. White, D. J. Williams, Chem.
Commun. 1998, 849–850; c) G. J. P. Britovsek, M. Bruce, V. C. Gibson,
B. S. Kimberley, P. J. Maddox, S. Mastroianni, S. J. McTavish, C. Redshaw,
G. A. Solan, S. Stromberg, A. J. P. White, D. J. Williams, J. Am. Chem. Soc.
1999, 121, 8728–8740; d) L. Guo, H. Gao, L. Zhang, F. Zhu, Q. Wu, Orga-
nometallics 2010, 29, 2118–2125; e) Z. Flisak, W. Sun, ACS Catal. 2015,
5, 4713–4724.
[8] a) D. Meinhard, M. Wegner, G. Kipiani, A. Hearley, P. Reuter, S. Fischer, O.
Marti, B. Rieger, J. Am. Chem. Soc. 2007, 129, 9182–9191; b) D. H. Cama-
cho, E. V. Salo, J. W. Ziller, Z. Guan, Angew. Chem. Int. Ed. 2004, 43,
1821–1825; Angew. Chem. 2004, 116, 1857–1861; c) D. H. Camacho, Z.
Guan, Macromolecules 2005, 38, 2544–2546; d) H. Zou, F. M. Zhu, Q.
Wu, J. Y. Ai, S. A. Lin, J. Polym. Sci. Part A 2005, 43, 1325–1330.
[9] a) S. Zai, F. Liu, H. Gao, C. Li, G. Zhou, L. Guo, L. Zhang, F. Zhu, Q. Wu,
Chem. Commun. 2010, 46, 4321–4323; b) S. Zai, H. Gao, Z. Huang, H.
Hu, H. Wu, Q. Wu, ACS Catal. 2012, 2, 433–440; c) H. Gao, H. Hu, F. Zhu,
Q. Wu, Chem. Commun. 2012, 48, 3312–3314; d) H. Hu, L. Zhang, H.
Gao, F. Zhu, Q. Wu, Chem. Eur. J. 2014, 20, 3225–3233; e) H. Gao, Y. Liu,
G. Li, Z. Xiao, G. Liang, Q. Wu, Polym. Chem. 2014, 5, 6012–6018; f) H.
Hu, H. Gao, D. Chen, G. Li, Y. Tan, G. Liang, F. Zhu, Q. Wu, ACS Catal.
2015, 5, 122–128; g) H. Hu, D. Chen, H. Gao, L. Zhong, Q. Wu, Polym.
Chem. 2016, 7, 529–537.
Synthesis of a-diamine nickel complexes
[ArÀNHÀC(Me)À(Me)CÀNHÀAr]NiBr2 (Ar=2,6-diisopropylphen-
yl) (1): A solution of ligand L1 (420 mg, 1.03 mmol) in 10 mL di-
chloromethane was added to a stirred suspension of (DME)NiBr2
(DME=1,2-dimethoxyethane; 308 mg, 1.00 mmol) in dichlorome-
thane (30 mL) under a nitrogen atmosphere at room temperature.
After the addition of the ligand, the solution began to turn dark
red. After stirring for 6 h, the solution was filtered through celite,
and the solvent of the filtrate was removed in vacuum. The residue
was recrystallized from dichloromethane–hexane to give a purple-
red powder in 63% yield (393 mg). Single crystals were grown
from a dichloromethane solution at room temperature in glove-
box. FAB-MS (m/z): 548, 547, 546 [MÀBr]+; 468, 467, 466 [MÀ2Br]+;
410, 409 [ligand]+; elemental analysis calcd for C28H44Br2N2Ni:
C 53.62, H 7.07, N 4.47; found: C 53.49, H 7.01, N 4.42.
[ArÀNHÀC(Me)À(Me)CÀNHÀAr]NiBr2(Ar=2,6-dimethylphenyl)
(2): Following the above procedure, the reaction of (DME)NiBr2
(308 mg, 1.00 mmol) and ligand L2 (305 mg, 1.03 mmol) gave
a yellow powder in 65% yield (333 mg). Single crystals were grown
from a dichloromethane solution at room temperature in a glove-
box. FAB-MS (m/z): 435, 434, 433 [MÀBr]+; 355, 354, 353 [MÀ2Br]+;
297, 296, 295 [ligand]+; elemental analysis calcd for C20H28Br2N2Ni:
C 46.65, H 5.48, N 5.44; found: C 46.51, H 5.41, N 5.38.
[ArÀNHÀC(Me)À(Me)CÀNHÀAr]NiBr2 (Ar=phenyl) (3): Following
the above procedure, the reaction of (DME)NiBr2 (308 mg,
1.00 mmol) and ligand ArÀNÀC(Me)À(Me)CÀNÀAr (Ar=phenyl;
250 mg, 1.04 mmol) gave a red powder in 69% yield (312 mg).
FAB-MS (m/z): 379, 378, 376 [MÀBr]+; 300, 299, 298[MÀ2Br]+; 242,
241[ligand]+; elemental analysis calcd for C16H20Br2N2Ni: C 41.88,
H 4.39, N 6.11; found: C 41.83, H 4.36, N 6.08.
Acknowledgements
This work was supported by grants from the National Natural
Science Foundation of China (NSFC; Projects 21374134 and
21274167), Natural Science Foundation of Guangdong Province
(1414050000552), Science and Technology Planning Project of
Guangdong Province (201510010070), Technology Innovation
Project of Educational Commission of Guangdong Province of
China (2013KJCX0002), the Fundamental Research Funds for
the Central Universities (15lgzd03), and CNPC Innovation Foun-
dation (2014D-5006-0502).
[10] S. B. Clara, P. T. Gomes, M. Gomes, T. Duarte, J. Organomet. Chem. 2014,
760, 101–107.
[11] C. M. Killian, L. K. Johnson, M. Brookhart, Organometallics 1997, 16,
2005–2007.
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[12] a) A. C. Gottfried, M. Brookhart, Macromolecules 2001, 34, 1140–1142;
b) A. C. Gottfried, M. Brookhart, Macromolecules 2003, 36, 3085–3100;
c) K. Zhang, Z. Ye, R. Subramanian, Macromolecules 2008, 41, 640–649;
d) Y. Zhang, Z. Ye, Chem. Commun. 2008, 1178–1180; e) J. D. Azoulay, Y.
Schneider, G. B. Galland, G. C. Bazan, Chem. Commun. 2009, 6177–6179.
[13] W. Sun, Adv. Polym. Sci. 2013, 258, 163–178.
[14] a) D. J. Tempel, L. K. Johnson, R. L. Huff, P. S. White, M. Brookhart, J. Am.
Chem. Soc. 2000, 122, 6686–6700; b) D. Zhang, E. T. Nadres, M. Brook-
hart, O. Daugulis, Organometallics 2013, 32, 5136–5143.
Received: May 24, 2016
Revised: July 12, 2016
Published online on && &&, 0000
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Homogeneous Catalysis
H. Liao, L. Zhong, Z. Xiao, T. Zheng,
H. Gao,* Q. Wu
&& – &&
a-Diamine Nickel Catalysts with
Nonplanar Chelate Rings for Ethylene
Polymerization
Novel a-diamine nickel complexes (see
picture; Ni green, N blue, Br red) with
nonplanar chelate rings catalyze ethyl-
ene polymerization in a living fashion.
The bulky catalyst with isopropyl sub-
stituents can be used to control the
branching topology of polyethylene
upon tuning the reaction temperature
and ethylene pressure.
Chem. Eur. J. 2016, 22, 1 – 9
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