New nickel(II) diimine complexes bearing phenyl and sec-phenethyl groups: Synthesis, characterization and ethylene polymerization behaviour

Article abstract of DOI:10.1002/aoc.3151

A series of nickel(II) catalysts containing phenyl and chiral sec-phenethyl groups, {[(4-R₁-2-R₂C₆H₂-Rfnet C)₂Nap]NiBr₂} (Nap: 1,8-naphthdiyl, R₁=Me, R₂=Ph (3a); R₁=Me, R₂=sec-phenethyl (3b); R₁=Cl, R₂=sec-phenethyl (3c); R₁=Me, R ₂=Me (3d) were synthesized and characterized. All organic compounds were fully characterized by FT-IR and NMR spectroscopy and elemental analysis. The single crystal for X-ray crystallography was isolated from 3a in CH ₂Cl₂/n-hexane under air; the crystal structure showed a binuclear complex 3a, in which each nickel atom was six-coordinate. The two nickel atoms together with two bromine atoms form a planar four-membered ring, with a bromine and H₂O axial ligands. These complexes, activated by diethylaluminum chloride and chiral nickel pre-catalysts rac-3c, exhibited good activities (up to 2.85×10⁶g PE (mol Ni h bar)^-1) for ethylene polymerization, and produced polyethylene products with a high degree of branching (up to 117 branched per 1000 carbons) at high temperature. The type and amount of branches of the polyethylenes obtained were determined by ¹H and ¹³C NMR spectroscopy. Copyright

Full text of DOI:10.1002/aoc.3151

Full Paper
Received: 25 January 2014
Revised: 27 February 2014
Accepted: 28 February 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3151
New nickel(II) diimine complexes bearing
phenyl and sec-phenethyl groups: synthesis,
characterization and ethylene polymerization
behaviour
Fuzhou Wanga,b, Jianchao Yuana*, Qingshan Lic, Ryo Tanakab,
Yuushou Nakayamab and Takeshi Shionob*
A series of nickel(II) catalysts containing phenyl and chiral sec-phenethyl groups, {[(4-R1–2-R2C6H2N¼C)2Nap]NiBr2} (Nap: 1,8-
naphthdiyl, R1 = Me, R2 = Ph (3a); R1 = Me, R2 = sec-phenethyl (3b); R1 = Cl, R2 = sec-phenethyl (3c); R1 = Me, R2 = Me (3d) were
synthesized and characterized. All organic compounds were fully characterized by FT-IR and NMR spectroscopy and elemental
analysis. The single crystal for X-ray crystallography was isolated from 3a in CH2Cl2/n-hexane under air; the crystal structure
showed a binuclear complex 3a′, in which each nickel atom was six-coordinate. The two nickel atoms together with two bromine
atoms form a planar four-membered ring, with a bromine and H2O axial ligands. These complexes, activated by
diethylaluminum chloride and chiral nickel pre-catalysts rac-3c, exhibited good activities (up to 2.85 × 106 g PE (mol Ni h bar)ꢀ1
)
for ethylene polymerization, and produced polyethylene products with a high degree of branching (up to 117 branched per
1000 carbons) at high temperature. The type and amount of branches of the polyethylenes obtained were determined by 1H
and 13C NMR spectroscopy. Copyright © 2014 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: Ni(II) complexes; sec-phenethyl; ethylene polymerization; crystal structure
geometry 9,10-dihydro-9,10-ethanoanthracene-11,12-diimine ligands
were successfully prepared and exhibited very high activity for
Introduction
The polymerization/oligomerization of ethylene and α-olefins
play pivotal roles in today’s polyolefin industry production, and
the polyethylene materials are tremendously important in our
daily life.[1] Bulky ortho-position aryl substituents α-diimine Ni(II)
complexes that show high catalytic activity for α-olefin polymer-
ization have been successfully developed by Brookhart and his
collaborators.[2–7] The key feature of these complexes lies in the
bulky α-diimine ligand that has steric hindrance in the axial direc-
tion of the metal coordination plane to suppress the associative
chain transfer.[2] Since the earlier studies, investigations have
increasingly focused on improvement of the catalytic properties
through modification of the structures of catalysts,[8–26] especially
on changes of the backbone substituents on the carbon atoms of
imine groups and replacement of the aniline moiety as well as
the resultant influence on ethylene polymerization. For instance,
norbornene (NB) homopolymerization with only B(C6F5)3 as
co-catalyst.[15] In particular, recent examples of ethylene polymeriza-
tion, especially using ‘sandwich’ (8-p-tolyl naphthyl α-diimine)nickel
(II) catalysts,[16] have yielded the most highly branched polyethylene
produced by Ni catalysts seen to date. However, the effect of a
para-position substituent of the styrene aniline group, especially a
para-position bulky electron-donating substituent of the styrene
aniline group, on the catalytic behaviour of α-diimine–nickel(II)
complexes and the property of resultant polymers has only been
reported in a limited number of cases.[27–29]
* Correspondence to: Jianchao Yuan, College of Chemistry and Chemical
Engineering, Northwest Normal University, Lanzhou 730070, China. E-mail:
jianchaoyuan@nwnu.edu.cn;
Rhinehart et al. reported
a robust 2,6-bis(diphenylmethyl)
substituted Ni(II) α-diimine catalyst,[13] which was highly active,
produced well-defined polyethylene and demonstrated remark-
able thermal stability in high-temperature ethylene polymerization.
Recently, a series of nickel(II) bromide catalysts with 2-{2,6-bis[di(4-
fluorophenyl)methyl]-4-chlorophenylimino}-3-aryliminobutane
ligands have been prepared and showed very high activity toward
ethylene polymerization, with activities of up to 107 g (PE) molꢀ1
(Ni) hꢀ1, and afforded highly branched polyethylene with a
bimodal distribution according to the result of Redshaw et al.[14]
Moreover, a series of nickel(II) complexes with three-dimensional
Takeshi Shiono, Graduate School of Engineering, Hiroshima University, Kagamiyama,
Higashi-Hiroshima 739-8527, Japan. E-mail: tshiono@hiroshima-u.ac.jp
a College of Chemistry and Chemical Engineering, Northwest Normal University,
Lanzhou 730070, China
b Graduate School of Engineering, Hiroshima University, Kagamiyama, Higashi-
Hiroshima 739-8527, Japan
c
State Key Laboratory of Elemento-Organic Chemistry, Nankai University,
Tianjin 300071, China
Appl. Organometal. Chem. (2014)
Copyright © 2014 John Wiley & Sons, Ltd.

F. Wang et al.
In previous work, using α-diimine nickel catalysts, we reported that
2,6-diphenyl-substituted[30] and bulky chiral 2,6-sec-diphenethyl-
substituted[31] α-diimine–Ni(II) complexes showed excellent activity
for ethylene polymerization upon activation with diethylaluminum
chloride (DEAC). In order to study the steric and electronic effects
at the metal centre on the catalyst activity, catalyst thermostability
and, in particular, on the microstructure of polyethylene, herein we
report the synthesis and characterization of three new α-diimine–Ni
(II) complexes of the type [NiBr2(Ar-BIAN)] (Ar-BIAN = bis(arylimino)
acenaphthene) bearing a phenyl or chiral sec-phenethyl or methyl
group in the ortho-aryl position with a methyl group in the para-aryl
position of the arylimino group. These nickel(II) complexes exhibit
good activity towards ethylene polymerization and produce highly
branched polyethylene. Furthermore, the impact of the polymeriza-
tion parameters and the nature of the ligands present on the
observed catalytic activity, as well as the properties of the polyethyl-
ene produced, have been investigated.
and the solvent was removed. The residue was purified by chroma-
tography on silica gel with petroleum ether/ethyl ester (v/v, 20:1) to
give 4-methyl-2-phenylaniline 1a (0.24 g, 66% yield). 1H NMR
(400 MHz, CDCl3): δ 7.33–7.38 (m, 4H, protons of C5, C10 and C11),
7.23–7.27 (m, 1H, proton of C9), 6.88–6.90 (m, 2H, proton of C3),
6.61 (d, J= 8.0Hz, 1H, proton of C2), 3.42 (s, 2H, ―NH2), 2.20 (s, 3H,
proton of C7). 13C NMR (400 MHz, CDCl3): δ 140.75 (C1), 139.60
(C8), 130.93 (C5), 129.04 (C10), 128.97 (C4), 128.70 (C3), 127.92 (C9),
127.78 (C11), 127.03 (C6), 115.85 (C2), 20.39 (C7). Anal. Calcd for
C13H13N: C, 85.21; H, 7.15; N, 7.64%. Found: C, 85.15; H, 7.17; N, 7.59%.
Synthesis of rac-4-methyl-2-sec-phenethylaniline 1b
rac-4-Methyl-2-sec-phenethylaniline 1b was synthesized according
to the literature.[31]
Synthesis of rac-4-chloro-2-sec-phenylethylaniline 1c
rac-4-Chloro-2-sec-phenylethylaniline 1c was synthesized according
to the literature.[31]
Experimental
Synthesis of Ligand
General Procedures and Materials
Synthesis of bis[N,N′-(4-methyl-2-phenylphenyl)imino]acenaphthene 2a
All manipulations were carried out under a nitrogen atmosphere using
standard Schlenk techniques. Methylene chloride and o-dichloroben-
zene were pre-dried with 4 Å molecular sieves and distilled from
CaH2 under dry nitrogen. Toluene, diethyl ether and 1,2-
dimethoxyethane (DME) were distilled from sodium/benzophenone
under nitrogen atmosphere. Anhydrous NiBr2 (99%), phenylboronic
acid (97%), Pd(OAc)2 and DEAC (0.9 M solution in toluene) were
obtained from Acros. 1,2-Acenaphthylenedione (98%), 4-methylaniline
(98%), 4-chloroaniline (97%), 2,4-dimethylaniline (98%) and Br2 (98%)
were purchased from Alfa Aesar and used without further purification.
[NiBr2(DME)] was synthesized according to the literature.[32]
NMR spectra were recorded at 400 MHz on a Varian Mercury Plus-
400 instrument, using TMS as internal standard. FT-IR spectra were
recorded on a Digilab Merlin FTS 3000 FT-IR spectrophotometer on
KBr pellets. The molecular weights and molecular weight distributions
(Mw/Mn) of the polymers were determined by gel permeation chroma-
tography–size-exclusion chromatography (GPC–SEC) with a Waters
Alliance GPCV2000 chromatograph, using 1,2,4-trichlorobenzene as
eluent, at a flow rate of 1.0mlminꢀ1 and operated at 140 °C.
N
N
Formic acid (0.2 ml) was added to a stirred solution of 1,2-
acenaphthylenedione (0.091 g, 0.50 mmol) and 4-methyl-2-
phenylbenzenamine (0.20 g, 1.10 mmol) in ethanol (10 ml). The
mixture was refluxed for 24 h, then cooled, and the precipitate
was separated by filtration. The solid was recrystallized from
EtOH/CH2Cl2 (v/v, 18:1), washed with cold ethanol and dried un-
1
der vacuum (0.22 g, 85% yield). H NMR (400 MHz, CDCl3): δ 7.72
(d, J = 8.2 Hz, 2H, proton of C14), 7.46 (m, 4H, protons of C16
and C15), 7.28 (s, 2H, proton of C5), 6.09–7.13 (m, 10H, protons
of C8, C9 and C10), 6.77 (d, J = 7.2 Hz, 2H, proton of C2), 6.69 (d,
J = 7.2 Hz, 2H, proton of C3), 2.44 (s, 6H, proton of C11). 13C
NMR (400 MHz, CDCl3): δ 160.43 (C12), 145.70 (C1), 139.65 (C18),
133.44 (C4), 130.65 (C7), 129.80 (C5), 129.56 (C6), 129.35 (C17),
129.03 (C16), 128.86 (C13), 128.66 (C9), 128.46 (C3), 127.88 (C8),
127.72 (C10), 127.41 (C15), 126.25 (C14), 122.53 (C2), 20.80
(C11). Anal. Calcd for C38H28N2: C, 89.03; H, 5.51; N, 5.46%. Found:
C, 88.95; H, 5.57; N, 5.29%. FT-IR (KBr): 1638 cmꢀ1(νC N).
Synthesis of Aniline Derivatives
Synthesis of 2-bromo-4-methylaniline a
2-Bromo-4-methylaniline a was synthesized according to the
literature.[33]
Synthesis
of
rac-bis[N,N′-(4-methyl-2-sec-phenethylphenyl)imino]
Synthesis of 4-methyl-2-phenylaniline 1a
acenaphthene 2b
NH2
N
N
Pd(OAc)2 (0.01 g, 0.04 mmol), 2-bromo-4-methylaniline (0.37 g,
2.00 mmol), K2CO3 (0.55 g, 4.00 mmol) and phenylboronic acid
(0.26 g, 2.10 mmol) were placed in a 100 ml flask and allowed to
stir at 25 °C for 24 h in the presence of 10 ml PEG-400. The
mixture was extracted three times with 10 ml diethyl ether. The
combined organic phase was dried over MgSO4 and filtered,
Using the same procedure as for the synthesis of 2a, 2b was
obtained as an orange powder (0.22 g, 78% yield). 1H NMR
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Appl. Organometal. Chem. (2014)

New nickel(II) diimine complexes for ethylene polymerization
(400 MHz, CDCl3): δ 8.09 (d, J = 8.4 Hz, 2H, proton of C16), 7.85 (d,
J = 8.4 Hz, 2H, proton of C18), 7.76 (m, 2H, proton of C17), 7.31
(s, 2H, proton of C5), 6.99 (d, J = 6.8 Hz, 4H, proton of C10), 6.74
(d, J = 7.6 Hz, 2H, proton of C2), 6.68 (m, 4H, proton of C11), 6.62
(d, J = 7.6 Hz, 2H, proton of C3), 6.48 (t, J = 7.2 Hz, 2H, proton of
C12), 4.31 (q, J = 7.2 Hz, 2H, proton of C8), 2.45 (s, 6H, proton of
C7), 1.56 (d, J = 7.2 Hz, 6H, proton of C13). 13C NMR (400 MHz,
CDCl3): δ 159.83 (C14), 146.60 (C1), 145.78 (C9), 142.99 (C20),
134.86 (C4), 131.86 (C6), 130.49 (C5), 128.65 (C19), 127.90 (C18),
127.83 (C15), 127.68 (C11), 127.56 (C10), 127.50 (C12), 124.97
(C3), 123.46 (C17), 121.71 (C16), 117.00 (C2), 40.34(C8), 21.34
(C7), 21.11 (C13). Anal. Calcd for C42H36N2: C, 88.69; H, 6.38;
N, 4.93%. Found: C, 88.75; H, 6.43; N, 5.01%. FT-IR (KBr):
1640 cmꢀ1(νC N).
for C42H36Br2N2Ni: C, 64.08; H, 4.61; N, 3.56%. Found: C, 64.12; H,
4.56; N, 3.63%. FT-IR (KBr): 1629 cmꢀ1 (νC N).
Synthesis of rac-{Bis[N,N′-(4-chloro-2-sec-phenethylphenyl)imino]acenaphthene}
dibromonickel 3c
Using the same procedure as for the synthesis of 3a, 3c was
obtained as a dark-red powder 3c (0.28 g, 85% yield). Anal. Calcd
for C40H30Br2Cl2N2Ni: C, 58.02; H, 3.65; N, 3.38%. Found: C, 57.97;
H, 3.69; N, 3.42%. FT-IR (KBr): 1628 cmꢀ1 (νC N).
Synthesis of bis[N,N′-(2,4-dimethylphenyl)imino]acenaphthene 2d
Bis[N,N′-(2,4-dimethylphenyl)imino]acenaphthene 2d was synthe-
sized according to the literature.[34]
Synthesis
of
rac-bis[N,N′-(4-chloro-2-sec-phenethylphenyl)imino]
acenaphthene 2c
Synthesis
of
{bis[N,N′-(2,4-dimethylphenyl)imino]acenaphthene}
dibromonickel 3d
{Bis[N,N’-(2,4-dimethylphenyl)imino]acenaphthene}dibromonickel
3d was synthesized according to the literature.[34]
Cl
N
N
Cl
X-Ray Structure Determinations
Single crystals of complex 3a′ suitable for X-ray analysis were
obtained at room temperature by dissolving nickel complex 3a
in CH2Cl2, following by slow layering of the resulting solution
with n-hexane in air. Data collections were performed at 296(2)
K on a Bruker SMART APEX diffractometer with a charge-coupled
device (CCD) area detector, using graphite monochromated Mo-
Kα radiation (λ = 0.71073 Å). The determination of crystal class and
unit cell parameters was carried out using the SMART program
package. The raw frame data were processed using SAINT and
SADABS to yield the reflection data file. The structures were
solved by using the SHELXTL program. Refinement was
performed on F2 anisotropically for all non-hydrogen atoms by
the full-matrix least-squares method. The hydrogen atoms were
placed at the calculated positions and were included in the struc-
ture calculation without further refinement of the parameters.
Crystal data, data collection and refinement parameters are listed
in Table 1.
Using the same procedure as for the synthesis of 2a, 2c was
obtained as an orange powder (0.23 g, 76% yield). 1H NMR
(400 MHz, CDCl3): δ 7.78 (d, J = 8.4 Hz, 2H, proton of C15), 7.73
(d, J = 8.0 Hz, 2H, proton of C17), 7.43–7.48 (m, 2H, proton of
C16), 7.24–7.30 (m, 4H, proton of C11), 7.05 (d, J = 7.6 Hz, 4H, pro-
ton of C3), 6.88 (d, J = 7.6 Hz, 2H, proton of C2), 6.53–6.78 (m, 6H,
protons of C10 and C12), 4.42 (q, J = 7.2 Hz, 2H, proton of C7),
1.58–1.63 (m, 6H, proton of C8). 13C NMR (400 MHz, CDCl3): δ
160.33 (C13), 147.62 (C1), 144.62 (C9), 143.21 (C19), 141.88 (C6),
136.94 (C4), 132.03 (C18), 130.53 (C17), 128.85 (C14), 127.90
(C5), 127.85 (C11), 127.66 (C10), 127.50 (C3), 124.97 (C16),
123.46 (C12), 118.46 (C15), 117.54 (C2), 40.19 (C7), 21.04 (C8).
Anal. Calcd for C40H30Cl2N2: C, 78.81; H, 4.96; N, 4.60%. Found:
C, 78.73; H, 5.02; N, 4.55%. FT-IR (KBr): 1639 cmꢀ1(νC N).
Synthesis of Nickel Complex 3a–d
Synthesis of {bis[N,N′-(4-methyl-2-phenylphenyl)imino]acenaphthene}
dibromonickel 3a
Ethylene Polymerization
[(DME)NiBr2] (0.12 g, 0.40 mmol) and ligand 2a (0.20 g, 0.40 mmol)
were combined in a Schlenk flask under a nitrogen atmosphere.
CH2Cl2 (20 ml) was added, and the reaction mixture was stirred at
room temperature for 24 h. The resulting suspension was filtered.
The solvent was removed under vacuum and the residue was
washed with diethyl ether (3 ×15 ml), and then dried under vacuum
at room temperature to give catalyst 3a (0.24 g, 83% yield). Anal.
Calcd for C38H28Br2N2Ni: C, 62.42; H, 3.86; N, 3.83%. Found: C,
62.31; H, 3.92; N, 3.90%. FT-IR (KBr): 1627 cmꢀ1 (νC N). Single crystals
of complex 3a′ suitable for X-ray analysis were obtained at room
temperature by dissolving the nickel complex in CH2Cl2, following
by slow layering of the resulting solution with n-hexane in air.
The polymerization of ethylene was carried out in a flame-dried
250 ml crown-capped pressure bottle sealed with neoprene
septa. After drying the polymerization bottle under nitrogen
atmosphere, 50 ml dry toluene was added to the polymeriza-
tion bottle. The resulting solvent was then saturated with a
prescribed ethylene pressure. The co-catalyst (DEAC) was then
added in Al/Ni molar ratios in the range of 200–1000 to the
polymerization bottle via a syringe. At this time, the solutions
were thermostated to the desired temperature and allowed
to equilibrate for 15 min. Subsequently, an o-dichlorobenzene
solution of Ni catalyst was added to the polymerization
reactor. The polymerization, conducted under a dynamic pres-
sure of ethylene (0.2 bar), was terminated with 100 ml of a
5% HCl–MeOH solution. The precipitated polymer was filtered,
washed with methanol and dried under vacuum at 60 °C to
constant weight.
Synthesis of rac-{Bis[N,N′-(4-methyl-2-sec-phenethylphenyl)imino]acenaphthene}
dibromonickel 3b
Using the same procedure as for the synthesis of 3a, 3b was
obtained as a dark-red powder (0.26 g, 86% yield). Anal. Calcd
Appl. Organometal. Chem. (2014)
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F. Wang et al.
Table 1. Crystal data and structure refinements of complex 3a′
X-ray Crystallographic Studies
Complex
3a′
Suitable crystals of complex 3a′ for X-ray diffraction was obtained
at room temperature by double layering a CH2Cl2 solution of the
complex 3a with n-hexane in air.
Empirical formula
Formula mass
Temperature (K)
Wavelength (Å)
Crystal size (mm3)
Crystal system
Space group
a (Å)
C
76H56Br4N4Ni2.2(H2O).2.5(CH2Cl2).O
1728.67
293
The molecular structure of complex 3a′ is shown in Figure 1.
Selected bond distances and angles are summarized in Table 2.
In the solid state, complex 3a′ clearly shows the dimeric structure
of 3a with two Ni centres connected by two bromide bridges. In
each unit, the two imine N atoms in Ar-DAB, three Br atoms and a
H2O molecule from the moisture coordinated to a Ni atom in a
six-coordinated distorted octahedral geometry. The two Br brid-
ges and the two imine groups bonded to the Ni centre in cis con-
figuration and the isolated Br ions and the coordinated water
linked to the same Ni atom in trans configuration. Both aryl rings
bonded to the iminic nitrogens of the α-diimine lie nearly per-
pendicular to the plane formed by the nickel and coordinated ni-
trogen atoms. The phenyl groups in the 2-position of the
benzenamine fragments in 3a′ point toward each other above
and below the plane, thus shielding the apical positions of the
Ni(II) centre.
0.71073
0.2 × 0.08 × 0.06
Triclinic
P ꢀ1
13.8380(7)
16.3253(5)
16.9637(7)
73.208(3)
87.828(4)
86.488(3)
3661.1(3)
2
b (Å)
c (Å)
α (°)
β (°)
γ (°)
V (Å3)
Z
Density (calcd) (mg cmꢀ3
Absorption coefficient
)
1.568
2.93
(mmꢀ1
F(000)
)
The structure of 3a′ is similar to those reported in the literature
for other similar [NiBr2(α-diimine)] compounds characterized by
X-ray diffraction, {bis[N,N′-4-bromo-2,6-dimethylphenyl)imino]
1742
Theta range for data
collection (°)
2.9 ꢀ 28.6
acenaphthene}dibromonickel[35]
and
{bis[N,N′-(2,4,6-
Limiting indices
ꢀ17 ≤ h ≤ 10; ꢀ21 ≤ k ≤ 21;
ꢀ22 ≤ l ≤ 22
trimethylphenyl)imino]acenaphthene}dibromonickel.[36] In fact,
the Ni1―N1 bond distances in complex 3a′ (2.086 Å) are similar
to those determined for these compounds (2.026 and 2.021Å, re-
spectively), as well as the Ni1―Br1 bond distances (2.5422 Å for
complex 3a′ vs. 2.329 and 2.323 Å, respectively). However, the
N1―Ni1―Br1 angles (92.33° for complex 3a′) are very different
from that determined for those compounds (113.32 and 114.4°, re-
spectively) due to the great steric hindrance of the ortho benzene
rings of complex 3a′ around the Ni(II) centre. The two imino
C―N bonds have a typical double bond character, with C―N
bond lengths of 1.276–1.288 Å. The aryl rings of each of the α-
diimines lie nearly perpendicular to the plane formed by the
nickel and the coordinated nitrogen atoms, and face each other
in a staggered conformation. In addition, there are two and a
half CH2Cl2 molecules and a crystallized H2O molecule in a
asymmetric unit in the 3a′ molecule.
Reflections collected
Independent reflections
Rint
32 291
16 463
0.044
Completeness to θ = 25.05°
Final R indices [I > 2θ(I)]
R indices (all data)
99.00%
R1 = 0.0640, wR2 = 0.1550
R1 = 0.0976, wR2 = 0.1766
Full-matrix least-squares on F2
1.06
Refinement method
Goodness of fit on F2
Max. and min. transmission
Largest diff. peak and
1.000 and 0.868
2.50 and ꢀ1.99
hole (e.Åꢀ3
)
Results and Discussion
Synthesis and Characterization of Ligands 2a–d and complexes
3a–d
Polymerization of Ethylene with Nickel Complexes 3a–d
The four α-diimine nickel(II) complexes 3a–d, activated by DEAC,
were tested as catalyst precursors for the polymerization of
ethylene, under the same reaction conditions. The results of the po-
lymerization experiments are shown in Table 3. Blank experiments
were carried out with DEAC alone under the same conditions, to
confirm the inability of DEAC for ethylene polymerization.
At 0 °C, the activity of complex 3a increased slightly with the
increase of [Al]/[Ni] ratio (runs 1–3), reached a maximum around
an [Al]/[Ni] ratio of 600 (run 3), and then decreased slightly
(runs 3–5). For a [Al]/[Ni] ratio of 600, the highest activities of
complex 3a (runs 3, 6–8) appeared around 20 °C, and an increase
in the polymerization temperature form 20 to 60 °C slightly
decreased the activity (runs 6–8). The molecular weight of
polymer decreased as the polymerization temperature increased
from 0 to 60 °C.
After the protection of the amino group by acetic acid, 4-
methylaniline was brominated to form 2-bromo-4-methylaniline
(a). The Suzuki coupling reaction of a and phenylboronic acid
catalysed by Pd(OAc)2 in PEG-400 led to the desired amine 4-
methyl-2-phenylaniline (1a), in 66% yield (Scheme 1). Reaction
of the p-substituted anilines with styrene at elevated tempera-
ture (160 °C) in the presence of CF3SO3H catalyst resulted in
the corresponding o-sec-phenethylanilines, rac-b and rac-c
(Scheme 1). The α-diimine ligands 2a–d were prepared by the
condensation of 2 equiv. of the aniline with 1 equiv. of 1,2-
acenaphthylenedione, usually in the presence of a formic acid
catalyst. Compounds 2a–d were characterized by elemental
analysis, and 1H and 13C NMR spectroscopy.
The reaction of equimolar amounts of NiBr2(DME) and the α-
diimine ligands 2a–d in CH2Cl2 led to the displacement of 1,2-
dimethoxyethane and afforded the catalyst precursors 3a–d as
moderately air-stable microcrystalline solids in high yields.
The performance of the α-diimine–Ni(II) complexes are signifi-
cantly affected by o-position substituents on the aniline rings
(Table 3). Complex rac-3c, bearing a chlorine substituent in the
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Appl. Organometal. Chem. (2014)

New nickel(II) diimine complexes for ethylene polymerization
Mn = 21.06 × 104 g molꢀ1, 0 °C, [Al]/[Ni] = 600) among our
four complexes.
Complexes 3a (bearing a phenyl group in the ortho-aryl
position), rac-3b (bearing a chiral sec-phenethyl group in the
ortho-aryl position) and 3d (bearing a methyl group in the
ortho-aryl position) exhibit significantly lower catalytic activity
highest activity: 3a, run 6, 1.96 × 106 g PE (mol Ni h bar)ꢀ1; rac-
3b, run 11, 2.76 × 106 g PE (mol Ni h bar)ꢀ1; and 3d, run 18,
0.94 × 106 g PE (mol Ni h bar)ꢀ1
.
Compared with complexes 3a and 3d/DEAC systems, which
showed the highest catalytic activity at 20 °C (such as 3d, run
18, activity: 0.94 × 106 g PE (mol Ni h bar)ꢀ1), complexes rac-3b
and rac-3c/DEAC systems showed the highest catalytic activity
at 40 °C (such as rac-3b, run 11, 2.76 × 106 g PE (mol Ni h
bar)ꢀ1); thus chiral nickel pre-catalysts rac-3b and rac-3c have
better thermal stability, even at 60 °C, and the rac-3b and rac-
3c/DEAC systems still maintain good activity and produce
higher-molecular-weight polyethylene.
It is known that ligand electronics have a dramatic effect on
catalytic activity and polymer molecular weight.[19–24] The
properties of resulting polyethylenes showed halide and poly-
merization temperature effects in that the polyethylenes pro-
duced with the chloride pre-catalyst rac-3c (runs 13–16 in
Table 3) had slightly higher molecular weights than those
produced with the methyl pre-catalyst rac-3b (runs 9–12 in
Table 3).
These results indicate that the rate of chain propagation is
greatly promoted by the bulky ortho-sec-phenethyl groups
of the ligand’s aryl rings. As a result, the following activity
trend can be summarized for our substituted pre-catalysts
under low ethylene pressure (0.2 bar), in the range 0–60 °C:
3c > 3b > 3a > 3d.
The type and amount of branches in the polyethylene
obtained by α-diimine nickel pre-catalysts depend not only
on the ligand structure but also on the reaction conditions,
such as reaction temperature end ethylene pressure.[2] Gener-
Scheme 1. Syntheses of α-diimine ligands 2a–d (*chiral carbon) and their
corresponding α-diimine nickel(II) dibromide complexes 3a–d.
para-aryl position and one bulky chiral sec-phenethyl group in
the ortho-aryl positions of the ligand, displayed the highest
catalytic activity of 2.85 × 106 g PE (mol Ni h bar)ꢀ1, and
produced the highest-molecular-weight polymer (run 13,
Table 2. Selected bond lengths (Å) and angles (°) for complex 3a′
Bond length
(Å)
Bond angle
(°)
Ni1–N1
Ni1–N2
Ni2–N3
Ni2–N4
Ni1–O1
Ni2–O2
Br1–Ni1
Br2–Ni1
Br3–Ni1
Br1–Ni2
Br2–Ni2
Br4–Ni2
N1–C1
2.086(4)
2.149(4)
2.135(4)
2.100(5)
2.126(4)
2.141(4)
2.5422(9)
2.4989(9)
2.5548(9)
2.5068(9)
2.5332(9)
2.5391(9)
1.439(7)
1.276(7)
1.285(7)
1.440(7)
1.288(7)
1.432(7)
1.280(7)
1.434(7)
Ni2–Br1–Ni1
Ni1–Br2–Ni2
N1–Ni1–O1
N1–Ni1–N2
O1–Ni1–N2
N1–Ni1–Br2
O1–Ni1–Br2
N2–Ni1–Br2
N1–Ni1–Br1
N2–Ni1–Br1
Br2–Ni1–Br1
N1–Ni1–Br3
N2–Ni1–Br3
Br2–Ni1–Br3
Br1–Ni1–Br3
N4–Ni2–N3
N4–Ni2–Br2
Br1–Ni2–Br2
Br1–Ni2–Br4
Br2–Ni2–Br4
88.15(3)
88.52(3)
91.73(16)
79.67(17)
91.95(15)
173.18(12)
82.92(11)
96.22(12)
92.33(12)
171.94(12)
91.65(3)
91.12(12)
89.76(12)
94.33(3)
N1–C14
N2–C25
N2–C26
N3–C51
N3–C39
N4–C62
N4–C63
91.39(3)
79.83(17)
92.15(12)
91.68(3)
91.29(3)
92.85(3)
Figure 1. Molecular structure of the catalyst precursor 3a′. Hydrogen
atoms have been omitted for clarity.
Appl. Organometal. Chem. (2014)
Copyright © 2014 John Wiley & Sons, Ltd.
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F. Wang et al.
Table 3. Polymerization of ethylene with complexes 3a–d/DEACa
Activityb
TOFc
Mn
Mw/Mn
Branchese/
1000 C
d
Run
Complex
[Al]/[Ni]
T (°C)
t
Yield
(g)
(h bar)ꢀ1
g molꢀ1
d
(min)
1
3a
200
400
600
800
1000
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
0
0
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
0.89
1.19
1.31
1.25
0.93
1.41
0.89
0.49
1.1
1.24
1.65
1.82
1.74
1.29
1.96
1.24
0.68
1.53
2.42
2.76
1.08
1.4
0.44
0.59
0.65
0.62
0.46
0.7
20.35
20.22
19.13
18.45
9.63
1.69
1.73
1.72
1.78
1.79
1.83
1.9
—
—
2
3a
3
3a
0
—
4
3a
0
—
5
3a
0
—
6
3a
20
40
60
0
12.93
9.13
84
7
3a
0.44
0.24
0.55
0.86
0.99
0.39
0.5
88
8
3a
7.67
2.02
1.72
1.75
1.81
1.96
1.71
1.73
1.85
1.98
1.75
1.79
1.94
2.13
108
—
9
rac-3b
rac-3b
rac-3b
rac-3b
rac-3c
rac-3c
rac-3c
rac-3c
3d
20.41
14.05
10.93
4.6
10
11
12
13
14
15
16
17
18
19
20
20
40
60
0
1.74
1.99
0.78
1.01
1.89
2.05
0.86
0.6
89
102
110
—
21.06
18.81
16.05
5.47
20
40
60
0
2.63
2.85
1.19
0.83
0.94
0.63
0.32
0.94
1.02
0.43
0.3
93
106
117
—
16.68
10.13
8.15
3d
20
40
60
0.68
0.45
0.23
0.34
0.22
0.11
73
3d
82
3d
2.67
99
aPolymerization conditions: n(Ni) = 3.60 μmol; ethylene relative pressure = 0.2 bar, ethylene absolute pressure = 1.2 bar; t = polymerization time;
solvent = toluene (50 ml); T = polymerization temperature.
bActivity in 106 g PE (mol Ni h bar)ꢀ1
.
cTurnover frequency in 105 mol ethylene (mol Ni h bar)ꢀ1
.
dMn in 104 g molꢀ1, determined by GPC.
eBranching density, branches/1000 C = (CH3/3)/[(CH + CH2 + CH3)/2] × 1000. CH3, CH2, CH refer to the intensities of the methyl, methylene and
1
methine resonances in H NMR spectra.[24]
ally, low ethylene pressure and high polymerization temperature
favour chain walking and afford highly branched polyethyl-
enes.[2] However, the effect of ligand structure on polyethylene
branching is much more complicated.
As shown in Table 3 and Figure 2, the catalyst system rac-
3c/DEAC, bearing one bulky sec-phenethyl in the ortho-aryl
position and an electron-withdrawing Cl substituent group in
the para-aryl position, generated polyethylene with high
degrees of branching. The total branching degrees of the
polymer samples prepared with rac-3c/DEAC (runs 14–16;
Figure 3. 13C NMR (CDCl3/o-dichlorobenzene, v/v = 1:3) spectrum of
polyethylene catalysed by rac-3c/DEAC at 40 °C (Table 3, run 15). Note
on labels: for xBy By is a branch of length y carbons, x is the carbon being
discussed, and the methyl at the end of the branch is numbered 1. Thus,
the second carbon from the end of a butyl branch is 2B4. xBy+ refers to
branches of length y and longer. The methylenes in the backbone are
labelled with Greek letters, which determine how far from a branch
point methine each methylene is; α denotes the first methylene next to
the methine. Thus γB1+ refers to methylenes γ from a branch of length
1 or longer.
Figure 2. 1H NMR (CDCl3/o-dichlorobenzene, v/v = 1:3) spectra of the
nanosized dendritic polyethylene catalysed by 3a, 3b, 3c and 3d/DEAC
at 40 °C (Table 3, runs 7, 11, 15, 19): ICH3, integrated intensity between
0.8 and 1.0 ppm; ICH2 + ICH, integrated intensity between 1.0 and 1.5 ppm.
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Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2014)

New nickel(II) diimine complexes for ethylene polymerization
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branching degree: 93, 106 and 117 branches/1000 C at 20, 40
and 60 °C, respectively) are higher than those observed for
rac-3b/DEAC (runs 10–12; branching degree: 91, 102 and 107
branches/1000 C at 20, 40 and 60 °C, respectively), 3a/DEAC
(runs 6–8; branching degree: 84, 88 and 108 branches/1000 C
at 20, 40 and 60 °C, respectively) and 3d/DEAC systems (runs
18–20; branching degree: 73, 82 and 99 branches/1000 C at
20, 40 and 60 °C, respectively).
The 13C NMR spectrum of the polyethylene prepared with rac-
3c/DEAC at 40 °C is shown in Figure 3. The number of branches
was calculated according to the literature,[37] and it was found
that polyethylene with 106 branches/1000 carbons (74 methyl,
11 ethyl, 9 propyl and 12 butyl or longer branches/1000 C) was
obtained at 40 °C (run 15 in Table 3). This result was consistent
with that calculated from the 1H NMR spectrum. The rationale
for the formation of the major types of branches (methyl, ethyl
and butyl) in the polyethylene obtained in this work (run 15) is
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A series of new α-diimine ligands and their Ni(II) complexes
have been prepared and characterized. Ligands 2a–c were
modified in an attempt to change steric effects and the
electronic density of the metal centre and eventually to
improve activity in the polymerization of ethylene and con-
trol the microstructure of polyethylene. The results obtained
show that the chiral complex rac-3c bearing one bulky sec-
phenethyl in the ortho-aryl position and an electron-with-
drawing Cl substituent group in the para-aryl position, acti-
vated by DEAC, produces a highly active catalyst system
for the polymerization of ethylene and highly branched
polyethylene at high temperature. Interestingly, the crystal
structure of 3a′ showed a binuclear complex in which each
nickel atom is six-coordinate. The two nickel atoms together
with two bromine atoms form a planar four-membered ring.
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Acknowledgments
We thank the National Natural Science Foundation of China
(20964003) and Chinese Ministry of Human Resources and Social
Security Overseas Students Science and Technology Activities
Merit-based Foundation (2009-416) for funding.
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Additional supporting information may be found in the online
version of this article at the publisher’s web-site.
CCDC 971546 contains the supplementary crystallographic data
for complex 3a′. The data can be obtained free of charge via
http://www.ccdc.cam.ac.uk, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. a,
1b, 1c, 2d and 3d are known compounds and characterization
data can be seen in the supporting information.
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Appl. Organometal. Chem. (2014)
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