New chiral α-diimine nickel(II) complexes bearing ortho-sec-phenethyl groups for ethylene polymerization

Article abstract of DOI:10.1002/aoc.2962

A series of new α-diimine nickel(II) catalysts bearing bulky chiral sec-phenethyl groups have been synthesized and characterized. The molecular structure of representative chiral ligand, bis[N,N′-(4-methyl-2,6-di-sec- phenethylphenyl)imino]-1,2-dimethylethane rac-1c and chiral complexes, {bis[N,N′-(4-methyl-2-sec-phenethylphenyl)imino]-2,3-butadiene} dibromidonickel rac-2a and bis{bis[N,N′-(4-methyl-2-sec-phenethylphenyl) imino]-2,3-butadiene}dibromidonickel rac-2b, were confirmed by X-ray crystallographic analysis. Complex rac-2c bearing two chiral sec-phenethyl groups in the ortho-aryl position and a methyl group in the para-aryl position, activated by diethylaluminum chloride (DEAC), showed highly catalytic activity for the polymerization of ethylene [4.12 × 10⁶ g PE (mol Ni.h.bar)^-1], and produced highly branched polyethylenes under low ethylene pressure (branching degree: 104, 118 and 126 branches/1000 C at 20, 40 and 60°C, respectively). Chiral 20-electron bis-α-diimine Ni(II) complex rac-2b also exhibited high activity toward ethylene polymerization [1.71 × 10⁶ g PE (mol Ni · h · bar)^-1]. The type and amount of branches of the polyethylenes obtained were determined by ¹H and ¹³C NMR. Copyright

Full text of DOI:10.1002/aoc.2962

Full Paper
Received: 1 November 2012
Revised: 29 November 2012
Accepted: 8 December 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.2962
New chiral a-diimine nickel(II) complexes
bearing ortho-sec-phenethyl groups for
ethylene polymerization
Fuzhou Wang, Jianchao Yuan*, Fengying Song, Jing Li, Zong Jia and
Bingnian Yuan
A series of new a-diimine nickel(II) catalysts bearing bulky chiral sec-phenethyl groups have been synthesized and
characterized. The molecular structure of representative chiral ligand, bis[N,N0-(4-methyl-2,6-di-sec-phenethylphenyl)
imino]-1,2-dimethylethane rac-1c and chiral complexes, {bis[N,N0-(4-methyl-2-sec-phenethylphenyl)imino]-2,3-butadiene}
dibromidonickel rac-2a and bis{bis[N,N0-(4-methyl-2-sec-phenethylphenyl)imino]-2,3-butadiene}dibromidonickel rac-2b, were
confirmed by X-ray crystallographic analysis. Complex rac-2c bearing two chiral sec-phenethyl groups in the
ortho-aryl position and a methyl group in the para-aryl position, activated by diethylaluminum chloride (DEAC),
showed highly catalytic activity for the polymerization of ethylene [4.12 ꢀ 106 g PE (mol Ni.h.bar)ꢁ1], and produced highly
branched polyethylenes under low ethylene pressure (branching degree: 104, 118 and 126 branches/1000 C at 20, 40
and 60ꢂC, respectively). Chiral 20-electron bis-a-diimine Ni(II) complex rac-2b also exhibited high activity toward
ethylene polymerization [1.71 ꢀ 106 g PE (mol Ni ꢃ h ꢃ bar)ꢁ1]. The type and amount of branches of the polyethylenes
obtained were determined by 1H and 13C NMR. Copyright © 2013 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: Ni(II) complexes; a-diimine ligand; crystal structure; chain walking polymerization; branched polyethylene
effects and electron densities at the metal center on the catalyst
activity, catalyst thermostability and, in particular, on the micro-
Introduction
The discovery of Ni(II) and Pd(II) a-olefin polymerization catalysts
containing a bulky a-diimine ligand, [MX2(a-diimine)] (M = Ni, Pd;
X = halide), by Brookhart and co-workers,[1–14] has stimulated
renewed interest in the chemistry of 1,4-diazadiene ligands and
their complexes. In contrast to metallocene catalysts based on early
transition metals,[15–18] Ni(II) and Pd(II) catalysts produced branched
polyethylene with different concentration and individual length of
branches,[1,12] exclusively from the ethylene monomer and accom-
modated even polar monomers, like acrylates.[11,13,14] This discovery
is beneficial to commercial applications and industrial production.
Chain walking ethylene polymerization with Ni(II)/Pd(II)-a-diimine
catalysts represents a new concept for synthesis of hyperbranched
or dendritic polyethylenes under low ethylene pressure.[19] Chain
walking polymerization (CWP) followed by atom transfer radical
polymerization (ATRP) can efficiently synthesize water-soluble
core–shell [core: polyethylene, PE; shell: oligo(ethylene glycol), OEG]
dendritic nanoparticles with tunable sizes and reactive surface
functionalities,[19] which could play a very important role in tumor-
targeted anti-cancer drugs.
structure of polyethylene were also investigated.
Experimental
General Considerations and Materials
All manipulations involving air-and/or water-sensitive compounds
were carriedout with standard Schlenk techniquesundernitrogen.
Methylene chloride and o-dichlorobenzene were pre-dried with
4 Å molecular sieves and distilled from CaH2 under dry nitrogen.
Toluene, diethyl ether and 1,2-dimethoxyethane (DME) were
distilledfrom sodium/benzophenoneunderN2 atmosphere. Anhy-
drous NiBr2 (99%) and diethylaluminum chloride (DEAC, 0.9 M
solution in toluene) were obtained from Acros. 2,3-Butanedione
(98%), 4-methylaniline(98%), 4-chloroaniline(97%), 2,4-dimethyla-
niline (98%) and 2,4,6-trimethylaniline (98%) were purchased from
Alfa Aesar and used without further purification. [NiBr2(DME)] was
synthesized according to the literature.[20]
Herein we describe our initial efforts at developing a series of
chiral nickel catalyst systems for ethylene polymerization.
In this work, we report the synthesis and characterization of
three new a-diimine Ni(II) complexes of the type[NiBr2(Ar-DAB)]
(Ar-DAB = N,N0-diaryl-1,4-diaza-1,3-butadiene) bearing one or
two bulky chiral sec-phenethyl groups in the ortho-aryl position
and different groups (methyl group and Cl group) in the para-aryl
position of the arylimino group. The influences of different steric
*
Correspondence to: Jianchao Yuan, Key Laboratory of Eco-Environment-
Related Polymer Materials of Ministry of Education, Key Laboratory of
Polymer Materials of Gansu Province, College of Chemistry and Chemical
Engineering, Northwest Normal University, Lanzhou 730070, China. E-mail:
jianchaoyuan@nwnu.edu.cn
Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education,
Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and
Chemical Engineering, Northwest Normal University, Lanzhou 730070, China
Appl. Organometal. Chem. (2013)
Copyright © 2013 John Wiley & Sons, Ltd.

NMR spectra were recorded at 400 MHz on a Varian Mercury Plus-
400 instrument, using tetramethylsilane as internal standard. The
molecular weights and molecular weight distributions (Mw/Mn) of the
polymers were determined by gel permeation chromatography–
size-exclusion chromatography (GPC-SEC) via a Waters Alliance
GPCV2000 chromatograph, using 1,2,4-trichlorobenzene as eluent,
at a flow rate of 1.0 mlminꢁ1 and operated at 140ꢂC.
obtained as a white powder (0.52 g, 82% yield). 1H NMR
(400MHz, CDCl3): d 7.02–7.26 (m, 10H, protons of C4, C5 and C7),
7.02 (s, 2H, protons of C6), 3.97 (m, 2H, proton of C9), 3.30
(s, 2H, —NH2), 2.16 (s, 3H, protons of C10), 1.57 (m, 6H, protons of
C11). 13C NMR (400 MHz, CDCl3): d 145.91 (C1), 139.74 (C2), 129.31
(C3), 128.64 (C4), 127.38 (C5), 126.89 (C6), 126.67 (C7), 126.09 (C8),
40.02 (C9), 22.28 (C10), 21.14 (C11). Anal. Calcd for C23H25N: C,
87.57; H, 7.99; N, 4.44. Found: C, 87.49; H, 8.12; N, 4.49. FTIR (KBr,
cmꢁ1): 3381 cmꢁ1, 3452 cmꢁ1 (-NH2).
Synthesis of 4-methyl-2-sec-phenethylaniline a
Synthesis of bis[N,N0-(4-methyl-2-sec-phenethylphenyl)imino]-1,2-dimethy-
lethane 1a
CF3SO3H (0.06g, 0.4 mmol), 4-methylaniline (0.21 g, 2.00 mmol),
styrene (0.31 g, 3.00 mmol) and xylene (1ml) were placed in a
10 ml Schlenk flask and stirred at 160ꢂC for 5 h. Volatile materials
were removed and the residue was purified by chromatography
on silica gel with petroleum ether–ethyl ester (v/v, 15:1) to give
4-methyl-2-sec-phenethylaniline as white microcrystals (0.32 g, 76%
yield). 1H NMR (400 MHz, CDCl3): d 7.30–7.42 (m, 5H, protons of C5,
C6 and C8), 7.23 (s, 1H, proton of C3), 7.02 (d, J= 8.0 Hz, 1H, proton
of C7), 6.64 (d, J=8.0 Hz, 1H, proton of C10), 4.15 (q, J= 7.2 Hz, 1H,
proton of C11), 3.36 (s, 2H, —NH2), 2.43 (s, 3H, protons of C12),
1.72 (d, J= 5.6 Hz, 3H, protons of C13). 13C NMR (400 MHz, CDCl3): d
146.22 (C1), 140.31 (C2), 130.01 (C3), 129.54 (C4), 129.02 (C5),
128.24 (C6), 127.46 (C7), 127.18 (C8), 126.92 (C9), 126.23 (C10),
40.17 (C11), 23.71 (C12), 21.85 (C13). Anal. Calcd for C15H11N:
C, 85.26; H, 8.11; N, 6.63. Found: C, 85.17; H, 8.19; N, 6.59. FT-IR
(KBr, cmꢁ1): 3370 cmꢁ1, 3437 cmꢁ1 (-NH2).
Formic acid (0.5 ml) was added to a stirred solution of 2,3-buta-
nedione (0.17 g, 2.00 mmol) and 4-methyl-2-sec-phenethylaniline
(0.84 g, 4.00 mmol) in methanol (20 ml). The mixture was refluxed
for 24 h, then cooled and the precipitate was separated by filtra-
tion. The solid was recrystallized from EtOH–CH2Cl2 (v/v, 6:1),
washed with cold ethanol and dried under vacuum. Yield:
0.81 g (86%). 1H NMR (400 MHz, CDCl3): d 7.16–7.19 (m, 6H,
protons of C7 and C10), 7.04–7.12 (m, 6H, protons of C6 and
C8), 7.01 (d, J = 8.0 Hz, 2H, protons of C11), 6.42 (d, J = 8.0 Hz,
2H, protons of C9), 4.08 (q, J = 7.2 Hz, 2H, protons of C12), 2.37
(s, 6H, protons of C13), 1.64 (s, 6H, protons of C15), 1.56 (d,
J = 7.2 Hz, 6H, protons of C14). 13C NMR (400 MHz, CDCl3): d
167.94 (C1), 146.52 (C2), 146.46 (C3), 134.99 (C4), 133.26 (C5),
128.14 (C6), 127.64 (C7), 127.52 (C8), 127.15 (C9), 125.71 (C10),
117.82 (C11), 39.96 (C12), 22.45 (C13), 21.78 (C14), 15.09 (C15).
Anal. Calcd for C34H36N2: C, 86.40; H, 7.68; N, 5.93. Found: C,
86.53; H, 7.54; N, 5.79. FT-IR (KBr): 1640 cmꢁ1 (C-N).
Synthesis of 4-chloro-2-sec-phenethylaniline b
Synthesis of bis[N,N0-(4-chloro-2-sec-phenethylphenyl)imino]-1,2-dimethy-
lethane 1b
Using the same procedure as for the synthesis of a, b was
obtained as a white powder (0.35 g, 76% yield). 1H NMR (400 MHz,
CDCl3): d 7.24–7.29 (m, 5H, protons of C4, C5 and C7), 7.23
(s, 1H, proton of C3), 7.00 (d, J = 8.0 Hz, 1H, proton of C6), 6.50
(d, J =8.0 Hz, 1H, proton of C10), 3.96 (q, J = 7.2Hz, 1H, proton of
C11), 3.38 (s, 2H, -NH2), 1.56 (d, J =6.8 Hz, 3H, protons of C12).
13C NMR (400 MHz, CDCl3): d 144.64 (C1), 142.81 (C2), 131.31 (C3),
128.80 (C4), 127.27 (C5), 127.05 (C6), 126.93 (C7), 126.55 (C8),
123.32 (C9), 117.14 (C10), 40.15 (C11), 21.66 (C12). Anal. Calcd for
C14H14ClN: C, 72.57; H, 6.09; N, 6.04. Found: C, 72.63; H, 5.97;
N, 6.15. FT-IR (KBr, cmꢁ1): 3365 cmꢁ1, 3442 cmꢁ1 (-NH2).
Using the same procedure as for the synthesis of 1a, 1b was
obtained as an orange powder (0.84 g, 82% yield). 1H NMR
(400 MHz, CDCl3): d 7.37 (d, J = 7.2 Hz, 2H, proton of C9), 7.18–7.19
(m, 6H, protons of C6 and C7), 7.11 (t, J = 8.0 Hz, 2H, proton of
C10), 7.03 (d, J = 7.2 Hz, 4H, proton of C8), 6.44 (d, J = 8.0 Hz, 2H, pro-
ton of C11), 4.02 (q, 2H, J = 7.2 Hz, 2H, proton of C12), 1.56–1.64 (m,
12H, protons of C13 and C14). 13C NMR (400 MHz, CDCl3): d 166.80
(C1), 146.28 (C2), 145.23 (C3), 136.96 (C4), 131.37 (C5), 130.21 (C6),
128.87 (C7), 127.55 (C8), 126.08 (C9), 118.78 (C10), 117.19 (C11),
40.16 (C12), 21.76 (C13), 13.66 (C14). Anal. Calcd for C32H30N2Cl2:
C, 74.85; H, 5.89; N, 5.46. Found: C, 74.78; H, 5.96; N, 5.53. FT-IR
(KBr): 1643 cmꢁ1 (C-N).
Synthesis of 4-methyl-2,6-di(sec-phenethyl)aniline c
Using the same procedure as for the synthesis of a [4-methyla-
niline (0.21 g, 2.00 mmol), styrene (0.62 g, 6.00 mmol)], c was
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Appl. Organometal. Chem. (2013)

New chiral a-diimine nickel(II) complexes for ethylene polymerization
Synthesis of bis[N,N0-(4-methyl-2,6-di-sec-phenethylphenyl)imino]-1,2-dimethy-
lethane rac-1c
4.74; N, 4.39. Single crystals of complex rac-2b suitable for X-ray
analysis were obtained at ꢁ30ꢂC by dissolving the nickel
complex in CH2Cl2, followed by slow layering of the resulting
solution with n-hexane. FT-IR (KBr): 1627 cmꢁ1 (C-N).
Synthesis of {bis [N,N0-(4-methyl-2,6-di-sec-phenylethyl) imino]-2,3-butadi-
ene}dibromidonickel 2c
Using the same procedure as for the synthesis of 2a, 2b was
obtained as a light-brown powder (0.72 g, 80% yield). Anal. Calcd
for C50H52N2Ni Br2: C, 66.77; H, 5.83; N, 3.11. Found: C, 66.83; H,
5.75; N, 3.04. FT-IR (KBr): 1628 cmꢁ1 (C-N).
Synthesis of {bis[N,N0-(2,4,6-trimethylphenyl)imino]-2,3-butadiene}dibro-
midonickel 2d
4-Methylbenzenesulfonic acid (20 mg, 0.1 mmol) was added
to a stirred solution of 2,3-butanedione (0.086 g, 1.00 mol) and
4-methyl-2,6-di-sec-phenethylaniline (0.66 g, 2.1 mmol) in tolu-
ene (20 ml). The mixture was refluxed for 16 h, and then the
solvent was removed. The residue was purified by chromatogra-
phy on silica gel with petroleum ether–ethyl ester (v/v, 20:1) to
give a yellow powder. Yield: 0.46 g (68 %). 1H NMR (400 MHz,
CDCl3): d 7.06–7.26 (m, 16H, protons of C6 and C8), 6.99-7.05
(m, 4H, protons of C9), 6.83–6.95 (s, 4H, protons of C7), 3.82 (q,
J = 7.6 Hz, 4H, protons of C10), 2.31 (s, 6H, protons of C11),
1.37–1.64 (m, 18 H, protons of C12 and C13). 13C NMR
(400 MHz, CDCl3): d 169.34 (C1), 146.05 (C2), 145.84 (C3), 133.03
(C4), 132.11 (C5), 128.47 (C6), 127.53 (C7), 126.79 (C8), 125.83
(C9), 39.94 (C10), 22.09 (C11), 21.35 (C12), 16.62 (C13). Anal.
Calcd for C50H52N2: C, 88.19; H, 7.70; N, 4.11. Found: C, 88.31;
H, 6.59; N, 5.05. FT-IR (KBr): 1633 cmꢁ1 (C-N). Single crystals of
ligand rac-1c suitable for X-ray analysis were obtained at
ꢁ30ꢂC by dissolving the ligand in CH2Cl2, followed by slow
layering of the resulting solution with n-hexane.
{Bis [N,N0-(2,4,6-trimethylphenyl)imino]-2,3-butadiene}dibromido-
nickel 2d was synthesized according to the literature.[21]
Synthesis of {Bis[N,N0-(2,4-dimethylphenyl)imino]-2,3-butadiene}dibromi-
donickel 2e
{Bis[N,N0-(2,4-dimethylphenyl)imino]-2,3-butadiene}dibromido-
nickel 2e was synthesized according to the literature.[22]
X-ray Structure Determinations
Single crystals of ligand 1c, complex 2a and complex 2b suitable
for X-ray analysis were obtained at ꢁ30ꢂC by dissolving the
ligand and nickel complex in CH2Cl2, following by slow layering
of the resulting solution with n-hexane. 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 MoKa radiation (l = 0.71073 Å). The determina-
tion 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 structure calculation without further
refinement of the parameters. Crystal data, data collection and
refinement parameters are listed in Table 1.
Synthesis of bis [N,N0-(2,4,6-trimethylphenyl)imino]-1,2-dimethylethane 1d
Bis [N,N0-(2,4,6-trimethylphenyl)imino]-1,2-dimethylethane 1d was
synthesized according to the literature.[21]
Synthesis of bis[N,N0-(2,4-dimethylphenyl)imino]-1,2-dimethylethane 1e
Bis[N,N0-(2,4-dimethylphenyl)imino]-2,3-butadiene 1e was synthesized
according to the literature.[22]
Synthesis of {bis[N,N0-(4-methyl-2-sec-phenethylphenyl)imino]-2,3-butadi-
ene}dibromidonickel rac-2a
[NiBr2(DME)] (0.31 g, 1.00 mmol), ligand 1a (0.47 g, 1.00 mmol)
and dichloromethane (30 ml) were mixed in a Schlenk flask and
stirred at room temperature for 18 h. The resulting suspension
was filtered. The solvent was removed under vacuum and the
residue was washed with diethyl ether (3 ꢀ 16 ml), and then dried
under vacuum at room temperature to give brown catalyst 2a
(0.59 g, 85% yield). Anal. Calcd for C34H36N2Ni Br2: C, 59.08; H,
5.25; N, 4.05. Found: C, 59.17; H, 5.14; N, 4.11. FT-IR (KBr):
1635 cmꢁ1 (C-N). Single crystals of complex rac-2a suitable for
X-ray analysis were obtained at ꢁ30ꢂC by dissolving the nickel
complex in CH2Cl2, followed by slow layering of the resulting
solution with n-hexane.
Ethylene Polymerization
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 N2 atmosphere, 50 ml
dry toluene was added to the polymerization 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 100–800 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 pressure of ethylene (0.2 bar), was terminated by quenching
the reaction mixtures with 100ml of a 2% HCl–MeOH solution.
The precipitated polymer was filtered, washed with methanol
and dried under vacuum at 60ꢂC to a constant weight.
Synthesis of bis{bis[N,N0-(4-chloro-2-sec-phenethylphenyl)imino]-2,3-buta-
diene}dibromidonickel rac-2b
Using the same procedure as for the synthesis of 2a, 2c was
obtained as a purple powder (0.53 g, 85% yield). Anal. Calcd for
C64H60Br2Cl4N4Ni: C, 61.72; H, 4.86; N, 4.50. Found: C, 61.61; H,
Appl. Organometal. Chem. (2013)
Copyright © 2013 John Wiley & Sons, Ltd.
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Table 1. Crystal data and structure refinements of chiral ligand rac-1c, complex rac-2a and complex rac-2b
Complex
2a
2b
1c
Empirical formula
C
34H36Br2N2Ni
C64H60Br2Cl4N4Ni
C50H52 N2
Formula mass
Temperature (K)
Wavelength (Å)
Crystal size (mm3)
Crystal system
Space group
a (Å)
345.59
1245.49
680.94
100
293
296
0.7107
0.7107
0.7107
0.36 ꢀ 0.32 ꢀ 0.14
Orthorhombic
Pbcm
0.33 ꢀ 0.31 ꢀ 0.28
Orthorhombic
Pcaa
0.23 ꢀ 0.21 ꢀ 0.19
Triclinic
P-1
8.3355 (3)
14.9305 (6)
24.2524 (10)
3018.3 (2)
4
16.8349 (7)
14.5175 (7)
24.3195 (10)
5943.7 (5)
4
8.332 (4)
12.392 (6)
20.605 (11)
2007.7 (18)
2
b (Å)
c (Å)
V (Å3)
Z
Density (calcd) (mg cmꢁ3
)
0.761
1.392
1.158
Absorption coefficient (mmꢁ1
)
1.66
1.89
0.06
F(000)
704
2552
732
Theta range for data collec. (ꢂ)
Limiting indices
3.2–28.6
2.9–28.5
2.2–28.3
ꢁ5 ≤ h ≤ 10,
ꢁ17 ≤ k ≤ 18,
ꢁ29 ≤ l ≤ 28
8432
ꢁ20 ≤ h ≤ 20,
ꢁ17 ≤ k ≤ 17,
ꢁ16 ≤ l ≤ 29
16 132
ꢁ22 ≤ h ≤ 22,
ꢁ24 ≤ k ≤ 25,
ꢁ9 ≤ l ≤ 9
14 147
Reflections collected
Independent reflections
Rint
2890
5 652
3 514
0.050
0.093
0.055
Completeness to θ = 25.20ꢂ
Final R indices [I > 2s(I)]
R indices (all data)
Goodness-of-fit on F2
Max. and min. transmission
Largest diff. peak and hole (e Åꢁ3
99.9%
99.6%
99.6%
R1 = 0.0481, wR2 = 0.1265
R1 = 0.0672, wR2 = 0.1429
0.92
R1 = 0.0567, wR2 = 0.1016
R1 = 0.2013, wR2 = 0.1541
0.91
R1 = 0.0840, wR2 = 0.1566
R1 = 0.1646, wR2 = 0.1839
1.029
0.9879 and 0.8953
0.925 and ꢁ0.390
1.000 and 0.8977
0.452 and ꢁ0.276
0.988 and 0.985
0.370 and ꢁ0.205
)
The molecular structure of chiral ligand rac-1c was determined
and the corresponding diagram is shown in (Fig. 1), while selected
bond distances and angles are summarized in Table 2. The X-ray
structure of ligand rac-1c exhibits trans conformation about the
central C—C bond of the ligand backbone. The existence of
two types of isomeric forms rac-1c–M1 and rac-1c–M2 shows that
rac-1c is a stereochemically non-rigid ligand and can change
slightly in solution. Bond lengths and angles are within the
expected range for a-diimines; for example, the bond distances
for the C24¼N1 [rac-1c–M1] or C49-N2 [(rac-1c–M2] double
bond and the central C(24)—C(24i) [rac-1c–M1] or C49–C49ii
[rac-1c–M2] single bond are 1.253(3) or 1.271(3) Å and 1.511(6) or
1.501(6) Å, which are very close to the values for other structurally
characterized free a-diimines.[23] Both C24 [rac-1c–M1] or C49
[rac-1c–M2] and C1 [rac-1c–M1] or C26 [rac-1c–M2] possess essen-
tially rather planar geometry (sp2 character), as shown by the
C24-N1-C1 [rac-1c–M1] or C49-N2-C26 [rac-1c–M2] angles {124.2(3)
[rac-1c–M1] or 121.3(3) [rac-1c–M1]), which are very close to 120ꢂ.
The molecular structure complex rac-2a was also determined
and the corresponding diagram is shown in (Fig. 2), while selected
bond distances and angles are summarized in Table 3. The structure
of complex rac-2a has pseudo-tetrahedral geometry about the
nickel center. In the solid state, the most interesting feature of
ligand rac-2a is the conformation of the substituents attached
to N1 and N1i. These groups are rotated about 180ꢂ from the
position they must occupy to chelate metal Ni. The rotation has
been confirmed by the crystal structure of its complex rac-2a.
The X-ray structures of ligand rac-1a and complex rac-2a
Results and Discussion
Synthesis and Characterization of Ligands 1a–e and Complexes
2a–e
Reaction of the p-substituted anilines with styrene at elevated
temperature (160ꢂC) in the presence of CF3SO3H catalyst resulted
in the corresponding o-sec-phenethylanilines rac-a–c (Scheme 1).
The ligands 1a, 1b, 1d and 1e were prepared by the condensa-
tion of equivalents of the appropriate aniline with one equivalent
of 2,3-butanedione, usually in the presence of a formic acid
catalyst. The a-diimine ligand 1c was synthesized by the Schiff
base condensation of 2,3-butanedione with two equiv. of the
o-sec-phenethylanilines in toluene at 80–110ꢂC using p-toluene-
sulfonic acid (p-TsOH) as catalyst. The compounds (1a–e) were
characterized by elemental analysis, H NMR and 13C NMR.
1
The reaction of equimolar amounts of NiBr2(DME) and the
a-diimine ligands 1a–e in CH2Cl2 led to the displacement of
1,2-dimethoxyethane and afforded the catalyst precursor 2a–e as a
moderately air-stable deep red microcrystalline solid in almost quan-
titative yield. Elemental analysis of complex 2a and ligand 1c fits the
molecular structure obtained by X-ray structural studies.
X-ray Crystallographic Studies
Suitable crystals of chiral ligand 1c, complex 2a and complex 2b
for X-ray diffraction were obtained at ꢁ30ꢂC by double layering a
CH2Cl2 solution of the ligand and complex with n-hexane.
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Appl. Organometal. Chem. (2013)

New chiral a-diimine nickel(II) complexes for ethylene polymerization
Table 2. Selected bond lengths (Å) and angles (ꢂ) for ligand rac-1c
Bond lengths
(Å)
Bond angles
(ꢂ)
C1—N1
1.436 (4)
1.529 (5)
1.253 (3)
1.511 (6)
1.531 (4)
1.529 (5)
1.534 (4)
1.434 (4)
1.519 (4)
1.271 (3)
1.501 (6)
1.526 (5)
1.530 (4)
1.510 (5)
C24—N1—C1
N1—C24—C24i
C2—C1—N1
124.2 (3)
116.7 (3)
117.7 (3)
120.8 (3)
126.8 (3)
112.0 (3)
116.5 (3)
121.3 (3)
117.9 (3)
118.4 (3)
120.3 (3)
113.6 (3)
125.8 (3)
117.9 (3)
C6—C7
C24—N1
C24—C24i
C2—C16
C7—C8
C6—C1—N1
N1—C24—C25
C9—C7—C6
C16—C17
C26—N2
C31—C32
C49—N2
C49—C49ii
C27—C41
C32—C33
C41—C42
C25—C24—C24i
C49—N2—C26
N2—C49—C49ii
C31—C26—N2
C27—C26—N2
C31—C32—C34
N2—C49—C50
C50—C49—C49ii
imino]acenaphthene}dibromidonickel.[21] In fact, the Ni–N bond
distances in complex rac-2a (1.982 Å) are similar to those
determined for these compounds (2.026 and 2.021 Å, respec-
tively), as well as the Ni–Br bond distances (2.338 Å for
complex (rac-2a vs. 2.3229 and 2.323 Å, respectively) and
the N–Ni-Br angles (107.25ꢂ for complex rac-2a vs. 113.32ꢂ
and 114.4ꢂ, respectively).
Previously, we reported the octahedral bis-a-diimine Ni(II)
complexes containing chloro-substituted aryl groups.[7] In this work,
the chiral octahedral bis-a-diimine Ni(II) complexes rac-2b containing
chloro-substituted aryl groups was also synthesized using the same
method. The molecular structure of complex rac-2b was confirmed
and the corresponding diagram is shown in (Fig. 3), while selected
bond distances and angles are summarized in Table 4.
In the solid state, the structure of complex rac-2b shows two
Ar-DAB and two bromide ligands featuring a six-coordinate
distorted octahedral geometry around the nickel center, having
a C2 molecular symmetry. The two a-diimine ligands coordinate
the nickel atom in the equatorial plane, their corresponding
DAB planes making a dihedral angle of 8.109ꢂ, whereas the two
bromide ligands lie in the axial positions, their bonds to the
nickel atom being perpendicular (89.699ꢂ) to the average plane
defined by the nitrogens and the nickel atoms. Its structure is
quite similar to that reported in the literature for the other
Scheme 1. Syntheses of chiral (*chiral carbon) a-diimine ligands 1a–e
and their corresponding a-diimine nickel(II) dibromide complexes 2a–e.
exhibit trans and cis conformation about the central C–C bond of
the backbone, respectively. Both aryl rings bonded to the iminic
nitrogens of the a-diimine lie nearly perpendicular to the plane
formed by the nickel and coordinated nitrogen atoms. Its structure
is similar to those reported in the literature for other similar
[NiBr2(a-diimine)] compounds characterized by X-ray diffraction,
{bis[N,N0-4-bromo-2,6-dimethylphenyl)imino]acenaphthene}
dibromidonickel[24] and {bis[N,N0-(2,4,6-trimethylphenyl)
Figure 2. Molecular structure of the catalyst precursor rac-2a. Hydrogen
Figure 1. Molecular structure of ligand rac-1c–M1. Hydrogen atoms
atoms have been omitted for clarity.
have been omitted for clarity.
Appl. Organometal. Chem. (2013)
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

Table 4. Selected bond lengths (Å) and angles (ꢂ) for complex rac-2b
Table 3. Selected bond lengths (Å) and angles (ꢂ) for complex rac-2a
(ꢂ)
Bond lengths
(Å)
Bond angles
(ꢂ)
Bond lengths
(Å)
Bond angles
Br1—Ni1
Ni1—Br1i
Ni1—N1
Ni1—N1i
Ni1—N2i
Ni1—N2
Cl1—C11
C31—C31i
N1—C14
N1—C15
N2—C30
N2—C31
C6—C7
2.5236 (6)
2.5236 (6)
2.129 (9)
2.129 (9)
2.194 (9)
2.194 (9)
1.722 (12)
1.54 (2)
N1—Ni1—Br1
N1i—Ni1—N1
N1—Ni1—N2
N1i—Ni1—N2
N2—Ni1—N2i
N1—C15—C15i
C14—N1—Ni1
C15—N1—Ni1
N1—Ni1—N2i
N1i—Ni1—N2i
N2i—Ni1—Br1
N2—Ni1—Br1
N2i—Ni1—Br1i
N2—Ni1—Br1i
N2—C31—C31i
C30—N2—Ni1
C31—N2—Ni1
88.3 (2)
75.6 (5)
Br1—Ni1
Br2—Ni1
Ni1—N1i
Ni1—N1
C16—C16i
C16—C17
C6—C8
2.3380 (10)
2.3056 (10)
1.982 (4)
1.982 (4)
1.508 (8)
1.485 (6)
1.518 (6)
1.445 (5)
Br2—Ni1—Br1
N1—Ni1—Br1
N1i—Ni1—Br1
N1—Ni1—Br2
N1i—Ni1—Br2
N1—Ni1—N1i
C1—N1—Ni1
C16—N1—Ni1
136.57 (4)
105.42 (10)
105.42 (10)
107.25 (10)
107.25 (10)
81.1 (2)
103.5 (2)
176.3 (2)
77.7 (5)
114.4 (7)
127.5 (8)
116.1 (7)
176.3 (2)
103.46 (19)
88.2(3)
120.8 (3)
N1—C1
114.7 (3)
1.472 (13)
1.324 (12)
1.420 (12)
1.232 (14)
1.512 (17)
1.571 (14)
1.528 (14)
1.48 (2)
92.0 (3)
92.0 (3)
C7—C8
88.1 (3)
C7—C9
119.0 (8)
128.3 (8)
111.4 (9)
C15—C15i
C15—C16
1.499 (13)
experiments carried out with DEAC alone, under similar conditions,
showed its inability to polymerize ethylene on its own.
At 0ꢂC, the activity of complex (S,R)-2a increases slightly with the
increase of [Al]/[Ni] ratio (runs 1–4), the maximum around [Al]/[Ni]
ratio 600 (run 4), and then the activity decreases slightly with the
increase of [Al]/[Ni] ratio (runs 4 and 5). For a ratio [Al]/[Ni] 600, an in-
crease in the polymerization temperature in the range 0–60ꢂC
slightly decreases the activity of the precatalyst (S,R)-2a (runs 4, 6–8).
The performance of the nickel pre-catalysts is also affected by
o-position substituents on the aniline rings of a-diimine Ni(II)
complexes (Table 5). Complex rac-2c, bearing a methyl substituent
in the para-aryl position and two bulky chiral sec-phenethyl groups in
the ortho-aryl positions of the ligand, displays the highest cata-
lytic activity, of 4.12 ꢀ 106 g PE (mol Ni ꢃ h ꢃ bar)ꢁ1, and produces
one of the highest molecular weights (run 14, Mw = 22.78 ꢀ 104 g
molꢁ1, 0ꢂC, [Al]/[Ni] = 600) among our five complexes.
Figure 3. Molecular structure of the catalyst precursor rac-2b.Hydrogen
atoms have been omitted for clarity.
Complexes rac-2a (bearing a methyl substituent in the para-aryl
position and one chiral sec-phenethyl groups in the ortho-aryl posi-
tions), rac-2b(bearingachlorinesubstituentinthepara-arylposition
and one chiral sec-phenethyl groups in the ortho-aryl positions) and
2d (bearing three methyl substituents in thepara-aryl and ortho-aryl
positions of the ligand), exhibit significantly lower catalytic activity
[NiX2(N,N0-diaryl-a-diimine)2] compound characterized by X-ray,
the bis[bis(N,N0-2,3-dichlorophenylimino)acenaphthene]dibromi-
donickel,[24,25] which contains two a-diimine ligands of the aryl-
BIAN type. In fact, the Ni—N bond distances in complex
rac-2b (2.129Å) are similar to that determined for that compound
(2.144Å), as well as the Ni—Br bond distances (2.5236 Å for
complex rac-2b vs. 2.5354 Å) and the two bromine atoms are
situated in the trans position (Br-Ni-Br angle, 179.87ꢂ for complex
rac-2b vs. 180ꢂ), whereas the halide ligands display a cis geome-
try in the majority of the other nickel complexes containing two
DAB-type ligands with non-aryl and non-bulky N-imine substitu-
ents.[25–27] The two imino C-N bonds have typical double bond
character with C-N bond lengths of 1.232–1.324 Å. The aryl rings
of each of the a-diimines lie nearly perpendicular to the plane
formed by the nickel and coordinated nitrogen atoms, and face
each other in a staggered conformation.
[the highest activity: rac-2a, run 4, 2.08 ꢀ 106 g PE (molNiꢃ h ꢃ bar)ꢁ1
;
rac-2b, run 9, 1.71 ꢀ 106 g PE (mol Niꢃ h ꢃ bar)ꢁ1; and 2d, run 19,
1.75 ꢀ 106 g PE (mol Ni ꢃ h ꢃ bar)ꢁ1].
Complex 2e, bearing only two methyl groups in the ortho-aryl
position, exhibits the lowest catalytic activity and produces the lowest
molecular weight among our three complexes [run 23, the highest
activity: 1.08 ꢀ 106 g PE (mol Niꢃ hꢃ bar)ꢁ1; Mw: 10.95ꢀ 104 gmolꢁ1].
These results indicate that the rate of chain propagation is
greatly promoted by the two 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 ethyl-
ene pressure (0.2 bar), in the range 0–60ꢂC: 2c> 2a ~ 2b~ 2c> 2e.
Compared with complexes 2d and 2e (such as 2e, run 25,
activity: 0.35 ꢀ 106 g PE (mol Niꢃ hꢃ bar)ꢁ1, Mw: 2.28ꢀ 104 gmolꢁ1),
complexes 2a–c (such as 2c, run 17, activity: 1.53 ꢀ 106 g PE
(mol Ni ꢃ h ꢃ bar)ꢁ1, Mw: 4.64 ꢀ 104 g molꢁ1) has better thermal
stability and produces higher-molecular-weight polyethylene,
even at 60ꢂC, the 2a–c/DEAC systems still keep high activity.
Polymerization of Ethylene with Nickel Complexes 2a–e
The five a-diimine nickel (II) complexes 2a–e, activated by DEAC,
were tested as catalyst precursors for the polymerization of ethylene,
under the same reaction conditions. The results of the polymerization
experiments are shown in Table 5. Noteworthy is the fact that blank
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New chiral a-diimine nickel(II) complexes for ethylene polymerization
Table 5. Polymerization of ethylene with complexes 2a–e/DEACa
Activityb
Mw
Mw/Mn
Branchesd
/1000 C
c
c
Run
Pre-catalyst
[Al]/
[Ni]
T
t
Yield
(g)
(ꢂC)
(min)
1
rac-2a
rac-2a
rac-2a
rac-2a
rac-2a
rac-2a
rac-2a
rac-2a
rac-2b
rac-2b
rac-2b
rac-2b
rac-2b
rac-2c
rac-2c
rac-2c
rac-2c
2d
100
0
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
0.93
0.98
1.12
2.08
1.13
1.97
1.34
1.08
2.05
1.80
0.92
0.86
0.81
2.47
2.21
1.56
0.92
0.85
1.05
0.64
0.36
0.60
0.65
0.45
0.21
0.93
0.98
1.12
2.08
1.13
1.97
1.34
1.08
1.71
1.50
0.77
0.72
0.68
4.12
3.68
2.60
1.53
1.42
1.75
1.07
0.60
1.00
1.08
0.75
0.35
22.40
21.38
20.81
20.42
3.58
1.68
1.63
1.64
1.51
1.87
1.88
1.90
2.01
1.75
1.82
1.84
1.96
1.89
1.73
1.81
1.82
1.85
1.67
1.72
1.67\
1.84
1.63
1.67
1.83
1.87
—
—
2
200
400
600
800
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
0
3
0
—
4
0
—
5
0
—
6
20
40
60
0
14.35
5.80
97
7
106
119
—
8
4.10
9
17.14
13.56
8.95
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
20
40
50
60
0
91
102
—
6.09
3.69
114
—
22.78
19.46
13.56
4.64
20
40
60
0
104
118
126
—
20.37
11.43
8.39
2d
20
40\
60
0
85
2d
94
2d
2.97
108
—
2e
15.40
10.95
7.84
2e
20
40
60
82
2e
92
2e
2.28
107
aPolymerization conditions: n(2a) = 5 mmol; n(2b) = 6 mmol; n(2c) = n(2d) = n(2e) = 3 mmol; 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 ꢃ h ꢃ bar)ꢁ1
.
cMw in 104 g molꢁ1, determined by GPC.
13ICH
CH CH
;
dEstimated by 1H NMR. [28] Branches=1000C ¼
ꢀ 1000
I
þI þI
CH
2
3
2
Interesting, new 20-electron chiral octahedral bis-a-diimine Ni(II)
complex rac-2b exhibited high activity toward ethylene polymeriza-
effect of ligand structure on polyethylene branching is much more
complicated.
tion [run 9, the highest activity: 1.71 ꢀ 106 g PE (mol Niꢃ hꢃ bar)ꢁ1
;
As shown in Table 5 and (Fig. 4), the catalyst system rac-2c/DEAC,
bearing two chiral bulky o-sec-phenethyl groups, generated polyeth-
ylene with high degrees of branching. The total branching degrees
of the polymer samples prepared with rac-2c/DEAC (runs 15–17;
branching degree: 104, 118 and 126 branches/1000 C at 20, 40 and
60ꢂC, respectively) are higher than those observed for rac-2b/DEAC
(runs 10, 11, 13; branching degree: 91, 102 and 107 branches/1000
C at 20, 40 and 60ꢂC, respectively), 2a/DEAC (runs 6–8; branching
degree: 97, 106 and 119 branches/1000 C at 20, 40 and 60ꢂC, respec-
tively), 2d/DEAC (runs 19–21; branching degree: 85, 94 and 108
branches/1000 C at 20, 40 and 60ꢂC, respectively), and 2e/DEAC
systems (runs 23–25; branching degree: 82, 92 and 107 branches/
1000 C at 20, 40 and 60ꢂC, respectively).
Mw: 17.14 ꢀ 104 g molꢁ1] under low ethylene pressure, which is
comparable to those of 16-electron [NiBr2(Ar-DAB)] catalyst
precursors 2d and 2e under the same experimental conditions.
These high activities may indicate that the active species operating
in the case of our 20-electron precatalyst rac-2b ([NiBr2(Ar-DAB)2])
is the same as that admitted for the corresponding 16-electron
species [NiBr2(Ar-DAB)].[12] In fact, similar to what was proposed
by other authors,[24] the bis-a-diimine nickel(II) used in the pres-
ent work may be involved in an equilibrium where one of the Ar-
DAB ligands dissociates, giving rise to a 16-electron complex
[NiBr2(Ar-DAB)], which can also originate from coordination/
abstraction from the pre-catalyst of one of such donor chelating
ligands to the Lewis acidic co-catalyst (AlEt2Cl) (Scheme 2). The
16-electron [NiBr2(Ar-DAB)] pre-catalyst can then follow the
classical polymerization mechanism put forward by Brookhart
and co-workers.[12]
The 13C NMR spectrum of the polyethylene prepared with the
catalyst rac-2a/DEAC at 40ꢂC shows in (Fig. 5). The resonances at
19.65 (71 branches/1000 C), 10.09 (9 branches/1000 C), 14.55
(5 branches/1000 C) and 14.10 (19 branches/1000 C) ppm were
attributed to the methyl branch, ethyl branch, propyl branch
and butyl or longer branch carbons, respectively.[29] This result
The type and amount of branches formed in the polymerization
of ethylene promoted by typical a-diimine nickel pre-catalysts
depend on reaction parameters such as the reaction temperature,
ethylene pressure and ligand structure.[12] Generally, low ethylene
pressure and high polymerization temperature favor chain-
walking and afford high-branched polyethylenes.[12] However, the
1
was consistent with that calculated from H NMR.
This difference, observed between complex rac-2c/DEAC and
the others, may be due to the presence of two chiral sec-
phenethyl groups in the para-aryl position of the ligand, which
Appl. Organometal. Chem. (2013)
Copyright © 2013 John Wiley & Sons, Ltd.
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Scheme 2. Possible dissociative equilibrium involving rac-2b and one of the Ar-DAB ligands, and initiation of the polymerization of ethylene.
Figure 5. 13C NMR spectrum (100 MHz, CDCl3–o-dichlorobenzene, 1:3 v/v) of
polyethylene catalyzed by rac-2a/DEAC at 40ꢂC (Table 5, run 7).
group in the para-aryl position, activated by diethylaluminum chlo-
ride (DEAC), showed highly catalytic activity for the polymeriza-
Figure 4. 1H NMR (400 MHz, CDCl3–o-dichlorobenzene, 1:3 v/v) spectra
tion of ethylene, and produced highly branched polyethylenes
of polyethylenes catalyzed by rac-2a, rac-2b and rac-2c/DEAC at 40ꢂC
under low ethylene pressure. The results indicate that the rate
(Table 5: runs 7, 11 and 16). ICH3: integrated intensity between 0.8 and
of chain propagation is greatly promoted by the two bulky
1.0 ppm; ICH2 + ICH: integrated intensity between 1.0 and 1.5 ppm.
ortho-sec-phenethyl groups of the ligand’s aryl rings. Complex
rac-2c/DEAC with two bulky ortho-sec-phenethyl groups may
better stabilize the transition state for chain walking, which
can effectively prohibit the rotations of C–N bonds. Complex rac-
2c/DEAC with two bulky ortho-sec-phenethyl groups may better
should afford a more branched polyethylene. Chiral octahedral
stabilize the transition state for chain walking, which should af-
bis-a-diimine Ni(II) complex rac-2b exhibited high activity
ford a more branched polyethylene.
toward ethylene polymerization, which is comparable to those
of 16-electron [NiBr2(Ar-DAB)] catalyst precursors 2d and 2e
under the same experimental conditions. These high activities
may indicate that the active species operating in the case of
Conclusions
20-electron precatalyst rac-2b is the same as that admitted for
the corresponding 16-electron species.
A series of new chiral a-diimine ligands and their chiral Ni(II) com-
plexes have been prepared and characterized. Ligands 1a–c were
modified in an attempt to change steric effects and the electronic
density of the metal center and eventually to improve the activity
in the polymerization of ethylene and control the microstructure.
The results obtained show that chiral complex rac-2c bearing two
chiral sec-phenethyl groups in the ortho-aryl position and a methyl
SUPPORTING INFORMATION
Supporting information can be found in the online version of this
article.
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Appl. Organometal. Chem. (2013)

New chiral a-diimine nickel(II) complexes for ethylene polymerization
[18] H. H. Brintzinger, D. Fischer, R. Mulhaupt, B. Rieger, R. Waymouth,
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We thank the National Natural Science Foundation of China
(20964003), Chinese Ministry of Education Returned Overseas
Students Scientific Research Foundation (2009-1341) 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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Appendix
CCDC 907762, CCDC 907763 and CCDC 907764 contain the supple-
mentary crystallographic data for ligand 1c, complex 2a and complex
2b. The data can be obtained free of charge via http://www.ccdc.
cam.ac.uk, or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or
e-mail: deposit@ccdc.cam.ac.uk. The NMR spectra of a, b, c, 1a, 1b,
and 1c are available as supporting information.
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Appl. Organometal. Chem. (2013)
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