Nickel(II)-α-diimine complexes containing naphthyl substituents for ethylene polymerization under low ethylene pressure

Article abstract of DOI:10.1007/s11243-013-9699-3

Two new α-diimine containing Ni(II) complexes, {bis[N,N′-(2,6- dimethyl-4-naphthylphenyl)imino]-1,2-dimethylethane}dibromonickel 3a and {bis[N,N′-(2-methyl-4-naphthylphenyl)imino]-1,2-dimethylethane} dibromonickel 3b were synthesized and characterized. Th

Full text of DOI:10.1007/s11243-013-9699-3

Transition Met Chem (2013) 38:341–350
DOI 10.1007/s11243-013-9699-3
Nickel(II)-a-diimine complexes containing naphthyl substituents
for ethylene polymerization under low ethylene pressure
•
Jianchao Yuan Zong Jia Jing Li
•
•
•
Fengying Song Fuzhou Wang Bingnian Yuan
•
Received: 22 November 2012 / Accepted: 15 January 2013 / Published online: 26 January 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract Two new a-diimine containing Ni(II) com-
plexes, {bis[N,N0-(2,6-dimethyl-4-naphthylphenyl)imino]-
1,2-dimethylethane}dibromonickel 3a and {bis[N,N0-
(2-methyl-4-naphthylphenyl)imino]-1,2-dimethylethane}dib-
romonickel 3b were synthesized and characterized. The crystal
structures of representative ligand 2a and its complex 3a were
determined by X-ray crystallography. Complex 3a bearing 2,6-
dimethyl and 4-naphthyl groups, activated by diethylaluminum
chloride (DEAC), shows high catalytic activity for the poly-
merization of ethylene [4.43 9 106 g PE/(mol Ni h bar)].
Interestingly, complexes 3a and 3b bearing the naphthyl sub-
stituent in the para-aryl position produced dendritic polyethyl-
enes (branching degree, 3a: 112, 118, and 147; 3b: 113, 127,
and 151 branches/1,000 C at 20, 40, and 60 °C, respectively).
The dendritic polyethylene particle size obtained by 3a and
3b/DEAC can be controlled in the 1–20 nm range under low
ethylene pressure (diameter, 3a: 18.31, 14.44, and 11.09; 3b:
12.29, 8.98 and 6.27 nm at 20, 40, and 60 °C, respectively) and
could be expected to produce a nano-targeted drug carrier after
modification with water-soluble oligo(ethylene glycol).
Introduction
‘‘Chain walking polymerization CWP’’ catalyzed by Ni(II)/
Pd(II)-a-diimine complexes has attracted increasing atten-
tion due to its ability to produce hyperbranched or dendritic
polymer under low ethylene pressure [1–24] (Scheme 1).
Nanosized dendrimers are artificial macromolecules with
tree-like structures and are synthesized from branched
monomer units in a stepwise manner [25]. Due to their
surface functional groups and highly branched architec-
tures, nanosized dendrimers have an enormous capacity for
solubilization of hydrophobic drugs and can be modified or
conjugated with various interesting guest molecules [25,
26]. Based on the ‘‘enhanced permeability and retention
(EPR) effect’’ [27–30], the nanosized dendrimers have
shown great promise in the development of anticancer drug
delivery systems [31]. However, it is very difficult to
prepare dendrimers through stepwise synthesis. Chain-
walking ethylene polymerization with Ni(II)/Pd(II)-a-dii-
mine catalysts represents a new concept for the synthesis of
hyperbranched or dendritic polyethylenes. CWP followed
by atom transfer radical polymerization (ATRP) can effi-
ciently synthesize water-soluble core–shell [core: polyeth-
ylene, PE; shell: oligo(ethylene glycol), OEG] dendritic
nanoparticles with tunable sizes and reactive surface
functionalities [32], which could play a very important role
in tumor-targeted anticancer drugs. A key part of this
approach is the synthesis of highly active late transition
metal catalysts, which could produce dendritic polyethyl-
ene and control the polyethylene particle diameter in the
nanometer range.
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-013-9699-3) contains supplementary
material, which is available to authorized users.
J. Yuan (&) ꢀ Z. Jia ꢀ J. Li ꢀ F. Song ꢀ F. Wang ꢀ B. Yuan
Key Laboratory of Eco-Environment-Related Polymer Materials,
Ministry of Education, Key Laboratory of Polymer Materials
of Gansu Province, College of Chemistry and Chemical
Engineering, Northwest Normal University,
In this work, we first report the synthesis and character-
ization of two new a-diimine Ni(II) complexes of the type
[NiBr2(Ar–DAB)] (Ar–DAB = N,N0-diaryl-1,4-diaza-1,3-
butadiene) bearing very bulky naphthyl groups (naphthyl
Lanzhou 730070, China
e-mail: jianchaoyuan@nwnu.edu.cn
123

342
Transition Met Chem (2013) 38:341–350
and 1-naphthyl boric acid (0.38 g, 2.20 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. Themixture was extracted three
times with 15 ml diethyl ether. The combined organic phase
was dried over MgSO4, filtered, and the solvent was
removed. The residue was purified by chromatography on
silica gel with petroleum ether/ethyl ester (v/v = 50:1) to
give 2,6-dimethyl-4-naphthylbenzenamine (0.35 g, 71 %
yield). 1H NMR (400 MHz, CDCl3): d 2.30 (s, 6H, –CH3),
4.13 (s, 2H, –NH2), 7.12 (s, 2H, benzenamine), 7.36–7.51
(m, 3H, naphthyl), 7.81 (d, 2H, naphthyl), 7.90 (d, 1H,
naphthyl), 8.01 (d, 1H, naphthyl). 13C NMR (400 MHz,
CDCl3): d 17.71 (carbon 1), 121.48 (carbon 2), 125.37
(carbon 3), 125.61 (carbon 4), 126.35 (carbon 5), 126.77
(carbon 6), 128.12 (carbon 7), 129.89 (carbon 8), 130.36
(carbon 9), 131.88 (carbon 10), 133.81 (carbon 11), 140.58
(carbon 12), 141.92 (carbon 13) (The numbered compounds
are shown in Chart S1 in the Supporting Information).
Scheme 1 Synthesis of nanosized polyethylene
group may behave either as an electron donor or as an
electron attractor) in the para-aryl position and different
groups (one or two electron-donating methyl groups) in the
ortho-aryl position of the arylimino group, in order to study
the influence of different steric effects and electron densities
at the metal center on the catalyst activity and, in particular,
on the microstructure and size of polyethylene.
Experimental
General procedures and materials
All manipulations involving air and/or moisture-sensitive
compounds were carried out with standard Schlenk techniques
under nitrogen. Methylene chloride and o-dichlorobenzene
Synthesis of 2-methyl-4-naphthylbenzenamine 1b
˚
were pre-dried with 4 A molecular sieves and distilled from
Pd(OAc)2 (0.01 g, 0.04 mmol), 4-bromo-2-methylbenzen-
amine (0.56 g, 3.00 mmol), K2CO3 (0.55 g, 4.00 mmol),
and 1-naphthyl boric acid (0.57 g, 3.30 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 15 ml diethyl ether. The combined organic
phase was dried over MgSO4, filtered, and the solvent was
removed. The residue was purified by chromatography
on silica gel with petroleum ether/ethyl ester (v/v = 30:1)
to give 2-methyl-4-naphthylbenzenamine (0.42 g, 60 %
yield). 1H NMR (400 MHz, CDCl3): d 2.12 (s, 3H, –CH3),
3.61 (s, 2H, –NH2), 6.55 (d, 1H, benzenamine), 6.81 (s, 1H,
benzenamine), 6.83 (d, 1H, benzenamine), 7.30–7.51 (m,
3H, naphthyl), 7.82 (d, 2H, naphthyl), 7.93 (d, 1H, naph-
thyl), 8.00 (d, 1H, naphthyl). 13C NMR (400 MHz,
CDCl3): d 17.18 (carbon 1), 110.08 (carbon 2), 114.77
(carbon 3), 116.31 (carbon 4), 122.17 (carbon 5), 124.42
(carbon 6), 125.51 (carbon 7), 126.27 (carbon 8), 126.73
(carbon 9), 126.90 (carbon 10), 128.16 (carbon 11), 128.64
(carbon 12), 129.52 (carbon 13), 132.12 (carbon 14),
132.79 (carbon 15) (The numbered compounds are shown
in Chart S1 in the Supporting Information).
CaH2 under dry nitrogen. Toluene, diethyl ether, and 1,2-
dimethoxyethane (DME) were distilled from sodium/benzo-
phenone under N2 atmosphere. Anhydrous NiBr2 (99 %),
1-naphthyl boric acid (97 %), Pd(OAc)2, and diethylaluminum
chloride (DEAC, 0.9 M solution in toluene) were obtained
from Acros. 2,3-Butanedione (98 %), 2,6-dimethylbenzen-
amine (98 %), 4-bromo-2-methylbenzenamine (98 %), and
4-bromo-2,6-dimethylbenzenamine (98 %) were purchased
from Alfa Aesar and used without further purification.
[NiBr2(DME)] was synthesized according to the literature [33].
NMR spectra were recorded at 400 MHz on a Varian Mer-
cury Plus-400 instrument, using TMS as internal standard. FTIR
spectra were recorded on a Digilab Merlin FTS 3000 FTIR
spectrophotometer on KBr pellets. 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 ml/min and operated at 140 °C. Effective hydrody-
namic diameters of the dendritic polyethylenes (1 mg/ml in
o-dichlorobenzene) were measured using a Zeta-pals dynamic
light-scattering detector (Brookhaven Instruments, Holtsville,
NY, USA) with a 15 mV incident beam at 676 nm at 20 °C.
Correlation functions were collected at a scattering angle of 90°.
Particle sizes were calculated using the multiple angle sizing
option of the instrument’s particle sizing software.
Synthesis of bis[N,N0-(2,6-dimethyl-4-
naphthylphenyl)imino]-1,2-dimethylethane 2a
Formic acid (0.5 ml) was added to a stirred solution of
2,3-butanedione (0.09 g, 1.00 mmol) and 2,6-dimethyl-4-
naphthylbenzenamine (0.49 g, 2.00 mmol) in methanol
(50 ml). The mixture was refluxed for 48 h, cooled, and the
precipitate was separated by filtration. The solid was re-
crystallized from EtOH/CHCl2 (v/v = 10:1), washed, and
Synthesis of 2,6-dimethyl-4-naphthylbenzenamine 1a
Pd(OAc)2 (0.01 g, 0.04 mmol), 4-bromo-2,6-dimethylben-
zenamine (0.40 g, 2.00 mmol), K2CO3 (0.55 g, 4.00 mmol),
123

Transition Met Chem (2013) 38:341–350
343
dried under vacuum. Yield: 0.30 g (55 %). 1H NMR
(400 MHz, CDCl3): d 2.15 (s, 12H, –CH3 of benzenamine),
2.22 (s, 6H, –N=C(CH3)–C(CH3)=N–), 7.25 (s, 4H, ben-
zenamine ring), 7.45–7.55 (m, 6H, naphthyl), 7.86 (d, 4H,
naphthyl), 7.92 (d, 2H, naphthyl), 8.03 (d, 2H, naphthyl).
13C NMR (400 MHz, CDCl3): d 16.13 (carbon 1), 17.95
(–carbon 2), 124.59 (carbon 3), 125.64 (carbon 4), 126.25
(carbon 5), 126.80 (carbon 6), 128.21 (carbon 7), 129.66
(carbon 8), 131.86 (carbon 9), 132.54 (carbon 10), 133.82
(carbon 11), 135.74 (carbon 12), 140.34 (carbon 13),
147.48 (carbon 14), 168.31 (carbon 15) (The numbered
compounds are shown in Chart S1 in the Supporting
Information). Anal. Calc. for C40H36N2: C, 88.20; H, 6.66;
N, 5.14. Found: C, 88.43; H, 6.47; N, 5.01. Single crystals
of ligand 2a suitable for X-ray analysis were obtained at
-30 °C by dissolving the ligand in CH2Cl2, followed by
slow layering of the resulting solution with n-hexane.
ethanol, and dried under vacuum. Yield: 1.16 g (88 %). 1H
NMR (400 MHz, CDCl3): d 2.00 (s, 12H, –CH3 of ben-
zenamine), 2.35 (s, 6H, –N=C(CH3)–C(CH3)=N–), 6.96 (d,
4H, benzenamine ring near methyl), 7.08 (t, benzenamine).
13C NMR (100 MHz, CDCl3): d 15.80 (carbon 1), 17.78
(carbon 2), 123.22 (carbon 3), 124.63 (carbon 4), 127.90
(carbon 5), 148.30 (carbon 6), 168.03 (carbon 7) (The
numbered compounds are shown in Chart S1 in the Sup-
porting Information). Anal. Calcd. for C20H24N2: C, 82.15;
H, 8.27; N, 9.58. Found: C, 82.27; H, 8.14; N, 9.43.
Synthesis of {bis[N,N0-(2,6-dimethyl-4-
naphthylphenyl)imino]-1,2-
dimethylethane}dibromonickel 3a
NiBr2(DME) (0.31 g, 1.00 mmol), ligand 2a (0.54 g,
1.00 mmol), and dichloromethane (40 ml) were mixed in a
Schlenk flask and 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 9 15 ml) and then dried under vacuum at room
temperature to give complex 3a 0.66 g (86 % yield).
Anal. Calcd. for C40H36Br2N2Ni: C, 62.95; H, 4.75; N,
3.67. Found: C, 62.81; H, 4.62; N, 3.54. FT-IR (KBr)
1,631 cm-1 (C=N). Single crystals of complex 3a suitable
for X-ray analysis were obtained at -30 °C by dissolving
the nickel complex in CH2Cl2, following by slow layering
of the resulting solution with n-hexane.
Synthesis of bis[N,N0-(2-methyl-4-
naphthylphenyl)imino]-1,2-dimethylethane 2b
Formic acid (0.5 ml) was added to a stirred solution of
2,3-butanedione (0.12 g, 1.40 mmol) and 2-methyl-4-naph-
thylbenzenamine (0.65 g, 2.80 mmol) in methanol (50 ml).
The mixture was refluxed for 48 h, cooled, and the pre-
cipitate was separated by filtration. The solid was recrys-
tallized from EtOH/CHCl2 (v/v = 10:1), washed, and dried
1
under vacuum. Yield: 0.41 g (57 %). H NMR (400 MHz,
CDCl3): d 2.24 (s, 6H, CH3 of benzenamine), 2.30 (s, 6H,
–N=C(CH3)–C(CH3)=N–), 6.82 (d, 2H, benzenamine),
7.26 (s, 2H, benzenamine), 2.28 (d, 2H, benzenamine),
7.35–7.56 (m, 6H, naphthyl), 7.87 (d, 4H, naphthyl), 7.93
(d, 2H, naphthyl), 8.02 (d, 2H, naphthyl). 13C NMR
(400 MHz, CDCl3): d 15.82 (carbon 1), 17.94 (carbon 2),
117.16 (carbon 3), 125.39 (carbon 4), 125.69 (carbon 5),
126,14 (carbon 6), 126.86 (carbon 7), 127.37 (carbon 8),
128.17 (carbon 9), 128.66 (carbon 10), 131.79 (carbon 11),
132.10 (carbon 12), 133.82 (carbon 13), 136.40 (carbon
14), 140.14 (carbon 15), 148.56 (carbon 16), 167.87 (car-
bon 17) (The numbered compounds are shown in Chart S1
in the Supporting Information). Anal. Calcd. for C38H32N2:
C, 88.34; H, 6.24; N, 5.42. Found: C, 88.15; H, 6.42; N,
5.21.
Synthesis of {bis[N,N0-(2-methyl-4-
naphthylphenyl)imino]-1,2-
dimethylethane}dibromonickel 3b
NiBr2(DME) (0.31 g, 1.00 mmol), ligand 2b (0.52 g,
1.00 mmol), and dichloromethane (40 ml) were mixed in a
Schlenk flask and 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 9 15 ml) and then dried under vacuum at room temper-
ature to give complex 3b (0.63 g, 85 % yield). Anal. Calcd.
for C38H32Br2N2Ni: C, 62.08; H, 4.39; N, 3.81. Found: C,
61.87; H, 4.28; N, 3.95. FT-IR (KBr) 1,624 cm-1 (C=N).
Synthesis of {bis[N,N0-(2,6-dimethylphenyl)imino]-1,2-
dimethylethane}dibromonickel 3c
Synthesis of bis[N,N0-(2,6-dimethylphenyl)imino]-1,2-
dimethylethane 2c
NiBr2(DME) (0.31 g, 1.00 mmol), ligand 2c (0.29 g,
1.00 mmol), and dichloromethane (40 ml) were mixed in a
Schlenk flask and 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 9 15 ml) and then dried under vacuum at room
temperature to give catalyst 3c 0.43 g (84 % yield). Anal.
Formic acid (0.5 ml) was added to a stirred solution of 2,
3-butanedione (0.35 g, 4.00 mmol) and 2,6-dimethylben-
zenamine (0.97 g, 8.00 mmol) in methanol (30 ml). The
mixture was stirred at 45 °C for 24 h, cooled, and the
precipitate was filtered off. The solid was recrystallized
from EtOH/CH2Cl2 (v/v = 10:1), washed with cold
123

344
Transition Met Chem (2013) 38:341–350
Calcd. for C20H24Br2 N2Ni: C, 47.02; H, 4.73; N, 5.48.
Found: C, 47.16; H, 4.59; N, 5.61. FT-IR (KBr)
1,632 cm-1 (C=N).
parameters. Crystal data, data collection, and refinement
parameters are listed in Table 1.
Procedure for the polymerization of ethylene
X-ray structure determinations
The polymerization of ethylene was carried out in a flame
Single crystals of ligand 2a and its complex 3a suitable for dried 250-ml crown-capped pressure bottle sealed with a
X-ray analysis were obtained at -30 °C by dissolving the neoprene septum. After drying the polymerization bottle
ligand and nickel complex in CH2Cl2, followed by slow
layering of the resulting solution with n-hexane. Data col-
lections were performed at 296(2) K on a Bruker SMART
under N2 atmosphere, 50 ml of dry toluene was added to
the polymerization bottle. The resulting solvent was then
saturated with a prescribed ethylene pressure. The co-cat-
alyst (DEAC) was then added in Al/Ni molar ratios in the
APEX diffractometer with a CCD area detector, using
˚
graphite monochromated MoKa radiation (k = 0.71073 A). range of 200–1,000 to the polymerization bottle via a
The determination of crystal class and unit cell parameters
was carried out by 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
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 polymeriza-
tion, conducted under a dynamic pressure of ethylene
(0.2 bar), was terminated by quenching the mixtures with
100 ml of a 2 % HCl–MeOH solution. The precipitated
polymer was filtered off, washed with methanol, and dried
under vacuum at 60 °C to a constant weight.
Table 1 Crystal data and structure refinements of ligand 2a and complex 3a
Empirical formula
Formula mass
C40H36N2
C40H36Br2N2Ni
544.71
763.24
Temperature (K)
296
293 K
˚
Wavelength (A)
0.71073
0.71073
Crystal size (mm3)
Crystal system
Space group
0.23 9 0.21 9 0.16
0.20 9 0.16 9 0.14 mm
Monoclinic
Orthorhombic, Pnma
P21/c
P21/m
˚
a (A)
11.214(7)
13.971(11)
˚
b (A)
9.274(6)
28.24(2)
˚
c (A)
15.628(9)
8.635(7)
3
V (A )
˚
1,584.9(16)
3,407(5)
Z
2
4
Density (calcd.) (mg/cm3)
Absorption coefficient (mm-1
F(000)
1.141
1.488
)
0.07
2.95
580
1,552
Theta range for data collection (°)
Limiting indices
h = 2.6–25.5°
-8 B h B 13, -11 B k B 9, -18 B l B 18
2.5–20.6
-16 B h B 15, -33 B k B 31, -8 B l B 10
Reflections collected
1,501
14,330
Independent reflections
Rint
2,900
3,058
0.044
0.129
Completeness to h = 25.50°
Final R indices [I [ 2h(I)]
R indices (all data)
98.10 %
99.00 %
R1 = 0.0631, wR2 = 0.1501
R1 = 0.1318, wR2 = 0.1885
Full-matrix least-squares on F2
1.02
R1 = 0.0799, wR2 = 0.1734
R1 = 0.1911, wR2 = 0.2283
Full-matrix least-squares on F2
1.06
Refinement method
Goodness of fit on F2
Maximum and minimum transmission
0.985 and 0.990
0.27 and -0.19
0.683 and 0.590
1.16 and -0.40
-3
˚
Largest difference peak and hole (e A
)
123

Transition Met Chem (2013) 38:341–350
345
Scheme 2 Syntheses of a-
diimine ligands 2a–c and their
corresponding a-diimine
nickel(II) dibromide complexes
3a–c
Results and discussion
Suitable crystals of ligand 2a and complex 3a for X-ray
diffraction were obtained at -30 °C by double layering a
CH2Cl2 solution of the ligand and complex with n-hexane.
The molecular structure of ligand 2a 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 2a exhibits trans-conformation about
the central C–C bond of the ligand backbone. Bond lengths
and angles are within the expected range for a-diimines; for
example, the bond distances for the C(19)=N(1) double bond
Synthesis and characterization of ligands
2a–c and complexes 3a–c
The Suzuki coupling reaction of 4-bromo-2,6-dimethyl-
benzenamine (4-bromo-2-dimethylbenzenamine) and
1-naphthyl boric acid catalyzed by Pd(OAc)2 in PEG-400
led to the desired amine 2,6-dimethyl-4-naphthylbenzena-
mine 1a (2-methyl-4-naphthylbenzenamine 1b) in 71 %
(60 %) yield (Scheme 2). The a-diimine ligands (2a–
c) were finally obtained by acid-catalyzed condensation
of the amines and 2,3-butanedione, and recrystallized
from EtOH/CH2Cl2 to afford microcrystalline solids. The
˚
and the central C(19)–C(19a) single bond are 1.272(3) A and
˚
1.485(5) A, which are very close to the values for other
structurally characterized free a-diimines [34]. Both C(19)
and C(1) possess essentially rather planar geometry (sp2
character), as shown by the C(19)–N(1)–C(1) angles [121.6
(2)], which are very close to 120°.
The molecular structure of complex 3a was also deter-
mined and the corresponding diagram is shown in Fig. 2,
while selected bond distances and angles are summarized
in Table 2. The structure of complex 3a has pseudo-
tetrahedral geometry about the nickel center, showing C2v
molecular symmetry. In the solid state, the most interesting
feature of ligand 2a is the conformation of the substituents
attached to N(1) and N(1a). These groups are rotated about
ligands (2a–c) were characterized by H- and 13C-NMR
1
and were found to be elementally pure. Elemental analysis
of ligand 2a fits the molecular structure obtained by the
X-ray structural studies (see below).
The reaction of equimolar amounts of NiBr2(DME) and
the a-diimine ligands (2a–c) in CH2Cl2 led to the displace-
ment of 1,2-DME and afforded the catalyst precursors (3a–
c) as a moderately air-stable microcrystalline solids in high
yields. Elemental analysis of complex 3a fits the molecular
structure obtained by the X-ray structural studies.
123

346
Transition Met Chem (2013) 38:341–350
Fig. 1 Molecular structure of
ligand 2a. Hydrogen atoms have
been omitted for clarity
˚
The C19–N1–C14 bond angle of complex 3a (119.7°) is
slightly smaller than that of its ligand 2a (121.6°) due to the
N–Ni coordination. 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.
The methyl groups in the 2,6-positions of the benzenamine
fragments in 3a point toward each other above and below
the plane, thus shielding the apical positions of the Ni(II)
center. Its structure is similar to those reported in the lit-
erature for other similar [NiBr2(a-diimine)] compounds
characterized by X-ray diffraction, {bis[N,N0-4-bromo-2,6-
dimethylphenyl)imino]acenaphthene}dibromonickel [35]
and {bis[N,N0-(2,4,6-trimethylphenyl)imino]acenaphthene}
dibromonickel [36]. In fact, the Ni–N bond distances in
Table 2 Selected bond lengths (A) and angles (°) for ligand 2a and
complex 3a
˚
(A)
Bond lengths
3a
2a
Ni1–N1a
Ni1–N1
2.001(7)
2.001(7)
Ni1–Br1
Ni1–Br2
C11–C12
C11–C16
C13–C14
C13–C17
C14–C15
C14–N1
C15–C18
C19–N1
C19–C19a
2.328(3)
2.340(3)
1.382(13)
1.418(14)
1.395(12)
1.496(13)
1.395(12)
1.470(10)
1.471(13)
1.280(10)
1.537(15)
1.380(4)
1.389(4)
1.388(4)
1.511(4)
1.391(4)
1.422(3)
1.501(4)
1.272(3)
1.485(5)
˚
complex 3a (2.001 A) are similar to those determined for
˚
these compounds (2.026 and 2.021 A, respectively), as
˚
well as the Ni–Br bond distances (2.328 A for complex 3a
˚
vs. 2.3229 and 2.323 A, respectively) and the N–Ni–Br
angles (116.1° for complex 3a vs. 113.32 and 114.4°,
respectively).
Bond angles
(°)
N1a–Ni1–N1
N1a–Ni1–Br1
N1–Ni1–Br1
N1a–Ni1–Br2
N1–Ni1–Br2
Br1–Ni1–Br2
C13–C12–C11
C12–C13–C14
C12–C13–C17
C14–C13–C17
N1–C19–C20
C20–C19–C19a
C19–N1–C14
80.2(4)
116.1(2)
116.1(2)
110.7(2)
110.7(2)
117.39(11)
122.6(10)
117.5(9)
119.6(10)
122.8(8)
127.2(7)
118.5(5)
119.7(7)
Polymerization of ethylene with nickel complexes 3a–c
The three a-diimine nickel (II) complexes 3a–c, activated
by DEAC, were tested as catalyst precursors for the poly-
merization of ethylene, under the same reaction conditions.
The results of the polymerization experiments are shown
in Table 3. Noteworthy is the fact that blank experiments
carried out with DEAC alone, under similar conditions,
showed its inability to polymerize ethylene on its own.
An increase of the [Al]/[Ni] ratio in the range 200–1,000
increases slightly the activity of complexes 3a and 3b, at
10 °C, which seem to go through a maximum around [Al]/
[Ni] = 600 (3a: runs 1–5; 3b: runs 10–14). For a ratio [Al]/
[Ni] = 600, the highest activities of complexes 3a (runs 3,
6–9) and 3b (runs 12, 15–18) appeared around 10 °C
[the highest activities of complex 3c (runs 19–22) appeared
around 0 °C].
122.1(3)
118.9(3)
120.6(3)
120.5(3)
125.3(3)
118.4(3)
121.6(2)
180° from the position they must occupy to chelate a metal
center. The rotation has been confirmed by the crystal
structure of its complex 3a. The X-ray structures of ligand
2a and complex 3a exhibit trans- and cis-conformation
about the central C–C bond of the backbone, respectively.
˚
The imino C=N bond length of complex 3a (1.280 A)
˚
is slightly larger than that of its ligand 2a (1.272 A).
The performances of the nickel precatalysts are signifi-
cantly affected by the position of the naphthyl and methyl
substituents on the aryl rings of the a-diimine Ni(II)
complexes 3a–c (Table 3). Complex 3a, bearing a naphthyl
123

Transition Met Chem (2013) 38:341–350
347
Fig. 2 Molecular structure of
complex 3a. Hydrogen atoms
have been omitted for clarity
substituent in the para-aryl position of the ligand (two
methyl groups in the ortho-aryl positions), displays the
highest catalytic activity, of 4.43 9 106 g PE/(mol Ni h bar),
and produces one of the highest molecular weights
(Mn = 10.13 9 104 g/mol, run 3, 10 °C, [Al]/[Ni] = 600)
among our three complexes. Complex 3b, bearing a
naphthyl substituent in the para-aryl position of the ligand
(only one methyl group in the ortho-aryl positions),
exhibits a significantly lower catalytic activity and pro-
duces a lower molecular weight [run 12, the highest
activity: 1.03 9 106 g PE/(mol Ni h bar); Mn: 7.86 9
104 g/mol]. These results indicate that the rate of chain
Table 3 Polymerization of ethylene with complexes 3a–c/DEAC
Run Complex [Al]/[Ni] T (°C) t (min) Yield (g) Activitya TOFb (h bar)-1 Mn (g/mold) Mw/Mnc Branchesd/ De (nm)
1,000 C
1
3a
3a
3a
3a
3a
3a
3a
3a
3a
3b
3b
3b
3b
3b
3b
3b
3b
3b
3c
3c
3c
3c
200
400
600
800
1,000
600
600
600
600
200
400
600
800
1,000
600
600
600
600
600
600
600
600
10
10
10
10
10
0
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
1.753
3.81
3.79
4.43
4.32
4.25
2.19
2.74
1.91
1.08
0.79
0.91
1.03
1.01
0.86
0.52
0.96
0.85
0.78
1.30
0.95
0.61
0.44
1.36
1.35
1.58
1.54
1.52
0.78
0.98
0.68
0.38
0.28
0.32
0.37
0.36
0.31
0.19
0.35
0.30
0.28
0.46
0.34
0.22
0.16
7.05
9.85
10.13
6.45
7.63
9.68
12.93
8.13
7.67
4.34
5.65
7.86
6.77
6.21
5.89
9.05
5.93
4.10
10.03
8.81
6.05
4.47
1.68
1.71
1.74
1.78
1.79
1.72
1.79
1.83
1.98
1.68
1.68
1.70
1.71
1.77
1.68
1.75
1.81
1.95
1.71
1.73
1.85
1.92
–
–
–
2
1.743
–
3
2.0377
1.986
–
–
4
–
–
5
1.9547
1.0089
1.2594
0.8765
0.4955
0.378
–
–
6
–
–
7
20
40
60
10
10
10
10
10
0
112
118
147
–
18.31 2.1
8
14.44 1.9
9
11.09 1.5
10
11
12
13
14
15
16
17
18
19
20
21
22
–
0.4345
0.4954
0.4864
0.4129
0.2497
0.4732
0.4059
0.3762
1.2976
0.9465
0.6124
0.4356
–
–
–
–
–
–
–
–
–
–
20
40
60
0
113
127
151
–
12.29 2.3
8.98 1.6
6.27 2.1
–
20
40
60
98
108
118
16.57 3.1
9.78 2.9
7.51 2.2
Polymerization conditions: n(3a) = 2.23 lmol, n(3b) = 2.24 lmol, n(3c) = 5.00 lmol, ethylene relative pressure = 0.2 bar, ethylene absolute
pressure = 1.2 bar, t polymerization time, solvent = toluene (50 ml), T polymerization temperature
a
Activity in 106 g PE/(mol Ni h bar)
b
Turnover frequency in 105 mol Ethylene/(mol Ni h bar)
c
Mn in 104 g/mol, determined by GPC
13ICH
d
e
1
Estimated by H NMR [37]. Branches/1,000 C =
3
þI
ꢁ 1; 000
I
þI
CH
CH CH
2
3
2
Estimated by particle size analyzer
123

348
Transition Met Chem (2013) 38:341–350
propagation is greatly promoted by the two ortho-methyl
groups of the ligand’s aryl rings. Complex 3c bearing only
two methyl groups in the ortho-aryl positions of the
ligand [run 19, the highest activity: 1.30 9 106 g PE/
(mol Ni h bar); Mn: 10.03 9 104 g/mol] exhibits an activity
close to complex 3b. This difference, observed between
complex 3a and 3c, may be partially due to the presence of
the naphthyl group in the para-aryl position of the ligand
(naphthyl group may behave either as an electron donor or
as an electron attractor). As a result, the following activity
trend can be summarized for our substituted precatalysts
under low ethylene pressure (0.2 bar), in the range 0–60 °C:
3a [ 3b * 3c.
Control of the polyethylene topology and size by a-
diimine ligand structures and polymerization conditions
The type and amount of branches formed in the polymeriza-
tion of ethylene promoted by typical a-diimine nickel pre-
catalysts depend on reaction parameters such as the reaction
temperature, ethylene pressure, and ligand structure[20].
Generally, low ethylene pressure and high polymerization
temperature favor the Chain Walking mechanism and afford
high branched polyethylenes [20]. However, the effect of
ligand structure on polyethylene branching is much more
complicated.
Fig. 3 1H NMR (CDCl3/o-dichlorobenzene, v/v = 1:3) spectra of
the nanosized dendritic polyethylene catalyzed by 3a, 3b, and 3c/
DEAC at 60 °C (Table 3, run 9, run 18, run 22). ICH integrated
3
intensity between 0.8 and 1.0 ppm, ICH þ ICH: integrated intensity
2
between 1.0 and 1.5 ppm
{bis[N,N0-(2,6-diisopropylphenyl)imino]-1,2-dimethylethane}
dibromonickel (30, 67, 80, and 90 branches/1,000 C, at 25,
50, 65, and 80 °C, respectively) [20], and {bis[N,N0-(2,6-diiso-
propylphenyl)imino]acenaphthene}dibromonickel (65 branches/
1,000 C, at 25 °C) [20], although the reaction conditions
are not exactly the same as in the present work. A possible
explanation is that the naphthyl group in the para-aryl
position of the ligand (naphthyl group may behave either as
an electron donor or as an electron attractor) may better
stabilize the transition state for monomer chain walking
than for insertion, which should afford a more dendritic
polyethylene.
Recent studies by Guan and coworkers [38–40] dem-
onstrated that in ethylene polymerization, the PE branching
topology could be simply controlled by the pressure of
ethylene, with linear PE being formed at high pressure and
dendritic PE being obtained at low pressure. So, in this
study, the low ethylene pressure (0.2 bar) was employed.
It is of particular interest that complexes 3a and 3b
bearing the naphthyl substituent in the para-aryl position of
the ligand produced the more dendritic polyethylenes than
that of complex 3c (Table 3; Fig. 3). The total branching
degrees of the polymer samples prepared with 3a/DEAC
(runs 7–9, branching degree: 112, 118, and 147 branches/
1,000 C at 20, 40, and 60 °C, respectively; diameter: 18.31,
14.44, and 11.09 nm at 20, 40, and 60 °C, respectively),
and 3b/DEAC (runs 16–18, branching degree: 113, 127,
and 151 branches/1,000 C at 20, 40, and 60 °C, respec-
tively; diameter: 12.29, 8.98, and 6.27 nm at 20, 40, and
60 °C, respectively) are much higher than that observed for
3c/DEAC system (runs 20–22, branching degree: 98, 108,
and 118 branches/1,000 C at 20, 40, and 60 °C, respectively;
diameter: 16.57, 9.78, and 7.51 nm at 20, 40, and 60 °C,
respectively). Also, the total branching degrees of the
polymer samples prepared with 3a and 3b/DEAC are higher
than those observed for similar precatalyst/DEAC systems
such as {bis[N,N0-(4-tert-butyl-diphenylsilyl-2,6-diisopro-
pylphenyl)imino]acenaphthene}dibromonickel (45 branches/
1,000 C, at 20 °C) [20] or precatalyst/MAO systems such as
As shown in Fig. 4, the number of branches was also
calculated according to the literature [41], and it was found
that the polyethylene with 144 branches/1,000 carbons (99
methyl, 12 ethyl, 8 propyl, and 25 butyl or longer branches/
1,000 C) was obtained at 60 °C (entry 8 in Table 3). This
1
result was consistent with that calculated from H NMR.
Conclusions
Two new a-diimine ligands 2a and 2b, and their Ni(II)
complexes 3a and 3b have been prepared and character-
ized. Ligands 2a and 2b 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 poly-
merization of ethylene and control the microstructure and
123

Transition Met Chem (2013) 38:341–350
349
Fig. 4 13C NMR (CDCl3/o-dichlorobenzene, v/v = 1:3) spectrum of
the nanosized dendritic polyethylene catalyzed by 3a/DEAC at 60 °C
(Table 3, run 9). Note on labels 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 labeled with Greek letters which
determine how far from a branch point methine each methylene is; a
denotes the first methylene next to the methine. Thus, cB1? refers to
methylenes c from a branch of length 1 or longer
References
size of the polyethylenes obtained. The results obtained
show that complex 3a, activated by DEAC, produces a
highly active catalyst system for the polymerization of
ethylene. Complexes 3a and 3b bearing the naphthyl sub-
stituent in the para-aryl position of the ligand produced
more nanosized dendritic polyethylene. A ligand bearing
a naphthyl group in the para-aryl position (naphthyl group
may behave either as an electron donor or as an electron
attractor) may better stabilize the transition state for
monomer chain walking than for insertion, which should
afford a more dendritic polyethylene.
1. Sun G, Hentschel J, Guan Z (2012) ACS Macro Lett 1:585–588
2. Zhang Z, Ye Z (2012) Chem Commun 48:7940–7942
3. Killian CM, Tempel DJ, Johnson LK, Brookhart M (1996) J Am
Chem Soc 118:11664–11665
¨
4. Popeney CS, Lukowiak MC, Bottcher C, Schade B, Welker P,
Mangoldt D, Gunkel G, Guan Z, Haag R (2012) ACS Macro Lett
1:564–567
5. Gao H, Hu H, Zhu F, Wu
48:3312–3314
6. Shi X, Zhao Y, Gao H, Zhang L, Zhu F, Wu Q (2012) Macromol
Rapid Commun 33:374–379
7. Johnson LK, Killian CM, Brookhart M (1995) J Am Chem Soc
117:6414–6415
Q (2012) Chem Commun
8. Yuan JC, Mei TJ, Gomes PT, Marques MM, Wang XH, Liu YF,
Miao CP, Xie XL (2011) J Organomet Chem 696:3251–3256
9. Yuan JC, Silva LC, Gomes PT, Campos JM, Riberio MR, Val-
erga PS, Chien JCW, Marques MM (2005) Polymer
46:2122–2132
Supplementary material
CCDC 905358 and 905359 contain the supplementary
crystallographic data for ligand 2a and complex 3a. These
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.
ˇ
ˇ ´ ´
10. Peleska J, Hostalek Z, Hasalıkova D, Merna J (2011) Polymer
´
52:275–281
11. Wegner MM, Ott AK, Rieger
43:3624–3633
B (2010) Macromolecules
12. Choi Y, Soares JBP (2010) Polymer 51:2271–2276
13. Camachoa DH, Guan Z (2010) Chem Commun 46:7879–7893
14. Guan Z, Popeney CS (2009) Top Organomet Chem 26:179–220
15. Takeuchi D, Fukuda Y, Park S, Osakada K (2009) Macromole-
cules 42:5909–5912
16. Li L, Jeon M, Kim SY (2009) J Mol Catal A Chem 303:110–116
17. Popeney C, Guan Z (2005) Organometallics 24:1145–1155
18. Liu FS, Hu HB, Xu Y, Guo LH, Zai SB, Song KM, Gao HY,
Zhang L, Zhu FM, Wu Q (2009) Macromolecules 42:7789–7796
Acknowledgments We thank the National Natural Science Foun-
dation 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.
123

350
Transition Met Chem (2013) 38:341–350
´
30. Danhier F, Feron O, Preat
148:135–146
19. Cherian AE, Rose JW, Lobkovsky EB, Coates GW (2005) J Am
Chem Soc 127:13770–13771
V
(2010)
J
Control Release
´
20. Ittel SD, Johnson LK, Brookhart
100:1169–1203
21. Johnson LK, Mecking S, Brookhart M (1996) J Am Chem Soc
118:267–268
22. Mecking S, Johnson LK, Wang L, Brookhart M (1998) J Am
Chem Soc 120:888–899
M
(2000) Chem Rev
31. Gillies ER, Frechet JMJ (2005) Drug Discov Today 10:35–43
32. Chen G, Huynh D, Felgner PL, Guan Z (2006) J Am Chem Soc
128:4298–4302
33. Ward LGL, Pipal JR (1972) Inorg Synth 13:162–163
34. Van KG, Vrieze K (1982) Adv Organomet Chem 21:151–239
35. Song CL, Tang LM, Li YG, Li XF, Chen J, Li YS (2006) J Polym
Sci Pol Chem 44:1964–1974
36. Maldanis RJ, Wood JS, Chandrasekaran A, Rausch MD, Chien
JCW (2002) J Organomet Chem 645:158–167
37. Meinhard D, Wegner M, Kipiani G, Hearley A, Reuter P, Fischer
S, Marti O, Rieger B (2007) J Am Chem Soc 129:9182–9191
38. Guan Z, Cotts PM, McCord EF, McLain SJ (1999) Science
283:2059–2062
39. Guan Z (2002) Chem Eur J 8:3086–3092
40. Cotts PM, Guan Z, McCord EF, McLain SJ (2000) Macromole-
cules 33:6945–6952
23. Gottfried AC, Brookhart
3085–3100
M (2003) Macromolecules 36:
24. Schrekker HS, Kotov V, Preishuber-Pflugl P, White P, Brookhart
M (2006) Macromolecules 39:6341–6354
25. Yang WJ, Cheng YY, Xu TW, Wang XY, Wen LP (2009) Eur J
Med Chem 44:862–868
26. Svenson S, Tomalia DA (2005) Adv Drug Deliv Rev 57:
2106–2129
27. Matsumura Y, Maeda H (1986) Cancer Res 46:6387–6392
28. Maeda H, Sawa T, Konno T (2001) J Control Release 74:47–61
29. Maeda H, Bharate GY, Daruwalla J (2009) Eur J Pharm Biop-
harm 71:409–419
41. Galland GB, Souza RF, Mauler RS, Nunes FF (1999) Macro-
molecules 32:1620–1625
123