Chiral naphthyl-α-diimine nickel(II) catalysts bearing sec -phenethyl groups: Chain-walking polymerization of ethylene at high temperature and stereoselective polymerization of methyl methacrylate at low temperature

Article abstract of DOI:10.1021/om400433t

A series of new naphthyl-α-diimine nickel(II) complexes, {bis[N,N′-(1-naphthyl)imino]-1,2-dimethylethane}dibromonickel (2a), {bis[N,N′-(2-methyl-1-naphthyl)imino]-1,2-dimethylethane}dibromonickel (2b), {bis[N,N′-(2-sec-phenethyl-1-naphthyl)imino]-1,2-dimethylethane} dibromonickel (rac-(RR/SS)-2c), {bis[N,N′-(2-methyl-1-naphthyl)imino] acenaphthene}dibromonickel (2d), and {bis[N,N′-(2-naphthyl)imino]-1,2- dimethylethane}dibromonickel (2e), were synthesized and characterized. The crystal structures of ligands 1b, rac-(RR/SS)-1c, 1d, 1e and their representative complexes rac-(RR/SS)-2c and 2d were determined by X-ray crystallography. These complexes, activated by diethylaluminum chloride (DEAC), were tested in the polymerization of ethylene and methyl methacrylate under mild conditions. Complex rac-(RR/SS)-2c, bearing chiral bulky sec-phenethyl groups in the o-naphthyl position, activated by diethylaluminum chloride (DEAC) shows highly catalytic activity for the polymerization of ethylene (2.81 × 10⁶ g PE/((mol of Ni) h bar)) and produced branched polyethylene (75 methyl, 9 ethyl, 5 propyl, and 19 butyl or longer branches/1000 C at 40 C). Interestingly, rac-(RR/SS)-2c could produce syndiotactic PMMA at low temperature (-30 C: rr 88.75%, mr 7.26%, mm 3.99%).

Full text of DOI:10.1021/om400433t

Article
pubs.acs.org/Organometallics
Chiral Naphthyl-α-diimine Nickel(II) Catalysts Bearing sec-Phenethyl
Groups: Chain-Walking Polymerization of Ethylene at High
Temperature and Stereoselective Polymerization of Methyl
Methacrylate at Low Temperature
Jianchao Yuan,* Fuzhou Wang, Weibing Xu, Tongjian Mei, Jing Li, Bingnian Yuan, Fengying Song,
and Zong Jia
Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, Key Laboratory of Polymer Materials of
Gansu Province, College of Chemistry & Chemical Engineering, Northwest Normal University, Lanzhou 730070, People’s Republic
of China
S
* Supporting Information
ABSTRACT: A series of new naphthyl-α-diimine nickel(II) complexes,
{bis[N,N′-(1-naphthyl)imino]-1,2-dimethylethane}dibromonickel (2a), {bis-
[N,N′-(2-methyl-1-naphthyl)imino]-1,2-dimethylethane}dibromonickel (2b),
{bis[N,N′-(2-sec-phenethyl-1-naphthyl)imino]-1,2-dimethylethane}-
dibromonickel (rac-(RR/SS)-2c), {bis[N,N′-(2-methyl-1-naphthyl)imino]-
acenaphthene}dibromonickel (2d), and {bis[N,N′-(2-naphthyl)imino]-1,2-
dimethylethane}dibromonickel (2e), were synthesized and characterized.
The crystal structures of ligands 1b, rac-(RR/SS)-1c, 1d, 1e and their
representative complexes rac-(RR/SS)-2c and 2d were determined by X-ray
crystallography. These complexes, activated by diethylaluminum chloride
(DEAC), were tested in the polymerization of ethylene and methyl
methacrylate under mild conditions. Complex rac-(RR/SS)-2c, bearing chiral
bulky sec-phenethyl groups in the o-naphthyl position, activated by
diethylaluminum chloride (DEAC) shows highly catalytic activity for the polymerization of ethylene (2.81 × 106 g PE/((mol
of Ni) h bar)) and produced branched polyethylene (75 methyl, 9 ethyl, 5 propyl, and 19 butyl or longer branches/1000 C at 40
°C). Interestingly, rac-(RR/SS)-2c could produce syndiotactic PMMA at low temperature (−30 °C: rr 88.75%, mr 7.26%, mm
3.99%).
N,N′-dinaphthyl-1,4-diaza-1,3-butadiene) and [NiBr2(Na-
BIAN)] (Na-BIAN = bis(naphthylimino)acenaphthene) bear-
ing different groups (H, methyl, and chiral sec-phenethyl group)
in the o-naphthyl position, in order to study the influence of
different steric effects and electron densities at the metal center
on the catalyst activity, on the microstructure of polyethylene,
and, in particular, on the stereoregularity structure of
poly(methyl methacrylate).
1. INTRODUCTION
Polymerization of α-olefin catalyzed by [NiII/PdIIX2(aryl-α-
diimine)] (X = halide) containing a bulky α-diimine ligand1−14
has stimulated renewed interest. In contrast to metallocene
catalysts based on early transition metals,15−18 Ni(II) and
Pd(II) catalysts produced branched polyethylene with different
concentrations and individual lengths of branches1,12 exclu-
sively from the ethylene monomer and accommodated even
polar monomers, such as acrylates.11,13,14 This discovery is
beneficial to commercial applications and industrial production.
Recently, Coates and co-workers proposed a new Brookhart
type catalyst for olefin polymerization, which bears a new class
of chiral anilines, and their incorporation in α-diimine Ni(II)
catalysts.19,20 However, naphthyl-α-diimine Ni(II)/Pd(II)
complexes have only been reported in limited cases.21 In
particular, naphthyl-α-diimine Ni(II)/Pd(II) complexes bearing
chiral sec-phenethyl groups have never been studied.
2. RESULTS AND DISCUSSION
2.1. Synthesis and Characterization of Ligands 1a−e
and Complexes 2a−e. The general synthetic routes of the
nickel(II) complexes 2a−e are shown in Scheme 1. Reaction of
the naphthylamine with styrene at elevated temperature (160
°C) in the presence of CF3SO3H catalyst resulted in the
corresponding o-sec-phenethyl naphthylamine compound rac-c
(Scheme 1). Ligands 1a,b,d,e were prepared by the
condensation of equivalents of the appropriate naphthylamine
with 1 equiv of 2,3-butanedione or acenaphthenequinone,
Recently, we reported the “chain-walking polymerization” of
ethylene using chiral22,23 or achiral7,8,24,25 aryl-α-diimine Ni(II)
complexes. Here, we first report the synthesis and character-
ization of a series of new chiral or achiral naphthyl-α-diimine
Ni(II) complexes of the types [NiBr2(Na-DAB)] (Na-DAB =
Received: May 21, 2013
© XXXX American Chemical Society
A
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a
1.273(2)/1.275(3)/1.271(2) and 1.501(3)/1.497(5)/1.492(3)
Å, respectively, which are very close to the values for other
structurally characterized free α-diimines.26 Both C12/C19/
C11 and C1/C9/C1 possess an essentially rather planar
geometry (sp2 character), as shown by the C12−N1−C1/
C19−N1−C9/C11−N1−C1 angles (120.80(16)/121.3(2)/
120.69(15)°), which are very close to 120°.
Scheme 1. Syntheses of α-Diimine ligands 1a−e and Their
Corresponding Bis-α-diimine Nickel(II) Dibromide
Complexes 2a−e
The molecular structure of complex rac-(RR/SS)-2c was also
determined, and the corresponding diagram is shown in Figure
1. The structure of complex rac-(RR/SS)-2c has pseudote-
Figure 1. Molecular structure of the catalyst precursor rac-(R,S)-2c.
Hydrogen atoms have been omitted for clarity. Selected bond lengths
(Å) and angles (deg) for complex rac-(R,S)-2c: Ni1−N1, 2.006(4);
Ni−N1A, 2.006(4); Ni−Br1, 2.3391(14); Ni−Br1A, 2.3390(14); N1−
C1, 1.443(5); N1−C19, 1.279(5); N1−Ni1−N1A, 81.1(2); N1−
Ni1−Br1A, 116.26(11); N1A−Ni1−Br1A, 104.83(11); N1−Ni1−Br1,
104.83(11); N1A−Ni1−Br1, 116.26(11); Br1A−Ni1−Br1, 125.30(7).
Selected torsion angle (deg): C2−C1−N1−Ni1, 89.5(5).
trahedral geometry about the nickel center. In the solid state,
the most interesting feature of ligand rac-(RR/SS)-1c is the
conformation of the substituents attached to N1 and N1A.
These groups are rotated about 180° from the position they
must occupy to chelate Ni. The rotation has been confirmed by
the crystal structure of its complex rac-(RR/SS)-2c. The X-ray
structures of ligand rac-(RR/SS)-1c and complex rac-(RR/SS)-
2c exhibit trans and cis conformations about the central C−C
bond of the backbone, respectively. The imino CN bond
length of complex rac-(RR/SS)-2c (1.279 Å) is slightly larger
than that of its ligand rac-(RR/SS)-1c (1.275 Å). The N1−
C19−C19A bond angle of complex rac-(RR/SS)-2c (115.1°) is
slightly less than that of its ligand rac-(RR/SS)-1c (116.5°) due
to the N−Ni coordination. Both naphthalene rings bonded to
the iminic nitrogens of the α-diimine lie nearly perpendicular to
the plane formed by the nickel and coordinated nitrogen atoms.
The sec-phenethyl groups in the 2-position of the naphthyl-
amine fragments in rac-(RR/SS)-2c 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 literature for other similar [NiBr2(α-diimine)] compounds
characterized by X-ray diffraction, {bis[N,N′-4-bromo-2,6-
dimethylphenyl)imino]acenaphthene}dibromonickel27 and
{bis[N,N′-(2,4,6-trimethylphenyl)imino]acenaphthene}-
dibromonickel.28 In fact, the Ni−N bond distances in complex
rac-(RR/SS)-2c (2.006 Å) are similar to those determined for
these compounds (2.026 and 2.021 Å, respectively), as well as
the Ni−Br bond distances (2.3390 Å for complex rac-(RR/SS)-
2c vs 2.3229 and 2.323 Å, respectively) and the N−Ni−Br
angles (116.26° for complex rac-(RR/SS)-2c vs 113.32 and
114.4°, respectively).
a
Asterisks denote the chiral carbons.
usually in the presence of a formic acid. Ligand 1c was
synthesized by the condensation of 2,3-butanedione with 2
equiv of the o-sec-phenethyl naphthylamine in toluene at 80−
110 °C using p-toluenesulfonic acid (p-TsOH) as catalyst.
Compounds 1a−e were characterized by IR, H NMR, 13C
1
NMR, and elemental analysis.
The reaction of equimolar amounts of NiBr2(DME) and the
naphthylamine α-diimine ligands 1a−e in CH2Cl2 led to the
displacement of 1,2-dimethoxyethane and afforded the catalyst
precursors 2a−e as moderately air-stable deep red micro-
crystalline solids in almost quantitative yields. Elemental
analyses of complexes 2c,d and ligands 1b−e fit the molecular
structure sobtained by X-ray structural studies.
2.2. X-ray Crystallographic Studies. Crystals of complex
rac-(RR/SS)-2c, complex 2d, and ligands 1b−e suitable for X-
ray diffraction were obtained at −30 °C by double-layering
CH2Cl2 solutions of the ligands and complexes with n-hexane.
The molecular structures of the ligands 1b, rac-(RR/SS)-1c,
and 1e were determined, and the corresponding diagrams are
shown in Figures SI-1−SI-4 (Supporting Information). The X-
ray structures of the ligands 1b, rac-(R,S)-1c, and 1e exhibit a
trans conformation about the central C−C bond of the ligand
backbone. Bond lengths and angles are within the expected
range for α-diimines; for example, the bond distances for the
C12N1/C19=N1/C11N1 double bonds and the central
C12−12A/C19−C19A/C11−C11A single bonds are
The free bis(2-methyl-1-naphthylimino)acenaphthene ligand
1d exhibits a cis conformation about the central C−C bond of
the ligand backbone instead of the trans conformation of
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ligands 1b, rac-(RR/SS)-1c, and 1e due to the rigid bis(imino)-
acenaphthene skeleton. The different configurations of the
imino units do not lead to important differences in the bond
lengths (C11−N1, 1.268(8); N1−C1, 1.400(9); C11−C11A,
1.504(13) Å), being similar to those in ligands 1b, rac-(RR/SS)-
1c, and 1e. Both C11 and C1 possess essentially rather planar
geometry (sp2 character), as shown by the C11−N1−C1 angles
(121.9(6)°), which are very close to 120°.
rings of the α-diimine Ni(II) complexes (Table 1). Complex
rac-(RR/SS)-2c, bearing one bulky chiral sec-phenethyl groups
in the o-naphthyl positions of the ligand, displays the highest
catalytic activity (2.81 × 106 g of PE/((mol of Ni) h bar)) and
produces one of the higher molecular weights (run 4, Mw =
15.5 × 104, 40 °C, [Al]/[Ni] = 800) among our five complexes.
Complexes 2a (bearing a hydrogen in the o-naphthyl positions
of the ligand, dimethylethane skeleton), 2b (bearing one
methyl groups in the o-naphthyl positions of the ligand,
dimethylethane skeleton), 2d (bearing one methyl groups in
the o-naphthyl positions of the ligand, acenaphthene skeleton),
and 2e (bearing a β-position in the naphthyldiimine, dimethyl-
ethane skeleton) exhibit significantly lower catalytic activity
(highest activity: 2a, run 16, 0.72 × 106 g of PE/((mol of Ni) h
bar); 2b, run 11, 1.10 × 106 g of PE/((mol of Ni) h bar); 2d,
run 20, 1.25 × 106 g of PE/((mol of Ni) h bar); 2e, run 25,
0.41 × 106 g of PE/((mol of Ni) h bar)).
The molecular structure complex 2d was determined, and
the corresponding diagram is shown in Figure 2. The imino
In comparison with complex 2b, complex 2d produces
slightly higher molecular weights and activities due to the
difference in the acenaphthene and dimethylethane skeletons.
In comparison with complex 2e, complex 2a produces slightly
higher molecular weights and activities due to the difference in
α- or β-position naphthyldiimines.
These results indicate that the rate of chain propagation is
greatly promoted by the bulky o-sec-phenethyl groups of the
ligand’s naphthyl rings (rac-(RR/SS)-2c). 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: 2c > 2b ≈ 2d > 2a > 2e.
Figure 2. Molecular structure of the catalyst precursor 2d. Hydrogen
atoms have been omitted for clarity. Selected bond lengths (Å) and
angles (deg) for complex 2d: Ni1−N1, 2.023(4); Ni−N1A, 2.023(4);
Ni−Br1, 2.3197(9); Ni−Br1A, 2.3197(9); C12−N1, 1.227(5); O1−
H1W, 0.902(18); O1−H2W, 0.910(18); N1−Ni1−N1A, 82.6(2);
N1−Ni1−Br1A, 105.34(10); N1A−Ni1−Br1A, 118.31(10); N1−
Ni1−Br1, 118.31(10); N1A−Ni1−Br1, 105.34(10); Br1A−Ni1−Br1,
121.09(5); N1−C12−C12A, 117.32(2); H1W−O1−H2W, 104(3).
In comparison with complexes 2a,b (60 °C: 2a, run 17,
activity 0.31 × 106 g of PE/((mol of Ni) h bar), Mw = 4.22 ×
104 g/mol; 2b, run 12, activity 0.38 × 106 g of PE/((mol of Ni)
h bar), Mw = 7.42 × 104 g/mol), complex rac-(RR/SS)-2c (60
°C: run 5, activity 2.50 × 106 g of PE/((mol of Ni) h bar), Mw
= 12.1 × 104 g/mol) has better thermal stability and produces
higher molecular weight polyethylene; even at 60 °C, the rac-
(RR/SS)-2c/DEAC systems still keep high activity.
The type and amount of branches formed in the polymer-
ization of ethylene promoted by typical α-diimine nickel
precatalysts depend on reaction parameters such as the reaction
temperature, ethylene pressure, and ligand structure.12
Generally, low ethylene pressure and high polymerization
temperature favor the chain walking and afford highly branched
polyethylenes.12 However, the effect of ligand structure on
polyethylene branching is much more complicated.
CN bond length of complex 2d (1.227 Å) is slightly less than
that of its ligand 1d (1.268 Å). The N1−C12−C12A bond
angle of complex 2d (117.32°) is slightly less than that of its
ligand 1d (120.3°) due to the N−Ni coordination. The Ni−N
bond distances in complex 2d (2.023 Å) are similar to that
determined for complex rac-(RR/SS)-2c (2.006 Å), as well as
the Ni−Br bond distances (2.3197 Å for complex 2d vs 2.3391
Å for complex rac-(RR/SS)-2c) and the N−Ni−Br angles
(118.31° for complex 2d vs 116.26° for complex rac-(RR/SS)-
2c). Both naphthalene rings bonded to the iminic nitrogens of
the α-diimine lie nearly perpendicular to the plane formed by
the nickel and coordinated nitrogen atoms. The methyl groups
in the 2-position of the naphthylamine fragments in 2d point
toward each other above and below the plane, thus shielding
the apical positions of the Ni(II) center.
As shown in Table 1 and Figure 3, the catalyst system rac-
(RR/SS)-2c/DEAC, bearing chiral bulky o-sec-phenethyl
groups, generated polyethylene with slightly lower degrees of
branching. The total branching degrees of the polymer samples
prepared with rac-(RR/SS)-2c/DEAC (runs 3−5, branching
degrees 86, 102, and 113 branches/1000 C at 20, 40, and 60
°C, respectively) are slightly lower than those observed for 2b/
DEAC (runs 8, 11, and 12, branching degrees 93, 107, and 117
branches/1000 C at 20, 40, and 60 °C, respectively), 2a/DEAC
(runs 15−17, branching degrees 100, 112, and 126 branches/
1000 C at 20, 40, and 60 °C, respectively), 2d/DEAC (runs 19,
21, and 22, branching degrees 99, 110, and 117 branches/1000
C at 20, 40, and 60 °C, respectively). These results (Table 1)
are consistent with the notion that the more electron-deficient
catalyst affords a more highly branched polymer.7 In addition,
2e/DEAC systems (runs 24−26, branching degrees 80, 94, and
107 branches/1000 C at 20, 40, and 60 °C, respectively)
2.3. Polymerization of Ethylene with Nickel Com-
plexes 2a−e. The five α-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 1. It is worth noting that blank experiments carried out
with DEAC alone, under similar conditions, showed its inability
to polymerize ethylene on its own.
At 20 °C, the activity of complex 2b increases slightly with an
increase of [Al]/[Ni] ratio (runs 6−8), the maximum being
reached around [Al]/[Ni] = 800 (run 8), and then the activity
decreases slightly with an increase of [Al]/[Ni] ratio (runs 8
and 9). For the ratio [Al]/[Ni] = 800, the activity increases first
up to 40 °C (runs 10, 8, and 11) and then decreases slightly in
the range 40−70 °C (runs 11−13).
The performances of the nickel precatalysts are significantly
affected by the ortho-position substituents on the naphthyl
C
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a
Table 1. Ethylene Polymerizations Using Nickel Precatalysts 2a−e Activated by DEAC
b
c
d
d
e
run
complex
[Al]/[Ni]
T (°C)
t (min)
yield (g)
activity
TOF (h bar)−1
Mw × 10−4
Mw/Mn
branches /1000 C
1
2c
2c
2c
2c
2c
2b
2b
2b
2b
2b
2b
2b
2b
2a
2a
2a
2a
2d
2d
2d
2d
2d
2e
2e
2e
2e
800
800
800
800
800
400
600
800
1000
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
0
10
20
40
60
20
20
20
20
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
10
0.68
1.01
1.92
2.02
1.80
0.49
0.74
0.76
0.61
0.46
0.79
0.27
0.12
0.25
0.31
0.52
0.22
0.44
0.63
0.90
0.82
0.29
0.19
0.24
0.41
0.15
0.94
1.40
2.67
2.81
2.50
0.68
1.03
1.06
0.85
0.64
1.10
0.38
0.17
0.35
0.43
0.72
0.31
0.61
0.87
1.25
1.14
0.40
0.26
0.33
0.57
0.21
0.34
0.50
0.95
1.00
0.89
0.24
0.37
0.38
0.30
0.23
0.39
0.13
0.06
0.12
0.15
0.26
0.11
0.22
0.31
0.45
0.41
0.14
0.09
0.12
0.20
0.07
22.1
20.9
19.4
15.5
12.1
18.5
17.8
19.5
16.0
21.8
13.6
7.42
3.92
18.5
16.0
4.68
4.22
22.1
20.8
18.7
15.5
8.01
13.0
10.9
6.64
4.05
1.8
1.3
1.8
1.6
1.7
1.6
1.7
1.7
1.6
1.6
1.8
2.0
2.0
1.6
1.6
2.0
1.8
1.8
1.7
1.6
1.6
1.7
1.8
1.9
1.7
1.8
2
3
85.7
101.6
113.3
4
5
6
7
8
92.7
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
40
60
70
0
106.8
117.4
20
40
60
0
100.1
111.9
126.4
20
30
40
60
0
99.3
109.9
116.9
20
40
60
79.6
93.9
106.7
a
Polymerization conditions and definitions: n(Ni) = 3.60 μmol; ethylene relative pressure 0.2 bar, ethylene absolute pressure 1.2 bar; t =
b
c
polymerization time; solvent toluene (50 mL); T = polymerization temperature. Activity in 106 g of PE/((mol of Ni) h bar). Turnover frequency
d
e
29
in 105 mol of ethylene/((mol Ni) h bar). Mw in 104 g/mol, determined by GPC. Estimated by H NMR:
1
1
3
I
CH3
branches/1000 C =
× 1000
ICH + ICH + ICH
2
3
2
4. The number of branches was calculated according to the
literature,30 and it was found that the polyethylene with 108
branches/1000 carbons (75 methyl, 9 ethyl, 5 propyl, and 19
butyl or longer branches/1000 C) was obtained at 40 °C (run 4
in Table 1). This result was consistent with that calculated from
1H NMR. The rationale for the formation of the major types of
branches (methyl, ethyl, and butyl) in the polyethylenes
obtained in this work is shown in Scheme 2.
2.4. Polymerization of Methyl Methacrylate (MMA)
with Nickel Complexes rac-(RR/SS)-2c and 2d. Polymer-
izations of MMA with complexes rac-(RR/SS)-2c and 2d in
combination with DEAC were carried out at −30, 0, 25, and 50
°C, and the results are summarized in Table 2. For the ratio
[Al]/[Ni] = 800, an increase in the polymerization temperature
in the range −30 to +30 °C increases the activity of the
precatalysts rac-(RR/SS)-2c and 2d (runs 1−3, 4−7). In
addition, the rate of chain propagation is greatly promoted by
the bulky o-sec-phenethyl groups of the ligand’s naphthyl rings
(rac-(RR/SS)-2c).
As shown in Table 2 and Figures 5 and 6, catalysts rac-(RR/
SS)-2c and 2d all gave syndiotactic-rich poly(methyl meth-
acrylate) (PMMA). At room temperature, the tacticities of
PMMA prepared with rac-(RR/SS)-2c/DEAC (run 3: rr
69.71%, mr 26.11%, mm 4.18%) are significantly higher than
those observed for 2d/DEAC (run 7: rr 58.71%, mr 32.26%,
mm 9.03%). The tacticities of PMMA prepared with 2d/DEAC
Figure 3. 1H NMR (400 MHz, CDCl3/1,2,4-trichlorobenzene, 1/3 v/
v) spectra of the polyethylenes catalyzed by rac-(RR/SS)-2c, 2d, 2b,
2a, and 2e/DEAC at 40 °C (Table 1s runs 4, 21, 11, 16 and 25): ICH
,
3
integrated intensity between 0.8 and 1.0 ppm; ICH + ICH, integrated
2
intensity between 1.0 and 1.5 ppm.
produced polyethylene with the lowest degrees of branching
among our five complexes.
The 13C NMR spectrum of the polyethylene prepared with
the catalyst rac-(RR/SS)-2c/DEAC at 40 °C is shown in Figure
D
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DEAC at high temperature (run 3, 25 °C) and for 2d/DEAC at
low temperature (run 5, 25 °C). A naphthyl ligand bearing
chiral bulky sec-phenethyl groups in the o-naphthyl position
may better control the stereoselective of monomer insertion,
which should afford a highly syndiotactic PMMA. A rationale
for the formation of the syndiotactic poly(methyl methacrylate)
catalyzed by rac-(RR/SS)-2c/DEAC is shown in Scheme 3.
3. CONCLUSION
A series of new naphthyl-α-diimine ligands and their Ni(II)
complexes have been prepared and characterized. Ligands 1a−e
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 methyl
methacrylate and control the microstructure of polyethylene, in
particular, the stereoregularity structure of poly(methyl
methacrylate). The results obtained show that the chiral
complex rac-(RR/SS)-2c, activated by DEAC, produces a highly
active catalyst system for the polymerization of ethylene and
highly branched polyethylene at high temperature. The rate of
chain propagation is promoted by the bulky o-sec-phenethyl
groups of the ligand’s naphthyl rings. Interestingly, the rac-
(RR/SS)-2c/DEAC catalyst system could produce syndiotactic
PMMA at low temperature. Complex rac-(RR/SS)-2c, bearing
chiral bulky sec-phenethyl groups in the o-naphthyl position,
may better control the stereoselectivity of monomer insertion,
which should afford highly syndiotactic PMMA.
Figure 4. 13C NMR (CDCl3/o-dichlorobenzene, 1/3 v/v) spectrum of
polyethylene catalyzed by rac-(RR/SS)-2c/DEAC at 40 °C (Table 1,
run 4). 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 labeled 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.
at lower temperature (run 5, −30 °C: rr 62.72%, mr 28.90%,
mm 8.38%) are slightly higher than those observed at high
temperature (run 7, 25 °C). Interestingly, the tacticities of
PMMA prepared with rac-(RR/SS)-2c/DEAC at lower temper-
ature (run 1, −30 °C: rr 88.75%, mr 7.26%, mm 3.99%) are
significantly higher than those observed for rac-(RR/SS)-2c/
4. EXPERIMENTAL SECTION
4.1. General Considerations. All operations were carried out
under an N2 atmosphere using standard Schlenk techniques unless
otherwise noted. Methylene chloride and o-dichlorobenzene were
predried with 4 Å molecular sieves and distilled from CaH2 under dry
nitrogen. Toluene, methyl methacrylate (MMA), diethyl ether, and
Scheme 2. Rationale for the Formation of the Major Types of Branches (Methyl, Ethyl, and Butyl) in the Polyethylenes
a
Obtained in This Work
a
Asterisks denote chiral carbons.
E
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a
Table 2. Polymerization Results for MMA Using rac-(RR/SS)-2c/DEAC and 2d/DEAC Catalytic Systems
d
triad fraction (%)
b
c
c
entry
precatalyst
T (°C)
yield (%)
Mn
Mw/Mn
rr
mr
mm
1
2
3
4
5
6
7
8
2c
2c
2c
2c
2d
2d
2d
2d
−30
0
10.3
18.6
27.9
25.7
8.2
27.6
29.5
19.8
10.7
18.2
20.9
14.2
8.5
1.2
1.2
1.3
1.5
1.2
1.5
1.6
1.6
88.75
7.26
3.99
25
50
−30
0
69.71
62.72
58.71
26.11
28.90
32.26
4.18
8.38
9.03
15.2
20.3
19.5
25
50
a
Polymerization conditions and definitions: n(2c) = n(2d) = 3.6 μmol, [Al]/[Ni] = 800; polymerization time 12 h; VMMA = 10 mL (ρ = 0.94 g/mL);
b
c
solvent toluene (30 mL); T = polymerization temperature. Defined as (mass of dry polymer recovered)/(mass of monomer used). Determined by
means of GPC. Observed in 13C NMR spectra of quaternary carbon resonance, from low to high field in the spectra (δ 45.8−44.8 ppm).31
d
Figure 5. 13C NMR (CDCl3) spectrum of syndiotactic PMMA obtained with rac-(RR/SS)-2c/DEAC catalyst at −30 °C.
4.2. Syntheses and Characterization. The structures and
numbering schemes for compounds c and 1a−e are given in Chart 1.
4.2.1. Synthesis of 2-(sec-Phenylethyl)naphthalen-1-amine (rac-
c). CF3SO3H (0.06 g, 0.40 mmol), 1-aminonaphtalene (0.29 g, 2.00
mmol), styrene (0.31 g, 3.00 mmol), and xylene (1 mL) were placed in
a 10 mL Schlenk flask and stirred at 160 °C for 5 h. The mixture was
extracted three times with 15 mL of diethyl ether. Volatile materials
were removed, and the residue was purified by chromatography on
silica gel with petroleum ether/ethyl ester (15/1 v/v) to give 2-(1-
1
phenylethyl)naphthalen-1-amine. Yield: 0.35 g (71%). H NMR (400
MHz, CDCl3, ppm): δ 1.68 (t, 3H, J = 7.6 Hz, protons of C12), 3.98
(br, s, 2H, −NH2), 4.22 (q, J = 6.8 Hz, 1H, protons of C11), 7.15−
7.28 (m, 5H, protons of C15, C13, and C14), 7.34−7.40 (m, 3H,
protons of C3, C4, and C7), 7.44 (t, 1H, J = 8.4 Hz, protons of C6),
7.67−7.70 (d, 1H, protons of C5), 7.76 (d, 1H, J = 6.4 Hz, protons of
Figure 6. 13C NMR (CDCl3) spectra of PMMA obtained with rac-
(RR/SS)-2c/DEAC and 2d/DEAC catalysts at 25 and −30 °C.
C8). 13C NMR (100 MHz, CDCl3, ppm): δ 21.60 (C12), 40.39
(C11), 120.95 (C4), 123.03 (C2), 123.07 (C8), 124.35 (C9), 124.44
(C6), 124.56 (C7), 126.32 (C3), 128.30 (C15), 128.68 (C5), 131.05
(C13), 137.52 (C14), 137.60 (C10), 145.67 (C1), 147.48 (C16).
1,2-dimethoxyethane (DME) were distilled from sodium/benzophe-
none under an N2 atmosphere. Anhydrous NiBr2 (99%) and
diethylaluminum chloride (DEAC, 0.9 M solution in toluene) were
obtained from Acros. Acenaphthenequinone (98%), 2,3-butanedione
(98%), 1-amino-2-methylnaphthalene (98%), 1-aminonaphthalene
(98%), and 2-aminonaphthalene (98%) were purchased from Alfa
Aesar and used without further purification. NiBr2 (DME) was
synthesized according to the literature.32
4.2.2. Synthesis of the Ligands 1a−e. Synthesis of Bis[N,N′-(1-
naphthyl)imino]-1,2-dimethylethane (1a). Formic acid (0.50 mL)
was added to a stirred solution of 2,3-butanedione (0.17 g, 2.00 mmol)
and 1-naphthylamine (0.57 g, 4.00 mmol) in methanol (25 mL). The
mixture was refluxed for 24 h and then cooled, and the precipitate was
separated by filtration. The solid was recrystallized from EtOH/
CH2Cl2 (7/1 v/v), washed with cold ethanol, and dried under vacuum.
1
Yield: 0.56 g (83%). H NMR (400 MHz, CDCl3, ppm): δ 2.31 (s,
NMR spectra were recorded at 400 (1H) and 100 (13C) MHz on a
Varian Mercury plus-400 instrument with TMS as internal standard.
FT-IR spectra were recorded on a Digilab Merlin FTS 3000 FTIR
spectrophotometer in KBr pellets. The molecular weights and
molecular weight distributions (Mw/Mn) of the polymers were
determined by gel permeation chromatography/size-exclusion chro-
matography (GPC/SEC) via a Waters Alliance GPCV2000 chromato-
graph, using 1,2,4-trichlorobenzene as eluent, at a flow rate of 1.0 mL/
min and 140 °C. The elemental content of samples was determined by
an elemental analyzer (Vaiio-EL106, Germany). Single-crystal
structures were determined on a Bruker APEX-II CCD diffractometer
with Mo Kα radiation (λ = 0.71073 Å) in the ω scan mode.
6H, protons of C12), 6.85 (d, 2H, J = 6.8 Hz, protons of C2), 7.48−
7.55 (m, 6H, protons of C3, C6, and C7), 7.66 (d, 2H, J = 8.4 Hz,
protons of C4), 7.82 (d, 2H, J = 7.6 Hz, protons of C5), 7.88 (d, 2H, J
= 8.0 Hz, protons of C8). 13C NMR (100 MHz, CDCl3, ppm): δ 15.93
(C12), 112.94 (C2), 123.34 (C4), 124.10 (C6), 125.47 (C7), 125.65
(C9), 125.75 (C5), 126.28 (C8), 128.04 (C3), 134.10 (C10), 146.96
(C1), 169.08 (C11). Anal. Calcd for C24H20N2: C, 85.68; H, 5.99; N,
8.33. Found: C, 85.45; H, 6.22; N, 8.48. FT-IR (KBr): 1639 cm−1
(νCN).
Synthesis of Bis[N,N′-(2-methyl-1-naphthyl)imino]-1,2-dimethyl-
ethane (1b). Using the same procedure as for the synthesis of 1a, 1b
was obtained as an orange powder. Yield: 0.59 g (81%). 1H NMR (400
F
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Scheme 3. Rationale for the Formation of the Syndiotactic Poly(methyl methacrylate) Catalyzed by rac-(RR/SS)-2c/DEAC
then the solvent was removed. The residue was purified by
Chart 1. Structure of Compounds c and 1a−e
chromatography on silica gel with petroleum ether/ethyl ester (20/1
1
v/v) to give a yellow powder. Yield: 0.35 g (64%). H NMR (400
MHz, CDCl3, ppm): δ 1.55−1.76 (m, 12H, protons of C12 and C18),
4.23 (q, J = 7.6 Hz, 2H, protons of C11), 7.11−7.14 (m, 2H, protons
of C3), 7.15−7.19 (m, 2H, protons of C16), 7.21−7.32 (m, 8H,
protons of C14 and C15), 7.42−7.45 (m, 2H, protons of C6), 7.47−
7.55 (m, 4H, protons of C7 and C4), 7.73−7.76 (m, 2H, protons of
C5), 7.83−7.91 (m, 2H, protons of C8). 13C NMR (100 MHz, CDCl3,
ppm): δ 16.38 (C18), 22.62 (C12), 40.85 (C11), 122.19(C6), 123.28
(C4), 124.02 (C7), 124.60 (C16), 125.20 (C3), 125.89 (C9), 126.32
(C8), 127.48 (C5), 127.67 (C14), 128.18 (C2), 128.70 (C15), 132.78
(C10), 144.67 (C13), 147.06 (C1), 170.17 (C17). Anal. Calcd for
C40H36N2: C, 88.20; H, 6.66; N, 5.14. Found: C, 88.37; H, 6.48; N,
5.29. FT-IR (KBr): 1633 cm−1 (νCN). Single crystals of ligand (S,R)-
1c 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.
Synthesis of Bis[N,N′-(2-methyl-1-naphthyl)imino]acenaphthene
(1d). Formic acid (0.50 mL) was added to a stirred solution of
acenaphthenequinone (0.55 g, 3.00 mmol) and 1-amino-2-methyl-
naphthalene (0.94 g, 6.00 mmol) in ethanol (30 mL). The mixture was
refluxed for 24 h and then cooled, and the precipitate was separated by
filtration. The solid was recrystallized from EtOH/CH2Cl2 (6/1 v/v),
washed with cold ethanol, and dried under vacuum to give the red
ligand. Yield: 1.17 g (85%). 1H NMR (400 MHz, CDCl3, ppm): δ 2.44
(s, 6H, protons of C11), 6.48 (d, 2H, J = 7.2 Hz, protons of C3), 7.22
(t, 2H, J = 7.6 Hz, protons of C7), 7.36 (t, 2H, J = 7.6 Hz, protons of
C6), 7.43−7.50 (m, 4H, protons of C13 and C4), 7.72 (d, 2H, J = 8.0
Hz, protons of C5), 7.80 (d, 2H, J = 8.4 Hz, protons of C8), 7.89 (m,
4H, protons of C14 and C12). 13C NMR (100 MHz, CDCl3, ppm): δ
18.05 (C11), 119.76 (C2), 123.10 (C4), 123.70 (C14), 124.27 (C6),
124.31 (C7), 125.39 (C13), 125.44 (C3), 125.69 (C9), 127.79 (C8),
128.12 (C5), 129.18 (C15), 129.23 (C12), 130.93 (C10), 131.34
(C17), 132.81 (C16), 145.55 (C1), 162.19 (C18). Anal. Calcd for
C34H24N2: C, 88.67; H, 5.25; N, 6.08. Found: C, 88.74; H, 5.19; N,
6.15. FT-IR (KBr): 1639 cm−1 (νCN). Single crystals of ligand 1d
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.
MHz, CDCl3, ppm): δ 2.16 (s, 6H, protons of C13), 2.26 (s, 3H,
protons of C11), 2.31 (s, 3H, protons of C11), 7.40 (d, 2H, J1 = 7.2
Hz, protons of C3), 7.44−7.50 (m, 4H, protons of C6 and C7), 7.56−
7.65 (m, 4H, protons of C4 and C5), 7.84−7.86 (m, 2H, protons of
C8). 13C NMR (100 MHz, CDCl3, ppm): δ 16.52 (C13), 17.82
(C11), 119.76 (C2), 122.61 (C6), 123.33 (C4), 124.15 (C7), 125.48
(C8), 126.06 (C9), 128.00 (C5), 128.98 (C3), 132.62 (C10), 144.52
(C1), 169.63 (C12). Anal. Calcd for C26H24N2: C, 85.68; H, 6.64; N,
7.69. Found: C, 85.54; H, 6.78; N, 7.74. FT-IR (KBr): 1638 cm−1
(νCN). Single crystals of ligand 1b 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.
Synthesis of Bis[N,N′-(2-sec-phenethyl-1-naphthyl)imino]-1,2-
dimethylethane (rac-(SS/RR)-1c). 4-Methylbenzenesulfonic acid (20
mg, 0.10 mmol) was added to a stirred solution of 2,3-butanedione
(0.086 g, 1.00 mol) and 2-sec-phenethyl-1-naphthylamine (0.52 g, 2.10
mmol) in toluene (20 mL). The mixture was refluxed for 16 h, and
Synthesis of Bis[N,N′-(2-naphthyl)imino]-1,2-dimethylethane
(1e). Using the same procedure as for the synthesis of 1a, 1e was
1
obtained as an orange powder. Yield: 0.56 g (83%). H NMR (400
MHz, CDCl3, ppm): δ 2.25 (s, 6H, protons of C12), 7.05 (d, 2H, J =
8.4 Hz, protons of C3), 7.19 (s, 2H, protons of C1), 7.41−7.45 (m,
2H, protons of C6), 7.49 (t, 2H, J = 6.8 Hz, protons of C7), 7.80 (d,
G
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2H, J = 8.4 Hz, protons of C5), 7.84 (d, 2H, J = 8.0 Hz, protons of
C8), 7.88 (d, 2H, J = 8.8 Hz, protons of C4). 13C NMR (100 MHz,
CDCl3, ppm): δ 15.68 (C12), 114.94 (C3), 119.98 (C1), 124.72 (C7),
126.42 (C6), 127.43 (C4), 127.78 (C5), 128.93 (C8), 130.67 (C10),
133.95 (C9), 148.57 (C2), 168.63 (C11). Anal. Calcd for C24H20N2:
C, 85.68; H, 5.99; N, 8.33. Found: C, 85.55; H, 6.15; N, 8.47. FT-IR
(KBr): 1638 cm−1 (νCN).
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, was terminated by
quenching the reaction mixtures with 100 mL of a 3% HCl−MeOH
solution. The precipitated polymer was filtered, washed with methanol,
and dried under vacuum at 60 °C to a constant weight.
4.5. Polymerization of MMA. MMA polymerizations were
carried out in a Schlenk tube (100 mL) with a connection to a
vacuum system. In a typical procedure, the required amounts of MMA,
DEAC, and precatalyst ([Al]/[Ni] = 800) were charged into a dry
Schlenk tube in this order under nitrogen flow. The polymerization
was carried out at constant temperature for 12 h. The resulting
solution was poured into acidified methanol (100 mL of a 5% v/v
solution of HCl). The polymer was then isolated by filtration and
washed with methanol before drying overnight at 60 °C. The polymer
yield was determined by gravimetry.
4.2.3. Synthesis of the Catalysts 2a−e. Synthesis of {Bis[N,N′-(1-
naphthyl)imino]-1,2-dimethylethane}dibromonickel (2a).
[NiBr2(DME)] (0.31 g, 1.00 mmol), ligand 1a (0.34 g, 1.00 mmol),
and dichloromethane (30 mL) were mixed in a Schlenk flask and
stirred at room temperature for 16 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 catalyst 2a. Yield: 0.50 g (91%). Anal.
Calcd for C24H20Br2NiN2: C, 51.94; H, 3.63; N, 5.05. Found: C, 51.86;
H, 3.78; N, 5.29. FT-IR (KBr): 1628 cm−1 (νCN).
Synthesis of {Bis[N,N′-(2-methyl-1-naphthyl)imino]-1,2-
dimethylethane}dibromonickel (2b). Using the same procedure as
for the synthesis of 2a, 2b was obtained as a purple powder. Yield: 0.54
g (93%). Anal. Calcd for C26H24Br2NiN2: C, 53.57; H, 4.15; N, 4.81.
Found: C, 53.76; H, 4.03; N, 4.59. FT-IR (KBr): 1624 cm−1 (νCN).
Synthesis of {Bis[N,N′-(2-sec-phenylethyl-1-naphthyl)imino]-1,2-
dimethylethane}dibromonickel (rac-(SS/RR)-2c). Using the same
procedure as for the synthesis of 2a, rac-(SS/RR)-2c was obtained
as a purple powder. Yield: 0.65 g (86%). Anal. Calcd for
C40H36Br2NiN2: C, 62.95; H, 4.75; N, 3.67. Found: C, 62.87; H,
4.96; N, 3.86. FT-IR (KBr): 1629 cm−1 (νCN). Single crystals of
complex (R,S)-2c suitable for X-ray analysis were obtained at −30 °C
by dissolving the complex in CH2Cl2, followed by slow layering of the
resulting solution with n-hexane.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, tables, and CIF files giving crystallographic data for
ligands 1b−e and complexes 2c,d. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*J.Y.: tel, +86-931-7971539; fax, +86-931-7971261; e-mail,
jianchaoyuan@nwnu.edu.cn.
■
Notes
The authors declare no competing financial interest.
Synthesis of {Bis[N,N′-(2-methyl-1-naphthyl)imino]-
acenaphthene}dibromonickel (2d). Using the same procedure as
for the synthesis of 2a, 2d was obtained as a dark red powder. Yield:
0.63 g (90%). Anal. Calcd for C34H28Br2N2NiO2: C, 57.11; H, 3.95; N,
3.92. Found: C, 56.95; H, 4.18; N, 4.19. FT-IR (KBr): 1625 cm−1
(νCN). Single crystals of complex 2d suitable for X-ray analysis were
obtained at −30 °C by dissolving the complex in CH2Cl2, followed by
slow layering of the resulting solution with n-hexane.
Synthesis of {Bis[N,N′-(2-naphthyl)imino]-1,2-dimethylethane}-
dibromonickel (2e). Using the same procedure as for the synthesis of
2a, 2e was obtained as a purple powder. Yield: 0.51 g (93%). Anal.
Calcd for C24H20Br2NiN2: C, 51.94; H, 3.63; N, 5.05. Found: C, 51.86;
H, 3.78; N, 5.19. FT-IR (KBr): 1627 cm−1 (νCN).
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(20964003), the Scientific Research Foundation for the
Returned Overseas Chinese Scholars of State Education
Ministry (2009-1341), and the Science and Technology
Activities Merit-based Foundation for the Returned Overseas
Chinese Scholars of State Human Resources and Social
Security Ministry (2009-416) for funding. We also thank the
Key Laboratory of Eco-Environment-Related Polymer Materi-
als of the Ministry of Education, Key Laboratory of Polymer
Materials of Gansu Province (Northwest Normal University),
for financial support.
4.3. X-ray Structure Determinations. Single crystals of ligands
1b−e, complex 2c, and complex 2d suitable for X-ray analysis were
obtained at −30 °C by dissolving the ligands and nickel complexes in
CH2Cl2, followed 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 CCD area detector, using
graphite-monochromated MoKα radiation (λ = 0.71073 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
calculated positions and were included in the structure calculation
without further refinement of the parameters. Crystal data, data
collection, and refinement parameters are given in Tables SI-1 and SI-2
(Supporting Information).
4.4. Ethylene Polymerization. Polymerization of ethylene was
carried out in a flame-dried 250 mL crown-capped pressure bottle
sealed with a neoprene septum. After the polymerization bottle was
dried under an N2 atmosphere, 50 mL of dry toluene was placed to the
polymerization bottle. The resulting solvent was then saturated with a
prescribed ethylene pressure. The cocatalyst (DEAC) was then added
in [Al]/[Ni] molar ratios in the range of 400−1000 to the
polymerization bottle via a syringe. At this time, the solutions were
thermostated to the desired temperature and allowed to equilibrate for
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dx.doi.org/10.1021/om400433t | Organometallics XXXX, XXX, XXX−XXX