Synthesis and characterization of the titanium complexes bearing two β-enaminoketonato ligands with electron withdrawing groups/modified phenyls and their behaviors for ethylene (co-)polymerization

Article abstract of DOI:10.1039/c001987a

A series of new titanium complexes with two asymmetric bidentate β-enaminoketonato [N,O] ligands (2b-t), [PhNC(CF₃)CHC(Ar)O] ₂TiCl₂ (2b, Ar = -C₆H₄F(o); 2c, Ar = -C₆H₄F(m); 2d, Ar = -C₆H₄F(p); 2e, Ar = -C₆H₄Cl(p); 2f, Ar = -C₆H ₄OMe(p); 2g, Ar = -C₆H₄CF₃(p); 2h, Ar = -C₆H₄CF₃(m); 2i, Ar = -C₆H ₄CF₃(o); 2j, Ar = -C₆H₄Cl(o); 2k, Ar = -C₆H₄Br(o); 2l, Ar = -C₆H₄I(o); 2m, Ar = -C₆H₃F₂(2,4); 2n, Ar = -C ₆H₃F₂(2,6); 2o, Ar = -C₆H ₃F₂(3,4); 2p, Ar = -C₆H₃F ₂(3,5); 2q, Ar = -C₆F₅; 2r, Ar = C ₆F₄OMe; 2s, Ar = -C₆H₃Cl ₂(2,6); 2t, Ar = -C₆H₃Cl₂(2,5)), have been synthesized based on substituted acetophenones. X-Ray analyses reveal that complexes 2h, 2k, 2m, and 2n adopt distorted octahedral geometry around the titanium center, in which the two chloride ligands are situated in the cis-orientation. 2s also adopts distorted octahedral geometry, but the two chloride ligands in it are situated in the trans-orientation due to the increase of the steric effect of the phenyl derived from the acetophenone. The influence of the substituent effects on catalyst performance, including catalytic activities and the molecular weight distribution of the polymers obtained, was investigated in detail. With modified methylaluminoxane (MMAO) as a cocatalyst, complexes 2b-r and 2t are active catalysts for ethylene polymerization at room temperature, and produce high molecular weight polymers. It is observed that the catalytic activities are significantly enhanced by introducing some electron-withdrawing groups, such as -F, -Cl and -CF₃, into the suitable positions of the phenyl ring close to the oxygen donor. It should be noted that complexes 2c-i, 2p, 2n and 2t are also capable of promoting the living copolymerization of ethylene with norbornene at room temperature, yielding high molecular weight copolymers with narrow molecular weight distributions (PDI = 1.05-1.30). The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c001987a

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Synthesis and characterization of the titanium complexes bearing two
b-enaminoketonato ligands with electron withdrawing groups/modified
phenyls and their behaviors for ethylene (co-)polymerization†
Wei-Ping Ye, Xin-Cui Shi, Bai-Xiang Li, Jing-Yu Liu,* Yue-Sheng Li, Yan-Xiang Cheng and Ning-Hai Hu
Received 1st February 2010, Accepted 5th July 2010
DOI: 10.1039/c001987a
A series of new titanium complexes with two asymmetric bidentate b-enaminoketonato [N,O] ligands
(2b–t), [PhN C(CF3)CHC(Ar)O]2TiCl2 (2b, Ar = –C6H4F(o); 2c, Ar = –C6H4F(m); 2d, Ar =
–C6H4F(p); 2e, Ar = –C6H4Cl(p); 2f, Ar = –C6H4OMe(p); 2g, Ar = –C6H4CF3(p); 2h, Ar =
–C6H4CF3(m); 2i, Ar = –C6H4CF3(o); 2j, Ar = –C6H4Cl(o); 2k, Ar = –C6H4Br(o); 2l, Ar = –C6H4I(o);
2m, Ar = –C6H3F2(2,4); 2n, Ar = –C6H3F2(2,6); 2o, Ar = –C6H3F2(3,4); 2p, Ar = –C6H3F2(3,5); 2q, Ar =
–C6F5; 2r, Ar = C6F4OMe; 2s, Ar = –C6H3Cl2(2,6); 2t, Ar = –C6H3Cl2(2,5)), have been synthesized
based on substituted acetophenones. X-Ray analyses reveal that complexes 2h, 2k, 2m, and 2n adopt
distorted octahedral geometry around the titanium center, in which the two chloride ligands are
situated in the cis-orientation. 2s also adopts distorted octahedral geometry, but the two chloride
ligands in it are situated in the trans-orientation due to the increase of the steric effect of the phenyl
derived from the acetophenone. The influence of the substituent effects on catalyst performance,
including catalytic activities and the molecular weight distribution of the polymers obtained, was
investigated in detail. With modified methylaluminoxane (MMAO) as a cocatalyst, complexes 2b–r and
2t are active catalysts for ethylene polymerization at room temperature, and produce high molecular
weight polymers. It is observed that the catalytic activities are significantly enhanced by introducing
some electron-withdrawing groups, such as –F, –Cl and –CF3, into the suitable positions of the phenyl
ring close to the oxygen donor. It should be noted that complexes 2c–i, 2p, 2n and 2t are also capable of
promoting the living copolymerization of ethylene with norbornene at room temperature, yielding high
molecular weight copolymers with narrow molecular weight distributions (PDI = 1.05–1.30).
We are interested in the design and synthesis of efficient
Introduction
transition metal catalysts for precise, controlled olefin polymer-
In the last decade, a significant amount of research has focused
ization. Previously, we reported the type of titanium catalysts
on the development of non-metallocene catalysts for olefin
chelating asymmetrical bidentate b-enaminoketonato ligands,
polymerization.1–24 To optimize catalyst performance and polymer
[ArN C(R1)CHC(R2)O]2TiCl2, and their use as catalyst pre-
properties, different catalytic systems have been investigated, re-
cursors for olefin polymerization.19–23 Both steric and electronic
sulting in the discovery of a number of highly active catalysts such
effects in the R1 and R2 groups showed greatly influence on the
as group 4 transition metal complexes2–8 and late transition metal
complexes.9–14 Especially in recent years, Fujita, Coates, and Pellec-
chia have developed a class of titanium catalysts bearing phenoxy-
imine ligands (named FI Catalysts) for olefin polymerizations.15–17
polymerization performance.19,20,22 The trifluoromethyl group in
the b-enaminoketonato ligands (R1) appears to be required for
the generation of high activity. Due to the fact that complex 2a
(R1 = CF3, R2 = Ph) exhibited living characteristics for ethylene
Introducing heteroatom(s) and/or heteroatom-containing sub-
polymerization and ethylene/norbornene or other monomer
stituent(s) in the ligand have led to the discovery of a new series
copolymerization,19,21 we therefore decided to extend the study by
of FI Catalysts having fluorine atom(s) in the ligand that display
introducing some substituents into the phenyl ring located in R2
unprecedented catalytic performance for the living polymerization
moiety and investigate the relationship between substituent effects
of ethylene.15,16 Subsequently, Coates and coworkers reported a
and catalytic performances. In this paper, we present the synthesis
facile synthesis of new titanium catalysts containing ancillary non-
and characterization of a series of new titanium complexes of the
fluorinated phenoxyketimine ligand of type, (PHI-R)2TiCl2 for
type [ArN C(CF3)CHC(R2)O]2TiCl2, as well as their behavior
the living ethylene polymerization.18 These results suggested that
towards ethylene and ethylene/norbornene copolymerization in
variation of the ligand structure may lead to profound changes in
the presence of MMAO.
the catalytic activity and the properties of the polymer.
State Key Laboratory of Polymer Physics and Chemistry, Changchun
Results and discussion
Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun,
130022, China. E-mail: ljy@ciac.jl.cn; Fax: +86-431-85262039
† Electronic supplementary information (ESI) available: Additional exper-
Synthesis and characterization of new titanium complexes
imental details. CCDC reference numbers 763881–763884 and 763886. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c001987a
The titanium complexes 2b–t were synthesized according to the
reaction sequence shown in Scheme 1.20–22 A tandem sequence of
9000 | Dalton Trans., 2010, 39, 9000–9007
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Scheme 1 General synthetic route of the titanium complexes used in this
study.
Claisen condensation between acetophenone derivatives and ethyl
trifluoroacetate and the condensation of the resulted b-diketone
with aniline in the acidic toluene was used to synthesize these
new b-enaminoketonates ligands 1b–t. Treatment of TiCl4 with
two equivalents of the lithium salts of 1b–t in dry diethylether
gave 2b–t in moderate to high yields. It is noticed that in the
process of the synthesis of complexes 2k, 2i and 2q, we chose
lithium diisopropylamide (LDA) as the base instead of n-BuLi to
avoid possible side reactions, such as halogen–lithium exchange
and nucleophilic substitution.25,26 The resultant complexes were
obtained as deep red or black crystalline solids.
Fig. 2 Molecular structure of complex 2k with thermal ellipsoids at 30%
probability level. Hydrogen atoms (except H8 and H24) are omitted for
clarity.
The molecular structures of complexes 2h, 2k, 2m, 2n and 2s
were confirmed by X-ray crystal analysis. The crystals suitable
for X-ray diffraction were grown from dichloromethane–hexane
solutions. The crystallographic data together with the collection
and refinement parameters are summarized in Table 1. The
molecular structures of these complexes are shown in Fig. 1–5,
respectively, and the selected bond lengths and bond angles are
summarized in Table 2.
Fig. 3 Molecular structure of complex 2m with thermal ellipsoids at 30%
probability level. Hydrogen atoms (except H8 and H24) are omitted for
clarity.
Fig. 1 Molecular structure of complex 2h with thermal ellipsoids at 30%
probability level. Hydrogen atoms (except H8 and H8A) are omitted for
clarity.
Fig. 4 Molecular structure of complex 2n with thermal ellipsoids at 30%
probability level. Hydrogen atoms (except H8 and H8A) are omitted for
clarity.
In the solid state, compound 2h, which has a CF3 group on the
meta position of the phenyl ring, adopts a distorted octahedral
geometry around the titanium center with trans oxygen atoms, cis
nitrogen atoms and cis chlorine atoms, which is the same as the case
reported for complex 2a.19 All of the bond distances and angles
around the titanium center of 2h are very close to the values of 2a
˚
˚
˚
(2h: Ti–O = 1.879(15) A, Ti–N = 2.207(2) A, Ti–Cl = 2.274(8) A, O–
Ti–O = 166.63(11)◦, Cl–Ti–Cl = 96.64(5)◦, N–Ti–N = 89.34(10)◦;
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Table 1 Crystal data and structure refinements of complexes 2h, 2k, 2m, 2n and 2s
2h 2k
C34H20Cl2F12N2O2Ti C32H20Br2Cl2F6N2O2Ti C32H18Cl2F10N2O2Ti C32H18Cl2F10N2O2Ti C32H18Cl6F6N2O2Ti
2m
2n
2s
Empirical formula
Formula weight
Crystal system
Space group
835.32
Monoclinic
P2/n
11.8417(9)
8.2093(6)
17.3695(12)
90.00
93.1170(10)
90.00
1686.0(2)
2
1.645
0.515
836
0.40 ¥ 0.17 ¥ 0.06
2.03 to 25.68
8793
3185 (Rint = 0.0200) 5809 (Rint = 0.0482)
3185/0/240
1.032
857.12
Monoclinic
P21/n
11.2111(10)
15.5277(14)
19.3685(17)
90.00
102.674(2)
90.00
3289.6(5)
4
1.731
2.919
1688
771.28
Monoclinic
P21/n
18.949(7)
8.403(3)
20.493(7)
90.00
100.738(5)
90.00
3206(2)
4
1.598
0.526
1544
771.28
Monoclinic
C2/c
21.721(2)
8.4944(8)
19.0168(17)
90.00
118.4900(10)
90.00
3083.8(5)
4
1.661
0.547
1544
837.08
Orthorhombic
Pbca
10.9398(6)
13.7340(8)
22.5112(13)
90.00
90.00
90.00
3382.2(3)
4
1.644
0.795
1672
0.29 ¥ 0.18 ¥ 0.08
1.81 to 25.35
16 839
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
Densitycalcd (Mg m-3)
Absorption coefficient/mm-1
F(000)
Crystal size/mm
0.21 ¥ 0.18 ¥ 0.10
1.70 to 25.03
16 828
0.31 ¥ 0.18 ¥ 0.16
1.62 to 25.03
15 682
5619 (Rint = 0.0508) 3035 (Rint = 0.0204) 3097 (Rint = 0.0200)
5619/22/430
1.147
0.1302, 0.2938
1.862 and -0.967
185(2)
0.37 ¥ 0.15 ¥ 0.11
2.13 to 26.03
8403
q range for data collection (◦)
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
5809/12/418
1.099
0.0874, 0.2359
4.761 and -0.983
185(2)
3035/0/222
1.052
0.0325, 0.0827
0.290 and -0.291
185(2)
3097/0/223
1.053
0.0451, 0.1188
1.154 and -0.706
187(2)
Final R indices [I > 2s(I)]: R1, wR2 0.0444, 0.1130
Largest diff. Peak and hole/e A
Measurement T/K
-3
˚
0.835 and -0.367
185(2)
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for complexes 2h, 2k, 2m, 2n and 2s
2h
2k
2m
2n
2s
Ti–O (1)
Ti–O (1A or 2)
Ti–N (1)
Ti–N (1A or 2)
Ti–Cl (1)
Ti–Cl (1A or 2)
N (1)–Ti–N (1A or 2)
O (1)–Ti–O (1A or 2)
Cl (1)–Ti–Cl (1A or 2)
N (1)–Ti–O (1)
N (1)–Ti–O (1A or 2)
N (1)–Ti–Cl (1)
1.8793(15)
1.8793(15)
2.207(2)
1.889(5)
1.859(5)
2.221(7)
2.180(7)
2.261(3)
2.282 (3)
87.8(2)
167.3(3)
97.11(11)
82.0(2)
1.871(6)
1.865(6)
2.207(8)
2.225(8)
2.248(3)
2.284(3)
83.3(3)
168.6(3)
98.34(14)
80.1(3)
1.8738(12)
1.8738(12)
2.2241(15)
2.2241(15)
2.2716(6)
2.2716(6)
89.37(7)
162.98(8)
96.35(3)
80.95(5)
1.8937(18)
1.8937(18)
2.145(2)
2.145(2)
2.2995(7)
2.2995(7)
180.00(1)
180.00(1)
180.0
2.207(2)
2.2738(8)
2.2738(8)
89.34(10)
166.63(11)
96.64(5)
81.20(7)
89.27(7)
84.34(8)
95.66(8)
90.51(6)
88.1(3)
87.88(19)
90.4(3)
166.9(2)
86.95(5)
175.19 (4)
87.35(5)
˚
˚
˚
˚
˚
˚
2a: Ti–O = 1.874(18) A, Ti–N = 2.209(2) A, Ti–Cl = 2.270(11) A, O–
Ti–O = 166.25(13)◦, Cl–Ti–Cl = 95.996(6)◦, N–Ti–N = 88.74(12)◦).
Complexes 2k, 2m, 2n and 2s, with substituent(s) on the ortho
position of the phenyl, show similar structures in the solid state.
For complexes 2k (Ar = o-BrPh), 2m (Ar = 2,4-2FPh) and 2n
(Ar = 2,6-2FPh), the two oxygen atoms are situated in the trans
position, while the two nitrogen atoms are in the cis position to
each other, and so do the two chlorine atoms around the central
titanium metal. However, all of these trans angles about the Ti
atom in 2s are constrained to 180◦ because the Ti atom lies on a
crystallographic center of symmetry. Complex 2s displays longer
(dc–c) is 4.346 A (2k), 4.064 A (2m) and 3.699 A (2n). The
˚
˚
interplanar distance (dp,p) is 3.581 A (2k), 3.528 A (2m) and 3.414
A (2n). The dihedral angle between ring planes is zero in the crystal
structures of the three complexes.
As shown in Table 3, the distances between ortho-substituted
fluorine atom and the olefinic proton are only 2.18/2.185 A (2m)
and 2.116 A (2n), suggesting the existence of weak intramolec-
˚
˚
˚
ular hydrogen bonding interactions between these atoms.27 The
intermolecular interactions (p–p stacking) and the intramolecu-
lar interactions (hydrogen bonding) result in the small torsion
angles (the torsion angle is the angle between the plane of
b-enaminoketonato and the substituted phenyl ring) the small
torsion angles in 2m (8.99◦, 13.59◦) and 2n (13.23◦). An obvious
difference in torsion angles for 2k was seen not only because
the N-aryl ring of the ligand is asymmetric, but also because
the p–p stacking interactions just occur in the one side of the
crystal structure. No p–p stacking interactions are observed in
complexes 2a and 2s, and the hydrogen bonding interaction in
˚
Ti–Cl and Ti–O bond distances of 2.2995(7) and 1.8937(18) A,
respectively, and shorter Ti–N bond distance of 2.145(2) A than
complex 2k (Ti–Cl(1) = 2.261(3), Ti–O(1) = 1.889(5), Ti–N(1) =
˚
˚
2.221(7) A).
It was interesting to note that there is overlapping of the
phenyl ring located in the R2 moiety in the crystal structure
of complexes 2k, 2m and 2n, indicating there is p–p stacking
interactions between them. The geometrical parameters of the
stacking contacts are as follows: the centroid–centroid distance
˚
2s is very weak as indicated by the distance (3.166 A) of Cl(2)–
H(8). From Table 3, we can also see that 2s has a larger torsion
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Table 4 Polymerization of ethylene by complexes 2a–t activated with
MMAO.a
Catalyst/
Entry mmol
Mnd/kg
Polymer/g Activityb Tmc/◦C mol-1
d
Mw/Mn
1
2
3
4
5
6
7
8
2a (1.0)ref 0.11
1.32
2.40
1.94
0.737
1.14
0.349
1.80
2.88
2.52
3.24
3.24
3.12
1.56
4.08
1.32
2.52
1.80
5.64
—
134
138
140
138
137
141
138
139
139
135
137
140
136
135
135
135
138
139
—
61.0
80.0
95.0
48.0
62.0
34.0
94.0
137
77.0
98.0
113
99.0
90.0
153
1.27
1.66
1.33
1.25
1.27
1.20
1.76
1.39
2.09
1.72
2.12
1.86
1.35
1.74
1.30
1.80
2.66
1.80
—
2b (1.0)
2c (1.0)
2d (1.0)
2e (1.0)
2f (1.0)
2g (1.0)
2h (1.0)
2i (1.0)
2j (1.0)
2k (1.0)
2l (1.0)
2m (1.0)
2n (1.0)
2o (1.0)
2p (1.0)
2q (1.0)
2r (1.0)
0.20
0.16
0.061
0.095
0.029
0.15
0.24
0.21
0.27
0.27
0.26
0.13
0.34
0.11
0.21
0.15
0.47
9
10
11
12
13
14
15
16
17
18
19
20
71.0
105
70.0
61.0
—
Fig. 5 Molecular structure of complex 2s with thermal ellipsoids at 30%
probability level. Hydrogen atoms (except H8 and H8A) are omitted for
clarity.
2s (10.0) trace
2t (1.0) 0.40
4.80
134
131
2.04
a Reaction conditions: 1 atm ethylene pressure, Al/Ti (mol ratio) = 2000,
toluene 50 mL, polymerization for 5 min, the results shown for each entry
are representative of at least three reproducible runs. b kg of PE/mmolTi·h.
c Melting temperature measured by DSC. d Number-average molecular
weight and polydispersity indexes of the resultant polyethylene determined
by GPC using polystyrene standard.
Table 3 Distances between ortho-substituted halogen atom and olefinic
◦
˚
proton (A) and torsion angles ( ) for complexes 2a, 2k, 2m, 2n and 2s
2a
2k
2m
2n
2s
Distances between ortho-substituted halogen atom and
˚
olefinic proton/A
F(4)–H(8)
F(9)–H(24)
2.180
2.185
F(5)–H(8)
Br(1)–H(8)
Br(2)–H(24)
2.116
13.23
2.824
2.512
Cl(2)–H(8)
3.116
65.14
Torsion angles (◦)
C(8)–C(9)–C(10)–C(15)
C(24)–C(25)–C(26)–C(27)
11.17 49.26
8.78
8.99
13.59
angle than 2a (2s, 65.14◦; 2a, 11.17◦), which is caused by bulkier
substituents on the ortho position of the N-aryl ring. These facts
indicate the intermolecular interaction (p–p stacking) and the
intramolecular interaction (hydrogen bonding) can be considered
as the determining factor for the association of molecules in the
solid state.27
Ethylene polymerization catalyzed by new titanium complexes
With MMAO as a cocatalyst, these new titanium complexes have
been investigated as catalyst precursors for ethylene polymeriza-
tion at room temperature under atmospheric pressure. The intro-
duction of an electron-withdrawing group (EWG) and an electron-
donating group (EDG) into the N-aryl moiety significantly affects
catalytic activity and the properties of the resultant polymers, as
shown in Table 4. The melting points, which range from 135–
140 ◦C, and the 13C NMR data for the polymer products are
consistent with linear polyethylene.
It is well-known that the ethylene polymerization by titanium
catalyst can be described as “coordination-migration” mechanism
like that shown in Scheme 2.28 The progress of polymerization can
be divided into two steps: “step a” (coordination of ethylene to
center metal) and “step b” (migration of ethylene to metal–carbon
Scheme 2 Potential mechanism of ethylene polymerization by the tita-
nium complex.
bond), and the catalytic activity is determined by the rate of both
steps as well as some kind of balance between them. Introduction
of an EWG into the phenyl ring close to the oxygen donor
makes ethylene coordinate to the titanium much easily, which
can accelerate “step a”. For fluorine-containing phenyl, the role
of the withdrawing inductive effect against donating conjugative
effect depends on the F-position. The inductive effect which favors
ethylene insertion is usually stronger than the conjugating effect
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Table 5 Copolymerization of ethylene/norbornene by complexes 2a–t activated with MMAO.a
Tgc/◦C
Mnd/kg mol-1
Mw/Mn
d
Entry
Catalyst/mmol
Polymer/g
Activityb
NB Incorp. (mol%)
1
2
3
4
5
6
7
8
2a
2c
2d
2e
2f
2g
2h
2i
0.860
0.806
0.641
0.755
0.157
0.468
0.837
0.366
0.494
1.39
1.72
1.61
1.28
1.52
0.313
0.940
1.67
0.73
0.99
2.78
1.13
44.3
45.3
39.4
42.1
46.7
41.9
46.9
44.8
40.2
47.1
45.3
121
124
115
118
132
119
130
126
115
135
130
340
297
224
256
51.3
152
289
29.2
167
480
185
1.17
1.25
1.09
1.14
1.07
1.21
1.19
1.24
1.28
1.10
1.21
9
10
11
2p
2n
2t
0.567
a Conditions: 3 mmol complex, 0.2 mol L-1 concentration of the norbornene, Al/Ti molar ratio = 2000 : 1, Vtotal = 50 mL, atmospheric pressure, 25 ◦C
polymerization for 10 min, the results shown for each entry are representative of at least two reproducible runs. b Activity in kg of polymer/mmolTi·h.
c Number-average molecular weight and polydispersity index determined by GPC using polystyrene standard.
in o-F and/or m-F-containing phenyl.29 Therefore, compared with
2a, 2b and 2c exhibited relatively high catalytic activity. Contrarily,
complex 2d with the p-F in phenyl ring displayed low catalytic
activity for ethylene polymerization due to the existence of strong
conjugating effect.
Introducing an extra chlorine atom in the phenyl of the ligands
also significantly influenced the catalyst behaviors. Complex 2s is
inactive for ethylene polymerization due to the trans arrangement
of the two chloride ligands (see Fig. 5), but 2t (2,5-2Cl) exhibits
higher catalytic activity (4.80 kg mmol-1Ti·h bar, entry 20) than 2j
(o-Cl).
In order to clarify the effect of a para-substituted group in the
phenyl ring on the catalytic performance, complexes 2e with a
p-Cl, 2f with a p-OMe and 2g with a p-CF3 were synthesized.
As expected, 2e and 2g showed higher catalytic activity (1.14 and
1.80 kg mmol-1Ti·h bar, respectively), while 2f exhibited much lower
catalytic activity (0.349 kg mmol-1Ti·h bar) than 2a. These results
support the assumption that the low catalytic activities of 2d results
from the conjugating effect of para-substituted fluorine atom. It is
worth noting that the polydispersity index (Mw/Mn = 1.20) of the
polymer obtained by 2f is obviously lower than that by 2a, which
implies that the electron-donating effect of the para-substituted
group can suppress b-H chain transfer and produces a narrower
polymer MWD. In addition, both 2h with m-CF3 and 2i with o-CF3
displayed high catalytic activities under the same conditions (2.88
and 2.52 kg mmol-1Ti·h bar, respectively). Moreover, complexes 2j–
l showed notable catalytic activities for ethylene polymerization
(2j, 3.24; 2k, 3.24; 2l, 3.12 kg mmol-1Ti·h bar). These results
suggested the existence of an electron-withdrawing group on the
ortho- and/or meta-substituted phenyl ring is a necessary factor
to improve the catalyst performance.
Complexes 2m–p containing two fluorine atoms in the phenyl
of the ligands were also synthesized. Complexes 2m (2,4-2F) and
2o (3,4-2F) exhibited comparable catalytic activities (2m, 1.56; 2o,
1.32 kg mmol-1Ti·h bar) with 2a. A significant increase in catalytic
activity was observed if 2n (2,6-2F) and 2p (3,5-2F) was used
(2n, 4.08; 2p, 2.52 kg mmol-1Ti·h bar). Obviously, the degenerating
effect of para-substituted fluorine atom counteracts the improving
function of the ortho- or meta-substituted one towards catalytic
performance. In order to further enhance the catalytic activity
of titanium complex [PhN C(CF3)CHC(Ar)O]2TiCl2, 2q and
2r were prepared and investigated for ethylene polymerization.
Complex 2q did not display surprising catalytic activity and
produced the polymer with broader MWD (1.80 kg mmol-1Ti·h
bar, Mw/Mn = 2.66, entry 17). However, 2r showed the highest
catalytic activity (5.64 kg mmol-1Ti·h bar, entry 18) among all
the bis(b-enaminoketonato) titanium complexes studied here.
Ethylene/norbornene copolymerization by the new titanium
complexes
Ethylene/norbornene copolymers are important in various fields
due to their remarkable material properties (e.g., high thermal
stability, high transparency, and high refractive indices). In our
previous study, complex 2a showed high activity for living ethy-
lene/norbornene copolymerization.12a Herein, we thus explored
ethylene/norbornene copolymerization by some representative
complexes. The typical results are summarized in Table 5. It
should be noted that complexes 2c–i, 2p, 2n and 2t are also
capable of promoting the living copolymerization of ethylene
with norbornene at room temperature, yielding high molecular
weight copolymers with narrow molecular weight distributions
(PDI = 1.05–1.30). These results indicated that the presence
of norbornene in the reaction medium and/or in the polymer
chain seems beneficial for suppressing the chain termination or
transfer.30 Complex 2c may be best suited as the catalyst precursor
for this copolymerization in terms of exhibiting a high catalytic
activity with efficient norbornene incorporation.
Conclusions
A series of new titanium complexes (2b–t) bearing two b-
enaminoketonato ligands with electron-withdrawing or electron-
donating group(s) have been synthesized and characterized. In
the solid state, complexes 2h, 2k, 2m, 2n and 2s adopt distorted
octahedral geometry around the titanium center. Complexes 2h,
2k, 2m and 2n display cis-configuration of the two chlorine
atoms around the titanium center, while complex 2s showed
trans-configuration of the two Ti–Cl bonds due to the steric
effect of the two ortho-substituted chlorine atoms on the phenyl
close to the oxygen donor. With MMAO as a cocatalyst,
all the titanium complexes, except for 2s, provide suitable
9004 | Dalton Trans., 2010, 39, 9000–9007
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precatalysts for ethylene polymerization, and produce high molec-
ular weight polymers with linear structures. Catalytic activity and
polymer properties are influenced significantly by the structure
of the b-enaminoketonato ligand. Both the electronic effect
(including conjugative and inductive effect) and the position
of the substituent on the ligand considerably influences the
polymerization behavior of the titanium complexes. Ortho- and/or
meta-halogenating considerably enhances the catalytic activity of
[PhN C(CF3)CHC(Ar)O]2TiCl2, but para-halogenating reduces
the catalytic activity for ethylene polymerization. Complexes 2c–
f, 2m and 2o produce high molecular weight polyethylenes with
relative narrow molecular weight distributions (Mw/Mn = 1.20–
1.35), which implies the electron-donating group(s) are beneficial
to obtaining the polymers with narrow MWD. In addition, the
representative substituted complexes are also capable of promot-
ing the living copolymerization of ethylene with norbornene at
room temperature to produce high molecular weight copolymers
with narrow molecular weight distributions (Mw/Mn = 1.05–1.30).
ammonium chloride solution, and then extracted with ether.
The combined organic layer was washed with brine, dried over
anhydrous sodium sulfate, filtered, and evaporated under reduced
pressure to afford a deep yellow oil. To a stirred solution of this
crude product in toluene (50 mL) were added aniline (5.6 mL,
60 mmol) and p-toluenesulfonic acid (ca. 20 mg) as a catalyst at
room temperature. The mixture was heated and refluxed, and the
water formed was removed azeotropically using a Dean–Stark
apparatus for 24 h. During the stirring period, a deep-yellow
solution was formed. Evaporation of toluene gave a deep yellow
oil. The crude product was purified by column chromatography
on silica gel with petroleum ether/ethyl acetate mixture (200 : 1) as
an eluent, affording 1b (9.27 g, 30.0 mmol) as a deep yellow oil in
75.0% yield. 1H NMR (300 MHz, CDCl3): d 11.54 (s, 1H, O–H),
8.29 (d, 1H, Ar), 7.51 (t, 1H, Ar), 8.29 (d, 1H, Ar), 7.36–7.28 (m,
4H, Ar), 7.08 (t, 1H, Ar), 6.82 (d, 2H, Ar), 6.51(s, 1H, = CH).
13C NMR (75 MHz, CDCl3): d 187.46, 159.84, 147.62, 136.54,
132.52, 129.49, 127.98, 125.30, 124.97, 123.41, 120.86, 117.17,
115.54, 94.98. Anal. Calcd. for C16H11F4NO: C, 62.14; H, 3.59;
N, 4.53; Found: C, 62.28; H, 3.64; N, 4.49. Complexes 1c–t were
prepared via a procedure similar to that for 2b, the 1H NMR and
the 13C NMR of these obtained complexes were available in the
ESI.†
Experimental
All air and moisture-sensitive compounds were manipulated using
standard Schlenk techniques or in a glove box under an argon
atmosphere. The NMR data of the ligands and the complexes
were obtained on a Bruker 300 MHz spectrometer at ambient
Titanium complexes synthesis.
[PhN C(CF3)CHC(o-FPh)O]2TiCl2 (2b)
˚
temperature with CDCl3 as a solvent (dried by MS 4 A). The
NMR data of the copolymers were obtained on a Varian Unity
400-MHz spectrometer at 125 ◦C with o-C6D4Cl2. Elemental
analysis was performed on a Perkin-Elmer Series II CHN/O
analyzer 2400. The DSC measurements were performed on a
Perkin-Elmer Pyris 1 differential scanning calorimeter at a rate
of 10 ◦C min-1. The number-average molecular weights (Mn) and
polydispersity indices (PDI) of the polymers were determined in
1,2,4-trichlorobenzene at 150 ◦C on a high temperature GPC using
PL-GPC 220 instrument equipped with three PLgel 10 mm mixed-
B LS columns.
Dried diethyl ether, hexane and toluene were purified from
an MBraun SPS system. The ethyl trifluoroacetate, potassium t-
butoxide, aniline and most of the acetophenone derivatives for
ligands synthesis were purchased from Aldrich Chemical or Alfa
Aesar Organics, and used without further purification. 2,3,5,6-
Tetrafluoro-4-methoxyacetophenone was prepared according to
the methods described in the literature.24 Commercial titanium
tetrachloride was distilled prior to use. The 1.6 M n-butyllithium
solution in hexane was purchased from Acros. Modified methy-
laluminoxane (MMAO, 7% aluminium in heptane solution) was
purchased from Akzo Nobel Chemical Inc. Commercial ethylene
was directly used for polymerization without further purification.
Complex 2a was synthesized according to the procedure reported
previously.
To a stirred solution of compound PhN C(CF3)CHC(o-FPh)OH
◦
(1b, 1.24 g, 4 mmol) in dried diethyl ether (20 mL) at -78 C, a
1.6 M n-butyllithium hexane solution (2.5 mL, 4 mmol) was added
dropwise over 5 min. The mixture was allowed to warm to room
temperature and stirred for 2.5 h. Then, the mixture was added
dropwise to TiCl4 (2 mmol) in dried diethyl ether (30 mL) at -78 ◦C
with stirring over 30 min. The mixture was allowed to warm to
room temperature and stirred overnight. The evaporation of the
solvent in vacuum yielded a crude product. Dried CH2Cl2 and n-
hexane were added to the solid residue, and the mixture was stirred
for 10 min. The filtration of the mixture gave complex 2b as a red
solid in 61% yield. 1H NMR (300 MHz, CDCl3): d 7.70 (td, 2H,
Ar–H), 7.46 (q, 2H, Ar–H), 7.29 (t, 2H, Ar–H), 7.20–7.04 (m, 6H,
Ar–H), 6.92 (t, 2H, Ar–H), 6.81–6.78 (m, 6H, Ar–H and CH).
Anal. Calcd. for C32H20Cl2F8N2O2Ti: C, 52.27; H, 2.74; N, 3.81;
Found: C, 52.18; H, 2.69; N, 3.86. Complexes 2c–2t were prepared
1
via a procedure similar to that for 2b, the H NMR and the 13C
NMR of these obtained complexes were available in the ESI.†
X-Ray crystallography†
Single crystals of complexes 2h, 2k, 2m, 2n and 2s suitable for
X-ray structure determination were grown from dichloromethane–
hexane solution at room temperature in a glove box, thus maintain-
ing a dry, O2-free environment. The X-ray diffraction data of the
complexes were collected on a Bruker Smart CCD diffractometer
equipped with graphite monochromated Mo-Ka radiation (l =
PhN C(CF3)CHC(o-FPh)OH (1b)
Ethyl trifluoroacetate (7.2 mL, 60 mmol) in ether (30 mL) was
added dropwise to a stirred suspension of t-BuOK (5.4 g, 48 mmol)
in dried ether (60 mL). The solution was stirred for 45 min at
room temperature. A solution of 2¢-chloroacetophenone (5.2 mL,
40 mmol) in ether (30 mL) was added, and the solution was stirred
overnight. The reaction mixture was poured into a saturated
˚
0.71073 A) at 187 K. Empirical absorption corrections were
applied to the data using the SADABS program. The structures
were solved by the direct method and refined by the full-matrix
least-squares on F2 using the SHELXTL-97 program.31–32 All of
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the non-hydrogen atoms were refined anisotropically. Crystallo-
graphic data and other pertinent information for the complexes
are summarized in Table 1. Selected bond lengths and angles are
listed in Table 2.
4 F. Gornshtein, M. Kapon, M. Botoshansky and M. S. Eisen,
Organometallics, 2007, 26, 497; P. Elo, A. Parssinen, M. Nieger, M.
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5 R. J. Long, V. C. Gibson, A. J. P. White and D. J. Williams, Inorg.
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6 L. Annunziata, D. Pappalardo, C. Tedesco and C. Pellecchia, Macro-
molecules, 2009, 42, 5572; L. Annunziata, D. Pappalardo, C. Tedesco
and C. Pellecchia, Organometallics, 2009, 28, 688.
7 V. Volkis, M. Rodensky, A. Lisovskii, Y. Balazs and M. S. Eisen,
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8 B. Lian, K. Beckerle, T. P. Spaniol and J. Okuda, Eur. J. Inorg. Chem.,
2009, 311; K. Itagaki, K. Kakinuki, S. Katao, T. Khamnaen, M.
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9 S. J. Luo, J. Vela, G. R. Lief and R. F. Jordan, J. Am. Chem. Soc., 2007,
129, 8946; D. Guironnet, P. Roesle, T. Ru¨nzi, I. Go¨ttker-Schnetmann
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Ethylene polymerization
Polymerization was carried out under atmospheric pressure in
toluene in a 150 ml glass reactor equipped with a mechanical
stirrer. Toluene (50 mL) was introduced into the argon-purged
reactor and stirred vigorously (600 rpm). The toluene was kept
at a prescribed polymerization temperature, and then the ethylene
gas feed was started. After 15 min, the polymerization was initiated
by the addition of a heptane solution of MMAO and a toluene
solution of one of the titanium complexes into the reactor with
vigorous stirring (600 rpm). After a prescribed time, isobutyl
alcohol (10 mL) was added to terminate the polymerization
reaction, and the ethylene gas feed was stopped. The resulted
mixture was added to the acidic methanol (1 mL concentrated of
HCl in 500 mL of methanol). The solid polyethylene was isolated
◦
by filtration, washed with methanol, and dried at 60 C for 24 h
in a vacuum oven.
Ethylene/norbornene copolymerization
The copolymerization was performed in a glass flask (150 mL)
equipped with a mechanical stirrer. Toluene (50 mL) was intro-
duced to the argon-purged reactor and stirred at 600 rpm. The
solvent was kept at the prescribed polymerization temperature,
and then the prescribed amount of norbornene was charged into
the reactor. The ethylene gas feed was started. After 15 min,
polymerization was initiated by the addition of a MMAO heptane
solution and then a solution of titanium complexes in toluene into
the reactor. The polymerization was quenched after the prescribed
time by the addition of isobutyl alcohol (5 mL). The resulting
mixture was added to the acidic methanol. The copolymer was
collected by filtration, washed with methanol, and then dried at
60 ◦C for 24 h in a vacuum.
12 M. A. Zuideveld, P. Wehrmann, C. Rohr and S. Mecking, Angew.
Chem., Int. Ed., 2004, 43, 869; D. Guironnet, T. Friedberger and S.
Mecking, Dalton Trans., 2009, 8929.
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Acknowledgements
16 P. D. Hustad and G. W. Coates, J. Am. Chem. Soc., 2002, 124, 11578;
A. F. Mason, J. Tian, P. D. Hustad, E. B. Lobkovsky and G. W. Coates,
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17 M. Lamberti, D. Pappalardo, A. Zambelli and C. Pellecchia, Macro-
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The authors are grateful for the support of the National Natural
Science Foundation of China (Nos. 20734002 and 20874096), and
by the Special Funds for Major State Basis Research Projects (No.
2005CB623800) from the Ministry of Science and Technology of
China.
18 S. Reinartz, A. F. Mason, E. B. Lobkovsky and G. W. Coates,
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20 L. M. Tang, T. Hu, Y. J. Bo, Y. S. Li and N. H. Hu, J. Organomet.
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