Highly active new α-diimine nickel catalyst for the polymerization of α-olefins

Article abstract of DOI:10.1016/j.jorganchem.2004.11.046

A new silylated α-diimine ligand, bis[N,N′-(4-tert-butyl- diphenylsilyl-2,6-diisopropylphenyl)imino]acenaphthene 3, and its corresponding Ni(II) complex, {bis[N,N′-(4-tert-butyl-diphenylsilyl-2,6- diisopropylphenyl)imino]acenaphthene}dibromonickel 4, have been synthesized and characterized. The crystal structures of 3 and 4 were determined by X-ray crystallography. In the solid state, complex 4 is a dimer with two bridging Br ligands linking the two nickel centers, which have square pyramidal geometries. Complex 4, activated either by diethylaluminum chloride (DEAC) or methylaluminoxane (MAO) produces very active catalyst systems for the polymerization of ethylene and moderately active for the polymerization of propylene. The activity values are in the order of magnitude of 10⁷ g PE (mol Ni [E] h)^-1 for the polymerization of ethylene and of 10⁵ g PP (mol Ni [P] h)^-1 for the polymerization of propylene. NMR analysis shows that branched polyethylenes (PE) are obtained at room or higher temperatures and almost linear PE is obtained at 0°C with 4/DEAC.

Full text of DOI:10.1016/j.jorganchem.2004.11.046

Journal of Organometallic Chemistry 690 (2005) 1314–1323
www.elsevier.com/locate/jorganchem
Highly active new a-diimine nickel catalyst for the
polymerization of a-olefins
a,
a
a
Hua-Rong Liu a,b, Pedro T. Gomes *, Sandra I. Costa , M. Teresa Duarte ,
a
a
R. Branquinho , Anabela C. Fernandes , James C.W. Chien ,
c
d
R.P. Singh , Maria M. Marques
a,
*
a
´ ´ ´
Centro de Quımica Estrutural, Departamento de Engenharia Quımica, Instituto Superior Tecnico, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal
b
Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China
c
Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA 01003, USA
d
Division of Polymer Chemistry, National Chemical Laboratory, Pune 411 008, India
Received 8 September 2004; accepted 29 November 2004
Abstract
A new silylated a-diimine ligand, bis[N,N0-(4-tert-butyl-diphenylsilyl-2,6-diisopropylphenyl)imino]acenaphthene 3, and its corre-
sponding Ni(II) complex, {bis[N,N0-(4-tert-butyl-diphenylsilyl-2,6-diisopropylphenyl)imino]acenaphthene}dibromonickel 4, have
been synthesized and characterized. The crystal structures of 3 and 4 were determined by X-ray crystallography. In the solid state,
complex 4 is a dimer with two bridging Br ligands linking the two nickel centers, which have square pyramidal geometries. Complex
4, activated either by diethylaluminum chloride (DEAC) or methylaluminoxane (MAO) produces very active catalyst systems for
the polymerization of ethylene and moderately active for the polymerization of propylene. The activity values are in the order of
magnitude of 107 g PE (mol Ni [E] h)ꢀ1 for the polymerization of ethylene and of 105 g PP (mol Ni [P] h)ꢀ1 for the polymerization
of propylene. NMR analysis shows that branched polyethylenes (PE) are obtained at room or higher temperatures and almost linear
PE is obtained at 0 ꢀC with 4/DEAC.
ꢁ 2004 Elsevier B.V. All rights reserved.
Keywords: a-Diimine nickel complex; Late transition-metal; Ethylene polymerization; Propylene polymerization
1. Introduction
ily useful for ethylene polymerization but also to
promote co-polymerizations of olefins with polar mono-
mers and functionalization of polyolefins [4–11]. In
addition, the a-diimine ligands are well-known to stabi-
lize organometallic complexes resulting in much more
stable late transition metal catalysts compared with
early transition metal catalysts [12]. This is beneficial
to commercial applications and industrial production.
In this work, we report the synthesis and character-
ization of a new silylated a-diimine ligand, in which
there is a –Si(tBu)Ph2 substituent at the para position
of the 2,6-dialkylaryl imine group, and its Ni dibromide
complex. We also describe the catalytic behavior of this
new complex towards ethylene and propylene polymeri-
There is a considerable interest in the development of
new late transition metal catalysts for the polymeriza-
tion of a-olefins since Brookhart and co-workers [1–3]
discovered highly active a-diimine nickel and palladium
catalysts which convert ethylene and a-olefins to high
molar mass polymers with unique microstructures.
These catalysts have been shown not only to be primar-
*
Corresponding authors. Tel.: +351 218419612/3; fax: +351
218414457/5.
E-mail addresses: pedro.t.gomes@ist.utl.pt (P.T. Gomes), pci011@
mail.ist.utl.pt (M.M. Marques).
0022-328X/$ - see front matter ꢁ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.11.046

H.-R. Liu et al. / Journal of Organometallic Chemistry 690 (2005) 1314–1323
1315
zation. One of the aims would be the modification of the
ligand structure in order to change the electronic density
at the metal center, thus inducing different activities,
polymer microstructures or degrees of co-monomers
incorporation.
have chosen the bulky tert-butyllithium to substitute
the Br, further preventing the reaction of the amine with
tert-butyldiphenylsilyl chloride in the next step. In fact,
none of the hydrogen atoms in the amine were substi-
tuted by the tert-butyldiphenylsilyl group. Compound
2 is obtained in low yield (25%) due to the simultaneous
formation of two unidentified by-products. Three dis-
tinct bands were obtained after separation by column
chromatography, being the desired product 2 isolated
from the middle fraction as white needles.
The new ligand 3 was prepared by condensation reac-
tion of acenaphthenequinone with the silylated aniline 2
according to a literature procedure [14] (see Scheme 2).
Recrystallization of the crude ligand from hot toluene
afforded air stable orange crystals in 64% yield.
2. Results and discussion
2.1. Syntheses and characterizations of ligand 3 and
complex 4
In order to synthesize the target silylated a-diimine li-
gand, the appropriate aniline 2 was first prepared by
bromination of diisopropylaniline at position 4, yielding
compound 1, followed by substitution reaction with tert-
butyldiphenylsilyl chloride. The reaction sequence lead-
ing to the synthesis of the novel compound 2 is depicted
in Scheme 1.
In the bromination reaction, Oskam and co-workers
[13] obtained 4-Br-2,6-iPr2-C6H2NH2 in 61% yield by
adding Br2 to a stirred slurry of 2,6-iPr2–C6H3NH3Cl
in glacial acetic acid. In the present work, a modification
of this synthetic procedure was introduced since the
ammonium salt was poorly soluble in glacial acetic acid.
Instead, we have used methanol/glacial acetic acid
(2:1(v/v)) as the reaction solvent, enabling the dissolu-
tion of the reagents and leading to an increased yield
of compound 1 (79%) in relation to the literature [13].
Usually, the amine group of compound 1 should be
protected before the substitution reaction of bromide
in position 4 by the tert-butyldiphenylsilyl group. How-
ever, in this case, the amine hydrogens seem to be not
very reactive due to steric hindrance of the bulky diiso-
propyl groups in both ortho positions. In addition, we
Complex 4 was prepared via two different procedures
[14] (Scheme 2): (a) one-pot reaction of NiBr2, acenaph-
thenequinone and 2,6-diisopropyl-4-tert-butyldiphenyl-
silyl-aniline 2 in glacial acetic acid, and (b) addition of
the diimine ligand 3 to [NiBr2(DME)] in CH2Cl2. It is
obtained in good yields (61% and 87%, respectively) as
a dark red-brown powder. Due to the geometry around
the nickel atom, this a-diimine Ni(II) complex is para-
1
magnetic and its H NMR is not assignable. Elemental
analysis of complex 4 fits the molecular structure ob-
tained by X-ray structural studies. This compound is
air stable but do require moisture-free solvents during
NH2
Pri
iPr
O
+
2
O
Si(tBu)Ph2
2
i
ii
NH3+Cl-
iPr
NH3+Cl-
iPr
Pri
Pri
i
Si(tBu)Ph2
Si(tBu)Ph2
ii
Pri
Pri
iPr
iPr
Br
N
N
Br
NH2
NH2
iii
Ni
Pri
Pri
iPr
iPr
Br
iii
N
N
Pri
iPr
Pri
iPr
Br
Si(tBu)Ph2
2
1
Si(tBu)Ph2
Si(tBu)Ph2
Scheme 1. Synthesis of compounds 1 and 2. Reagents: (i) 2,6-di-i-
propylaniline hydrochloride (1 equiv) in MeOH/CH3COOH, Br2 (1
equiv), room temperature, 2.5 h. (ii) NaOH in water/Et2O, room
temperature. (iii) LitBu (3 equiv) in THF, ꢀ70 ꢀC to room temper-
ature, overnight, then Ph2(tBu)SiCl (1 equiv), ꢀ20 ꢀC to room
temperature, overnight.
3
4
Scheme 2. Synthesis of compounds 3 and 4: (i) in MeOH, formic acid
(catalytic amount), room temperature, 3 days; (ii) in glacial acetic acid,
reflux, 4 h, then anhydrous NiBr2 (1 equiv), reflux, 16 h; (iii)
[NiBr2(DME)] (1 equiv), in CH2Cl2, room temperature, overnight.

1316
H.-R. Liu et al. / Journal of Organometallic Chemistry 690 (2005) 1314–1323
Table 1
handling. The use of glacial acetic acid does not interfere
with the formation of the nickel complex, which suggests
that it is acid-stable with respect to decomposition. In
addition, the tert-butyldiphenylsilyl group is stable to
hydrolysis. This highly polarizable substituent also con-
fers increased solubility properties to the complex since,
compared to its non-silylated analogue, it is fairly solu-
ble in toluene.
˚
Selected bond distances (A) and angles (ꢀ) for ligand 3 and the
corresponding nickel complex 4
3
4
N1–C1
N1–C13
1.274(7)
1.488(7)
1.274(10)
1.458(10)
1.280(11)
1.444(12)
1.556(12)
1.459(12)
1.479(12)
1.394(12)
1.423(12)
2.412(2)
2.447(2)
2.470(2)
2.106(8)
2.073(8)
3.49(2)
N2–C11
N2–C41
1.308(7)
1.367(8)
C1–C11
C1–C2
1.536(7)
1.490(7)
Recrystallization of ligand 3, by cooling a toluene
solution overnight to ꢀ20 ꢀC, gave crystals suitable
for X-ray crystal structure determination. The molecular
structure of 3 is shown in Fig. 1, and selected bond
lengths and angles are presented in Table 1. The ligand
exhibits a pseudo C2v symmetry with different electron
density distributions on both sides of the ligand, visible
when comparing the C@N imine and the N–Caryl bonds
C11–C10
C10–C12
C12–C2
1.491(7)
1.423(10)
1.403(9)
–
Ni–Br1
Ni–Br2
–
Ni–Br2a
Ni–N1
Ni–N2
–
–
–
Niꢁ ꢁ ꢁNia
–
118.3(5)
˚
(respectively, 1.274(7), 1.308(7) A) and 1.488(7), 1.367(8)
˚
C1–N1–C13
N1–C1–C11
C1–C11–N2
C11–N2–C41
N1–C13–C14
N1–C13–C18
N2–C41–C42
N2–C41–C46
N1–Ni–N2
N2–Ni–Br1
N1–Ni–Br1
N2–Ni–Br2
N1–Ni–Br2
Br1–Ni–Br2
N2–Ni–Br2a
N1–Ni–Br2a
Br1–Ni–Br2a
Br2–Ni–Br2a
a
118.6(9)
116.8(10)
116.4(9)
117.0(9)
121.8(10)
116.1(10)
122.2(11)
116.7(12)
80.5(3)
A). The relatively long C1–C11 bond distance, 1.536(7)
˚
118.4(5)
120.6(4)
A, compared to the expected Csp2–Csp2 single bond, 1.48
˚
A [15], is due to the constraints existing in the five mem-
119.1(5)
117.4(5)
bered ring. The bis(imino)acenaphthene skeleton is pla-
nar as shown by the torsion angles N1–C1–C11–N2 and
C2–C1–C11–C10 of ꢀ1.8(7) and 0.1(7)ꢀ, respectively.
This planarity is extended to the imine–aryl bonds,
proved by the low values of the torsion angles C1–
C11–N2–C41 and C11–C1–N1–C13 of 2.1(5) and
1.3(5)ꢀ. Both aryl rings, practically parallel to each other
(dihedral angle of 2.3(2)ꢀ), are almost perpendicular to
the bis(imino)acenaphthene plane (dihedral angles of
79.3(2) and 80.3(2)). Due to the overall stereochemical
constraints of the molecule, the N atoms of the imine
groups display short interactions with the hydrogen
atoms of the isopropyl substituents of the phenyl rings,
both above and below the plane of the molecule (C–
Hꢁ ꢁ ꢁN, interactions with Hꢁ ꢁ ꢁN distances in the order
118.2(5)
118.6(6)
121.8(6)
–
–
–
–
–
–
–
–
–
–
96.8(2)
96.5(2)
158.0(2)
91.5(2)
104.48(7)
90.8(3)
158.9(2)
103.67(7)
89.55(6)
Symmetry transformation 1 ꢀ x, ꢀy, ꢀz + 1.
Complex 4 was also characterized by X-ray crystallog-
raphy. Crystals were grown by slow diffusion of hexane
into a CH2Cl2 solution of 4. The molecular structure is de-
picted in Fig. 2 with selected bond lengths and angles in
Table 1. It shows a dimeric structure composed by two
centrosymmetrically related monomeric moieties, where
˚
of 2.412(4), 2.471(6), 2.496(6) and 2.532(4) A). These
structural features compare well with those found in
the non-silylated analogue (iPPBIAN) [16] or in the p-
tolyl derivative (p-Tol-BIAN) [17] and related com-
pounds referred therein.
Fig. 1. Molecular structure of ligand 3. Thermal ellipsoids are given with 40% probability level. Hydrogen atoms are omitted for clarity.

H.-R. Liu et al. / Journal of Organometallic Chemistry 690 (2005) 1314–1323
1317
Fig. 2. Molecular structure of dimeric 4. Thermal ellipsoids are given with 30% probability level. Hydrogen atoms are omitted for clarity. (a) Global
view of the dimer. (b) View of one of moieties.
two bromine atoms (Br2) are bridging the two nickel cen-
˚
ters (3.49(2) A apart), which display a square pyramidal
2.2. Polymerization results
˚
geometry. Each apical Br1 atom is 2.812(4) A away from
the basal square plane defined by the two nitrogen and the
˚
two bromine atoms, and the Ni atom stays at 0.414(3) A
Complex 4 was tested as catalyst precursor for the
polymerization of ethylene. The results listed in Table
2 show that diethylaluminium chloride (DEAC) and
methylaluminoxane (MAO) are better activators than
trimethylaluminium (TMA) or tri-iso-butylaluminium
(TIBA) for this new a-diimine nickel complex, consis-
tent with other a-diimine nickel complexes [14], [20].
The concentration of 4 (entries LNi3–LNi7) [22], the
corresponding [Al]/[Ni] ratio (entries LNi8–LNi12) and
the polymerization temperature (entries LNi13–LNi16)
seem not to affect significantly the activities of ethylene
polymerization. However, the microstructure of PE
and consequently their thermal properties are seriously
affected by the polymerization temperature (Table 3).
It is shown that the polyethylene branching increases
with increasing polymerization temperature. Simulta-
neously, for polymerization runs carried out at room
temperature, the total branching of the polymer samples
obtained with MAO is smaller than that with DEAC. As
can be seen in Table 3, their thermal properties support
this conclusion. Actually, the decrease in the polymeri-
zation temperature gave more crystalline polymers, with
from this plane. This dimeric structure is similar to
those found for the non-silylated analogue [bis(N,N0-2,-
6-diisopropylphenylimino)acenaphthene]dibromonickel
([NiBr2(iPPBIAN)]) [18] and for [bis(N,N0-2-tert-butyl-
phenylimino)acenaphthene]dibromonickel [14], in oppo-
sition to the monomeric structures obtained for other
bis(arylimino)acenaphthene nickel complexes, where the
nickel centre presents a tetrahedral geometry [14,19].
The Br bridges are unsymmetrical (Ni–Br2 = 2.447(2)
˚
and 2.470(2) A) and somewhat longer than the apical
˚
Ni–Br1 bond length of 2.412(2) A. Compared to the free
ligand, the planar geometry of the bis(imino)acenaphtene
skeleton is maintained upon coordination to the Ni atom,
but the electron delocalization of one of the sides of the li-
gand precursor is lost, leading to a symmetrical coordi-
nated diimine ligand. As already noted for the free
ligand, a small intramolecular Nꢁ ꢁ ꢁH–C distance is ob-
served in the complex, whose values (Nꢁ ꢁ ꢁH) range from
˚
2.370(9) to 2.433(9) A.

1318
H.-R. Liu et al. / Journal of Organometallic Chemistry 690 (2005) 1314–1323
Table 2
Polymerization of ethylene catalyzed by 4a
Entry
Co-catalyst
[Ni] (lmol lꢀ1
)
[Al]/[Ni]
Time (min)
Temp (ꢀC)
Yield (g)
Activitye · 10ꢀ7 (g PE (mol Ni [E] h)ꢀ1
)
LNi1
LNi2
MAO
DEAC
DEAC
DEAC
DEAC
DEAC
DEAC
DEAC
DEAC
DEAC
DEAC
DEAC
DEAC
DEAC
DEAC
TMA
50
50
500
250
720
720
720
720
720
360
720
1440
1860
2880
720
720
720
720
720
720
720
–
7
17
17
2.239
1.863
0.610
0.759
1.043
0.700
0.552
0.554
0.515
0.567
0.511
0.626
0.380
0.464
0.309
0.124
0.128
0.610
0.668
–
1.997
2.170
7.711
5.906
5.206
4.207
6.495
7.010
6.671
6.751
5.713
6.461
3.348
4.824
4.304
1.322
1.405
6.058
5.989
–
5.36
9.79
7.95
6.20
3.43
1.31
5
LNi3
LNi4
2.52
5.04
10.1
15.1
20.2
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
5.04
58.6
42
19.5
19.5
19.5
19.5
19.5
20.5
20.5
20.5
20.5
20.5
0
LNi5
LNi6
LNi7
LNi8
LNi9
4.89
5.32
5.66
6.13
6.17
6.07
6.97
5.90
5.74
6.35
7.03
35
LNi10
LNi11
LNi12
LNi13
LNi14
LNi16
LNi17
LNi18
LNi19
LNi20
LNi21
Ref. [5]b
Ref. [5]c
Ref. [20]d
20
60
18
18
TIBA
MAO
DEAC
–
18
18
18
25
MAO
MAO
MAO
DEAC
2100
3500
2000
500
0.257
0.437
0.207
0.160
25
25
8.3
1.6
60
60
25
a
Conditions: toluene (50 ml), PE = 15 psig.
Work performed in our laboratory, using similar conditions of the present work although higher catalyst concentrations; catalysts: b[NiBr2-
b,c
c
(MesBIAN)], [NiBr2(iPPBIAN)].
d
Results reported by another group using the catalyst [NiBr2(iPPBIAN)].
E concentration determined according to the solubility parameters listed in ASPEN [21]. [E] = 0.3801 M at 17 ꢀC; [E] = 0.3771 M at 18 ꢀC;
e
[E] = 0.3726 M at 19.5 ꢀC; [E] = 0.3711 M at 20 ꢀC; [E] = 0.3696 M at 20.5 ꢀC; [E] = 0.3559 M at 25 ꢀC; [E] = 0.2410 M at 60 ꢀC.
Table 3
Microstructure and thermal properties of selected polyethylene samples
No. branches/1000 Ca
DHf (J gꢀ1 b
)
Tonset (ꢀC)
DTb (ꢀC)
b
b,c
Tmax (ꢀC)
Entry
LNi13 (DEAC, 0 ꢀC)
LNi14 (DEAC, 20 ꢀC)
LNi16 (DEAC, 60 ꢀC)d
LNi19 (MAO, 18 ꢀC)
Traces
45
92
182.1 (111.1)
82.5 (71.1d)
1.5 (–)
120.1 (109.8)
82.1 (22.5d)
33.7 (–)
134.9 (130.1)
92.9 (80.3b)
47.1 (–)
ꢂ70 (ꢂ70)
ꢂ80e (ꢂ80)
ꢂ30 (–)
34
108.2 (78.7)
89.5 (71.0)
109.9 (99.0)
ꢂ60f (ꢂ80)
a
Determined by 1H NMR [24].
b
Determined by DSC; scans performed from 30 to 200 ꢀC, scan rate: 10 ꢀC/min; values for a second scan between brackets.
The peak value of the melting endotherm (ideally taken as the temperature at which the largest and most perfect crystals are melting) is
c
frequently assigned as the melting temperature, i.e. Tm ” Tmax. DT values give an approximate idea of the melting range [25].
d
Scan performed from 15 to 200 ꢀC, scan rate: 10 ꢀC/min.
Two peaks observed.
Shoulder observed.
e
f
higher melting temperatures and higher enthalpies of fu-
sion. Sample LNi16 was the only PE soluble in THF at
room temperature (GPC data: Mn = 7.78 · 104, Mw/
Mn = 1.79), implying the observed higher extent of
branching and/or a lower molecular weight due to a
higher polymerization temperature, where chain isomer-
ization and b-hydride elimination reactions are favored.
Complex 4 was also tested for the polymerization of
propylene. In this case, a solution of 4 stored at ꢀ4 ꢀC
for 3 days showed a similar activity to a freshly prepared
one, meaning that no decomposition occurred. From
Table 4, it is shown that the activity for propylene poly-
merization is about 102 times lower than that of the
polymerization of ethylene (compare entry LNi19 with
LNi3P). Both the activity and molecular weight increase
with the increase of temperature in the range ꢀ20–20 ꢀC.
However, at 40 ꢀC a decrease, either in the activity or in
the Mn, is observed. Thus, the highest activity and the
highest molecular weight were obtained at a polymeriza-
tion temperature of 20 ꢀC. The 13C NMR spectrum of
polypropylene (entry LNi5P) shows that atactic PP
was obtained [26–28].

H.-R. Liu et al. / Journal of Organometallic Chemistry 690 (2005) 1314–1323
1319
Table 4
Polymerization of propylene catalyzed by 4a
Entry
Cocatalyst
[Ni] (lmol lꢀ1
)
[Al]/[Ni]
T (ꢀC)
Yield (g)
Activity · 10ꢀ5 (g PP (mol Ni [P]h)ꢀ1
)
Mn/10ꢀ4
Mw/Mn
LNi5Pb
LNi2Pb
LNi3Pb
LNi6Pc
LNi7Pc
LNi8Pc
LNi9Pc
a
DEAC
DEAC
DEAC
DEAC
DEAC
DEAC
DEAC
20.2
20.2
20.2
50.0
50.0
50.0
50.0
720
720
720
360
360
360
360
ꢀ20
0
0.116
0.280
0.393
0.206
0.517
0.700
0.448
0.99
2.79
5.66
0.66
2.02
3.50
3.18
12.9
13.9
21.5
10.0
19.8
16.1
10.7
1.21
1.52
1.51
1.24
1.31
1.73
1.81
20
ꢀ20
0
20
40
Conditions: toluene (50 ml), PP = 15.7 psig, 1 h.
b
c
Catalyst solution stored in toluene, at ꢀ4 ꢀC for 3 days.
Catalyst solution stored in toluene, at ꢀ4 ꢀC for 1 day.
The new a-diimine nickel complex, activated by
MAO or DEAC, shows much higher activities for
ethylene polymerization than those observed in our lab-
oratory for the [bis(N,N0-dimesitylimino)acenaphthene]-
dibromonickel ([NiBr2(MesBIAN)]) and [bis(N,N0-2,
6-diisopropylphenylimino)acenaphthene]dibromonickel
([NiBr2(iPPBIAN)]) [5] in similar conditions except for
the catalyst concentrations (see Table 2). It also exhibits
higher activities than those shown by other similar Ni
catalysts reported either by us [9,14] or other groups
[2,18,20], although in some of these studies the experi-
mental conditions are not exactly the same. This differ-
ence, observed between the present catalyst and the
others, may be partially due to the presence of the elec-
tron donor –Si(tBu)Ph2 group in the 4-position of the
2,6-dialkylarylimine group. In fact, the activity of these
catalysts depends not only on the coordination step (p-
complex formed between the metal atom of the catalyst
and the double bond of the monomer), but also on the
activation energy of the migratory insertion step. The
higher the electrophilicity of the metal centre the stron-
ger the r-bond between the monomer and the metal
atom in the p-complex. However, the higher the back-
bonding of the metal to the p anti-bonding orbital of
the olefin the higher the insertion rate, since a weaker
carbon-carbon double bond has to be broken during
this step. Thus, the increase of the electron-donor char-
acter of the metal atom is expected to increase the inser-
tion rate while the increase of its electrophilicity is
expected to increase the strength of the r-bond of the
p-complex. Therefore, both factors must be balanced
when predicting the effect of an electron donor substitu-
ent on the catalyst activity. In the present case, some
theoretical studies (PM3 and DFT) have been per-
formed and preliminary results show that, in fact, the
stability of the p-complex is lower than that of the
[NiBr2(MesBIAN)] [29]. Studies on the activation ener-
gies of the insertion reactions are underway.
tained with this new complex 4, activated either by
MAO or DEACis slightly lower than that observed for
the polyethylene samples obtained with ([NiBr2(Mes-
BIAN)]) [9]. In fact, while the total number of branches
is ca. 3% and 5% when the catalyst systems 4/MAO and
4/DEAC are used, it turns to ca. 9% and 10% for the sys-
tems [NiBr2(MesBIAN)]/MAO and [NiBr2(MesBIAN)]/
DEAC, respectively.
3. Conclusion
A new silylated a-diimine ligand 3 and the corre-
sponding Ni(II) complex 4 have been prepared and
characterized. The ligand was modified in an attempt
to change the electronic density of the metal center
and eventually to improve the activities in either the a-
olefin polymerisation or the copolymerisation with polar
monomers. The results obtained showed that complex 4,
activated either by diethylaluminum chloride (DEAC)
or by methylaluminoxane (MAO), produces highly ac-
tive catalyst systems for ethylene polymerization and
moderately active for propylene polymerization. The
higher activities obtained in comparison with the non-
silylated a-diimine nickel complexes may be partially
attributed to the electron donor effect of the –Si(tBu)Ph2
aryl substituent. Investigations on the copolymerization
catalysis of the new a-diimine nickel complex are now in
progress.
4. Experimental
4.1. General procedures and materials
All manipulations were carried out under an atmo-
sphere of nitrogen using glove box and standard Schlenk
techniques, unless stated otherwise. Solvents were pre-
˚
It is also found that complex 4 shows higher activity
for propylene polymerization than either of the above
mentioned a-diimine nickel complexes [2,9,14]. The total
number of branches of the polyethylene samples ob-
dried over activated 4 A molecular sieves or sodium wire
(for toluene) and then distilled under an atmosphere of
nitrogen from sodium/benzophenone (diethyl ether and
tetrahydrofuran), sodium/potassium alloy (toluene) or

1320
H.-R. Liu et al. / Journal of Organometallic Chemistry 690 (2005) 1314–1323
calcium hydride (methanol, dichloromethane and n-hex-
ane), and stored under nitrogen. Deuterated solvents
(C6D6, CD2Cl2 and CDCl3) for NMR samples were
the addition was completed. The resulting white precip-
itate was collected and washed with water, then dried in
a vacuum oven at room temperature to give the desired
product in 87% yield.
˚
dried with activated 4 A molecular sieves, degassed by
freeze-pump-thaw cycles and stored in ampoules over
˚
activated 4 A molecular sieves.
4.3. Synthesis of compound 4-bromo-2,6-diisopropyl-
aniline (1)
The NMR spectra were recorded on a Varian Unity
300 (1H, 300 MHz; 13C, 75.43 MHz) spectrometer and
referenced internally using residual proton- (1H) and
carbon-(13C) solvent resonances relative to tetrameth-
ylsilane (TMS, d = 0 ppm). NMR analyses of the poly-
ethylenes were performed on samples dissolved in a
mixture of 1,2,4-trichlorobenzene/C6D6 (v/v = 1:1) at
110 ꢀC, and the internal reference was provided by hex-
amethyldisiloxane (d1H 0.058, d13C 1.9 relative to
TMS). Polypropylene NMR analyses were performed
on samples dissolved in CDCl3 at 50 ꢀC and referenced
to internal TMS. Elemental analyses were performed on
a Fisons Instrument Mod EA-1108 analyzer by the Lab-
Compound 1 was obtained by modification of a re-
ported procedure [13,31]. Br2 (8.64 ml, 0.168 mol) was
added via a dropping funnel to a stirred solution of
2,6-isopropylaniline hydrochloride (35.9 g, 0.168 mol)
in methanol (100 ml) and glacial acetic acid (50 ml).
The resulting mixture was further stirred for 2 h. The
precipitated salt ([4-Br–C6H4–NH3][Cl]) was filtered
and washed with fresh acetic acid, air dried and recrys-
tallized from hot benzene. The free base was prepared
by adding aqueous NaOH and diethyl ether to the solid
salt. The ether phase was dried over MgSO4, filtered,
and the diethyl ether removed via rotary-evaporation
to yield a crude brown liquid product. At last, purified
by trap-to-trap distillation, colorless 1 was obtained in
79% yield. 1H NMR (300 MHz; CD2Cl2): dH 1.22
´
´
oratorio de Analises in this Institute. Differential Scan-
ning Calorimetry (DSC) analyses of the polymers were
performed on a Setaram DSC121 calorimeter. In order
to minimize the differences in the thermal history of
the samples, the polymers were subjected to heating/
cooling/heating cycles under argon, between 15 or 20
ꢀC and 200 ꢀC using heating and cooling rates of 10
ꢀC/min. After the first heating scan, the samples re-
mained at 200 ꢀC for 10 min. The results obtained in
the first and second heating runs were registered and
are presented. During the experiments the sample holder
was continuously purged with argon. Molecular weight
measurements were performed by Gel Permeation Chro-
matography/Size Exclusion Chromatography (GPC/
SEC) on a Waters 150 CV chromatograph at 30 ꢀC. A
series of three columns Waters Ultrastyragel HR-1,
HR-3 and HR-4 (10 lm; 30 · 0.8 cm) were used, and
3
(12H, d, CH3, JHH = 6.9 Hz), 2.86 (2H, hept, CH,
3JHH = 6.9 Hz), 3.75 (2H, br, NH2), 7.09 (2H, s, ArH).
4.4. Synthesis of compound 2,6-diisopropyl-4-tert-
butyldiphenylsilyl-aniline (2)
To a stirred solution of 1 (14.52 g, 56.7 mmol) in
THF (180 ml) at ꢀ70 ꢀC was slowly added dropwise
of a 1.7 mol dmꢀ3 solution of tert-butyllithium in pen-
tane (100 ml, 0.17 mol). The solution was stirred over-
night and let warm up to room temperature. Then a
solution of tert-butyldiphenylsilyl chloride (14.7 ml,
56.7 mmol) in THF (80 ml) was added dropwise via can-
nula at ꢀ20 ꢀC. The mixture was stirred overnight at
room temperature and hydrolyzed with 500 ml of aque-
ous saturated NH4Cl. The organic phase was separated,
and the aqueous layer was extracted twice with CHCl3.
The combined organic phases were dried over MgSO4
and filtered, and the solvent removed. The residue was
purified in a chromatography column (L = 40 cm) eluted
by hexane/diethyl ether (v/v = 6:1) to give white needle
crystals of 2 in 25% yield. 1H NMR (300 MHz; CD2Cl2):
dH 1.14 (9H, s, CH3), 1.16 (12H, d, CH3, 3JHH = 6.9 Hz),
2.89 (2H, hept, CH, 3JHH = 6.9 Hz), 3.88 (2H, br, NH2),
7.19 (2H, s, ArH), 7.3–7.6 (10H, m, ArH).
the eluant was THF at a flow rate of 1 cm3 minꢀ1
The calibration was made using polystyrene standards
(TSK Tosoh Co.).
.
Reagents as 2,6-isopropylaniline, tert-butyllithium,
tert-butyldiphenylsilyl chloride, diethylaluminium chlo-
ride, trimethylaluminium, tri-iso-butylaluminium (Al-
drich Chemical Company), acenaphthenoquinone,
bromine, formic acid, concentrated hydrochloric acid,
anhydrous NiBr2 (Acros Organics), glacial acetic acid
(Merck) and methylaluminoxane (Akzo Chemical
Company) were purchased and used as received.
[NiBr2(DME)] (DME = dimethoxyethane) was pre-
pared according to the literature [30].
4.5. Synthesis of ligand bis[N,N0-(4-tert-butyl-
diphenylsilyl-2,6-diisopropylphenyl)imino]acenaphthene
(3)
4.2. Synthesis of 2,6-diisopropyl-aniline hydrochloride
Under air atmosphere, concentrated hydrochloric
acid (50 ml, excess) was added dropwise to 2,6-isopropy-
laniline (50 ml, 0.265 mol) dispersed in water (250 ml).
The reaction mixture was further stirred for 1 h after
Acenaphthenequinone (0.25 g, 1.37 mmol), methanol
(50 ml) and a slight excess of compound 2 (1.25 g, 3.01
mmol) were placed in a 100 ml flask and allowed to stir

H.-R. Liu et al. / Journal of Organometallic Chemistry 690 (2005) 1314–1323
1321
for 3 days in the presence of a formic acid (a few drops).
CH2Cl2 and water were added and the layers were sep-
arated. The resulting organic layer was dried with
MgSO4, filtered and the solvent was removed to give
the crude new ligand as an orange solid. The impure li-
gand was recrystallized from hot toluene to give orange
crystals of 3 in 64% yield. 1H NMR (300 MHz; CD2Cl2):
the resulting deep red solution was filtered via a cannula
to another Schlenk tube. This procedure was repeated
twice and the solvent was removed in vacuum to give
a dark brown-red powder 4 in 61% yield, that is suitable
for elemental anlaysis and polymerization use. Crystals
of complex 4 suitable for X-ray studies were obtained
by dissolving a small amount of 4 in CH2Cl2 double-lay-
ered with hexane. The mixture was allowed to sit undis-
turbed overnight. The resulting red crystals formed at
the sides of the tube and the filtrate was decanted. The
crystals were used immediately for X-ray and elemental
3
dH 0.90 (12H, s, CH3), 1.15 (12H, d, CH3, JHH = 5.4
Hz), 1.25 (18H, s, CH3), 2.94 (4H, hept, CH,
3JHH = 5.4 Hz), 6.64 (2H, s, ArH), 7.1–7.9 (28H, m,
ArH). Anal. Calc. for C68H76N2Si2: C, 83.55; H, 7.84;
N, 2.87. Found: C, 83.80; H, 8.40; N, 2.90%.
analysis.
Anal.
Calc.
for
C68H76Br2N2NiSi2 Æ
0.25CH2Cl2: C, 67.34; H, 6.33; N, 2.30. Found: C,
4.6. Synthesis of complex bis[N,N0-(4-tert-butyl-
diphenylsilyl-2,6-diisopropylphenyl)imino]acenaphthene
(4)
67.36; H, 6.71; N, 2.35%.
(b) [NiBr2(DME)] (0.27 g, 0.88 mmol), the ligand 3
(0.85 g, 0.87 mmol) and CH2Cl2 (50 ml) were combined
in a Schlenk tube and stirred at room temperature for 16
h. The resulting suspension was filtered and the solvent
of the filtrate removed under vacuum. The resulting so-
lid was washed with Et2O (20 ml · 3) to give brown-red
complex 4 in 87% yield.
Complex 4 was prepared using two procedures:
(a) Acenaphthenequinone (0.5 g, 2.75 mmol) and a
slight excess of compound 2 (2.5 g, 6.04 mmol) were
placed in a Schlenk tube followed by 100 ml glacial ace-
tic acid, forming a brown-red solution within minutes.
This mixture was heated to reflux for 4 h. Anhydrous
NiBr2 (0.61 g, 2.75 mmol) was added as a solid to this
mixture and the reaction was allowed to reflux over-
night. The solvent was removed in vacuum and the
resulting solid was washed with diethyl ether (20
ml · 3). The residue was extracted with CH2Cl2 and
4.7. Polymerization procedure
Polymerizations of ethylene and propylene were car-
ried out in a flame dried 250 ml crown capped pressure
bottle sealed with neoprene septum and pump filled with
nitrogen atmosphere. To this polymerization bottle
Table 5
Crystal data, data collection and refinement parameters for 3 and 4
Compound
3
4
Empirical formula
Formula weight
Crystal color
C
977.49
68H76N2Si2
C68H76Br2N2NiSi2
1196.02
Red
Orange
0.5 · 0.1 · 0.08
Monoclinic
P21
Crystal size (mm)
Crystal system
Space group
0.50 · 0.45 · 0.15
Monoclinic
P21/n
˚
a (A)
8.794(4)
30.428(8)
11.545(4)
100.98(2)
3033(2)
2
14.669(4)
31.549(8)
14.939(4)
115.45(2)
6243(3)
˚
b (A)
˚
c (A)
b (ꢀ)
3
˚
V (A )
Z
4
1.273
Dcalc (g cmꢀ3
l (mmꢀ1
F(000)
)
1.070
0.821
)
1.670
2496
1052
4.16–66.98
h range (ꢀ)
Limiting indices
1.63–24.95
ꢀ17 6 h 6 0; ꢀ37 6 k 6 0; ꢀ15 6 l 6 17
0 6 h 6 10; ꢀ31 6 k 6 0; ꢀ13 6 l 6 13
Completeness to h (%)
Reflections collected
Unique data [I > 2r(I)]
Goodness of fit
94.4
5566
98.2
11353
5214 (Rint = 0.0128)
1.049
645
10904 (Rint = 0.1390)
0.815
Refined parameters
R1 [I > 2r(I)]
wR2 [I > 2r(I)]
R1,wR2 all data
676
0.0935
0.0626
0.1624
0.0798
0.3407/0.1134
–
0.0761/0.1712
ꢀ0.04(6)
0.593 and ꢀ0.325
Abs. struct. parameter
ꢀ3
˚
Larg. peak/hole (e A
)
0.433 and ꢀ0.619

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H.-R. Liu et al. / Journal of Organometallic Chemistry 690 (2005) 1314–1323
50 ml of dry toluene were added and the resulting sol-
vent was then saturated at a monomer relative pressure
of 1 bar, which was maintained throughout the polymer-
ization reactions. Then, the co-catalyst was added in the
proper Al/Ni ratio via a glass syringe. At this time, the
solutions were brought to the desired temperatures
and allowed to equilibrate for 15 min. After this, the
corresponding amount of a toluene solution of the Ni
catalyst was added to the polymerization reactors. The
polymerizations were terminated after several minutes
for ethylene and 1 h for propylene by quenching the
mixture with 150 ml of a 2% HCl/methanol solution.
The obtained polymers were then filtered, washed sev-
eral times with 50 ml portions of methanol and dried
in a vacuum oven at 50 ꢀC for 3 days.
32771/99 and POCTI/QUI/42015/2001 and Grants
SFRH/BPD/3522/2000 and PRAXIS XXI/BD/19638/
99, respectively, to H-R.L. and to S.I.C.), FSE and
FEDER. We are grateful to DST, India, and GRICES,
Portugal, for grants under Indo-Portuguese Collabora-
tion for this investigation. We appreciate the support
by Borealis in the polymerization equipment.
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˚
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˚
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information may be obtained free of charge from: The
Director, CCDC, 12 Union Road, Cambridge CB2
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higher or lower catalyst concentrations in the feed, respectively.
Acknowledgements
This work was funded by Fundac¸a˜o Luso-Americana
para o Desenvolvimento (Projet 122/2000), Fundac¸a˜o
ˆ
para a Ciencia e Tecnologia (Projects POCTI/QUI/

H.-R. Liu et al. / Journal of Organometallic Chemistry 690 (2005) 1314–1323
1323
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