Trinuclear Fe(II)/Ni(II) complexes as catalysts for ethylene polymerizations

Article abstract of DOI:10.1016/j.cattod.2010.10.084

A new trimetallic heterotrinuclear catalyst bearing two Fe and one Ni atoms of the general formula {(2,6-C₁₂H₁₇)NC-(C ₇H₉N)[FeCl₂]C-NC-(C₂₃H ₃₄)-C}₂N-C-(C₁₀

Full text of DOI:10.1016/j.cattod.2010.10.084

Catalysis Today 164 (2011) 80–87
Contents lists available at ScienceDirect
Catalysis Today
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Trinuclear Fe(II)/Ni(II) complexes as catalysts for ethylene polymerizations
Bijal Kottukkal Bahuleyan, Kyoung Ju Lee, So Hyun Lee, Yinshan Liu, Weihua Zhou, Il Kim∗
The WCU Center for Synthetic Polymer Bioconjugate Hybrid Materials, Department of Polymer Science and Engineering, Pusan National University, Jangjeon-dong,
Geumjeong-gu, Busan 609-735, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 June 2010
Received in revised form 22 October 2010
Accepted 23 October 2010
Available online 27 November 2010
A new trimetallic heterotrinuclear catalyst bearing two Fe and one Ni atoms of the general formula
{(2,6-C12H17)N C–(C7H9N)[FeCl2]C–N C–(C23H34)–C}2 N–C–(C10H6)–C N[NiBr2] has been synthe-
sized successfully. For comparison, monometallic Ni(II) and bimetallic Fe(II) complexes are also
investigated. When activated with methylaluminoxane or common alkyl aluminums such as ethyl alu-
minum sesquichloride and triethylaluminum, all catalysts polymerize ethylene with activities exceeding
105 g-PE/mol-M h bar at 30 ◦C. The heteronuclear trimetallic complexes showed higher catalytic activity
and longer life time, especially at high temperature, than monometallic Ni(II) and homonuclear bimetallic
Fe analogues, due to the cooperative effect of the neighboring metals. The molecular weight, polydisper-
sity and the degree of branching of polyethylene were drastically influenced by the type of the metal
complex and experimental parameters.
Keywords:
Trimetallic catalysts
Ethylene polymerization
Late transition metals
Ligands
© 2010 Elsevier B.V. All rights reserved.
Transition metal chemistry
1. Introduction
pounds including functionalized polyolefins. It is assumed that the
active species of these homogeneous systems are a single metal
Multimetallic complexes are reasonable models, like enzymes,
to achieve superior reactivity and selectivity, in part, by their
efficacy in creating highly local reagent concentrations and con-
formationally advantageous active site–substrate proximities and
interactions [1,2]. On the basis of these aspects, multinuclear
metallocenes have been reported to provide novel and versatile
polymerization catalysts by cooperative effects between multi-
metallic centers [2–4]. Within the context of the rapidly advancing
and technologically significant field of homogeneous single-site
olefin polymerization catalysis [5–7], the conjecture that coopera-
tive effects involving two or more metal centers in close proximity
might achieve more efficient chain propagation to yield novel poly-
mer architectures.
Homogeneous catalyst systems based on late transition metal
catalysts are currently regarded as being one of the interest-
ing research fields in olefin polymerization catalysts due to their
unique reactivity patterns and unusual catalytic properties [8–13].
Recently, a range of sterically encumbered multidentate binucle-
ating ligands such as organo-bridged iminopyridines, ␣-diimines,
and iminophenolates have been found to be efficient systems
achieving active dinickel, dipalladium and diiron catalysts for olefin
polymerizations [11,14–18]. They are also expected to behave as
promising catalysts in producing a wide range of olefinic com-
center with a defined structure. However, the behavior of these
complexes in polymerization was similar to mononuclear com-
plexes and specific properties of late transition metal multinuclear
complexes are yet unknown. The catalytic activities, sterioregu-
larities, molecular weights and molecular weight distributions of
the obtained polymer might be controlled by modifying the ligand
structure of the monometallic complexes, and these might also be
tuned by arranging multimetal centers at spatially confined posi-
tions. Although several syntheses of heterometalic complexes are
known, only a few of them have been employed as polymerization
catalysts [14,19–21].
Here we would like to report on the synthesis and character-
ization of a new class of multinuclear Ni(II)/Fe(II) complexes as
catalysts for ethylene polymerizations. The Ni(II) and Fe(II) sys-
tems are so particular in that (1) the Ni(II)/Pd(II) diimine based
catalysts are highly active and can convert ethylene directly to
highly branched polyethylene without the addition of expensive
␣-olefins, such as 1-hexene and 1-octene [11,22] and (2) the
Fe(II)/Co(II) bis(imino)pyridine based catalysts are more active
than its Ni(II) and Fe(II) monometallic counterparts. This new
Fe(II)–Ni(II)–Fe(II) trinuclear trimetallic catalyst, when activated by
various cocatalysts like methylaluminoxane (MAO) and common
alkyl aluminums such as ethyl aluminum sesquichloride (EASC) and
triethylaluminum (TEA), resulted in polyethylene (PE) with unique
microstructure and broad molecular weight distribution, due to the
formation of multiple active sites from each Fe(II) and Ni(II) metal
centers.
∗
Corresponding author. Tel.: +82 51 510 2466; fax: +82 51 513 7720.
E-mail address: ilkim@pusan.ac.kr (I. Kim).
0920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.10.084

B.K. Bahuleyan et al. / Catalysis Today 164 (2011) 80–87
81
2. Experimental
C(NAr)CH3], 2.74 [sept, 2H, CH(CH3)(CH3)], 2.81 [s, 3H, C(O)CH3],
7.08–7.22 (m, 3H, CH Ar), 7.96 (t, 1H, CH Ar), 8.16 (d, 1H, CH
Ar), 8.58 (d, 1H, CH Ar) ppm. 13C NMR (75 MHz, CDCl3): ı 17.7,
23.5, 23.9, 26.4, 29.0, 123.2, 123.7, 124.4, 125.2, 136.4, 138.4,
142.9, 153.1, 156.2, 167.8, 200.8 ppm. Calcd. for C21H26N2O
(322.45): C 78.22, H 8.13, N 8.69, O 4.96; Found: C 78.39, H 8.17,
N 8.84, O 4.6.
2.1. Materials
All reactions were performed under
a purified nitrogen
atmosphere using standard glove box and Schlenk techniques.
Polymerization grade of ethylene (SK Co., Korea) was purified by
passing it through columns of Fisher RIDOXTM catalyst and molec-
˚
ular sieve 5 A/13X. All organic solvents purchased from Aldrich
2.3.2. 4-((4E)-4-(1-(6-((E)-1-(2,
Chemical Co. were purified by known procedures and stored
over molecular sieves (4 A). All reagents used in this study were
purchased from Aldrich Chemical Co. and used without further
purification. MAO (8.4 wt% total Al solution in toluene) was donated
by LG Chemicals, Korea and was used without further purification.
6-diisopropylphenylimino)ethyl)pyridine-2-yl)ethyldene
amino)-3,5-diisopropylbenzyl)-2,6-diisopropylbenzenamine (B)
˚
Compound B
(1.6 g, 5 mmol) and 4,4ꢀ-methylenebis(2,6-
diisopropylaniline) (5.5 g, 15 mmol) were dissolved in 15 mL of
xylene and stirred at 100 ◦C until it become a clear solution.
Acetic acid (100 ␮L) was added, and stirred for 5 days. The solu-
tion was dried under reduced pressure. The solid precipitate was
washed with cold methanol several times and the product was
purified via column chromatography [5% ethyl acetate (EA)/n-
hexane-alumina]. The solvent was evaporated to afford the product
as a dark yellow powder in 60% yield. 1H NMR (300 MHz, CDCl3):
ı 1.27 [d, 6H, CH(CH3)(CH3)], 1.28 [d, 6H, CH(CH3)(CH3)], 2.30
[d, 6H, CH(CH3)(CH3)], 1.32 [d, 6H, CH(CH3)(CH3)], 1.38 [d, 6H,
CH(CH3)(CH3)], 1.40 [d, 6H, CH(CH3)(CH3)], 2.2 [s, 6H, C(NAr)CH3],
2.68 [sept, 2H, CH(CH3)(CH3)], 2.72 [sept, 2H, CH(CH3)(CH3)], 2.86
[sept, 2H, CH(CH3)(CH3)], 3.56 (s, 2H, NH2), 3.88 (s, 2H, CH2),
6.82 (s, 2H, CHAr), 6.94 (s, 2H, CHAr), 7.09 (m, 3H, CHAr), 7.84 (t,
1H, CHAr), 8.42 (dd, 1H, CHAr) ppm. 13C NMR (75 MHz, CDCl3):
ı 16.13, 21.51, 21.99, 22.20, 26.83, 26.95, 27.17, 27.28, 122.21,
122.58, 122.80, 130.32, 131.47, 134.54, 134.71, 135.29, 136.92,
143.20, 145.44, 154.02, 154.20, 165.88, 166.09. Anal. Calcd. for
C46H62N4 (671.01): C 82.34, H 9.31, N 8.35; Found: C 82.34, H 9.31,
N 8.35.
2.2. Characterization
1H NMR and 13C NMR spectra of ligands were recorded on a
Varian Gemimi-2000 (300 MHz for 1H NMR, 75 MHz for 13C NMR)
spectrometer. All chemical shifts were reported in parts per mil-
lion (ppm) relative to residual CHCl3 (ı 7.24) for 1H and relative to
CDCl3 (ı 77.00) for 13C. 1H and 13C NMR spectra of PE were taken in
C6H4Cl2 at 135 ◦C on an Inova-500 (500 MHz, 125 MHz) spectrome-
ter. The elemental analysis of complexes was carried out using Vario
EL analyzer. Metal concentrations (Fe and Ni) of the metal com-
plexes were determined using inductively coupled plasma-atomic
emission spectrometry (ICP-AES, Perkin-Elmer Optima 3300DV).
Analytical thin layer chromatography (TLC) was conducted using
Merck 0.25 mm silica gel 60F pre-coated aluminum plates with flu-
orescent indicator UV254. Ligands were purified by a Combi-Flash
(Companion) auto-column machine. Elemental analysis was car-
ried out using Vario EL analyzer and UV–vis spectra were recorded
on a Shimadzu UV-1650PC UV–vis spectrophotometer at 30 ◦C.
FT-IR analyses were carried out in Shimadzu IR Prestige-21 spec-
trophotometer.
2.4. Synthesis of trinuclear ligand C
The molecular weight (MW) and polydispersity index (PDI)
of PE were determined by GPC (PL-GPC220/FTIR) in 1,2,4-
trichlorobenzene/THF using polystyrene columns as standard.
Thermal analysis of PE was carried out by differential scanning
calorimetry (Perkin-Elmer DSC, model: Pyris 1) at 10 ◦C/min heat-
ing rate under nitrogen atmosphere. The results of the second
scan were reported to eliminate any difference in sample history.
The branching numbers for PE were determined by 1H NMR spec-
troscopy using the ratio of the number of methyl groups to the
overall number of carbons and were reported as branches per thou-
sand carbons.
Compound B (1.5 g, 2.2 mmol) and acenaphthenequinone
(0.18 g, 1 mmol) were dissolved in 10 mL of xylene and stirred at
100 ◦C until it becomes a clear solution. Acetic acid (100 ␮L) was
added, and sealed solution was stirred for 5 days. It was purified
by auto-column using hexane and ethyl acetate as solvents. The
product was then isolated and dried under vacuum to give 0.5 g
(33%) trinuclear ligand as an orange powder. IR (KBr): ꢀ 1640 cm−1
(C N). 1H NMR (300 MHz, CDCl3): ı 0.873 [d, 12H, CH(CH3)(CH3)],
0.90 [d, 12H, CH(CH3)(CH3)], 1.09 [d, 12H, CH(CH3)(CH3)], 1.11 [d,
12H, CH(CH3)(CH3)], 1.14 [d, 12H, CH(CH3)(CH3)], 1.16 [d, 12H,
CH(CH3)(CH3)], 2.21 [s, 6H, C(NAr)CH3], 2.25 [s, 6H, C(NAr)CH3],
2.71 [sept, 4H, CH(CH3)(CH3)], 2.73 [sept, 4H, CH(CH3)(CH3)], 2.96
[sept, 4H, CH(CH3)(CH3)], 4.07 (s, 2H, CH2), 6.73 (m, 4H, CHAr), 7.00
(m, 4H, CHAr), 7.08 (m, 6H, CHAr), 7.87 (m, 2H, CHAr), 8.44 (m, 4H,
CHAr) ppm. 13C NMR (75 MHz, CDCl3): ı 16.16, 21.92, 22.10, 22.18,
22.31, 22.47, 27.24, 27.54, 40.42, 121.16, 121.96, 122.39, 123.49,
127.77, 128.58, 130.06, 134.19, 134.60, 134.74, 135.60, 135.82,
143.21, 144.35, 145.43, 154.03, 154.16, 160.06, 165.92, 166.19,
178.47. Anal. Calcd. for C104H126N8 (1487.01): C 83.94, H 8.53, N
7.53; Found: C 83.94, H 8.53, N 7.53.
2.3. Synthesis of ligand precursors
The syntheses of trinuclear ligand (C) and metal monometal-
lic Ni(II) complex D, bimetallic Fe(II) complex E and trimetallic
Ni(II)/Fe(II) complex F are summarized in Scheme 1.
2.3.1. 1-(6-((E)-1-(2,6-diisopropylphenylimino)ethyl)pyridin-2-
yl)ethanone (A)
Compound A was synthesized by reacting 2,6-diacetylpyridine
(1.630 g, 10.0 mmol) with 2,6-diisopropylaniline (1.70 mL,
9.0 mmol) in methanol (15 mL). A catalytic amount of formic
acid was added. The yellow precipitate obtained was filtered
and washed with cold methanol. The solid was suspended in
refluxing ethanol and the resulting mixture was filtered in hot
condition. The solvent was removed from the filtrate under
reduced pressure to give pure compound A as pale yellow crys-
2.5. Synthesis of complexes
2.5.1. Monometallic Ni(II) complex D
Trinuclear ligand C (0.15 g, 0.1 mmol) and (DME)NiBr2 (0.03 g,
0.1 mmol) (1:1) were stirred overnight in CH2Cl2 (20 mL) at room
temperature. The resulting mixture was filtered and the solvent
was removed under reduced pressure. The complex precipitated in
ether was filtered, washed and dried under vacuum at 60 ◦C. The
brown powder was obtained in more than 90% yield. The NMR spec-
troscopic analysis was not informative due to the paramagnetic
tals in 67% yield (2.17 g, 6.73 mmol). m.p. 182–184 ◦C. IR: ꢀ(C
N)
1648 cm−1, ꢀ(C
1698 cm−1
.
1H NMR (300 MHz, CDCl3): ı 1.16
O)
[d, 6H, CH(CH3)(CH3)], 1.17 [d, 6H, CH(CH3)(CH3)], 2.28 [s, 3H,

82
B.K. Bahuleyan et al. / Catalysis Today 164 (2011) 80–87
Scheme 1. Synthesis of trinuclear ligand and mono-, bi- and tri-metallic complexes: (i) MeOH/formic acid; (ii) 4,4ꢀ-methylenebis(2,6-diisopropylaniline), xylene/acetic acid;
(iii) acenaphthenequinone, xylene/acetic acid; (iv) (DME)NiBr2/CH2Cl2; (v) FeCl2/THF; (vi) (DME)NiBr2/CH2Cl2.
nature of the metal complexes. Anal. Calcd. for C104H126Br2N8Ni:
C, 73.19; H, 7.44; N, 6.57; Ni, 3.44. Found: C, 73.81; H, 7.54; N, 6.45.
Ni, 3.47 wt% (ICP-AES). IR (KBr): ꢀ 1641, 1615 cm−1 (C N).
7.29; N, 6.43; Fe, 6.41. Found: C, 71.81; H, 7.45; N, 6.41. Fe, 6.52 wt%
(ICP-AES). IR (KBr): ꢀ 1641, 1617 cm−1 (C N).
2.5.3. Trinuclear trimetallic 2Fe(II)/Ni(II) complex F
2.5.2. Bimetallic Fe(II) complex E
The complex E (0.2 g, 0.11 mmol) and (DME)NiBr2 (0.04 g,
0.13 mmol) were added together in a Schlenk flask containing
20 mL of CH2Cl2 at 30 ◦C and stirred for 24 h to ensure complete
metallation. The complex precipitated in diethyether was filtered
and washed with ether and dried in vacuum to get dark green
powder (yield > 90%). The NMR spectroscopic analysis was not
informative due to the paramagnetic nature of the metal com-
plexes. Anal. Calcd. for C104H126Br2Cl4Fe2N8Ni: C, 63.72; H, 6.48;
Trinuclear ligand C (0.3 g, 0.2 mmol) and ferrous chloride
tetrahydrate (0.08 g, 0.4 mmol) were stirred overnight with THF
(20 mL) at room temperature. The precipitated complex in ether
was washed, filtered and dried under vacuum at 60 ◦C. The dark
green powder was yielded above 90%. The NMR spectroscopic anal-
ysis was not informative due to the paramagnetic nature of the
metal complexes. Anal. Calcd. for C104H126Cl4Fe2N8: C, 71.72; H,

B.K. Bahuleyan et al. / Catalysis Today 164 (2011) 80–87
83
N, 5.72; Fe, 5.70; Ni, 2.99. Found: C, 64.34; H, 6.45; N, 5.41. Ni, 2.86;
the trinuclear ligand C bearing both N2 ␣-diimine and N3 pyri-
dine diimine might give a mixture of complexes as illustrated in
Fig. 1. Fortunately, as reported by Bianchini and coworkers [28]
and Wang and coworkers [29], when the FeCl2 was coordinated
to a ligand C, the pyridine diimine was predominantly favored to
be coordinated. In order to investigate the selectivity of the coor-
dination of Fe and Ni, preliminary tests have been performed by
using homonuclear N2 ␣-diimine [(o,oꢀ-iPr2Ar-DAB) (Ar-DAB = 1,4-
bis(aryl)-2,3-dimethyl-1,4-diaza-1,3-butadiene)] and N3 pyridine
diimine [(2-iPrC6H4N C(Me))2C5H3N] ligands. The reaction of
0.9 equiv. of FeCl2 with the mixture of 1 equiv. of N2 ␣-diimine and
1 equiv. of N3 pyridine diimine ligand in THF gave only N3 pyridine
diimine Co(II) complex. In a similar reaction by using 0.9 equiv.
of (DME)NiBr2 in CH2Cl2, only N2 ␣-diimine Ni(II) complex was
obtained as a pure product. These results indicate that in the pres-
ence of both pyridine diimine and ␣-diimine ligands, iron has a
high selectivity to the tridentate ligand and nickel to the bidentate
ligand to give a desired product indicated in Fig. 1. Although these
simple tests cannot be direct evidences for the formation of desired
complex F (Scheme 1), other results such as element analysis, ICP-
AES, IR spectroscopy, and ethylene polymerizations further support
the selectivity of Ni to N2 ligand and Fe to N3 ligand at least in the
preparation conditions employed in this study.
Thus, the complexes were synthesized by the addition of trin-
uclear ligand to appropriate metal salt [FeCl2 or (DME)NiBr2]
according to Scheme 1. Two equivalents of FeCl2 were strictly used
for the preferential coordination to tridentate ligand to form the
complex E. To the homobimetallic Fe complex, one equivalent of
(DME)NiBr2 was added to obtain the final trinuclear complex F. It
may be noted that both metallations were carried out separately.
For comparison, mononuclear Ni(II) complex D was also prepared
by treating the trinuclear ligand with one equivalent of (DME)NiBr2.
Various efforts to achieve metal complexes that are suitable for
crystallography were unsuccessful possibly due to the steric bulk
of the complexes; however, the formation of the desired com-
plexes was traced by element analysis and ICP-AES. In addition,
the extremely low solubility of the metallic complexes with the
NNN–NN–NNN ligands in either hydrocarbon or chlorinated sol-
vents precluded their NMR characterization. Heating the solution
to increase the solubility of the complexes resulted in forming some
decomposed products that cannot be suitably assigned. The IR spec-
tra of the metal complexes display medium absorption bands in the
range ꢀ ∼ 1615 cm−1, which is the absorption region for ꢀ(C N).
Bands assigned to the free ligand C N stretching vibrations were
observed in the range ꢀ ∼ 1640 cm−1. In general, the bands in the
complexes are shifted to lower wavenumbers which is a criterion
of the coordination of the ligands to the Ni(II) and/or Fe(II) ion.
The IR spectra of monometallic Ni(II) complex D and bimetallic
Fe(II) complex E showed two ꢀ(C N) bands. For example, compared
with one single ꢀ(C N) band at 1640 cm−1 assigned to free imine
group of ligand C, the medium absorption bands at 1615 cm−1 (for
D) and 1617 cm−1 (for E) were assigned for the Ni(II) coordinated
␣-diimine groups and the Fe(II) coordinated pyridine imine groups,
respectively, and more intense ones at 1641 cm−1 for the free pyri-
dine imine and ␣-diimine groups. This indicates that not all of the
nitrogen atoms of the potentially NNN–NN–NNN ligand involved
in the coordination to the nickel or iron center. The element anal-
ysis data and nickel and iron contents measured by ICP confirm
that only one equivalent nickel or iron coordinates to each ligand
to form complex D and E.
Fe, 5.77 wt% (ICP-AES). IR (KBr): ꢀ 1615 cm−1 (C N).
2.6. Polymerization procedure
Ethylene polymerizations were performed in a 250 mL round-
bottom flask equipped with a magnetic stirrer and a thermometer.
The catalyst was added to the flask and the reactor was charged
with toluene (80 mL) by using syringe. The reactor was immersed
in a constant temperature bath previously set to desired temper-
ature. When the reactor temperature had been equilibrated to the
bath temperature, ethylene was introduced into the reactor after
removing nitrogen gas under vacuum. When no more absorption
of ethylene into toluene was observed, the cocatalyst was injected
into the reactor and then the polymerization was started. Poly-
merization rate was determined at every 0.01 s from the rate of
consumption, measured by a hotwire flowmeter (model 5850 D
from Brooks Instrument Div.) connected to a personal computer
through an A/D converter. Polymerization was quenched by the
addition of methanol containing HCl (5 vol.%) and then the unre-
acted monomer was vented. The polymer was washed with an
excess amount of methanol and dried in vacuum at 50 ◦C. To make
a worthy comparison of the effect of metal selection on catalytic
activity and polymer structure and properties, all data were col-
lected under similar conditions.
3. Results and discussion
3.1. Synthesis and characterization of the ligands and complexes
Trinuclear NNN–NN–NNN ligand was synthesized by a three
step condensation reaction according to Scheme 1. Acenaph-
thenequinone and 2,6-diacetyl pyridine were utilized to provide
bidentate and tridentate coordination vacancies, respectively. 4,4ꢀ-
Methylenebis(2,6-diisopropylaniline) and 2,6-diisopropylaniline
provide symmetrical environment for Ni center and unsymmetrical
one for Fe centers. The methylene group in 4,4ꢀ-methylenebis(2,6-
diisopropylaniline) serves as a bridge between the tridentate
and bidentate nuclei. Compound A (Scheme 1) was synthe-
sized according to the reported procedure [23]. After column
purification, compound A was treated with 4,4ꢀ-methylenebis(2,6-
diisopropylaniline) at 110 ◦C using a little glacial acetic acid as
promoter in a minimal solvent to give the expected unsymmetrical
compound B. Trinuclear ligand C was prepared by the conven-
tional Schiff-base condensation of 2.2 equiv. of compound B with
acenaphthenequinone. Trinuclear ligand C was purified by col-
umn chromatography [2% ethyl acetate/n-hexane-alumina]. This
trinuclear ligand was characterized by 1H NMR, 13C NMR and IR
spectroscopies and elemental analysis.
Taking a glance at the structure of the trinuclear ligand C for the
preparation of metal complexes with different metals, one of the
most ambiguous problems related with the structure of the com-
plexes can be realized. In fact late transition metal catalysts, such
as bidentate (N2) ␣-diimine nickel and palladium catalyst devel-
oped by Brookhart [8] and tridentate (N3) pyridine diimine iron
and cobalt catalysts by Brookhart and Gibson [24,25], have received
a lot of attention because they can produce novel polyolefins with
unique properties. When activated with MAO for the ethylene poly-
merizations, ␣-diimine nickel and palladium catalyst yield highly
branched polyethylene (PE) and pyridine diimine iron and cobalt
complexes produce highly linear PE. However, tridentate pyridine
diimine nickel complexes have been successfully prepared and uti-
lized them for the vinyl polymerization of norbornene [26], and
bidentate ␣-diimine cobalt complexes have also been prepared for
ethylene polymerization to give a moderate activity [27]. As a result
3.2. UV–vis spectroscopic studies
The UV–vis spectroscopic investigation of modified ␣-diimine
Ni(II) catalyst/MAO systems has been recently reported [13,30].
This technique was found to be very effective for the observa-

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B.K. Bahuleyan et al. / Catalysis Today 164 (2011) 80–87
Fig. 1. Possible coordinations of nickel and iron to the ligand comprised of bidentate (N2) ␣-diimine and tridentate (N3) pyridine diimine and desired trinuclear Fe(II)/Ni(II)
complex.
tion of the successive elementary steps leading to the formation of
active species in metal (Fe or Ni) complex/MAO system [35]. Fig. 2
shows the UV–vis spectra of Ni(II), bimetallic Fe(II) and trimetallic
Fe(II)/Ni(II) complexes activated by MAO at [Al]/[M] = 300 (M = Fe
and/or Ni) at 30 ◦C. Since all the complexes show limited solu-
bility in toluene, the UV investigation was carried out in CH2Cl2
at nitrogen atmosphere. The addition of MAO to the CH2Cl2 solu-
tion of the Ni(II) complex D results in the formation of three new
absorption peaks (Fig. 2(a)). It was reported that the active species
could exhibit absorptions at 523 and 565 nm, whereas the species
corresponding to the band at 710 nm are likely inactive for poly-
merization [13]. The appearance of a UV absorption band centered
at 624 nm for Fe(II) complex E corresponds to the active site formed
by Fe(II) in the presence of MAO. Further the UV spectrum of the
Fe–Ni trimetallic catalyst F shows both the absorption bands cor-
responding to active species of Ni and Fe, indicating the existence
of Fe and Ni in the complex. It may be noted from Fig. 2(c) that
the absorption peak for usual deactivation species of Ni complex
around 700 nm is not appeared for complex F [13]. This is akin to
the polymerization results (vide infra). The activity of the trimetallic
catalyst is much higher than Ni and bimetallic Fe catalysts.
3.3. Polymerization of ethylene
Fig. 2. UV–vis absorption spectra of reaction mixtures of precursors (a) Ni based
catalyst (D), (b) Fe based bimetallic catalyst (E), and (c) Fe and Ni based trimetallic
Homogeneous catalysis is one of the most rapidly developing
areas of synthetic organic chemistry. There are strong incentives
catalyst (F) with MAO ([Al]/[M] = 300; M = Fe and/or Ni) in CH2Cl2 at 30 ◦C.

B.K. Bahuleyan et al. / Catalysis Today 164 (2011) 80–87
85
Table 1
Selected results of ethylene polymerizationa.
d
e
Entry
Catalyst
Cocatalyst ([Al/M])b
Tp (◦C)
Activityc (10−4
)
Yield (g)
Mn (10−4
)
PDId
3.5
2.7
4.9
Tm (◦C)
Branches/1000 Cf
1
2
3
4
5
6
7
8
D
D
E
E
F
F
F
F
F
F
F
F
F
MAO(300)
EASC(300)
MAO(300)
EASC(300)
MAO(300)
MAO(300)
MAO(300)
MAO(300)
EASC(300)
TEA(300)
30
30
30
30
30
10
50
70
30
30
30
30
30
1.2
17.5
24.1
0.4
28.3
14.6
16.2
11.5
8.6
9.9
4.6
13.9
19.5
0.6
2.8
3.8
0.2
4.2
2.7
2.9
1.8
1.5
1.3
1.0
2.1
3.1
36.3
56.2
205.7
249.2
1.6
5.3
1.5
1.1
65.0
0.7
n.d.g
n.d.
103
92
3
133.3
134.2
130.1
133.5
131.5
128.9
n.d
130.8
132.7
134.3
132.2
5.7
3
73.2
46.4
54.9
19.6
3.0
144.0
6.0
6.3
17
10
25
59
134
33
32
30
25
9
10
11
12
13
MAO(50)
MAO(100)
MAO(500)
38.3
45.5
1.6
70.2
a
Polymerization conditions: catalyst = 2 (mol, toluene = 80 mL, time = 30 min and PC
= 1.3 bar.
H
2
4
b
c
d
e
f
Al EASC (ethylaluminum sesquichloride), MAO (methylaluminoxane) or TEA (triethylaluminum); M = Ni for complex D, Fe for E and (Fe + Ni) for F.
Average rate of polymerization over the period of polymerization.
Polydispersity index determined by gel permeation chromatography.
Determined by differential scanning calorimetry (heating rate: 10 ◦C/min).
Total degree of branching determined by 1H NMR spectroscopy.
g
Not detected.
to achieve environmentally friendly, high-economy processes and
much research is performed to develop more selective catalysts.
The most selective catalysts have been found in nature. The elu-
cidation of the structure of the active sites of many enzymes and
the nature of the key intermediates in enzyme catalyzed processes
has both contributed to dramatic progress in recent years. It has
been shown that a number of these active sites contain two metal
ions, that operate cooperatively [31]. As a result, dinuclear com-
plexes containing two metals in close proximity have become the
subject of extensive investigations in order to design new trimetal-
lic catalysts [32]. In order to investigate the scope of homo- and
hetero-nuclear multimetallic enchainment cooperative effects in
ethylene polymerization, the rate of polymerization of purified
ethylene was followed using Fe(II) and Ni(II) complexes, in a dry
toluene between 10 and 70 ◦C in combination with various cocat-
alysts like MAO, EASC and TEA. A partial pressure of 1.3 bar was
maintained by supply of ethylene, the consumption being mea-
sured every 0.1 s. In this way the characteristic rate curves can be
followed consisting of rate growth and rate decay. Fig. 3 shows rate
curves obtained for Fe and Co catalyst systems using 0.6 mmol MAO,
2 ␮mol catalyst. The results of polymerization are also summarized
in Table 1.
There is a gradual increase in the rate of polymerization (Rp) in
the build-up period where the Rp increases rapidly to a maximum
until the almost stationary state is reached directly. Comparing
the activity as the average rate of polymerization (Rp,avg) over
a polymerization period, homonuclear bimetallic Fe(II) catalyst
E and heteronuclear trimetallic Fe(II)/Ni(II) catalyst F show sig-
nificantly greater activities, Rp,avg = 24.1 × 104 g-PE/mol-Fe h bar
(Entry 3 in Table 1) and 28.3 × 104 g-PE/mol-(Fe + Ni) h bar (Entry
5), respectively, than monometallic Ni(II) catalyst D (1.2 × 104 g-
PE/mol-Ni h bar, Entry 1). These results show the cooperative
nuclearity effect, probably minimizing monometallic “back coordi-
nation” of the last inserted monomer to the parent cationic metal
centers [3]. Taking a careful look at the Rp’s obtained by using D–F,
the activity of F is similar to the sum of the activity of D and E. The
formation of active and inactive metal sites was traced by UV–vis
spectral analysis (vide supra). Catalyst D and E yield high MW
polymers, the number average molecular weight (Mn) of 36,300
and 205,700, respectively, with the polydispersity index (PDI) of
3.5 and 4.9, respectively. On the other hand trinuclear trimetallic
Fe(II)/Ni(II) catalyst F yields low MW polymer (Mn = 1600) with a
very broad polydispersity (PDI = 73.2). This is another evidence of
the cooperative effect of the trimetallic complexes in an adverse
direction. Even though it is too premature to conclude the effect
on the MW, the sharply decreased MW value indicates that the
cooperative effects provided by the two different metals crowded
Fig. 3. Rate of polymerization (Rp) versus time plots of ethylene polymerizations
catalyzed by (a) complex D, (b) complex E, and (c) complex F, in combination
with MAO ([MAO]/[M] = 300; M = Fe and/or Ni). Polymerzation conditions: catalyst
Fig. 4. The complicated cooperations among three metals in a hetero-trimetallic
catalyst, in which the three metals perform their own tasks influenced by the bulky
ligands, incoming monomers and propagating polymer chains.
amount = 2 ␮mol, toluene solvent = 80 mL, PC
= 1.3 bar and temperature = 30 ◦C.
H
2
4

86
B.K. Bahuleyan et al. / Catalysis Today 164 (2011) 80–87
with bulky substituents on the aryl rings and by neighboring prop-
agating chains (Fig. 4) play a significant role in controlling the rate
of propagation and chain termination, thus influencing both activ-
ity and MW of the polymer. In addition each metal has its own
characteristic active sites with different ␤-hydrogen transfer and
chain transfer to Al, resulting in the formation of polymer with low
MW and broad polydispersity. The low MW comes from acceler-
ated chain terminations and the broad polydispersity from the fact
that each metal has its own kinetics.
1 with 2 in Table 1) [13,15], while Fe(II) bis(imino)pyridine
complex is more active with MAO than with EASC (compare
Entry 3 with 4). The catalytic activity of F/EASC system shows
much less activity (8.6 × 104 g-PE/mol-(Fe + Ni) h bar, Entry 9)
than F/MAO (28.3 × 104 g-PE/mol-(Fe + Ni) h bar, Entry 5). Together
with these results, considering the former system yields highly
branched PE and the latter yields PE with moderate branches (17
branches/1000 C), the EASC activates predominantly Ni centers
and the MAO activates both Ni and Fe centers. In addition, the
activity (9.9 × 104 g-PE/mol-(Fe + Ni) h bar, Entry 10) and the degree
of branching (33 branches/1000 C) achieved by TEA demonstrate
the TEA also activate both metal centers. Out of various concen-
trations of MAO used, the polymerization activity increases from
4.6 × 104 g-PE/mol-(Fe + Ni) h bar (Entry 11) at 50 equiv. of MAO to
28.3 × 104 g-PE/mol-(Fe + Ni) h bar (Entry 5) at 300 equiv. and then
decreases along with higher amount of MAO (Entry 13). The amount
of MAO to achieve the maximum activity is much lower than that
needed in conventional metallocene catalyst systems [5–7,33], in
which a gross excess (>10,000) of MAO is frequently needed.
Similar to the conventional metallocene and Ziegler–Natta cat-
alysts [5–7], the MW was found to decrease as MAO concentration
increases due to the chain transfer to MAO. It is interesting to note
that at low MAO concentration, say up to [MAO]/[Fe + Ni] = 100,
the polydispersity is found to be narrow (PDI = 6.0–6.3 (Entries 11
and 12)); however, at high MAO concentrations, the PDI values
increase above 70. The polydispersity is found to be very high for
the trimetallic Fe(II)/Ni(II) catalyst when compared to monometal-
lic Ni(II) or homonuclear bimetallic Fe(II) analogues. For example,
while Ni(II) and Fe(II) catalysts produce PE with PDI of 3.5 and
4.9, respectively (Entries 1 and 3), the trimetallic catalyst produces
PE with very broad polydispersity system (PDI = 73.2 (Entry 5)) at
the same conditions. The maximum PDI of 144 is recorded when
TEA is used as a cocatalyst (Entry 10). These results demonstrate
that the polydispersity is tunable by a suitable choice of heteronu-
clear multimetallic complexes together with the suitable selection
of coactivators. The broad polydispersity of PE is favorable for the
processibility, such as the high shear viscosity and low melt exten-
sional viscosity of the product. The PE obtained by the trimetallic
Fe(II)/Ni(II) catalyst is moderately branched, predominantly influ-
enced by Ni(II) center, since the Fe(II) catalyst gives only highly
linear PE. This observation was supported by DSC and NMR mea-
surements. The Tm values were not so affected by the Ni(II) catalyst,
for example, Tm value shifted from 133 to 130 for E and F respec-
tively (Entries 3 and 5 in Table 1).
The cooperation of two metals in a heterotrimetallic system is an
attractive approach because the each metal can perform different
tasks. Recent advances in olefin polymerization catalysis using Fe
are mainly based on developments around the bis(imino)pyridine
systems [11]. This family of catalysts has attracted great interest,
both in academia and in industry, since the Fe catalyst based on
bis(imino)pyridine ligands with bulky aryl substituents is one of the
most active examples. The catalysts show exceptionally high activi-
ties for ethylene polymerization, producing strictly linear, high MW
polymer [11]. The similar behavior was observed for the bimetallic
Fe catalyst (see Entry 3 in Table 1). The steric bulk provided by the
substituents on the imino nitrogen donors plays a pivotal role in
preventing the formation of bis-chelate complexes and controlling
the rates of propagation and chain termination, thus influencing
both activity and molecular weight of the polymer. The family of
cationic ␣-diimine Ni(II) catalyst has also continued to attract much
interest. The steric and electronic properties of the ␣-diimine ligand
can be readily adjusted by modifying the imino carbon and nitrogen
substituents, and hence a large number of structural variations have
been reported in the academic and patent literature [11]. The Ni(II)
␣-diimine catalysts bearing isopropyl substituents on the aryl rings
yield highly branched PE in the ethylene homopolymerizations.
The similar behavior was observed for the monometallic Ni(II) cat-
alyst (see Entry 1 in Table 1). The formation of highly branched
polymer is the result of a series of ␤-hydrogen elimination/re-
insertion reactions via the ␤-agostic cationic alkyl intermediate
[8,11]. Considering these, it is worthy investigating the ethylene
polymerization behavior of the complex F bearing Ni(II) ␣-diimine
complex as well as two Fe(II) bis(imino)pyridine complexes in some
more detail.
Ethylene polymerizations were carried out with trinuclear com-
plex F at temperatures between 10 and 70 ◦C in combination with
various cocatalysts such as MAO, EASC and TEA. The results of poly-
merization are also summarized in Table 1. The polymerizations
were carried out at very low catalyst concentrations; say 2 ␮mol
of complex F, in 80 mL toluene to avoid the occurrence of mass
transport limitations. The active catalysts are generated in situ in
toluene by the addition of cocatalyst to the complex solution in the
presence of ethylene. As the polymerization temperature increases,
the degree of branching increases and the polymer MW and melt-
ing point decreases along with the decrease in activity as expected
(Entries 5–8, Table 1), due to the deactivation of active species at
high temperature [33]. The catalyst shows low activity at low tem-
perature. At low temperature the catalytic structure might have
blocked the bulky MAO accessing to the metal centers. As the
temperature increases, the closed structures are slowly opened
for the easy access of the cocatalyst. Even at high temperature,
say 70 ◦C, the trimetallic catalyst showed considerable activity of
(11.5 × 10−4 g-PE/mol-(Fe + Ni) h bar, Entry 8) and this may be due
to the cooperative effect of adjacent metal centers present in the
complex F. Note that its monometallic Fe and Ni(II) counterparts
show only limited activity at this conditions [8,9,11,16].
4. Conclusions
A new trinuclear ligand was synthesized successfully and
utilized to coordinate late transition metals leading to a het-
erotrimetallic system. This Fe(II)/Ni(II) based trimetallic complexes
were effective catalyst precursors for the polymerization of
ethylene using various cocatalysts, with activities exceeding 105 g-
PE/mol-metal h bar. Formation of active species resulting from the
activation of Fe and Ni(II) complexes in the presence of MAO were
studied by UV–vis spectroscopy. The trimetallic complexes showed
high catalytic activity and a long life time, especially at high tem-
perature, when compared to its bimetallic Fe(II) and monometallic
Ni(II) analogues due to The cooperative effect of the heteronuclear
Fe(II)/Ni(II) complex yielded PEs of moderate MW and of very broad
polydispersity that were also influenced by the polymerization
parameters such as the type and the amount of cocatalyst and tem-
perature. The degree of branching was tunable from highly linear
to highly branched by the type of catalyst, monometallic, homonu-
clear bimetallic and heteronuclear trimetallic, and polymerization
parameters.
The nature of alkylaluminum cocatalyst has an apparent
effect on the catalytic activity and product distribution [34].
Chlorinated alkylaluminum as a coactivator has been reported
to be more effective for homogeneous Ni(II) ␣-diimine com-
plexes than the most commonly used MAO (compare Entry

B.K. Bahuleyan et al. / Catalysis Today 164 (2011) 80–87
87
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