Optimization of ethylene trimerization using catalysts based on TiCl3/half-sandwich ligands

Article abstract of DOI:10.1002/aoc.4666

Catalytic trimerization of ethylene using three titanium-based complexes [η⁵-C₉H₆C(R)thienyl]TiCl₃ with various types of bridges (R?=?cyclo-C₅H₁₀ (C1), cyclo-C₄H₈ (C2) and (CH₃)₂ (C3)) has been successfully optimized and compared. First of all, three benzofulvene precursors, C₉H₆C(R), were synthesized. Then the corresponding indenyl-based ligands were obtained via the reaction of the precursors with thienyllithium. The final titanium-based catalysts display a distorted tetrahedral geometry, as expected for Ti(IV), with the ligand coordinated with a hemilabile behaviour. The structures of the compounds were confirmed on the basis of various analyses. The effect of catalyst concentration, ethylene pressure, reaction temperature and nature of the bridge as the significant factor affecting coordination and orientation of thienyl group relative to the metal centre on 1-hexene (1-C₆) productivity and selectivity was investigated. Results revealed that the bulky bridge groups such as cyclo-C₅H₁₀ and cyclo-C₄H₈ are appropriate for ethylene trimerization due to the closer coordination of sulfur atom with Ti, especially in cationic state, and catalyst C2 with cyclo-C₄H₈ bridge exhibits moderate productivity equal to 785?kg 1-C₆?(mol Ti)^?1?h^?1. According to the results, ethylene at a pressure of 10?bar, 50°C and 1.5?μmol of catalyst were selected as the best conditions for obtaining 1-C₆ with high productivity and selectivity. The presence of indenyl enhances the thermal stability of the catalysts and preserves their activity in higher temperatures such as 50 and 80°C.

Full text of DOI:10.1002/aoc.4666

Received: 10 May 2018
DOI: 10.1002/aoc.4666
Revised: 6 July 2018
Accepted: 30 September 2018
F U L L P A P E R
Optimization of ethylene trimerization using catalysts
based on TiCl3/half‐sandwich ligands
Leila Azimnavahsi
| Zahra Mohamadnia
Department of Chemistry, Institute for
Advanced Studies in Basic Sciences
(IASBS), Zanjan 45137‐66731, Iran
Catalytic trimerization of ethylene using three titanium‐based complexes [η5‐
C9H6C(R)thienyl]TiCl3 with various types of bridges (R = cyclo‐C5H10 (C1),
cyclo‐C4H8 (C2) and (CH3)2 (C3)) has been successfully optimized and com-
pared. First of all, three benzofulvene precursors, C9H6C(R), were synthesized.
Then the corresponding indenyl‐based ligands were obtained via the reaction
of the precursors with thienyllithium. The final titanium‐based catalysts dis-
play a distorted tetrahedral geometry, as expected for Ti(IV), with the ligand
coordinated with a hemilabile behaviour. The structures of the compounds
were confirmed on the basis of various analyses. The effect of catalyst concen-
tration, ethylene pressure, reaction temperature and nature of the bridge as the
significant factor affecting coordination and orientation of thienyl group
relative to the metal centre on 1‐hexene (1‐C6) productivity and selectivity
was investigated. Results revealed that the bulky bridge groups such as
cyclo‐C5H10 and cyclo‐C4H8 are appropriate for ethylene trimerization due to
the closer coordination of sulfur atom with Ti, especially in cationic state,
and catalyst C2 with cyclo‐C4H8 bridge exhibits moderate productivity equal
Correspondence
Zahra Mohamadnia, Department of
Chemistry, Institute for Advanced Studies
in Basic Sciences (IASBS), Zanjan 45137‐
66731, Iran.
Email: z.mohamadnia@iasbs.ac.ir
to 785 kg 1‐C6 (mol Ti)−1 −1. According to the results, ethylene at a pressure
h
of 10 bar, 50°C and 1.5 μmol of catalyst were selected as the best conditions for
obtaining 1‐C6 with high productivity and selectivity. The presence of indenyl
enhances the thermal stability of the catalysts and preserves their activity in
higher temperatures such as 50 and 80°C.
KEYWORDS
1‐hexene, catalyst, ethylene trimerization, productivity, titanium
1 | INTRODUCTION
market demand. Thus, the transformation of a statistical
ethylene oligomerization process into selective tri‐ or
Linear α‐olefins are some of the most important products
of petrochemical industries which have many applica-
tions in detergents, lubricants and surfactants, and as
co‐monomers for copolymerization with ethylene to pro-
duce linear low‐density polyethylene (LLDPE).[1,2] Con-
ventional full‐range production technology of ethylene
oligomerization normally results in a wide distribution
of α‐olefins and does not match the incessantly increasing
tetramerization seems highly desirable. Due to the higher
cost of 1‐hexene (1‐C6) isolated from oil and better
mechanical properties of LLDPE in which 1‐C6 is used as
a co‐monomer, great attention has been focused on the
selective synthesis of 1‐C6 via the trimerization of
ethylene catalysed by metal‐based catalysts.[3,4] The first
prevalent utilized catalyst system contains a chromium
atom as a central core such as Philips trimerization
Appl Organometal Chem. 2018;e4666.
wileyonlinelibrary.com/journal/aoc
© 2018 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4666

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AZIMNAVAHSI AND MOHAMADNIA
catalyst.[5,6] This catalyst is the only ethylene trimerization
system that has been commercialized up to now. The cat-
alyst consists of a chromium complex, 2,5‐dimethylpyrrole
and triethylaluminium as co‐catalyst.[3]
the metal centre has a tight connection with the surface
and they are compact with each other, either the perfor-
mance of the catalyst or catalyst selectivity decreases.[16]
Bercaw's group produced LLDPE from an ethylene‐
only feed over a tandem catalyst system consisting of a
phenoxyimine titanium trimerization catalyst and a
silylene‐linked Cp/amido titanium polymerization cata-
lyst co‐supported on the same MAO/silica particles.
The level of 1‐C6 incorporation can be controlled by
varying the ethylene pressure. Also, mechanistic studies
of selective trimerization of linear α‐olefins over a sup-
ported titanium catalyst were conducted by Bercaw and
co‐workers.[17,18]
In the study reported here, due to the environmental
impacts of residual chromium in final products and the
possibility of catalytic ethylene trimerization under mild
condition, a series of indenyl half‐sandwich titanium
complexes, namely [η5‐C9H6C(R)thienyl]TiCl3, bearing
thienyl group were synthesized for the first time and
the influence of hemilabile coordination on ethylene
trimerization was studied. The effect of the bridge, as one
of the significant factors affecting the coordination and
orientation of thienyl group relative to the metal centre,
on the productivity and selectivity of 1‐C6 was investi-
gated. Moreover, the specific steric and electronic effects
of the indenyl ring instead of Cp on the catalytic ethylene
trimerization behaviour and potential of the reaction at
high temperatures were investigated. The influence of var-
ious parameters such as ethylene pressure, reaction tem-
perature and catalyst concentration on 1‐C6 productivity
was studied. The application of the synthesized titanium‐
based catalysts in tandem polymerization of ethylene for
LLDPE production will be reported in the future.
As well as the chromium‐based catalysts,[7,8]
trimerization catalysts can be obtained from other transi-
tion metals, mainly those from groups 4 and 5 of the
periodic table, namely Zr, Ti, V and Ta. The reasons for
their application are catalytic improvements and also
the avoidance of environmental impacts of chromium
residues from catalysts in final products. Probably, the
main interest is related to catalysts with titanium as a
central atom. A significant innovation was developed in
2001, when Hessen's group reported a highly selective
arene‐substituted Cp titanium complex catalyst, namely
[(η5‐C5H4C(Me)2RTiCl3]/MAO, that can be switched
from an ethylene polymerization catalyst into an
ethylene trimerization catalyst, producing 1‐C6, by simply
changing the ligand substituent R from methyl to
phenyl group.[9] In 2002, the same group synthesized
[(η5‐C5H3R(bridge)ArTiCl3] complexes activated by
methylaluminoxane (MAO) co‐catalyst that produced
1‐C6 with an excellent productivity and selectivity (up to
99%).[10,11] The only by‐products are C10 fraction and a
trace amount of polyethylene. This discovery led to much
interest of the titanium‐mediated trimerization reaction.
Huang's group developed a half‐sandwich titanium
complex bearing pendant thienyl group that was able to
effectively trimerize ethylene to form 1‐C6 with a high
activity (220 kg 1‐C6 (mol Ti)−1 −1). The coordination
h
of the sulfur atom to the titanium centre is responsible
for the selective trimerization of ethylene.[12]
Also other half‐sandwich titanium complexes with a
pendant ethereal group, activated by MAO, have been
used for trimerization of ethylene with high selectivity
(>90%) and moderate activity (25–57 kg (mol Ti)−1 −1).
h
2 | EXPERIMENTAL
2.1 | Materials
The pendant ethereal group is essential for the selective
formation of 1‐C6.[13] In 2013, a series of half‐sandwich
indenyl titanium complexes, [Ind(bridge)Ar]TiCl3,
bearing pendant arene group on indenyl ring were
synthesized and used for the catalytic trimerization of
ethylene.[14] Indenyl‐based systems show temperature
tolerance; they can catalyse ethylene trimerization with
both high productivity and 1‐C6 selectivity even at
80°C.[15]
Two titanium‐based catalyst systems supported on
silica via various methods including grafting through a
covalent Ti─O─Si bond and grafting through a pendant
flexible tether from a Cp ligand were synthesized by
Pinkas and co‐workers in 2015.[16] The second method
of grafting led to an inactive catalyst in ethylene
trimerization, probably due to the close contact between
the catalytically active species and the surface. When
All reactions were performed under an inert atmosphere
using Schlenk techniques and a glovebox equipped with
a purifier unit. The modified MAO (MMAO) co‐catalyst
(7 wt% in toluene), 2‐thienyllithium (1 M in tetrahydrofu-
ran), n‐butyllithium (n‐BuLi; 2.5 M in n‐hexane), tita-
nium tetrachloride (TiCl4) and molecular sieve were
obtained from Aldrich (Germany). Indene, pyrrolidine,
cyclohexanone, cyclopentanone, acetone, ethanol, n‐
hexane, diethyl ether, toluene, Na2CO3, MgSO4, NaCl,
Na2SO4, sodium and NaOH were purchased from Merck
(Germany). Ethylene was provided by Bandar Imam
Petrochemical Company (Iran) and purified by passing
through NaOH, activated silica gel and molecular sieve
(3 Å) columns. Toluene, n‐hexane, methanol and diethyl

AZIMNAVAHSI AND MOHAMADNIA
3 of 9
ether were refluxed over sodium with benzophenone as
an indicator and then were distilled under argon atmo-
sphere prior to use.
2.2 | Characterization of ligand, catalyst,
trimerization product and polyethylene
Fourier transform infrared (FT‐IR) spectroscopy was car-
ried out by means of a Bruker Vector 22 FT‐IR spectrom-
eter (Germany) in the range 400–4000 cm−1 to confirm
the structure of the precursors, ligands and catalysts.
The samples were prepared by mixing with powdered
KBr in the form of pellets under ambient conditions
SCHEME 1 General procedure for synthesis of benzofulvene
precursors, ligands and corresponding titanium‐based catalysts
1
and measured in transmission mode. The H NMR and
13C NMR spectra of ligands were recorded with a Bruker
400 MHz Ultrashield NMR instrument (Germany) at
room temperature. The samples for 1H NMR and 13C
NMR analysis were dissolved in CDCl3. Elemental analy-
sis was performed using a Vario EL III CHNOS elemental
analyser. UV–visible absorption spectra of samples were
2.3.1 | 1‐Cyclohexylidene‐1H‐indene (F1)
Benzofulvene F1 was obtained as white crystals in 42%
yield. FT‐IR (KBr, υmax, cm−1): 3480 (adsorbed water),
3014, 3018 and 3068 (sp2 C─H), 2850 and 2925 (sp3
C─H), 1930–1780 (overtone of aromatic ring), 1621
(C═C), 1446 (CH2), 742 and 746 (═C─H). 1H NMR
(400 MHz, CDCl3, δ, ppm): 1.71–1.79 (2H, m,
C9H6C(cyclo‐C5H10)), 1.79–1.9 (4H, m, C9H6C(cyclo‐
C5H10)), 2.74 (2H, t, C9H6C(cyclo‐C5H10)), 3.05 (2H, t,
C9H6C(cyclo‐C5H10)), 6.8 (1H, d, C9H6C(cyclo‐C5H10)),
6.95 (1H, d, C9H6C(cyclo‐C5H10)), 7.2–7.25 (2H, m,
C9H6C(cyclo‐C5H10)), 7.38 (1H, d, C9H6C(cyclo‐C5H10)),
7.9 (1H, d, C9H6C(cyclo‐C5H10)). 13C NMR (100 MHz,
CDCl3, δ, ppm): 26.5, 28.2, 28.9, 32.3, 34.6 (CH2), 121.2,
123.8, 124.7, 126.1, 127.4, 128.4 (CH), 133.8, 136, 144.7,
152.4 (Cq) (Figures S1–S3).
measured using
a
UV–visible spectrophotometer
(Pharmacia Biotech Ultrospec 4000) and acquired
between 200 and 800 nm. Determination of amount of
metal was performed using inductively coupled plasma
analysis (3410ARL, Switzerland). The trimerization reac-
tion mixtures were analysed using a GC system (Varian
CP 3800) fitted with a 30 m × 0.32 mm i.d. capillary CP
volamine column and a flame ionization detector.
2.3 | General procedure for synthesis of
benzofulvene precursors C9H6C(R)
(R = cyclo‐C5H10 (F1), cyclo‐C4H8 (F2),
(CH3)2 (F3))
Benzofulvene precursors F1–F3 were synthesized accord-
ing to the Stone and Little method[19] with relatively few
modifications (Scheme 1). In a 50 ml round‐bottom flask,
indene (0.58 ml, 5 mmol) and pyrrolidine (0.25 ml,
3 mmol) were dissolved in methanol (2 ml) and stirred
for 10 min. Under argon atmosphere, cyclohexanone or
other ketones such as cyclopentanone and acetone
(2 mmol) was added dropwise to the stirred solution and
the solution was stirred for 12 h. The mixture was
quenched by adding acetic acid (0.18 ml, 3 mmol) and
diluted using diethyl ether (10 ml). The organic layer was
washed with deionized water (2 × 10 ml) followed by brine
(3 × 10 ml), dried over anhydrous MgSO4, filtered, concen-
trated under vacuum and purified using column chroma-
tography with n‐hexane as eluent to give compounds
F1–F3. The pure obtained fulvenes were characterized
using 1H NMR, 13C NMR and FT‐IR spectroscopies.
2.3.2 | 1‐Cyclopentylidene‐1H‐indene (F2)
Benzofulvene F2 was obtained as a yellow solid in 31%
yield. FT‐IR (KBr, υmax, cm−1): 3200–3600 (adsorbed
water), 3083 (sp2 C─H), 2964 and 2870 (sp3 C─H),
1930–1780 (overtone of aromatic ring), 1600 (C═C),
1
1452 (CH2), 804 (═C─H). H NMR (400 MHz, CDCl3, δ,
ppm): 1.8–1.88 (2H, m, C9H6C(cyclo‐C4H8)), 1.92–2.03
(2H, m, C9H6C(cyclo‐C4H8)), 2.93 (2H, t, C9H6C(cyclo‐
C4H8)), 3.01 (2H, t, C9H6C(cyclo‐C4H8)), 6.7 (1H, d,
C9H6C(cyclo‐C4H8)), 6.8 (1H, d, C9H6C(cyclo‐C4H8)),
7.21–7.25 (2H, m, C9H6C(cyclo‐C4H8)), 7.34–7.37 (1H, m,
C9H6C(cyclo‐C4H8)), 7.59–7.63 (1H, m, C9H6C(cyclo‐
C4H8)). 13C NMR (100 MHz, CDCl3, δ, ppm): 26.03,
26.88, 33.02, 34.45 (CH2), 121.02, 122.68, 124.68, 125.99,
128.30, 128.60 (CH), 133.12, 135.93, 143.34, 154.75 (Cq)
(Figures S4–S6).

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AZIMNAVAHSI AND MOHAMADNIA
Elemental analysis (%): calculated for C19H20S (found): C
81.38 (80.95), H 7.19 (7.49), S 11.43 (11.56).
2.3.3 | 1‐(Propan‐2‐ylidene)‐1H‐indene
(F3)
Benzofulvene F3 was obtained as yellow oil in 35% yield.
FT‐IR (KBr, υmax, cm−1): 3200–3600 (adsorbed water),
3040–3090 (CHaromatic), 2911 and 2861 (CHaliphatic),
1930–1780 (overtone of aromatic ring), 1635 (C═C),
1450 (CH2), 742 and 752 (C─Hout‐of‐plane). 1H NMR
(400 MHz, CDCl3, δ, ppm): 2.33 (3H, s, C9H6C(CH3)2),
2.46 (3H, s, C9H6C(CH3)2), 6.81 (1H, d, C9H6C(CH3)2),
6.88 (1H, d, C9H6C(CH3)2), 7.22–7.26 (2H, m,
C9H6C(CH3)2), 7.41 (1H, m, C9H6C(CH3)2), 7.82 (1H, m,
C9H6C(CH3)2). 13C NMR (100 MHz, CDCl3, δ, ppm):
22.94, 25.00 (CH3), 121.03, 123.48, 124.71, 126.01,
127.61, 128.35 (CH), 135.74, 136.67, 143.35, 143.94 (Cq)
(Figures S7–S9).
2.4.2 | 2‐[1‐(3H‐Inden‐1‐yl)cyclopentyl]
thiophene (L2)
Ligand L2 was obtained as yellow oil in 12% yield. FT‐IR
(KBr, υmax, cm−1): 3100–3600 (H2O), 3089 (CHaromatic),
2956 and 2873 (CHaliphatic), 1650 1450, (C═C), 750
1
(═C─H). H NMR (400 MHz, CDCl3, δ, ppm): 1.8–2.5
(8H, m, [C9H6C(cyclo‐C4H8)]C4H3S), 2.9 (2H, d,
[C9H6C(cyclo‐C4H8)]C4H3S)), 6.7–7.6 (8H, [C9H6C(cyclo‐
C4H8)]C4H3S. 13C NMR (100 MHz, CDCl3, δ, ppm):
24.14, 26.01, 26.89, 32.97, 34.45 (CH2), 39.72
C9H6C(cyclo‐C4H8), 121.09, 122.68, 124.69, 125.99,
128.29, 128.59, 128.92, 133.13 (CH), 133.30, 135.94,
143.31, 154.75 (Cq) (Figures S13–S15). Elemental analysis
(%): calculated for C18H18S (found): C 81.15 (81.84), H
6.81 (7.11), S 12.04 (11.05).
2.4 | General procedure for synthesis of
ligands [C9H6C(R)]C4H3S (R = cyclo‐C5H10
(L1), cyclo‐C4H8 (L2), (CH3)2 (L3))
2.4.3 | 2‐[1‐(3H‐Inden‐1‐yl)‐1‐methylethyl]
Indenyl‐based ligands were prepared according to a
slightly modified literature procedure (Scheme 1).[15]
Under argon atmosphere, benzofulvens (0.5 mmol) in
diethyl ether was added dropwise to a stirred solution of
2‐thienyllithium (2 mmol) in diethyl ether (5 ml) at
−40°C. The mixture was warmed to room temperature,
stirred for 24 h and hydrolysed with 10 ml of ice–water.
The water layer was extracted with light petroleum (three
times), and the combined organic layers were dried with
anhydrous MgSO4. The solvent was removed under vac-
uum, and purified using column chromatography with
n‐hexane as eluent to afford pure ligands L1–L3.
thiophene (L3)
Ligand L3 was obtained as yellow oil in 12% yield. FT‐IR
(KBr, υmax, cm−1): 3100–3600 (H2O), 3075 (CHaromatic),
3000–2800 (CHaliphatic), 1662 (C═C), 1467 (CH2bending),
1
769 (═C─Hbending). H NMR (400 MHz, CDCl3, δ, ppm):
1.8 (6H, s, [C9H6C(CH3)2]C4H3S), 3.42 (2H, d,
[C9H6C(CH3)2]C4H3S)), 6.4 (1H, t, [C9H6C(CH3)2]
C4H3S)), 6.73–6.78 (1H, m, [C9H6C(CH3)2]C4H3S), 6.78–
6.83 (1H, m, [C9H6C(CH3)2]C4H3S), 7.20–7.26 (2H, m,
[C9H6C(CH3)2]C4H3S), 7.27–7.31 (2H, m, [C9H6C(CH3)2]
C4H3S), 7.33–7.38 (1H, m, [C9H6C(CH3)2]C4H3S),
7.58–7.64 (1H, m, [C9H6C(CH3)2]C4H3S). 13C NMR
(100 MHz, CDCl3, δ, ppm): 30.83 (CH3), 37.28 (CH2),
39.1 [C9H6C(CH3)2]C4H3S, 122, 123.1, 123.2, 123.8,
124.2, 125.6, 126.5, 127.3 (CH), 143.5, 145, 151.5, 154
(Cq) (Figures S16–S18). Elemental analysis (%): calcu-
lated for C16H16S (found): C 79.95 (80.26), H 6.71 (6.48),
S 13.34 (13.26).
2.4.1 | 2‐[1‐(3H‐Inden‐1‐yl)cyclohexyl]thio-
phene (L1)
Ligand L1 was obtained as a white solid in 24% yield. FT‐
IR (KBr, υmax, cm−1): 3100–3600 (adsorbed water), 3000
(CHaromatic), 2938 and 2911 (CHaliphatic), 1664 (C═C),
1
1467 (CH2bending), 700–800 (═C─H). H NMR (400 MHz,
CDCl3, δ, ppm): 1.5–1.64, 1.67–1.76, 2.2–2.3, 2.47–2.53
(10H, m, [C9H6C(cyclo‐C5H10)]C4H3S), 3.42 (2H, d,
[C9H6C(cyclo‐C5H10)]C4H3S)), 6.47 (1H, t, [C9H6C(cyclo‐
C5H10)]C4H3S)), 6.92–6.95 (2H, m, [C9H6C(cyclo‐C5H10)]
C4H3S), 7.12–7.16 (3H, m, [C9H6C(cyclo‐C5H10)]C4H3S),
7.27–7.31 (1H, m, [C9H6C(cyclo‐C5H10)]C4H3S), 7.45–
7.47 (1H, m, [C9H6C(cyclo‐C5H10)]C4H3S). 13C NMR
(100 MHz, CDCl3, δ, ppm): 22.91, 26.34, 37.40, 38.08
(CH2), 43.48 (C9H6C(cyclo‐C5H10)]C4H3S), 122.2, 123.08,
123.78, 123.82, 124.04, 125.48, 126.29, 129.44 (CH),
143.71, 145.22, 149.92, 153.76 (Cq) (Figures S10–S12).
2.5 | Synthesis and characterization of
catalysts {[η5‐C9H6C(R)]C4H3S}TiCl3
(R = cyclo‐C5H10 (C1), cyclo‐C4H8 (C2),
(CH3)2 (C3))
Under argon atmosphere, a solution of n‐BuLi (0.5 mmol)
in n‐hexane was added dropwise to a solution of the syn-
thesized ligands L1–L3 (0.5 mmol) in 5 ml of dried n‐
hexane at −40°C. Then the mixture was warmed to room
temperature and stirred overnight. Then titanium

AZIMNAVAHSI AND MOHAMADNIA
5 of 9
tetrachloride (0.5 mmol) was added to the reaction mix-
ture at −78°C. The mixture colour changed from white
to red immediately. Then, the red solution was warmed
to room temperature and stirred overnight.
3 | RESULTS AND DISCUSSION
3.1 | Characterization of catalysts using
UV–visible spectroscopy
The solvent and unreacted TiCl4 were removed under
reduced pressure and the residue was extracted with
dried n‐hexane. After cooling the filtrate to −20°C, the
red crystals of catalyst appeared (Scheme 1). Catalyst C1
was obtained as red crystals in 45% yield. Elemental anal-
ysis (%): calculated for C19H19STiCl3 (found): C 52.62
(53.48), H 4.42 (4.13), S 7.39 (7.86), Ti 11.04 (12.21), Cl
24.53 (—). FT‐IR (KBr, υmax, cm−1): 3360 (adsorbed
water), 2964 and 2933 (CHaliphatic), 1616 (C═C), 1461
(CH2bending), 831 (═C─H) (Figure S19). Catalyst C2 was
obtained as an orange solid in 30% yield. Elemental anal-
ysis (%): calculated for C18H17STiCl3 (found): C 51.52
(52.41), H 4.08 (3.88), S 7.64 (7.08), Ti 11.41 (12.65), Cl
25.35 (—). FT‐IR (KBr, υmax, cm−1): 3357 (adsorbed
water), CHaliphatic overlapped with water peak, 1652
(C═C), 1457 (CH2bending), 860 (═C─H), 468 (Ti─Cl) (Fig-
ure S20). Catalyst C3 was obtained as an orange solid in
35% yield. Elemental analysis (%): calculated for
C16H15STiCl3 (found): C 48.83 (49.83), H 3.84 (3.74), S
8.15 (7.53), Ti 12.16 (11.56), Cl 27.02 (—). FT‐IR (KBr,
When the ligands react with TiCl4, the obtained com-
plexes were isolated as a red powder. The {[η5‐C9H6C(R)]
C4H3S}TiCl3 complexes display a distorted tetrahedral
geometry, as expected for Ti(IV), with the ligand coordi-
nated with hemilabile behaviour.[20,21] Figure 1 compares
the absorption spectra of L1 with the corresponding
titanium‐based complexes. In general, for these types of
complexes two tetrahedral and square‐planar structures
are proposed. Due to the presence of the bulky indenyl
group in the ligand coordinated to the titanium centre
and electrostatic repulsion between them, the bonds make
distorted tetrahedral geometry in order to minimize the
potential energy and maximize stability of the molecules.
According to the valence bond theory, for titanium‐
based complexes with electron configuration of d0,
valence orbitals of a central atom are a mixture of sp3 or
d3s due to the closer distance of s to the d orbital relative
to p. As shown in Figure 1, d–d transitions are not seen for
all complexes. The absorption in the UV region around
290–300 nm is related to π → π* (intra‐ligand) transitions.
In the case of the catalysts, in addition to the intra‐ligand
transition, the ligand‐to‐metal charge transfer transition
at around 240 nm is also observed. Due to the expansion
of the resonance system, the intra‐ligand transition in all
catalysts has shifted to higher wavelengths.
υ
max, cm−1): 3388 (adsorbed water), 3019 (CHaliphatic),
1612 (C═C), 1492 (CH2bending), 769 (═C─H), 433, 484
(Ti─Cl) (Figure S21).
2.6 | Trimerization of ethylene
The catalytic ethylene trimerization reactions were per-
formed in a stainless steel research reactor. The tempera-
ture and pressure of the high‐pressure reactor were
controlled. Before each run, the reactor was dried in an
oven at 130°C for 3 h, vacuumed for 30 min and purged
with three cycles of argon–vacuum. The reactor was then
preheated, charged with toluene and the desired amount
of MMAO, pressurized with ethylene, and stirred at the
required reaction temperature. After 10 min, equilibrium
was reached. At that time, a specific volume of the cata-
lyst solution in toluene was injected into the reactor to
start the ethylene trimerization. The reaction temperature
and ethylene pressure were kept constant during the
reaction. After stirring the reaction mixture for 30 min
at a certain temperature, the reactor was cooled to
−10°C. Then the temperature was slowly increased to
0°C and the off‐gas was collected in specific balloons for
GC analysis. Then the liquid phase containing 1‐C6 and
other by‐products was collected, stored in a refrigerator
to avoid the volatilization of 1‐C6 and finally analyzsd
using GC. The solid polyethylene was collected by filtra-
tion, washed with MeOH, dried and weighed.
FIGURE 1 Comparison between UV–visible absorption spectra
of indenyl‐based ligand (L1) and corresponding catalysts (C1–C3)

6 of 9
AZIMNAVAHSI AND MOHAMADNIA
depending on the conditions, measured selectivity overall
by mass). A generalized mechanism for activation by
co‐catalyst and ethylene trimerization via metallacycle
intermediates is shown in Scheme 3. The active cationic
species will undergo ethylene insertions into the Ti─Me
bonds and produce cationic bis(n‐alkyl) species. These
groups are in balance (through β‐hydrogen elimination)
with hydride–olefin species. Presence of thienyl group
causes remove of alkene group and reduction elimination
creates CH3CH2R species that makes a cationic intermedi-
ate of Ti(II). The produced intermediate reacts with two
molecules of ethylene and produces metallacyclopentane
intermediates. Insertion of the third ethylene molecule will
result in the production of metallacycloheptane intermedi-
ates. From here, insertion of further ethylene into the
metallacyclopentane produces larger ring metallacycles,
while the breakdown of the metallacycle at any point can
produce linear α‐olefins. Thus the selectivity of the process
is controlled by the relative stability of the differently sized
metallacycles and their tendency to either decompose or
grow via ethylene insertion. For selective ethylene
trimerization, the release of 1‐C6 is fast, inhibiting further
ring growth and thus formation of higher α‐olefins.[23,24]
A trace amount of polyethylene was found as by‐product
under different reaction conditions.
3.2 | Catalytic ethylene trimerization
using synthesized catalysts (C1–C3)
A 200 ml double‐wall stainless steel high‐pressure reactor
equipped with an external circulation bath, a mechanical
stirrer and a temperature controller was used for ethylene
trimerization. The ethylene trimerization reaction using
the titanium‐based catalysts activated by MMAO afforded
1‐C6. Transition metal‐based trimerization catalysts
usually are not able to produce α‐olefins alone and must
be activated by suitable co‐catalysts. Among the important
co‐catalysts,
perfluoroarylboranes,
fluoroarylalanes,
tritylaluminium alkyls and MAO are of note. Various struc-
tures such as linear, rings, two‐dimensional structures and
three‐dimensional clusters were proposed for the composi-
tion of MAO, all of which signify a possible combination of
MAO (─Al(Me)─O─)n units. It is generally accepted that
the co‐catalyst, among other functions such as removal
water and oxygen contaminant and easy reduction of tita-
nium, facilitates alkyl abstraction from the catalyst precur-
sor to yield an anionic co‐catalyst fragment [RX−] and a
cationic metal fragment [LnM+], which in combination rep-
resents the active catalytic system as an ion pair represented
by [LnM+][RX−] (Scheme 2).[22] These activated cationic
species are responsible for ethylene trimerization.
The MMAO‐activated system was found to be highly
active and selective for ethylene trimerization. An analy-
sis of the liquid fraction by GC revealed that ethylene
trimerization by catalyst/MMAO under various condi-
tions yielded 1‐C6 with excellent selectivity (80–97%
The results of the catalytic ethylene trimerization
using C1–C3 (20 ml of toluene, 30 min reaction time,
600 rpm, Al/Ti = 1000) are summarized in Table 1. The
reaction was carried out at three catalyst concentrations
(0.5, 1.5 and 3 μmol), at 30°C and 5 bar ethylene pressure.
SCHEME 2 Proposed mechanism for the key role of MAO in catalyst activation
SCHEME 3 General mechanism for
ethylene trimerization via metallacycle
intermediates

AZIMNAVAHSI AND MOHAMADNIA
7 of 9
TABLE 1 Ethylene trimerization results with titanium‐based catalysts (C1–C3)
[Catalyst
Productivity
amount
(μmol)
Temperature Pressure 1‐Hexene PE
C8+
(kg 1‐C6
Selectivity of
1‐C6 (wt%)b
Entry Catalyst
(°C)
(bar)
(mg)
(mg) (mg) (mol Ti)−1 h−1 a
)
1
2
3
4
5
6
7
0.5
1.5
3
1.5
1.5
1.5
1.5
30
30
30
50
80
50
50
5
5
5
5
5
173
625
527
655
753
757
761
20
10
30
50
80
40
40
11
6
695
833
351
84.8
97.5
91.8
89.9
85.9
92.3
92.6
17
23
43
23
21
873
1004
1009
1014
8
10
8
9
10
11
12
1.5
1.5
1.5
1.5
1.5
30
50
80
50
50
5
5
5
8
10
390
513
589
589
63
30
40
70
60
60
14
24
36
29
26
520
684
785
785
840
89.7
88.9
84.7
86.9
88.0
13
14
15
16
1.5
1.5
1.5
1.5
30
50
80
50
5
5
5
8
226
387
478
490
30
50
60
50
17
23
34
26
301
516
637
653
82.8
84.1
83.6
86.6
17
1.5
50a
10b
459
60
32
612
83.3
aAl/Ti = 1000, time = 30 min, rpm = 1100, solvent = toluene (20 ml).
bPercentage of 1‐C6 overall by mass.
According to Table 1 (entries 1–3), the productivity and
selectivity rise with increasing catalyst concentration
from 0.5 to 1.5 μmol due to the increasing number of cat-
alytic active sites. Considering the constant value of Al/Ti
ratio, with further increasing catalyst concentration from
1.5 to 3 μmol, due to the co‐catalyst adsorption on the cat-
alytic centre surfaces in competition with the monomer,
the productivity decreases (Figure 2). MMAO acts as a
poison at higher alkylaluminium concentrations. The
subsequent experiments were performed at a catalyst con-
centration of 1.5 μmol.
Temperature is another factor that must be optimized.
For all synthesized catalysts, it can be seen that the pro-
ductivity of 1‐C6 increases with increasing temperature
during 30–80°C (Figure 3). This means that the
trimerization active species of the indenyl complexes
can keep their stability even at 80°C. For the indenyl,
the titanium is more lacking of electrons, and more
favourable to coordinate with the sulfur atom. At low
temperature, the coordination between sulfur atom and
the titanium cation is tighter, so the productivity is low
although obtaining a relatively high selectivity. On
increasing the temperature to 80°C, the coordination is
weak enough and shows the most appropriate hemilabile
behaviour for ethylene trimerization. This shows that the
indenyl–thienyl complexes exhibit higher temperature
resistance. In comparison with Cp‐based catalysts that
are sensitive to high temperatures, our results show that
the presence of indenyl group increases the temperature
stability of the synthesized catalysts. On the other hand,
increasing the temperature led to the further production
of polyethylene as by‐product. For example, catalyst C1
FIGURE 2 Effect of the catalyst concentration on productivity;
Al/Ti = 1000, time = 30 min, rpm = 1100, solvent = toluene (20 ml)

8 of 9
AZIMNAVAHSI AND MOHAMADNIA
at 50°C, gives relatively high 1‐C6 productivity equal to
873 kg 1‐C6 (mol Ti)−1 h−1 and a reasonable amount of
polyethylene equal to 50 mg.
The positive role of ethylene pressure on catalyst pro-
ductivity is evident from Table 1. Higher productivity was
obtained with increasing the ethylene pressure to 8 and
10 bar (Table 1, entries 4, 6 and 7 for C1, entries 9, 11
and 12 for C2, entries 14, 16 and 17 for C3). The higher
solubility of ethylene at a higher pressure as well as better
coordination of ethylene with active sites are the main
reasons for increasing productivity under higher ethylene
pressures (Figure 4). The difference of the productivity at
8 and 10 bar is not great.
According to similar works on these types of catalysts,
there is no coordination between S atom and Ti centre in
normal condition, but after activation with the MAO co‐
catalyst, this is achieved and 1‐C6 is selectively formed.[12]
The bridge between the indenyl and thienyl groups has
an important role in the orientation of the thienyl group
relative to the titanium metal centre and the size of
C(Ind)C(bridge)C(thienyl) angle. With activation of the cat-
alyst using MMAO co‐catalyst, the bridge will press the
thienyl group to a certain extent to approach the low‐
valence metal. So the bridge moiety with different substitu-
ents has a direct effect on the coordination between the sul-
fur atom and titanium. So the productivity of C1 with
C(cyclo‐C5H10) as bridge group is higher than that of similar
complexes with CMe2 as the bridge group.
FIGURE 3 Effect of temperature on catalyst productivity; Al/
Ti = 1000, time = 30 min, rpm = 1100, solvent = toluene (20 ml),
catalyst concentration = 1.5 μmol, ethylene pressure = 5 bar
Results reveal that the bulky bridge groups are
appropriate for ethylene trimerization due to the closer
coordination of sulfur atom with Ti, especially in the
cationic state (Figure 5). Catalyst C2 with C(cyclo‐C4H8)
bridge exhibits moderate productivity of around 786 kg
FIGURE 4 Effect of ethylene pressure on catalyst productivity;
Al/Ti = 1000, time = 30 min, rpm = 1100, solvent = toluene (20 ml)
1‐C6 (mol Ti)−1 h−1
.
FIGURE 5 Effect of various bridges on catalyst productivity; Al/Ti = 1000, time = 30 min, rpm = 1100, solvent = toluene (20 ml), catalyst
concentration = 1.5 μmol, temperature = 80°C, ethylene pressure = 5 bar

AZIMNAVAHSI AND MOHAMADNIA
9 of 9
[7] Z. Mohamadnia, E. Ahmadi, M. Nekoomanesh, H. Salehi,
According to the results, 10 bar ethylene pressure,
50°C and 1.5 μmol catalyst amount were selected as the
best conditions for obtaining both high activity and selec-
tivity toward 1‐C6 production with high commercial cost.
Catal. Lett. 2011, 141, 474.
[8] Z. Mohamadnia, E. Ahmadi, M. Nekoomanesh, Catal. Lett.
2011, 141, 1191.
[9] P. J. W. Deckers, B. Hessen, J. H. Teuben, Angew. Chem. 2001,
113, 2584.
4 | CONCLUSIONS
[10] P. J. W. Deckers, B. Hessen, J. H. Teuben, Organometallics
2002, 21, 5122.
High selectivity and productivity of 1‐C6 was obtained
using the three titanium‐based catalysts C1–C3. The pro-
ductivity of 1‐C6 increases with the temperature rising to
80°C, which shows the stability of the catalyst structure at
high temperatures. The higher solubility of ethylene at
higher pressure as well as better coordination of ethylene
with active sites is the main reasons for increasing pro-
ductivity under higher ethylene pressures. Results reveal
that the bulky bridge groups C(cyclo‐C5H10) and C(cyclo‐
C4H8) are appropriate for ethylene trimerization due to
the closer coordination of sulfur atom with Ti, especially
in the cationic state.
[11] E. Otten, A. A. Batinas, A. Meetsma, B. Hessen, J. Am. Chem.
Soc. 2009, 131, 5298.
[12] J. Huang, W. Tianzhi, Q. Yanlong, Chem. Commun. 2003,
22, 2816.
[13] T. Wu, Y. Qian, J. Huang, J. Mol. Catal. A 2004, 214, 227.
[14] Y. Zhang, H. Ma, J. Huang, J. Mol. Catal. A 2013, 373, 85.
[15] Y. Zhang, C. Wang, T. Wu, M. J. Huang, J. Mol. Catal. A 2014,
387, 20.
[16] V. Varga, T. Hodík, M. Lamač, M. Horáček, A. Zukal, N.
Žilková, W. O. Parker Jr., J. Pinkas, J. Organometal. Chem.
2015, 777, 57.
[17] D. C. Aluthge, A. Sattler, M. A. Al‐Harthi, J. A. Labinger, J. E.
Bercaw, ACS Catal. 2016, 6, 6581.
ACKNOWLEDGEMENTS
[18] D. K. Steelman, D. C. Aluthge, M. C. Lehman, J. A. Labinger, J.
E. Bercaw, ACS Catal. 2017, 7, 4922.
The authors thank Mr Biglari for his efforts in the NMR
analysis of the samples and the Institute for Advanced
Studies in Basic Sciences (IASBS) for its financial and
spiritual support.
[19] K. J. Stone, R. D. Little, J. Org. Chem. 1984, 49, 1849.
[20] I. Saeed, S. Katao, K. Nomura, Organometallics 2008, 28, 111.
[21] R. Bína, I. Císařová, M. Pavlišta, I. Pavlík, Appl. Organometal.
Chem. 2004, 18, 262.
[22] W. J. van Rensburg, J. A. van den Berg, P. J. Steynberg, Organ-
ometallics 2007, 26, 1000.
ORCID
[23] D. S. McGuinness, Chem. Rev. 2011, 111, 2321.
Leila Azimnavahsi https://orcid.org/0000-0002-3728-5033
[24] P. R. Elowe, M. Cassandra, P. G. Pringle, S. K. Spitzmesser, J. E.
Zahra Mohamadnia
https://orcid.org/0000-0002-1937-
Bercaw, Organometallics 2006, 25, 5255.
4980
SUPPORTING INFORMATION
REFERENCES
Additional supporting information may be found online
in the Supporting Information section at the end of the
article.
[1] Y. V. Kissin, Polymers of higher olefins, in Kirk‐Othmer
Encyclopedia of Chemical Technology, John Wiley, New York
2005.
[2] K. Hideki, K. Miura, K. Yamamoto, T. Sakuragi, T. Sugiyama,
M. Ohshima, H. Miura, Catalysts 2013, 3, 125.
[3] J. T. Dixon, M. J. Green, F. M. Hess, D. H. Morgan,
How to cite this article: Azimnavahsi L,
Mohamadnia Z. Optimization of ethylene
trimerization using catalysts based on TiCl3/half‐
sandwich ligands. Appl Organometal Chem. 2018;
e4666. https://doi.org/10.1002/aoc.4666
J. Organometal. Chem. 2004, 689, 3641.
[4] P. A. R. Breuil, M. Lionel, O. B. Hélène, Catal. Lett. 2015,
145, 173.
[5] J. R. Briggs, J. Chem. Soc. Chem. Commun. 1989, 11, 674.
[6] R. M. Manyik, W. E. Walker, T. P. Wilson, J. Catal. 1977, 47, 197.