Experimental and DFT study on titanium-based half-sandwich metallocene catalysts and their application for production of 1-hexene from ethylene

Article abstract of DOI:10.1016/j.mcat.2021.111636

Different types of [Ind-C(R)-Phenyl]TiCl₃ catalysts based on pendant arene containing indenyl (Ind) ligand bearing various types of bridges (R=cyclo‐C₅H₁₀ (C1), (CH₃)₂ (C2), 4-tBu-cyclo‐C₅H₉ (C3), and cyclo‐C₆H₁₂ (C4)) have been synthesized, and used in the ethylene trimerization to 1-hexene in the presence of methyl aluminoxane (MAO) as co-catalyst. The reaction conditions were first optimized in C2 catalyst case, where the highest 1-hexene product was achieved at the catalyst concentration, temperature and ethylene pressure of 1.5× 10^?3 M, 40 °C, and 8 bar, respectively. During this optimization and under specific reaction conditions, a switching behavior from ethylene trimerization to polymerization was also detected, as an undesired reaction. At the optimized conditions, synthesized catalysts showed the following trend toward both 1-hexene yield and selectivity: C1>C2>C3>C4. Then, to shed light on the possible reaction mechanisms and to confirm the activity trend obtained in experimental section, density functional theory (DFT) calculations were employed. In this line, obtained results for activity trend in the simulation studies fit well with the experiments. According to both experimental and DFT results, the highest catalytic activity was observed in the presence of the catalyst with a cyclohexane middle bridge (C1).

Full text of DOI:10.1016/j.mcat.2021.111636

Molecular Catalysis 509 (2021) 111636
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Experimental and DFT study on titanium-based half-sandwich metallocene
catalysts and their application for production of 1-hexene from ethylene
,
Sajjad Gharajedaghia, Zahra Mohamadniaa *, Ebrahim Ahmadib, Mohamadreza Marefatb,
,
,
,*
Gerard Parerasc d, Sílvia Simonc, Albert Poaterc *, Naeimeh Bahri-Lalehe
a Polymer Research Laboratory, Department of Chemistry, Institute for Advanced Studies in Basic Science (IASBS), Gava Zang, Zanjan 45137-66731, Iran
b Polymer Research Laboratory, Department of Chemistry, Faculty of Science, University of Zanjan, Zanjan, Iran
c
`
`
Institut de Química Computacional i Catalisi and Departament de Química, Universitat de Girona, c/Maria Aurelia Capmany 69, Girona, Catalonia 17003, Spain
d School of Chemistry, University College Cork, College Road, Cork, Ireland
e Polymerization Engineering Department, Iran Polymer and Petrochemical Institute (IPPI), P.O. Box 14965/115, Tehran, Iran
A B S T R A C T
Different types of [Ind-C(R)-Phenyl]TiCl3 catalysts based on pendant arene containing indenyl (Ind) ligand bearing various types of bridges (R=cyclo-C5H10 (C1),
(CH3)2 (C2), 4-tBu-cyclo-C5H9 (C3), and cyclo-C6H12 (C4)) have been synthesized, and used in the ethylene trimerization to 1-hexene in the presence of methyl
aluminoxane (MAO) as co-catalyst. The reaction conditions were first optimized in C2 catalyst case, where the highest 1-hexene product was achieved at the catalyst
concentration, temperature and ethylene pressure of 1.5 × 10ꢀ 3 M, 40 ◦C, and 8 bar, respectively. During this optimization and under specific reaction conditions, a
switching behavior from ethylene trimerization to polymerization was also detected, as an undesired reaction. At the optimized conditions, synthesized catalysts
showed the following trend toward both 1-hexene yield and selectivity: C1>C2>C3>C4. Then, to shed light on the possible reaction mechanisms and to confirm the
activity trend obtained in experimental section, density functional theory (DFT) calculations were employed. In this line, obtained results for activity trend in the
simulation studies fit well with the experiments. According to both experimental and DFT results, the highest catalytic activity was observed in the presence of the
catalyst with a cyclohexane middle bridge (C1).
Introduction
chromium metal has attracted more attention in the recent years [23,
24]. This metal is the main center of Phillips’ Cr-pyrrolide catalysts [25].
BP’s (o-OMe)PNP catalysts [26], Sasol’s PNP/SNS trimerization cata-
lysts [27,28], and PNP tetramerization catalysts [29]. Catalysts based on
other transition metals such as Zr, Ti, V, Ta, or Ni have been less studied
[30,31].
The oligomerization of linear terminal alkenes is one of the signifi-
cant problems in the linear α-olefins (LAOs) production in both industry
and academia [1–6], together with hydrogenation of alkanes [7]. Linear
alpha olefins, normally produced by ethylene oligomerization processes
[8–11], and by Fischer–Tropsch synthesis followed by purification [12],
found applications as co-monomers in high density and linear low
density polyethylene (HDPE and LLDPE) production, and as detergents,
plasticizers, surfactants, and lubricants.[13,14]. Traditional technology
for LAO synthesis is based on a full-range production process in which
ethylene oligomerizes to achieve a broad range of the products. It is a
non-selective approach and cannot match the constantly growing mar-
ket demand. Accordingly, the change of a statistical ethylene oligo-
merization process into selective approach appears highly demanded
[13–15]. The selective oligomerization of ethylene has recently attrac-
ted considerable attention [16,17]. In this regard, the catalytically se-
lective trimerization of ethylene to 1-hexene has been extensively
studied [18–23]. Amongst all the systems, the catalysts based on
Hessenʼs group in 2001 for the first time reported that the change of
R from methyl to phenyl in [(
η
5-C5H4C(Me)2RTiCl3]/MAO, switches the
reaction from ethylene polymerization into ethylene trimerization and
facilitates formation of 1-hexene as the major product. The hemilabile
behavior of cyclopentadienyl ligand with the arene group is the main
reason of this significant change in catalyst behavior [32]. Deckers et al.
in 2002 synthesized a new family of highly active catalysts for the tri-
merization of ethylene based on (arene-cyclopentadienyl) titanium
complexes [(ƞ5-C5H3R-(bridge)-ArTiCl3] activated by MAO co-catalyst.
Selectivity to produce 1-hexene not only depends on the presence of the
arene pendant group but also the bridge nature between cyclo-
pentadienyl (Cp) and arene. In the absence of arene, polyethylene was
the main product [33,34]. Huang’s group synthesized a half-sandwich
* Corresponding authors.
E-mail addresses: z.mohamadnia@iasbs.ac.ir (Z. Mohamadnia), albert.poater@udg.edu (A. Poater), n.bahri@ippi.ac.ir (N. Bahri-Laleh).
https://doi.org/10.1016/j.mcat.2021.111636
Received 3 April 2021; Received in revised form 7 May 2021; Accepted 8 May 2021
Available online 28 May 2021
2468-8231/© 2021 The Authors.
Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

S. Gharajedaghi et al.
Molecular Catalysis 509 (2021) 111636
Scheme 1. Titanium-based catalysts C1–C4 (in blue the bridge).
titanium complex containing pendant thienyl group and used it in
ethylene trimerization. In the reported results, they affirmed the
important role of thiophene in ethylene trimerization [35]. Cp-based
ligands have been widely studied as important ligands in organome-
tallic chemistry and most studies on Cp modification can be focused on
the type of the bearing pendant group on it. In this regard, in 2004,
Huang et al. used half-sandwich titanium complexes with the pendant
ethereal group activated by MAO for ethylene trimerization [36]. In
2013, Zhang et al. synthesized half-sandwich indenyl-based titanium
complexes [Ind-(bridge)-Ar]TiCl3 bearing pendant arene group on the
indenyl ring and examined the selective ethylene trimerization in the
presence of MAO co-catalyst [37–39]. In the following of previous
works, Zhang et al. synthesized another series of half-sandwich inden-
yl-based titanium complexes with the thienyl group (Cp(Ind)-bridge--
thienyl]TiCl3, which showed high selectivity in ethylene trimerization
and its conversion to 1-hexene [23]. In 2015, Varga et al. synthesized
and characterized two titanium-based heterogeneous catalysts using
different methods including grafting through a covalent Ti-O-Si bond as
well as through a pendant flexible tether from the Cp ligand [40]. The
catalyst synthesized by the second method did not show any activity in
ethylene trimerization because the active species were too close to the
support surface [40]. In 2015, Duchateau et al. synthesized different
types of phenoxy-imine titanium catalyst according to the Fujita method
and examined homogeneous and heterogeneous types [41]. Despite the
high activity and selectivity of the synthesized titanium catalysts, very
little polyethylene was produced as a by-product. In this work, an
attempt was also made to stabilize the catalyst and prevent the forma-
tion of the polymer, while maintaining the desired catalyst activity and
selectivity. In this line, MAO co-catalyst and phenoxy-imine titanium
catalysts were stabilized using a two-step process on silica carrier [41].
In 2019, Mohamadnia et al. successfully synthesized and identified three
titanium-based catalysts {[ƞ5-C9H6-C(R)]-C4H3S}TiCl3 with different
bridges (cyclohexane, cyclopentane and dimethyl) active in ethylene
trimerization [29]. Factors affecting the catalyst activity in the pro-
duction of 1-hexene including catalyst concentration, ethylene pressure,
and reaction temperature were optimized [29].
In the following of our research on the selective trimerization of
ethylene, due to the high importance of α-olefins in the petrochemical
industry, here a series of indenyl half-sandwich titanium complexes,
namely [Ind-C(R)-phenyl]TiCl3 was synthesized for possible application
in ethylene trimerization process. The main aim is to fulfill the process at
mild operating conditions such as low pressure and temperature to
reduce operator-related risks, and to reach high economic efficiency by
reducing catalyst consumption. In this regard, the effect of the arene
bridge and indenyl ring, temperature, ethylene pressure, and MAO and
catalyst concentrations on the catalytic efficiency was investigated.
Furthermore, the effect of ligand type on the 1-hexene selectivity was
also examined using DFT simulations by considering the energy path for
each catalyst (C1-C4, see Scheme 1), during ethylene oligomerization
process [42–44].
Experimental
Materials
All manipulations of water- and/or air-sensitive compounds were
performed using standard Schlenk and glove-box techniques under
deoxygenated argon or nitrogen. The modified MAO (MMAO) co-
Scheme 2. General procedure for synthesis of benzofulvene precursors, ligands and corresponding titanium-based catalysts.
2

S. Gharajedaghi et al.
Molecular Catalysis 509 (2021) 111636
catalyst (7 wt% in toluene), titanium tetrachloride (TiCl4), n-butyl-
lithium (n-BuLi; 2.5 M in n-hexane), phenyllithium (1.5 M in dibutyl
ether), and molecular sieve were obtained from Aldrich (Germany).
Indene, pyrrolidine, 4-tert-butylcyclohexanone, cyclohexanone, cyclo-
heptanone, 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, respectively. Methanol, n-hex-
ane, toluene and diethyl ether were dried and vacuum-distilled using
calcium hydride (CaH2) and sodium metal consecutively before use.
NMR (400 MHz, CDCl3, δ, ppm): 2.31 (3H, s, C9H6-C(CH3)2), 2.54 (3H, s,
C9H6-C(CH3)2), 6.84 (1H, d, C9H6-C(CH3)2), 6.91 (1H, d, C9H6-C(CH3)2),
7.25–7.32 (2H, m, C9H6-C(CH3)2), 7.41 (1H, d, C9H6-C(CH3)2), 7.82
(1H, d, C9H6-C(CH3)2). 13C NMR (100 MHz, CDCl3, δ, ppm): 22.84,
25.00 (CH3), 121.03, 123.52, 124.71, 126.02, 127.61, 128.35 (CH),
135.74, 136.70, 143.38, 143.97 (Cq). FT-IR (KBr, υmax, cmꢀ 1): 3600
(adsorbed water), 3030–3090 (sp2 C-H), 2913 and 2854 (sp3 C-H),
1930–1780 (overtone of aromatic ring), 1630 (C=C), 1450 (CH2), 727
and 750 (=C-H) (Figs. S4–S6).
Characterization of C9H6-C(4-tBu-cyclo-C5H9) (F3)
1
Characterization of ligands, catalysts, trimerization products
Fulvene (C19H24, F3) was obtained as yellow oil in 75% yield. H
NMR (400 MHz, CDCl3, δ, ppm): 0.9 (9H, s, C9H6-C(4-tBu-cyclo-C5H9)),
1.4 (3H, m, C9H6-C(4-tBu-cyclo-C5H9)), 2.2 (2H, t, C9H6-C(4-tBu-cyclo-
C5H9)), 2.4 (2H, m, C9H6-C(4-tBu-cyclo-C5H9)), 3.2 (1H, d, C9H6-C(4-
tBu-cyclo-C5H9)), 3.8 (1H, d, C9H6-C(4-tBu-cyclo-C5H9)), 6.86 (1H, d,
C9H6-C(4-tBu-cyclo-C5H9)), 6.96 (1H, d, C9H6-C(4-tBu-cyclo-C5H9)),
7.24-7.30 (2H, m, C9H6-C(4-tBu-cyclo-C5H9)), 7.40 (1H, d, C9H6-C(4-
tBu-cyclo-C5H9)), 7.95 (1H, d, C9H6-C(4-tBu-cyclo-C5H9)). 13C NMR (100
MHz, CDCl3, δ, ppm): 27.6 (CH3), 28.1, 29.3, 31.5, 33.8 (CH2), 47.5,
121.4, 124, 124.8, 126.1, 127.5, 128.3 (CH), 32.8, 133.6, 136, 144.8,
152.1 (Cq). FT-IR (KBr, υmax, cmꢀ 1): 3040–3090 (sp2 C-H), 2870 and
2956 (sp3 C-H), 1940–1780 (overtone of aromatic ring), 1627 (C=C),
1446 (CH2), 727 and 748 (=C-H) (Figs. S7–S9).
The 1H NMR and 13C NMR spectra have been recorded by the Bruker
400 MHz Ultra shield NMR instrument (Germany) at room temperature.
The progress of the catalyst synthesis and trimerization reactions was
followed by thin-layer chromatography (TLC) and gas chromatography
GC system (Varian CP 3800), respectively. The inductively coupled
plasma analysis (ICP), model 3410 ARL made in Switzerland, was used
to determine the metal components of the catalyst. The UV-visible
spectrophotometer (Pharmacia Biotech Ultrospec 4000) was used to
further examine the spectral characteristics of synthetic complexes.
Elemental analysis was performed using a Vario EL III CHNS elemental
analyzer.
General procedure for the synthesis of benzofulvene precursors C9H6-C(R)
(R=cyclo-C5H10 (F1), (CH3)2 (F2), 4-tBu-cyclo-C5H9 (F3), cyclo-C6H12
(F4))
Characterization of C9H6-C(cyclo-C6H12) (F4)
1
Fulvene (C16H18, F4) was obtained as yellow oil in 75% yield. H
NMR (400 MHz, CDCl3, δ, ppm): 1.68 (4H, m, C9H6-C(cyclo-C6H12)),
1.86 (2H, m, C9H6-C(cyclo-C6H12)), 1.97 (2H, m, C9H6-C(cyclo-C6H12)),
2.93 (2H, t, C9H6-C(cyclo-C6H12)), 3.17 (2H, t, C9H6-C(cyclo-C6H12)),
6.87 (1H, d, C9H6-C(cyclo-C6H12)), 6.95 (1H, d, C9H6-C(cyclo-C6H12)),
7.25–7.32 (2H, m, C9H6-C(cyclo-C6H12)), 7.41 (1H, d, C9H6-C(cyclo-
C6H12)), 7.81 (1H, d, C9H6-C(cyclo-C6H12)). 13C NMR (100 MHz, CDCl3,
δ, ppm): 26.2, 28.6, 28.8, 29.5, 34.5, 34.9 (CH2), 121.2, 123.6, 124.7,
The different fulvene precursors F1–F4 were synthesized with a
slight change according to the method proposed by Stone and Little
(Scheme 2) [45]. For this purpose, freshly distilled indene (5 mmol,
0.58 mL) and freshly distilled pyrrolidine (3 mmol, 0.25 mL) were dis-
solved in 2 mL of methanol under argon atmosphere at ambient tem-
perature. Then, different ketones)2 mmol(, such as cyclohexanone,
cycloheptanone, 4-t-butyl cyclohexanone, and acetone, were added
dropwise to the stirred solution and the reaction mixture was stirred for
12 h. At the end of the reaction, acetic acid (3 mmol, 0.18 mL) was added
to neutralize the residual base, and dilution was performed with diethyl
ether (10 mL). To separate the remaining indene, and other unreacted
materials, extraction was performed with deionized water (3 × 10 mL)
followed by brine (2 × 10 mL). Finally, the water remaining in the
organic phase was dried by anhydrous MgSO4. Synthesized fulvenes
were purified using column chromatography by silica gel (petroleum
ether as eluent). The pure fulvenes were characterized using 1H NMR,
13C NMR, and FT-IR spectroscopies.
124.9, 126, 127.2 (CH), 128.3, 136.1, 144.1, 154 (Cq). FT-IR (KBr, υmax
,
cmꢀ 1): 3010–3090 (sp2 C-H), 2852 and 2921 (sp3 C-H), 1790-1940
(overtone of aromatic ring), 1627 (C=C), 1448 (CH2), 721 and 750
(=C-H) (Figs. S10–S12).
General procedure for synthesis of ligands [C9H6-C(R)]-C6H5 (R=cyclo-
C5H10 (L1), (CH3)2 (L2), 4-tBu-cyclo-C5H9 (L3), cyclo-C6H12 (L4))
Indenyl-based ligands were prepared according to a slightly modified
literature method (Scheme 2) [26]. Solution of synthetic fulvenes de-
rivatives (0.5 mmol) in diethyl ether (3 mL) was added dropwise to the
phenyllithium solution in dibuthyl ether (2 mmol, 1.3 mL, 1.9 M) in 5
mL of dry diethyl ether under argon atmosphere at -40 ◦C. The mixture
was stirred at room temperature for 12 h. After one day, the reaction
mixture was hydrolyzed by 10 mL of cold water. The aqueous layer was
extracted with light petroleum ether (three times), and the organic layer
was dried with anhydrous MgSO4. The solvent was removed under
vacuum. The obtained L1–L4 ligands were purified using column chro-
matography via petroleum ether as eluent.
Characterization of C9H6-C(cyclo-C5H10) (F1)
Fulvene (C15H16, F1) was obtained as white crystals in 70% yield. 1H
NMR (400 MHz, CDCl3, δ, ppm): 1.70–1.79 (2H, m, C9H6-C(cyclo-
C5H10)), 1.79–1.9 (4H, m, C9H6-C(cyclo-C5H10)), 2.75 (2H, t, C9H6-C
(cyclo-C5H10)), 3.05 (2H, t, C9H6-C(cyclo-C5H10)), 6.79 (1H, d, C9H6-C
(cyclo-C5H10)), 6.94 (1H, d, C9H6-C(cyclo-C5H10)), 7.17–7.27 (2H, m,
C9H6-C(cyclo-C5H10)), 7.37 (1H, d, C9H6-C(cyclo-C5H10)), 7.9 (1H, d,
C9H6-C(cyclo-C5H10)). 13C NMR (100 MHz, CDCl3, δ, ppm): 26.40,
28.10, 28.76, 32.29, 34.48 (CH2), 121.05, 123.73, 124.64, 126.01,
127.30, 128.25 (CH), 133.65, 135.87, 144.56, 152.46 (Cq). FT-IR (KBr,
υmax, cmꢀ 1): 3010, 3014 and 3064 (sp2 C-H), 2848 and 2921 (sp3 C-H),
1780–1930 (overtone of aromatic ring), 1619 (C=C), 1443 (CH2), 723
and 740 (=C-H) (Figs. S1–S3).
Characterization of [C9H7-C(cyclo-C5H10)]-C6H5 ligand (L1)
Ligand (C21H22, L1) was obtained as a white solid in 91% yield. 1H
NMR (400 MHz, CDCl3, δ, ppm): 1.4–1.55 (1H, m, [C9H7-C(cyclo-
C5H10)]-C6H5), 1.51–1.66 (5H, m, [C9H7-C(cyclo-C5H10)]-C6H5),
2.23–2.25 (2H, m, [C9H7-C(cyclo-C5H10)]-C6H5), 2.39–2.42 (2H, m,
[C9H7-C(cyclo-C5H10)]-C6H5), 3.44 (2H, d, [C9H7-C(cyclo-C5H10)]-
C6H5), 6.56 (1H, t, [C9H7-C(cyclo-C5H10)]-C6H5), 7.02–7.1 (3H, m,
[C9H7-C(cyclo-C5H10)]-C6H5), 7.15–7.21 (1H, m, [C9H7-C(cyclo-C5H10)]-
C6H5), 7.28–7.31 (2H, m, [C9H7-C(cyclo-C5H10)]-C6H5), 7.4–7.49 (3H,
Characterization of C9H6-C(CH3)2 (F2)
Fulvene (C12H12, F2) was obtained as a yellow oil in 70% yield. 1H
3

S. Gharajedaghi et al.
Molecular Catalysis 509 (2021) 111636
m, [C9H7-C(cyclo-C5H10)]-C6H5). 13C NMR (100 MHz, CDCl3, δ, ppm):
23.03, 26.61, 36.46, 37.48 (CH2), 44.55 [C9H7-C(cyclo-C5H10)]-C6H5,
122.35, 123.64, 123.86, 125.35, 125.67, 127.06, 128.13, 129.61 (CH),
143.93, 145.22, 147.38, 149.96 (Cq). FT-IR (KBr, υmax, cmꢀ 1): 2974 (sp2
C-H), 2933 and 2921 (sp3 C-H), 1610 (C=C), 1461 (CH2bending),
700–800 (=C-H) (Figs. S13–S15).
temperature, white milky salt was obtained. Then, 0.5 mmol (0.05 mL)
of TiCl4 was added to the reaction mixture at -78 ◦C. As soon as the TiCl4
was added, a dark red solution was obtained. The liver red solution was
maintained at room temperature for 12 h. The unreacted solvent and
TiCl4 were then removed by vacuum and the residue were dissolved in
10 mL of dry petroleum ether and then centrifuged. After cooling the
◦
solution containing the catalyst to -20 C, the catalysts were obtained
(Scheme 2). Catalyst C1 was obtained as dark red crystals in 70% yield.
Elemental analysis (%): calculated for C21H21TiCl3 (found): C 58.99
(59.30), H 4.95 (5.12), Ti 11.19 (10.53), Cl 24.87 (—). Catalyst C2 was
obtained as dark red solid in 65 % yield. Elemental analysis (%):
calculated for C18H17TiCl3 (found): C 55.79 (54.90), H 4.42 (4.40), Ti
12.35 (11.95), Cl 27.44 (—). Catalyst C3 was obtained as dark red solid
in 68% yield. Elemental analysis (%): calculated for C25H29TiCl3
(found): C 62.08 (62.22), H 6.04 (6.11), Ti 9.90 (9.12), Cl 21.99 (—).
Catalyst C4 was obtained as dark red solid in 73% yield. Elemental
analysis (%): calculated for C22H23TiCl3 (found): C 59.83 (59.92), H
5.25 (5.22), Ti 10.84 (9.93), Cl 24.08 (—).
Characterization of [C9H7-C(CH3)2]-C6H5 ligand (L2)
1
Ligand (C18H18, L2) was obtained as a yellow oil in 90% yield. H
NMR (400 MHz, CDCl3, δ, ppm): 1.75 (6H, s, [C9H7-C(CH3)2]-C6H5),
3.47 (2H, d, [C9H7-C(CH3)2]-C6H5), 6.54 (1H, t, [C9H7-C(CH3)2]-C6H5),
6.73–6.78 (1H, d, [C9H7-C(CH3)2]-C6H5), 7.01–7.08 (1H, t, [C9H7-C
(CH3)2]-C6H5), 7.10–7.17 (1H, t, [C9H7-C(CH3)2]-C6H5), 7.20–7.26 (1H,
t, [C9H7-C(CH3)2]-C6H5), 7.27–7.34 (2H, t, [C9H7-C(CH3)2]-C6H5),
7.35–7.42 (2H, d, [C9H7-C(CH3)2]-C6H5), 7.48–7.52 (1H, d, [C9H7-C
(CH3)2]-C6H5). 13C NMR (100 MHz, CDCl3, δ, ppm): 29.65 (CH3)2, 37.8
(CH2), 40.5 [C9H7-C(CH3)2]-C6H5, 122.34, 123.70, 123.96, 125.47,
125.82, 126.21, 127.42, 128.29 (CH), 143.79, 145.28, 148.15, 152.35
(Cq). FT-IR (KBr, υmax, cmꢀ 1): 3072 (sp2 C-H), 2800–3000 (sp3 C-H),
1677 (C=C), 1465 (CH2bending), 780 (=C-H) (Figs. S16–S18).
Trimerization of ethylene
The ethylene trimerization reactions were performed using synthetic
titanium catalysts in a pressurized steel reactor equipped with a me-
chanical stirrer. The pressure and temperature inside the reactor and the
mixer speed were controlled by the reactor’s digital displays. In this
regard, the reactor was first purged with dried pure argon at 120 ◦C for 2
h. It was reached the desired temperature and the solvent and MMAO
were injected subsequently. After 10 min, the solution containing
catalyst/solvent was injected and trimerization was started by charging
the reactor with ethylene monomer. The reaction temperature and
ethylene pressure were fixed constant throughout the process. After 30
min, the reactor was cooled to -10 ◦C. Then, the liquid phase including,
1-C6 and probable by-products were collected and analyzed using GC
instrument. The produced polyethylene by-product was also washed
with acidified ethanol (3% HCl), and dried under vacuum at 60 ◦C to a
constant weight.
Characterization of [C9H7-C(4-tBu-cyclo-C5H9)]-C6H5 ligand (L3)
Ligand (C25H30, L3) was obtained as a white solid in 92% yield. 1H
NMR (400 MHz, CDCl3, δ, ppm): 0.8 (9H, s, [C9H7-C(4-tBu-cyclo-C5H9)]-
C6H5), 1.1–1.2 (1H, m, [C9H7-C(4-tBu-cyclo-C5H9)]-C6H5), 1.3–1.53
(2H, m, [C9H7-C(4-tBu-cyclo-C5H9)]-C6H5), 1.6–1.80 (2H, t, [C9H7-C(4-
tBu-cyclo-C5H9)]-C6H5), 1.86–2.07 (2H, t, [C9H7-C(4-tBu-cyclo-C5H9)]-
C6H5), 2.58–2.85 (2H, m, [C9H7-C(4-tBu-cyclo-C5H9)]-C6H5), 3.5 (2H, d,
[C9H7-C(4-tBu-cyclo-C6H9)]-C6H5)), 6.63–6.79 (1H, t, [C9H7-C(4-tBu-
cyclo-C5H9)]-C6H5), 6.9–7.2 (4H, m, [C9H7-C(4-tBu-cyclo-C6H9)]-C6H5),
7.2–7.34 (2H, m, [C9H7-C(4-tBu-cyclo-C6H9)]-C6H5), 7.3-7.5 (3H, m,
[C9H7-C(4-tBu-cyclo-C5H9)]-C6H5). 13C NMR (100 MHz, CDCl3, δ, ppm):
24.1 (CH2), 27.5 [C9H7-C(4-tBu-cyclo-C5H9)]-C6H5, 32.5, 36.9 (CH2),
37.7 (CH2), 44.5, 48 [C9H7-C(4-tBu-cyclo-C5H9)]-C6H5, 122.5, 123.5,
123.9, 125.3, 125.7, 126.2, 128.2, 131.5 (CH), 144.1, 145.1, 147.5,
148.9 (Cq). FT-IR (KBr, υmax, cmꢀ 1): 3010–3085 (sp2 C-H), 2974 (sp3 C-
H), 1606 (C=C),1458 (CH2bending), 700–800 (=C-H) (Figs. S19–S21).
Computational details
DFT static calculations were performed at B3LYP level [46] with the
Gaussian16 package [47]. The electronic configuration of the system
was described with the standard split valence basis set with a polariza-
tion function for all the atoms (def2SVP keyword in Gaussian) of Ahl-
richs and co-workers [48]. Geometry optimizations were performed
without symmetry constrain, and analytical frequency calculations
performed the characterization of the local stationary points. These
frequencies were used to calculate unscaled zero-point energies as well
as thermal corrections and entropy effects at 298 K and 1 atm. The
transition states were located using the synchronous transit-guided
quasi-Newton (QST3) approach and the extrema have been checked
by analytical frequency calculations. All transition states have associ-
ated only one imaginary frequency. Solvent effects were estimated in
single point energy calculations on the gas phase optimized structures
based on the polarizable continuous solvation model (PCM) [49], as
implemented in Gaussian16, using toluene (Tol) as a solvent. Energies
were obtained using the B3LYP functional [46], in conjunction with the
triple-ζ basis set cc-pVTZ for all the atoms [50], together with the
Grimme D3 correction term [51] to the electronic energy. The reported
free energies in this work include energies obtained at the
B3LYP/cc-pVTZ level of theory corrected with zero-point energies,
thermal corrections and entropy effects evaluated at 298 K, achieved at
the B3LYP/def2SVP level, without translational entropy corrections
[52].
Characterization of [C9H7-C(cyclo-C6H12)]-C6H5 ligand (L4)
Ligand (C22H24, L4) was obtained as a white solid in 90% yield. 1H
NMR (400 MHz, CDCl3, δ, ppm): 1.57–1.92 (8H, m, [C9H7-C(cyclo-
C6H12)]-C6H5), 2.24–2.50 (4H, m, [C9H7-C(cyclo-C6H12)]-C6H5), 3.48
(2H, d, [C9H7-C(cyclo-C6H12)]-C6H5), 6.51–6.55 (1H, t, [C9H7-C(cyclo-
C6H12)]-C6H5), 6.75–6.79 (1H, d, [C9H7-C(cyclo-C6H12)]-C6H5),
6.98–7.01 (1H, t, [C9H7-C(cyclo-C6H12)]-C6H5), 7.07–7.10 (1H, t, [C9H7-
C(cyclo-C6H12)]-C6H5), 7.15–7.19 (1H, t, [C9H7-C(cyclo-C6H12)]-C6H5),
7.25–7.31 (2H, t, [C9H7-C(cyclo-C6H12)]-C6H5), 7.36–7.38 (2H, d,
[C9H7-C(cyclo-C6H12)]-C6H5), 7.43–7.54 (1H, d, [C9H7-C(cyclo-C6H12)]-
C6H5). 13C NMR (100 MHz, CDCl3, δ, ppm): 24.07, 31.63, 37.36, 39.11
(CH2), 47.80 [C9H7-C(cyclo-C6H12)]-C6H5, 122.5, 123.58, 123.87,
125.37, 125.57, 126.69, 127.49, 128.22 (CH), 144.09, 145.27, 148.77,
151.83 (Cq). FT-IR (KBr, υmax, cmꢀ 1): 3025 (CHaromatic, sp2 C-H), 2971
and 2861 (CHaliphatic, sp3 C-H), 1610 (C=C), 1467 (CH2bending), 792 (=C-
H) (Figs. S22–S24).
Synthesis and characterization of [C9H6-C(R)-C6H5]TiCl3 complexes with
various types of bridges (R=cyclo-C5H10 (C1), (CH3)2 (C2), 4-tBu-cyclo-
C5H9 (C3), and cyclo-C6H12 (C4))
For the synthesis of the desired catalysts, ligands L1–L4 (0.5 mmol)
in dry petroleum ether (5 mL), was added dropwise to a solution of the
0.5 mmol n-BuLi (0.2 mL, 2.5 M) solution in petroleum ether at -70 ◦C
under an argon atmosphere. After mixing for four hours at the same
4

S. Gharajedaghi et al.
Molecular Catalysis 509 (2021) 111636
1.6
1.2
0.8
0.4
0.0
C1
L1
200
300
400
500
600
700
800
Wavelength (nm)
Fig. 1. Comparison between UV–visible absorption spectra of L1 ligand and corresponding catalyst (C1)
Scheme 3. Proposed mechanism for the key role of MAO in catalyst activation.
[55], and in agreement the relative stability of the complex increases.
Results and discussion
According to the valence bond theory, orbitals of the valence layer of the
central atom in the tetrahedral structure are hybridized either as sp3 or
as d3s [29]. In our synthesized titanium-based catalysts as well as TiCl4,
the electron arrangement of the central atom is d0. As shown in Fig. 1,
due to the d0 spin of Ti atom in the synthesized catalysts, d→d transi-
tions were not observed for the studied complexes. Actually, the peaks
Characterization of catalysts using UV–visible spectroscopy
In the structure of synthetic catalysts, [C9H6-C(R)-C6H5]TiCl3, due to
the presence of halide groups and bulky indenyl moiety, the coordina-
tion number of Ti is IV [53,54]. In general, for this group of complexes,
tetrahedral and square-planar structures are observed. Due to the pres-
ence of bulky ligands in the structure and electrostatic repulsion be-
tween them, the complex tends to have a deviated tetrahedral structure.
For the latter geometry, the steric hindrance has the minimum value
observed at 260–280 nm were related to π→π (intra-ligand) transitions.
In addition to this type of transition, the ligand-to-metal transition at
about 240 nm was also detected. However, due to the increase in the
length of the resonance system, the intra-ligand transition in all catalysts
Table 1
Ethylene trimerization results with C2 catalyst.
a
Entry
Al/Ti
T (˚C)
Ethylene pressure (bar)
1-C6 wt%
1-Hexene (g)
1-Octene (g)
PE (g)
Activity (kg 1-C6.molTiꢀ 1.hꢀ 1
)
Selectivity (wt.% of 1-C6)b
1
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
500
20
20
20
20
40
60
80
40
40
40
40
40
40
3
1.20
2.46
5.10
1.46
10.34
11.22
2.06
3.91
6.86
2.01
1.80
4.10
9.80
0.208
0.426
0.884
0.253
1.792
1.945
0.357
0.676
1.190
0.348
0.311
0.711
1.699
0.004
0.007
0.011
0.004
0.040
0.034
0.001
0.008
0.017
0.001
0.001
0.001
0.030
0.03
0.14
0.30
5.60
0.43
0.49
0.10
0.10
0.25
5.70
0.003
Trace
0.38
277
85.95
74.34
73.98
4.32
2
5
568
3
8
1179
337
4
12
8
5
2389
2593
476
79.36
78.77
77.95
86.22
81.67
5.76
6
8
7
8
8
3
901
9
5
1586
465
10
11
12
13
12
8
416
98.73
99.85
80.56
1000
1500
8
948
8
2265
a
b
Time = 30 min, rpm = 1100, solvent = toluene (20 mL).
Percentage of 1-C6 overall by mass.
5

S. Gharajedaghi et al.
Molecular Catalysis 509 (2021) 111636
Selectivity of 1-C6 (wt.%)
Productivity kg 1-C6/mol-Ti h
(
)
100
80
60
40
20
0
3000
2500
2000
1500
1000
500
0
20
40
60
80
Temperature (0C)
Fig. 2. Effect of reaction temperature on C2 catalyst productivity; Al/Ti= 2000, time= 30 min, rpm= 600, solvent =toluene (20 mL), catalyst concentration= 1.5
μ
mol, ethylene pressure= 8 bar.
Selectivity of 1-C6 (wt.%)
Productivity (kg 1-C6/mol-Ti h)
100
80
60
40
20
0
3000
2500
2000
1500
1000
500
0
5
12
8
3
Ethylene Pressure (bar)
Fig. 3. Effect of ethylene pressure on the catalyst productivity; Al/Ti= 2000, time= 30 min, rpm= 600, solvent = toluene (20 mL), catalyst (C2) concentration= 1.5
μ
mol, temperature= 40 ◦C.
has shifted to higher wavelengths (Figs. S25–S27).
performed at four temperatures of 20, 40, 60, and 80 ◦C with 1.5
μmol of
C2 catalyst solution, and 8 bar ethylene pressure. From Fig. 2, it is clear
that for reaction temperature there is an optimal value of 60 ◦C, in which
catalyst activity and 1-C6 selectivity have their maximum amount of
2593 kg 1-C6/mol-Ti h and 78.77%, respectively. It was reported that
the pendant ring in the complex structure can coordinate easily to the
electron deficient cationic Ti species due to the higher catalyst flexibility
at elevated temperatures which subsequently causes a substantial
decrease in the catalyst activity [33,58,59].
Ethylene trimerization results using synthesized catalysts (C1–C4)
The ethylene trimerization reaction using the as-synthesized tita-
nium-based catalysts activated by MMAO afforded 1-hexene and some
by-products. The key role of co-catalyst, between other functions such as
removal of oxygen and water and eliminating environmental pollution,
is to reduce the oxidation state of titanium metal and to facilitate the
production of cationic species [56]. In general, co-catalyst facilitates
alkyl abstraction from the catalyst pioneer to yield an anionic co-catalyst
species [RXꢀ ] and a cationic metal species [LnM+], which together
represent the active catalytic system as an ion pair with [LnM+][RXꢀ ]
formula (Scheme 3) [54]. The MMAO-activated system was highly
active and selective in ethylene trimerization reaction [57]. In fact, an
analysis of the liquid fraction by GC disclosed that ethylene trimeriza-
tion via catalyst/MMAO under different position achieved 1-hexene
with high selectivity.
Due to the negligible productivity difference in the productivity
values at the reaction temperatures of 40 ◦C and 60 ◦C (2389 and 2593
kg 1-C6/mol-Ti h, respectively), the temperature of 40 ◦C was chosen for
the next studies. In the next step, effect of ethylene pressure was taken
into account. According to Fig. 3, increasing the ethylene pressure from
3 to 8 bar, the activity of the C2 catalyst increased due to the enhanced
solubility of ethylene gas at higher pressures. However, after P = 8 bar,
the reaction switched to ethylene polymerization, so that at P = 12 bar,
the activity decreased to 337, and 465 kg 1-C6/mol-Ti h, at the T = 20
and 40 ◦C (entries 4 and 10, Table 1), respectively. Therefore, the next
experiments were conducted at P = 8 bar as the optimum ethylene
pressure value.
The results of ethylene oligomerization using the C2 catalyst are
shown in Table 1. First, the effect of reaction temperature on catalyst
activity was investigated (entries 3, 5-7). To do this, the reaction was
6

S. Gharajedaghi et al.
Molecular Catalysis 509 (2021) 111636
Selectivity of 1-C6 ( wt.% )
Productivity (kg 1-C6/mol-Ti h
)
100
80
60
40
20
0
1400
1200
1000
800
600
400
200
0
3
5
12
8
Ethylene Pressure ( bar )
Fig. 4. Effect of ethylene pressure on catalyst productivity; Al/Ti= 2000, time= 30 min, rpm= 600, solvent = toluene (20 mL), catalyst (C2) concentration= 1.5
μ
mol, temperature= 20 ◦
C
Selectivity of 1-hexene(wt.%)
Productivity
(kg 1-C6/mol-Ti h)
3000
2500
2000
1500
1000
500
100
80
60
40
20
0
0
500
1000
1500
2000
Al/Ti
Fig. 5. Effect of ethylene pressure on catalyst productivity; time= 30 min, rpm= 600, solvent= toluene (20 mL) catalyst (C2) concentration= 1.5
ature= 40 ◦C, ethylene pressure= 8 bar
μmol, temper-
Worth mentioning, effect of ethylene pressure on C2 catalyst activity
and 1-C6 selectivity was considered at T = 20 ◦C, as well (entries 1–4,
Table 1 and Fig. 4). Notably, in this condition the same trend as it at
T=40◦C was observed. Indeed, the highest catalyst activity and 1-C6
selectivity of 1179 kg 1-C6.mol Tiꢀ 1.hꢀ 1 and 74% were obtained at P = 8
bar (entry 3) after which the process was switched to the polymerization
reaction with catalyst activity of 1294 kg PE/mol-Ti h.
activity decreased due to the poisoning of catalytic sites. Therefore, Al/
Ti=2000 molar ratio was selected as the desired value, due to high ac-
tivity of C2 catalyst toward 1-C6 formation.
According to the results, C2, pressure of 8 bar, T = 40 ◦C and catalyst
dosage of 1.5 μmol were selected as the optimum reaction conditions for
achieving high catalytic activity and 1-hexene selectivity. Finally, the
effect of bridge type on the catalytic efficiency was elucidate. According
to the previous studies conducted in these catalyst systems, there is no
coordination between dangling arene (phenyl) and Ti metal at the pri-
mary precatalyst. However, after activation with the MAO co-catalyst, it
happens easily which facilitates the formation of 1-C6 product [26,60].
In the following, effect of Al/Ti molar ratio on the C2 catalyst activity
and selectivity was investigated, Fig. 5. Obviously, by increasing Al/Ti
molar ratio up to 2000, catalyst activity increases due to the formation of
more catalytic active sites. As the Al/Ti ratio raised above 2000, the
Table 2
Ethylene trimerization results with titanium-based catalysts (C1–C4).
Catalyst
Selectivity (wt.% of 1- C6)b with PE
Selectivity (wt.% of 1- C6) without PE
Activity (kg 1-C6/mol-Ti h)a
PE (g)
1-C8 (g)
1-C6 (g)
1-C6 (wt%)
C1
C2
C3
C4
80.30
79.35
77.94
30.25
98.14
97.76
97.85
99.2
3244
2389
2129
989
0.55
0.426
0.425
1.7
0.048
0.03
2.4328
1.792
14.03
10.34
9.21
0.035
0.006
1.5979
0.7421
4.28
a
b
Al/Ti = 2000, time = 30 min, rpm = 600, solvent = toluene (20 mL), catalyst concentration = 1.5
Percentage of 1-C6 overall by mass.
μ
mol, ethylene pressure =8 bar
7

S. Gharajedaghi et al.
Molecular Catalysis 509 (2021) 111636
Selectivity of 1-C6 ( wt.% )
Productivity
(
kg 1-C6/mol-Ti h )
100
80
60
40
20
0
4000
3500
3000
2500
2000
1500
1000
500
0
C1
C4
C2
C3
Fig. 6. Effect of various bridges on catalyst productivity; Al/Ti= 2000, time= 30 min, rpm= 600, solvent= toluene (20 mL), catalyst concentration= 1.5
temperature= 40 ◦C, ethylene pressure= 8 bar
μmol,
Scheme 4. General mechanism for ethylene trimerization via metallacycle intermediates.
The bridge between the indenyl and phenyl moieties has a significant
effect on the direction of phenyl ring relative to the titanium metal
center and the size of C(Ind)C(bridge)C(phenyl) angle. When the cata-
lyst is activated by the co-catalyst, the dangling phenyl will go toward
the cationic metal with a low oxidation number. Therefore, the type of
bridge and its size have a direct effect on the coordination of arene and
Ti. In the C1 with the bridge C(cyclo-C5H10), it has stronger coordination
than the same complex with the CMe2 bridge. Catalyst C4 has the most
space constraints since the pendant arene moiety has a very strong co-
ordination with Ti. Consequently, this can prevent ethylene from
approaching the metal active site. That is, the bridge has a dual effect on
catalyst efficiency. While the cycloheptane bridge leads to the largest
spatial hindrance and angular pressures, and thus low activity, the
catalyst with a cyclohexane bridge with stable chair structure, high
activity and selectivity. Table 2 shows the trimerization results for
C1–C4 catalysts (see Scheme 1 for the detail of the catalysts).
According to Table 2, it is noteworthy that, regardless of the polymer
produced in the trimerization, the selectivity for 1-hexene in all catalysts
was higher than 90%. However, considering the unwanted polymer by-
product, the selectivity of 1-hexene for all four catalysts decreased. 1-
hexene selectivity for catalyst C4 was obtained due to the production
of 1.7 g of polyethylene in the reaction of only 30% (Fig. 6).
8

S. Gharajedaghi et al.
Molecular Catalysis 509 (2021) 111636
possible that as a result of the ring-opening reaction 1-butene will be
formed through two different reaction pathways. The first mechanism is
the β-hydrogen transfer to Ti, forming a hydride, and the subsequent
elimination, the second mechanism is the transfer of intramolecular
β-hydrogen and the formation of M3, which means a reduction of the
metal center again to Ti(II), and the olefin is bonded to titanium. Since
1-butene is not formed, instead the reaction will lead to the formation of
M4 and the third ethylene molecule will be coordinated. After ethylene
insertion (M5), the reaction likely continues with a ring-opening (M6) to
finally produce 1-C6 (experimentally observed). However, a new
ethylene molecule may bond to titanium and M7 may be formed, leading
to the release of 1-octene or even higher olefins such as 1-decene.
However, the latter species was not observed experimentally. After
considering the general path shown in Scheme 4, the structure of the
catalysts was optimized. Since 1-hexene was observed experimentally as
the main product and 1-octene as the by-product, only the energy of
these steps was investigated for the four catalysts included in Fig. 5. The
energy diagram for ethylene oligomerization was then obtained for the
four catalysts. In summary, the catalysts can evolve via four main stages
of release of 1-butene, the formation of a seven-membered ring, the
release of 1-hexene, and the formation of a nine-membered ring.
The reaction mechanism proposed in Scheme 4 has been studied for
the four different catalysts C1–C4. In general, results in Fig. 7 are similar
for the different candidates. In all cases, the formation of the seven-
membered metallacycle (M5) through an insertion of a third molecule
of ethylene (T4) is more kinetically favorable than the release of a 1-
butene molecule through the intramolecular β-hydride transfer (T2B).
This difference is more significant in systems C2 (9.5 kcal•molꢀ 1) and
C4 (8.3 kcal•molꢀ 1) than in systems C1 (2.7 kcal•molꢀ 1) and C3 (3.2
kcal•molꢀ 1). These observations match with the experimental evidence
reported above, since it has not been observed any release of 1-butene
molecules. After the formation of the seven-membered ring (M5) the
mechanism proposes both the insertion of a fourth ethylene molecule
subsequently forming a nine-membered metallacycle (M8) or, the
release of a 1-hexane molecule through, again, an intramolecular β-hy-
dride elimination, leading to M6, being the latter, the winner not ther-
modynamically, but kinetically speaking. The transition state involving
the intramolecular migration of the β-hydrogen (T5) (see Fig. 8a) is thus
lower in energy than the transition state involving the formation of the
nine-membered metallacycle (T7) (see Fig. 8b). Matter of fact, in all
cases the expected product is the 1-hexene while 1-octane becomes only
a by-product. The catalysts showing better selectivity towards 1-hexene
release is the system C1 as the difference in energy is around 2.9
kcal•molꢀ 1, being C4 the one showing less selectivity as this difference
is only 0.8 kcal•molꢀ 1, with C2 and C3 in between, with values of 1.6
and 2.2 kcal•molꢀ 1, respectively. Again, these observations match with
the experimental data reported before.
Fig. 7. Gibbs free energy profile (kcal•molꢀ 1) for the ethylene oligomerization
by catalysts (a) C1, (b) C2, (c) C3 and (d) C4, relative to compound M0 and
considering toluene as a solvent.
According to the summarized data included in Table 3 (all the
optimized geometries and more relevant distances are collected in
Table S1 in the supporting information), the energy barrier of ΔETS for
the formation of the seven-membered ring is lower than for the release
of 1-butene. On the other hand, the energy barrier of ΔETS to release 1-
hexene is lower than for the formation of a nine-membered ring. Because
the amount of ΔE2 is less than that of ΔE1, the reaction tends to form 1-
hexene. Calculations for other catalysts are shown in Table 3, indicating
that the most feasible path is the formation of 1-hexene. The C4 catalyst
has a lower selectivity than other catalysts. Anyway, still the next kinetic
barrier leading to M9 is higher, and thus the 1-octene formation is even
more disfavored than the 1-hexene one, for the four catalysts C1-C4.
To evaluate the sterics among the series C1–C4 the %VBur of the rate
determining intermediate M5 was evaluated [62,63]. The values are
26.9, 25.4, 27.0 and 26.3%, respectively. Even though the cyclohexane
based systems C1 and C3 have a more hindered metal center [64], the
difference is scarce and not enough to describe any trend (see
Tables S28–S31 for further details) [65–67]. Nor can emphasis be placed
on weak interactions such as non-covalent ones [68], since for the four
Evaluation of ethylene oligomerization reaction using as-synthesized half-
sandwich titanium-based catalysts by applying density functional theory
(DFT)
In the last part of our study, to compare quantitatively the effect of
ligand type on the energy values of ethylene trimerization reaction path,
and shed light on the structural parameters, DFT calculations were
carried out. In this regard, the main responsible and effective steps in the
reaction pathways were considered (Scheme 4).
The catalyst activation steps by the MAO co-catalyst (Scheme 3) were
not considered and M0 is our starting active catalyst (see Scheme 4), in
which the titanium has an oxidation state II. Thus, it has a severe elec-
tron deficiency and tends to coordinate rapidly with the two ethylene
molecules leading to M1 (note that in the energy profile the insertion of
only one ethylene molecule (M1’) has also been considered) [61], from
which a five-membered metallacycle is formed (M2), switching the
corresponding oxidation of the metal center from Ti(II) to Ti(IV). It is
9

S. Gharajedaghi et al.
Molecular Catalysis 509 (2021) 111636
Fig. 8. Optimized geometries for the transition states T5 (a) and T7 (b) for catalyst C1 (main distances in Å).
Acknowledgments
Table 3
Summary of relative energies (in kcal•molꢀ 1) obtained for different stages of
ethylene oligomerization by C1-C4 titanium-based catalysts.
This work was supported by the Iran National Science Foundation
(INSF) through the grant number of 98020308. A.P. is a Serra Húnter
Fellow and ICREA Academia Prize 2019, and thanks the Spanish
MINECO for project PGC2018-097722-B-I00 and CTQ2017-85341-P. G.
P. gratefully acknowledges the support of Institut de Química Compu-
Catalyst name
C1
C2
C3
C4
1-Bu release
C7 ring formation
∆E1
24.1
21.4
2.7
24.9
15.4
9.5
18.9
15.7
3.2
23.7
15.4
8.3
`
tacional i Catalisi (IQCC) and the computer resources and technical
1-Hex release
16.7
19.6
2.9
16.7
18.3
1.6
12.0
14.2
2.2
18.1
18.9
0.8
support provided by the Barcelona Supercomputing Center (BSC).
C
9 ring formation
∆E2
1-Oct release
24.1
18.6
25.0
19.9
Supplementary materials
Supplementary material associated with this article can be found, in
the online version, at doi:10.1016/j.mcat.2021.111636.
catalysts the H-bonds are almost identical.
Conclusions
References
High selectivity and productivity of 1-hexene production was ob-
tained using the four titanium-based catalysts C1–C4, in good compar-
ison with recent work [69]. Ethylene trimerization was performed using
these synthetic catalysts with changes in MAO concentration, reaction
temperature, and ethylene pressure. Using the C2 catalyst, the concen-
[1] Y.K. Kim, J. Park, S. Yoon, S.B. Park, Selective ethylene trimerization with a
cyclopentadienyl-arene titanatrane catalyst, Transit. Met. Chem. 37 (2012)
439–444.
[2] A. Hanifpour, N. Bahri-Laleh, M. Nekoomanesh-Haghighi, A. Poater, Coordinative
chain transfer polymerization of 1-decene in the presence of a Ti-based diamine bis
(phenolate) catalyst: a sustainable approach to produce low viscosity PAOs, Green
Chem. 22 (2020) 4617–4626.
tration of 1.5 μm for the C2 catalyst, the selectivity, and activity of the
[3] Y.V. Kissin, Oligomerization reactions of 1-hexene with metallocene catalysts:
detailed data on reaction chemistry and kinetics, Mol. Catal. 463 (2019) 87–93.
[4] J. Skupinska, Oligomerization of a-olefins to higher oligomers, Chem. Rev. 91
(1991) 613–648.
three catalysts C1, C3, and C4 were also examined in this optimal con-
centration. The activity of all four catalysts increased with increasing
temperature to 60 ◦C, indicating the thermal resistance of synthetic
catalysts at high temperatures. Increasing ethylene pressure always in-
creases the solubility of ethylene and increases the activity, which was
also true for the synthesized titanium-based catalysts but, the C2 catalyst
moves from ethylene trimerization to ethylene polymerization at an
ethylene pressure of 12 bar at 20 and 40 ◦C.
[5] R. Sun, L. Liu, R.H. Cheng, Z. Liu, B.P. Liu, Effects of ether electron donors on Cr/
PNP-catalyzed ethylene selective oligomerization, J. Mol. Catal. 33 (2019)
103–112.
[6] A. Hanifpour, N. Bahri-Laleh, M. Nekoomanesh-Haghighi, A. Poater, Group IV
diamine bis(phenolate) catalysts for 1-decene oligomerization, Mol. Catal. 493
(2020), 111047.
[7] D.P. Solowey, M.V. Mane, T. Kurogi, P.J. Carroll, B.C. Manor, M.H. Baik, D.
J. Mindiola, A new and selective cycle for dehydrogenation of linear and cyclic
alkanes under mild conditions using a base metal, Nat. Chem. 9 (2017) 1126–1137.
[8] J. Ferreira, R. Zilz, I.S. Boeira, S.M. da Silva, A.C. Casagrande, O.L. Casagrande,
Chromium complexes based on thiophene–imine ligands for ethylene
oligomerization, Appl. Organomet. Chem. 33 (2019) e4697.
Declaration of Competing Interest
The authors state that “There are no conflicts to declare”.
[9] V.C. Gibson, S.K. Spitzmesser, Advances in non-metallocene olefin polymerization
catalysis, Chem. Rev. 103 (2003) 283–315.
[10] D.S. McGuinness, Olefin oligomerization via metallacycles: dimerization,
trimerization, tetramerization, and beyond, Chem. Rev. 111 (2011) 2321–2341.
10

S. Gharajedaghi et al.
Molecular Catalysis 509 (2021) 111636
[11] R.A. Hackler, G. Kang, G.C. Schatz, P.C. Stair, R.P. Van Duyne, Analysis of TiO2
atomic layer deposition surface chemistry and evidence of propene oligomerization
using surface-enhanced raman spectroscopy, J. Am. Chem. Soc. 141 (2018)
414–422.
[38] N. Bahri-Laleh, A. Hanifpour, S.A. Mirmohammadi, A. Poater, M. Nekoomanesh-
Haghighi, G. Talarico, L. Cavallo, Computational modeling of heterogeneous
Ziegler-Natta catalysts for olefins polymerization, Interaction of common
cocatalysts in Ziegler-Natta catalyzed olefin, Prog. Polym. Sci. 84 (2018) 89–114.
[39] M. Fallah, N. Bahri-Laleh, K. Didehban, A. Poater, Interaction of common
cocatalysts in Ziegler-Natta catalyzed olefin polymerization, Appl. Organomet.
Chem. 34 (2020) e5333.
´
[12] M. Rouen, P. Queval, E. Borre, L. Falivene, A. Poater, M. Berthod, F. Hugues,
´
L. Cavallo, O. Basle, H. Olivier-Bourbigou, M. Mauduit, Selective metathesis of
α
-Olefins from bio-sourced fischer–tropsch feeds, ACS Catal. 6 (2016) 7970–7976.
ˇ
ˇ
´ˇ
´
[13] I.A. Hussein, T. Hameed, B.F.A. Sharkh, K. Mezghani, Miscibility of hexene-LLDPE
and LDPE blends: influence of branch content and composition distribution,
Polymer 44 (2003) 4665–4672.
[40] V. Varga, T. Hodík, M. Lamac, M. Horacek, A. Zukal, N. Zilkova, W.O. Parker,
J. Pinkas, Homogeneous and heterogeneous cyclopentadienyl-arene titanium
catalysts for selective ethylene trimerization to 1-hexene, J. Organomet. Chem. 777
(2015) 57–66.
[14] I.A. Hussein, T. Hameed, Influence of branching characteristics on thermal and
mechanical properties of Ziegler–Natta and metallocene hexene linear low-density
polyethylene blends with low-density polyethylene, J. Appl. Polym. Sci. 97 (2005)
2488–2498.
[41] F.F. Karbach, J.R. Severn, R. Duchateau, Effect of aluminum alkyls on a
homogeneous and silica-supported phenoxy-imine titanium catalyst for ethylene
trimerization, ACS Catal. 5 (2015) 5068–5076.
[15] C. Bariashir, C. Huang, G.A. Solan, W.H. Sun, Recent advances in homogeneous
chromium catalyst design for ethylene tri-, tetra-, oligo- and polymerization,
Coord. Chem. Rev. 385 (2019) 208–229.
[42] A. Hanifpour, N. Bahri-Laleh, M. Nekoomanesh-Haghighi, A. Poater, 1-Decene
oligomerization by new complexes bearing diamine-diphenolates ligands: effect of
ligand structure, Appl. Organomet. Chem. 35 (2021) e6227.
[16] C. Wang, J.L. Huang, Catalytic trimerization of ethylene with highly active half-
sandwich titanium complexes bearing pendant p-fluorophenyl groups, Chin. J.
Chem. 24 (2006) 1397–1401.
[43] A. Jalali, M. Nekoomanesh-Haghighi, S. Dehghani, N. Bahri-Laleh, Group IV
diamine bis(phenolate) catalysts for 1-decene oligomerization, Appl. Organomet.
Chem. 34 (2020) e5338.
[17] M. Soheili, Z. Mohamadnia, B. Karimi, Switching from ethylene trimerization to
ethylene polymerization by chromium catalysts bearing sns tridentate ligands:
process optimization using response surface methodology, Catal. Lett. 148 (2018)
3685–3700.
[44] R. Lapenta, A. Buonerba, E. Luciano, F.D Monica, A. De Nisi, M. Monari, A. Grassi,
C. Capacchione, S. Milione, Phenylene-bridged OSSO-type titanium complexes in
the polymerization of ethylene and propylene, ACS Omega 3 (2018) 11608–11616.
[45] K.J. Stone, R.D. Little, An exceptionally simple and efficient method for the
preparation of a wide variety of fulvenes, J. Org. Chem. 49 (1984) 1849–1853.
[46] P.J. Stephens, F.J. Devlin, C.F. Chabalowski, M.J. Frisch, Ab Initio Calculation of
Vibrational Absorption and Circular Dichroism Spectra Using Density Functional
Force Fields, J. Phys. Chem. 98 (1994) 11623.
[18] D.S. McGuinness, P. Wasserscheid, W. Keim, C. Hu, U. Englert, J.T. Dixon,
C. Grove, Novel Cr-PNP complexes as catalysts for the trimerisation of ethylene,
Chem. Commun. (2003) 334–335.
[19] D.S. McGuinness, P. Wasserscheid, W. Keim, D. Morgan, J.T. Dixon, A. Bollmann,
H. Maumela, F. Hess, U. Englert, First Cr(III)ꢀ SNS complexes and their use as
highly efficient catalysts for the trimerization of ethylene to 1-hexene, J. Am.
Chem. Soc. 125 (2003) 5272–5273.
[47] M.J. Frisch, G.W. Trucks, H.B. Schlegel, et al., Gaussian 16, Revision C.01,
Gaussian, Inc., Wallingford CT, 2016.
¨
[48] A. Schafer, H. Horn, R. Ahlrichs, Fully optimized contracted Gaussian basis sets for
[20] M. Fallahi, E. Ahmadi, A. Ramazani, Z. Mohamadnia, Trimerization of ethylene
catalyzed by Cr-based catalyst immobilized on the supported ionic liquid phase,
J. Organomet. Chem. 848 (2017) 149–158.
atoms Li to Kr, J. Chem. Phys. 97 (1992) 2571–2577.
[49] V. Barone, M. Cossi, Quantum calculation of molecular energies and energy
gradients in solution by a conductor solvent model, J. Phys. Chem. A 102 (1998)
1995–2001.
[21] M. Marefat, E. Ahmadi, Z. Mohamadnia, Influence of ionic liquid counterions on
activity and selectivity of ethylene trimerization using chromium-based catalysts in
biphasic media, Appl. Organomet. Chem. 34 (2020) e5874.
[50] R.A. Kendall, T.H. Dunning, R.J. Harrison, Electron affinities of the first-row atoms
revisited. Systematic basis sets and wave functions, J. Chem. Phys. 96 (1992) 6796.
[51] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, A consistent and accurate ab initio
parametrization of density functional dispersion correction (DFT-D) for the 94
elements H-Pu, Chem. Phys. 132 (2010), 154104.
[22] R. Arteaga-Müller, H. Tsurugi, T. Saito, M. Yanagawa, S. Oda, K. Mashima, New
tantalum ligand-free catalyst system for highly selective trimerization of ethylene
affording 1-hexene: new evidence of a metallacycle mechanism, J. Am. Chem. Soc.
131 (2009) 5370–5371.
[52] L. Falivene, G. Talarico, V. Barone, Unraveling the role of entropy in tuning
unimolecular vs. bimolecular reaction rates: the case of olefin polymerization
catalyzed by transition metals, Mol. Catal. 452 (2018) 138–144.
[53] I. Saeed, S. Katao, K. Nomura, Synthesis and structural analysis of
(Cyclopentadienyl)(pyrrolide)titanium(IV) complexes and their use in catalysis for
olefin polymerization, Organometallics 28 (2009) 111–122.
[23] M. Fallahi, E. Ahmadi, Z. Mohamadnia, Effect of inorganic oxide supports on the
activity of chromium-based catalysts in ethylene trimerization, Appl. Organomet.
Chem. 33 (2019) e4975.
[24] E. Naji-Rad, M. Gimferrer, N. Bahri-Laleh, M. Nekoomanesh-Haghighi, R. Jamjah,
A. Poater, Exploring basic components effect on the catalytic efficiency of chevron-
phillips catalyst in ethylene trimerization, Catalysts 8 (2018) 224.
[25] R.M. Manyik, W.E. Walker, T.P. Wilson, A soluble chromium-based catalyst for
ethylene trimerization and polymerization, J. Catal. 47 (1977) 197–209.
[26] Y. Zhang, C. Wang, T. Wu, M.J. Huang, Highly selective trimerization of ethylene
with half-sandwich cyclopentadienyl and indenyl titanium complexes bearing
pendant thienyl group, J. Mol. Catal. A Chem. 387 (2014) 20–30.
[27] E. Ahmadi, Z. Mohamadnia, M.N. Haghighi, High productive ethylene
trimerization catalyst based on CrCl3/SNS ligands, Catal. Lett. 141 (2011)
1191–1198.
[54] R. Bína, I. Císarova, M. Pavlista, I. Pavlík, Crystallographic report: the [bis(η5
-
ˇ
´
ˇ
cyclopentadienyl)titanium(IV)-bis(L-methionine)] dichloride, Appl. Organomet.
Chem. 18 (2004) 262–263.
[55] L. Falivene, Z. Cao, A. Petta, L. Serra, A. Poater, R. Oliva, V. Scarano, L. Cavallo,
Towards the online computer-aided design of catalytic pockets, Nat. Chem. 11
(2019) 872–879.
[56] W.J. van Rensburg, J.A. van den Berg, P.J. Steynberg, Role of MAO in Chromium-
Catalyzed Ethylene Tri- and Tetramerization: a DFT study, Organometallics 26
(2007) 1000–1013.
[28] P.J. Deckers, B. Hessen, J.H. Teuben, Switching a catalyst system from ethene
polymerization to ethene trimerization with a hemilabile ancillary ligand, Angew.
Chem. Int. Ed. 40 (2001) 2516–2519.
[57] A.N.J. Blok, P.H.M. Budzelaar, A.W. Gal, Mechanism of ethene trimerization at an
ansa-(Arene)(cyclopentadienyl) titanium fragment, Organometallics 22 (2003)
2564–2570.
[29] A. Bollmann, K. Blann, J.T. Dixon, F.M. Hess, E. Killian, H. Maumela, D.
S. McGuinness, D.H. Morgan, A. Neveling, S. Otto, M. Overett, Ethylene
tetramerization: a new route to produce 1-octene in exceptionally high
selectivities, J. Am. Chem. Soc. 126 (2004) 14712–14713.
[58] P. Foster, J.C. Chien, M.D. Rausch, Highly stable catalysts for the stereospecific
polymerization of styrene, Organometallics 15 (1996) 2404–2409.
[59] J.C. Flores, J.S. Wood, J.C. Chien, M.D. Rausch, [1-(2-Phenylethyl)-2,3,4,5-
tetramethylcyclopentadienyl]- titanium compounds. synthesis and their use for the
syndiospecific polymerization of styrene, Organometallics 15 (1996) 4944–4950.
[60] P.R. Elowe, C. McCann, P.G. Pringle, S.K. Spitzmesser, J.E. Bercaw, Nitrogen-
linked diphosphine ligands with ethers attached to nitrogen for chromium-
catalyzed ethylene tri- and tetramerizations, Organometallics 25 (2006)
5255–5260.
[30] L. Azimnavahsi, Z. Mohamadnia, Optimization of ethylene trimerization using
catalysts based on TiCl3/half-sandwich ligands, Appl. Organomet. Chem. 33 (2019)
e4666.
[31] L. Chen, G. Li, Z. Wang, S. Li, M. Zhang, X. Li, Ethylene oligomerization over nickel
supported silica-alumina catalysts with high selectivity for C10+ products, Catalysts
10 (2020) 180.
[61] A. Saidi, M.K. Samantaray, A. Poater, M. Tretiakov, L. Cavallo, J.M. Basset,
Metathesis of classical and functionalized olefins catalyzed by silica-supported
single-site well-defined W and MO pre-catalysts, ChemCatChem 12 (2020)
6067–6075.
[32] P.J.W. Deckers, B. Hessen, J.H. Teuben, Switching a catalyst system from ethene
polymerization to ethene trimerization with a hemilabile ancillary ligand, Angew.
Chem. Int. Ed. 40 (2001) 2516–2519.
[33] P.J.W. Deckers, B. Hessen, J.H. Teuben, Catalytic trimerization of ethene with
highly active cyclopentadienyl-arene titanium catalysts, Organometallics 21
(2002) 5122–5135.
[62] A. Poater, B. Cosenza, A. Correa, S. Giudice, F. Ragone, V. Scarano, L. Cavallo,
SambVca: a web application for the calculation of buried volumes of N-heterocyclic
carbene ligands, Eur. J. Inorg. Chem. (2009) 1759–1766.
[34] E. Otten, A.A. Batinas, A. Meetsma, B. Hessen, Versatile coordination of
cyclopentadienyl-arene ligands and its role in titanium-catalyzed ethylene
trimerization, J. Am. Chem. Soc. 131 (2009) 5298–5312.
[63] L. Falivene, R. Credendino, A. Poater, A. Petta, L. Serra, R. Oliva, V. Scarano,
L. Cavallo, SambVca 2. A web tool for analyzing catalytic pockets with topographic
steric maps, Organometallics 35 (2016) 2286–2293.
[35] J. Huang, T. Wu, Y. Qian, Ethylene trimerization with a half-sandwich titanium
complex bearing a pendant thienyl group, Chem. Commun. 22 (2003) 2816–2817.
[36] T. Wu, Y. Qian, J. Huang, Hydrogenation of fast pyrolyis oil and model compounds
in a two-phase aqueous organic system using homogeneous ruthenium catalysts,
J. Mol. Catal. A Chem. 214 (2004) 227–236.
[64] J.A. Luque-Urrutia, A. Poater, The fundamental noninnocent role of water for the
hydrogenation of nitrous oxide by PNP pincer RU-based catalysts, Inorg. Chem. 56
(2017) 14383–14387.
[65] J. Poater, M. Gimferrer, A. Poater, Covalent and ionic capacity of MOFs to sorb
small gas molecules, Inorg. Chem. 57 (2018) 6981–6990.
[37] Y. Zhang, H. Ma, J. Huang, Highly selective ethylene trimerization catalyzed by
half-sandwich indenyl titanium complexes with pendant arene groups and MAO,
J. Mol. Catal. A Chem. 373 (2013) 85–95.
[66] L. Falivene, L. Cavallo, G. Talarico, Buried volume analysis for propene
polymerization catalysis promoted by group 4 metals: a tool for molecular mass
prediction, ACS Catal. 5 (2015) 6815–6822.
11

S. Gharajedaghi et al.
Molecular Catalysis 509 (2021) 111636
[67] A. Poater, L. Cavallo, Comparing families of olefin polymerization precatalysts
using the percentage of buried volume, Dalton Trans. (2009) 8878–8883.
[68] L. Falivene, L. Cavallo, G. Talarico, The role of noncovalent interactions in olefin
polymerization catalysis: a further look to the fluorinated ligand effect, Mol. Catal.
494 (2020), 111118.
[69] A. Cordier, P.A. Breuil, T. Michel, L. Magna, H. Olivier-Bourbigou, J. Raynaud,
C. Boisson, V. Monteil, Titanium-based phenoxy-imine catalyst for selective
ethylene trimerization: effect of temperature on the activity, selectivity and
properties of polymeric side products, Catal. Sci. Technol. 10 (2020) 1602–1608.
12