Trimerization of ethylene to 1-hexene with titanium complexes bearing phenoxy-imine ligands with pendant donors combined with MAO

Article abstract of DOI:10.1021/om1003368

New Ti complexes bearing phenoxy-imine ligands with pendant aryl-OMe donors have been developed for ethylene trimerization to produce 1-hexene. These Ti complexes combined with methylaluminoxane selectively trimerize ethylene to form 1-hexene with exceptionally high activity (e.g., 6.59 tons of 1-hexene/((g of Ti) h)).

Full text of DOI:10.1021/om1003368

2394 Organometallics 2010, 29, 2394–2396
DOI: 10.1021/om1003368
Trimerization of Ethylene to 1-Hexene with Titanium Complexes Bearing
Phenoxy-Imine Ligands with Pendant Donors Combined with MAO
Yasuhiko Suzuki, Shinsuke Kinoshita, Atsushi Shibahara, Seiichi Ishii,
Kazumori Kawamura,* Yoshihisa Inoue, and Terunori Fujita*
Research Center, Mitsui Chemicals, Inc., 580-32 Nagaura, Sodegaura, Chiba 299-0265, Japan
Received April 22, 2010
Summary: New Ti complexes bearing phenoxy-imine ligands
with pendant aryl-OMe donors have been developed for
ethylene trimerization to produce 1-hexene. These Ti com-
plexes combined with methylaluminoxane selectively trimerize
ethylene to form 1-hexene with exceptionally high activity
(e.g., 6.59 tons of 1-hexene/((g of Ti) h)) .
Ethylene oligomerization normally results in the forma-
tion of a range of R-olefins following a Schulz-Flory
distribution. Thus, the development of catalysts capable of
selectively forming a desired R-olefin would be of great
significance. In particular, the selective trimerization of
ethylene to 1-hexene has received a great deal of attention
because of the importance of this comonomer in the manu-
facture of linear low-density polyethylene (LLDPE).1
Figure 1. Ti complexes employed in this study.
Since the first report on Cr-catalyzed selective ethylene
trimerization by Manyik and co-workers, a large number of
highly active and selective Cr-based catalysts for this con-
version have been developed.2,3 Additionally, a great deal
of effort has been exerted into developing non-Cr-based
*To whom correspondence should be addressed. E-mail: Terunori.
Fujita@mitsui-chem.co.jp (T.F.).
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Figure 2. Molecular structure of complex 3.
catalysts for ethylene trimerization, resulting in the intro-
duction of high-performance catalysts based on a number of
transition metals, including Zr,4 Ti,5 and Ta.6
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Our research, which is based on a ligand-oriented catalyst
design concept, has resulted in the discovery of phenoxy-
imine ligated early-transition-metal complexes (aka FI cat-
alysts) for the controlled (co)polymerization of olefinic
monomers.7 Notably, FI catalysts, after activation, can
polymerize ethylene with very high efficiency independent
of the central metals employed (Ti, Zr, Hf, V, Cr, etc.),
suggesting the significant potential of a phenoxy-imine
ligand for efficient ethylene insertion. Therefore, in recent
years, there has been increased interest in the further devel-
opment of FI catalysts and related complexes for the oligo-
merization/polymerization of ethylene.7,8
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Ed. 2001, 40, 2516–2519. (b) Deckers, P. J. W.; Hessen, B.; Teuben, J. H.
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A.; Hessen, B. J. Am. Chem. Soc. 2009, 131, 5298–5312. (d) Huang, J.; Wu, T.;
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r
pubs.acs.org/Organometallics
Published on Web 05/03/2010
2010 American Chemical Society

Communication
Organometallics, Vol. 29, No. 11, 2010 2395
Table 1. Catalytic Ethylene Conversion with Complexes 1-3 with MAOa
selectivity (wt %)
C2H4 pressure
(MPa)
activity
(kg/((g of Ti) h))
1-hexene productivity
(kg of 1-hexene/((g of Ti) h))
entry
complex
C6
C10
PE
1
2
3
1
2
3
3
3
3
0.8
0.8
0.8
1.6
3.2
5.0
5
138
169
654
2720
7140
76.6
86.2
91.4
93.2
93.9
92.3
0
23.4
1.6
2.1
1.0
0.5
0.4
4
119
155
609
2550
6590
12.2
6.4
5.8
5.6
7.3
4b
5b
6b
a Conditions: cyclohexane, 30 mL (b150 mL); complex, 0.5 μmol; MAO, 5.0 mmol as Al; 30 °C; 60 min.
As part of a program to develop new catalysts for olefin
insertion based on FI catalysts, we have investigated early-
transition-metal complexes with phenoxy-imine ligands
bearing pendant donors. Such research has led to the dis-
covery of Ti complexes incorporating phenoxy-imine li-
gands with pendant aryl-OMe donors that can selectively
transform ethylene to 1-hexene with extremely high produc-
tivity, representing the first examples of phenoxy-imine
ligated catalysts for selective ethylene trimerization. There-
fore, we herein describe the catalytic performance of these Ti
complexes.
Scheme 1. Plausible Mechanism for the Formation of 1-Hexene
Shown in Figure 1 are Ti complexes 1-3 used in this study,
which incorporate phenoxy-imine ligands with pendant
donors (tridentate phenoxy-imine ligands). The tridentate
phenoxy-imine ligands were readily synthesized by Schiff
base condensations between the corresponding salicylalde-
hydes and primary amines. Complex formation was achieved
by the treatment of the tridentate ligand with TiCl4 (1, 27%;
2, 59%; 3, 82%). The 1H NMR spectra of complexes 2 and 3
exhibited a pair of singlet peaks assigned to the methyl
protons of a cumyl group (complex 2) or AB system peaks
derived from the methylene protons of an adamantyl group
(complex 3), consistent with the facial coordination of the
tridentate ligands. Indeed, an X-ray crystallographic analysis has
demonstrated that complex 3 adopts a distorted-octahedral
structure with the tridentate phenoxy-imine ligand coordinated
(119 kg of 1-hexene/((g of Ti) h)). This result indicates that
the introduction of the OMe group in place of the OPh group
converted a poorly active ethylene trimerization catalyst into
a highly active and selective ethylene trimerization catalyst,
illustrating how a minor change in the ligand structure can
have a dramatic effect on catalyst performance.9
A plausible mechanism for the formation of 1-hexene is
presented in Scheme 1, which is founded on a metallacyclic
mechanism that involves Ti(II) and Ti(IV) species.5,10 The
mechanism involves the coordination of two ethylene mole-
cules, oxidative coupling to yield a metallacyclopentane,
insertion of a third ethylene molecule to form a metallacy-
cloheptane, and β-H elimination/reductive elimination (or
concerted 3,7-H transfer) to form 1-hexene.
We postulate that the OMe donor may play a crucial role
in the formation and/or stabilization of the Ti(II) species
that is presumably the initial active species for ethylene
trimerization.8b,f,9 Scheme 2 displays a possible mechanism
for the formation of the Ti(II) species, which is the key
species for the reaction. The mechanism involves the initial
formation of a cationic dimethyl Ti(IV) species, insertion of
ethylene to form a cationic dialkyl Ti(IV) species, and (a)
intramolecular β-H transfer or (b) β-H elimination/reductive
elimination to form the Ti(II) species.5,10
˚
in a facial fashion (bond distances: Ti-phenoxy-O = 1.827(2) A,
˚
˚
Ti-imine-N = 2.197(3) A, Ti-aryl-O = 2.182(2) A) (Figure 2).
The results of catalytic ethylene conversion experiments
using complexes 1 and 2 with methylaluminoxane (MAO)
under 0.8 MPa of ethylene pressure are given in Table 1
(entries 1 and 2). Complex 1, bearing a pendant aryl-OPh
donor, produced 1-hexene (76.6 wt %) displaying low activ-
ity along with a considerable amount of PE. To our surprise,
however, complex 2, with a pendant OMe donor, after
activation yielded a catalyst with much higher selectivity
for 1-hexene (86.2 wt %) with extremely high productivity
Further research on Ti complexes incorporating phenoxy-
imine ligands with aryl-OMe donors has resulted in the
development of complex 3 having an adamantyl group ortho
to the phenoxy-O. Under 0.8 MPa of ethylene pressure,
complex 3 with MAO exhibited higher 1-hexene selectivity
(91.4 wt %) and activity (155 kg of 1-hexene/((g of Ti) h)) than
(8) (a) Huang, J.; Lian, B.; Qian, Y.; Zhou, W.; Chen, W.; Zheng, G.
Macromolecules 2002, 35, 4871–4874. (b) Owiny, D.; Parkin, S.; Ladipo,
F. T. J. Organomet. Chem. 2003, 678, 134–141. (c) Suzuki, Y.; Terao, H.;
Fujita, T. Bull. Chem. Soc. Jpn. 2003, 76, 1493–1517. (d) Hu, W.-Q.; Sun,
X.-L.; Wang, C.; Gao, Y.; Tang, Y.; Shi, L.-P.; Xia, W.; Sun, J.; Dai, H.-L.; Li,
X.-Q.; Yao, X.-L.; Wang, X.-R. Organometallics 2004, 23, 1684–1688. (e)
Jones, D. J.; Gibson, V. C.; Green, S. M.; Maddox, P. J.; White, A. J. P.;
William, D. J. J. Am. Chem. Soc. 2005, 127, 11037–11046. (f) Pennington,
D. A.; Clegg, W.; Coles, S. J.; Harrington, R. W.; Hursthouse, M. B.; Hughes,
D. L.; Light, M. E.; Schormann, M.; Bochmann, M.; Lancaster, S. J. Dalton
Trans. 2005, 561–571. (g) Qi, C.-H.; Zhang, S.-B.; Sun, J.-H. J. Organomet.
Chem. 2005, 690, 3946–3950. (h) Wang, C.; Ma, Z.; Sun, X.-L.; Gao, Y.;
Guo, Y.-H.; Tang, Y.; Shi, L.-P. Organometallics 2006, 25, 3259–3266. (i)
Salata, M. R.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 12–13. (j) Kirillov,
E.; Roisnel, T.; Razavi, A.; Carpentier, J.-F. Organometallics 2009, 28,
2401–2409.
(9) DFT calculations suggested that the Ti-OR bond distance of 2
˚
˚
(2.31 A) is shorter than that of 1 (2.43 A), which might be responsible for
the difference in catalytic performance between the OMe and OPh
donors.
(10) (a) Blok, A. N. J.; Budzelaar, P. H. M.; Gal, A. W. Organome-
tallics 2003, 22, 2564–2570. (b) de Bruin, T. J. M.; Magna, L.; Raybaud, P.;
Toulhoat, H. Organometallics 2003, 22, 3404–3413. (c) Tobisch, S.; Ziegler,
T. Organometallics 2003, 22, 5392–5405.

2396 Organometallics, Vol. 29, No. 11, 2010
Suzuki et al.
Scheme 2. Possible Mechanism for the Formation
of a Ti(II) Species
formation of the metallacyclopentane species via a pair of
coordinated ethylene molecules.
GC-MS together with 1H and 13C NMR studies have
suggested that, as byproducts other than PE, branched
decenes (cotrimers of the 1-hexene produced with ethylene)
were formed, 2-butyl-1-hexene being predominant (ca. 90 wt %).
Considering that the formation of 2-butyl-1-hexene is vir-
tually second order in ethylene pressure (entries 3-6), 2-butyl-
1-hexene is produced by a 1,2-insertion of 1-hexene into the
metallacyclopentane to form a 3-butylmetallacycloheptane,
followed by β-H elimination/reductive elimination (or con-
certed 3,7-H transfer) (see the Supporting Information).
Further studies, including a detailed mechanistic investiga-
tion, are underway.
complex 2 (Table 1, entry 3). Increasing the ethylene pressure
to 5.0 MPa gave a significant enhancement of 1-hexene
productivity (6.59 tons of 1-hexene/((g of Ti) h), TOF =
3.7 Â 106/h) with higher selectivity (92.3 wt %).11 This activity
represents one of the highest values reported to date for
ethylene trimerization catalysts. In fact, this activity is 2 orders
of magnitude greater than that seen with the commercially
employed Cr-based catalyst (Phillips catalyst) under similar
ethylene pressure conditions.
Ethylene pressure studies have revealed a second-order
dependence of productivity on ethylene pressure (entries
3-6) (see the Supporting Information), which is consistent
with the metallacyclic mechanism (Scheme 1).10 This cataly-
tic behavior may suggest that the rate-determining step is the
In summary, we have developed new Ti complexes for
ethylene trimerization to produce 1-hexene, which are sup-
ported by phenoxy-imine ligands with pendant aryl-OMe
donors (tridentate phenoxy-imine ligands). These com-
plexes combined with MAO can selectively transform ethy-
lene into 1-hexene with unprecedented productivity (e.g.,
6.59 tons of 1-hexene/((g of Ti) h)). The results reported herein
further demonstrate the unique catalytic behavior of transi-
tion-metal complexes incorporating phenoxy-imine-based
ligands (FI-type ligands) for ethylene insertion reactions.7,8
Supporting Information Available: Text, figures, and a CIF
file giving details of the synthesis of 1-3, crystallographic data
for 3, catalytic ethylene conversion experiments, a plausible
mechanism for the formation of 2-butyl-1-hexene, and DFT
calculations. This material is available free of charge via the
Internet at http://pubs.acs.org.
(11) A 1-hexene-rich environment is probably responsible for the
1-hexene selectivity at 5.0 MPa ethylene pressure (92.3 wt %) being
lower than that at 3.2 MPa ethylene pressure (93.9 wt %), since the
amount of byproduct decenes formed is largely a product of 1-hexene
concentration.