Ethylene trimerization with a half-sandwich titanium complex bearing a pendant thienyl group

Article abstract of DOI:10.1039/b309749h

A half-sandwich titanium complex with a pendant thienyl group, activated by methylalumoxane (MAO), can trimerize ethylene to 1-hexene with considerable activity and high selectivity; the coodination of the sulfur atom to the titanium center is proposed to be responsible for the selective trimerization of ethylene.

Full text of DOI:10.1039/b309749h

Ethylene trimerization with a half-sandwich titanium complex bearing a
pendant thienyl group†
Jiling Huang,* Tianzhi Wu and Yanlong Qian
The Laboratory of Organometallic Chemistry, East China University of Science and Technology, 130
Meilong Road, Shanghai, 200237, China. E-mail: qianling@online. sh. cn; Fax: +86-21-5428 2375;
Tel: +86-21-5428 2375
Received (in Cambridge, UK) 13th August 2003, Accepted 23rd September 2003
First published as an Advance Article on the web 13th October 2003
A half-sandwich titanium complex with a pendant thienyl
group, activated by methylalumoxane (MAO), can trimerize
ethylene to 1-hexene with considerable activity and high
selectivity; the coodination of the sulfur atom to the titanium
center is proposed to be responsible for the selective
trimerization of ethylene.
It is noted that the selectivity of the catalyst is temperature-
dependent. At 0 °C, only a trace of polymer is produced and the
selectivity is very high (95%), but when we increase the reaction
temperature, although the activity for trimerization is retained,
the selectivity decreases dramatically because the amount of
polymer increases. It is interesting that, when the temperature is
raised to 80 °C, 1/MAO shows considerable activity not only for
trimerization but also for polymerization.
1-Hexene, which is used as a comonomer for synthesis of linear
low-density polyethylene (LLPDE), can be produced by
catalytic trimerization of ethylene. Presently, most catalytic
trimerization systems are based on chromium compounds.1 To
find an alternative catalyst system based on other metals is of
academic and industrial interest. Hessen and co-workers
reported that titanium complexes with an arene-pendant
cyclopentadienyl ligand in combination with methylalumoxane
(MAO) can selectively trimerize ethylene with high activity.2
The arene-pendant cyclopentadienyl ligand is likely to exhibit
hemilabile behavior, stabilizing the titanium center of the
The MAO activated half-sandwich titanium complex has
been shown to be a multisite system containing Ti(IV), Ti(III
)
and Ti(II) species.6 The Ti(II) species was proposed to be the
initial active species for the trimerization of ethylene.7 Hessen
elucidated that the pendant arene group may promote the
formation of the titanium(II) species and provide it with
additional stability.2 In our case, the thiophene moiety could
coordinate to the titanium center when complex 1 is activated by
MAO, and this coordination may also make the formation of the
titanium(II) species much more favourable, which leads to the
highly selective trimerization of ethylene. But at high tem-
peratures, the thiophene partly dissociates from the titanium
center, so the selectivity of 1-hexene decreases.
6
activated species by h coordination and slipping to make room
for the incoming ethylene.
Modification of the Cp ring by introducing a pendant group
that has the ability to coordinate to a metal center is of current
interest. All kinds of ligands such as alkenyl, phenyl, alkoxyl,
O-donor, N-donor, and P-donor groups have attracted much
attention.3 However, less attention has been paid to S-donor
ligands.3d
As mentioned above, thiophene can coordinate to the metal
1
5
center by two modes, h -S coordination and h coordination. In
the 1/MAO catalyst system, the two coordination modes are
both favourable with respect to molecular structure. Our
attention was focused on clarifying which of the two coordina-
tion modes is essential for ethylene trimerization. In order to
obtain some evidence, we synthesized complex 2 (Scheme 2).
In complex 2, the thiophene moiety is linked to the Cp ring
through a carbon bridge at the 3-position, thus, the sulfur atom
is away from the titanium center, which makes it impossible for
the sulfur atom to coordinate to the titanium center. However in
Thiophene is a special ligand. In contrast to dialkyl sulfides,
thiophene is a weak S-donor, so it can be expected to only
weakly coordinate to metal centers.4 Furthermore, as an
electron-rich aromatic system, thiophene can coordinate to
5
metal centers by h coordination.5 If we introduce the thiophene
moiety into a cyclopentadienyl ring, it is envisaged that the
thiophene-cyclopentadienyl ligand may serve as a hemilabile
ligand in the catalytic trimerization of ethylene. With this in
mind, we designed and synthesized a titanium complex with a
thiophene-pendant cyclopentadienyl ligand, complex 1‡
(Scheme 1).
5
complex 2, the possibility of h -coordination still exists. We
studied the catalytic behavior of 2 in combination with MAO in
ethylene conversion, compared with 1/MAO. The results are
shown in Table 2.
Entries 1 and 2 describe the catalytic behavior of 2/MAO. In
2/MAO, polymer is produced as the major product and the
In complex 1, the thiophene moiety is linked to the Cp ring
through a carbon bridge at the 2-position.
Complex 1 with MAO was tested in ethylene conversion. The
results are listed in Table 1.
From the results shown in Table 1, as we expected, 1/MAO
exhibits considerable activity as well as high selectivity for
ethylene trimerization. In addition to 1-hexene, a small amount
of butene (2–3 wt%) is produced. No other oligomers such as C8
and C10 olefins are detected. In most cases, polymer is
produced. The highest activity for trimerization is 220 Kg
mol21 Ti h21 (under 8 atm of ethylene pressure and at 30 °C).
Table 1 Results of ethylene conversion with 1/MAOa
Selectivity
Pressure/
Entry T/°C atm
Productivityb of 1-hexene
1-Hexene/g Polymer/g of 1-hexene (wt%)
1
2
3
4
0
30
80
30
5
5
5
8
0.090
0.117
0.102
0.165
Trace
0.017
0.206
0.022
120
156
136
220
> 95
84
33
86
a Reaction conditions: 1.5 mmol catalyst; Al : Ti = 1000 : 1; 20 ml toluene
solvent; 30 min run time. b In Kg mol21 Ti h21
.
Scheme 1
† Electronic supplementary information (ESI) available: syntheses of
compounds 1 and 2. See http://www.rsc.org/suppdata/cc/b3/b309749h/
Scheme 2
2816
CHEM. COMMUN., 2003, 2816–2817
This journal is © The Royal Society of Chemistry 2003

Table 2 Results of ethylene conversion with 2/MAO, compared with
1/MAOa
catalyst precursor for ethylene trimerization. The coordination
of the sulfur atom is likely to be essential for the selective
trimerization of ethylene. Our findings indicate that s-donor
ligands attached to the cyclopentadienyl ring can also play an
important role in ethylene trimerization, which will greatly
expand the library of catalysts for ethylene trimerization.
The authors gratefully acknowledge the Special Funds for
Major State Basic Research Projects (G 1999064801) and the
Research Fund for the Doctoral Program of Higher Education
(No. 20020251002) for financial support.
Selectivity
Productivityb of 1-hexene
Entry Complex T/°C 1-Hexene/g Polymer/g of 1-hexene (wt%)
1
2
3
4
2
2
1
1
0
0.009
30 0.014
0.090
30 0.117
0.040
0.094
Trace
0.017
12
19
120
156
18
12
> 95
84
0
a Reaction conditions: 1.5 mmol catalyst; Al : Ti = 1000 : 1; 5 atm ethylene
pressure; 20 ml toluene solvent; 30 min run time. b In Kg mol21 Ti h21
.
Notes and references
‡
Selected data for 1: 1HNMR (500 MHz, CDCl3) d 7.25 (m, 1H, a-
activity with respect to trimerization is low. The fact that the
thiophene-H ); d 7.00 (m, 2H, b-thiophene-H); d 6.98 (ps t, 3JHH = 2.9 Hz,
2H, Cp–H ); d 6.79 (pst, 3JHH = 2.9 Hz, 2H, Cp–H ); d 2.60 (d, 2JHH = 12.9
Hz, 2H, a-CHeq); d 2.14 (m, 2H, a-CHax); d 1.66 (br, 3H, b-and g-CH2); d
1.27–1.54 (m, 3H, b-and g-CH2). Anal. C15H17Cl3STi, found (calc) C:
47.28 (46.96) H: 4.49 (4.47).
catalytic behavior of 2/MAO dramatically differs from that of
5
1/MAO indicates that it is not the h coordination mode but the
1
h -S coordination mode that plays an important role in ethylene
trimerization. Very recently, Bianchini and co-workers reported
that Co(II) complexes with pyridinimine ligands bearing a
thienyl group can serve as efficient catalysts for oligomerization
of ethylene.8 They suggested that the sulfur atom would be
likely to play an active role during the oligomerization of
ethylene. While in our case, by comparing the catalytic behavior
of 1/MAO and 2/MAO, a further conclusion can be drawn that
the coordination of the sulfur atom to the titanium center plays
an important role during the trimerization of ethylene.
Hessen proposed a mechanism involving titanacycle inter-
mediates accounting for the selective formation of 1-hexene.
Recently, a theoretical study of ethylene trimerization at a
cationic (C6H5CH2C5H4)Ti fragment supported this mecha-
nism.9 By analogy, a possible mechanism proposed in our case
is shown in Scheme 3.
Selected data for 2: 1HNMR (500 MHz, CDCl3) d 7.33 (dd, 2JHH = 5.0
Hz, 3JHH = 2.9 Hz, 1H, a-thiophene-H ); d7.19 (dd, 2JHH = 2.9 Hz, 3JHH
= 1.2 Hz, 1H, a-thiophene-H ); d 7.07 (dd, 2JHH = 5.0 Hz, 3JHH = 1.2 Hz,
1H, b-thiophene-H); d 6.90 (pst, 3JHH = 2.9 Hz, 2H, Cp–H ); d 6.78 (pst,
3JHH = 2.8 Hz, 2H, Cp–H ); d 2.59 (d, 2JHH = 12.9 Hz, 2H, a-CHeq); d 2.03
(m, 2H, a-CHax); d1.53–1.70 (br, 3H, b-and g-CH2); d 1.30–1.44 (m, 3H, b-
and g-CH2). Anal. C15H17Cl3STi, found (calc.) C: 47.00 (46.96) H: 4.61
(4.47).
1 (a) J. R. Briggs, US Pat. Appl. 4668838, 1987; (b) J. R. Briggs, Chem.
Commun., 1989, 11, 674; (c) Y. Araki, H. Nakamura, Y. Nanba and T.
Okano, US Pat. Appl. 5856612, 1997; (d) D. F. Wass, PCT Int. Appl. WO
0204119, 2002; (e) A. Carter, S. A. Cohen, N. A. Cooley, A. Murphy, J.
Scutt and D. F. Wass, Chem. Commun., 2002, 8, 858.
2 (a) P. J. W. Deckers, B. Hessen and J. H. Teuben, Angew. Chem., Int. Ed.,
2001, 40, 2516; (b) P. J. W. Deckers, B. Hessen and J. H. Teuben,
Organometallics, 2002, 21, 5122.
3 (a) J. Okuda, Comments Inorg. Chem., 1994, 16, 185; (b) P. Jutiz and U.
Siemeling, J. Organomet. Chem., 1995, 500, 178; (c) U. Siemeling,
Chem. Rev., 2000, 100, 1495; (d) H. Butenschön, Chem. Rev., 2000, 100,
1527.
In summary, we have found that a titanium complex with a
thienyl-cyclopentadienyl ligand can serve as an efficient
4 (a) R. J. Angelici, Acc. Chem. Res., 1988, 21, 387; (b) J. L. Burmeister,
Coord. Chem. Rev., 1990, 105, 77.
5 (a) M. F. Balley and L. F. Dahl, Inorg. Chem., 1965, 4, 1306; (b) J. R.
Lockemeyer, T. B. Rauchfuss, A. L. Rheighold and S. R. Wilson, J. Am.
Chem. Soc., 1989, 111, 8828.
6 J. C. W. Chien, Z. Salajka and S. Dong, Macromolecules, 1992, 25,
3199.
7 C. Pellecchia, D. Pappalardo and G. J. Gruter, Macromolecules, 1999, 32,
4491.
8 C. Bianchini, G. Mantovani, A. Meli, F. Migliacci and F. Laschi,
Organometallics, 2003, 22, 2545.
9 A. N. J. Blok, P. H. M. Budzelaar and A. W. Gal, Organometallics, 2003,
22, 2564.
Scheme 3 Proposed mechanism for the formation of 1-hexene.
CHEM. COMMUN., 2003, 2816–2817
2817