First Cr(III)-SNS complexes and their use as highly efficient catalysts for the trimerization of ethylene to 1-hexene

Article abstract of DOI:10.1021/ja034752f

Cr(III) complexes of tridentate SNS ligands have been prepared and evaluated as catalysts for ethylene trimerization, with several giving very high activity and excellent selectivity toward 1-hexene when activated with methylaluminoxane. The new complexes

Full text of DOI:10.1021/ja034752f

Published on Web 04/15/2003
First Cr(III)-SNS Complexes and Their Use as Highly Efficient Catalysts for
the Trimerization of Ethylene to 1-Hexene
David S. McGuinness,*,†,§ Peter Wasserscheid,* Wilhelm Keim, David Morgan, John T. Dixon,
,†
†
‡
‡
‡
‡
‡
Annette Bollmann, Hulisani Maumela, Fiona Hess, and Ulli Englert
Institut f u¨ r Technische Chemie und Makromolekulare Chemie der RWTH Aachen, Worringerweg 1,
5
9
2074 Aachen, Germany, Sasol Technology (Pty) Ltd, R & D DiVision, 1 Klasie HaVenga Rd, Sasolburg,
570 South Africa, and Institut f u¨ r Anorganische Chemie der RWTH Aachen, Professor Pirlet Strasse 1,
52074 Aachen, Germany
Received February 19, 2003; E-mail: wasserscheidp@itmc.rwth-aachen.de
The selective trimerization of ethylene to 1-hexene is an area of
much recent interest due to the importance of this comonomer in
Scheme 1
1
the production of polyethylene. The trimerization route largely
avoids the production of unwanted olefins that conventional
transition metal oligomerization processes produce. We have
recently communicated that Cr(III) complexes of tridentate PNP
ligands (I), along with MAO cocatalyst, are highly active and
2
selective catalysts for this transformation. However, the high cost
of secondary alkyl phosphine precursors for the synthesis of these
ligands makes their use in a potential technical process difficult.
Added to this, the toxicity of phosphines and their sensitivity toward
oxidation, which makes inert atmosphere techniques necessary for
ligand synthesis, represent serious drawbacks for this and other1
phosphine-containing systems. In trying to increase the attractive-
ness of this system, we therefore became interested in alternatives
that could replace the phosphorus donor in these ligands, while
still maintaining the essential attributes of the donor set. For this
purpose, it occurred to us that the thioether donor group may be
suitable.4
Complexes 1-4 represent new additions to a rare class of
complexes that display Cr -thioether bonding.
Initial catalytic tests to probe the activity of the complexes were
carried out in a 75 mL autoclave, immersed in a oil bath heated to
III
7,8
d,3
8
0 °C. The results of this catalytic testing of 1-3 for ethylene
trimerization are given in Table 1.
Along with MAO as cocatalyst, all complexes gave rise to highly
active and selective catalysts for the trimerization of ethylene to
1
-hexene. Table 1 shows that over a 30 min run, TOFs (of ethylene
conversion) of between ca. 75 000 and 95 000 were obtained with
total C selectivities of 93-94% (entries 1-3). Furthermore, the
6
hexene fraction was composed of over 99.3% 1-hexene in each
case. The methyl-substituted and ethyl-substituted complexes gave
rise to catalysts with the same activity and selectivity, while
complex 3 was significantly more active than the complexes with
shorter alkyl chains. It should be noted here that the procatalyst 3
was noticeably more soluble in toluene than were 1 and 2, and an
activity contribution from this effect cannot be ruled out.
After these initial tests, complexes 2 and 4 were taken for further
optimization, and the results are shown in Table 2. These reactions
were carried out in a 300 mL Parr autoclave fitted with a gas
entraining stirrer and internal temperature control, allowing the
temperature to be maintained at close to 85 (complex 2) and 90 °C
In principle, an appropriately chosen thioether group should be
able to mimic a phosphine group in many circumstances. In the
PNP system (I) with Cr as the metal center, it is thought that the
phosphine arms operate as soft donor ligands which are capable of
facile association-dissociation equilibria, and from this respect the
donor properties of thioether groups should be similar. In this report,
we detail the synthesis of new SNS complexes of Cr and their use
as highly active catalysts for the selective trimerization of ethylene
to 1-hexene.
(
complex 4) during the reaction. Results using complex 2 reveal
that very low concentrations of catalyst in this system led to
extremely high activities and unparalleled selectivity toward C
Previous results with PNP ligands I revealed that alkyl groups
with a low steric demand on phosphorus give the most active
catalysts; we therefore targeted SNS ligands with like substitution.
6
.
For instance, with 280 equiv of MAO, an activity of 160 840 g/g
of Cr/h was obtained (TOF ) 298 900), with a selectivity toward
C of 98.4%, of which 99.7% was 1-hexene. Trimerization is best
6
conducted at an ethylene pressure of 30-50 bar. At higher pressure
(70 bar), no increase in activity results, but a greater amount of
polymer is formed (entry 6).
5
Ligands II-V were prepared by a literature method that allows
simple large-scale benchtop synthesis in high yield, using technical
solvents and inexpensive reagents. This ease of synthesis is in
6
contrast to the preparation of ligands I. Most importantly, the
dramatically lower cost of this ligand makes it superior in terms of
Results using the more soluble complex 4 reveal that by using
similar pressures, temperatures, and chromium concentrations, we
could achieve high catalyst activity in the presence of a very low
amount of MAO (30-100 equiv). This, as alluded to earlier, may
in part be due to the enhanced solubility in toluene that the longer
alkyl chain affords. The ability of the catalyst to operate at low
cocatalyst loadings becomes crucially important in the potential
3 3
potential technical application. When reacted with CrCl (thf) , the
immediate formation of complexes 1-4 was observed, which were
isolated as blue-green powders in 87-96% yield (Scheme 1).
†
Institut f u¨ r Technische Chemie.
‡
Sasol Technology.
§
Present address: Departement of Chemistry, Imperial College London,
Exhibition Rd., South Kensington, London, SW7 2AY, U.K.
5272
9
J. AM. CHEM. SOC. 2003, 125, 5272-5273
10.1021/ja034752f CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Ethylene Trimerization with Complexes 1-3a
Å] are likewise similar to the Cr-P distances [2.4660(12),
2.4678(12) Å] and indicate a similar degree of donor-Cr bonding.
The Cr-N distance in 2 [2.1059(18) Å] is somewhat shorter than
that in the PNP analogue [2.139(3) Å], but overall it can be said
that the metal-ligand bonding in 2 and the PNP analogue is very
similar. This supports our supposition that the thioether group can
make an effectiVe substitute for the phosphine donor group in
certain circumstances.
entry
run time
(min)
PE
hexenes
(wt %)
1-hexene
(wt %)b
TOF
(
complex)
(wt %)
TON
(h-1)c
1
2
3
4
5
6
(1)
(2)
(3)
(1)
(2)
(3)
30
30
30
60
60
60
0.11
0.66
0.97
0.32
0.27
0.50
94
94
93
95
95
93
99.7
99.7
99.3
99.8
99.7
99.7
38 530
37 630
47 730
49 400
59 780
59 220
77 050
75 260
95 470
49 400
59 780
59 220
In summary, we have developed highly efficient new catalysts
for the trimerization of ethylene which are characterized by a simple
and inexpensive synthesis. This, coupled with the high activity,
excellent selectivity, and performance in the presence of low
amounts of MAO, means that the potential of this system being
employed to produce 1-hexene on a large scale is promising. The
high cost of ligand is often a limiting factor in the application of
new homogeneous catalysts based on phosphine ligands; however,
we have demonstrated here that thioether donor groups can make
effective substitutes. The application of sulfur-based ligands in
a
Conditions: 11.0-12.0 µmol of complex, 600-650 MAO, 25 mL of
b
toluene, 40 bar of ethylene, 80 °C. Selectivity for 1-hexene as a percentage
of total C6 fraction. TOF of ethylene conversion averaged over complete
run.
c
Table 2. Ethylene Trimerization with 2 and 4a
µ
of Cr
mol
equiv
pressure
(bar)
PE
C
6
1-C
6
TOF
productivity
(g/g of Cr/h)
(h-1
entry
of MAO
(wt %) (wt %) (wt %)
)
1
2
3
4
5
6
7
8
9
(2) 4.6 280
(2) 4.3 120
(2) 12.0 200
(2) 20.0 120
(2) 20.0 280
(2) 20.0 120
(4) 12
(4) 12
(4)
30
30
50
30
30
70
45
45
45
0.16 98.4 99.7 298 900 160 840
0.38 97.1 99.8 104 870 56 260
0.10 98.0 99.7 211 820 113 950
0.14 96.6 99.6 113 550 61 060
0.14 96.7 99.6 190 560 102 470
1.15 96.4 99.7 86 480 46 660
0.30 97.2 99.7 263 757 142 035
4
homogeneous catalysis has recently been reviewed. This report
showed that, on the whole, sulfur ligands have received very little
attention, and the instances where catalysis has been studied with
sulfur-based ligands have almost exclusively involved late transition
metals. We have shown here for the first time the positive effect
sulfur ligands are capable of having on an early transition metal
for olefin oligomerization. While it appears sulfur-based ancillary
ligands have, to an extent, been overlooked, these ligands have great
potential in many types of homogeneous catalysis, and this aspect
of their chemistry is under further investigation.
100
30
50
1.51 97.4 99.7 147 619
1.11 98.7 99.8 153 482
82 654
85 950
8
a
Conditions: 100 mL of toluene, 30 min.
Acknowledgment. D.S.M. and P.W. thank Dr Mike Green and
the members of the Sasol ethylene trimerization group for fruitful
discussion, and Sasol Technology Ltd. for financial support.
Supporting Information Available: Crystallographic data, in CIF
format, for structure 2. Preparation of 1-4 and X-ray structure
determination (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
Figure 1. Molecular structure of 2. Selected bond distances (Å) and angles
deg): Cr-N 2.1059(18), Cr-S1 2.4508(7), Cr-S2 2.4556(7), Cr-Cl1
.2985(8), Cr-Cl2 2.3184(7), Cr-Cl3 2.3167(7), N-Cr-S1 83.07(5),
(
2
N-Cr-S2 82.90(5), S1-Cr-Cl1 97.20(2), S2-Cr-Cl1 96.85(2), N-Cr-
References
Cl1 179.71(5), N-Cr-Cl2 85.82(6), N-Cr-Cl3 88.64(6).
(
1) (a) Yang, Y.; Kim, H.; Lee, J.; Paik, H.; Jang, H. G. Appl. Catal., A:
Gen. 2000, 193, 29. (b) Deckers, P. J. W.; Hessen, B.; Teuben, J. H.
Angew. Chem., Int. Ed. 2001, 40, 2516. (c) Andes, C.; Harkins, S. B.;
Murtuza, S.; Oyler, K.; Sen, A. J. Am. Chem. Soc. 2001, 123, 7423. (d)
Carter, A.; Cohen, S. A.; Cooley, N. A.; Murphy, A.; Scutt, J.; Wass, D.
F. Chem. Commun. 2002, 858.
industrial use of this and other systems. The results presented in
Table 2 show that the complexes give activity comparable to those
of the best trimerization systems available, but that significantly
enhanced selectivity is achieved. While trimerization systems of
(2) McGuinness, D. S.; Wasserscheid, P.; Keim, W.; Hu, C.; Englert, U.;
like high activity1b,d give <93% selectivity to C
Dixon, J. T.; Grove, C. Chem. Commun. 2003, 334.
3) Wu, F.-J. U.S. Patent 5,811,618, 1998.
6
, the very high
(
selectivities obtained with this system represent an important
advance in terms of process economy.
(4) Bay o´ n, J. C.; Claver, C.; Masdeu-Bult o´ , A. M. Coord. Chem. ReV. 1999,
193-195, 73.
(5) Konrad, M.; Meyer, F.; Heinze, K.; Zsolnai, L. J. Chem. Soc., Dalton
There are very few examples of structurally characterized
Trans. 1998, 199.
III 7
8
(6) Danopoulos, A. A.; Wills, A. R.; Edwards, P. G. Polyhedron 1990, 9,
thioether complexes of Cr , and, in all but one, the thioether donor
forms part of a macrocyclic ring. Single crystals of 2 suitable for
X-ray diffraction studies were grown by slow evaporation of a
nitromethane solution of the complex. The molecular structure of
2413.
(7) (a) Pope, S. J. A.; Champness, N. R.; Reid, G. J. Chem. Soc., Dalton
Trans. 1997, 1639. (b) K u¨ ppers, H.-J.; Wieghardt, K. Polyhedron 1989,
8, 1770. (c) Champness, N. R.; Jacob, S. R.; Reid, G.; Frampton, C. S.
Inorg. Chem. 1995, 34, 396. (d) Bruce, J. I.; Gahan, L. R.; Hambley, T.
W.; Stranger, R. Chem. Commun. 1993, 702. (e) Grant, G. J.; Rogers, K.
E.; Setzer, W. N.; VanDerveer, D. G. Inorg. Chim. Acta 1995, 234, 35.
8) Pattanayak, S.; Das, D. K.; Chakraborty, P.; Chakravorty, A. Inorg. Chem.
1995, 34, 26.
2
, along with selected bond lengths and angles, is shown in Figure
9
1. It is instructive to compare the ligand bonding parameters of 2
(
with those in the closely related PNP-Cr trimerization catalyst
2
(9) Crystal data: monoclinic space group P2
1
/c, a ) 7.6255(12), b )
CrCl
3
{HN(CH
2 2
CH PPh
2
)
2
}, which incorporates ligand I. In both
3
1
3.059(5), c ) 14.3703(10) Å, â ) 90.790(11)°, V ) 1430.9(6) Å , Z )
-3 -1
complexes, the Cr displays the expected octahedral coordination
geometry, with the tridentate ligand coordinated in a meridonal
fasion. The chelate bite angles of the SNS ligand in 2 [83.07(5)°,
4, D
c
) 1.633 g‚cm , µ ) 1.622 mm , F(000) ) 724, 2θmax ) 54°,
4
013 reflections, 3126 independent data. Convergence for 138 parameters
at wR2 ) 0.0857, R1 ) 0.0351, GOF ) 1.074 for all data and R1 )
0.0309 for 2846 reflections with I > 2(I). Residual electron density was
3
0
.439 and -0.549 e‚Å-
.
8
2.90(5)°] compare similarly to those in the PNP complex
[81.08(8)°, 82.07(8)°]. The Cr-S distances [2.4508(7), 2.4556(7)
JA034752F
J. AM. CHEM. SOC.
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VOL. 125, NO. 18, 2003 5273