Ethylene trimerisation and tetramerisation catalysts with polar-substituted diphosphinoamine ligands

Article abstract of DOI:10.1039/b412432d

Chromium-based catalyst systems with polar-substituted diphosphinoamine ligands are selective for either trimerisation or tetramerisation of ethylene, depending on the position of the polar groups on the aryl rings. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b412432d

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COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Ethylene trimerisation and tetramerisation catalysts with polar-
substituted diphosphinoamine ligands{
Matthew J. Overett,* Kevin Blann, Annette Bollmann, John T. Dixon, Fiona Hess, Esna Killian,
Hulisani Maumela, David H. Morgan, Arno Neveling and Stephanus Otto
Received (in Cambridge, UK) 12th August 2004, Accepted 25th October 2004
First published as an Advance Article on the web 10th December 2004
DOI: 10.1039/b412432d
Chromium-based catalyst systems with polar-substituted
diphosphinoamine ligands are selective for either trimerisation
or tetramerisation of ethylene, depending on the position of the
polar groups on the aryl rings.
New technologies for the selective oligomerisation of ethylene to
valuable 1-alkenes such as 1-hexene and 1-octene remain an
imperative in the co-monomer industry.1 To this end, we have
previously reported several new chromium-based catalysts for the
selective trimerisation of ethylene to 1-hexene.2
Recently, a highly active ethylene trimerisation system, gener-
ated in situ using a chromium source, a diphosphinoamine ligand
of the type Ar2PN(Me)PAr2 (Ar 5 ortho-methoxyaryl) and an
alkyl aluminoxane activator, was reported.3 It was claimed that the
ortho-methoxy groups act as pendant donors to the metal centre,4
and are essential for catalytic activity. We subsequently reported
the use of related chromium–diphosphinoamine systems for the
Changing the pattern of aryl substitution progressively from
trimerisation of ethylene to 1-hexene5 and the unprecedented
ortho (1, runs 1 and 2) to meta (2, runs 3 and 4) to para (3, runs 5
selective tetramerisation of ethylene to 1-octene.6 ortho-
and 6) changes the selectivity of the catalyst from selective
Substitution of the diphosphinoamine ligand Ar2PN(R)PAr2 with
trimerisation to predominantly tetramerisation (50% C8 in run 5).
non-coordinating groups (Ar 5 ortho-alkylaryl) surprisingly
The selectivities obtained with ligand 3 were thus closer to those
resulted in a highly selective ethylene trimerisation system, while
obtained with the unsubstituted analogue 46 (runs 7 and 8) than
using an unsubstituted ligand (Ar 5 Ph) gave tetramerisation of
with the electronically similar 1. meta-Substituted ligand 2 gave
ethylene to 1-octene with selectivities of up to 71%. The nature and
poor activity and higher PE formation, but C6 and C8 selectivities
position of substituents on the ligands’ aryl groups were thus
obeyed the trend described above.
As reported for the Ph2PN(R)PPh2 tetramerisation systems,6
found to be crucial to the selectivity of these oligomerisation
reactions, and it is clear that pendant coordination through aryl
replacement of the N-methyl by the bulkier isopropyl group
donor-substituents is not alone essential for catalytic activity. We
improves both catalytic activity and selectivity to C6 and C8. The
report here the results of a study into the role of polar aryl-
best tetramerisation results were thus obtained with 5, which gave
substitution in chromium–diphosphinoamine ethylene trimerisa-
a C8 selectivity of 68% with activity of 112700 g/g Cr.h in one
example (run 10). Compared with the unsubstituted analogue 66
tion and tetramerisation systems.
The active catalyst was obtained by introducing a mixture of a
(runs 11 and 12), para-methoxy substituted ligand 5 results in
chromium(III) source, typically Cr(acac)3, and the ligand to an
similar C8 and higher C6 selectivities, although at lower activity.
autoclave containing solvent and methylaluminoxane (MAO). The
Other significant product components include cyclic compounds,
autoclave was then pressurised with ethylene and maintained at
particularly methylcyclopentane and methylenecyclopentane in the
pressure and temperature throughout the course of the reaction.
C6 fraction, and a complex array of C10 to C14 oligomers resulting
Ligands 1–9 were prepared and tested under two sets of
from co-trimerisation and co-tetramerisation reactions of ethylene
conditions, and the results are shown in Table 1.
with 1-hexene or 1-octene.
In general, higher activities are observed at 45 bar, 45 uC than at
30 bar, 65 uC. This can be attributed to both the higher ethylene
pressure and reduced catalyst deactivation at lower temperatures.
{ Electronic supplementary information (ESI) available: experimental
details of the new ligand syntheses and catalyst screening runs. See http://
www.rsc.org/suppdata/cc/b4/b412432d/
*matthew.overett@sasol.com
With some of the ligands (2–7), higher PE formation occurs at
45 bar, 45 uC, and, in general, higher C8 at the expense of C6 is
observed at 45 bar, 45 uC compared with 30 bar, 65 uC.
622 | Chem. Commun., 2005, 622–624
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Table 1 Ethylene oligomerisation results
1-hexene in
C6 fraction
(%)
1-octene in
C8 fraction C10–C14
Pressure
(bar)
Temperature Activity
(uC)
PE
(wt %)
C6
(wt %)
C8
(wt %)
Runa
Ligand
(g/g Cr.h)
(%)
(wt %)
1
2b
3
4
5
1
1
2
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9
30
45
30
45
30
45
30
45
30
45
30
45
30
45
30
45
30
45
65
45
65
45
65
45
65
45
65
45
65
45
65
45
65
45
65
45
25000
159600
12800
25400
45200
54600
26500
44000
72300
112700
11700
272400
154000
45600
76300
243900
15800
159300
, 1
, 1
11
19
, 1
12
1
40
1
3
, 1
1
1
7
, 1
, 1
, 1
, 1
91
82
30
16
26
17
25
4
39
24
33
17
70
48
74
63
47
27
. 99
. 99
67
55
47
35
39
23
88
74
87
70
. 99
. 99
98
98
89
7
13
33
22
50
38
59
22
51
68
61
68
2
. 99
, 1
5
9
10
11
14
10
11
8
4
6
7
26
33
20
19
3
. 99
92
90
94
91
94
92
99
99
. 99
99
. 99
. 99
99
98
94
98
6
7c
8
9
10d
11c
12e
13
14
15
16
17
18
a
6
6
17
47
63
72
5
Standard conditions unless otherwise stated: 0.033 mmol Cr(acac)3, 2 eq. ligand, 300 eq. activator, 100 ml toluene solvent, 30 minutes (or
b
time taken to fill the reactor if less). 0.02 mmol Cr(THF)3Cl3. 0.033 mmol Cr(THF)3Cl3. 0.015 mmol Cr(THF)3Cl3, 1.2 eq. ligand.
c
d
e
0.022 mmol Cr(acac)3.
We have shown that the selectivity of ortho-alkyl substituted
diphosphinoamine oligomerisation systems can be shifted from
trimerisation to tetramerisation by reducing the number of such
substituents stepwise from four to zero.5 The selectivity in these
systems is thus clearly mediated by a steric effect. To establish
whether the selectivity of the ortho-methoxy substituted diphos-
phinoamine systems is determined by a similar steric effect or
by pendant coordination as proposed,3 ligands with two (7) and
one (8) ortho-methoxy groups were prepared and evaluated.
Interestingly, only a small shift in selectivity towards C8 is observed
on reducing the number of ortho-methoxy substituents (runs 13–
16). Ligand 8 (runs 15 and 16) remains predominantly selective
for trimerisation, with a maximum of 17% C8 observed. By
comparison, the sterically similar ortho-ethyl substituted ligand 95
resulted in a C8 selectivity of 63% under the same conditions
(run 18).
b-hydride transfer. In the case of ortho-methoxy substituents,
competitive coordination by the pendant donors may retard the
coordination and insertion of ethylene into the metallacycle. In this
way, the trimerisation behaviour of these two groups of catalysts
may be rationalised. Further experimental and theoretical
investigations into the mechanism and selectivity of these selective
oligomerisation reactions and the nature of the catalytic species are
ongoing.
In conclusion, we have shown that the nature, position and
number of aryl-substituents on diphosphinoamine ligands play an
important role in determining the selectivity of chromium
catalysed oligomerisation reactions. ortho-Substitution at the
diphosphinoamine aryl rings is key to the switch between
tetramerisation and trimerisation selectivity, which may be
mediated by either steric crowding around the catalytic centre
(non-polar substitution) or by pendant coordination of a donor
substituent.
From these results it is clear that two separate effects can
mediate the switch from tetramerisation to trimerisation. Firstly,
steric bulk in the immediate vicinity of the metal centre favours
trimerisation, and a direct relationship between the extent of
crowding (number of ortho substituents) and the preference for
trimerisation may be observed.5 Secondly, however, a coordination
effect may also be deduced from the predominantly trimerisation
behaviour of 8. Only a single ortho-methoxy group is required to
interact with the metal centre, possibly as a hemilabile donor. This
explains why reduction in the number of potentially coordinating
substituents does not significantly affect the preference of the
system for trimerisation.
The authors thank Sasol Technology (Pty) Ltd for permission to
publish this work.
Matthew J. Overett,* Kevin Blann, Annette Bollmann, John T. Dixon,
Fiona Hess, Esna Killian, Hulisani Maumela, David H. Morgan,
Arno Neveling and Stephanus Otto
Sasol Technology (Pty) Ltd, R&D Division, 1 Klasie Havenga Road,
Sasolburg, 1947, South Africa. E-mail: matthew.overett@sasol.com;
Fax: 27 (0)11 522 9696; Tel: 27 (0)16 960 5407
Notes and references
1 For a recent review see: J. T. Dixon, M. J. Green, F. M. Hess and
D. H. Morgan, J. Organomet. Chem., 2004, 689, 3641.
It seems reasonable to assume that 1-hexene and 1-octene are
formed via a common metallacycloheptane intermediate.7 Thus it
may be postulated that the selectivity towards these products is
determined by the relative rates of b-hydride transfer8 (1-hexene)
vs. further ethylene insertion to form a metallacyclononane
(1-octene) from this species. In the case of ortho-alkyl substituted
ligands, steric bulk around the catalytic centre may constrain the
ring into a more favourable conformation for metal-mediated
2 (a) H. Mahomed, A. Bollmann, J. Dixon, V. Gokul, L. Griesel,
C. Grove, F. Hess, H. Maumela and L. Pepler, Appl. Catal., A: Gen.,
2003, 255, 355; (b) D. H. Morgan, S. L. Schwikkard, J. T. Dixon,
J. J. Nair and R. Hunter, Adv. Synth. Catal., 2003, 345, 939; (c)
D. S. McGuinness, P. Wasserscheid, W. Keim, C. Hu, U. Emglert,
J. T. Dixon and J. J. C. Grove, Chem. Commun., 2003, 334; (d)
D. S. McGuinness, P. Wasserscheid, W. Keim, D. H. Morgan,
J. T. Dixon, A. Bollmann, H. Maumela, F. M. Hess and U. Englert,
J. Am. Chem. Soc., 2003, 125, 5272.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 622–624 | 623

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3 A. Carter, S. A. Cohen, N. A. Cooley, A. Murphy, J. Scutt and
D. F. Wass, Chem. Commun., 2002, 858.
M. J. Overett, A. M. Z. Slawin, P. Wasserscheid and S. Kuhlmann,
J. Am. Chem. Soc., 2004, 126, 14712; (b) K. Blann, A. Bollmann,
J. T. Dixon, A. Neveling, D. H. Morgan, H. Maumela, E. Killian,
F. M. Hess, S. Otto, L. Pepler, H. A. Mahomed and M. J. Overett,
Pat. Appl., WO 2004/056479, 2002 (to Sasol Technology (Pty)
Ltd).
7 J. R. Briggs, J. Chem. Soc., Chem. Commun., 1989, 674.
8 Original mechanistic proposals were for a formal b-hydride elimination
to the metal, followed by reductive elimination of the resultant hexenyl
hydride species. More recent calculations for Ti and Cr trimerisation
systems have supported a concerted metal-assisted hydride transfer from
C2 to C6 of the metallacycloheptane. See (a) Z. Yu and K. N. Houk,
Angew. Chem., Int. Ed., 2003, 42, 808; (b) W. J. van Rensburg, C. Grove´,
J. P. Steynberg, K. B. Stark, J. J. Huyser and P. J. Steynberg,
Organometallics, 2004, 23, 1207.
4 A subsequently published single crystal structure of [Ar2PN-
(Me)PAr2]CrCl3 (Ar 5 o-methoxyphenyl) shows the ligand coordinat-
ing in a tridentate (P,P,O) mode. See T. Agapie, S. J. Schofer,
J. A. Labinger and J. E. Bercaw, J. Am. Chem. Soc., 2004, 126, 1304.
5 (a) K. Blann, A. Bollmann, J. T. Dixon, F. H. Hess, E. Killian,
H. Maumela, D. H. Morgan, A. Neveling, S. Otto and M. J. Overett,
Chem. Commun., 2004, DOI: 10.1039/b412431f, preceding article; (b)
K. Blann, A. Bollmann, J. T. Dixon, A. Neveling, D. H. Morgan,
H. Maumela, E. Killian, F. M. Hess, S. Otto, L. Pepler, H. A. Mahomed
and M. J. Overett, Pat. Appl., WO 2004/056477, 2002 (to Sasol
Technology (Pty) Ltd).
6 (a) A. Bollmann, K. Blann, J. T. Dixon, F. M. Hess, E. Killian,
H. Maumela, D. S. McGuinness, D. H. Morgan, A. Neveling, S. Otto,
624 | Chem. Commun., 2005, 622–624
This journal is ß The Royal Society of Chemistry 2005