Phosphanyl ureas as ligands for ethylene oligomerization: A cascade reaction

Article abstract of DOI:10.1080/10426500802004352

The phosphanyl urea ligands MeNHC(O)NMePR2 (1: R = Ph and 2: R = 2,4-tBu2C6H3O) form effective ethylene oligomerization catalysts with [Ni(cod)2] (cod = 1,5 cyclooctadiene). Whereas the primary product with 1 is butene, 2 initiates a cascade reaction leading to hexenes and octenes. In the first reaction cycle ethylene is dimerized to butene, which serves as feedstock for the second dimerization cycle to the final products hexenes and octenes. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426500802004352

This article was downloaded by: [Florida International University]
On: 29 December 2014, At: 00:47
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,
UK
Phosphorus, Sulfur, and Silicon
and the Related Elements
Publication details, including instructions for
authors and subscription information:
http://www.tandfonline.com/loi/gpss20
Phosphanyl Ureas as Ligands
for Ethylene Oligomerization: A
Cascade Reaction
Olaf Kühl a , Peter Lobitz a & Normen Peulecke a
a Institut für Biochemie, Ernst Moritz Arndt
Universität Greifswald , Greifswald, Germany
Published online: 07 Oct 2008.
To cite this article: Olaf Kühl , Peter Lobitz & Normen Peulecke (2008) Phosphanyl
Ureas as Ligands for Ethylene Oligomerization: A Cascade Reaction, Phosphorus,
Sulfur, and Silicon and the Related Elements, 183:11, 2777-2783
To link to this article: http://dx.doi.org/10.1080/10426500802004352
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the
information (the “Content”) contained in the publications on our platform.
However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness,
or suitability for any purpose of the Content. Any opinions and views
expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the
Content should not be relied upon and should be independently verified with
primary sources of information. Taylor and Francis shall not be liable for any
losses, actions, claims, proceedings, demands, costs, expenses, damages,
and other liabilities whatsoever or howsoever caused arising directly or
indirectly in connection with, in relation to or arising out of the use of the
Content.

This article may be used for research, teaching, and private study purposes.
Any substantial or systematic reproduction, redistribution, reselling, loan,
sub-licensing, systematic supply, or distribution in any form to anyone is
expressly forbidden. Terms & Conditions of access and use can be found at
http://www.tandfonline.com/page/terms-and-conditions

Phosphorus, Sulfur, and Silicon, 183:2777–2783, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500802004352
Phosphanyl Ureas as Ligands for Ethylene
Oligomerization: A Cascade Reaction
¨
Olaf Kuhl, Peter Lobitz, and Normen Peulecke
Institut fu¨r Biochemie, Ernst Moritz Arndt Universita¨t Greifswald,
Greifswald, Germany
The phosphanyl urea ligands MeNHC(O)NMePR (1: R = Ph and 2: R = 2,4-
2
tBu C H O) form effective ethylene oligomerization catalysts with [Ni(cod) ] (cod
2
6
3
2
= 1,5 cyclooctadiene). Whereas the primary product with 1 is butene, 2 initiates a
cascade reaction leading to hexenes and octenes. In the first reaction cycle ethylene
is dimerized to butene, which serves as feedstock for the second dimerization cycle
to the final products hexenes and octenes.
Keywords Cascade reaction; ethylene oligomerization; homogenous catalysis; nickel;
phosphino urea; phosphino amine; transition metal
INTRODUCTION
There is great interest to develop catalytical systems for the copoly-
merization of ethylene with other olefins.1 A particularly challenging
task is to generate the comonomer from ethylene during the reaction.
This can be achieved by a few tandem processes recently published by
Bazan et al.,2 in which two catalysts operate simultaneously. A com-
mon drawback of commercial ethylene polymerization systems is the
need for a cocatalyst to activate the precatalyst species prior to the
catalytic chain reaction. These activators are metal containing species
that are employed in huge excess (up to 1000 fold) and thus create a
major waste and product purification problem. This and reproducibility
problems are the reasons for a great industrial interest to find ethylene
oligomerization and polymerization processes that dispense with the
need for cocatalysts.3
Received 6 February 2008; accepted 16 February 2008.
We thank Prof J. Heinicke, for his ongoing support and the use of his laboratory
facilities.
Address correspondence to PD Dr. Olaf Ku¨hl, Institut fu¨r Biochemie, Ernst Moritz
Arndt Universita¨t Greifswald, Felix-Hausdorff-Strasse 4, D-17489 Greifswald, Germany.
E-mail: dockuhl@gmail.com
2777

2778
O. Ku¨ hl et al.
RESULTS AND DISCUSSION
The ligands 1 and 2 were prepared from N,Nꢀ-dimethyl urea and the
appropriate chlorophosphine or chlorophosphite in THF with triethyl
amine as auxiliary base (Scheme 1).4 Toluene or dichloromethane as
solvents lead in the case of chlorodiphenylphosphine to the disubsti-
tuted product.
SCHEME 1 Synthesis of the ligands.
Reaction of the ligands 1 and 2 with [Ni(cod)2] in toluene affords
dark orange solutions. When these solutions are transferred into a steel
autoclave with magnetic stirring and 50 bar ethylene pressure and are
then placed into a 100◦C hot oil bath, ethylene is oligomerized to C4-C8
olefins after a short initiation period. Attempts to carry out the reaction
at lower temperatures failed.
The structure of the active catalyst is unknown, but it can be as-
sumed that ligand 1 or 2 binds to nickel with the phosphorus terminus
displacing one of the COD ligands. Oxidative addition of the N-H group
to nickel then renders the catalytically active Ni-H bond. Keim et al.
have established a mechanism for the formation of the active catalyst
from [Ni(COD)2] that might very well be operative in the present case.5
The two ligands 1 and 2 were carefully selected for their relative
electronic properties and steric requirements. 2 has the sterically more
demanding and the more electron withdrawing phosphorus group com-
pared to 1. With the Brookhart catalysts,6 the steric bulk of the ligand
provides steric shielding of the metal’s axial positions usually resulting
in ethylene polymerization rather than oligomerization for sterically
less demanding ligands. 2 is obviously incapable of effectively shield-
ing both axial positions at nickel since the steric bulk is entirely at the
phosphorus atom leaving the nitrogen end wide open for chain transfer

Phosphanyl Ureas as Ligands for Ethylene Oligomerization
2779
SCHEME 2 Proposed mechanism of the cascade reaction using ligand 2.
reactions. However, it was hoped to see a difference in chain length of
the resulting oligomer distribution by going from 1 to 2. Similarily, a
decrease in electron density on the metal due to a more electron with-
drawing ligand is expected to result in faster ethylene consumption and
thus greater TOF values.7,8
Running the ethylene oligomerization reactions under equal
conditions—100 µmol catalyst, 50 bar initial ethylene pressure, 100◦C

2780
O. Ku¨ hl et al.
16
14
12
10
8
16
14
12
10
8
6
4
2
0
6
4
2
0
t in sec
t in sec
FIGURE 1 Pressure time plot for ethylene oligomerization with ligand 1 (left)
and 2 (right).
oil bath temperature, and toluene as solvent—is accompanied indeed by
different product distributions and a marked difference in the reaction
rates. Whereas 3 yields predominantly 1-butene and n-hexenes with a
small amount of octenes and an average TOF of 6,000 mol ethylene h−1
(mol Ni)−1, 4 results in branched hexenes with a much smaller butene
fraction and larger share of branched octenes with a significantly in-
creased average TOF of 15,000 mol ethylene h−1 (mol Ni)−1 during the
first cycle 9. The second cycle displays significantly smaller TOF.
Surprisingly, the main difference between the two ligands was not
the chain length or the difference in reaction rates, but the preference
of branched oligomers for 4 as opposed to linear ones for 3. The pressure
time plots for the two batch oligomerizations (see Figure 1) also display
a marked difference. Whereas the plot for 3 shows normal exponential
decay, the plot for 4 displays a transition point where the rate of decay
becomes noticeably smaller. As the pressure time plots usually show
decay down to a base value (4 mV) corresponding to the butene formed
during the reaction, any pressure above this base value originates from
the ethylene present in the reaction mixture. A transition point in the
pressure time plot thus indicates a sudden decrease in the ethylene
uptake rate and thus the occurrence of a second reaction that does not
rely on ethylene as a major feedstock.
Uptake of oligomers formed during the reaction cannot be shown
from the product distribution simply because branching can have two
possible causes.7 As the oligomer chain grows, a series of β-hydrogen
eliminations and subsequent reinsertion normally referred to as “chain
walking”10 leads to isomerization and inner olefins that after adding
another ethylene unit result in a branched product. The same product
may be obtained by inserting an olefinic oligomer into a metal ethyl
bond. The former process is clearly ethylene oligomerization, whereas
the latter can best be referred to as ethylene/oligomer cooligomeriza-
tion.

Phosphanyl Ureas as Ligands for Ethylene Oligomerization
2781
80
70
60
50
40
30
20
10
0
70
60
50
40
1-
Anteil
2-
3-
30
20
10
0
Buten
Hexene
Octene
Butene
Hexene
Octene
FIGURE 2 Product distribution for ethylene oligomerization with ligand 1
(left) and 2 (right).
Analysis of the pressure time plot can be helpful in distinguishing
between the two mechanisms. In chain walking isomerization has to
be fast compared to chain propagation and thus the reaction depends
on the ethylene uptake. The result is a single rate pressure time plot
without any transition points. If during the reaction the uptake and
insertion of any oligomer formed becomes significant, then the reaction
continues via oligomer uptake at the expense of ethylene consumption.
This results in more shallow pressure decay and thus a transition point
in the pressure time plot as seen for 4.
From the product distribution (see Figure 2) for the two ligands it can
be seen that the chain length for the products from 4 is essentially one
ethylene group longer than for 3. To infer from this, that the steric re-
quirements of 2 result in an increased preference for chain propagation
compared to 1, could be ill advised. It might only reflect the difference in
feedstock used by the two dimerization catalysts. Catalyst 3 dimerizes
ethylene to butenes with a significant amount of trimerization occur-
ring, whereas 4 seemingly generates octenes by dimerization of butene
rather than chain propagation using ethylene as feedstock.
The process observed with 4 can be explained as a cascade reac-
tion (see Scheme 2). In the first step the catalyst dimerizes ethylene to
butene that is used as feedstock in the second dimerization reaction.
The butyl complex initially formed can then either use ethylene to form
hexenes in a heterodimerization reaction or a second molecule of butene
in a homodimerization reaction to form octenes. Such reactions are rare
in ethylene oligomerization reactions and usually occur only as a mi-
nor side reaction at prolonged reaction times.7,11 Here incorporation of
higher olefins is the main reaction after only 20 min.
The second process in the cascade is slower than the first as seen
from the pressure time plot in combination with the product distribu-
tion. However, as butene binds more strongly to nickel than ethylene12
the butyl complex is formed and cycle 1 is suppressed, once a critical

2782
O. Ku¨ hl et al.
concentration of butene is reached or the solution is sufficiently depleted
of ethylene due to low solubility of ethylene in toluene.
Even in the second cycle uptake of ethylene by the intermediate butyl
complex is more rapid than that of butene as can be seen from the
product distribution (far more hexene than octene is formed).
EXPERIMENTAL
All operations with phosphines and catalyst solutions were carried
out under carefully dried, oxygen-free argon, using Schlenk tech-
niques. Toluene was ketyl-dried and distilled before use. Ni(COD)2,13
phosphanylurea4 and phosphitylurea4 were synthesized as reported;
all other chemicals were purchased. Ethylene (99.5%, Air Liquide) was
used without further treatment. GC analyses were carried out using a
gas chromatograph Hewlett Packard 5890, column HP-5(30 m) (cross
linked 5% PhMe silicone), 40–150◦C, 10 min isotherm, 4◦C/min. The
turnover frequency (TOF) was determined as average TOF over the ef-
fective reaction time. The effective reaction time was monitored with
online pressure registration (HEJU pressure sensor 1–100 bar from
Juchheim connected to digital multimeter and PC) with the end point
detected when a stable pressure was reached (pressure caused by low
boiling point products, i.e., butene).
Ethylene Oligomerization—General Procedure
Compound 1 (27.8 mg, 100 µmol) or compound 2 (29.0 mg, 100 µmol)
and [Ni(COD)2] (27.5 mg, 100 µmol) were dissolved in toluene (each in 8
mL) at 0◦C, combined, and stirred for 5 min at 0◦C and for 10 min at 20◦C
to give orange or yellow solutions. The solutions were transferred by a
syringe (Teflon canula) to a stainless-steel autoclave (75 mL), equipped
with a Teflon coated magnetic stirrer, gas inlet and sample inlet valves,
mechanic or electronic pressure gauge, and a safety diaphragm. Ethy-
lene was added (pstart ca. 50 bar), and the autoclave was placed into a
preheated bath (100◦C) and heated overnight (ca. 15 h). After cooling
unreacted ethylene was allowed to escape, volatiles were condensed in
a cooling trap (−78◦C); mass loss was usually <2%.
REFERENCES
[1] F. Speiser, P. Braunstein, L. Saussine, and R. Welter, Organometallics, 23, 2613
(2004).
[2] (a) Z. J. A. Komon, G. M. Diamond, M. K. Leclerc, V. Murphy, M. Okazaki, and G. C.
Bazan, J. Am. Chem. Soc., 124, 15280 (2002); (b) Z. J. A. Komon, X. Bu, and G. C.

Phosphanyl Ureas as Ligands for Ethylene Oligomerization
2783
Bazan, J. Am. Chem. Soc., 122, 1830 (2000); (c) R. W. Barnhart, G. C. Bazan, and T.
Mourey, J. Am. Chem. Soc., 120, 1082 (1998).
[3] (a) C. Wang, S. Friedrich, T. R. Younkin, R. T. Li, R. H. Grubbs, D. A. Bansleben, and
M. W. Day, Organometallics, 17, 3149 (1998); (b) S. Y. Desjardins, K. J. Cavell, J.
L. Hoare, B. W. Skelton, A. N. Sobolev, A. W. White, and W. J. Keim, J. Organomet.
Chem., 544, 163 (1997); (c) J. L. Hoare, K. J. Cavell, R. Hecker, B. W. Skelton, and
A. W. White, J. Chem. Soc., Dalton Trans., 2197 (1996).
[4] (a) O. Ku¨hl, P. Lo¨nnecke, Inorg. Chem., 41, 4315 (2002); (b) O. Ku¨hl, Dalton Trans.,
949 (2003); (c) O. Ku¨hl, W. Langel, Inorg. Chem. Commun., 6, 74 (2003); (d) S.
Blaurock, O. Ku¨hl, and E. Hey-Hawkins, Organometallics, 16, 807 (1997); (e) O.
Ku¨hl, S. Blaurock, Inorg. Chem., 43, 6543 (2004); (f) O. Ku¨hl, S. Blaurock, and T.
Carls, Inorg. Chem., 45, 1723 (2006); (g) O. Ku¨hl, Coord. Chem. Rev., 250, 2867
(2006).
[5] M. Peuckert and W. Keim, Organometallics, 2, 594 (1983).
[6] (a) G. J. P. Britovsek, V. C. Gibson, and D. F. Waas, Angew. Chem. Int. Ed., 38,
428 (1999); (b) A. L. McKinnon and M. C. Baird, J. Organomet. Chem., 683, 114
(2003); (c) C. Bianchini, L. Gonsalvi, W. Oberhauser, D. Se´meril, P. Bru¨ggeler, and
R. Gutmann, Dalton Trans., 3869 (2003).
[7] S. A. Svejda and M. Brookhart, Organometallics, 18, 65 (1999).
[8] (a) W.-H. Sun, W. Zhang, T. Gao, X. Tang, L. Chen, Y. Li, and X. Jin, J. Organomet.
Chem., 689, 917 (2004); (b) J. M. Malinoski and M. Brookhart, Organometallics, 22,
5324 (2003).
[9] The average TOF was measured over the entire period of ethylene uptake. As the
amount of available ethylene decreased in the course of the reaction, the maximum
TOF of the reaction is significantly higher.
[10] M. D. Leatherman, S. A. Svejda, L. K. Johnson, and M. Brookhart, J. Am. Chem.
Soc., 125, 3068 (2003).
[11] (a) B. L. Small and M. Brookhart, J. Am. Chem. Soc., 120, 7143 (1998); (b) F. Speiser,
P. Braunstein, and L. Saussine, Organometallics, 23, 2625 (2004).
[12] J. Heinicke, M. Ko¨hler, N. Peulecke, M. He, M. K. Kindermann, W. Keim, and G.
Fink, Chem. Eur. J., 9, 6093 (2003), and references cited therein.
[13] R. A. Shum, Inorg. Synth. 15, 5 (1974).