An efficient keim-type catalyst based on an electron-poor P,O-chelate; tuning the selectivity of ethylene oligomerisation towards short α-olefins

Article abstract of DOI:10.1002/ejic.200401015

The tertiary phosphane Ph₂P-pzONa, in which the phosphorus atom is substituted by an electron-withdrawing pyrazolonato unit, reacts with trans-[NiPhCl(PPh₃)₂] to afford quantitatively trans-P,P′-[NiPh(Ph₂P-pzO)(

Full text of DOI:10.1002/ejic.200401015

FULL PAPER
An Efficient Keim-Type Catalyst Based on an Electron-Poor P,O-Chelate;
Tuning the Selectivity of Ethylene Oligomerisation towards Short α-Olefins
Pierre Kuhn,[a] David Sémeril,[a] Catherine Jeunesse,[a] Dominique Matt,*[a] Pierre Lutz,*[b]
and Richard Welter[c]
Keywords: Nickel P,O complexes / 4-Phosphanylpyrazolonate / Oligomerisation / Phosphorus ylides / Electron-withdrawing
ligands / X-ray diffraction
The tertiary phosphane Ph2P-pzONa, in which the phospho-
rus atom is substituted by an electron-withdrawing pyrazol-
onato unit, reacts with trans-[NiPhCl(PPh3)2] to afford quan-
titatively trans-P,PЈ-[NiPh(Ph2P-pzO)(PPh3)] (4), the first
Keim-type catalyst derived from an amide. In the absence of
any cocatalyst, complex 4 converts ethylene in high selectiv-
ity and activity into α-olefins. The product distribution is
drastically shifted towards lower oligomers when compared
to the outcome of the reaction carried out with the conven-
tional SHOP catalyst. Thus, when operating at 1 bar, over
99 wt.-% of the oligomers formed are C4–C16 oligomers. In
ethylene-free solvents, 4 was shown to slowly undergo re-
ductive elimination to produce the stabilised phosphorus
ylide Ph3P-pzO (5). Both compounds (4 and 5) were charac-
terised by single-crystal X-ray diffraction analyses.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
nickel complexes with electron-rich P,O-chelates,[10,11] while
a few nickel complexes in which the P,O ligand behaves as
a neutral 4-electron donor have also been studied.[12]
Introduction
Square-planar nickel(ii) complexes of the general for-
mula trans-P,PЈ-[NiAr{Ar2PCH=C(O)RЈ}(PR3)] (1; R,
RЈ = aryl, alkyl) have been extensively studied over the last
25 years, mainly because of their ability to oligomerise eth-
ylene into α-olefins under mild conditions.[1–7] The discov-
ery of this catalyst type stretches back to the early seventies,
when researchers at SHELL discovered that mixing Ni0
complexes with phosphorus ylides, e.g. Ph3PCH=C(O)Ph
(2), produced highly active oligomerisation catalysts.[8]
Later, Keim et al. showed that reaction of 2 with Ni(cod)2
in the presence of PPh3 resulted in an oxidative addition
reaction with cleavage of a P–Ph(ylide) bond and subse-
quent formation of the phosphanyl-enolato complex
[NiPh{Ph2PCH=C(O)Ph}(PPh3)] (1a), known as Keim’s
oligomerisation catalyst.[9] Since Keim’s pioneering studies,
a number of related P,O-nickel complexes (P,O = 3-electron
chelate) have been prepared and studied, but surprisingly
little is known about variants in which the P,O-chelate be-
haves as an electron-poor donor, although the complex
[NiH{Ph2PCH2C(CF3)2O}PCy3] was reported to be inac-
tive. Note that several recent publications have focussed on
We now describe the convenient, direct synthesis of the
new Keim-type catalyst 4 starting from a PIII derivative,
namely the sodium pyrazolonato-phosphane Ph2P-pzONa
(3·Na).[13] Our investigations show how the presence of the
strong electron-withdrawing pyrazolonato ring,[14] known
to be stronger than CF3, drastically drives the product dis-
tribution towards shorter olefins. The present study furthers
demonstrates that “Keim catalysts” may undergo reductive
elimination with formation of a phosphorus ylide. The re-
ported complex 4 is the first example of a Keim-type cata-
lyst derived from an amide.
[a] Université Louis Pasteur, Laboratoire de Chimie Inorganique
Moléculaire, UMR 7513 CNRS,
1 rue Blaise Pascal, 67008 Strasbourg Cedex France
E-mail: dmatt@chimie.u-strasbg.fr
[b] Institut Charles Sadron, UPR 22 CNRS,
6 rue Boussingault, 67083 Strasbourg Cedex
[c] DECMET, UMR 7513,
4 rue Blaise Pascal, 67070 Strasbourg Cedex France
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
Eur. J. Inorg. Chem. 2005, 1477–1481
DOI: 10.1002/ejic.200401015
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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P. Kuhn, D. Sémeril, C. Jeunesse, D. Matt, P. Lutz, R. Welter
FULL PAPER
Results and Discussion
Reaction of [NiPhCl(PPh3)2] with one equiv. of Ph2P-
pzONa in toluene afforded complex 4 in ca. 90% yield. It
is noteworthy that this reaction constitutes a rare case in
which a 1a-analogue was obtained from a phosphanylenol-
ate rather than a phosphorus ylide. The 31P NMR spectrum
of 4 displays an AB quartet (δA/δB = 20.2/–8.6 ppm) with a
J(PPЈ) coupling constant of 280 Hz, typical for trans-ar-
ranged phosphorus atoms. The phosphorus signal of the P-
enolato moiety (δ = –8.6 ppm) is strongly shifted to high
field with respect to that of other diphenylphosphanylenol-
ates reported in the literature, thus reflecting the high elec-
tron-withdrawing nature of the pyrazolonato ring. The so-
lid-state structure of 4 was elucidated by an X-ray diffrac-
tion study (Figure 1), which reveals a significant deviation
from the usual square-planar coordination geometry ob-
served in other [NiPh(PO)PPh3] complexes. Thus, while the
C(23) and oxygen atoms lie on one side of the mean-squares
coordination plane, the two P atoms are turned towards the
other side [distances to the plane: C(23) –0.154(1);
O –0.160(1); P1 +0.184(1); P2 +0.185(1) Å]. Consistent with
the electronic features of the pyrazolonato ring, the Ni–O
and the Ni–P(1) bonds [1.955(1) Å and 2.210(1) Å, respec-
tively] are somewhat longer than their counterparts in 1a
(1.914 Å; 2.168 Å). The shortness of the P(1)–C(2) bond
[1.770(2) Å] is in keeping with electron delocalisation within
the PO chelate.
Figure 1. Molecular structure of complex 4. Important distances
[Å] and angles [°]: NiC(23) 1.885(2); NiO 1.955(1); NiP(1) 2.210(1);
NiP(2) 2.224(1); P(1)C(2) 1.770(2); P(1)C(17) 1.820(2); P(1)C(11)
1.823(2); OC(1) 1.296(2); C(2)C(1) 1.393(2); C(2)C(3) 1.419(3);
N(1)C(1) 1.370(2); N(1)N(2) 1.400(2) P(1)NiP(2) 167.26(2); C(23)
NiO 173.83(7).
Interestingly, we noted that on standing, colourless crys-
tals slowly formed from toluene solutions containing 4. An
X-ray analysis of the product reveals the formation of the
phosphorus ylide 5 (Figure 2). Surprisingly, a similar re-
ductive elimination reaction had not been reported for 1a.
A careful reinvestigation of the latter complex showed that,
in fact, 1a undergoes the same slow transformation in tolu-
ene, resulting in the formation of Ph3PCH=C(O)Ph. Poss-
ibly, this transformation is triggered by the presence of trace
amounts of air, since we found that bubbling air through a
C6D6 solution of 4 rapidly produced 5.
Figure 2. Molecular structure of the phosphorus ylide 5. Important
distances [Å]: P(1)C(2) 1.736(2); P(1)C(23) 1.797(3); P(1)C(17)
1.806(2); P(1)C(11) 1.808(2); OC(1) 1.249(3); C(2)C(1) 1.430(3);
C(2)C(3) 1.429(3); N(1)C(1) 1.400(3); N(1)N(2) 1.395(3).
6600 molethylene·molNi–1·h–1 were observed when operating
in toluene at 70 °C under a constant ethylene pressure of
5 bar and a nickel concentration of 0.25 μmol/mL (cf. TOF
= 7500 for 1a) [2 h experiments]. At this pressure, the selec-
tivity in the α-olefins surpassed 95% (Table 1).
The most striking feature of 4 involves the product distri-
bution which considerably shifts towards lower oligomers
Typically for a Keim-type catalyst, complex 4 displays in comparison with the outcome of Keim’s catalyst. Thus,
high activity in the low pressure oligomerisation of ethylene under the conditions outlined above, ca. 72 wt.-% of the
without requiring any activating agent to trigger the oligomers produced after 60 min were C4–C16 olefins; this
reaction. Turnover frequencies (TOF) as high as fraction represents only 15 wt.-% of the total amount of
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Eur. J. Inorg. Chem. 2005, 1477–1481

An Efficient Keim-Type Catalyst Based on an Electron-Poor P,O-Chelate
Table 1. Ethylene oligomerisation with 1a and 4. General conditions: toluene 30 mL, 70 °C, calibrated on heptane.
FULL PAPER
Ni complex
P(C2H4)
[bar]
Time
[h]
BF3
[μmol]
TOF
C4–C16 distribution
[wt.-%]
(7.5 μmol)[a]
[molethylene·molNi–1·h–1]
1a
1a
1a
1a[a]
1a[a]
4
1
5
5
20
20
1
5
5
20
20
2
2
1
1
1
2
2
1
1
1
3400
7500
15500
19900
9400
1800
6600
9700
6600
3800
60.2
15.3
15.3
9.6
7.5
7.5
9.6
98.9
45.2
72.5
25.1
97.0
4
4
4[a]
4[a]
[a] Batch experiments, toluene 20 mL, 70 °C.
5 cm-thick alumina column and stored under nitrogen over mo-
lecular sieves (4 Å). IR spectra were measured with a Perkin–Elmer
oligomers produced with 1a in the same period of time.
When applying a constant ethylene pressure of 1 bar, this
ratio rose to 99 wt.-% (versus 60% for 1a). The perform-
ances of 4 and 1a were also compared with those in batch
experiments carried out at higher pressures. Thus, when ap-
plying an initial pressure of 20 bar ([Ni] = 0.25 μmol/mL),
the amount of C4–C16 olefins produced with 4 after 1 h was
25 wt.-% of the total amount of oligomers, versus only
9.6% for 1a. It must be mentioned here that a SHOP cata-
1
1605 spectrometer. Routine H and 31P{1H} spectra were recorded
with FT Bruker instruments (AC-300 or Bruker AM–400).
13C{1H} were recorded with an FT Bruker AC-200 spectrometer.
1H NMR spectra are referenced to residual protiated solvents (δ
=7.26 ppm for CDCl3 and 5.32 ppm for CD2Cl2); 13C chemical
shifts are reported relative to deuterated solvents (δ =77.0 ppm for
CDCl3 and 53.8 ppm for CD2Cl2); 31P NMR spectroscopic data
are given relative to external H3PO4. The catalytic solutions were
lyst in which the oxygen atom was involved in hydrogen analysed with a Varian 3900 gas chromatograph equipped with a
WCOT fused silica column (25 m, 0.32 mm inside diameter,
0.25 mm film thickness). High temperature size exclusion
chromatography (HT SEC) measurements were performed at
150 °C with a “PL220” apparatus (Column set HT-MixedB-TCB-
01) in 1,2,4-trichlorobenzene (with 0.2% Irganox). Calibration was
made with linear PS samples. Ph2P-pzONa[13] and [NiPhCl-
(PPh3)2][20,21] are reported according to procedures reported in the
literature.
bonding has been reported to also favour β elimination,
hence the formation of shorter oligomers, but not to such
an extent.[2] Interestingly, the observed effect can be drasti-
cally enhanced by using BF3 as an additive. For example,
in experiments where stoichiometric amounts of BF3 were
used (batch autoclave, Pinitial = 20 bar, [4] = 0.25 μmol/mL),
the fraction of C4–C16 olefins obtained after 1 h represented
97% of all oligomers formed. It appears plausible that the
BF3 unit binds to the pyrazole ring, probably through a
nitrogen atom,[15] hence increasing the electron deficiency
of the nickel atom. Overall, the effect observed by adding
BF3 is reminiscent of the properties of [Ni{Ph2PC6H4C(O)
O}(Me-2-allyl)],[16] which upon addition of B(C6F5)3 shifts
ethylene oligomerisation towards the formation of low-
weight olefins (butene and hexene).[17]
Finally, we noted that mixing complex 4 with Ni(cod)2
in excess afforded, as expected,[11,18,19] a catalyst which
polymerises ethylene. Its activity compares with that of 1a,
but analysis of the polymer reveals that the polyethylene
chains are considerably shorter (Mw = 650 gmol–1) than
those obtained with 1a (Mw = 4500 gmol–1) under similar
conditions. These findings confirm the remarkable elec-
tronic properties of the phosphanyl-pyrazolonato ligand.
Further investigations are in progress which aim at the use
of electron poor P,O chelators that allow the controlled pro-
duction of industrially relevant α-olefins in the absence of
any cocatalyst.
[NiPh{Ph2P-pzO}(PPh3)] (4): To a stirred suspension of Ph2P-
pzONa (0.766 g, 2.14 mmol) in toluene (30 mL) was added a solu-
tion of [NiPhCl(PPh3)2] (1.491 g, 2.14 mmol) in toluene (40 mL).
After stirring for 4 h, the mixture was filtered through Celite in
order to remove NaCl. The solution was concentrated to ca.
10 mL. On standing, dark yellow crystals formed overnight. After
removal of the supernatant solution, the crystals were washed suc-
cessively with cold toluene (5 mL) and pentane (10 mL), and then
dried in vacuo. Yield: 1.088 g, 60%. Addition of pentane to the
mother liquor afforded further amounts of 4 as a yellow powder
which increases the yield to 90 %. C46H38N2NiOP2·C7H8 (Mr =
755.46 + 92.14 = 847.60): calcd. C 75.1, H 5.47, N 3.31; found C
74.6, H 5.39, N 3.34. IR (KBr): 1592.6 (m), 1515.5 (s), 1495.0 (s),
1
1425.8 s cm–1. H NMR (300 MHz, C6D6): δ = 8.25–6.44 (35 H,
arom. H), 2.02 (s, 3 H, CH3) ppm. 13C{1H} NMR (75 MHz,
CDCl3): δ = 140.76–118.06 (arom. C), 89.83 (d, J(P,C) = 63 Hz,
PCH), 14.77 (s, CH3) ppm. 31P{1H} NMR (121 MHz, CDCl3): δ
= 20.2 and –8.6 (AB spin system, J(PPЈ) = 280 Hz).
Ph3P-pzO (5): [NiPh{Ph2P-pzO}(PPh3)] (4; 0.100 g, 0.13 mmol)
was dissolved in toluene (5 mL). Upon standing in air, crystals of
Ph3P-pzO appeared after 48 h. The supernatant was removed by
suction, and the crystals were dried under high vacuum. Yield:
0.056 g, 70%. C28H23N2OP (Mr = 434.48): calcd. C 77.41, H 5.34,
N 6.45; found C 77.49, H 5.38, N, 6.41. 31P{1H} NMR (121 MHz,
Experimental Section
CDCl3): δ = 10.6 ppm (lit.: 12.6[22]
)
General: All manipulations were performed in Schlenk-type flasks
under dry nitrogen. Solvents were dried by conventional methods
and distilled immediately prior to use. CDCl3 was passed down a
X-ray Crystal Structure Determination of 4·toluene: Single crystals
of 4 suitable for X-ray diffraction analysis were obtained from a
Eur. J. Inorg. Chem. 2005, 1477–1481
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P. Kuhn, D. Sémeril, C. Jeunesse, D. Matt, P. Lutz, R. Welter
FULL PAPER
Table 2. Crystallographic data for 4·toluene and 5·benzene.
Formula
Formula weight
C46H38N2NiOP2·C7H8 (4·toluene)
847.57
C28H23N2OP·C6H6 (5·benzene)
512.56
Crystal system
Space group
Triclinic
P1
Monoclinic
C2/c
¯
a [Å]
b [Å]
c [Å]
α [°]
10.939(1)
13.283(1)
16.156(1)
72.29(5)
29.361(5)
9.314(1)
22.758(5)
90
β [°]
75.29(5)
87.71(5)
2161.2(3)
2
1.302
0.565
888
119.06(5)
90
5440(3)
γ [°]
V [Å3]
Z
8
Dcalcd. [g·cm–3]
μ(Mo-Kα) [mm–1]
F(000)
1.252
0.131
2160
Crystal Size [mm]
Temperature [K]
Radiation [Å]
θ min/max [°]
Data set
Total, Unique data,
R(int)
0.08×0.10×0.13
173(2)
Mo-Kα 0.71069
1.4/30.0
–15/15; –17/18; –22/21
34952, 12654
0.035
0.06×0.07×0.09
173(2)
Mo-Kα 0.71069
1.6/30.0
–41/36; 0/13; 0/32
7963, 7962
0.036
Observed data [I Ͼ 2.0σ(I)]
Number of reflections,
Number of parameters
R1, wR2, S
9904
12654
532
0.0442, 0.1468, 0.993
–0.664, 0.453
5436
7962
333
0.0781, 0.2297, 1.156
–0.864, 1.829
Min. and max. residual density [e·Å–3]
concentrated toluene solution of the complex. Data were collected
on a Nonius KappaCCD diffractometer (graphite Mo-Kα radia-
tion, λ = 0.71073 Å). The structure was solved by direct methods
with the SHELXS-97 program and refined by full-matrix least-
squares techniques (SHELXL-97) on F2.[23] Hydrogen atoms were
located by using the geometric method. The crystal data and struc-
tural refinement details are listed in Table 2. The compound was
found to crystallise with one molecule of toluene (CCDC 242626).
magnetic stirrer and heating (temperature-controlled oil bath), at
constant ethylene pressure. The autoclave was heated at 100 °C un-
der vacuum for 1 h before use and purged with ethylene. The reac-
tion temperature refers to the bath temperature.
In a typical run, the reactor was charged at ambient temperature
under an ethylene atmosphere, with
a solution of catalyst
(7.5 μmol) in toluene (15 mL), followed by a solution of Ni(cod)2
(75 μmol) in toluene (15 mL). The autoclave temperature was
raised to 70 °C, and pressurised to 5 bar. The reaction was
quenched by venting the autoclave, and the reaction mixture was
subsequently poured into a stirring solution of methanol (300 mL)
and concentrated HCl (10 mL) to precipitate the polymer. The
white powder was isolated by filtration, washed with methanol, and
dried in vacuo at 50 °C. By using Keim’s catalyst (1a), 1.80 g of a
polymer was obtained and was characterised by Mw = 4500 gmol–1
and Mw/Mn = 1.87 (Catalyst activity: 240 g·mmolcat–1·h–1). The
polymer (1.51 g) obtained with 4 has the following characteristics:
X-ray Crystal Structure Determination of 5·benzene: Single crystals of
5 suitable for X-ray diffraction analysis were obtained from a toluene
solution of the complex on standing in air. Data collection and struc-
ture solution were as for compound 4. The compound
crystallised with one molecule of benzene. The benzene molecules
occupy special positions (–1). CCDC-242627 and CCDC-242626
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Mw
=
650 gmol–1 and Mw/Mn
=
1.31 (Activity:
Catalytic runs
201 g·mmolcat–1·h–1).
Ethylene oligomerisation: The low pressure catalytic runs (Յ5 bar)
were carried out in a Büchi miniclave (200 mL) equipped with a
Teflon-coated magnetic stirrer. For higher pressures, a stainless
steel autoclave was used (100 mL). Both reactors were dried under
vacuum at 100 °C for 1 h before use. In a typical procedure, a
Schlenk flask was charged with the complex (7.5 μmol) and toluene
(10 mL). The resulting solution was injected into the autoclave to-
gether with additional toluene (20 mL). The autoclave was then
pressurised with ethylene and heated at 70 °C. After completion of
the reaction, the autoclave was cooled with an ice bath and de-
pressurized over 1 h. The solution was analysed by GC. Heptane
(200 μL) was used as internal reference. For details see Tables S1–
S3.
Supporting Information Available: See also footnote on the first
page of this article. Ethylene oligomerisation with the Keim-type
complex 1a (constant pressure) (Table S1). Ethylene oligomeri-
sation with Keim-type complex 4 (constant pressure) (Table S2).
Comparison of ethylene oligomerisation with 1a and 4 in batch
experiments (Table S3).
Acknowledgments
P. K. thanks the Ministère de l’Education Nationale et de la Re-
cherche Scientifique for a grant. The authors thank Prof. Dr. R.
Müllhaupt and U. Westphal for their help with the HT SEC mea-
surements.
Ethylene Polymerisation: Polymerisations were carried out in a
mini-Büchi glass reactor (200 mL), equipped with a Teflon-coated
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An Efficient Keim-Type Catalyst Based on an Electron-Poor P,O-Chelate
FULL PAPER
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