New diphosphine ligands based on diphenyl ether for the Pd-catalyzed CO/ethene copolymerization

Article abstract of DOI:10.1039/b503764f

The catalytic activity and selectivity of palladium(II) complexes of new, flexible bidentate ligands in the CO/ethene copolymerization reaction have been found to change considerably with the steric properties of the ligands. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b503764f

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New diphosphine ligands based on diphenyl ether for the Pd-catalyzed
CO/ethene copolymerization{
Maria Caporali,a Christian Mu¨ller,a Bastiaan B. P. Staal,a Duncan M. Tooke,b Anthony L. Spekb and
Piet W. N. M. van Leeuwen*a
Received (in Cambridge, UK) 15th March 2005, Accepted 29th April 2005
First published as an Advance Article on the web 9th June 2005
DOI: 10.1039/b503764f
The catalytic activity and selectivity of palladium(II) complexes
of new, flexible bidentate ligands in the CO/ethene copoly-
merization reaction have been found to change considerably
with the steric properties of the ligands.
The palladium-catalyzed carbonylation of ethene is a reaction of
great potential from an industrial point of view and it has also
attracted academic interest for many years.1 With this process, a
wide range of products can be obtained, ranging from methyl
propanoate, a useful starting material for the preparation of
methyl methacrylate,2 to thermoplastic polymers, called polyke-
tones, which have valuable chemico-physical properties.3 Studies
carried out by van Leeuwen4 et al. on alcoholysis of acyl
palladium(II) complexes in methanol point out that complexes
bearing rigid bulky diphosphines, which adopt trans coordination,
showed no alcoholysis, while complexes containing flexible ligands
that are cis or can adopt cis coordination do undergo alcoholysis
and it was concluded that a cis geometry was needed for an
intramolecular ester formation. When the steric bulk of the
diphosphine is increased, the reaction rate is enhanced. Other
authors have recently asserted that the methanolysis of acyl–Pd(II)
complexes proceeds via an intramolecular mechanism.5 In this
paper, we focus on flexible bidentate diphosphines with variable
steric bulk containing a diphenyl ether (DPE) backbone. Indeed,
the variation of the steric hindrance allows modulation of the bite
angle and the flexibility of the ligand (i.e. the geometry of
coordination, cis or trans) which strongly influence the rates of the
insertion and termination reactions. Our aim is to understand the
influence of the steric properties of the ligand in the CO/C2H4
copolymerization. The synthesis of the ligands has been carried out
using 4,49-dimethyldiphenyl ether as starting material, according to
Scheme 1.
Scheme 1
The palladium(II) bis-acetonitrile complexes [(P,P)Pd-
(CH3CN)2]2+ 2[CF3SO3]2 were used for the catalytic reactions.
The triflate ion is a non-coordinating anion towards palladium in
methanol. The desired complexes can be prepared in a straightfor-
ward manner either by reaction of the precursor
Pd(CH3CN)4(CF3SO3)2 with ligand L1, L2 or L3, or by reaction
of the ligand with (1,5-COD)PdCl2 and subsequent reaction with
AgCF3SO3, according to Scheme 2.
Single crystals suitable for X-ray diffraction{ of the compound
(L2)PdCl2 could be obtained by crystallization from pentane and
the molecular structure7 is depicted in Fig. 1. The compound is a
cis-complex exhibiting a P–Pd–P angle of 97.67(3)u.
Catalytic experiments with the new complexes were carried out
Ortho-lithiation and subsequent reaction with ClP(NEt2)2 gave
the diethylamino phosphane, which by reaction with HCl,
afforded the 2,29-bis(dichlorophosphino)-4,49-dimethyldiphenyl
ether. Reduction with LiAlH4 to the corresponding 2,29-bis-
(phosphino)-4,49-dimethyldiphenyl ether and double Michael
addition to acrylonitrile gave the desired cyanoethylphosphane
L1. Coupling of the 2,29-bis(dichlorophosphino)-4,49-dimethyl-
diphenyl ether with aryllithio derivatives on the other hand
afforded the more bulky ligands L2 and L3, respectively. DPEphos
was used as a reference in our studies.6
under mild conditions and the results have been summarized in
{ Electronic supplementary information (ESI) available: experimental
details. See http://www.rsc.org/suppdata/cc/b5/b503764f/
*p.w.n.m.v.leeuwen@tue.nl
Scheme 2
3478 | Chem. Commun., 2005, 3478–3480
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reaction rates by many orders of magnitude, while changes in
electronic properties gave only small changes in rate.9
Entries 1, 2 and 6 in Table 1, show that the growing steric bulk
of the ligand favours the formation of short oligomers i.e. there is
an increase of termination rate respect to the insertion rate. Ligand
L3, the most sterically demanding of all ligands, gives mainly
products of methoxycarbonylation of alkene instead of polymers.
However, entry
4 and 5, which corresponds to complex
[(L2)Pd(NCCH3)2]2+ in absence and in presence of triflic acid
respectively, shows higher productivity in polyketones compared to
complexes [(DPEphos)Pd(NCCH3)2]2+ and [(L3)Pd(NCCH3)2]2+
while less methyl propionate is produced. This behaviour was not
expected on the basis of previous studies. It should be noted here
that the low overall activity of the complex [(L1)Pd(NCCH3)2]2+
might be due to intra- or intermolecular coordination of the
pendant cyano group to a Pd center, which is currently under
investigation.
Fig. 1 X-ray crystal structure of the complex (L2)PdCl2. Displacement
The CO/ethene copolymers, obtained from the catalytic
1
experiments, were analysed by H and 13C NMR and by Maldi-
ellipsoids shown at the 50% probability level. Hydrogen atoms omitted for
˚
clarity. Selected bond lengths (A) and angles (deg): Pd–P(1): 2.2738 (11),
Tof-MS in order to characterize the end groups of the polymer
chains. The analysis reveals that each chain has a methyl ester (E)
and an ethylketone (K) end group (EK), and neither ketone-
ketone (KK) nor ester-ester end-groups (EE) were detected. In
addition, the structure of the polymer is perfectly alternating (see
supplementary material{).
In order to analyze the molecular weight distribution of the
polyketones, the logarithm of intensities of the peaks obtained by
Maldi-Tof and the weight fraction have been plotted versus the
mass (m/z). The resulting curve shows the behaviour of a Schulz–
Flory curve and the Schulz–Flory constant, a, was found to be
equal to 0.90 (Fig. 2).
Pd–P(2): 2.2416(11), P(1)–Pd–P(2): 97.67(3), Cl(1)–Pd–Cl(2): 91.26(4).
Table 1. Ligands L2 and L3 show increasing steric bulk due to the
tert-butyl and the phenyl substituents in meta position of the
aromatic ring. This might give rise to different bite angles of
the ligands, which consequently affects the product distribution.
Indeed, as shown in Table 1, the product distribution is
dramatically different going from ligand L1, the least hindered
diphosphine, to ligand L3, the bulkiest one; the former gives only
polymers, while the latter gives mainly methyl propanoate and a
small amount of low molecular weight oligomers. As expected,
there is a competition between chain growth and chain termination
rates: the bisphosphine L1 which contains cyanoethyl moieties and
considered electronically as being in between arylphosphines and
arylphosphites,8 but sterically much less demanding, is expected to
adopt a cis-coordination to the palladium centre which favours
multiple insertion reactions of carbon monoxide and ethene, while
the bulky ligand L3 may disfavour the formation of long chains.
A recent study concerning reductive elimination reactions of
palladium(II) complexes with mono- or bidentate phosphines
showed that an increase of the steric hindrance led to higher
In conclusion, the steric bulk of the new, fluxional ligands
control the rates of reaction and the molecular weight of the final
products of the CO/C2H4 copolymerization. The trend toward
more MP with increasing cone angles is the same as that found for
diphosphines containing C1–3 bridges.1
This work was supported by the European Commission
(HPRN-CT-2002-00196). The authors wish to thank prof. dr. D.
Vogt (TU Eindhoven) for helpful discussions and Degussa for a
generous loan of palladium.
Table 1 Comparative catalytic results for complexes 1–3
CF3SO3H
(equiv)
Entry
Ligand
MPa
n.d.
267.0
498.5
91.7
133.7
119.7
395.9
Oliga
1
2
3
4
5
6
7
a
L1
DPEphos*
DPEphos
L2
L2
L3
L3
0
0
20
0
20
0
20
20
28
34
153
165
9
6
Grams of product per gram of Pd per hour. MP: methyl
propanoate, Olig: precipitated fraction. *DPEphos
2,29-bis(diphenylphosphino)diphenyl ether. Conditions: catalyst:
5
b
Fig. 2 Weight fraction distribution for CO/ethene copolymer obtained in
the presence of complex [(L2)Pd(NCCH3)2(CF3SO3)2] entry 4, Table 1, a)
theoretical curve calculated according to Schulz–Flory distribution, b)
curve obtained from experimental data.
0.02 mmol, MeOH: 10 mL, p(CO) 5 p(ethene): 25 bar (T: 25 uC),
Reaction time: 2 h, T: 60 uC, 1100 rpm, stainless steel reactor with
volume of 75 mL.
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Maria Caporali,a Christian Mu¨ller,a Bastiaan B. P. Staal,a
Duncan M. Tooke,b Anthony L. Spekb and Piet W. N. M. van
Leeuwen*a
3 A. Sen, Z. Jiang and J. T. Chen, Macromolecules, 1989, 22, 2012;
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Z. Freixa, P. C. J. Kamer, K. Goubitz, J. Fraanje, M. Lutz and
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aDepartment of Chemical Engineering and Chemistry, Technische
Universiteit Eindhoven, P.O. Box 513, 5600 MB, Eindhoven, The
Netherlands. E-mail: p.w.n.m.v.leeuwen@tue.nl; Fax: +31 40 245 5054;
Tel: +31 40 247 3040
bBijvoet Center for Biomolecular Research, Dept. Crystal and Structural
Chemistry, Utrecht University, 3584 CH, Utrecht, The Netherlands
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Notes and references
6 M. Kranenburg, Y. E. M. van der Burgt, P. C. J. Kamer, P. W. N.
M. van Leeuwen, K. Goubitz and J. Fraanje, Organometallics, 1995, 14,
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{ CCDC 267390. See http://www.rsc.org/suppdata/cc/b5/b503764f/ for
crystallographic data in CIF or other electronic format.
7 Crystal data: C70H96Cl2OP2Pd, Mr 5 1192.71, orthorhombic, space
˚
1 A. Sen, Catalytic Synthesis of Alkene-Carbon Monoxide Copolymers and
Cooligomers, Catalysis by Metal Complexes, Volume 27, Kluwer AP,
Dordrecht, 2003; P. W. N. M. van Leeuwen, Homogeneous Catalysis;
Understanding the Art, Kluwer AP, Dordrecht, 2004, Chapter 12;
E. Drent and P. H. M. Budzelaar, Chem. Rev., 1996, 96, 663.
2 W. Clegg, G. R. Eastham, M. R. J. Elsegood, R. P. Tooze and
X. L. Wang, Chem. Commun., 1999, 1877; K. Whinston, W. Clegg,
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R. Whyman and S. Zacchini, Organometallics, 2002, 21, 1832; R. A.
M. Robertson and D. J. Cole-Hamilton, Coord. Chem. Rev., 2002, 225,
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group Pbca, a 5 25.241 (3), b 5 20.053 (4), c 5 26.004 (2) A, V 5
13162 (3) A , T 5 150 K, Z 5 8, Dcalcd 5 1.204 g cm23, F(000) 5 5072,
3
˚
m (MoKa) 5 0.452 mm21, crystal dimensions 0.02 6 0.22 6 0.24 mm.
Of 164826 reflections measured, 8606 unique reflections were used in the
refinement. Final R 5 0.0363 (wR2 5 0.0874) S 5 1.02.
8 The x-value of L1 is considered to be ca. 17, thus in between
arylphosphines (y13) and arylphosphites (y22). See also: C. A. Streuli,
Anal. Chem., 1960, 32, 985; W. M. A. Henderson and C. A. Streuli,
J. Am. Chem. Soc., 1960, 82, 5791.
9 G. Mann, Q. Shelby, A. H. Roy and J. F. Hartwig, Organometallics,
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3480 | Chem. Commun., 2005, 3478–3480
This journal is ß The Royal Society of Chemistry 2005