Synthesis of anionic methylpalladium complexes with phosphine-sulfonate ligands and their activities for olefin polymerization

Article abstract of DOI:10.1039/b512452m

The synthesis of anionic methylpalladium complexes with phosphine-sulfonate ligands and their activities for olefin polymerization are examined. The methylpalladium complexes are frequently used for mechanistic investigation of the copolymerization. The c

Full text of DOI:10.1039/b512452m

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Synthesis of anionic methylpalladium complexes with phosphine–sulfonate
ligands and their activities for olefin polymerization†
Takuya Kochi, Kenji Yoshimura and Kyoko Nozaki*
Received 2nd September 2005, Accepted 31st October 2005
First published as an Advance Article on the web 14th November 2005
DOI: 10.1039/b512452m
Reaction of diarylphosphinobenzene-2-sulfonic acids with
tertially amines, followed by addition of [PdMeCl(cod)], pro-
vided anionic methylpalladium(II) complexes with bidentate
phosphine–sulfonate ligands, which show high activity for
copolymerization of ethylene with methyl acrylate.
linear copolymer of ethylene and alkyl acrylates5a and a non-
perfectly alternating copolymer of ethylene and CO.5b In their
reports, the palladium catalysts were formed in situ by combina-
tion of diarylphosphinobenzene-2-sulfonic acid with Pd(dba)2 or
Pd(OAc)2. Very recently, Rieger et al. reported the first examples
of structurally-characterized palladium complexes with bidentate
phosphine–sulfonate ligands, which gave higher catalytic activity
and a higher number of extra ethylene insertions in the non-
perfectly alternating copolymerization of ethylene and CO.6 To
improve the catalyst system, our group has concurrently attempted
to synthesize palladium complexes with diarylphosphinobenzene-
2-sulfonic acids. Herein we report the first synthesis of methylpal-
ladium(II) complexes with bidentate phosphine–sulfonate ligands.
Preliminary results for the copolymerization of ethylene and
methyl acrylate are also described.
Treatment of diphenylphosphinobenzene-2-sulfonic acid 1a
with diisopropylethylamine in dichloromethane, followed by ad-
dition of PdMeCl(cod), afforded the anionic methylpalladium
complex 2a in 94% yield (Scheme 2).7 Similarly, the reaction
of o-methoxyphenyl substituted ligand 1b gave methylpalladium
complex 2b in 92% yield. When triethylamine was used instead of
diisopropylethylamine for the reaction of 1a, the corresponding
methylpalladium complex 2aꢀ was also obtained in 89% yield.
Olefin polymerization with late transition-metal complexes has
attracted much interest1 due to their lower oxophilicity relative to
early transition metals, and many catalysts have been developed
to polymerize various olefins and to copolymerize non-polar
olefins with carbon monoxide2 or with polar olefins.3 In order to
achieve higher activity and further understanding of the reaction
mechanism, structurally-characterized organometallic complexes
have been isolated in many cases before the polymerization.
In the case of palladium catalysts with bidentate ligands, the
corresponding monomethyl complexes4 have generally been recog-
nized as convenient catalyst precursors, because simple generation
(Scheme 1) of a vacant site at the cis-position to the methyl group,
followed by coordination of an olefin, would readily initiate the
coordination polymerization. These methylpalladium complexes
have also been frequently used for mechanistic investigation of the
copolymerization.
Scheme 1 Generation of a vacant site on monomethyl palladium
complexes with bidentate ligands.
In the past decade, significant efforts have been devoted to
developing late transition-metal catalysts capable of producing
highly functionalized polymers by copolymerization of non-
polar olefins with readily-available polar olefins. For example,
Brookhart’s diimine palladium complexes can copolymerize ethy-
lene or propylene with polar vinyl monomers such as methyl
acrylate.3c,d The copolymers produced were highly branched and
methyl acrylate was incorporated only at the chain end.
Scheme 2 Preparation of anionic methylpalladium(II) complexes with
phosphine–sulfonate ligands.
The structure of 2aꢀ was determined by X-ray crystallography‡
to reveal that the phosphine–sulfonate ligand coordinates to
palladium in a bidentate fashion, similar to Rieger’s dicyclopenta-
dienylethoxy palladium complex (Fig. 1). The palladium complex
adopts a square planar geometry and the methyl group is located
at the trans-position to the oxygen atom of the sulfonate. It is
interesting that a chloride ion is bound to the palladium to form
an anionic methylpalladium complex, considering simple loss of
the chloride ion would give a neutral complex. The synthesis of 2a
can also be performed in acetonitrile, meaning that acetonitrile
did not replace the chloride ion on the palladium. Anionic
methylpalladium(II) complexes are relatively rare, and as far as we
In 2002, Pugh and coworkers have developed palladium cat-
alysts which can produce unique polymers such as a highly
Department of Chemistry and Biotechnology, Graduate School of Engineer-
ing, The University of Tokyo, Tokyo 113-8656, Japan. E-mail: nozaki@
chembio.t.u-tokyo.ac.jp; Fax: +81 3 5341 7261; Tel: +81 3 5841 7261
† Electronic supplementary information (ESI) available: Synthetic proce-
dures and characterization data for 2a, 2aꢀ and 2b; X-Ray data for 2aꢀ. See
DOI: 10.1039/b512452m
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Copolymerization of ethylene and methyl acrylate was then
investigated with catalyst◦2 (Table 1). When the copolymerization
was carried out at 80 C with NaBArf4, 2b showed higher
activity than 2a, as is consistent with Pugh’s result (entry 2,
3).5a,9,10 Screening of additives such as NaBArf4 and AgOTf was
performed to examine the effectiveness in removing the chloride
ion from the palladium metal of 2b. Effect of the additives on
the copolymerization was quite notable. For example, the catalytic
activity of the copolymerization with NaBArf4 at 80 ◦C (entry 2)
was twice as high as that reported by Pugh with Pd(dba)2 and 1b in
the same reaction conditions,5a while the reactions with AgOTf or
no additive showed much lower activities (entry 6, 8). However, the
highest comonomer incorporation was observed with no additive,
and particularly, the reaction at 80 ◦C provided the copolymer
with 16% of methyl acrylate incorporated in the polymer chain
(entry 8). An increase in reaction temperature generally leads to
an enhancement in catalytic activity and a reduction of polymer
molecular weight, but no gain or a decrease in the activity was
observed at high temperature possibly due to decomposition of
the catalyst (entry 1, 6, 8). The highest molecular weight (12 500)
Fig. 1 Structure of complex 2aꢀ. Hydrogen atoms are omitted for clarity.
Thermal ellipsoids are shown at the 50% probability level. Selected bond
˚
˚
lengths (A) and angles (A): Pd(1)–C(1) 2.051(4); Pd(1)–O(1) 2.184(3);
Pd(1)–P(1) 2.2195(12); Pd(1)–Cl(1) 2.3997(12); Cl(1) · · · N(1) 3.268(5).
◦
was obtained in the reaction with no additive at 50 C for 72 h.
Addition of galvinoxyl had little influence on the activity of the
reaction with no additive at 80 ◦C, while significantly reduced
activity (4 g mmol−1 h−1) was observed for the reaction with
NaBArf4. Espinet and co-workers have reported that galvinoxyl
not only serves as a radical trap but can react with a Pd–H bond.11
The observed reduction of activity with NaBArf4 can be attributed
to the frequent chain transfer reaction via b-hydride elimination
and generation of a Pd–H bond, relative to those in the reaction
with no additive.
In conclusion, the first syntheses of methylpalladium(II) com-
plexes with phosphine–sulfonate bidentate ligands were achieved.
An X-ray structure of 2aꢀ revealed that the complexes were rare
examples of anionic methylpalladium(II) complexes. We further
examined the copolymerization of ethylene and methyl acrylate
with the catalysts and found interesting effects of additives on the
catalytic activity, the molecular weight and the degree of methyl
acrylate incorporation. These preliminary copolymerization re-
sults indicate that further examination of additives, ligands, bases
used to prepare the catalysts, and reaction conditions would lead
to an improved catalyst system for production of the unique highly
linear copolymer of ethylene and methyl acrylate.
know, all of the previously reported examples possess a dianionic
ligand and a methyl group on palladium.8 The nitrogen atom of
˚
the ammonium ion is in close contact with the chloride (3.27 A)
and the ammonium hydrogen should be located at a reasonable
position for a hydrogen bond with the chloride ligand.
Notable temperature dependence was observed in the 1H NMR
spectra of complex 2b. At room temperature the two methoxy
groups of 2b gave one sharp singlet at 3.58 ppm, which split
into two broad signals at 3.49 and 3.66 ppm on cooling at
−60 ◦C. This behavior can be accounted for by considering the
conformational interchange of the aryl groups which are arranged
in an unsymmetric structure as disclosed by the crystallographic
study on complex 2aꢀ (Scheme 3).
Scheme 3 Conformational interchange of the aryl groups of 2b.
Table 1 Copolymerization of ethylene and methyl acrylate with 2a
‘
Methyl acrylate
Entry Catalyst Additive
Temp/◦C Yield/g Activity/g mmol−1 h−1 TOF/h−1b incorporation (mol%)c Mn
Mw/Mn
1
2
3
4
5
6
7
8
9
2b
2b
2a
2b
2b
2b
2b
2b
2b
2b
2bd
NaBArf4 (0.01 mmol) 90
NaBArf4 (0.01 mmol) 80
NaBArf4 (0.01 mmol) 80
NaBArf4 (0.01 mmol) 70
NaBArf4 (0.01 mmol) 60
2.59
2.61
0.94
2.00
1.40
0.49
0.96
0.24
0.30
0.19
0.57
17
17
6.3
13
9.3
3.3
6.4
1.6
2.0
1.3
0.8
490
490
200
400
290
90
180
43
55
13
13
7
9
8
14
14
16
15
14
13
3200 1.64
4100 1.87
2700 1.46
6200 1.54
6100 2.44
2400 1.54
4500 1.61
4500 1.61
6800 1.61
8500 2.11
12 500 1.79
AgOTf (0.02 mmol)
80
60
80
70
50
50
AgOTf (0.02 mmol)
None
None
None
None
10
11
35
22
a Conditions: 0.01 mmol of catalyst 2; ethylene pressure, 3.0 MPa; 2.5 mL of methyl acrylate; solvent, 2.5 mL of toluene; reaction time, 15 h. b TOF
(turnover frequency): mol olefin/(mol Pd·h). c Determined by 1H NMR spectroscopy. d Copolymerization was carried out for 72 h.
2 6 | Dalton Trans., 2006, 25–27
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Acknowledgements
We are grateful to Prof. Takayuki Kawashima and Dr Makoto
Yamashita (the University of Tokyo) for the X-ray crystal analysis.
Notes and references
‡ Crystal data for 2aꢀ: C25H33ClNO3PPdS, M = 600.40, monoclinic, a =
◦
˚
˚
˚
14.186(6) A, b = 10.679(5) A, c = 17.678(8) A, b = 92.549(2) , V =
3
˚
2675.5(19) A , T = 120(2) K, space group P21/c, Z = 4, l(Mo-Ka) =
0.958 mm−1, 14896 reflections measured, 4364 unique (Rint = 0.0255)
which were used in all calculations. The final wR(F2) was 0.1208 (all data).
CCDC reference number 283231. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b512452m
7 Rieger et al. reported that
a stoichiometric reaction between
PdMeCl(cod) and a diarylphosphinobenzene-2-sulfonic acid derivative
did not provide any methylpalladium complex with the phophine–
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1
9 The H NMR spectrum of the volatile materials collected by vaccum
transfer of the crude mixture showed no byproduct such as oligomers
of ethylene and/or methyl acrylate.
10 The activity of the reaction with 2b for 3 h was 29 g mmol−1 h−1. The
copolymerization reaction slows down as it proceeds and the amounts
of the remaining monomers decrease.
11 A. C. Alve´niz, P. Espinet, R. Lo´pez-Ferna´ndez and A. Sen, J. Am.
Chem. Soc., 2002, 124, 11278.
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