Catalysts for Suzuki polycondensation: Ionic and "quasi-ionic" amphipathic palladium complexes with self-phase-transfer features

Article abstract of DOI:10.1002/chem.201202162

Poly(9,9-dioctylfluorene) (PFO) with M_n values above 100 000 g mol^-1 in a toluene/water system and M_n values up to 600 000 g mol^-1 in a THF/water system has been obtained by improved Suzuki polycondensation using a new kind of amphipathic palladium catalyst with self-phase-transfer features, which could overcome the disadvantage caused by the immiscible biphasic mixture and accelerate the transmetalation step (see figure). Copyright

Full text of DOI:10.1002/chem.201202162

COMMUNICATION
DOI: 10.1002/chem.201202162
Catalysts for Suzuki Polycondensation: Ionic and “Quasi-Ionic” Amphipathic
Palladium Complexes with Self-Phase-Transfer Features
Jing Li,[a, b] Hongwei Fu,[a, b] Pan Hu,[a, b] Zilong Zhang,[a] Xiao Li,*[a] and
Yanxiang Cheng*[a]
Dedicated to Prof. Dr. Max Herberhold on the occasion of his 75th birthday
Suzuki polycondensation (SPC) is one of the most
common ways to synthesize conjugated polyarylenes poly-
mers,[1] such as derivatives of poly(para-phenylene)[2] and
ing long polymer chains after several catalytic cycles, the
much greater hydrophobicity makes the process even more
difficult, resulting in chain stop and transfer leading to the
formation of low molecular weight polymers.
polyACHTUNGTRENNUNG
(fluorene),[3] which are potentially useful candidates as
active layers in light emitting diodes and photovoltaic devi-
ces.[4] Compared with many developments in the small mole-
cule Suzuki coupling methodology,[5] the most recent SPC
reactions still utilize only slight modifications of Schlꢀterꢁs
original protocol,[6] which is based on the equally prolific
small molecule Suzuki coupling reaction between an aryl
halide and an aryl boronic acid,[7] often resulting in molecu-
lar weights limited to the 103 to 104 gmolÀ1 range.[8]
Procedures used for these polymerization processes com-
monly employ biphasic reaction mixtures.[9] This is because
the reactants and coupling products are always soluble in or-
ganic solvents, whereas inorganic bases are soluble in the
water phase, which can facilitate the otherwise slow trans-
metalation of the boronic acid by replacing the halide in the
coordination sphere of the palladium complex,[10] or by
forming a more reactive boronate anion [ArB(OH)3]À with
stronger nucleophilicity to the Pd center.[11] In this system,
however, a chief obstacle is that the majority of organic sol-
vents are immiscible with water, which is intensely unfavora-
ble to the transmetalation step, because the step occurs at
the interface of the two phases. The oxidative addition (O-
A), usually the rate-controlling step, occurs smoothly in the
organic phase, but the intermediate produced in the first
step cannot contact well with the inorganic base or the boro-
nate anion in the water phase because of its more favorable
oil solubility. In particular, for the PdII intermediate contain-
In fact, the conditions could be improved by selecting
partly water-miscible organic solvents such as THF,[12]
adding phase-transfer agents such as tricaprylammonium
chlorides (Aliquat 336),[13] or using water-soluble sulfonated
analogues of triphenylphosphine such as sodium 3-(di
AHCTUNGTRENNUNG
ACHTUNGTRENNUNGphen-
ylphosphanyl) benzenesulfonate (LSS-3).[14] In these cases,
we think a primary reason for the improvement could be
that the base or the boronate anion could readily interact
with the Pd center and accelerate the transmetalation step.
Inspired by these results, we designed and synthesized a
class of phosphine ligands containing weaker hydrophilic
groups and the corresponding Pd complexes, which were ex-
pected to produce a similar effect to the above successful
examples with the aid of their amphipathic property
(Scheme 1). Finally, we were surprised to achieve even
higher molecular weight polymers with Mn values above
100000 gmolÀ1 in toluene/water systems and Mn values up
to 600000 gmolÀ1 in THF/water systems due to the smooth-
er transmetalation step.
We firstly choose the commercial hydrophilic ligand LSS-
3 to compose Pd0 and PdII complexes, which were then ap-
plied in a SPC reaction for the synthesis of poly(9,9-dioctyl-
fluorene) (PFO) as a model polymer. The catalytic perform-
ances are listed in Tables 1 and 2. The results obtained using
the similar catalyst (Table 1, entry 2) are analogous to litera-
ture results, in which the gel permeation chromatography
(GPC) molar masses of the polymer PFO were Mw
ꢀ85000 gmolÀ1, Mn ꢀ37000 gmolÀ1 in a toluene/water/etha-
nol mixture for 48 h,[14b] and lower than those (Mn
[a] J. Li, H. Fu, P. Hu, Z. Zhang, Dr. X. Li, Prof. Y. Cheng
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences
ꢀ48000 gmolÀ1) using [Pd
ACHTUGNRTEN(NUGN PPh3)4] with the phase-transfer
catalyst Aliquat 336, but still much higher than those with-
out Aliquat 336.[15] Compared with the toluene/water system
(below 10000 gmolÀ1, Table 1, entries 1 and 6), in partially
miscible THF/water system the molecular weight increased
to 70000 gmolÀ1 (Table 2, entries 1 and 6) using the conven-
tional PdCl2L2 and PdL4. Moreover, with the aid of hydro-
Changchun 130022 (P.R. China)
Fax : (+86)431-85262572
E-mail: xiaoli@ciac.jl.cn
yanxiang@ciac.jl.cn
[b] J. Li, H. Fu, P. Hu
Graduate School, Chinese Academy of Science
Beijing 100039 (P.R. China)
philic groups, PdCl2ACTHNUTRGNEN(UG LSS-3)2 and PdCAHTUNGTREN(NNGU LSS-3)3 as catalysts can
achieve higher molecular weights with Mn values above
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202162.
400000 gmolÀ1 (Table 2, entries 2 and 7). The facts revealed
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PdACHTNURTGNE(NUG LSS-3)3 it even showed no
advantage (Table 1, entries 2
and 7).
It is speculated that the
rather good hydrophilic proper-
ty of the ionic sulfonated group
(-SO3Na) might contribute the
water solubility of the com-
plexes and the fact that they
are hard to dissolve in weak
polar organic solvents, which
would inhibit the oxidative ad-
dition step and initiation of the
catalytic cycle. Enlightened by
Scheme 1. Synthesis of poly(9,9-dioctylfluorene) (PFO) as a model polymer to test the catalytic performance
and structures of ligands used.
the idea that the corresponding neutral acid ligands, with
better lipophilicity and also good hydrophilicity, would
result in the acidic hydrogen being deprotonated under the
basic conditions of the polymerization,[16] we conceived that
the process might be improved by replacing ionic ligands
with such neutral acidic ones. Therefore, a single sulfonic
acid group (-SO3H) or carboxylic acid group (-COOH) was
incorporated into the PPh3 parent to form LS-3, LC-4, and
LC-3 as amphipathic ligands. Because the neutral -SO3H
and -COOH would ionize during the polymerization, we
called this type of ligands “quasi-ionic” ligands. Finally, we
had a series of paired amphipathic catalysts, including PdII
Table 1. Reaction conditions[a] and product molecular weight[b] of the
polymerization in toluene/water biphasic system.
Entry
Cat.
Mn/104
Mw/104
PDI
Yield [%][c]
[d]
1
2
3
4
5
6
7
8
PdCl2L2
0.58
3.51
14.57
12.25
17.76
0.50
0.51
0.47
5.07
0.54
0.59
10.23
52.27
49.24
52.59
0.51
0.52
0.48
14.59
0.56
1.04
2.92
3.59
4.02
2.96
1.03
1.03
1.01
2.88
1.05
–
PdCl
PdCl
PdCl
PdCl
PdL4
N
92.1
88.8
81.9
92.3
[d]
–
–
–
[d]
Pd
Pd
Pd
Pd
N
[d]
9
10
86.9
[d]
–
[a] Reaction conditions: All polymerization reactions were carried out in
toluene (8 mL) and aqueous 2m K2CO3 (2 mL) with monomers (1 mmol)
and catalyst (1.5 mol%) at 1008C for 12 h under argon. [b] The polymer
molecular weight was determined by GPC analysis calibrated with poly-
styrene standards, with THF as the eluent with the flow rate at
and Pd0 complexes, with ionic ligands: PdCl
Pd(LSS-3)3 and quasi-ionic ligands: PdCl2A(LS-3)2 and Pd-
(LS-3)3, PdCl2A(LC-4)2 and Pd(LC-4)3, and PdCl2A(LC-3)2 and
Pd(LC-3)3.
2ACHTUNGTRENN(UNG LSS-3)2 and
A
CHTUNGTRENNUNG
A
N
G
CHTUNGTRENNUNG
AHCTUNGTRENNUNG
As shown in Table 1, the amphipathic PdII complexes
could be used as catalysts to obtain polymers with higher
molecular weights than hydrophobic PdCl2L2 and PdL4 in
toluene/water systems. The Mn obtained from polymeriza-
tions with the PdII quasi-ionic ligands (Table 1, entries 3–5
and Figure 1) increased even higher (Mn >100000 gmolÀ1)
than with the ionic ligand (Table 1, entry 2 and Figure 1) as
we had speculated. The amphipathic Pd0 complexes failed to
promote polymerization in this system, presumably due to
their extraordinarily good water solubility and no dissolubil-
ity in toluene, which hindered the O-A step due to the poor
1.0 mLminÀ1
. [c] After purification and precipitation in methanol.
[d] Due to the low molecular weight, the polymer was hard to weigh.
Table 2. Reaction conditions[a] and product molecular weight[b] of the
polymerization in a THF/water biphasic system.
Entry
Cat.
Mn/104
Mw/104
PDI
Yield [%][c]
1
2
3
4
5
6
7
8
PdCl2L2
7.49
42.63
45.41
57.17
39.92
7.42
47.54
19.76
59.20
41.91
34.22
159.48
167.29
194.29
82.55
27.81
190.23
99.06
4.57
3.74
3.68
3.40
2.07
3.75
4.00
5.01
3.41
2.17
90.7
94.3
82.2
90.1
87.1
77.1
78.4
80.6
79.1
78.5
PdCl
PdCl
PdCl
PdCl
PdL4
G
Pd
Pd
Pd
Pd
C
9
10
201.57
91.00
[a] Reaction conditions: All polymerization reactions were carried out in
THF (8 mL) and aqueous 2m K2CO3 (8 mL) with monomers (1 mmol)
and catalyst (1.5 mol%) at 808C for 12 h under argon. [b] The polymer
molecular weight was determined by GPC analysis calibrated with poly-
styrene standards, with THF as the eluent with the flow rate at
1.0 mLminÀ1. [c] After purification and precipitation in methanol.
that using the hydrophilic system (or groups and phase-
transfer agent) really was an effectual method to improve
the polymerization as we had anticipated. However, in tol-
uene/water systems, the molecular weight was still under
Figure 1. GPC chromatograms for polymerizations run with different PdII
catalysts in a toluene/water system.
100000 gmolÀ1 with PdCl
2ACHTNUTRGENNG(U LSS-3)2 as the catalyst, and for
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Catalysts for Suzuki Polycondensation
COMMUNICATION
contact between the monomer and catalyst. With Pd
(LC-4)3,
The conclusion has been verified indirectly by the meas-
urement of the UV/Vis spectrum in the same reactive sys-
tems as polymerization (including toluene, H2O, K2CO3 aq.
(2m), THF, and DMF/Toluene (2/1), see the Supporting In-
formation). If there was no or very slight absorbance of
some catalyst in the same solution/mixture, we could deduce
that the catalyst hardly dissolved in this system. The amphi-
pathic catalysts had macroscopic insolubility in toluene, as
shown in Figures S2a and S3a (in the Supporting Informa-
When using these amphipathic catalysts in the THF/water
system, the best results were obtained with PdCl2ACTHNUTRGNEN(UG LC-4)2
and PdACHTUNGTRENNUNG(LC-4)3 with which polymer products with molecular
weights of nearly up to 600000 gmolÀ1 (Table 2, entries 4
and 9 and Figure 2) were obtained. For other amphipathic
ones, except Pd
N
tion). The Pd0 complexes except Pd
ACTHNGUTERN(NUG LC-4)3 did totally not
dissolve in toluene, while all of the corresponding PdII ones
could dissolve partly in it. Moreover, from the inset of Fig-
ure S2a (in the Supporting Information), the quasi-ionic PdII
complexes, PdCl
showed a little better performance than the ionic PdCl2-
(LSS-3)2 as we had tentatively thought. PdCl2A(LSS-3)2 and
(LS-3)2 were water soluble, and PdCl2A(LC-4)2 and
2A(LC-3)2 turned out to be soluble in K2CO3 aqueous
2ACHTUNGNERT(NUNG LS-3)2, PdCl2AHCTNURGTEGNUN(N LC-4)2, and PdCl2ACHTUNGTRENNUNG(LC-3)2,
A
CHTUNGTRENNUNG
PdCl
PdCl
N
CHTUNGTRENNUNG
solution (Figure S2b and c in the Supporting Information).
All of the Pd0 amphipathic complexes easily dissolved com-
pletely in water, not least in K2CO3 aqueous solution (Fig-
ure S3b and c in the Supporting Information). It confirmed
our speculation that the rather good water solubility and
nearly no solubility in toluene was responsible for their dis-
appointing activity in the toluene/water system. All amphi-
pathic Pd complexes showed better solubility properties in
THF (Figures S2d and S3d in the Supporting Information),
Figure 2. GPC chromatograms for polymerizations run with different Pd0
catalysts in a THF/water system.
whereas PdACHTUNGRTENNG(U LSS-3)3 was partly soluble in THF and PdACHTUNGTRENNUNG(LS-
3)3 showed no solubility. The illustrations for solubility were
consistent with the polymerization experimental results in-
cluding the homogeneous DMF/toluene (2/1) system, in
which all of the hydrophobic and amphipathic catalysts dis-
solved completely (Figures S2e and S3e in the Supporting
Information).
above 400000 gmolÀ1 (Table 2, entries 2, 3, 5, 7, and 10).
Compared with only rare polymer precipitation in the tol-
uene/water system, solid polymer product began to precipi-
tate from the THF layer after 6 h, which might be due to
the ultra-high molecular weight of the polymers and lower
solubility in THF than in toluene. From the results, we pre-
sumed that the solubility of amphipathic catalysts were im-
proved in THF and the para-substituted hydrophilic group
would further facilitate the transmetalation step. Here we
also come to the conclusion that besides the rate-determin-
ing step (oxidative addition) of the catalytic cycle in the pal-
ladium-catalyzed SMC,[10] the transmetalation step has a
great influence on the process as well, which has, disappoint-
ingly, always been ignored.
We also tested this series of catalysts in homogeneous
DMF/toluene systems with KF as a base (Table 3 in the Sup-
porting Information). The new amphipathic catalysts, with
both PdII and Pd0, showed no advantage over traditional
ones in the polymerization, which indicated that the ach-
ievement of high molecular weight was mainly from the dra-
matic improvement in the transmetalation step in micro-
cosm and also that these Pd complexes exhibited a special
self-phase-transfer feature. The PdII intermediate produced
in the oxidative addition can be anchored to the two-phase
interface with the aid of the hydrophilic groups, which
makes the components in the water phase react much more
rapidly with it, resulting in efficient entry into the catalytic
cycle.
In summary, palladium(II) and palladium(0) complexes
coordinated by phosphine ligands modified by introducing
weak acidic groups on the phenyl ring are highly efficient
and effective in promoting Suzuki polymerization. The am-
phipathic nature of these complexes not only introduces
self-phase-transfer features, but also to stabilizes low-valent
metals. Ultra-high molecular weight polymers PFO (Mn) up
to 600000 gmolÀ1 were achieved by using these amphipathic
palladium complexes as catalysts. Their dominant virtue rel-
ative to the classical Pd catalysts is that the complexes may
accelerate the transmetalation step in the catalytic cycle,
which is also the key step for synthesizing high molecular
weight polymers. Considering the better solubility of the
polymer product in toluene, studies concerning further im-
provement of the molecular weight in toluene/water systems
are currently underway by introducing neutral hydrophilic
groups for the balance of the water and oil solubility of the
amphipathic catalysts.
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X. Li, Y. Cheng et al.
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Experimental Section
Typical experimental procedure for the palladium-catalyzed Suzuki poly-
condensation: Toluene (8 mL) and a 2m aqueous solution of K2CO3
(2 mL), or THF (8 mL) and a 2m aqueous solution of K2CO3 (8 mL)
were added to a mixture of monomers (1 mmol; purity>99.7%) and cat-
alyst (1.5 mol%) under an argon atmosphere. The reaction was stirred
and refluxed for 12 h. After workup, the mixture was poured into metha-
nol. The precipitate was collected by filtration and then dissolved in
CH2Cl2. The solution was washed with water and dried with anhydrous
sodium sulfate. After filtration, the solution was concentrated to an ap-
proximate volume and poured into methanol to obtain polymer fibers.
The resulting polymers obtained after vacuum drying were weighed as
yield data and determined by GPC. Other experimental details, such as,
materials, experimental techniques, synthesis of catalysts, UV/Vis spec-
trum and the purity of the monomers, are given in the Supporting Infor-
mation.
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This work is supported by the National Natural Science Foundation of
China (Nos. 21174141 and 51073152) and the 973 Project
(2009CB623600).
Keywords: amphiphiles
·
palladium
·
phase-transfer
catalysis · polymers · Suzuki polycondensation
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Received: June 19, 2012
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Catalysts for Suzuki Polycondensation
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Palladium
J. Li, H. Fu, P. Hu, Z. Zhang, X. Li,*
Y. Cheng* ...................... &&&& — &&&&
Catalysts for Suzuki Polycondensation:
Ionic and “Quasi-Ionic” Amphipathic
Palladium Complexes with Self-Phase-
Transfer Features
Poly(9,9-dioctylfluorene) (PFO) with
Mn values above 100000 gmolÀ1 in a
toluene/water system and Mn values up
to 600000 gmolÀ1 in a THF/water
system has been obtained by improved
Suzuki polycondensation using a new
kind of amphipathic palladium catalyst
with self-phase-transfer features, which
could overcome the disadvantage
caused by the immiscible biphasic mix-
ture and accelerate the transmetalation
step (see figure).
Chem. Eur. J. 2012, 00, 0 – 0
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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