Synthesis of novel polyfluorene with defined group in the center using aryl dipalladium complex as an initiator

Article abstract of DOI:10.1016/j.jorganchem.2013.04.016

Dinuclear Pd(II) complex DiPd-DPE with a bridging ligand C ₆H₄OC₆H₄-4,4′ was successfully synthesized by double oxidative addition reaction of Pd(PCy₃) ₄ with 4,4′-dibromodiphenyl ether in good yield. X-ray crystal structure analysis reveals that the Pd atoms have the nearly square-planar coordination geometry with the similar bond lengths of two Pd-Br as well as four Pd-P, implying that two Pd(II) centers have similar reactivity. Using the dinuclear Pd(II) complex DiPd-DPE as an initiator, novel polyfluorene (PF) with a defined group (C₆H₄OC₆H₄-4, 4′) in the middle of polymer main chain was prepared, demonstrated by MALDI-TOF analysis.

Full text of DOI:10.1016/j.jorganchem.2013.04.016

Journal of Organometallic Chemistry 738 (2013) 55e58
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Note
Synthesis of novel polyfluorene with defined group in the center using aryl
dipalladium complex as an initiator
,
,
,
a
a,
*
Hongwei Fu a b, Jing Li a b, Zilong Zhang a, Hongmei Zhan a , Xiao Li , Yanxiang Cheng
*
a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, PR China
b University of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e i n f o
a b s t r a c t
Dinuclear Pd(II) complex DiPdeDPE with a bridging ligand C6H4OC6H4-4,40 was successfully synthesized
by double oxidative addition reaction of Pd(PCy3)4 with 4,40-dibromodiphenyl ether in good yield. X-ray
crystal structure analysis reveals that the Pd atoms have the nearly square-planar coordination geometry
with the similar bond lengths of two PdeBr as well as four PdeP, implying that two Pd(II) centers have
similar reactivity. Using the dinuclear Pd(II) complex DiPdeDPE as an initiator, novel polyfluorene (PF)
with a defined group (C6H4OC6H4-4,40) in the middle of polymer main chain was prepared, demonstrated
by MALDI-TOF analysis.
Article history:
Received 28 February 2013
Received in revised form
9 April 2013
Accepted 10 April 2013
Keywords:
Palladium complex
Catalyst
Ó 2013 Elsevier B.V. All rights reserved.
Chain-growth polycondensation
Conjugated polymer
1. Introduction
reaction due to its relatively poor stability, which must be strictly
operated and stored under inert atmosphere at low temperature.
Conjugated polymers have drawn a great deal of attention owing
to their potential applications in organic light-emitting diodes
(OLEDs) [1e8], field-effect transistors (OFETs) [9e11] and photovol-
taic cells (OPVs) [12,13]. These polymers have been commonly syn-
thesized by conventional step-growth polycondensation [14e16],
displaying broad molecular weight distribution. Chain-growth
polymerization, however, could address this issue and obtaining
polymers with controlled molecular weight and low polydispersity
[17e20]. The representative work involves the Ni-catalyzed poly-
condensation of Grignard-type monomer which was extensively
used in the synthesis of regioregular poly(3-alkylthiopherens)
(P3ATs) as an electron-donating material for OPVs [20e23]. The
controlled regioregular polythiophenes with low polydispersity
show superior electronic and optical properties over the regioran-
dom analogs. Recently, Yokozawa reported a three-coordinate
complex [(tBu3P)Pd(Ph)Br] used as an initiator in the chain-growth
SuzukieMiyaura polycondensation reaction of AB-type aromatic
monomer. The polycondensationproceeded from the defined phenyl
unit derived from the catalyst [(tBu3P)Pd(Ph)Br] and achieved poly-
merswith well-defined end groups[24e26]. Nevertheless, evenwith
these advantages, the unsaturated-coordination catalyst [(tBu3P)
Pd(Ph)Br] has not been widely investigated in polymerization
Inspired by these ideas, we speculate that if the polymerization
starts from a saturated-coordination dinuclear palladium initiator
with a bridging functional group between two Pd centers and
proceeds in a chain-growth polymerization, the defined functional
group would serve as an initiator unit and be incorporated in the
center of polymer main chain. Therefore, we focus on the devel-
opment of dipalladium catalyst [27e31], in which the choice of
phosphine ligand is very critical. The bulky tricyclohexyl phosphine
(PCy3) was chosen as the auxiliary ligand because it has high ac-
tivity in the reductive elimination step and could effectively inhibit
the arylearyl exchange side reaction without the additional phos-
phine ligand [32e35]. In addition, diphenyl ether with simple
structure and sufficient space is chosen as the bridging ligand.
Finally, We successfully synthesized the dinuclear palladium
complex DiPdeDPE by double oxidative addition of 4,40-dibro-
modiphenyl ether to Pd(PCy3)4, and preliminarily investigate the
polymerization behavior of AB-type fluorene monomer using
DiPdeDPE as an initiator (Scheme 1).
2. Results and discussions
2.1. Synthesis of dipalladium complex DiPdeDPE
Based on the consideration of thermal stability, we initially
attempted to prepare the dinuclear Pd(II) complex DiPdeDPE at
* Corresponding authors. Tel.: þ86 431 85262106; fax: þ86 431 85262572.
E-mail addresses: hmzhan@ciac.jl.cn (H. Zhan), yanxiang@ciac.jl.cn (Y. Cheng).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.04.016

56
H. Fu et al. / Journal of Organometallic Chemistry 738 (2013) 55e58
PCy3
Pd Br
PCy3
Br
O
Br
Br
O
toluene,85 oC
Pd-DPE
N2H4 H2O
Pd(PCy3)4
PdCl2 + PCy
3
toluene,120 o
C
PCy3
Br Pd
PCy3
PCy3
Pd Br
PCy3
Br
O
Br
O
toluene,120 oC
DiPd-DPE
DiPd-DPE
O
B
O
O
Br
n
n
THF/18-Crown-6/KF/H2O
C8H17
C8H17
C8H17
C8H17
C8H17
C8H17
PF-DPE
Scheme 1. Synthesis of dipalladium catalyst DiPdeDPE and polymer PFeDPE.
85 ꢀC by double oxidative addition reaction of 4,40-dibromodi-
phenyl ether with Pd(PCy3)4 generated in situ in a ratio of 1:2.5 in
toluene, however, only mononuclear complex PdeDPE was iso-
lated in good yield, confirmed by 1H NMR spectrum and X-ray
crystallography. Increase of the reaction temperature to 120 ꢀC
leads to the target dipalladium complex DiPdeDPE in 72% yield,
which means that the double oxidative addition could be carried
out smoothly at higher temperature [24]. The chemical structure
of DiPdeDPE was determined by 1H NMR and 31P NMR. 31P NMR
spectrum shows a singlet at 19.3 ppm, indicative of its symmetric
structure, which is in accordance with a trans configuration hav-
ing four magnetically equivalent PCy3 ligands. The dipalladium(II)
complex DiPdeDPE has better stability compared with mono-
meric three-coordination arylpalladium(II) halide complex
[(tBu3P)Pd(Ph)Br], which renders the operation easier. Indeed,
complex DiPdeDPE can be weighted, transferred in air, and
recrystallized from the solvents without distillation treatment.
Furthermore, it is worth noting that the dipalladium(II) complex
DiPdeDPE can be preserved in the Schlenk flask under argon at
room temperature for two month with the catalytic activity
unchanged.
2.3. Synthesis and polymerization behavior study of polyfluorene
The polymerization of AB-type monomer 2-(7-bromo-9,9-
dioctylfluoren-2-yl)-1,3,2-dioxaborinane with dipalladium(II)
complex DiPdeDPE was carried out at 25 or 75 ꢀC for 12 h in the
presence of KF/18-crown-6. THF with a small amount of water for
dissolving KF was chosen as the reaction solvents because they
could form a homogeneous system facilitating the polymerization
process. After the reaction was quenched with 1 M hydrochloric
acid, the crude product was filtered and washed with acetone via a
Soxhlet apparatus to remove low-molecular-weight fractions. Mo-
lecular weights and polydispersity indices (PDI) of polyfluorene
PFeDPE were determined by gel permeation chromatography
(GPC) analysis with a polystyrene standard calibration. The number
average molecular weight (Mn) of PFeDPE (25 ꢀC) is 12,320 g/mol
with a PDI of 1.84, and PFeDPE (75 ꢀC) exhibits a higher Mn of
15,670 g/mol with a slightly broader PDI of 2.09. These results
reveal that high temperature is beneficial for the proceeding of
SuzukieMiyaura coupling polymerization of fluorene-based
monomer.
The polymerization behavior was further evaluated by MALDI-
TOF mass spectrometry. The mass spectrum for polyfluorene ob-
tained at 75 ꢀC (Fig. 3) shows a series of major peaks accompanied
with two sets of minor peaks. All of the mass differences between
2.2. X-ray characterization of DiPdeDPE and PdeDPE
The molecular structures of complexes DiPdeDPE and PdeDPE
are shown in Figs. 1 and 2, and selected bond distances and angles
were summarized in Table 1. Complex DiPdeDPE adopts the ex-
pected trans configuration with a nearly square-planar geometry
around central Pd atoms, supported by the bond angle of C1ePd1e
Br1, P1ePd1eP2, C10ePd2eBr2 and P3ePd2eP4 (166.15e173.80ꢀ),
which are approximate to 180ꢀ. The four cis angles around each Pd
atom are close to orthogonal (89.76e93.67ꢀ and 87.10e92.48ꢀ,
respectively). The two planes defined by Pd1Br1P1P2C1 and
Pd2Br2P3P4C10 are twisted with dihedral angle of 50.71ꢀ. Each
phenyl ring is almost perpendicular to the adjacent Pd(II)-centered
plane with the dihedral angles of 85.74 and 84.83ꢀ. In addition, the
ꢀ
ꢀ
bond length of Pd1eBr1 [2.5324(7) A] and Pd2eBr2 [2.5104(9) A]
are similar to each other as well as the four PdeP bond distances.
All of the results illustrate that two Pd(II) centers should have
similar chemical reaction activity. Accordingly, the bridging group
C6H4OC6H4-4,40 would be introduced in the center of polymer main
chain when the dipalladium(II) complex DiPdeDPE is used as an
initiator in the polycondensation reaction.
Fig. 1. An ORTEP drawing of DiPdeDPE showing the atom-labeling scheme and 30%
probability thermal ellipsoids, H atoms and solvent molecule are omitted for clarity.

H. Fu et al. / Journal of Organometallic Chemistry 738 (2013) 55e58
57
Fig. 3. MALDI-TOF mass spectrum of PFeDPE obtained at 75 ꢀC.
Fig. 2. An ORTEP drawing of PdeDPE showing the atom-labeling scheme and 30%
probability thermal ellipsoids, H atoms are omitted for clarity.
the active Pd species should transfer and carry out the oxidative
addition with the end CeBr, however, it is possible for the active Pd
species to form free active ones. These species may in turn initiate
the polymerization of monomer to give PF without DPE group. In
spite of this, there exists a problem to speculate the precise mole-
cule structures of these peaks based on their absolute mass because
the corresponding m/z is not consistent with either the molecule
weight of pure fluorene oligomers or the sum of central group
C6H4OC6H4-4,40 (168.2) plus any fluorene unit. Therefore, further
work using the aryl Pd complexes with the functional group as
initiator is in progress so as to discern reasonable supporting
information.
the adjacent main peaks are almost 388.6 such as 2502.0e
2113.5 ¼ 388.5, 2890.5e2502.0 ¼ 388.5., corresponding to the
characteristic mass value of fluorene unit, which demonstrates that
9,9-dioctylfluorene is the repeating unit of afforded polymer. On
the other hand, the major peaks correspond to polymer with
diphenyl ether group (C6H4OC6H4-4,40) in the center, derived from
complex DiPdeDPE, and two hydrogen atoms at both ends, derived
from the Pd complex end groups by quenching with hydrochloride
acid (designed as DPE/H/H) [24e26,36]. For example, the mass of
the 9-mer with DPE/H/H should be 3667.6 in theory and a signal
peak at 3667.6 is observed in fact. The number of repeating unit for
major signal peaks from left to right increases from 5 to 16. Ac-
cording to these results, we can conclude that the defined group
(C6H4OC6H4-4,40) is to a large degree incorporated into the middle
of the polymeric main chain, implying that the polymerization at
75 ꢀC mostly proceeds in a chain-growth manner.
3. Conclusions
In summary, we have successfully prepared a stable dinuclear
palladium complex DiPdeDPE by double oxidative addition reac-
tion of Pd(PCy3)4 generated in situ with 4,40-dibromodiphenyl ether
in good yield. Furthermore, a new-type conjugated polymer PFe
DPE bearing diphenyl ether group in the center of polymer main
chain was obtained using the complex DiPdeDPE as an initiator.
The polymerization at high temperature starts from the diphenyl
ether moiety derived from dipalladium complex DiPdeDPE and to
a large extent proceeds in a chain-growth manner even with PCy3
as auxiliary ligand. By reasonable modification of the bridging
ligand of dinuclear palladium complex, we think, the polymers
with the functional group in the center can be developed via the
chain-growth polycondensation, and their photophysical proper-
ties might be tuned by the central group.
The polymer obtained at 25 ꢀC was also investigated by MALDI-
TOF analysis. Unfortunately, the signal peaks containing the central
C6H4OC6H4-4,40 group could not be found in the mass spectrum.
However, the repeating unit of the obtained polymer is still 9,9-
dioctylfluorene unit, deducing from the mass differences between
the adjacent peaks (388.6). The result indicates the polymerization
behavior at low temperature (25 ꢀC) is the traditional direct
coupling of AB-type fluorene monomer rather than the typical
chain-growth process. This behavior was also found in the poly-
merization carried out at 75 ꢀC, deducing from two sets of minor
peaks of the MALDI-TOF spectrum. According to the mechanism of
the chain-growth polymerization, after the reduction elimination,
4. Experimental
4.1. General materials and measurements
Table 1
ꢀ
ꢀ
Selected bond lengths [A] and angles [ ] for DiPdeDPE and PdeDPE.
All the reactions were carried out under argon atmosphere us-
ing standard Schlenk techniques. The solvents were dried and
distilled under argon prior to use. All chemicals were purchased
from the commercial sources such as Aldrich, J&K and Acros. 1H
NMR spectra with TMS as internal reference and 31P NMR spectra
with 85% H3PO4 as external reference were taken on a Bruker AV
400 NMR spectrometer at room temperature. Elemental analyses
were performed on a BioRad elemental analysis system. Matrix-
assisted laser desorption/ionization time-of-flight (MALDI-TOF)
spectra were measured on an Autoflex III Bruker MALDI-TOF mass
spectrometer.
Complex
DiPdeDPE
PdeDPE
Pd1eC1
Pd1eP1
Pd1eP2
Pd1eBr1
2.008(5) Pd2eC10
2.3525(14) Pd2eP3
2.3687(14) Pd2eP4
2.5324(7) Pd2eBr2
2.019(5) PdeC1
2.3520(16) PdeP1
2.3637(15) PdeP2
2.5104(9) PdeBr1
2.023(3)
2.3681(7)
2.3559(7)
2.5394(3)
C1ePd1eP1
C1ePd1eP2
P1ePd1eP2 169.60(5) P3ePd1eP4
C1ePd1eBr1 166.15(16) C10ePd2eBr2 173.80(16) C1ePdeBr1 177.54(7)
P1ePd1eBr1 91.91(4) P3ePd1eBr2
P2ePd1eBr1 87.10(4) P4ePd1eBr2
89.76(14) P2ePd1eBr1
93.67(15) C10ePd2eP3
87.10(4) C1ePdeP1
89.91(15) C1ePdeP2
171.42(5) P1ePdeP2 177.16(2)
92.14(7)
90.64(7)
89.87(4) P1ePdeBr1 87.115(18)
92.48(4) P2ePdeBr1 90.083(19)

58
H. Fu et al. / Journal of Organometallic Chemistry 738 (2013) 55e58
4.2. Synthesis of dinuclear Pd(II) complex DiPdeDPE
Acknowledgments
To a Schlenk flask containing PdCl2 (0.44 g, 2.48 mmol) and PCy3
(3.50 g, 12.48 mmol) at room temperature were added dry toluene
(45 mL). The mixture was heated to 120 ꢀC under argon and then
Hydrazine hydrate (20 mL) was added dropwise to the solution. The
mixture was stirred at 120 ꢀC for 30 min to give a yellow solution.
After cooled to the room temperature, the supernatant was trans-
ferred into a solution of 4,40-dibromodiphenyl ether (0.33 g,
1.01 mmol) in toluene (10 mL) under Ar atmosphere. After stirring
of the mixture at 120 ꢀC for 24 h, the solvent was evaporated under
reduced pressure. The residue was washed with Et2O (3 ꢁ 10 mL) to
give a white solid. The crude product was recrystallized from
CH2Cl2/Et2O to give DiPdeDPE (1.19 g, 72%) as colorless crystals. 1H
This work is supported by the National Natural Science Foun-
dation of China (Nos. 21174141 and 51073152) and 973 Project
(2009CB623600).
Appendix A. Supplementary material
CCDC 923881 and 923882 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
NMR (400 MHz, CD2Cl2,
d
/ppm): 7.18 (d, J ¼ 7.9 Hz, 4H, phenylene),
6.58 (d, J ¼ 7.9 Hz, 4H, phenylene), 2.07e1.93 (m, 12H, PCy3), 1.85e
References
1.77 (m, 24H, PCy3), 1.68e1.47 (m, 60H, PCy3), 1.18e0.97 (m, 36H,
PCy3). 31P{1H} NMR (162 MHz, CDCl3,
d/ppm): 19.3 (s). Anal. Calcd.
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4.3. Synthesis of polyfluorene PFeDPE
To a Schlenk flask containing the monomer 2-(7-bromo-9,9-
dioctylfluoren-2-yl)-1,3,2-dioxaborinane (0.28 g, 0.51 mmol), 18-
crown-6 (0.53 g, 2.01 mmol), KF (0.12 g, 2.01 mmol) were added a
mixture of THF/H2O (8 mL/1 mL) in that order. To another Schlenk
containing dinuclear Pd(II) complexes DiPdeDPE (33.3 mg,
0.02 mmol) were added THF (7 mL). The solution of DiPdeDPE was
transferred into the solution of monomer in THF/H2O. The reaction
mixture was stirred at 25 or 75 ꢀC for 12 h. The resulting mixture
was cooled to room temperature, and then poured into dichloro-
methane (200 mL). The dilute aqueous HCl (10 mL, 1 M) was added.
After stirring at room temperature for 30 min, the mixture was
washed with water and the organic layer was dried over anhydrous
Na2SO4. The solvents were removed under reduced pressure. The
residue was precipitated from dichloromethane/MeOH to give the
crude polymer. Then the solid was loaded into Soxhlet apparatus
and washed with acetone (12 h). The remaining polymer was dried
over in a high vacuum drying oven overnight. Yield: 86%.
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4.4. X-ray structure determination of DiPdeDPE
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