Multifunctional redox polymers: Electrochrome, polyelectrolyte, sensor, electrode modifier, nanoparticle stabilizer, and catalyst template

Article abstract of DOI:10.1002/anie.201403062

Simple "click" polycondensation metallopolymers of redox-robust bis(ethynyl)biferrocene (biFc) and di(azido) poly(ethylene glycol) (PEG400 and PEG1000) were designed for multiple functions including improvement of water solubility and biocompatibility, the introduction of mixed valency and sensing capabilities, and as nanoparticle stabilizers for catalysis.

Full text of DOI:10.1002/anie.201403062

Angewandte
Chemie
DOI: 10.1002/anie.201403062
Biferrocene Polymers
Multifunctional Redox Polymers: Electrochrome, Polyelectrolyte,
Sensor, Electrode Modifier, Nanoparticle Stabilizer, and Catalyst
Template**
Christophe Deraedt, Amalia Rapakousiou, Yanlan Wang, Lionel Salmon, Melanie Bousquet,
and Didier Astruc*
Dedicated to Alan H. Cowley on the occasion of his 80th birthday
Abstract: Simple “click” polycondensation metallopolymers
of redox-robust bis(ethynyl)biferrocene (biFc) and di(azido)
poly(ethylene glycol) (PEG400 and PEG1000) were designed
for multiple functions including improvement of water solu-
bility and biocompatibility, the introduction of mixed valency
and sensing capabilities, and as nanoparticle stabilizers for
catalysis.
reasonable dimesions to make use of the enhanced perme-
ability and retention (EPR) effect to achieve accumulation in
cancer cells.[5] The PEG chains and biferrocene (BiFc) units
have been assembled by the copper-catalyzed alkyne–azide
1,3-cycloaddition (CuAAC) reaction (“click” chemistry),[6,7]
generating 1,2,3-triazole (trz) linkages. Indeed, these hetero-
cyclic nitrogen ligands are known to sense and stabilize
transition-metal ions such as PdII and AuIII, which in turn are
precursors for useful metal nanoparticles (NPs) if the polymer
can protectively encapsulate and stabilize them.[8] Small Pd
NPs and Au NPs are efficient catalysts for a variety of
reactions, and their stabilization through the triazole-con-
taining polymer would be a fast and efficient way to provide
templated catalysts. Depending on the AuIII to Au0 reduction
mode, the size of the Au NPs might be tuned for various
applications in catalysis or biomedicine, in which the PEG
coating provides additional benefits. Another interesting
property of the biferrocene polymers is that the parent unit
itself reduces AuIII to the polymer-stabilized Au NPs.
The synthesis of the copolymers PEG-trz-biFc 4 [poly-
(PEG400-biFc)] and 5 [poly(PEG1000-biFc)] was achieved by
the simple CuAAC “click” reaction between bis-azido-
poly(ethylene glycol) with molecular weights of 400 or 1000
gmolÀ1 (PEG400 and PEG1000; 1 and 2, respectively)[9] and 1,1’-
biethynyl-biFc 3.[10] The biferrocene unit provides the electro-
chemical properties (recognition ability, polyelectrolytic and
polyelectrochromic features) while the PEG chains of vari-
able length introduce tunable solubility into the polymer. The
1,2,3-triazole ring constructed by “click” chemistry stabilizes
active palladium nanoparticles for catalysis of the Suzuki–
Miyaura[11] reaction, which was performed with as little as 2 to
20 ppm of Pd. Additionally, gold nanoparticles were used to
efficiently catalyze the reduction of 4-nitrophenol.
T
he development of sustainable and biocompatible nano-
materials that have a variety of functions is highly desirable.[1]
Here we propose a significant step toward this goal by using
biferrocene units to construct water-soluble polymers, which
can be used for a variety of applications including sensing and
catalysis. Nowadays, a variety of ferrocene-based polymers
are known, which exhibit very useful electrochromic proper-
ties. For example, orange ferrocene polymers that turn blue
upon oxidation to ferricenium were synthesized by ring-
opening polymerization of ferrocenophanes by Manners and
co-workers.[2] To strengthen the electron-deficient ferrice-
nium groups in polymers, we have now synthesized biferro-
cene[3,4] polymers in which the electron-releasing ferrocene
units stabilize the adjacent ferricenium units to produce
robust blue-green mixed-valence biferrocenium polymers. In
addition to the electrochromic properties, these new metal-
lopolymers have been constructed with poly(ethylene glycol)
(PEG) chains that not only introduce water solubility and
biocompatibility to the nanomaterials but also provide
[*] C. Deraedt, A. Rapakousiou, Y. Wang, Prof. D. Astruc
ISM, UMR CNRS N85255, Univ. Bordeaux
33405 Talence, cedex (France)
E-mail: d.astruc@ism.u-bordeaux.fr
Homepage: http://astruc.didier.free.fr
M. Bousquet
The “click” polymerization (polycondensation) between
1 or 2 and diethyne 3 is carried out at 408C over 2 days in
THF/H2O (3:2) using the Sharpless–Fokin catalyst consisting
of CuSO4, H2O (5 equiv), and sodium ascorbate (NaAsc;
Scheme 1). After removal of the copper ions the polymers are
precipitated from Et2O providing 4 as a red-orange air-stable
film and 5 as a paste in 78% and 58% yield, respectively (see
the Supporting Information, SI).
These neutral metallopolymers have been fully charac-
terized by NMR, UV/Vis, and IR spectroscopy, matrix-
assisted laser desorption/ionization time-of-flight (MALDI-
TOF) mass spectrometry, elemental analysis (EA), cyclic
Laboratoire de Chimie des Polymꢀres Organiques
UMR CNRS N8 5629, Univ. Bordeaux
33607 Pessac, cedex (France)
Dr. L. Salmon
Laboratoire de Chimie de Coordination UPR CNRS N88241
31077 Toulouse cedex (France)
[**] Helpful discussions with Dr. J. Ruiz (Univ. Bordeaux) and financial
support from the Univ. Bordeaux and Toulouse III, the CNRS, and
the Ministꢀre de l’Enseignement Supꢁrieur et de la Recherche
(Ph.D. grant to C.D.) are gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403062.
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molecular properties: because of
their redox properties and as
stable and isolable mixed-valence
compounds, they can act as molec-
ular electron storage systems.
Cyclic voltammetry involving the
bis(trz-Fc) units has been applied to
sense ions with potential applica-
tions such as the fabrication of
modified electrodes. The solubiliza-
tion properties of the two polymers
with distinct PEG lengths are com-
plementary; indeed 4 is soluble only
in CH2Cl2, CHCl3, DMF, whereas
due to the long PEG chain, 5 is
soluble in many more solvents
including THF, MeOH, EtOH,
and H2O.
The cyclic voltammograms of 4
and 5 measured with a Pt electrode
in CH2Cl2 using 0.1m [nBu4N][PF6]
as the supporting electrolyte and
decamethylferrocene,
[Cp*2Fe]
(Cp* = h5-C5Me5), as the internal
reference show two reversible
waves at 0.440 and 0.785 V vs.
Scheme 1. Synthesis of the PEG-biFc polymers 4 and 5 and the mixed-valence polymers 6–9).
voltammetry (CV), and size-exclusion chromatography
(SEC) (SI). Mass spectrometry shows that 4 is composed of
at least eight co-units, which corresponds to 15 trz rings. No
higher polymer has been observed by mass spectrometry
presumably due to saturation of the detector with small
oligomers. The polydispersity of PEG1000 prevents the obser-
vation of 5 by mass spectrometry. No chain terminus (CH2-N3
[Cp*2Fe]+/0 (Figure 1a) and 0.420 and 0.800 V vs. [Cp*2Fe]+/0
(SI), respectively. The presence of only two reversible waves,
like in biferrocene itself, implies that the two Fc moieties of
one biFc unit are dependent on each other, whereas each biFc
unit is independent of other biFc units. One interesting
ꢀ
or -C CH) has been observed in the NMR spectra of either
polymer. The polymeric nature of 4 and 5 was also confirmed
by IR spectroscopy (no vibrations corresponding to azide and
alkyne groups were found) and by CV measurements
(estimation for 4: 62 Æ 12 co-units; 5: 131 Æ 20 co-units; SI).
The dispersity of the polymers was measured by SEC analyses
(4: D = 1.24 and 5: D = 1.27) and is particularly low for
a polycondensation process, which makes it possible to
exclude the formation of macrocyles. If any macrocycles
would be formed in small quantities, they would be removed
during the precipitation from ether.[12]
The polymers exhibit a variety of interesting features.
First of all, the synthesis is very simple, because only one step
is needed from known products, and the CuAAC reaction
conveniently yields products, which are easy to purify.
Secondly, the use of PEG units introduces biocompatibility,
while at the same time the easy availability and simple
synthesis from the starting materials 1 and 2 is maintained.
Additionally, the use of linear PEG minimizes steric bulk
around the trz-biFc-trz units. Thirdly, the “click” chemistry
provides triazolyl groups, which are important for the binding
of metals and the stabilization of metal NPs. The crucial role
of the trz rings in the catalysis by metal NPs is exemplified
below.[13] Finally, the presence of biFc units results in multiple
stable redox states, which is the most interesting feature of
polymers 4 and 5. These polymers have multifaceted supra-
Figure 1. a) Cylic voltammogram of 4 in CH2Cl2; reference electrode:
Ag; working and counter electrodes: Pt; scan rate: 0.2 VsÀ1; support-
ing electrolyte: [nBu4N][PF6]. Two reversible waves are present, one at
0.440 V and a second at 0.785 V. b) Progressive adsorption of 4 upon
scanning around the biFc area. c) The modified Pt electrode at various
scan rates in a CH2Cl2 solution, which only contains the supporting
electrolyte). d) Splitting of the second oxidation wave upon addition of
Pd(OAc)2 to 4 (1 equiv of PdII per BiFc unit).
2
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feature of 4 is its progressive adsorption on the electrode, as
can be seen from the cyclic voltammogram (Figure 1b). This
allows the facile formation of robust metallopolymer-modi-
fied electrodes upon scanning back and forth around the biFc
potential zone (Figure 1c). Variation of the intensity with the
scan rate during the electrode modification provides a linear
function as it would be expected for a correctly modified
electrode (SI). The adsorption of 5 on the electrode is weaker
than that of 4 owing to the better solubility of 5 and its
ferricenium form. The CV behavior of 4 allows its use and
that of the corresponding modified electrode for ion recog-
nition. After addition of Pd(OAc)2 to a solution of 4,
a splitting of 70 mV of the second biFc oxidation wave is
observed, reflecting the coordination of PdII to the nitrogen
atoms of the trz ligand. This modifies the electron density of
the Fc centers (Figure 1d). The complexation of Pd(OAc)2 by
trz can also be monitored by 1H NMR spectroscopy in CDCl3/
MeOH (this solvent mixture was also used in the Pd NP
Figure 2. Mçssbauer spectrum of 6 at zero field and 78 K showing the
localized class II mixed valency at 107 sÀ1 with both ferrocenyl (doublet
I.S.=0.536 mmsÀ1 vs. Fe; Q.S.=2.192 mmsÀ1) and ferricinium
groups (doublet I.S.=0.490 mmsÀ1 vs. Fe; Q.S.=0.571 mmsÀ1).
The presence of triazole units in the polymer chains allows
the coordination of 1 equiv PdII or AuIII ions per heterocycle
ligand (see above and SI) and further reduction leads to the
formation of polymer-stabilized Pd NPs and Au NPs,
respectively. These nanomaterials were shown to be active
catalysts for various reactions. In addition, it was shown that
the longer PEG chains in 5 (PEG1000) result in the solubiliza-
tion of these stabilized NPs in water.
The Suzuki–Miyaura reaction has been conducted in the
presence of Pd NPs stabilized by 4 and 5. Polymer 4 is mixed
with Pd(OAc)2 (1 equiv per triazole unit) in CHCl3/MeOH (2/
1) and stirred for 5 min. Then NaBH4 (10 equiv per PdII) is
added to the solution, leading to the formation of black Pd
NPs. These stable Pd NPs at a loading of 0.25 mol% Pd are
efficient in the Suzuki–Miyaura cross-coupling reaction of
various bromoarenes with phenylboronic acid (Table 1) in
CHCl3/MeOH (2:1) at 908C for 16 h. The water-soluble Pd
NPs stabilized by 5 are prepared by mixing aqueous solutions
of 5 and K2PdCl4 at 208C and stirring for 5 min (1/1 Pd/
triazole stoichiometry) followed by the quick addition of
a NaBH4 solution (10 equiv per PdII). The yellow solution of
PdII-5 instantaneously turned orange-brown (SI), confirming
the formation of Pd NPs. Transmission electron microscopy
(TEM) reveals that the size of Pd NPs stabilized by 5 is (2.3 Æ
0.6) nm after they had been stored for one month, which is
ideal for efficient catalysis (Figure 3).
The Suzuki–Miyaura reaction has also been performed
with this catalyst system, using the “greener” solvent mixture
EtOH/H2O (1:1) at 808C for 24 h with only 20 ppm of Pd (Pd
NPs). The results are summarized in Table 1 and demonstrate
the high activity of 5-PdNPs. A loading of only 20 ppm Pd is
sufficient to obtain good yields using both activated as well as
deactivated bromoarenes. Moreover, the amount of Pd can be
further reduced. In the case of the coupling of 4-bromoace-
tophenone and phenylboronic acid at 808C only 2 ppm of Pd
was necessary to achieve a reaction yield of 82% after 36 h
(TON = 410000, TOF = 11400 hÀ1). The use of such a low
amount of catalyst is very rare in the Suzuki–Miyaura
reaction, further underlining the interesting properties of
the nanomaterial described here. A comparative table (SI)
positions 5-PdNPs as one of the best catalysts known for the
Suzuki–Miyaura reaction.
1
catalysis, see below). The H NMR signal of the acetate is
shifted upon addition of 4. This shift is observed until 1 equiv
Pd(OAc)2 per triazolyl ring has been added (SI). The 1:1
stoichiometry of PdII/trz ligand proves to be important in
catalysis (see below), when Pd NPs resulting from the
reduction of trz-coordinated PdII species are used.
The proximity of the Fc moieties in the BiFc units implies
another application, which is the synthesis of stable mixed-
valence polymers affording electrochromic and polyelectro-
lyte materials. The mixed-valence polymers 6 and 7 were
quantitatively synthesized by stoichiometric exergonic reac-
tions of 4 in CH2Cl2 with [Cp2Fe][PF6] and [Cp2Fe][BF4],
respectively, because the redox potential of [Cp2Fe]+/0 is
higher than the first oxidation potential of the biFc+/0 units.
An additional driving force for these reactions is the
precipitation of the mixed-valence polymers 6 and 7 from
CH2Cl2. The acetonitrile-soluble polymers 6 and 7 are blue-
green, whereas the starting material 4 is orange. Likewise, the
mixed-valence polymer 8 was synthesized from 5 in the
presence of 1 equiv [Cp2Fe][BF4] per biFc unit and fully
characterized (SI). The mixed-valence polyelectrolytes 6 and
7 are insoluble in water, whereas 8 is water-soluble. The three
polymers were characterized by UV/Vis, FTIR, near-IR, and
Mçssbauer spectroscopy. The FTIR spectrum of 6 inconven-
À
iently contains a band of PF6 at 841 cmÀ1, but both the
À
ferrocenyl and the ferricinium groups of the BF4 polymeric
salts 7 and 8 are detected at 818 cmÀ1 (nFc) (compare to
819 cmÀ1 in 4) and 832 cmÀ1 (nFc*), respectively (SI), indicating
a localized mixed valency even at high IR frequency (1013 sÀ1),
which was already observed for the parent biferrocenium.
Further proof of this class II mixed valency is the presence of
absorption bands in the near-infrared range corresponding to
a transition from the ground state to the intervalence charge-
transfer (IVCT) state (SI). The recorded Mçssbauer spectrum
of 6 (Figure 2) at zero field and 78 K further confirms the
localized class II mixed valency with the presence of both the
ferrocenyl and the ferricinium groups, which was expected
since this spectroscopic method covers lower frequencies
(107 sÀ1) than IR. Finally in the UV/Vis spectra the lmax of 6–8
(ca. 600 nm) was strongly shifted relative to the lmax of 4 and 5
(ca. 456 nm) (SI).
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Table 1: Results of the Suzuki–Miyaura reaction of bromoarenes in the
presence of Pd NPs stabilized by 4 and 5.
Bromoarene Conditions[a] Conv. [%][b] Yield [%][c] TON [hÀ1
]
TOF [hÀ1
]
R=
H
CH3
A (PdNP-4) 100
A (PdNP-4) 100
A (PdNP-4) 98
A (PdNP-4) 100
A (PdNP-4) 100
A (PdNP-4) 100
B (PdNP-5) 100
B (PdNP-5) 100
B (PdNP-5) 100
B (PdNP-5) 100
B (PdNP-5) 100
B (PdNP-5) 100
99
97
97
99
99
99
90
80
78
91
91
99
396
388
388
396
396
396
45000
40000
39000
45500
45500
49500
24.8
24.3
24.3
24.8
24.8
24.8
1875
1667
1625
1896
1896
2062
OCH3
NO2
CHO
COCH3
H
CH3
OCH3
NO2
CHO
COCH3
[a] Conditions A: the reactions were carried out with 1 mmol of bromoar-
ene, 1.5 mmol of phenylboronic acid, 2 mmol of K3PO4, and 0.25 mol% of
Pd NPs stabilized by 4 in 3 mL of CHCl3/MeOH (2:1) for 16 h at 908C.
Conditions B: the reactions were carried out with 1 mmol of bromoarene,
1.5 mmol of phenylboronic acid, 2 mmol of K3PO4, and 0.002 mol% of Pd
NPs stabilized by 5 in 6 mL of EtOH/H2O (1:1) for 24 h at 808C. [b] The
conversion was determined by 1H NMR analysis. [c] Yield of isolated
product.
Figure 4. a) TEM image of Au NPs synthesized by reduction of AuIII
with 5. b) Size distribution of the Au NPs present in the TEM picture
(average size of 98 Au NPs: (12Æ2) nm).
(Figure 4). Scheme 2 shows the reduction and stabilization
of both 5-PdNPs and 5-AuNPs. The Au NPs (0.6–1.2 mol%)
were used to catalyze the reduction of 4-nitrophenol to 4-
aminophenol by an excess of NaBH4; 4-aminophenol is
a potential industrial intermediate in the production of many
analgesic and antipyretic drugs, anticorrosion lubricants, and
hair dyeing agents.[14] When 1.2 mol% of Au NPs is used, the
reaction is complete within 350 seconds, which corresponds to
a kapp value of 6.0 ꢀ 10À3 sÀ1. The reaction rate constant kapp is
calculated using the rate equation Àln(Ct/C0) = kapp t (the
catalyst exhibits pseudo-first-order kinetics; SI).
In conclusion, the two copolymers 4 and 5 synthesized by
“click” polycondensation show multifunctional properties as
polyelectrolytes and electrochromes, redox sensors for tran-
sition-metal cations, electrode modifiers, and excellent tem-
plates for the synthesis of efficient Pd and Au nanoparticle
catalysts. The biferrocene unit provides a more stable
ferricenium form than the parent ferricenium, which makes
the polymers ideal as class II mixed-valence polyelectrolytes
and electrochromes. The proximity of the triazole ring to the
biFc units facilitates redox sensing as well as the under-
standing of the stoichiometry of metal cation complexation.
The stabilization and solubilization of Pd NPs and Au NPs by
easily available polymers yield extremely efficient catalysts
for current organic reactions.
Figure 3. a) TEM image of Pd NPs stabilized by 5. b) Size distribution
of the Pd NPs present in the TEM picture (average size of 103 Pd
NPs: (2.3Æ0.6) nm).
Polymer 5 is oxidized to the mixed-valence water-soluble
polyelectrolyte 9 when 1 equiv HAuCl4 is added per 3 equiv
biferrocene unit according to the redox stoichiometry,
thereby forming Au0 atoms that give polymer-stabilized Au
NPs. Due to the plasmon absorption, which is detected by
UV/Vis spectroscopy at 534 nm (SI) the Au NPs solution
presents a purple coloration. TEM analysis shows that these
Au NPs have an average size of around (12 Æ 2) nm
Received: March 6, 2014
Published online: && &&, &&&&
Keywords: biferrocenes · click chemistry ·
.
heterogeneous catalysis · nanoparticles · polymers
4
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Scheme 2. Synthesis of Pd NPs (left) and Au NPs (right).
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!

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Angewandte
Communications
Communications
Biferrocene Polymers
Very versatile: New metallopolymers were
designed and prepared by “click” poly-
condensation between bis(ethynyl)-
biferrocene and poly(ethylene glycol)
(PEG400 and PEG1000) fragments. These
can be used for multiple functions and
uses including the improvement of water
solubility and biocompatibility, the stabi-
lization of Pd and Au nanoparticles, in
catalysis, electrode derivatization, and
sensing, and as a polyelectrochrome and
polyelectrolyte.
C. Deraedt, A. Rapakousiou, Y. Wang,
L. Salmon, M. Bousquet,
D. Astruc*
&&&& — &&&&
Multifunctional Redox Polymers:
Electrochrome, Polyelectrolyte, Sensor,
Electrode Modifier, Nanoparticle
Stabilizer, and Catalyst Template
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!