Electron Flow in Large Metallomacromolecules and Electronic Switching of Nanoparticle Stabilization: Click Ferrocenyl Dentromers that Reduce AuIII to Au Nanoparticles

Article abstract of DOI:10.1002/chem.201802289

Click ferrocenyl-terminal dentromers, a family of arene-cored dendrimers with triple branching (9-Fc, 27-Fc, 81-Fc, and 243-Fc), reduce Au^III to ferricinium dentromer-stabilized Au nanoparticles (AuNPs). Cyclic voltammetry studies in CH₂

Full text of DOI:10.1002/chem.201802289

A Journal of
Accepted Article
Title: Electron flow in large metallomacromolecules and electronic
switch of nanoparticle stabilization: new click ferrocenyl
dentromers that reduce Au(III) to Au nanoparticles
Authors: Didier Astruc, Qi Wang, Fangyu Fu, Angel M Martinez-
Villacorta, Sergio Moya, Lionel Salmon, Jaime Ruiz, Julien
Hunel, and Amélie Vax
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Chem. Eur. J. 10.1002/chem.201802289
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Electron flow in large metallomacromolecules and electronic
switch of nanoparticle stabilization: new click ferrocenyl
dentromers that reduce Au(III) to Au nanoparticles
[
a]
[a]
[b]
[b]
[c]
[d]
Qi Wang, Fangyu Fu, Angel M. Martinez-Villacorta, Sergio Moya, Lionel Salmon, Amélie Vax,
[
a]
[a]
[a]
Julien Hunel, Jaime Ruiz, and Didier Astruc*
Dedication ((optional))
Abstract: Click ferrocenyl-terminal dentromers, a family of arene-
cored dendrimers with triple branching (9-Fc, 27-Fc, 81-Fc and 243-
Fc) reduce Au(III) to ferricinium dentromer-stabilized Au
to both their reversible cyclic voltammetry and the useful redox
[
6]
and functional chemistry of ferrocene that have applications as
[
7]
[8]
[9]
catalysts,
nanoreactors,
sensors,
[11]
electrochemical
[
10]
[12]
nanoparticles (AuNPs). Cyclic voltammetry studies in CH
2
Cl
2
show
mediators, molecular electrodes and biomedical vectors.
In spite of many electrochemical studies, however, ferrocenyl-
terminated dendrimers have not been used so far as redox
agents for chemical transformations. Here we introduce the first
use of ferrocenyl-terminated dendrimers as electron reservoirs
to reduce Au(III) to Au nanoparticles that become stabilized by
ferricinium chloride dendrimers. Therefore we have synthesized
a new third-generation ferrocenyl (Fc)-terminated dendrimer
reversible CV waves with some adsorption for the 243-Fc dentromer
and a number of redox groups found, 255 ± 25, using the Bard-
Anson method, close to the theoretical number of 243. The
dentromers reduce aqueous HAuCl
4
to water-soluble ferricinium
chloride dentromer-stabilized gold nanoparticles (AuNPs) with core
sizes between 30 and 47 nm. These triazolylferricinium dentromer-
stabilized AuNPs are reduced by cobaltocene to cobalticinium
chloride and ferrocene dentromer-weakly stabilized AuNPs together
with red shift of the AuNP plasmon. The weakness of the AuNP
containing
a
theoretical number of 243 terminal 1,2,3-
triazolylferrocenyl tethers (243-Fc) using Cu-catalyzed azide
[
13]
stabilization is characterized by dentromer extraction with CH
2
Cl
2
alkyne cycloaddition (CuAAC-type “click” chemistry as well as
along with irreversible AuNP agglomeration for the 9, 27 and 81-
ferrocenyl dentromer, only the 243-ferrocenyl dentromer-AuNP
withstanding this process. Altogether this demonstrates the
electronic switch of the dentromer-mediated AuNP stabilization.
other known smaller Fc-terminated dendrimers of zeroth- (9-Fc),
[
14]
first- (27-Fc) and second generation (81-Fc).
In the present article the synthetic redox chemistry of Fc-
terminated redox dendrimers and dendrimer-stabilized large Au
nanoparticles is presented. Indeed the reaction of Au(III) with the
Fc-triazolyl-terminated dendrimer produces large water-soluble
AuNPs in a size range useful in biomedical applications. In order
to rapidly produce a large number of ferrocenyl groups at the
dendrimer periphery we are using mesitylene as a starting arene
Introduction
Electron flow mechanisms in nanoscale materials has recently
+
core. The CpFe -induced nona-allylation of mesitylene is known
[
1]
been a subject of scrutiny in physics, chemistry and biology.
Metallomacromolecules containing precisely-localized redox
to burst the core methyl groups into nine branches. Further triple
branching, involving Williamson and hydrosilylation reactions,
[
2]
relay centers are especially appropriate for such investigations.
m
provides 3 branches, n = m-2 being the dendrimer generation
Ferrocenyl-terminated dendrimers are among the most precise
metal-containing polymers and have attracted the attention of
number n in Gn. Whereas most dendrimers such as the PAMAM
[3e,15]
dendrimers
involve a construction based on double
organometallic,
electrochemistry
and
macromolecular
[16]
[17]
branching,
the triple branching
starting from the 1,3,5
[
3-5]
communities for more than two decades.
This interest is due
tribranched arene core presented here provides super
dendrimers that have been called “dentromer” [in Greek dentro
[
18]
(
δεντρο) = tree, and meros (µεροσ) = part of] a term that will
[a]
Q. Wang, F. Fu, Dr J. Hunel, Dr J. Ruiz, Prof. D. Astruc
ISM, UMR CNRS No. 5255
be further used in this article.
Univ. Bordeaux
33405 Talence Cedex, France
E-mail: didier.astruc@u-bordeaux.fr
A. M. Martinez-Villacorta, Dr S. Moya
Soft Matter Nanotechnology Lab
CIC biomaGUNE
Paseo Miramon 182, 20014, Donostia-San Sebastian, Spain
Dr L. Salmon
[b]
[c]
[d]
Results and Discussion
Synthesis and characterizations of the ferrocenyl
dentromers.
Laboratoire de Chimie de Coordination
UPR CNRS 8241
+
The known 9-allyl core was synthesized using the CpFe
induced nona-allylation of mesitylene followed by visible-light
31077 Toulouse Cedex, France
[
19]
photolysis.
Hydrosilylation reaction of this 9-allyl core with
Dr A. Vax
LCPO UMR 5629
chloromethyldimethylsilane catalyzed by the Karstedt catalyst in
16 avenue Pey Berland, 33600 Pessac, France
diethyl ether gives the 9-Cl intermediate, which upon reaction
Supporting information for this article is given via a link at the end of
the document.
3 3
with NaN provides 9-N that reacts with ethynylferrocene by
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click chemistry, generating the G0-triazolylferrocenyl dentromer
[
14]
9
-Fc (Figure 1 and Scheme 1)
The construction of the G0, G1, G2 and G3
chloromethyl(dimethyl)silyl dentromers is carried out by iteration
using Williamson reaction with the known triallyl phenol
[
19]
dendron,
then hydrosilylation of the resulted allyl-terminated
[
20]
dentromer with chloromethyldimethylsilane.
The terminal
chloro groups are then substituted by azido groups using sodium
azide providing azido-terminated dentromers that react further
with ethynylferrocene, yielding the known G1 (27-Fc) and G2
(
81-Fc) dentromers with 27 and 81 1,2,3-triazolylferrocenyl
termini, respectively (Figure and Scheme 1). The new
1
dentromer 243-Fc has now been synthesized using the same
iteration of reactions according to Scheme 1. These ferrocenyl
1
13
dentromers are characterized by H and C NMR; see the
supporting information (SI), by their electrochemistry and
electron-transfer chemistry (vide infra) and Size Exclusion
Chromatography, SEC with PDI = 1.03 for 243-Fc (Figure S10).
Scheme 1. Synthesis of the Fc-dentromers 9-Fc, 27-Fc, 81-Fc and 243-Fc.
Figure 1. Planar representation of the structures of the 1,2,3-
triazolylferrocenyl dentromers 9-Fc, 27-Fc, 81-Fc and 243-Fc.
AFM studies.
Atomic force microscopy (AFM) studies of 243-Fc have been
conducted on a solid-state sample using glass surfaces that
have a negative surface charge density for clean substrate
imaging. A diluted THF solution of the dentromer is deposited on
a glass surface by spin coating (800-1000 rpm for one minute,
0
.1% w/w) followed by evaporation. The height of the dentromer
layer that is quite uniform represents a monolayer of single-
dentromer height of 10 nm for the 243-Fc dentromer, whereas a
width of 80 nm shows the formation of various dentromer
aggregates on the glass surface (Figure 2).
Figure 2. AFM images of 243-Fc on glass support shown in (a) two
dimensions and (b) three dimensions; (c) The diameters are 80 nm on
average, and the height is 10 nm. AFM was operated in the tapping mode with
a resonance frequency of around 200 MHz. The tip has a radius of 10 nm.
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Cyclic voltammetry studies.
The 243-Fc dentromer was further studied by cyclic
[
21]
voltammetry
internal reference.
recorded in CH Cl
2
using decamethylferrocene (FeCp* ) as the
[
22]
The cyclic voltammograms (CVs) are
2
. This 1,2,3-triazolylferrocenyl dentromer
2
shows, with some adsorption (i
and electrochemically reversible CV wave (∆E
0.555 V vs. decamethylferrocene in CH
apparently equivalent ferrocenyl groups, which is the same E1/2
c
/i
a
= 1.7), a single, chemically
= 60 mV) at E1/2
Cl for all the
p
=
2
2
[
14]
value as that of the 9-Fc, 27-Fc and 81-Fc dentromers. All the
ferrocenyl redox potentials of the various redox groups appear
equivalent, although a tiny difference among all of them is due to
the extremely small electrostatic factor that distinguishes all
these redox potentials. The distinction between these various
redox potentials is so small that it is not apparent, and therefore
[
21,23]
only a single CV wave is observed.
reversibility reflects fast electron transfers between the electrode
The electrochemical
[
21]
and the dentromer within the electrochemical time scale. This
is taken into account by fast rotation of the dentromer bringing
all the redox centers in turn near the electrode within the
electrochemical time scale and/or fast electron hopping among
[
24]
the redox sites (Figure 3a).
Determination of the number of electrons included in this CV
[
23]
wave using the Bard-Anson equation
with decamethyl-
ferrocene as the internal reference yields a value of 255 ± 25
electrons for 243-Fc, thus in good agreement with the theoretical
number. The experimental value observed in slight excess is not
due to adsorption that seems to selectively affect the cathodic
wave, since the calculation of the number of redox centers was
carried out using the anodic wave intensity that is not marred by
Figure 3. CVs of the 243-Fc dentromer in CH
Bu N][PF ]: (a) CV of 243-Fc (4 mg) in CH Cl : reversible wave at E1/2 = 0.555
V [vs. FeCp* as an internal reference (Cp* = η -C
b) modified Pt electrode at various scan rates in CH
2
Cl
2
containing 0.1 M [n-
4
6
2
2
5
-1
2
5
Me
5
), scan rate: 0.2 V·s ];
Cl and (c) plot of the
(
2
2
wave intensity as a function of scan rate (linearity showing the expected
behavior of an adsorbed dentromer).
1
13
the adsorption (vide supra). The H and C NMR spectra are
consistent along the dentromer construction; thus it is assumed
that the number of lacking ferrocenyl redox centers, if any, is
minimal.
FeCp*
2
) by 0.015 V towards anodic values compared to that of
), thus a ferricinium salt would not
ferrocene (0.45 V vs. FeCp*
2
be the best to oxidize the ferrocenyl dentromers.
Acetylferricinium hexafluorophosphate, a classic oxidant, was
Adsorption on the platinum electrode is probed by scanning
around the oxidation potentials of the peripheral ferrocenyl
groups approximately 20 cycles and therefore modified the Pt
electrode. The 243-Fc dentromer-modified electrode was
+
/0
selected, because the oxidation potential of FcCOMe is 0.80 V
vs. FeCp* , which would in principle insure exergonic, electron
2
transfers by 0.245 V, thus favorable. Acetylferricinium
hexafluorophosphate was synthesized according the Geiger’s
method and used to oxidize the 9-Fc and 243-Fc dentromers in
[
25]
produced in this way.
The electrochemical behavior of this
Cl containing only the supporting
modified electrode in CH
2
2
electrolyte (Figure 3b,c) gave rise to a well-defined, symmetrical
redox wave that was characteristic of a surface-confined redox
couple, with the expected linear relationship of peak current with
various scan rates: 50, 100, 200, 300 mV/s. The stability of the
modified electrode towards electrochemical recycling was
demonstrated by the fact that repeated scans did not change the
shape and intensity of the CVs. By comparison, the quality of the
modified electrodes increases with the increase of generation in
the series: 9-Fc < 27-Fc < 81-Fc < 243-Fc (Figures S11-13 and
Table S2). See the surface coverage and the full width data at
half maximum for the surface wave of all the modified electrodes
with the different Fc dentromers in the SI (Table S2).
2 2
CH Cl at r.t. under nitrogen (see the experimental section). This
finally resulted in the production of insoluble dark-blue solids,
this color being characteristic of ferricinium, and the compounds
were characterized by EPR and Mössbauer spectroscopies.
Mössbauer spectroscopy.
This technique is not very sensitive to the ferrocene substituents,
but on the other hand it is very sensitive to the oxidation state,
-
1
giving a QS value around 2.4 mm·s for the doublet of ferrocene
-
1
derivatives and a value near QS = 0 mm·s (singlet or very
[
26]
narrow doublet) for ferricinium compounds.
This distinction
allows to easily recognize ferrocene-type and ferricinium-type
components in the spectrum. The zero-field Mössbauer
Oxidation of the ferrocenyl dentromers by acetylferricinium
hexafluorophosphate.
6
spectrum of dentromer 9-FcPF was recorded at 80K for a
sample obtained according to the procedure described in the
experimental section, leaving a dark-blue powder. The
The weakly electron-withdrawing 1,2,3-triazolyl group shifts the
redox potential of the ferrocenyl dentromers (0.555 V vs.
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+
-
6
Figure 4. Zero-field Mössbauer spectrum of the dentromer 9-Fc PF at 80K.
main compound is represented by a doublet for 9-FcPF
6
(IS =
.564 mm·s ; QS = 0.413 mm·s ). Although this major complex
in the spectrum is the awaited 9-ferricinium PF dentromer (80 ±
%), the presence of another minor component of the
Figure 5. EPR spectra at 80K of dentromer as powders (a) 9-FcPF
243-FcPF6.
6
and (b)
-
1
-1
0
6
5
5
+
-
ferrocenyl-type (20 ± 3%) indicates that oxidation was not
complete. In another similar reaction, a slight excess (5% vs.
stoichiometry) of acetylferricinium hexafluorophosphate was
used, and an analogous Mössbauer results was obtained with
6
17-electron d ferricinium at g = 2.09 for 9-Fc PF and g = 2.065
+
- [28]
for 243-Fc PF .
6
A signal at g = 4.245 is also observed
corresponding to high-spin Fe(III) that probably reflects some
decomposition, but this signal is not observed for 9-FcPF which
6
might signify a higher instability in several days at r.t. for 243-
FcPF compared to 9-FcPF .
(
15 ± 5%) of ferrocenyl-type signal. These % values are
approximate, because absorption energy is not necessarily
exactly the same for both oxidation states even at low
temperature. This lack of complete oxidation is possibly due to
insufficient driving force from the acetyl ferricinium oxidant
resulting in the formation of random mixed-valent Fe(II)/Fe(III)
dentromers with a small amount of Fe(II). Indeed the driving
force between the acetylferricinium oxidant and the dentromer
ferrocenyl groups, although exergonic, is only about 0.245 V.
Thus electron-transfer sometimes occurs eventually with long
distance, for instance if ferrocenyl groups back fold toward
the dentromer center (but not only), which becomes difficult in
the case of weak driving force. It is also possible that the
electrostatic factor, that renders electron transfer more difficult
along the increase of the positive charges in the dentromer,
inhibits electron transfer with the last ferrocenyl branches. A
similar phenomenon has been observed in the oxidation of
redox groups in dendrimers by the Balzani and Stoddart
6
6
Oxidation of the click ferrocenyl dentromers by Au(III) to
click ferricinium dentromer-stabilized gold nanoparticles
(AuNPs)
The reaction of the dentromer 9-Fc with HAuCl (stoichiometry:
4
1 equiv. Fc/1 equiv. HAuCl ) occurs within 5 min at r.t. upon
4
stirring in a homogeneous THF/H O solution (1/19 ml). A color
2
change from light yellow to red-purple is observed, the reaction
time becoming longer upon increasing the dentromer generation
[
29,30]
number. Finally a red-purple AuNP
solution is obtained for
each dentromer generation (Figure S15). These red-purple
solutions are characterized by UV-vis. spectroscopy and
transmission electron microscopy (TEM). The UV/vis. spectra
show the characteristic surface plasmon band (SPB) of Fc
dentromer-stabilized AuNP/Fc ranging from 536 to 549 nm, the
SPB being progressively red-shifted as the generation number
increases (Figure 6). The AuNP core sizes of these AuNPs
determined by TEM are between 30 and 47 nm (Figures S16-
19). The dynamic light scattering measurement for 81-Fc has
[
27]
groups.
Electron Paramagnetic Resonance (EPR).
The dentromers 9-FcPF and 243-FcPF
similarly by oxidation of the neutral ferrocenyl dentromers by
[
8b]
6
6
are synthesized
been shown to reflect a diameter size of 9 nm, thus the AuNP
cores appear to be larger than the dentromers. This means that
the AuNPs are stabilized around their periphery by several
dentromers.
+
-
FcCOMe PF
6 2 2
in CH Cl at r.t. (see experimental section) for
electron paramagnetic resonance (EPR) measurements as
powders at 80K. The EPR spectra show a low spin Fe(III) signal
that is classic for the orthorhombic distortion characteristic of the
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+
-
Figure 6. (a) UV-vis. absorption and (b) color of AuNPs stabilized by Fc Cl -
dentromers.
Reactions of the ferricinium chloride dentromer-stabilized
AuNPs with cobaltocene.
In order to further characterize this macromolecular system we
[
31,32]
conducted reaction under nitrogen between cobaltocene
in
THF and the purple THF/water (1/19 ml) solution of all the
ferricinium dentromer-stabilized AuNPs (Scheme 2). The neutral
1
9-electron late transition-metal sandwich complexes of the first
raw of the transition metals are known as strongly reducing
[
32,33]
electron-reservoir complexes.
shown on the UV-vis. spectra and on the pictures of the reaction
media including after extraction with CH Cl (Figures 7 and S20-
3). The addition of cobaltocene to the 9, 27 and 81-ferricinium
This reaction process is
2
2
2
dentromer-stabilized AuNPs provokes the immediate formation
of a purple precipitate and a slightly purple solution showing a
much weaker SPB than before this reaction (Figures S21-23).
+
-
For 9-Fc Cl -AuNP, this SPB is significantly red-shifted from 546
nm to 559 nm and is observed together with the band of
cobalticinium at 400 nm (Figures S21, red curve b). Upon
+
-
extraction with CH
2
Cl
2
the purple precipitate and purple color of
Scheme 2. Synthesis of AuNP-243-Fc Cl in THF/water and its reaction with
cobaltocene. On the other hand for the other dentromer-stabilized AuNPs (9-
the solution disappear, which means decomposition of the
AuNPs that also appear as a brown insoluble material at the
+
-
+
-
+
-
2 2
Fc Cl , 27-Fc Cl and 81-Fc Cl ) extraction of b with CH Cl (last step) leads
to AuNP agglomeration and reformation of AuNP-free Fc dentromer (see SI).
2 2
interphase. Indeed the CH Cl solution shows the ferrocenyl
band at 430 nm in the UV-vis. spectrum (blue curve d), and its
1
H NMR spectrum confirms the presence of only the ferrocene
The situation is very different in the case of the large 243-
ferricinium dentromer-stabilized AuNP. Following the reaction of
dentromer (Figure S20). After this extraction, the aqueous
solution still contains cobalticinium chloride as shown by its
cobalticinium band at 398 nm in the UV-vis. spectrum (green
curve c). This means that after the addition of cobaltocene to the
+
-
AuNP-243-Fc Cl with cobaltocene, the clear homogeneous
solution changes to a suspension that floculates to a red-purple
precipitate. Upon shaking a few seconds it becomes a turbid
red-purple solution for which the AuNP plasmon band in the UV-
vis. spectrum is observed at 548 nm, although it is less intense
than before cobaltocene addition (Figure 7, red curve b). The
cobalticinium band is observed therein around 400 nm. Then
9
-ferricinium dentromer-stabilized AuNPs, the precipitate shows
the insolubility of the 9-ferrocenyl-dentromer-stabilized AuNPs,
the ferrocenyl groups being hydrophobic contrary to the
2 2
ferricinium groups. The facile extraction using CH Cl also
shows the weakness of the stabilization of the AuNPs by the 9-
Fc dentromer contrasting with the robustness of the stabilization
of these AuNPs by the 9-ferricinium dentromer, the latter
benefiting from the electrostatic interaction. This series of
after addition of CH
2
Cl
2
the cobaltocenium band is also
observed at 402 nm in the UV-vis. spectrum of the aqueous
phase (green curve c). The 243-Fc dentromer is not extractable
2 2
with CH Cl from the AuNP with which it possibly forms a
4
experiments shows that upon reaction with HAuCl , the 9-Fc
composite film because of its bulk (Scheme 2 and Figure 7)
contrasting with the lower dentromer generations.
dentromer is oxidized to 9-ferricininium dentromer that
electrostatically stabilizes AuNPs in THF/water, and that
cobaltocene reduces the 9-ferricinium dentromer back to the 9-
Fc dentromer.
The multi-electron transfer reactions are taken into account by
the favorable driving force of the electron transfer from the 1,2,3-
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0
triazolylferrocenyl groups (E (Fe(II)/Fe(III) = 0.55 V) to Au(III)
which dendrimers are known to largely contribute due to the
[
34]
(
E° Au(III)/Au(0) = 0.93 V) for the formation of ferricinium
encapsulation properties.
Reaction of the ferricinium
0
+/0
dentromer-stabilized AuNPs, then from CoCp
2
(E (CoCp
2
) =
dentromer-stabilized AuNPs with cobaltocene provides a large
driving force of about 1.5 V for reduction of the ferricinium
groups back to ferrocenyl groups. This reaction considerably
weakens the AuNP stabilization by cancellation of the
electrostatic effect, as shown by the dentromer extraction with
-
0.9 V) to the 1,2,3-triazolylferricinium dentromer-stabilized
AuNPs. The formation of the AuNPs is essentially driven by the
enormous electrostatic effect stabilizing the AuNPs. This strong
effect is cancelled upon reaction with cobaltocene back re-
forming the neutral ferrocenyl dentromers. In summary the
AuNP electrostatic stabilization is switched upon redox change.
CH
2 2
Cl that destroys the AuNP ensemble. The AuNP are partly
stabilized by the Cl ligands provided by the precursor HAuCl
4
during their redox synthesis, but these experiments show that
this stabilization is not sufficient, and that the electrostatic
stabilization provided by the ferricinium-terminated dentromers is
so efficient that these AuNPs are stable for months. On the other
hand much AuNP destabilization occurs upon returning to the
neutral Fc dentromers. The dentromer generation influences this
destabilization that is achieved with the 9-, 27- and 81-Fc-
AuNPs, whereas the 243-Fc dentromer withstands extraction
and AuNP agglomeration. Multi-electron transfers from such
ferrocenyl dentromers and their electronic consequences
represent an impressive effect of the switch of the electrostatic
factor; they are also reminiscent of multiple single-electron
transfer processes in other nanomaterials such as fullerenes
+
-
+
-
Figure 7. (a) UV-vis. absorption of (a) AuNP-243-Fc Cl , (b) AuNP-243-Fc Cl
cobaltocene giving AuNP-243-Fc and cobalticinium chloride, (c) H O phase
after extraction of the solution b with CH Cl . Note that both solutions a and b
+
2
[
35]
and nanoclusters.
2
2
show the purple color of the AuNPs with some variation illustrated by the SPB
red shift of b).
Experimental Section
Concluding remarks
General Data. For general data including solvents, apparatuses,
compounds, reactions, spectroscopies, and CV, see the SI. Gn indicates
the generation number n.
A family of four Fc-terminated click metallodentromers, 9-Fc, 27-
Fc, 81-Fc and 243-Fc was synthesized with a specific type of
+
Synthesis of dentromer 243-Fc: The syntheses of the different
generations of Fc-dentromers are gathered in the SI. The 243-Fc
dentromers whose construction starts with the CpFe -induced
nona-allylation of mesitylene followed by successive triple
branching at each generation. These syntheses include the new
-
3
dentromer is synthesized from 243-N
The 243-N dentromer and ethynyl ferrocene (1.5 equiv. per branch) are
dissolved in tetrahydrofuran (THF), then water is added (1:1 THF/water).
CuSO is added (1.2 equiv. per branch, 1M aqueous solution) to this
3
(0.3 g, 4.7×10 mmol, see SI.).
3
2
43-Fc for which the AFM studies show a dentromer height of
1
0 nm and a width of 80 nm due to aggregation, whereas cyclic
4
voltammetry studies show full chemical and electrochemical
reversibility despite its large size. The modified Pt electrodes
built with the ferrocenyl dentromers are all the more stable as
the dentromer generation increases. Using the Bard-Anson
equation the number of ferrocenyl redox centers was found
experimentally to be close (255) to the theoretical one (243).
reaction mixture, followed by dropwise addition of a freshly prepared
solution of sodium ascorbate (2.4 equiv. per branch, 1M aqueous
solution). The reaction mixture is stirred at r.t. for 16 h. After removing
THF in vacuo, dichloromethane and an aqueous ammonia solution are
added. The mixture is allowed to stir for 1 h in order to remove all Cu(I)
trapped inside the dentromer. The organic phase is washed twice with
water, dried with sodium sulfate, filtered over paper, and the solvent is
removed in vacuo. The product is precipitated using dichloromethane/
pentane. The product 243-Fc is obtained as an orange-red powder in
n
The synthesis of this series of four dentromers with 3 ferrocenyl
termini (n = m + 2, m being the generation number) allows to
evaluate the variation of the dentromer properties from
generation to generation in particular with respect to collective
electron transfers to a strong oxidant such as Au(III). With a
weaker oxidant such as an acetylferricinium salt, however, the
oxidation reactions appear incomplete as shown by zero-field
Mössbauer spectroscopy due to the weaker driving force of the
reaction producing random mixed valent dentromer systems.
The stabilization of AuNPs by ferricinium-terminated dentromers
is achieved for all the dentromer generations, but the reaction is
all the slower as the generation number increases. This results
in the formation of large AuNP between 30 nm and 47 nm with
progressive red shift of the SPB as the generation number
increases. This range of sizes is that used in nanomedicine for
-3
1
7
3
7.9% yield (0.42 g, 3.7×10 mmol). H NMR (300 MHz, CDCl ): δ 7.31
(s, 243H), 7.15 (s, 234H), 6.89 (s, 234H), 4.73 (s, 486H), 4.27 (s, 486H),
4.07 (s, 1215H), 3.85 (s, 486H), 3.53 (s, 234H), 1.62 (s, 720H), 1.10 (s,
1
3
7
1
6
20H), 0.61 (s, 720H), 0.10 (s, 2160H); C NMR (76 MHz, CDCl
3
): δ
59.13, 146.16, 138.71, 127.10, 119.68, 113.49, 75.73, 69.55, 68.56,
2
9
6.54, 60.29, 43.01, 41.86, 40.80, 17.45, 14.84, -3.80, -4.45; Si NMR
, 59.6 MHz): δ 2.90 (SiCH N), 0.37 ppm (SiCH O).
(
CDCl
3
2
2
+
-
Procedure for the preparation of 243-Fc PF
6
: The 243-Fc dentromer,
+
-
3 6
dry dichloromethane and CH COFc PF
(1 equiv. per branch) are
successively added into a Schlenk flask under nitrogen, then the solution
is stirred for 16 h at r.t. Dichloromethane is removed in vacuo, then
diethyl ether is added (3 x 10 mL) to wash the product 3 times, then
dichloromethane is added (3 x 10 mL) to further wash the product 3 times
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Chemistry - A European Journal
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+
-
due to the poor solubility of the dentromer Fc PF
6
. The product remains
B. Delavaux-Nicot, Dendrimers Towards Catalytic, Material and
Biomedical Uses, Chichester, UK, 2011; e) D. A. Tomalia, J. B.
Christensen, U. Boas, Dendrimers, Dendrons and Dendritic Polymers,
Cambridge University Press, Cambridge, U. K. 2012; f) S. Campagna,
P. Ceroni, F. Puntoriero, Eds. Designing Dendrimers. Wiley, Hoboken,
NJ, 2012.
in the Schlenk flask as an insoluble dark-blue powder.
General procedure for the oxidation of the Fc-dentromers by Au(III)
+
-
yielding Fc Cl -stabilized AuNPs: 18 mL of water is added into a
Schlenk flask, and a solution of HAuCl ·3H O (1mg in 1 mL H O) is
4
2
2
[
[
4]
5]
For a comprehensive review of the design and functions of dendrimers,
see: D. Astruc, E. Boisselier, C. Ornelas, Chem. Rev. 2010, 110, 1857-
added under stirring; then a solution of the Fc-dentromer (3 equiv. Fc
branch per Au) dissolved in 1 mL THF is injected into the flask, and the
solution is allowed to stir overnight at r.t. These AuNP solutions are
further characterized by TEM (see the SI), the SPB recorded in the UV-
vis. spectrum and their reaction with cobaltocene.
1
959.
For reviews on ferrocenyl-terminated dendrimers, see: a) C. M. Casado,
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pp. 219-262.
+
-
General procedure for the reaction of Fc Cl -stabilized AuNPs with
cobaltocene: After the preparation of AuNPs stabilized by the
ferricinium-terminated dentromers, the AuNP solution is degassed for 15
[
[
6]
7]
For a recent review on ferrocene, see: D. Astruc, Eur. J. Inorg. Chem.
min, and the UV-vis. spectrum is recorded under N
cobaltocene (1 equiv. per Fc) in THF (1 ml) is added to the AuNP
solution under N , and the new UV-vis. spectrum is recorded after stirring
for 3 min. Then CH Cl is added to this mixture leading to the extraction
2
. Freshly prepared
2
017, 1, 6-29.
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2
2
2
the Fc-dentromer in the organic phase (in the cases of the 9-Fc, 21-Fc
and 27-Fc dentromers, and the destabilized AuNPs aggregate), which is
1
further characterized by H NMR, whereas cobalticinium chloride is
characterized in the aqueous phase c by its UV-vis. spectrum (in all
cases). In the case of the 243-Fc-dentromer-stabilized AuNPs (b) in
Scheme 2, no decomposition is observed.
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Qi Wang, Fangyu Fu, Angel
[
b]
M. Martinez-Villacorta, Sergio
[
b]
[c]
Moya, Lionel Salmon, Amélie
Large electron flows
using redox reagents
switch Au nanoparticles
stabilization by
[d]
[a]
VAX, Julien Hunel, Jaime
[a]
[a]
Ruiz, and Didier Astruc*
Page No. – Page No.
dentromers between
strong and weak
Electron flow in large
depending on the
metallomacromolecules and
electronic switch of
electrostatic factor and
dentromer generation
nanoparticle stabilization: new
click ferrocenyl terminated
dentromers that reduce Au(III)
to charge-dependent
dentromer-stabilized Au
nanoparticles
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