Click syntheses of 1,2,3-triazolylbiferrocenyl dendrimers and the selective roles of the inner and outer ferrocenyl groups in the redox recognition of ATP2- and Pd2

Article abstract of DOI:10.1002/anie.201004756

This way and that: Triazolylbiferrocenyl dendrimers can be synthesized with up to 729 termini. Oxidation results in isolable mixed-valence dendrimers (see picture). In electrochemical studies, these species can recognize ATP ^2- and Pd^II, and they can be used to stabilize Pd nanoparticle catalysts.

Full text of DOI:10.1002/anie.201004756

Communications
DOI: 10.1002/anie.201004756
Dendrimers
Click Syntheses of 1,2,3-Triazolylbiferrocenyl Dendrimers and the
Selective Roles of the Inner and Outer Ferrocenyl Groups in the Redox
Recognition of ATP2ꢀ and Pd2+**
Rodrigue Djeda, Amalia Rapakousiou, Liyuan Liang, Nicola Guidolin, Jaime Ruiz, and
Didier Astruc*
Ferrocenyl (Fc) dendrimers[1] and polymers[2] have attracted
much attention owing to their multielectron redox properties
and functions as biosensors,[3] virus-like receptors,[4a] models
of molecular batteries,[4b] and colorimetric sensors.[2] Fc-
terminated dendrimers belong to the large family of redox-
active metallodendrimers that may eventually mimic related
nanometer-sized biological processes and provide useful
energy-relevant devices.[5]
In typical examples, Nishihara and co-workers have
extensively studied the efficient electrodeposition of acylbiFc
nanodevices in gold nanoparticles,[6] and recently, the group of
Reinhoudt and Ravoo elegantly demonstrated that small
acylbiFc dendrimers form inclusion complexes with self-
assembled monolayered b-cyclodextrin as “molecular print-
boards”.[7] Biferrocene itself can be incorporated without
covalent bonding as a biferrocenium charge-transfer complex
with the arylimino groups of Yamamotoꢀs arylazidomethine
dendrimers.[8] From these studies, it has become clear that the
redox properties of the biFc derivatives, which include three
easily accessible oxidation states, are much richer than those
of the Fc group.
We have envisaged taking advantage of the possibility of
stabilizing class II mixed-valence biferrocenium cations and
exploiting specific properties of the two Fc groups therein.[9,10]
Herein we report 1) the CuI-catalyzed azide alkyne “click”
cycloaddition (CuAAC)[11] of the knew alkyne ethynylbifer-
rocene 1 with five generations of azido-terminated dendrim-
ers containing 3n terminal tethers, from n = 2 (G0, 9 termini) to
n = 6 (G4, 729 termini), leading to the formation of large 1,2,3-
triazolylFc-terminated dendrimers, 2) the isolation and full
characterization of the first mixed-valence dendrimer, 3) the
redox recognition, with positive dendritic effects, of both the
ATP2ꢀ anion using the outer Fc groups of the dendrimer and
PdII using the inner Fc groups of the dendrimers, and 4) the
role of the mixed-valence metallodendrimers.
The new complex 1 was synthesized in 50% overall yield
from acetylbiferrocene[12] in a reaction that parallels that
known for the synthesis of ethynylferrocene[13] (see the
Supporting Information). The CuAAC reaction (Scheme 1)
was carried out using CuSO4/sodium ascorbate as the CuI
source between the terminal alkyne 1 and five generations G0
to G4 of arene-centered dendrimers dend-N3 constructed
according to 1!3 connectivity[14] and containing in theory
respectively 9 (G0), 27 (G1), 81 (G2), 243 (G3), and 729 (G4)
azido termini.[15,16]
The five triazolylbiFc dendrimers of generations G0–G4
are yellow solids that are soluble in dichloromethane and
THF but insoluble in hydrocarbons, diethyl ether, and
acetonitrile. They are air-stable and thermally stable, except
that G3-243 slowly becomes insoluble in all solvents, which we
tentatively attribute to supramolecular polymerization owing
to interpenetration of the terminal tethers among dendrimers.
This irreversible catenation is selective to the interbranch
spacing for this precise dendritic generation, and remarkably
this phenomenon is not observed when the interbranch
spacing is larger (in G2-81) or smaller (in G4-729).
Besides standard 1H and 13C NMR spectroscopy, infrared
spectroscopy (G0–G4), and MALDI TOF mass spectrometry
(G0-9: m/z calcd 5086.98; found 5087.11, M+, 100%), these
metallodendrimers were also characterized by elemental
analysis, size exclusion chromatography (SEC, Figure 1),
and dynamic light scattering (DLS); the latter two techniques
show the mass and size progression (Table 1).
The DLS measurements show that the size increase is
reduced between G3-243 and G4-729 as compared to large size
increases between the generations up to G3, which must be
due to steric congestion at the dendrimer surface in G4-729
forcing extensive backfolding of the rather large triazolylbiFc
termini inside the dendrimer.[17]
Cyclic voltammetry (CV) of the five metallodendrimers
was recorded on Pt in CH2Cl2 using 0.1m [nBu4N][PF6] as the
supporting electrolyte and decamethylferrocene (FcH*) as
the internal reference. Each dendrimer shows two reversible
waves at 0.43 and 0.75 V vs. FcH*+/0 (Figure 2a), with the
adsorption of the dendrimers on the electrode surface
dramatically growing with increasing dendrimer generation.
For instance, D(EpcꢀEpa), which has a value of 60 mV for G0-9
[*] R. Djeda, A. Rapakousiou, L. Liang, Dr. J. Ruiz, Prof. D. Astruc
Institut des Sciences Molꢀculaires, UMR CNRS N85255
Universitꢀ Bordeaux 1, 33405 Talence Cedex (France)
Fax: (+33)5-4000-2995
E-mail: d.astruc@ism.u-bordeaux1.fr
Homepage: http://astruc.didier.free.fr
N. Guidolin
Laboratoire de Chimie des Polymꢁres Organiques
UMR CNRS N8 5629
Universitꢀ Bordeaux 1, 33607 Pessac Cedex (France)
[**] Helpful assistance and discussion of Mꢂssbauer data with Prof.
Azzedine Bousseksou (LCC, Toulouse), EPR data with Mattieu
Duttine (ICMCB, Pessac), and syntheses with Dr. A. K. Diallo (ISM),
and financial support from the Universitꢀ Bordeaux I, the CNRS,
and the ANR are gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004756.
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Scheme 1. The CuAAC reaction and structures of dendrimers G0-9, G1-27, and G2-81.
Figure 1. Size exclusion chromatograms of the triazolylbiferrocenyl
dendrimers (tR = retention time). The polydispersity indices (PDI) are
1.01 (G0-9), 1.02 (G1-27), 1.15 (G2-81), and 1.08 (G3-263). G4-729
appeared to be too large for SEC analysis, but its chloromethylsilyl-
terminated G4 precursor provided a satisfactory SEC trace with
PDI=1.03 (see the Supporting Information).
Figure 2. Cyclic voltammograms of G2-81: a) in CH2Cl2, [nBu4N][PF6]
0.1m; see Table 2 for data and conditions; b) progressive adsorption
upon scanning around the biFc area; c) spitting of the first CV wave
upon addition of ATP (the second wave is not represented, because its
scanning destroys the first one); d) addition of Pd(OAc)2 provoking
the splitting of the second wave.
Table 1: Hydrodynamic diameters of the metallodendrimers (obtained
by DLS), and calculated diffusion coefficient, volume, and density.
[c]
Product[a] MM[b]
2ꢃRh
D[d]
Volume[e]
[m3]
d[f]
[m2 sꢀ1
]
[kgmꢀ3
]
G1-27
G2-81
G3-243
G4-729
16927
8.6
52522 14.7
159306 25.2
479928 27.2
10.84ꢃ10ꢀ14
6.34ꢃ10ꢀ14
3.70ꢃ10ꢀ14
3.33ꢃ10ꢀ25 84
16.63ꢃ10ꢀ25 52
83.79ꢃ10ꢀ25 32
3.43ꢃ10ꢀ14 105.36ꢃ10ꢀ25 76
at 258C, progressively drops to 0 for G3-243 and G4-729
(Table 2), signifying complete adsorption at the first scan.
This result is an advantage for the facile formation of robust
metallodendrimer-modified electrodes (Figure 2b) upon
scanning around the biFc potential zone. The 1,2,3-triazolyl
group is an electron-withdrawing substituent for the inner Fc
[a] The hydrodynamic diameters of G0-9 could not be obtained by DLS
because it is below the lower limit of the technique. [b] MM: molecular
mass (gmolꢀ1). [c] 2xRh: hydrodynamic diameter (nm) measured in THF
at 258C. [d] D: diffusion coefficient. [e] considering the globular shape of
3
the dendrimer as a perfect sphere (V=(4/3)pRh ). [f] d: density.
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Communications
Table 2: Cyclic voltammetry data for the triazolylbiferrocenyl dendri-
mers.[a]
E1/2 [V][b]
D[b]
Ipc/Ipa
E1/2(V)[c]
D[c]
Ipc/Ipa
[b]
[c]
Product
G0-9
0.430
0.430
0.430
0.430
0.430
0.060
0.045
0.005
0.000
0.000
1.25
1.6
1.8
1.9
2
0.740
0.745
0.745
0.750
0.740
0.060
0.030
0.010
0.000
0.000
3
G1-27
G2-81
G3-243
G4-729
3.2
1.3
1.5
1.8
[a] Solvent: CH2Cl2; electrolyte [nBu4N][PF6] 0.1m; scan rate: 0.200 Vsꢀ1
;
258C; working and counter electrodes: Pt; quasi-reference electrode: Ag;
all half-wave potentials E1/2 are given versus the internal reference system
[Fe(h5-C5Me5)2]+/0. [b] (EpcꢀEpa) for the first wave. [c] (EpcꢀEpa) for the
second wave. See also Ref. [19b].
Figure 3. A) UV/Vis spectra of G1-27 and [G1-27][PF6]27. B) EPR spectra
of [G1-27][PF6]27 a) experimental; b) simulated; gk =3.413, g? =1.885,
T=4 K; B=magnetic field.
group, whereas the outer Fc group only bears the electron-
releasing inner Fc group. Thus, if the mixed valence is
localized (see below), the first oxidation wave at 0.43 V vs.
FcH* can be assigned to the outer Fc groups that are easier to
oxidize, whereas the second one at 0.74 V vs. FcH* can be
assigned to the inner Fc groups. This assignment is confirmed
by the remarkable selectivity of redox molecular recognition
studies with the inner and the outer Fc groups.
lized by the G0–G2 biferrocenium dendrimers absorb at 530–
550 nm (see the Supporting Information). The PdNPs formed
in this way efficiently catalyze the Miyaura–Suzuki coupling
of PhB(OH)2 with PhI at 208C for 24 h at a concentration of
0.1% Pd catalyst. These PdNPs are stable for a few hours.
Although PdNPs did not form upon addition of Pd(OAc)2 to
the electrochemical cell, they formed upon further addition of
methanol that reduced PdII to G1-27-protected PdNPs. At
Indeed, addition of the adenosyl triphosphate salt
[nBu4N]2[ATP] to the electrochemical cell containing G2-81
provokes a splitting of the outer Fc CV wave at 0.43 V owing,
to some extent, to ion-pairing interaction[18] between [ATP]2ꢀ
and [outer Fc]+, reflected by the new part of this CV wave
(Figure 2c). Geiger and co-workers have shown that ion
pairing can also have an important influence on the separa-
tion between the two CV waves of binuclear or multinuclear
redox systems. For instance, these authors demonstrated that
exchanging the ion-pairing BF4 or PF6 anion in the supporting
electrolyte for the non-ion-pairing anions BAr4 (Ar= C6F5 or
meta-C6H3(CF3)2) provoked an increase of the splitting
between the CV waves of biferrocene, oligoferrocenes, and
other polynuclear complexes.[19a–c] We have compared the
cyclic voltammograms of biferrocene[9,19d] and benzyltriazo-
lylbiferrocene[20] with those of the triazolylbiFc dendrimers
using both electrolytes [nBu4N][PF6] and [nBu4N][B{meta-
C6H3(CF3)2)}4], and we also observed a large increase of the
E2ꢀE1 value (480 mV) using the later. However, this
enhancement is subjected to a negative dendritic effect,[19b]
as opposed to the dramatic positive dendritic effect found in
the recognition of the ATP anion described above, owing to
synergistic additional supramolecular interactions that are
enhanced by dendritic effects.
On the other hand, addition of Pd(OAc)2 provokes the
splitting of the CV wave at 0.74 V owing to coordination of
the triazolyl ligand[20] attached to the inner Fc group, while the
CV wave of the outer Fc group is left unchanged (Figure 2d).
This splitting by 80 mV is due to the dendritic effect, because
it is not observed at all with benzyltriazolylbiferrocene.[20] If
the PdII cations are introduced as [Pd(MeCN)4][PF6]2, an
analogous phenomenon is still observed, but a purple
precipitate immediately forms, owing to the oxidized den-
drimers that stabilize the Pd nanoparticles (PdNPs). The
oxidized dendrimer and the PdNPs absorb in the UV/Vis
spectrum around 600 nm (Figure 3), and these PdNPs stabi-
ꢀ
0.1% Pd vs. substrates, yields of 33% to 42% of C C
coupling product were obtained in 24 h at 258C for PdNPs
stabilized by G0–G2 dendrimers, and this yield reached 53%
ꢀ
of C C coupled product upon diluting the G1-PdNPs ten
times to a concentration of 0.01% Pd. Both the generation
near-independency and the yield increase upon dilution are
consistent with a leaching mechanism that generates catalyti-
cally active Pd atoms in solution,[21] which has already been
observed with dendrimer-supported PdNPs.[21b,c]
In contrast to triazolylbiFc-terminated dendrimers, tria-
zolylmonoFc-terminated dendrimers did not reduce [Pd-
(MeCN)4][PF6]2, thus showing the better efficiency of the
outer Fc groups to reduce [Pd(MeCN)4][PF6]2 compared to
triazolyl-substituted Fc groups either in triazolylmonoFc-
terminated dendrimers or in triazolylbiFc-terminated den-
drimers. This finding also shows that mixed-valence triazo-
lylbiFc-terminated dendrimers stabilize catalytically active
PdNPs.
To determine the electronic structure of these mixed-
valence triazolylbiFc-terminated dendrimers, we oxidized G1-
27 with [FcH][PF6], which has a redox potential only slightly
more positive (by 70 mV) than the redox potential of the
outer Fc groups of the metallodendrimers. We therefore
reasoned that precipitation of the mixed-valence polycationic
metallodendrimer should completely drive the redox equilib-
rium toward its formation. Indeed, the addition of 27 equiv-
alents [FcH][PF6] to G1-27 in CH2Cl2 and subsequent addition
of diethyl ether provided soluble ferrocene and the dark-blue
dendritic complex [G1-27][PF6] (Scheme 2), which was iso-
lated as a powder subsequent to reprecipitation by addition of
diethyl ether to a MeCN solution. [G1-27][PF6]27 was soluble
in MeCN but almost insoluble in CH2Cl2, and it was
characterized by satisfactory elemental analysis and by UV/
Vis and EPR spectroscopy (Figure 3), which showed the
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(1 equiv per branch) was added at 08C, with subsequent dropwise
addition of a freshly prepared solution of 1m sodium ascorbate
(2 equiv per branch). The brown solution at 08C changed to orange at
room temperature. The reaction mixture was allowed to stir for 24 h
under N2 at room temperature. Then CH2Cl2 (100 mL) was added
followed by the addition of an aqueous solution of ammonia. The
mixture was allowed to stir for 10 min in order to remove all the
copper salts trapped inside the dendrimer. The organic phase was
washed twice with water, dried over sodium sulfate, and filtered, and
the solvent was removed under vacuum. The product was then
washed with methanol to remove the excess alkyne and precipitated
from CH2Cl2 with methanol and then with pentane, giving a yellow
powder. G0-9: 0.134 g, yield 80%. Elemental analysis calcd (%) for
Scheme 2.
classic spectrum of d5 ferrocenium, and by Mꢁssbauer
spectroscopy under zero field at 77 K (Figure 4) and 293 K
(see the Supporting Information). Both spectra show a
localized mixed-valence state, as for parent biferrocenium
itself[9] at this frequency (10ꢀ8 s).
C261H291Fe18N27Si9: C 61.56, H 5.72; found: C 60.58, H 7.91. The G1–
G4 dendrimers were obtained analogously (see the Supporting
Information).
Synthesis of the mixed-valence dendrimer [G1-27][PF6]:
A
solution of ferrocenium hexafluorophosphate (0.043 g, 0.128) in
CH2Cl2 (50 mL) was added to a solution of G1-27 (0.080 g, 0.005) in
CH2Cl2 (5 mL) at room temperature, formation of a dark-blue
precipitate was observed, and the mixture was allowed to stir under
N2 for 1 h at room temperature. Complete precipitation of the
product was obtained by dropwise addition of diethyl ether (50 mL)
to the reaction mixture. After filtration under N2 on Celite, the solid
was dried under vacuum to give [G1-27][PF6]27 as a dark-blue powder
(0.062 g, 66.3% yield). Elemental analysis calcd (%) for
C882H1029Fe54N81O9Si36P27F162: C 50.70, H 5.01; found: C 49.74, H
5.05. From the filtrate, 0.023 g (96%) ferrocene was recovered.
Received: July 31, 2010
Published online: September 20, 2010
Figure 4. Mꢂssbauer spectra of [G1-27][PF6]27 at zero field and 80 K.
Outer quadrupole doublet (inner Fc, FeII): isomer shift (IS)=0.506(3),
quadrupole splitting (QS)=2.224(6). Central doublet (outer ferroce-
nium group): IS=0.51(1); QS=0.491(17). See the spectrum at 293 K
in the Supporting Information, also showing localized mixed valence.
Keywords: dendrimers · metallocenes ·
.
mixed-valent compounds · nanoparticles · redox chemistry
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In conclusion, the first “click” synthesis, isolation, and
characterization of five generations of triazolylbiFc dendrim-
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the first mixed-valence dendrimers have also been deter-
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