"Click" dendrimers: Synthesis, redox sensing of Pd(OAc) 2, and remarkable catalytic hydrogenation activity of precise Pd nanoparticles stabilized by 1,2,3-triazole-containing dendrimers

Article abstract of DOI:10.1002/chem.200701410

"Click" dendrimers containing 1,2,3-triazolyl ligands that coordinate to Pd^II(OAc)₂ have been synthesized in view of catalytic applications, Five of these dendrimers contain ferro-cenyl termini directly attached to the triazole ligand in order to monitor the number of Pd^II that are introduced into the dendrimers by cyclic voltammetry. Reduction of the Pd^II-triazole dendrimers by using NaBH₄ or methanol yields Pd nanoparticles (PdNPs) that are stabilized either by several dendrimers (G₀, DSN) or by encapsulation inside a dendrimer (G₁ and G₂: DEN), as confirmed by TEM. Relative to PAMAM-DENs (PAMAM = poly(amidoamine)), the "click" DSNs and DENs show a remarkable efficiency and stability for olefin hydrogenation under ambient conditions of various substrates. The influence of the reductant of Pd ^II bound to the dendrimers is dramatic, reduction with methanol leading to much higher catalytic activity than reduction with NaBH₄. The most active NPs are shown to be those derived from dendrimer G₁, and variation of its termini groups (ferrocenyl, alkyl, phenyl) allowed us to clearly delineate, optimize, and rationalize the role of the dendrimer frameworks on the catalytic efficiencies. Finally, hydrogenation of various substrates catalyzed by these PdNPs shows remarkable selectivity features.

Full text of DOI:10.1002/chem.200701410

DOI: 10.1002/chem.200701410
“Click” Dendrimers: Synthesis, Redox Sensing of Pd(OAc)2, a nd
G
Remarkable Catalytic Hydrogenation Activity of Precise Pd Nanoparticles
Stabilized by 1,2,3-Triazole-Containing Dendrimers
[a]
CµtiaOrnelas,
Jaime Ruiz Aranzaes,[a] Lionel Salmon,[b] and Didier Astruc*[a]
50
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Chem. Eur. J. 2008, 14, 50 – 64

FULL PAPER
Abstract: “Click” dendrimers contain-
inside a dendrimer (G1 and G2: D EN), dramatic, reduction with methanol
ing 1,2,3-triazolyl ligands that coordi-
as confirmed by TEM. Relative to
PAMAM–DENs (PAMAM=poly(ami-
doamine)), the “click” DSNs and
DENs show a remarkable efficiency
and stability for olefin hydrogenation
under ambient conditions of various
substrates. The influence of the reduc-
tant of PdII bound to the dendrimers is
leading to much higher catalytic activi-
ty than reduction with NaBH4. The
most active NPs are shown to be those
derived from dendrimer G1, and varia-
tion of its termini groups (ferrocenyl,
alkyl, phenyl) allowed us to clearly de-
lineate, optimize, and rationalize the
role of the dendrimer frameworks on
the catalytic efficiencies. Finally, hydro-
genation of various substrates cata-
lyzed by these PdNPs shows remark-
able selectivity features.
nate to PdII
(OAc)2 have been synthe-
A
sized in view of catalytic applications.
Five of these dendrimers contain ferro-
cenyl termini directly attached to the
triazole ligand in order to monitor the
number of PdII that are introduced into
the dendrimers by cyclic voltammetry.
Reduction of the PdII–triazole den-
drimers by using NaBH4 or methanol
yields Pd nanoparticles (PdNPs) that
are stabilized either by several den-
drimers (G0, DSN) or by encapsulation
Keywords: dendrimers · ferrocene ·
heterogeneous catalysis · hydroge-
nation · nanoparticles · palladium
Introduction
The synthesis of NPs stabilized by dendrimers was first re-
ported in 1998[10] either by encapsulation of the NP within a
single dendrimer (dendrimer-encapsulated NPs, DENs) or
by surrounding the NPs by several dendrimers (dendrimer-
stabilized NPs, DSN). PAMAM-OH or PPI-modified den-
drimers (PAMAM-OH=poly(amidoamine)-OH, PPI=poly-
Dendrimers[1] occupy a privileged place among branched
macromolecules,[2] because they are multifacet monodisperse
macromolecular compounds, the supramolecular properties
of which[3] have potential applications in medicinal chemis-
try[4] and others fields of nanosciences, such as sensors[5] and
green catalysts.[6] Dendrimers have indeed been shown to
encapsulate a variety of substrates of interest,[3] as exempli-
fied by the concept of “molecular box”.[3a] The incorporation
of metals into the dendrimer backbone brought a variety of
novel properties in dendrimer chemistry and was in particu-
lar applied to catalysis.[7] Since the turn of the millennium,
the interest in nanoparticle (NP) catalysts has considerably
increased, because this class of catalysts appears as one of
the most promising solutions towards efficient reactions
under mild, environmentally benign conditions in the con-
text of green chemistry.[8] The stabilization of NPs for fur-
ther catalytic use has been achieved by a variety of means
including polymers, ligands, surfactants, ionic liquids, and su-
percritical micro-emulsions.[9] While all these stabilization
media provide small, catalytically efficient NPs, one of the
very best ways to control particle sizes and morphologies in
order to understand and optimize the NP catalytic activity is
that using dendrimers.[7]
A
encapsulate the NPs; the interior amido or amino groups
first bind the transition-metal cations followed by its reduc-
tion to metal atoms forming NPs that remain inside the den-
drimer.[7,11] The dendrimer structure is thus used as template
to prepare monodispersed NPs. Depending on the encapsu-
lated metal NPs, DENs showed unusual properties as cata-
[15]
lysts for hydrogenation[7,12–14] and C C coupling reactions.
À
Hydrogenation reactions in organic synthesis and industrial
applications are an essential research field devoted to the
design of more selective catalysts, such as transition-metal
NP-based catalysts.[16]
Palladium is one of the most efficient metals in catalysis,
and Pd–DENs have been widely investigated.[7] The catalytic
activity of the Pd–DENs evidently depends on their nature
(PAMAM, PPI, or other), generation, size, and the kind of
functional group at the dendrimer periphery.
Crooks et al. pioneered[7] the use of DENs as catalysts
and reported the hydrogenation of allylic alcohols in aque-
ous solutions, presenting catalytic efficiencies around
500 molH
2 (molPd)À1 hÀ1; TOF, turn over frequency) for
R
monometallic Pd DENs.[13] Bimetallic DENs have also been
tested in catalysis, in aqueous conditions, giving TOF values
[a] C. Ornelas, Dr. J. R. Aranzaes, Prof. D. Astruc
Nanosciences and Catalysis Group
of 1300 molH
2 (molPd)À1 hÀ1 both with Au/Pd-DENs for hy-
U
Institut des Sciences MolØculaires, UMR CNRS N8 5255
UniversitØ de Bordeaux I, 351 Cours de la LibØration
33405 Talence Cedex (France)
drogenation of allyl alcohol[17] and Pd/Rh-DENs for partial
hydrogenation of 1,3-cyclooctadiene.[18] Esumi et al.[19] modi-
fied PAMAM dendrimers with hydrophobic groups in order
to study the formation of DENs in organic solvents (M=
Au, Pt). Crooks et al. used this synthetic approach to synthe-
size hydrophobic Pd–DENs, using modified–PPI dendrimers
and tested them as catalysts in organic solvents.[14] These
Pd–DENs gave initial TOFs, for hydrogenation of 1-hexene,
Fax : (+33)540-00-6646
E-mail: d.astruc@ism.u-bordeaux1.fr
[b] Dr. L. Salmon
Laboratoire de Chimie de Coordination
UPR CNRS N8 8241, 205 Route de Narbonne
31077 Toulouse Cedex 04 (France)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
of 120 molH
2 (molPd)À1 hÀ1 in CHCl3/MeOH (2:1) and were
N
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51

D. Astruc et al.
being quite instable under hydrogenation conditions (precip-
itation after 20 min).[14] However, all these studies have
been carried out exclusively with PAMAM or PPI dendri-
click chemistry. Then, the CuI-induced click reaction be-
tween 4 and 5 in water/THF yielded the first-generation 27-
allyl dendrimer 6, assembled with nine 1,2,3-triazole links; it
is characterized by its molecular peak at 3937.42 ([M+Na]+)
in the MALDI TOF mass spectrum (calcd for
A
taining dendrimers with several generations to examine and
optimize the key parameters governing the catalytic proper-
ties, that is, efficiency, selectivity, and stability.
C
234H327N27O9Si9Na: 3938.04). Repetition of this sequence of
reactions yield the 27-azido intermediate 8, then the 81-allyl
second-generation dendrimer 9, containing 36 triazole links
in two layers (9 + 27) (see Scheme 1). Note, that whereas
this “click” reaction is usually catalytic in CuI (5% CuI is
used by most authors), the present click dendrimer synthesis
requires a stoichiometric amount of CuI, because CuI re-
Our group has recently reported, in a preliminary form,
the synthesis of some poly-1,2,3-triazolylferrocenyl[20] den-
drimers assembled by “click” chemistry;[21] these dendrimers
contain triazole units and are able to complex metal cations.
One of our goals is to investigate the NP encapsulation
properties of these new hydrophobic, heterocyclic-ligand-
containing dendrimers, in order to examine the influence of
the structural and functional dendritic features on the cata-
lytic efficiency and stability. Another goal is for these den-
drimers to serve as sensors inter alia of metal ions so as to
control the stoichiometry and number of PdII that can be in-
troduced into each dendrimer. Successful sensing of cationic
transition-metal ions could indeed so far be achieved.
In the present article, we report new poly-1,2,3-triazolyl
dendrimers including dendrimers that contain ferrocenyl ter-
mini.[20] These dendrimers were assembled and functional-
ized by click chemistry[21] and thus all contain 1,2,3-triazole
units. They are able to 1) complex PdII, 2) stabilize and en-
capsulate PdNPs, and 3) produce highly active and size-se-
lective DENs catalysts for hydrogenation. The variation of
the dendritic structure in the interior backbone and periph-
ery of the dendrimers allows to investigate the influence of
these structural variations on the catalytic properties of the
new DENs. The dramatic effect of the nature of the reduc-
ing agent (NaBH4 vs. methanol) opposite to that known in
polymer-stabilized PdNP catalysis and the remarkably size-
selective properties of the catalysts with various olefin sub-
strates are also reported and discussed in this article.
mains trapped inside the dendrimer and is only removed as
+
CuI
A
This feature is further confirmed by the recognition and ti-
tration studies of the click dendrimers with CuI.[20] On the
other hand, an advantage of this procedure variation is that
the click reaction is much faster here than in the standard
procedure (0.5 h at 208C instead of 16 h).
Reactivity studies carried out at different temperatures, to
study the regioselectivity of the click reaction in the dendrit-
ic construction, show that the formation of the undesired
1,5-disusbtituted 1,2,3-triazole isomer is favored by an in-
crease in temperature. For example, at 508C, both 1,4- and
1,5-isomers are obtained in a 7:3 ratio and can be easily
identified by their different NMR signals for the triazole
proton (d=7.49 ppm for the 1,4-isomer and d=7.62 ppm for
the 1,5-isomer, see the Supporting Information p. 44). The
click dendrimers reported here do not present any trace of
1,5-disubstituted 1,2,3-triazole isomer. This selectivity is ach-
ieved by adding the sodium ascorbate dropwise at 08C, this
procedure minimizes the heating caused by the exothermic
click reaction and therefore avoids the formation of the 1,5-
isomer.
These dendrimers can be further functionalized with ethy-
nylferrocene, also by “click chemistry”, generating triazolyl-
ferrocenyl dendrimers.[20]
Results and Discussion
To study the dendritic effects on the type of NP stabiliza-
tion and on the catalysts performance, three different gener-
ations of poly-1,2,3-triazolylferrocenyl dendrimers were syn-
thesized: G0 with nine terminal ferrocenyl-triazole units (12-
G0), G1a with 36 triazole units (9 interior + 27 terminal)
(13-G1a) and G2a with 117 triazole units (9+27 interior +
81 terminal) (14-G2a) (see Scheme 2). The nona-triazolylfer-
rocenyl dendrimer 12-G0 is characterized by its molecular
peak at 3408.18 ([M]+) in the MALDI TOF mass spectrum
(calcd for C171H219N27Si9Fe9: 3408.12); all the rest of dendri-
mer series was further characterized by size-exclusion chro-
matography (SEC), showing the size progression and poly-
dispersity lower than 1.02.
Synthesis and functionalization of the triazolyl dendrimers:
To efficiently complex palladium(II) and improve the stabi-
lization of PdNPs, dendrimers containing triazole ligands
were designed by “click chemistry”.[20]
The dendritic construction starts with the known nona-al-
lylation of [FeCp(h6-mesitylene]
[PF6], quantitatively yield-
A
ing the nona-allyl dendritic core 1 on a large scale subse-
quent to visible-light photolysis that removes the metal
moiety.[22] Hydrosilylation of terminal olefinic bonds of the
nona-allyl core, with HSiMe
ACHTREUNG
regioselectively gives the nonachloromethyl
AHCTREUNG
(dendri-(CH2Cl)9) intermediate, 3,which upon reaction with
NaN3 provides the nona-azide 4.
The role of the interior triazole units on the encapsulation
and catalytic efficiency of the PdNPs was also studied with
poly-1,2,3-triazolylferrocenyl dendrimers that do not contain
interior triazole units, and that only have triazolylferrocenyl
units at the periphery.
The
known
phenoltriallyl
dendronic
brick
p-
HOC4H4C(CH2CH=CH2)3 (2), obtained by one-pot reaction
of [FeCp(h6-p-chlorotoluene]
[PF6] with allyl bromide and
R
tBuOK,[22] is functionalized using propargyl bromide at the
phenol focal point, giving the new dendron 5 suitable for
Dendrimers assembled by the Williamson coupling reac-
tion[22] (dendri-(CH2Cl)27 and dendri-(CH2Cl)81) provide,
52
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Dendrimers
FULL PAPER
Scheme 1. “Click” assembly of dendrimers.i) 1. CH2=CHCH2Br, KOH, THF, RT, 3 d; 2. hnvis, MeCN; ii) 1. HSiCH2(CH3)2Cl, Kartsted catalyst, diethyl
G
ether, RT, 13 h; 2. NaN3, DMF, 16 h, 808C; iii) CH2=CHCH2Br, tBuOK, THF, RT, 5 d; iv) propargyl bromide, acetone, reflux, 16 h; v) CuSO4, sodium as-
corbate, THF/H2O, RT, 30 min; vi) 1. HSiCH2(CH3)2Cl, Kartsted catalyst, dietehyl ether, RT, 13 h; 2. NaN3, D MF, 16 h, 808C; vii) CuSO4, sodium ascor-
U
bate, 5, THF/H2O, RT, 30 min.
upon reaction with NaN3, the azido dendrimers dendri-
(CH2N3)27 (15) and dendri-(CH2N3)81 (16) that further react
with ethynylferrocene by “click” chemistry yielding G1b (17-
G1b) with 27 triazolyl-ferrocenyl termini and G2b (18-G2b)
with 81 triazolyl-ferrocenyl termini, respectively (full draw-
ings of the structures of 15 and 16 are available in the Sup-
porting Information).
Both 81-triazolylferrocenyl dendrimers (14-G2a and 18-
G2b) are also characterized by dynamic light scattering
(DLS) that provide the diameters of these dendrimers in di-
chloromethane: 14-G2a: 12Æ0.5 nm, 18-G2b: 9Æ0.5 nm.
The poly-1,2,3-triazolylferrocenyl dendrimers present a
bulky periphery due to the ferrocenyl group located at the
termini of the dendritic tethers. To confirm the crucial role
of the bulky periphery and to understand the influence of
the ferrocenyl group on the stabilization of the PdNP and
catalyst performances, the 27 ferrocenyl terminal units of
17-G1b were replaced by 27 dendronic-9-propyl units (21-
G1c) (bulky) and by 27 phenyl units (22-G1d) (not bulky).
The synthesis of these dendrimers was also carried out by
“click chemistry” between the dendri-(CH2N3)27 (16) and the
ethynyl-dendronic-9-propyl (20) and phenylacetylene (see
Scheme 3).
All the seven poly-1,2,3-triazolyl dendrimers reported
here were used to stabilize PdNPs (characterized by trans-
mition electron microscopy, TEM) and tested as catalysts in
hydrogenation of styrene at 0.1% mol Pd. Different types of
PdNP stabilization caused by the variation of dendrimer
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D. Astruc et al.
Scheme 2. Synthesis of the 1,2,3-triazolylferrocenyl dendrimers by “click Chemistry”. i) CuSO4, sodium ascorbate, THF/H2O, RT, 30 min, ethynylferro-
cene.
structures and the influence of the dendritic structure on the
catalytic performances are discussed further in this article.
ment, was used as a redox monitor. The poly-1,2,3-triazolyl-
ferrocenyl dendrimers show a single, fully reversible cyclic
voltammetry (CV) wave for all the equivalent (but distant)
ferrocenyl groups, the potentials of which are similar and
the electrostatic factor being very weak.[24] Complexation of
Complexation of Pd
ACHTREUNG
dendrimer: The complexation of Pd
AHCTREUNG
groups of the poly 1,2,3-triazolylferrocenyl dendrimers was
monitored by cyclic voltammetry,[23] and UV/Vis and
1H NMR spectroscopy. In the cyclic voltammetry studies,
the ferrocenyl group, directly attached to the triazole frag-
Pd(OAc)2 to the poly-1,2,3-triazolylferrocenyl dendrimers in
A
CHCl3/MeOH (2:1) gives a new reversible CV wave that ap-
pears at a potential more positive than that of the initial
wave, showing that the Pd–dendrimer assembly is more dif-
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Dendrimers
FULL PAPER
wave does not decrease before
20 PdII fragments are added per
dendrimer. This amount is
lower than the two first-genera-
tions (36 triazoles), indicating
that PdII starts binding to the
ferrocenyltriazole at the periph-
ery before the second triazole
layer is saturated. This means
that complexation of the inner
layers occurs somewhat selec-
tively, but this selectivity is not
as complete as for 13-G1a. The
outer triazole rings bear a fer-
rocenyl ring that carries more
steric bulk than the inner tri-
azole rings that are connected
to the tethers by two methylene
groups, and this steric effect
alone could account for the ob-
served selectivity.
For complexation of the tri-
A
presence of methanol is re-
quired, since no interaction is
observed in CH2Cl2 or CHCl3
(Figure 30 of the Supporting In-
formation). It is known that
methanol (as well as water)
readily breaks the trimeric spe-
cies [{Pd(OAc)2}3] upon deche-
T
lating one or more acetate li-
gands.[26] After one of the ace-
tate ligands is dechelated, the
triazole ligand binds the metal,
thereby accelerating the reduc-
tion of PdII to Pd0, (see NMR
data, Figures 26–32 of the Sup-
porting Information). Indeed, a
few minutes after addition of
Pd(OAc)2 to the dendrimer in
N
methanol, the UV/Vis spectra
of the dendrimers show a new
band at 278 nm that corre-
sponds to the start of the PdNP
formation and stabilization by
Scheme 3. Synthesis of the dendron 20 and dendrimers 21-G1c and 22-G1d. i) HSi(CH2CH2CH3)3, Karsted cata-
lyst, diethyl ether, RT, 15 d; ii) propargyl bromide, acetone, reflux, 16 h; iii) CuSO4, sodium ascorbate, 20,
THF/H2O, RT, 5 h; iv) CuSO4, sodium ascorbate, phenylacetylene, THF/H2O, RT, 30 min.
ficult to oxidize than the dendrimer alone. Titrations of Pd-
(OAc)2 with poly-1,2,3-triazolylferrocenyl dendrimers indi-
cates that each triazole unit binds one Pd(OAc)2 (see
the triazole interaction (Figure 8 of the Supporting Informa-
tion). This band remains after the formation of the PdNPs
(either using NaBH4 or methanol as reducing agent). The
ACHTREUNG
AHCTREUNG
Table 1 and Figure 1 for CV data). Moreover, the titration
graphs show that, for 13-G1a, which possesses two triazole
layers, the intensity of the ferrocenyl wave does not vary
before 25% (i.e., 9 out of 36) of the PdII species is added to
the dendrimer solution. This means that the inner triazole
layer (nine triazoles) binds PdII first, and the ferrocenyltri-
UV/Vis spectrum of Pd(OAc)2 also shows the complete dis-
A
appearance of the initial band at 399 nm and the appearance
of a large band at 300 nm after 30 min, indicating the initial
formation of the PdNPs. After 16 h, evolution of the UV/
Vis spectrum to a monotonic typical PdNP spectrum is ob-
served; a similar spectrum is obtained upon PdNP reduction
1
G
with NaBH4 (see Figure 2). The H NMR data also show the
rapid reduction of PdII to Pd0 in the presence of methanol
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55

D. Astruc et al.
Table 1. Cyclic voltammetry results of the recognition of the Pd
by the poly-1,2,3-triazolylferrocenyl dendrimers.
G
and triazole: after 10 min of reaction, a decrease of the in-
tensity of the dendrimer NMR signals is observed, showing
only small broad peaks (Figure 32 of the Supporting Infor-
mation). However, the presence of NPs (in the absence of
NaBH4) is only observed after 16 h, indicating that the for-
mation of NPs under these conditions is slowed by the par-
tial stabilization of Pd0 by the triazole ligands. The TEM
data confirm the formation of the NPs using either metha-
nol or NaBH4 as a reducing agent.
Recognition of Pd(OAc)2
A
[c]
No.
E1/2[b] (EpaÀEpc) E1/2 (EpaÀEpc) DE1/2
K(0)/K(+)
triazole[a] [V]
[V]
[V]
12-G0
13-G1a
14-G2a 117
9
36
0.540 (0.050)
0.540 (0.030)
0.540 (0.030)
0.640 (0.030) À0.100 53
0.625 (0.020) À0.085 29
0.595 (0.020) À0.055
9
[a] Number of triazole units (interior + exterior). [b] E1/2 =(Epa +Epc)/2
versus FeCp2*, Cp*=h5-C5Me5 (in V). Electrolyte: [nBu4N]
AHCTREUNG
solvent: chloroform/methanol (2:1); working and counter electrodes: Pt;
quasi-reference electrode: Ag; internal reference: FeCp2*; scan rate:
Synthesis and characterization of PdNPs stabilized by the
1,2,3-triazolyl dendrimers: All the seven poly-1,2,3-triazolyl
dendrimers reported here were used to stabilize PdNPs in
CHCl3/MeOH (2:1) with two different reducing agents:
NaBH4 (method 1) and methanol (method 2). In method 1,
0.200 VsÀ1
; 208C. [c] Difference between values of E1/2 before (3rd
column) and after (4th column) titration. [d] Ratios of apparent associa-
tion constants; error=10%; DE1/2 =0.058 log(K(0)/K(+)) at 208C.[25]
the dendrimer was mixed with Pd
azole unit) in CHCl3/MeOH (2:1), followed by addition of
NaBH4 (10 equiv per Pd). Since methanol is able to reduce
(OAc)2 (1 equiv per tri-
AHCTREUNG
Figure 1. Top: Titration of Pd
ACHTREUNG
tammetry. The CVs were obtained i) before addition Pd
A
ACHTREUNG
&
0.65 equiv per triazole); and iii) at the end of titration of Pd
ACHTREUNG
~
and increase of the new CV wave ( ) versus the number of equiv of Pd
ACHTREUNG
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Chem. Eur. J. 2008, 14, 50 – 64

Dendrimers
FULL PAPER
Figure 2. a) UV/Vis spectrum of a freshly prepared solution of Pd(OAc)2
G
(2.97”10À3 m) in CHCl3/MeOH (2:1); b) UV/Vis spectrum of a solution
of 12-G0 (9.78x10À5 m) and 9 equiv of Pd(OAc)2 (8.80”10À4 m) in CHCl3/
U
MeOH (2:1) after 30 min [using a solution of dendrimer (9.78”10À5 m) in
CHCl3/MeOH (2:1) as blank]; c) UV/Vis spectrum of the PdNPs ob-
tained by reduction with NaBH4 (a similar spectrum was obtained using
MeOH as the reductant).
Pd
ACHTREUNG
the mixture of dendrimer and Pd
AHCTREUNG
(2:1) without adding NaBH4. Reduction of PdII using only
methanol is much slower, and it required 16 h of stirring
(method 2). The formation of the NPs was observed by the
color change of the solution from yellow to golden brown
and confirmed by UV/Vis spectroscopy. The size of all the
NPs was obtained by transmition electron microscopy
(TEM).
Table 2. Size of the PdNPs stabilized by the triazolyl dendrimers ob-
tained by TEM.
PdNP
No. Pd
Calculated
diameter[b]
[nm]
Diameter
method 1[c]
[nm]
Diameter
method 2[c]
[nm]
atoms[a]
Due to its small size with an open structure, 12-G0 cannot
encapsulate a PdNP. Thus, 12-G0 forms interdendrimer-sta-
bilized PdNPs (DSN-12-G0) in which the NP surface is stabi-
lized by several dendrimers. The TEM data show that, in
this case, the PdNPs obtained by reduction with only metha-
nol are larger with 12-G0 than with 13-G1a or 14-G2a. Also,
the nature of the reductant has a crucial influence on the
size of the NPs formed. Upon reduction with methanol, 12-
G0 forms DSNs with a diameter of 2.8Æ0.3 nm (766 Pd
atoms stabilized by 85 dendrimers), whereas reduction with
NaBH4 affords 12-G0-DSN with a diameter of 1.2Æ0.2 nm
DSN-12-G0
9
36
117
36
27
81
27
27
40
–
1.2Æ0.2
1.1Æ0.2
1.6Æ0.3
1.1Æ0.2
1.0Æ0.3
1.3Æ0.3
1.0Æ0.3
2.0Æ0.4
1.1Æ0.2
2.8Æ0.3
1.3Æ0.2
1.6Æ0.3
1.3Æ0.3
1.1Æ0.3
1.4Æ0.3
1.1Æ0.3
2.5Æ0.3
–
DEN-13-G1a
DEN-14-G2a
DEN-14-G2a-36
DEN-17-G1b
DEN-18-G2b
DEN-21-G1c
DSN-22-G1d
DEN-PAMAM
1.0
1.5
1.0
0.9
1.3
0.9
–
1.0
[a] Number of Pd atoms per dendrimer. [b] Calculated using the equation
n=4pr3/3 Vg, in which n is the number of Pd atoms, r is the radius of the
Pd nanoparticle and Vg is the volume of one Pd atom (15 Š3).[28] [c] Di-
ameter obtained by TEM.
(60 Pd atoms stabilised by seven dendri
mers).[28] This size
A
difference can be taken into account, as in polymer chemis-
try, by the stronger reducing power of NaBH4 than metha-
nol, because no encapsulation of the DSNs drives the size
control for 12-G0. On the other hand, as expected, 13-G1a
and 14-G2a form very small intradendrimer-encapsulated
PdNPs (DENs). Their sizes (measured by TEM) correspond
to the calculated sizes[28] for the number of Pd atoms inside
the dendrimer, that is, the same as that of triazole ligands
for 13-G1a (36) and 14-G2a (117; see Table 2). In constrast
to the DSN-12-G0, we find that for DEN-13-G1a and DEN-
14-G2a, the size of the PdNPs is about the same regardless
of whether NaBH4 or methanol is used (Table 2). This result
is well taken in account by the fact that in DENs the size of
the PdNPs is governed by the number Pd atoms inserted in
the dendrimers before reduction, not by the reducing power
of the reductant. The dramatic influence of the nature of
the reductant on the size of DSN-12-G0 also confirms the
different type of stabilization, DSN versus DENs, when
comparing 12-G0 with 13-G1a and 14-G2a.
To compare DENs formed from 13-G1a and 14-G2a with
the same number of Pd atoms in the PdNPs (i.e. 36), anoth-
er DEN was synthesized, setting only 36 Pd equivalents in
the dendrimer 14-G2a (DEN-14-G2a-36). This experiment
should help us to understand the role of the dendritic struc-
Chem. Eur. J. 2008, 14, 50 – 64
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www.chemeurj.org
57

D. Astruc et al.
ture in the NP formation, stabilization, and catalytic activity.
According to the TEM data, these DENs have a size that is
similar to that of DEN-13-G1a (1.1Æ0.2 nm), confirming
that these NPs contain about 36 Pd atoms.
(PAMAM-NH2-G4 modified with 1,2-epoxydodecane)[19a] to
form Pd-DENs. The DEN-Pd-PAMAM was mixed with
40 equivalents of Pd(OAc)2 per dendrimer in CHCl3/MeOH
N
(2:1), and NaBH4 was added.
For the dendrimers that do not contain triazole units in
the interior, 17-G1b and 18-G2b, the TEM data show that
they are also able to form DENs, and the sizes obtained are
close to those calculated for the number of triazole ligands
in the dendrimers (see Table 2).
According to TEM data, G1 and G2 of both classes of fer-
rocenyl dendrimers (Gna and Gnb, n=1 and 2) form DENs.
Thus, after the complexation of ferrocenyltriazolyl unit to
PdII, the ferrocenyl group plays an important role. It blocks
the periphery, which retains the Pd0 atoms inside the dendri-
mer, and consequently leads to the formation of very small
PdNPs. This demonstrates that the functionalization of den-
drimers with triazolylferrocenyl groups has two major ad-
vantages: 1) they can sense the binding of the metal to the
triazole fragment in the dendrimer and 2) they block the pe-
riphery by stabilizing the nanoparticles inside the dendri-
Catalytic efficiency and stability of the DSNs and DENs
based on the 1,2,3-triazolyl dendrimers: The DSNs and
DENs formed with the new poly-1,2,3-triazolyl-ferrocenyl
dendrimers prove to be efficient catalysts for hydrogenation
of styrene at 0.1% mol Pd. By using NaBH4 as the reducing
agent (method 1), the new DSNs and DENs can be com-
pared to DENs based on the commercial dendrimers more
often used as catalysts under the same conditions.
The turn over frequencies (TOF) obtained for styrene hy-
drogenation at 0.1% mol Pd with DEN-13-G1a are about
six times larger (310 molH
A
based on PAMAM (56 molH
A
Table 3. Catalytic efficiency (TOF values) and stability (TON values) ob-
tained for all the new catalysts for styrene hydrogenation at 0.1% mol
Pd.
ACHTREUNG
PdNP
TOF[a] [molH2
Method 1
A
]
TON
G1d are also able to stabilize the PdNPs. As expected, and
according to the TEM data, the bulky 21-G1c forms DENs
and the non-bulky 22-G1d forms DSNs, because the phenyl
terminal groups are not bulky enough to encapsulate the
NPs (see Figure 3 and Table 2).
To compare the catalytic activity of the new PdNPs with
those of Pd-DENs derived from the PAMAM dendrimers
studied by Crooksꢁ group, under the same conditions, we
used a known hydrophobic PAMAM-modified dendrimer
Method 1 Method 2
DSN-12-G0
200
310
200
280
330
210
1780
995
56
30000
10000
20000
7400
31500
9300
16650
10000
9400
21150
76170
35000
–
DEN-13-G1a
DEN-14-G2a
DEN-14-G2a-36
DEN-17-G1b
DEN-18-G2b
DEN-21-G1c
DSN-22-G1d
DEN-PAMAM
9800
20400
78420
38580
7500
[a] The catalytic activity of the PdNPs was investigated for the hydroge-
nation of styrene at 0.1% Pd, in CHCl3/MeOH (2:1), 258C and 1 atm H2.
Reactions were followed by GPC, and TOF values were determined
based on the yield of ethylbenzene formation.
Other known DENs based on the PPI-modified-dendrimers
were reported to be very unstable under hydrogenation con-
ditions in organic solvents (turn over number, TON of only
40).[14] On the other hand, the new “click” DENs are very
stable under catalytic conditions: they can be re-used in ten
catalytic cycles (TONꢀ10000) for DEN-13-G1a and 20 cata-
lytic cycles for DEN-14-G2a (TONꢀ20000). The largest
TONs are obtained with DSN-12-G0 (TONꢀ30000), al-
though the TOF is lower than that found with DEN-13-G1a
(Table 3).
The catalytic activity of the DSNs and DENs synthesized
by method 2 (methanol as reducing agent) is much higher
than that of DENs synthesized by method 1, with TOF
values of 1620 molH
2 (molPd)À1 hÀ1 (13-G1a) (Table 3),
G
whereas the opposite was known with polymers.[16]
The large increase of catalytic activity found with these
“click” DENs compared to DENs prepared from the com-
mercial dendrimers PAMAM and PPI is shown to be due to
both the type of dendrimer and nature of the reductant, but
we can also assign the contribution of each of these two pa-
Figure 3. TEM image (left) and size distribution (right) of DEN-21-G1c
(top) and DSN-22-G1d (bottom) (both were prepared by method 2—re-
duction of PdII using methanol).
58
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Chem. Eur. J. 2008, 14, 50 – 64

Dendrimers
FULL PAPER
rameters influencing the increase of catalytic activity. On
one hand, the newly designed dendritic framework with tri-
Using method 1, the catalytic activity of the DENs that
do not contain ferrocenyl groups (DEN-21-G1c and DEN-
22-G1d) is much higher than that of the ferrocenyl-contain-
ing DENs (this difference is small when method 2 is used,
however). The rationalization of this point is not straightfor-
ward, but the interaction between BH3 (a Lewis acid) and
ferrocenyl (a Lewis base) could possibly play a role, particu-
larly a steric one. Indeed, the increase of steric bulk that re-
sults from this interaction may slow down the introduction
of the olefin substrate into the dendritic framework.
A
activity, by comparison with commercial dendrimers under
the same conditions. On the other hand, the nature of the
reductant, methanol, is also responsible for another large in-
crease of catalytic activity, as shown in Table 3. All the pre-
vious literature data with DENs had been recorded with
NaBH4, possibly because NaBH4 had provided better cata-
lytic results than methanol in the cases of studies with poly-
mer-stabilized NPs.[16] The better results obtained with
NaBH4 relative to methanol as a reducing agent in polymer-
stabilized NPs were due to the fact that NaBH4, being a
stronger reductant than methanol, produced smaller NPs
that were more active in hydrogenation catalysis than the
larger polymer-stabilized NPs produced with methanol. In
contrast, we find that, with the “click” DENs, the size of the
PdNPs obtained from TEM data is about the same regard-
less of whether NaBH4 or methanol is used (Table 2). This
result is taken in account by the fact that the size of the
PdNPs is governed only by the number Pd atoms in the
DENs, which is the same as that of triazole ligands for G1
and G2 species. Methanol does not leave inhibiting residues
on the PdNPs surface subsequent to reduction, whereas
The DEN-21-G1c appears to be the most efficient catalyst
(TOF=3390 molH2
(molPd)À1 hÀ1), because it is bulky
R
enough to encapsulate the PdNPs, forming very small NPs.
However, the size and mobility of its periphery groups
makes the PdNP surface more accessible to substrates for
catalysis, relative to DENs containing rigid ferrocenyl
groups. These DENs exhibit a very high stability with im-
pressive TON values. Indeed, they can be re-used for 80 cat-
alytic cycles (TONꢀ80000). The catalytic efficiency and sta-
bility obtained for all the DSNs and DENs based on 1,2,3-
triazolyl dendrimers are gathered in Table 3.
After catalysis, the poly-1,2,3-triazolylferrocenyl dendri-
A
easily recovered from the reaction media by simple extrac-
tion with dichloromethane and precipitation in methanol.
We found that the dendritic structures do not suffer any sig-
NaBH4 does (B
(OCH3)3 and Na+).[29] Also note that, if the
G
nature of the reductant has a strong influence on the cataly-
sis kinetics, it has no significant influence on the stabilities
of the “click” DENs as shown by the high TON values
found with both reductants. The catalytic efficiency of
DEN-G1 and DEN-G2 species confirm the trend according
which the catalytic activity is a function of the size of the
PdNPs, that is, the catalytic reactions are faster for the
smaller NPs than for the larger ones. We also examined the
influence of the dendrimer size on the catalytic activity of
“click” DENs. The comparison of the catalytic activity of
DEN-14-G2a-36 with those of DEN-13-G1a (36 Pd) and
DEN-14-G2a (117 Pd) shows that DEN-14-G2a-36 is more
efficient than DEN-14-G2, but less efficient than DEN-13-
G1. This allows us to state that the catalytic efficiency of the
DENs depends not only on the number of Pd atoms of each
NP, but also on the dendrimer structure that encapsulates it.
The steric factor is indeed of great importance, the larger
dendrimer G2 species slowing the catalysis kinetics due to
steric effects of the large dendrimer framework.[7] We can
conclude that, with this family of click dendrimers, the G1
species is the optimal generation to form DEN catalysts. It
is large enough to encapsulate NPs, and it is able to form
very small NPs that are very active in catalysis.
1
nificant structural change (confirmed by H NMR spectros-
copy) and they can be re-used as catalyst supports.
Catalytic selectivity with DEN-G1afor the hydrogenation of
various olefin substrates including conjugated dienes and tri-
enes: The DEN-13-G1a/method 2 was tested in the hydroge-
nation of various olefin substrates under ambient conditions,
including conjugated dienes and trienes, to study the selec-
tivity properties of this catalyst. The TOF values obtained
are gathered in Table 4. With this new catalyst, styrene and
allyl alcohol were converted (100%) to ethylbenzene and 1-
propanol, respectively, with high selectivity of the catalyst
for substituted olefins. During the hydrogenation of 1-
hexene, the conversion to hexane is 43%, because the 2-
hexene isomer is also produced.[30] The hydrogenation of in-
ternal conjugated dienes and trienes using the DEN-13-G1a
catalyst was 100% selective for the formation of monoenes,
showing the selectivity of the partial hydrogenation of cyclic
and linear internal conjugated dienes. Finally, size selectivity
of DEN-13-G1a was investigated by comparing its catalytic
activity on hydrogenation of cyclohexadiene and ergosterol
(that contains a cyclohexadiene cycle in its structure). We
found that DEN-13-G1a catalyzes the partial hydrogenation
of cyclohexadiene to cyclohexene, but no hydrogenation is
observed in the case of ergosterol. Moreover, only cyclohex-
adiene is semihydrogenated upon reaction of a mixture of
cyclohexadiene and ergosterol. This clearly shows the size
selectivity performed by the DEN-13-G1a catalyst; ergoster-
ol is not able to enter into the dendrimer structure so as to
locate its double bonds onto the NP surface (see Figure 4).
There is no significant difference between the catalytic ef-
ficiency of both classes of triazolylferrocenyl dendrimers
(with and without interior triazole units), 17-G1b and 18-G2b
being slightly more efficient than 13-G1a and 14-G1a respec-
tively, probably due to the small difference of the number of
palladium atoms by NP (36 in 13-G1a and 27 in 17-G1b; 117
in 14-G2a and 81 in 18-G2b). This feature shows the impor-
tance of a bulky periphery provided by ferrocenyl group in
the encapsulation of NPs by dendrimers.
Chem. Eur. J. 2008, 14, 50 – 64
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www.chemeurj.org
59

D. Astruc et al.
Table 4. Hydrogenation of olefins catalyzed by DEN-13-G1a/method 2.
Conclusion
Substrate
Products[a]
Yield [%] TOF[b]
A full account of the synthesis and characterization of a va-
riety of novel click dendrimers is provided in this article.
The engineering of these novel ligand-containing dendrimers
and precise redox sensing of PdII coordinated to the triazole
ligands, leads to the production of various dendrimer-encap-
sulated PdNPs (DENs) with a pre-organized number of Pd
atoms that is confirmed by TEM data for first- and second-
gernation dendrimers (G1 and G2). The TEM data also con-
firm that 12-G0 is too small to encapsulate PdNPs, and the
PdNPs are stabilized by several dendrimers around the
PdNPs (DSNs) in this case.
100
100
1620
1160
43
57
1800
2400
78
22
810
230
This study provides precise DENs and DSNs that are
highly efficient, stable, and size-selective hydrogenation cat-
alysts, and the various structural influences on the catalytic
performances are rationalized. The dramatic and specific
structure-dependent role of the reductant is clearly elucidat-
ed, which leads to a significant improvement in our under-
standing of the catalytic reactions with PdNPs. The results
will be useful for comparison with other types of catalysis,
and the parameters obtained here should serve further for
the design of highly selective heterobimetallic hydrogena-
tion catalysts of industrial relevance.
100
100
1150
530
100
130
480[c]
Experimental Section
no hydrogenation
–
General data: Reagent-grade diethyl ether (used in hydrosilylation reac-
tions) was predried over Na foil and distilled from sodium/benzophenone
under nitrogen immediately prior to use. Syntheses of the NPꢁs were car-
ried out by using Schlenk techniques with degased solvents. PAMAM-
NH2-G4 was purchased from Sigma-Aldrich and modified with 1,2-epoxy-
dodecane according to reference [29]. Ethynylferrocene, phenylacetylene,
and all the olefinic substrates were purchased from Sigma-Aldrich. Ergo-
sterol was purchased from Alfa Aesar.
[a] Reactions are performed at 258C/1 atm H2; followed by GPC in the
case of styrene and by NMR spectroscopy in all other cases. [b] TOF
values are determined on basis of the yield of formation of the final
product. [c] The NMR data do not allow to determinate the relative
quantities of the two isomers formed. [d] Ergosterol was left under hy-
drogenation conditions during four days.
Cyclic voltammetry measurements: All electrochemical measurements
were recorded under nitrogen atmosphere. Conditions: solvent: CHCl3/
MeOH; temperature: 208C; Supporting electrolyte: [nBu4N]
working and counter electrodes: Pt; reference electrode: Ag; internal
reference: FeCp2* (Cp*=h5-C5Me5); scan rate: 0.200 VsÀ1
A
.
Transmission electron microscopy (TEM): Samples were prepared plac-
ing a drop of a 4.41”10À4 m solution (CHCl3/MeOH 2:1) of PdNP (con-
centration in mol Pd) on a holey-carbon-coated Cu TEM grid. The size
of the nanoparticles was measured using the software sigmascanpro (for
each sample, about 100 nanoparticles were measured).
General procedure for the hydrosilylation reactions: The polyolefin den-
drimer, diethyl ether, dimethylchloromethylsilane (2 equiv per branch)
and Kartsted catalyst (0.1%) were successively introduced into
a
Schlenck flask under a nitrogen atmosphere. The reaction solution was
stirred at 258C for 16 h. For dendrimers containing triazole units, several
additions of catalyst and longer time reactions were needed to complete
reactions (due to possible partial complexation of Pt0 of the catalyst by
the triazole units). The reaction was followed by 1H NMR spectroscopy;
at the end of the reaction, the solvent was removed under vacuum, the
catalyst residue was removed by flash chromatography, and the dendri-
mer was precipitated using dichloromethane/pentane.
General synthesis of azido dendrimers: The chloroalkyl-dendrimer and
sodium azide (2 equiv per branch) were heated at 808C for 16 h in dry
DMF. The solvent was removed under vacuum; the crude product was
dissolved in dichloromethane, washed twice with water, dried with
sodium sulfate, and filtered over paper; and the solvent was removed
Figure 4. Schematic representation of the size selectivity of DEN-13-G1a.
60
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Chem. Eur. J. 2008, 14, 50 – 64

Dendrimers
FULL PAPER
under vacuum. The dendrimer was precipitated with dichloromethane/
methanol.
18H; CH2O), 3.86 (s, 18H; SiCH2N), 2.71 (s, 54H; CH2Cl), 1.59 (s, 72H;
CH2CH2CH2Si), 1.06 (s, 72H; CH2CH2CH2Si), 0.57 (s, 72H;
CH2CH2CH2Si), 0.041 ppm (s, 216H; Si
(CH3)2CH2Cl); 13C NMR (CDCl3,
A
General procedure for the “click” reactions: The azido dendrimer
(1 equiv) and the alkyne (1.5 equiv per branch) were dissolved in tetrahy-
drofuran (THF) and water was added (1:1 THF/water). At 208C, CuSO4
was added (1 equiv per branch, 1m aqueous solution), followed by the
dropwise addition of a freshly prepared solution of sodium ascorbate
(2 equiv per branch, 1m aqueous solution). The solution was allowed to
stir for 30 min at room temperature. After removing THF under vacuum,
dichloromethane and an aqueous ammonia solution were added. The
mixture was allowed to stir for 10 min in order to remove all the CuI
75.0 MHz): d=156.0 (OCAr), 143.9 (Cq of triazole), 140.1 (substituted
arene core C), 136.9 (arom Cq of the dendron), 127.4, 114.1 (unsubsti-
tuted arene C of the dendron), 123.6 (CH of triazole), 62.1 (CH2O), 44.0
(CH2CH2CH2Si), 41.8 (SiCH2N), 41.0 (benzylic quaternary C of the core
and dendron), 17.5 (CH2CH2CH2Si), 14.4 (CH2CH2CH2Si), À3.81 (Si-
A
N
8
7
trapped inside the dendrimer as Cu
(NH3)6+. The organic phase was
A
washed twice with water, dried with sodium sulphate, and filtered, and
the solvent was removed under vacuum. The product was washed with
pentane in order to remove the excess of alkyne and precipitated using
dichloromethane/pentane.
AHCTREUNG
AHCTREUNG
CH2O), 3.86 (s, 18H; SiCH2N), 2.71 (s, 54H; CH2Cl), 1.61 (s, 72H;
CH2CH2CH2Si), 1.07 (s, 72H; CH2CH2CH2Si), 0.55 (s, 72H;
Synthesis of 4: The 9-azido dendrimer 4 was synthesized from 3 (0.190 g,
0.130 mmol) by using the general procedure for azido dendrimers. The
product 4 was obtained as a colorless waxy product in 99% yield
CH2CH2CH2Si), 0.023 ppm (s, 216H; Si
(CH3)2CH2Cl); 13C NMR (CDCl3,
G
75.0 MHz): d=156.0 (OCAr), 143.9 (Cq of triazole), 140.1 (substituted
arene core C), 136.9 (arom Cq of the dendron), 127.4, 114.1 (unsubsti-
tuted arene C of the dendron), 123.6 (CH of triazole), 60.3 (SiCH2O),
43.1 (CH2CH2CH2Si), 41.8 (SiCH2N), 41.0 (benzylic quaternary C of the
core and dendron), 17.5 (CH2CH2CH2Si), 14.9 (CH2CH2CH2Si), À3.86
1
(0.196 g, 0.129 mmol). H NMR (CDCl3, 300 MHz): d=6.94 (s, 3H; arom
CH), 2.72 (s, 18H; CH2N3), 1.62 (s, 18H; SiCH2CH2CH2), 1.07 (s, 18H;
SiCH2CH2CH2), 0.58 (s, 18H; SiCH2CH2CH2), 0.042 ppm (Si(CH3)2);
T
13C NMR (CDCl3, 75.0 MHz): d=145.7 (arom Cq), 121.5 (arom CH), 43.9
(SiCH2CH2CH2), 41.9 (Cq), 41.1 (CH2N3), 17.7 (SiCH2CH2CH2), 15.0
(Si
(CH3)2CH2N), À4.41 ppm (Si
(CH3)2CH2N3); 29Si NMR (CDCl3,
59.6 MHz): d=3.33 (SiCH2N3), 2.80 ppm (SiCH2N); elemental analysis
calcd (%) for C315H570N108O9Si36: C 53.85, H 8.18; found: C 53.25, H 8.00;
IR: n˜ =2093 cmÀ1 (N3).
(SiCH2CH2CH2), À4.04 ppm (Si
N
G
C 49.86, H 8.57; found: C 49.31, H 8.30. IR: n˜ =2093 cmÀ1 (N3).
Synthesis of 9: The 81-allyl dendrimer 9 was synthesized from 8 (0.050 g,
0.00712 mmol) and 5 (0.230 g, 0.865 mmol) by using the general proce-
dure for “click” reactions. The product 9 was obtained as a colorless
waxy product in 67% yield (0.069 g, 0.00477 mmol). 1H NMR (CDCl3,
300 MHz): d=7.41 (s, 81H; triazole CH), 7.18, 6.90 (brs, 144H; p-C6H4),
5.52 (m, 81H; CH=CH2), 5.13 (s, 72H; CH2O), 4.96 (brs, 162H; CH=
CH2), 3.84 (s, 72H; SiCH2N), 2.41 (brs, 162H; CH2CH=CH2), 1.59 (s,
72H; CH2CH2CH2Si), 1.07 (s, 72H; CH2CH2CH2Si), 0.57 (s, 72H;
Synthesis of 5: The phenoltriallyl dendronic brick p-HOC4H4C(CH2CH=
CH2)3 (1.02 g, 4.46 mmol) and Cs2CO3 (2.18 g, 6.69 mmol) were intro-
duced in a Schlenck flask and acetone (30 mL) and propargyl bromide
(0.58 mL of a 80% solution in toluene, 5.36 mmol) were added. The mix-
ture was refluxed at 658C for 16 h. The solvent was removed under
vacuum, the crude product was dissolved with dichloromethane and
washed with water. The organic layer was dried with sodium sulfate, fil-
trated, and the solvent was removed under vacuum. The product was pu-
rified by silica column chromatography using pentane as eluent. A color-
less oil was obtained (1.178 g, 99% yield). 1H NMR (CDCl3, 300 MHz):
d=7.27, 6.98 (2d, J=11 Hz, 4H; p-C6H4), 5.60 (m, 3H; HC=CH2), 5.05
(m, 6H; HC=CH2), 4.69 (s, 2H; CH2CCH), 2.54 (s, 1H; CCH), 2.46 ppm
(d, J=10 Hz, 2H; CH2HC=CH2); 13C NMR (CDCl3, 75.0 MHz): d=154.2
(OCq), 137.3 (arom Cq), 133.2 (HC=CH2), 126.3, 113.0 (arom CH), 116.3
(HC=CH2), 77.5 (CH2CCH), 74.1 (CH2CCH), 54.4 (CH2CCH), 41.4
(C(CH2HC=CH2)3), 40.6 ppm (CH2HC=CH2); elemental analysis calcd
(%) for C19H22O: C 85.67, H 8.32; found: C 85.82, H 8.62.
CH2CH2CH2Si), 0.029 ppm (s, 54H; Si
(CH3)2); 13C NMR (CDCl3,
G
75.0 MHz): d=156.2 (OCAr), 143.8 (Cq of triazole), 138.2 (substituted
arene core C), 136.6 (arom Cq of the dendron), 134.6 (inner C=C), 127.7,
114.2 (unsubstituted arene C of the dendron), 123.8 (CH of triazole),
117.6 (outer C=C), 62.1 (CH2O), 43.8 (CH2CH2CH2Si), 42.7 (SiCH2N),
À
41.9 (CH2 CH=CH2), 41.1, 40.3 (benzylic Cq of the core and dendron),
17.5 (CH2CH2CH2Si), 14.7 (CH2CH2CH2Si), À3.99 ppm (Si
A
NMR (CDCl3, 59.6 MHz): d=2.86 ppm (Si(CH3)2CH2N); elemental anal-
C
ysis calcd (%) for C828H1164N108O36Si36: C 69.95, H 8.25; found: C 69.06, H
8.12.
Synthesis of 6: The 27-allyl dendrimer 6 was synthesized from 4 (0.127 g,
0.0835 mmol) and 5 (0.334 g, 1.25 mmol) by using the general procedure
for “click” reactions. The product 6 was obtained as a colorless waxy
product obtained in 92% yield (0.300 g, 0.0768 mmol). 1H NMR (CDCl3,
300 MHz): d=7.50 (s, 9H; triazole CH), 7.20, 6.91 (2d, J=11 Hz, 36H;
p-C6H4), 5.53 (m, 27H; CH=CH2), 5.10 (s, 18H; CH2O), 4.97 (m, 54H;
CH=CH2), 3.86 (s, 18H; SiCH2N), 2.40 (d, J=10 Hz, 54H; CH2CH=
CH2), 1.64 (s, 18H; CH2CH2CH2Si), 1.10 (s, 18H; CH2CH2CH2Si), 0.64 (s,
Synthesis of 10: The 81-chloro dendrimer 10 was synthesized from 9
(0.050 g, 0.00345 mmol) by using the general procedure for hydrosilyla-
tion reactions. The product 10 was obtained as a colorless waxy product
in 59% yield (0.047 g, 0.00203 mmol). 1H NMR (CDCl3, 300 MHz): d=
7.50 (s, 81H; triazole CH), 7.15, 6.89 (2d, J=11 Hz, 144H; p-C6H4), 5.11
(s, 72H; CH2O), 3.86 (s, 72H; SiCH2N), 2.71 (s, 162H; CH2Cl), 1.59 (s,
234H; CH2CH2CH2Si), 1.06 (s, 234H; CH2CH2CH2Si), 0.57 (s, 234H;
CH2CH2CH2Si), 0.041 ppm (s, 702H; Si
(CH3)2CH2Cl); 13C NMR (CDCl3,
H
18H; CH2CH2CH2Si), 0.039 ppm (s, 54H; Si
(CH3)2); 3C NMR (CDCl3,
N
75.0 MHz): d=156.1 (OCAr), 143.8 (Cq of triazole), 140.4 (substituted
arene core C), 137.1 (arom Cq of the dendron), 127.3, 114.1 (unsubsti-
tuted arene C of the dendron), 123.3 (CH of triazole), 62.0 (CH2O), 44.1
(CH2CH2CH2Si), 41.7 (SiCH2N), 41.0 (benzylic Cq of the core and den-
dron), 17.6 (CH2CH2CH2Si), 14.5 (CH2CH2CH2Si), À3.84 (Si-
75.0 MHz): d=156.2 (OCAr), 143.9 (Cq of triazole), 138.3 (substituted
arene core C), 136.9 (arom Cq of the dendron), 134.6 (inner C=C), 127.7,
114.2 (unsubstituted arene C of the dendron), 123.7 (CH of triazole),
117.5 (outer C=C), 62.0 (CH2O), 43.9 (CH2CH2CH2Si), 42.7 (SiCH2N),
41.9 (CH2-CH=CH2), 41.0, 40.4 (benzylic Cq of the core and dendron),
(CH3)2); 29Si
(CH3)2CH2N); MALDI-TOF: m/z
(CH3)2CH2N), À4.51 ppm (Si
E
17.7 (CH2CH2CH2Si), 14.9 (CH2CH2CH2Si), À3.83 ppm (Si
59.6 MHz): d=3.57 (SiCH2Cl), 2.83 ppm (SiCH2N).
NMR (CDCl3, 59.6 MHz): d=2.81 (Si
ACHTREUNG
calcd for C234H327N27O9Si9: 3915.05; found: 3937.42 [M+Na]+; elemental
analysis calcd (%) for C234H327N27O9Si9: C 71.79, H 8.42; found: C 71.03,
H 8.28.
Synthesis of 11: The 81-azido dendrimer 11 was synthesized from 10
(0.047 g, 0.00203 mmol) by using the general procedure for azido den-
drimers. The product 11 was obtained as a colorless waxy product with
91% yield (0.043 g, 0.00185 mmol). 1H NMR (CDCl3, 300 MHz): d=7.42
(s, 81H; triazole CH), 7.16, 6.90 (2d, J=11 Hz, 144H; p-C6H4), 5.11 (s,
72H; CH2O), 3.86 (s, 72H; SiCH2N), 2.71 (s, 162H; CH2Cl), 1.61 (s,
234H; CH2CH2CH2Si), 1.07 (s, 234H; CH2CH2CH2Si), 0.55 (s, 234H;
Synthesis of 7: The 27-chloro dendrimer
7 was synthesized from 6
(0.180 g, 0.0460 mmol) by using the general procedure for hydrosilylation
reactions. The product 7 was obtained as a colorless waxy product in
71% yield (0.192 g, 0.0327 mmol). 1H NMR (CDCl3, 300 MHz): d=7.51
(s, 9H; triazole CH), 7.15, 6.89 (2d, J=10 Hz, 36H; p-C6H4), 5.11 (s,
CH2CH2CH2Si), 0.023 ppm (s, 702H; Si
(CH3)2CH2Cl); 13C NMR (CDCl3,
G
Chem. Eur. J. 2008, 14, 50 – 64
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www.chemeurj.org
61

D. Astruc et al.
75.0 MHz): d=156.1 (OCAr), 143.7 (Cq of triazole), 140.2 (substituted
arene core C), 136.7 (arom Cq of the dendron), 127.3, 114.2 (unsubsti-
tuted arene C of the dendron), 123.6 (CH of triazole), 62.0 (CH2O), 43.1
(CH2CH2CH2Si), 41.6 (SiCH2N), 41.1 (benzylic quaternary C of the core
and dendron), 17.5 (CH2CH2CH2Si), 14.6 (CH2CH2CH2Si), À3.90 (Si-
C
N
(CH3)2CH2N), À4.39 ppm (Si
N
N
59.6 MHz): d=3.33 (SiCH2N3), 2.81 ppm (SiCH2N); elemental analysis
calcd (%) for C1071H1893N351O36Si117: C 54.62, H 8.10; found: C 53.67, H
8.01; IR: n˜ =2093 cmÀ1 (N3).
Synthesis of 12-G0: The 9-ferrocenyltriazole dendrimer 12-G0 was synthe-
sized from
4 (0.100 g, 0.0659 mmol) and ethynylferrocene (0.187 g,
0.890 mmol) by using the general procedure for “click” reactions. The
product 12-G0 was obtained as an orange powder in 94% yield (0.211 g,
1
0.0619 mmol). H NMR (CDCl3, 300 MHz): d=7.38 (s, 9H; triazole CH),
6.91 (s, 3H; Ar core), 4.70, 4.27, 4.04 (s, 81H; Cp), 3.83 (s, 18H;
SiCH2N), 1.59 (s, 18H; CH2CH2CH2Si), 1.06 (s, 18H; CH2CH2CH2Si),
C
N
0.59 (s, 18H; CH2CH2CH2Si), 0.045 ppm (s, 54H; Si
A
(CDCl3, 75.0 MHz): d=146.1 (Cq of triazole), 120.0 (CH of triazole), 75.7
(Cq of Cp), 69.5, 68.6, 66.5 (CH of Cp), 43.8 (CH2CH2CH2Si), 41.7 (ben-
zylic Cq of the core), 40.8 (CH2N), 17.6 (CH2CH2CH2Si), 14.8
G
(CH2CH2CH2Si), À3.74 ppm (Si
N
ACHTREUNG
G
C171H219N27Si9Fe9: 3408.12; found: 3408.18 [M]+; elemental analysis calcd
(%) for C171H219N27Si9Fe9: C 60.26, H 6.48; found: C 59.47, H 6.67; poly-
dispersity obtained by SEC <1.02.
Synthesis of 13-G1a: The 27-ferrocenyltriazole dendrimer 13-G1a was syn-
thesized from 8 (0.040 g, 0.00569 mmol) and ethynylferrocene (0.048 g,
0.230 mmol) by using the general procedure for “click” reactions. The
product 13-G1a was obtained as an orange waxy product in 71% yield
(0.051 g, 0.00404 mmol). 1H NMR (CDCl3, 300 MHz): d=7.50 (s, 9H;
inner triazole CH), 7.29 (s, 27H; outer triazole CH), 7.14, 6.87 (brs, 36H;
p-C6H4), 5.08 (s, 18H; CH2O), 4.70, 4.26, 4.03 (s, 243H; Cp), 3.82 (s,
54H; SiCH2N), 1.56 (s, 72H; CH2CH2CH2Si), 1.04 (s, 72H;
CH2CH2CH2Si), 0.57 (s, 18H; CH2CH2CH2Si), 0.053 ppm (s, 54H; Si-
G
A
dendron),
40.8
30.3
outer triazole), 143.7 (Cq of inner triazole), 139.8 (aromatic Cq of the
dendron), 127.4, 114.2 (unsubstituted arene C of the dendron), 123.7 (CH
of inner triazole), 119.7 (CH of outer triazole), 75.7 (Cq of Cp), 69.5,
68.6, 66.5 (CH of Cp), 62.1 (CH2O), 43.1 (CH2CH2CH2Si), 41.8, 40.9
(benzylic quaternary C of the core and dendron), 40.8 (SiCH2N), 17.4
ACHTREUNG
ACHTREUNG
(CH2CH2CH2Si), 14.8 (CH2CH2CH2Si), À3.86 ppm (Si
ACHTREUNG
(CDCl3, 59.6 MHz): d=2.97 ppm (Si
tained by SEC <1.02.
ACHTREUNG
Synthesis of 14-G2a: The 81-ferrocenyltriazole dendrimer 14-G2a was syn-
thesized from 11 (0.030 g, 0.00128 mmol) and ethynylferrocene (0.033 g,
0.156 mmol) by using the general procedure for “click” reactions. The
product 14-G2a was obtained as an orange waxy product in 56% yield
(0.029 g, 0.000715 mmol). 1H NMR (CDCl3, 300 MHz): d=7.59 (s, 36H;
inner triazole CH), 7.29 (s, 81H; outer triazole CH), 7.13, 6.89 (brs,
144H; p-C6H4), 5.12 (s, 72H; CH2O), 4.69, 4.26, 4.03 (s, 729H; Cp), 3.82
(s, 162H; SiCH2N), 1.57 (s, 234H; CH2CH2CH2Si), 1.04 (s, 234H;
CH2CH2CH2Si), 0.57 (s, 234H; CH2CH2CH2Si), 0.056 ppm (s, 54H; Si-
(CH3)2). 13C NMR (CDCl3, 75.0 MHz): d=156.1 (OCAr), 146.5 (Cq of
ACHTREUNG
outer triazole), 143.1 (Cq of inner triazole), 138.3 (aromatic Cq of the
dendron), 127.4, 114.2 (unsubstituted arene C of the dendron), 123.7 (CH
of inner triazole), 120.0 (CH of outer triazole), 76.1 (Cq of Cp), 69.9,
68.9, 66.7 (CH of Cp), 62.1 (CH2O), 43.1 (CH2CH2CH2Si), 40.4 (benzylic
Cq of the dendron), 40.6 (SiCH2N), 17.4 (CH2CH2CH2Si), 14.8
À4.4 ppm (Si
ACHTREUNG
0.54 ppm
C612H840O9N81Si36Fe27: C 61.28, H 7.06; found: C 61.70, H 7.16.
Synthesis of 19: The phenoltriallyl dendron 3 (0.194 g, 0.851 mmol), dry
diethyl ether, tripropylsilane (0.808 g, 5.11 mmol, 2 equiv per branch) and
the Kartsted catalyst (0.1%) were successively introduced into
(CH2CH2CH2Si), À3.82 ppm (Si
C
a
ACHTREUNG
Schlenck flask under a nitrogen atmosphere. The reaction solution was
stirred at 258C for 15 days (several additions of catalyst were necessary
to complete the reaction). The solvent was removed under vacuum, and
the product was purified by silica chromatography by using pentane/di-
ethyl ether (95.5) as eluent. The product was obtained as a colorless oil
(0.238 g, 0.338 mmol) with 40% yield. 1H NMR (CDCl3, 300 MHz): d=
7.13, 6.76 (d, J=11 Hz, 4H; aromatic CH), 4.90 (s, 1H; OH), 1.59 (m,
6H; CqCH2), 1.29 (m, 18H; SiCH2CH2CH3), 1.03 (m, 6H;
CH2CH2CH2Si), 0.93 (m, 27H; SiCH2CH2CH3), 0.48 ppm (m, 18H;
hydrodynamic diameter obtained by DLS: 12Æ0.5 nm.
Synthesis of 15: The 27-azido dendrimer 15 was synthesized from 27-
chloro-dendrimer (0.100 g, 0.0163 mmol) by using the general procedure
for azido dendrimers. The product was obtained as a colorless waxy prod-
uct in 99% yield (0.101 g, 0.0161 mmol). 1H NMR (CDCl3, 300 MHz):
d=7.16, 6.89 (2d, J=11 Hz, 36H; p-C6H4), 3.53 (s, 18H; SiCH2O), 2.71
(s, 54H; CH2N3), 1.62 (s, 72H; CH2CH2CH2Si), 1.09 (s, 72H;
CH2CH2CH2Si), 0.55 (s, 72H; CH2CH2CH2Si), 0.0061 (s, 54H; Si-
62
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 50 – 64

Dendrimers
FULL PAPER
CH2SiCH2CH2CH3); 13C NMR (CDCl3, 75.0 MHz): 152.8 (C-OH), 140.3
(aromatic Cq), 127.6, 114.5 (aromatic CH), 43.2 (Cq), 42.4
(CH2CH2CH2Si), 18.6 (SiCH2CH2CH3), 17.8 (SiCH2CH2CH2), 17.4
(SiCH2CH2CH3), 15.4 (SiCH2CH2CH3), 13.5 ppm (SiCH2CH2CH2); 29Si
NMR (CDCl3, 59.62 MHz): d=2.16 ppm (SiCH2CH2CH3); elemental
analysis calcd (%) for C43H86OSi3: C 73.42, H 12.32; found: C 73.50, H
12.02.
chloroform (1.2 mL) was added. Chloroform (0.8 mL) and methanol
(2 mL) were added in order to obtain a solution 8.82”10À4 m in Pd, 2:1
(CHCl3/MeOH). The solution was stirred for 5 min, NaBH4 (2 mg,
5.87x10À2 mmol, 10 equiv per Pd) was added, and the yellow solution
turned to golden brown indicating the nanoparticle formation.
Method 2: A solution of dendrimer 12-G0 (2.0 mg, 5.87x10À4 mmol) in
chloroform (2 mL) was introduced into a Schlenk flask under nitrogen.
of a 4.5x10À3 m A solution of Pd(OAc)2 (1.2 mg, 5.28x10À3 mmol, 1 equiv
N
Synthesis of 20: The dendron 19 (0.230 g, 0.327 mmol) and Cs2CO3
(0.160 g, 0.490 mmol) were introduced in a Schlenck flask and acetone
(30 mL) and propargyl bromide (0.05 mL of a 80% solution in toluene,
0.399 mmol) were added. The mixture was refluxed at 658C for 16 h. The
solvent was removed under vacuum, the crude product was dissolved
with dichloromethane and washed with water. The organic layer was
dried with sodium sulfate and filtered over paper; the solvent was re-
moved under vacuum. The product was purified by silica column chroma-
tography with pentane as eluent. A colorless oil was obtained (0.237 g,
per triazole) in chloroform (1.2 mL) was added. Chloroform (0.8 mL)
and methanol (2 mL) were added in order to obtain a solution 8.82”
10À4 m in Pd, 2:1 (CHCl3/MeOH). The solution was stirred for 16 h and
turned from yellow to golden brown indicating the nanoparticle forma-
tion.
Hydrogenation reactions: The nanoparticles were freshly prepared in a
Schlenck flask in order to obtain a solution 8.82”10À4 m (in Pd), in
CHCl3/MeOH (2:1), and 1000 equiv of the substrate was added. The
Schlenck flask was filled with H2 (1 atm) and the solution was allowed to
stir at 258C. For re-use of the catalyst, the substrate was added to the re-
action solution until the catalyst was no longer active. Calculation of the
turn over frequency (TOF) was carried out by using several samples of
the solution that were extracted at different reaction times and analyzed.
Calculation of the turn over number (TON) was carried out by using the
sum of substrate that reacted in all the catalytic cycles, after analyzing
the final reaction solution.
1
98% yield). H NMR (CDCl3, 300 MHz): d=7.20, 6.90 (d, J=10 Hz, 4H;
aromatic CH), 4.67 (s, 2H; CH2CCH), 2.49 (CH2CCH), 1.61 (m, 6H;
CqCH2), 1.26 (m, 18H; SiCH2CH2CH3), 1.04 (m, 6H; CH2CH2CH2Si),
0.93 (m, 27H; SiCH2CH2CH3), 0.45 ppm (m, 18H; CH2SiCH2CH2CH3);
13
À
C NMR (CDCl3, 75.0 MHz): d=155.0 (arom C O), 141.0 (arom Cq),
127.3, 113.9 (aromatic CH), 78.8 (CH2CCH), 75.0 (CH2CCH), 55.7
(CH2CCH), 43.1 (Cq), 42.3 (CH2CH2CH2Si), 18.5 (SiCH2CH2CH3), 17.8
(SiCH2CH2CH2), 17.3 (SiCH2CH2CH3), 15.3 (SiCH2CH2CH3), 13.4 ppm
(SiCH2CH2CH2);
29Si
NMR
(CDCl3,
59.62 MHz):
d=4.34
(SiCH2CH2CH3); elemental analysis calcd (%) for C46H88OSi3: C 74.52,
H 11.96; found: C 74.67, H 11.82.
Synthesis of 21-G1c: The dendrimer 21-G1c was synthesized from 16
(0.030 g, 0.0048 mmol) and the dendron 20 (0.141 g, 0.190 mmol) by using
the general procedure for “click” reactions. The product 21-G1c was puri-
fied by precipitation in dichloromethane/methanol, and was obtained as
a colorless waxy product in 97% yield (0.123 g, 0.0046 mmol). 1H NMR
(CDCl3, 300 MHz): d=7.46 (s, 27H; triazole CH), 7.18, 6.89 (brs, 144H;
arom CH), 5.13 (s, 54H; triazole-CH2O), 3.86 (s, 54H; SiCH2N), 3.54 (s,
18H; inner-CH2O), 1.59 (m, 234H; CH2CH2CH2Si), 1.26 (m, 486H;
SiCH2CH2CH3), 1.04 (m, 234H; CH2CH2CH2Si), 0.91 (m, 729H;
SiCH2CH2CH3), 0.65 (m, 234H; CH2CH2CH2Si), 0.45 (m, 486H;
Acknowledgements
We are grateful to Dr. Eric Cloutet (LCPO, Bordeaux) for helpful assis-
tance in the SEC and DLS experiments, and to Fundażo para a CiÞncia
e a Tecnologia (FCT), Portugal (Ph.D. grant to C.O.), the Institut Univer-
sitaire de France (IUF, DA), the CNRS, and the UniversitØ Bordeaux I
for financial support.
SiCH2CH2CH3), 0.070 ppm (s, 216H; Si
(CH3)2); 13C NMR (CDCl3,
G
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1990, 102, 119; Angew. Chem. Int. Ed. Engl. 1990, 29, 138; b) G. R.
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75.0 MHz): d=156.2 (OCAr), 141.1 (Cq of triazole), 139.8 (arom Cq of the
dendron), 127.8, 114.3 (arom CH of the outer dendron), 125.9, 113.8
(arom CH of the inner dendrons), 123.7 (CH of triazole), 75.5 (triazole-
CHO), 62.6 (inner CH2O), 43.6 (CH2CH2CH2Si), 42.8 (benzylic quaterna-
ry
(SiCH2CH2CH3), 17.4 (CH2CH2CH2Si), 14.2 (CH2CH2CH2Si), 13.9
(CH3)2); elemental analysis calcd (%) for
C of the dendrons), 38.5 (SiCH2N), 19.0 (SiCH2CH2CH3), 17.8
(SiCH2CH2CH3), À3.58 ppm (Si
ACHTREUNG
C1530H2919O36Si117N81: C 69.83, H 11.18; found: C 69.11, H. 11.16.
Synthesis of 22-G1d: The dendrimer 22-G1d was synthesized from 16
(0.050 g, 0.0079 mmol) and phenylacetylene (0.033 g, 0.321 mmol) by
using the general procedure for “Click” reactions. The product 22-G1d
[3] a) J. F. G. A. Jansen, E. M. M. de Brabander-van den Berg, E. W.
Meijer, Science, 1994, 266, 1226–1229; b) V. Percec, G. Johansson,
G. Ungar, J. P. Zhou, J. Am. Chem. Soc. 1996, 118, 9855–9866;
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2000, 39, 1285–1288.
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K. Kono, K. Maruyama, T. Takagishi, Bioconjugate Chem. 2000, 11,
910–917; K. Sadler, J. P. Tam, J. Biotechnol. 2002, 90, 195–229; M. J.
Cloninger, Curr. Opin. Chem. Biol. 2002, 6, 742–748; S. E. Stiriba,
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was obtained as
a colorless waxy product in 95% yield (0.068 g,
0.0075 mmol). 1H NMR (CDCl3, 300 MHz): d=7.79, 7.37 (m, 135H;
arom CH of phenyl), 7.53 (s, 27H; triazole CH), 7.12, 6.83 (brs, 36H;
arom CH), 3.85 (s, 54H; SiCH2N), 3.50 (s, 18H; inner-CH2O), 1.58 (m,
72H; CH2CH2CH2Si), 1.08 (m, 72H; CH2CH2CH2Si), 0.56 (m, 72H;
CH2CH2CH2Si), 0.050 ppm (s, 216H; Si
(CH3)2); 13C NMR (CDCl3,
A
75.0 MHz): d=159.1 (Cq of phenyl), 135.7, 130.8, 125.5 (CH of phenyl),
151.5 (OCAr), 147.3 (Cq of triazole), 139.8 (aromatic Cq of the dendron),
127.8, 113.5 (arom CH of dendron), 125.5 (CH of triazole), 60.3 (CH2O),
43.9 (CH2CH2CH2Si), 42.9 (benzylic quaternary C of the dendrons), 34.2
(SiCH2N), 17.4 (CH2CH2CH2Si), 14.2 (CH2CH2CH2Si), À3.88 ppm (Si-
A
H
tal analysis calcd (%) forC504H705N81Si36O9: C 66.86, H 7.85; found: C
66.17, H 7.81.
General procedure for the preparation of the PdNPꢀs (the procedure is
described using preparation of DSN-12-G0 as an example)
Method 1: A solution of dendrimer 12-G0 (2.0 mg, 5.87”10À4 mmol) in
chloroform (2 mL) was placed in a Schlenk flask under nitrogen. A solu-
tion of Pd
(OAc)2 (1.2 mg, 5.28”10À3 mmol, 1 equiv per triazole) in
A
Chem. Eur. J. 2008, 14, 50 – 64
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63

D. Astruc et al.
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Received: September 6, 2007
Published online: November 30, 2007
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