The effect of ligand denticity in size-selective synthesis of calix[n]arenestabilized gold nanoparticles: A multitechnique approach

Article abstract of DOI:10.1002/chem.201001039

A series of gold nanoparticles (AuNPs) stabilized by monodentate, bidentate, and tridentate thiolate calix[n]arene ligands 1-3 was prepared by using the Brust-Schiffrin two-phase direct synthesis and characterized with NMR spectroscopy, elemental analysis

Full text of DOI:10.1002/chem.201001039

FULL PAPER
DOI: 10.1002/chem.201001039
The Effect of Ligand Denticity in Size-Selective Synthesis of Calix[n]arene-
Stabilized Gold Nanoparticles: A Multitechnique Approach
Luca Pescatori,[a] Alice Boccia,[b] Flavio Ciesa,[a] Francesca Rossi,[c] Vincenzo Grillo,[c, d]
Arturo Arduini,[a] Andrea Pochini,[a] Robertino Zanoni,*[b] and Andrea Secchi*[a]
Abstract: A series of gold nanoparti-
cles (AuNPs) stabilized by monoden-
tate, bidentate, and tridentate thiolate
calix[n]arene ligands 1–3 was prepared
by using the Brust–Schiffrin two-phase
direct synthesis and characterized with
NMR spectroscopy, elemental analysis,
troscopy (XPS). The experimental data
show that the particular multidentate
structure of calix[n]arene derivatives 2
and 3 introduces a control element in
the preparation of the gold nanoparti-
cles that allows, in the particular exper-
imental conditions here reported, to
obtain very small (ꢀ1 nm) AuNPs.
These are the first experimental find-
ings that identify a role of ligand “den-
ticity” in the determination of the nu-
clearity of nanoparticles.
Keywords: calixarenes · denticity ·
gold · ligand effects · nanoparticles
transmission
electron
microscopy
(TEM), and X-ray photoelectron spec-
Introduction
those characterized by a core of gold atoms and stabilized
with a layer of thiolate ligands have attracted the largest in-
terest of the scientific community because of their facile
preparation, stability, and solubility in both aprotic and
protic solvents.[4]
Ligand-stabilized nanoparticles (NPs)[1] represent an emerg-
ing class of organic–inorganic hybrid materials.[2] Unlike col-
loids of noble-metal nanoparticles,[1d,3] these species are con-
stituted by a discrete aggregate of metal atoms stabilized by
a shell of organic molecules (arranged like a monolayer
around the metal surface) that maintain their stability in so-
lution and prevent aggregation phenomena. Among the ple-
thora of ligand-stabilized nanoparticles synthesized so far,
A very attractive topological property of ligand-stabilized
NPs is the possibility to anchor onto the metallic core a dis-
crete number of suitable receptors in a radial tridimensional
arrangement.[5] In this context, we have shown that AuNPs
stabilized with thiolate calix[4]arene ligands can be success-
fully employed as multivalent hosts for the recognition of
organic salts both in organic[6] and aqueous media.[7] Anoth-
er aspect of potentially great importance is associated with
quantum size effects that arise from the confinement of
electrons in very small objects (ꢀ1 nm). Despite the docu-
mented potentiality of these systems,[8] the development of
reliable direct synthetic protocols for the synthesis of stable
and lipophilic AuNPs characterized both by very small core
diameter and potential recognition properties of the organic
shell still remains a challenging task.[9] The combination of
these features could indeed lead to the manufacture of
nanoscale devices with potential applications as sensors,
switches, and new materials that have tunable proper-
ties.[4,5,10]
[a] Dr. L. Pescatori, Dr. F. Ciesa, Prof. A. Arduini, Prof. A. Pochini,
Dr. A. Secchi
Dipartimento di Chimica Organica e Industriale
and Unitꢀ INSTM Sez. 4 – UdR Parma, Universitꢀ di Parma
viale Usberti 17/a, 43124 Parma (Italy)
Fax : (+39)0521-905472
E-mail: andrea.secchi@unipr.it
[b] A. Boccia, Prof. R. Zanoni
Dipartimento di Chimica
Universitꢀ degli Studi di Roma ꢁLa Sapienza’
p.le Aldo Moro 5, 00185 Roma (Italy)
Fax : (+39)06490324
E-mail: robertino.zanoni@uniroma1.it
[c] Dr. F. Rossi, Dr. V. Grillo
IMEM-CNR, viale Usberti 37/A, 43124 Parma (Italy)
The synthesis of lipophilic AuNPs is usually obtained
through the reduction of aurate salts in the presence of a
thiolate ligand. The formation of the stabilized nanoparticles
could be considered the result of two competitive processes:
growth of the metallic core and gold surface passivation due
[d] Dr. V. Grillo
Centro S3, CNR-Istituto Nanoscienze
via Campi 213/A, 41125 Modena (Italy)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001039.
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to the presence of the thiols in solution.[11] Murray et al.
have shown that the core size of n-alkylthiol-stabilized
AuNPs is strongly affected by the concentration of the reac-
tants, temperature, and addition rate of the reductant.[12]
Tsukuda et al. later proposed that the isolated nanoparticles
correspond to kinetically trapped intermediates of the grow-
ing gold core.[13] Nevertheless, the effect of the ligand “den-
ticity” on the growth of the nanoparticles cores has not been
systematically investigated yet. A perusal of the literature
data only offers evidence that multidentate thiolate ligands
can promote the preparation of more stable AuNPs,[14] but it
could be also envisioned that multidentate ligands may also
exert a relevant kinetic effect on the growth of the gold
core.
In this paper we report a one-pot direct synthesis and de-
tailed characterization (X-ray photoelectron spectroscopy
(XPS), NMR spectroscopy, TEM, and high-angle annular
dark-field scanning transmission electron microscopy
(HAADF-STEM)) of a series of AuNPs stabilized with a
series of multipodand thiolate calix[n]arene ligands (1–3)
that bear one, two, or three convergent w-undecanthiol
chains on their lower rim, respectively. This study shows
that the “ligand denticity” represents a size-control element
in the synthesis of these new NPs.
Scheme 1. Reactions and conditions: i) CH3COSH, AIBN, toluene,
reflux, 5 h; ii) K2CO3, acetone, reflux, 48 h; iii) K2CO3, KI, CH3CN,
reflux, 96 h; iv) 10% HCl, THF/water, reflux, 48 h.
such functions yield RS–Au interactions more stable than
those formed by thioether functions (R2S–Au).[15] The calix-
AHCTUNGTREG[NNUN n]arene macrocycle is known for its large synthetic versatil-
ity,[16] and both rims of these molecular platforms might in
principle be used for the insertion of convergent thiol func-
tionalities.[17] However, the lower rim functionalization
offers several advantages. First, the calixarene aromatic cavi-
ties, which represent potential recognition units, are exposed
to the bulk and not toward the surface. Second, the position
of the recognition units with respect to the gold surface can
be modulated by using thiolate alkyl chains of variable
length.
The thiolate calix[4]arene ligands 1 and 2 that bear one or
two convergent w-undecanthiol chains,[18] respectively, were
synthesized by exploiting the different acidity of the calix[4]-
arene phenolic OH groups, which allows the regiochemical
Results and Discussion
Synthesis of the thiolate calixar-
ene ligands: The thiolate calix-
ACHTUNGTRENNUNG[n]arene ligands (1–3) were syn-
thesized according to Schemes 1
and 2 and fully characterized.
The use of thiol groups (SH) as
the macrocycle anchoring point
on the gold surface was dictated
Scheme 2. Reactions and conditions: i) K2CO3, CH3CN, reflux, 96 h; ii) CH3COSH, AIBN, toluene, reflux, 5 h;
by previous observations that iii) 10% HCl, THF/water, reflux, 48 h.
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Calix[n]arene-Stabilized Au Nanoparticles
FULL PAPER
insertion of a variable number (up to 4) of alkyl chains.[19]
This was accomplished by varying the nature of the base
employed during the alkylation process (K2CO3) and by
choosing the appropriate molar ratio between the calix[4]ar-
ene and the alkylating agent. In 1 the sulfur anchoring
group was inserted in the alkyl chain before the alkylation
of the macrocycle lower rim (see Scheme 1). In particular,
undec-10-enyl 4-methylbenzenesulfonate (4) was reacted
with thioacetic acid in presence of the radical initiator azobi-
sisobutyronitrile (AIBN). The resulting S-11-(tosyloxy)un-
decyl ethanethioate (5) was used for the regioselective alky-
lation of the lower rim of calix[4]arene 6. Using 0.8 equiv of
both the base (K2CO3) and of tosylate 5 in dry acetone
heated to reflux, the monoalkylated calix[4]arene 7 was ob-
tained in 80% yield. The final acid-catalyzed hydrolysis of
the thioester group afforded the monodentate ligand 1 in
72% overall yield. The synthesis of the bidentate ligand 2
was accomplished by reacting calix[4]arene 6 with 2 equiv of
undec-10-enyl 4-methylbenzenesulfonate (4) in acetonitrile
using K2CO3 as the base. The thioacetyl group was then in-
serted in the terminal position of the two w-undecenyl
chains present on the lower rim of 8 through an anti-Marko-
vinkov addition of thioacetic acid. As seen for 1, the acid-
catalyzed hydrolysis of the resulting thioacetyl groups af-
forded the target bidentate ligand 2 in 44% overall yield.
The “tridentate” thiolate calix[6]arene ligand 3 was pre-
pared in 37% overall yield by using the same synthetic strat-
egy previously adopted for 2, but with the trimethoxy cal-
ix[6]arene 10, which has the proper symmetry, as starting
compound (see Scheme 2).
protons that are not magnetically equivalent. The three AB
systems in 1:2:1 integral ratios that arise from the bridging
methylene groups finally confirmed that 1 also adopts a
cone conformation on the NMR spectroscopic timescale. As
seen for 2, the methylene group in a position a to SH gives
rise to a quartet signal that integrates two protons centered
1
at d=2.51 ppm. The H NMR spectrum of 3 taken in CDCl3
(see the Supporting Information) shows two singlets, each
integrating six protons, at d=7.28 and 6.64 ppm for the pro-
tons of the differently substituted phenolic rings. The pres-
ence of an AB system of two doublets for the axial and
equatorial protons of the methylene bridging units at d=
4.58 and 3.40 ppm, respectively, together with a broad sin-
glet at dꢀ2.2 ppm suggested that in CDCl3 the calix[6]arene
macrocycle predominantly assumes a flattened cone confor-
mation on the NMR spectroscopic timescale. As seen in
other studies,[20] in this conformation the macrocycle pos-
sesses three methylated phenolic rings bent and oriented
inward with the methoxy groups towards the cavity, whereas
those bearing the w-undecanthiol chains are almost parallel
(see Scheme 2).
Synthesis of the calixarene-stabilized AuNPs: A series of
calixarene-stabilized AuNPs was prepared by adopting the
two-phase method reported by Brust–Schiffrin.[21] To dis-
close the role of the ligand denticity on the size dispersion
of the resulting coated NPs, we took advantage of the semi-
nal paper published by Murray for n-dodecanthiol-stabilized
AuNPs (see the Experimental Section).[12c] In particular,
three sets of nanoparticles were prepared for each calixar-
ene ligand (1–3) by using different S/Au ratios (3:1, 1:3, and
1:6; see Table 1) in the reaction mixtures. In these ratios, S
1
The H NMR spectrum of 2, taken in CDCl3 (see the Sup-
porting Information), shows a singlet for the two OH pro-
tons at d=8.29 ppm. The methylene bridges of the macrocy-
cle give rise to a unique AB system of two doublets (d=
4.40 and 3.45 ppm) for the axial and equatorial protons, re-
spectively. This pattern of signals is typical for a calix[4]ar-
ene macrocycle that adopts a cone conformation in solution
and confirms that the two w-undecanthiol chains are orient-
ed on the same side of the macrocycle. The methylene pro-
tons in a position a to the SH group are visible as a quartet
that integrates four protons at d=2.59 ppm. The signal of
the SH protons is totally hidden by the strong absorption of
the aliphatic protons of the two alkyl chains. To verify the
presence of this resonance, the solution of CDCl3 and 2 was
submitted to a 2D COSY NMR spectroscopy experiment.
In the corresponding 2D spectrum, two relevant symmetric
cross-peaks are indeed present; they show the coupling be-
tween the quartet assigned to the methylene protons adja-
cent to the SH group and the resonance of the latter that is
found at dꢀ1.4 ppm (see the Supporting Information).
The 1H NMR spectrum in CDCl3 of monodentate 1 is
characterized by a complicated pattern of signals due the
absence of symmetry elements in the structure of this ligand
(see the Supporting Information). Nevertheless, the mono-
functionalization of the lower rim was confirmed by the
presence in the spectrum of two singlets at d=9.74 and
9.43 ppm in an integral ratio of 1:2 assigned to three OH
Table 1. Composition and mean core size of the calixarene-stabilized
AuNPs.
Entry NP
Ligand/
S/Au
(theor)[a]
S/Au
A
Organic
[%][c]
dTEM
À
designation AuCl4
G
(exptl)[b] fraction
[nm][d]
molar
ratio
1
2
3
4
5
6
7
8
9
Cx4S1/Au
(1s)
Cx4S1/Au
(1m)
Cx4S1/Au
(1l)
Cx4S2/Au
(2s)
Cx4S2/Au
(2m)
Cx4S2/Au
(2l)
Cx6S3/Au
(3s)
Cx6S3/Au
(3m)
3:1
3:1
1:3
1:6
3:1
1:3
1:6
3:1
1:3
1:6
0.45
0.18
0.16
0.88
0.37
0.2
60
37
25
64
46
28
77
50
32
ACHTUNGTRENNUNG
1:3
ACHTUNGTRENNUNG
1:6
ACHTUNGTRENNUNG
1.5:1
1:6
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
1:12
1:1
ACHTUNGTRENNUNG
1.09
0.38
0.18
ACHTUNGTRENNUNG
1:9
ACHTUNGTRENNUNG
Cx6S3/Au
(3l)
1:18
ACHTUNGTRENNUNG
[a] Equivalents of alkylthiol chains per aurate. [b] Determined through
elemental analysis. [c] Determined through TGA or elemental analysis.
[d] Determined through TEM measurements (meanÆstandard devia-
tion).
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A. Secchi, R. Zanoni et al.
expresses the equivalents of thiolate alkyl chains present in
solution regardless of the nature of the multidentate ligand.
In this way, the outcomes of the synthesis carried out using
the same S/Au ratio become independent of the absolute
amount of the calixarene employed (see Table 1) and, most
importantly, are directly comparable with the literature data
relative to monodentate thiolate ligands.
After precipitation from their reaction mixture, all the
batches of the isolated NPs were contaminated with both
tetraoctylammonium bromide (TOABr), used during the re-
action as phase-transfer catalyst, and unreacted calixarene
ligand. To have reproducible results, a purification proce-
dure was devised. The black powder of the nanoparticles ob-
tained after the precipitation with ethanol was suspended in
a 5:1 mixture of ethanol and dichloromethane, sonicated,
and centrifuged at 14000 rpm for at least 20 min. This purifi-
cation cycle was repeated until no free calixarene ligand was
detected (by TLC) in the supernatant solution. The effec-
tiveness of this purification method has been verified
through elemental analysis and 1H NMR spectroscopic
measurements (vide infra). The small nanoparticles pre-
pared with a large excess amount of the calixarene ligand
(S/Au=3:1) suffered, as expected by an important contami-
nation of the free ligand. Their purification required a fur-
ther chromatographic step with a 9:1 mixture of dichlorome-
thane/methanol as eluent.
Characterization of AuNPs: The core-size distribution of
the nine sets of synthesized AuNPs was determined through
TEM measurements (see Table 1). Each set of nanoparticles
was designated as CxmSn/Au (nc), in which m (4 or 6) and n
(1 to 3) identify the calixarene macrocycle and its “dentici-
ty,” respectively. To distinguish among nanoparticles stabi-
lized with the same ligand but prepared using different S/Au
ratios, the index c (s, m, or l) was used. This index would
represent the relative core size (s=small, m=medium, or
l=large) of the nanoparticles with respect to the 3:1, 1:3,
and 1:6 S/Au molar ratio used for their syntheses (vide
supra). The analysis of the TEM images showed that for
each ligand used, regardless of its denticity, the mean core
size (dTEM) of the corresponding nanoparticles was inversely
proportional to the S/Au ratio employed during the synthe-
sis (see entries 1–3, 4–6, and 7–9 in Table 1). This was an ex-
pected result since the effect of the ligand concentration on
the size of the resulting nanoparticles has been already veri-
fied.[12]
More interesting results were obtained by comparing the
TEM images taken from the three sets of nanoparticles 1s,
2s, and 3s, which were prepared using an identical S/Au
ratio of 3:1, but with the monodentate 1, the bidentate 2,
and the tridentate 3 calixarene as protecting thiolate ligand,
respectively (see Figure 1a–c). The core-size distribution dia-
grams that correspond to each TEM image clearly show that
the nanoparticles present in samples 2s and 3s are endowed
with a mean core size (dTEM ꢀ1 nm; see Figure 1b and c)
smaller than those present in 1s (dTEM ꢀ1.5 nm; see Fig-
ure 1a).
Figure 1. TEM images and core-size distribution diagrams of calixarene-
stabilized AuNPs prepared using an S/Au ratio of 3:1. a) 1s (dTEM =(1.5Æ
0.3) nm, N=550); b) 2s (dTEM =(1.0Æ0.3) nm, N=800); and c) 3s
(dTEM =(1.0Æ0.2) nm, N=690). d) HAADF-STEM image and core-size
distribution diagram of 2s (dSTEM =(1.2Æ0.3) nm, N=270). The image
was acquired at 1.5ꢃ magnification, with an image size of 1024ꢃ1024
pixels (pixel dimension=0.86 ꢄ).
We propose that the observed reduction in the core size
of the nanoparticles stabilized with the multidentate ligands
2 and 3 is due to a convergent arrangement of the (two or
three for 2 or 3, respectively) thiolate chains to the growing
À
gold core, after the formation of the first Au S bond, which
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Calix[n]arene-Stabilized Au Nanoparticles
FULL PAPER
increases the passivation rate.[22] In fact, a constant S/Au sto-
ichiometric ratio of 3:1 was chosen in the synthesis of the
full series of 1s, 2s, 3s to keep the number of thiol chains
constant by adding the proper amount of the ligand (see the
(see Figure 2d) with those, somewhat broader, present in
the spectra of the three nanoparticles shows that the calixar-
ene macrocycle did not undergo significant changes upon in-
troduction onto the gold surface. On the other hand, the
broadness of the resonances of the nanoparticles increases
on passing from 2s to 2l, which is in agreement with previ-
ous findings reported in the literature.[12c] This is due to sev-
eral concomitant effects: an average arrangement of the cal-
ix[4]arene macrocycles around the gold core that reflects
the polydispersity of the size of the nanoparticles, and the
predictable slow tumbling motion of the NPs in solution
that reduces the transverse relaxation time.[27] As expected,
the anchoring process of the thiolate SH groups on the sur-
face of the metal induces the disappearance of the diagnos-
tic signal of the methylene protons of the CH2 group adja-
cent to the S atom directly attached to the gold surface,
which is visible in the free ligands as a quartet centered at
around d=2.5 ppm (see Figure 2d).
À
effective ligand/AuCl4 molar ratios of Table 1). This effect
was also verified, although to a lesser extent, in the sets of
nanoparticles m and l that were respectively prepared with
S/Au ratios of 1:3 and 1:6 (see entries 2, 5, and 8 and 3, 6,
and 9, respectively, in Table 1).[23]
A large relative abundance of NPs in the diameter range
0.9–1.2 nm was found in the 2s and 3s samples by TEM
measurements, which hints at Au cores mainly composed of
25 to 55 gold atoms.[24] However, TGA and elemental analy-
ses carried out on 2s and 3s revealed a high percentage of
organic fraction and experimental S/Au ratios close to one
(see Table 1). The latter results suggest that in 2s and 3s are
also present smaller NPs, such as Au13 covered by six and
four calixarene units, respectively.[25] Since the Au13 nano-
clusters are characterized by very small icosahedral gold
cores (dꢀ0.8 nm), their presence in the 2s sample was veri-
fied through HAADF-STEM analyses.[26] The Z contrast
allows one to clearly distinguish the particles as bright ob-
jects on a dark background that corresponds to the light
carbon-support film. Statistical analyses of the size of the
NPs give a distribution of diameters cantered around
1.2 nm, but the presence of subnanometric NPs is confirmed,
with the 0.75–1 nm fraction being almost half of the largest
fraction, 1.25–1.5 nm (see Figure 1d).
XPS measurements: An XPS analysis has been conducted
on the three series of s, m, and l NPs and on the ligand-ex-
changed Au11 nanoclusters (2ex). The latter nanoparticles
were obtained through a ligand-place-exchange reaction by
reacting, in CH2Cl2, Au11ACTHNUTRGEN(UGN PPh3)7Cl3 undecagold clusters with
an excess amount of thiolate calix[4]arene 2 (see the Experi-
mental Section).[9c,28] The relevant XPS regions for the in-
vestigated systems are those of Au 4f and S 2p, which
appear as complex peaks and required curve-fitted compo-
nents (Figures 3 and 4). As evidenced by a rigid energy shift
of the overall spectrum, the nanoparticles experienced static
charging under X-rays to different amounts, which were
quantified and corrected by taking the distinct energy sepa-
rations between the C 1s peak components, respectively, due
to the graphite tip and the NPs. Such static charging only af-
fected m and s NPs, to comparable extents.
NMR spectroscopic measurements: All the nanoparticles
1
were sufficiently soluble in chloroform to allow for H NMR
spectroscopic analysis. In Figure 2, the 1H NMR spectro-
scopic stack plot in CDCl3 (300 MHz) of the NPs 2s, 2m,
and 2l along with the “free” calix[4]arene 2 used as the pro-
tecting ligand (see the Supporting Information for the NMR
spectra of the other NPs) of the NPs is depicted. An overall
matching of the resonances present in the spectrum of 2
Theoretically reconstructed Au 4f peaks invariably show
three spin–orbit split components (Table 2), whereas two
À
main components were expected, one due to the Au S
À
bonds and the second to the Au Au (both central and sur-
face atoms) bonds. Their assignment is further complicated
by the fact that the three Au components happen to follow
at different binding energies (BEs) depending on the nano-
particles size. We assign the Au electronic states for l, m,
and s NPs by first considering the typical energy separation
7
between Au 4f = components, as extracted from the relevant
2
I
I
III
literature:[29] Au Au ((1.3Æ0.3) eV) and Au Au ((1.7Æ
0.3) eV). These values are consistently reproduced by peaks
Au(a), Au(b), and Au(c) in Table 2, which are present with
different relative ratios in the series depending on the mean
dimension of NPs. The minor Au(c) component can be as-
À
À
À
signed to the AuIII species that likely derive from AuCl4 . It
is expected on the simple basis of the decreasing trend of
surface/bulk atomic ratio in a nanoparticle as a function of
its increasing nuclearity that the Au(a)/Au(b) ratio is the
largest for l and the smallest for s NPs, as indeed was found,
with the m ones lying definitely closer to the latter
Figure 2. 1H NMR spectroscopic stack plot (300 MHz, CDCl3) of a) 2,
b) 2s, c) 2m, and d) 2l.
7
(Figure 3). The main Au 4f = component in all the spectra is
2
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11093

A. Secchi, R. Zanoni et al.
bond, accompanied in the case
of s NPs by a small contribution
from distinct sulfur species. We
assign this minor component to
disulfide bridges that connect
distinct calixarenes,[30] since
NMR spectroscopic analysis ex-
cludes the alternative assign-
ment in terms of free thiols, be-
cause of the absence of SH
groups in the NPs.
Relevant quantitative results
(S/Au and Au(b)/Au(a)) are re-
ported in Table 2. We note that
the S/Au ratios determined by
means of XPS spectroscopy are
usually higher than those deter-
mined by means of elemental
analysis (see Tables 1 and 2),
most likely because of the
photon attenuation exerted by
the ligand shell on the Au core.
The S 2p major component due
À
Figure 3. Au 4f XPS spectral regions of a) 2s, b) 2m, c) 2l, and d) 2ex. Experimental curve (dots) and Au
to the thiolate Au bond is also
7
5
4f = , = curve-fitted components: Au(a) (dashed line), Au(b) (solid line), and Au(c) (dotted line).
2
2
fully compatible with the struc-
tural motif (“staple”) first seen
in the external organic shell of
the X-ray structure of the Au25
nanoparticles stabilized with
phenylethanthiolate groups re-
ported by Murray[24b] and con-
firmed in subsequent reports.[31]
We note that Au BEs and rela-
tive quantitative ratios for 2ex
are closely comparable to 2s
and 3s, and distinct from 2m,
2l, and 1s. More in detail, the
S/Au and A(b)/Au(a) ratios ob-
tained for 2ex, and expected
for an Au11 cluster, are only re-
produced in the cases of 2s and
3s, within the experimental
error.
7
As for BEs, Au(a) 4f = values
2
for s and m NPs fall in the
range of 82.8–83.5 eV, that is, at
consistently negative BE shift
with respect to bulk Au
(84.0 eV). Their interpretation
in terms of a negatively charged
Au central atom is supported
by previous literature reports.[32]
We can take the relative abun-
3
1
2
Figure 4. S 2p XPS spectral regions of a) 2s, b) 2m, c) 2l , and d) 2ex. Experimental curve (dots) and S 2p = , =
2
À
À
curve-fitted components: S Au (solid line), S S (dotted line).
3
invariably associated with a relative shift to the main S 2p =
dance of Au(a) together with S/Au ratios as an indirect mea-
sure of the nuclearity of the nanoparticles, thus allowing for
a distinction to be made between s, m, and l. In fact, the ex-
perimental S/Au and Au(b)/Au(a) ratios are inversely pro-
2
component equal to (78.3Æ0.1) eV, a value found in the lit-
erature as characteristic for S-bound Au atoms.[29] The S 2p
À
spectra present a major component due to the thiolate Au
11094
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Chem. Eur. J. 2010, 16, 11089 – 11099

Calix[n]arene-Stabilized Au Nanoparticles
FULL PAPER
Table 2. XPS binding energies for Au 4f and S 2p and relative quantitative ratios for different series of calixar-
ene-stabilized AuNPs.
tained, and that ligand denticity
can play a role in addressing
the synthetic outcomes towards
the nanodimensional range.[33]
In fact: 1) the mean diameters
calculated for the NPs stabi-
lized with the monodentate cal-
ix[4]arene ligand 1 (1s, 1m, and
1l) are always larger than those
calculated for the NPs loaded
with the bidentate 2 and triden-
7
3
2
Au 4f =
S 2p =
S/Au
ACHTUNGTRENNUNG
(XPS)[a]
2
Entry NP designation
Au(b)/Au(a)[a] dTEM [nm][b]
Label BE [eV] Label BE [eV]
Au(a) 82.9
1
2
3
4
5
6
Cx4S1/Au (1s)
Au(b) 84.7
Au(c) 86.7
Au(a) 83.3
S–Au 162.9
0.7
1.0
1.0
0.6
0.3
1.5
6
G
Cx4S2/Au (2ex) Au(b) 84.6
Au(c) 87.1
S–Au 163.0
S–Au 163.2
8
Au(a) 83.5
Cx4S2/Au (2s)
Au(b) 84.8
Au(c) 86.5
Au(a) 82.8
S–S
165.3
10[d]
tate
3 thiolate calixarene li-
Cx4S2/Au (2m) Au(b) 84.1
Au(c) 85.6
S–Au 162.4
5
gands that were obtained using
the same experimental condi-
tions and the same equivalents
of thiolate alkyl chains present
in solution. The TEM measure-
ments also show that the diam-
eter of the NPs present in the
samples 1s, 1m, and 1l is com-
parable, in terms of mean diam-
eter and size distribution, to
those of NPs prepared by
Au(a) 84.0
Cx4S2/Au (2l)
Cx6S3/Au (3s)
Au(b) 85.5
Au(c) 86.8
Au(a) 82.8
Au(b) 84.1
Au(c) 85.4
S–Au 162.3
S–Au 162.3
0.1
9
S–S
163.6
[a] Quantitative ratios (associated uncertainty of Æ10%). [b] From Table 1, reported for clarity. [c] Not deter-
mined. [d] Average value calculated from different 2s batches.
portional to the size of the nanoparticles. This trend is con-
sistent with the observation that the smaller the core size of
the NPs, the larger the percentage of surface Au atoms
bound to S. In addition to that, we could tentatively infer
the nuclearity of the NPs by comparing the present results
with the expected values in the notable cases of previously
reported Au11, Au13, Au25(SR)18À, Au38/Au40, Au55, Au68,
Au102, and Au144 clusters.[31a]
On the basis of the data in Table 2 on the S/Au and
Au(b)/Au(a) ratios, the nuclearities of 2s and 3s are both
likely assigned to Au11 or Au13 (which can be hardly distin-
guished by XPS only), whereas 2m NPs belong to higher-
nuclearity species. As regards to 2l, the S/Au and Au(b)/
Au(a) ratios are too small to find a correspondence with the
above range of Au nuclearity, thus hinting at a larger range
of nanoparticle diameters. We also note that, in the case of
2l, although TEM measurements clearly distinguish a larger
average dimension of the particles, XPS results are distinct
from the analogous s and m, but they could be mainly relat-
ed to the outer shell of the particle, since its experimental
diameter range becomes comparable to the inelastic mean
free path for Au 4f photoelectrons at the photon energy ap-
plied here.
Murray from the monodentate dodecanthiol ligand under
similar experimental conditions.[12c] 2) XPS results support a
distinct size for sample 1s with respect to 2s and 3s (the
latter two being closely comparable). Actually, the positions
in energy of Au 4f peak components, and their relative
quantitative ratios, are clearly distinct in the two subsets,
and can be interpreted in the light of the literature as size-
dependent effects. 3) Elementary analysis of the 2s and 3s
samples shows the presence of NPs with nuclearities around
1 nm.
These results open new possibilities for the synthesis of
NPs that merge the high potential of calix[n]arene receptors
with those associated to quantum size effects. Studies are
undergoing in our laboratory to apply these achievements in
the preparation of working devices.
Experimental Section
Chemicals: All reactions were carried out under nitrogen; all solvents
were freshly distilled under nitrogen and stored over molecular sieves for
1
at least 3 h prior to use. H and 13C NMR spectra were recorded using in-
struments operating at 300 and 75 MHz, respectively. Mass spectra were
recorded in ESI mode. Melting points are uncorrected. Undec-10-enyl 4-
methylbenzenesulfonate 4,[34] calix[4]arene 6,[35] 38,40,42-trimethoxy-p-
tert-butylcalix[6]arene 10,[36] and AuPPh3Cl[37] were prepared according to
published procedures.
Conclusion
Synthesis of the thiolate calix[4]arene ligands
In summary, the synthesis and characterization of a series of
AuNPs stabilized with calixarene derivatives functionalized
with alkylthiol chains on their lower rim has been reported.
The group of experimental results that have emerged from
the different techniques adopted here constitutes a sound
basis to demonstrate that, by making use of suitable calixar-
enes, a full series of AuNPs of different sizes can be ob-
Thioate 5: A tip of a spatula of AIBN was added to a solution of tosylate
4 (5 g, 15.4 mmol) and thioacetic acid (5.9 g, 77 mmol) in dry toluene
(200 mL). After heating at reflux for 5 h, the reaction was quenched by
addition of water (200 mL). The separated organic phase was dried over
Na2SO4 and the solvent evaporated to dryness under reduced pressure.
The solid residue was purified by column chromatography (silica gel,
eluent: hexane/ethyl acetate 9:1) to afford 5 as a yellow low-melting solid
(90%). M.p. 28.0–29.58C; 1H NMR (300 MHz, CDCl3): d=7.74 (d, J=
Chem. Eur. J. 2010, 16, 11089 – 11099
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www.chemeurj.org
11095

A. Secchi, R. Zanoni et al.
8.3 Hz, 2H), 7.30 (d, J=8.3 Hz, 2H), 3.97 (t, J=6.6 Hz, 2H), 2.81 (t, J=
7.2 Hz, 2H), 2.40 (s, 3H), 2.37 (s, 3H), 1.7–1.4 (m, 4H), 1.4–1.1 ppm (m,
14H); 13C NMR (75 MHz, CDCl3): d=195.0, 144.4, 133.3, 130.5, 128.3,
71.0, 31.0, 29.9, 29.7 (2 resonances), 29.5, 29.4, 29.3, 29.2 (2 resonances),
25.7, 22.0 ppm; MS (ESI): m/z (%): 347 (100) [M+Na]; elemental analy-
sis calcd (%) for C20H32O4S: C 59.96, H 8.05, S 16.01; found: C 59.57, H
8.15, S 16.23.
The organic phase was separated, washed with brine up to neutrality, and
dried over Na2SO4. After the removal of the solvent under reduced pres-
sure, the solid residue was purified by column chromatography (silica gel,
eluent: n-hexane/CH2Cl2 8:2) to afford 11 as a white solid (55%). M.p.
133–1358C; 1H NMR (300 MHz, CDCl3, cone conformer): d=7.29 (s,
6H), 6.64 (s, 6H), 5.9–5.7 (m, 3H), 5.1–4.8 (m, 6H), 4.58 (d, J=14.1 Hz,
6H), 3.88 (t, J=6.4 Hz, 6H), 3.40 (d, J=14.1 Hz, 6H), 2.20 (s, 9H), 2.1–
2.0 (m, 6H), 2.0–1.8 (m, 6H), 1.6–1.5 (m, 6H), 1.39 (s, 27H), 1.4–1.3 (m,
30H), 0.79 ppm (s, 27H); 13C NMR (75 MHz, CDCl3, cone conformer):
d=154.5, 152.1, 145.5, 145.2, 139.2, 133.6, 133.3, 127.9, 123.4, 114.1, 73.0,
60.2, 34.2, 33.9, 33.8, 31.6, 31.3, 31.2, 30.4, 29.7, 29.6, 29.5, 29.1, 28.9,
26.2 ppm; MS (ESI): m/z (%): 1495 (100) [M+Na]; elemental analysis
calcd (%) for C102H150O6: C 83.15, H 10.32; found: C 83.20, H 10.52.
Calix[4]arene 7: A solution of calix[4]arene 6 (0.42 g, 1 mmol), K2CO3
(0.11 g, 0.8 mmol), and 5 (0.26 g, 0.8 mmol) in dry acetone (70 mL) was
poured into a small glass autoclave filled with nitrogen. After sealing the
autoclave, the reaction mixture was heated at reflux at 808C for 48 h.
After this period, the mixture was cooled at room temperature and the
solvent evaporated to dryness under reduced pressure. The solid residue
was dissolved in a 10% (w/v) aqueous solution of HCl (100 mL) and
CH2Cl2 (100 mL). The separated organic phase was washed with water
up to neutrality, dried over Na2SO4, and evaporated to dryness under re-
duced pressure. The crude product was purified by column chromatogra-
phy (silica gel, eluent: n-hexane/CH2Cl2 7:3) to afford 7 as a white solid
(80%). M.p. 53.0–54.08C; 1H NMR (300 MHz, CDCl3): d=9.75 (s, 1H),
9.44 (s, 2H), 7.1–7.0 (m, 8H), 6.87 (t, J=7.5 Hz, 1H), 6.7–6.6 (m, 3H),
4.37 (d, J=13 Hz, 2H), 4.28 (d, J=14 Hz, 2H), 4.16 (t, J=7.2 Hz, 2H),
3.47 (d, J=14 Hz, 2H), 3.46 (d, J=13 Hz, 2H), 2.86 (t, J=7.2 Hz, 2H),
2.32 (s, 3H), 2.2–2.1 (m, 2H), 1.8–1.6, 1.6–1.5, and 1.5–1.3 ppm (3m,
16H); 13C NMR (75 MHz, CDCl3): d=196.0, 151.4, 150.8, 149.2, 134.2,
129.2, 128.8, 128.7 (2 resonances), 128.3, 126.0, 121.9, 120.8, 77.1, 31.8,
31.4, 30.6, 29.8, 29.4 (2 resonances), 29.1, 28.8, 25.8 ppm; MS (ESI): m/z
(%): 653 (100) [M+1], 675 (50) [M+Na]; elemental analysis calcd (%)
for C41H48O5S: C 75.43, H 7.41, S 4.91; found: C 75.22, H 7.50, S 4.56.
Calix[6]arene 12: A tip of a spatula of AIBN was added to a solution of
11 (1 g, 0.68 mmol) and thioacetic acid (0.37 g, 4.9 mmol) in dry toluene
(100 mL). After heating at reflux for 5 h, the reaction was quenched by
addition of water (100 mL). The separated organic phase was dried over
Na2SO4. After the removal of the solvent under reduced pressure, the
solid residue was purified by column chromatography (silica gel, eluent:
n-hexane/ethyl acetate 95:5) to afford 12 as a white solid (80%). M.p.
125–1278C; 1H NMR (300 MHz, CDCl3, cone conformer): d=7.29 (s,
6H), 6.66 (s, 6H), 4.59 (d, J=15.0 Hz, 6H), 3.88 (t, J=6.5 Hz, 6H), 3.40
(d, J=15.2 Hz, 6H), 2.87 (t, J=7.3 Hz, 6H), 2.32 (s, 9H), 2.22 (s, 9H),
2.0–1.8 (m, 6H), 1.6–1.4 (m, 6H), 1.42 (s, 27H), 1.4–1.1 (m, 42H),
0.80 ppm (s, 27H); 13C NMR (75 MHz, CDCl3, cone conformer): d=
196.0, 154.4, 152.0, 145.6, 145.2, 133.8, 133.6, 133.2, 127.9, 123.3, 73.0,
60.1, 34.2, 34.0, 33.9, 31.6, 31.4, 31.1, 30.6, 30.4, 29.6, 29.5, 29.4, 29.1, 28.8,
26.3, 26.2 ppm; MS (ESI): m/z: 1723 [M+Na]; elemental analysis calcd
(%) for C108H162O9S3: C 76.23, H 9.52, S 5.65; found: C 76.35, H 9.43, S
5.81.
Calix[4]arene 8: A mixture of calix[4]arene 6 (3 g, 7 mmol), K2CO3
(2.9 g, 21 mmol), tosylate 4 (7 g, 21 mmol), and KI (cat.) in CH3CN
(150 mL) was stirred and heated under reflux. After four days, the sol-
vent was evaporated under vacuum and the solid residue was dissolved in
CH2Cl2. The organic phase was washed with H2O up to neutrality and
dried over Na2SO4. After evaporation of the solvent under reduced pres-
sure, the resulting crude product was purified by column chromatography
(silica gel, n-hexane/ethyl acetate 9:1) to give 8 as yellowish solid (70%).
M.p. 120–1228C; 1H NMR (300 MHz, CDCl3): d=8.24 (s, 2H), 7.05 (d,
J=7 Hz, 4H), 6.91 (d, J=7 Hz, 4H), 6.72 (t, J=7 Hz, 2H), 6.62 (t, J=
7 Hz, 2H), 5.9–5.7 (m, 2H), 5.1–4.9 (m, 4H), 4.32 (d, J=14 Hz, 4H), 3.99
(t, J=6 Hz, 4H), 3.37 (d, J=14 Hz, 4H), 2.2–2.0 (m, 8H), 1.8–1.6 (m,
4H), 1.6–1.2 ppm (m, 20H); 13C NMR (CDCl3, 75 MHz): d=155.2, 150.8,
138.0, 132.3, 127.6, 127.2, 127.0, 124.0, 117.7, 112.9, 76.4, 32.6, 30.2, 28.8,
28.4, 28.3, 27.9, 24.8 ppm; MS (ESI): m/z (%): 729 [M+H]; elemental
analysis calcd (%) for C50H64O4: C 82.37, H 8.85; found: C 82.45, H 8.52.
General procedure for the removal of the thioacetyl protecting groups:
A solution of the appropriate calix[n]arene derivative (1 mmol) in a mix-
ture of THF (20 mL) and HCl (10% w/v in H2O, 20 mL) was heated at
reflux for 48–72 h. After cooling to room temperature, the mixture was
extracted with CH2Cl2 (30 mL). The resulting organic phase was separat-
ed, washed with water up to neutrality, dried over Na2SO4, and evaporat-
ed to dryness under reduced pressure.
Calix[4]arene 1: The oily residue obtained from the hydrolysis of 7 was
purified by column chromatography (silica gel, eluent: n-hexane/CH2Cl2
7:3) to afford 1 as a white solid (90%). M.p. 87.0–88.08C; 1H NMR
(300 MHz, CDCl3): d=9.75 (s, 1H), 9.43 (s, 2H), 7.1–7.0 (m, 8H), 6.87 (t,
J=7.5 Hz, 1H), 6.7–6.6 (m, 3H), 4.37 (d, J=13 Hz, 2H), 4.28 (d, J=
14 Hz, 2H), 4.15 (t, J=7.2 Hz, 2H), 3.47 (d, J=14 Hz, 2H), 3.46 (d, J=
13 Hz, 2H), 2.6–2.5 (m, 2H), 2.2–2.1 (m, 2H), 1.8–1.6, 1.6–1.5, and 1.5–
1.3 ppm (3m, 16H); 13C NMR (75 MHz, CDCl3): d=151.4, 150.8, 149.2,
134.2, 129.3, 128.8, 128.7 (2 resonances), 128.4, 126.0, 121.9, 120.9, 77.4,
34.0, 31.9, 31.4, 29.9, 29.5 (2 resonances), 29.4, 29.1, 28.4, 25.9, 24.6 ppm;
MS (ESI): m/z (%): 611 (10) [M+1], 634 (85) [M+Na], 650 (60) [M+K];
elemental analysis calcd (%) for C39H46O4S: C 76.68, H 7.59, S 5.25;
found: C 75.62, H 7.46, S 4.81.
Calix[4]arene 9: A catalytic amount of AIBN was added to a solution of
calix[4]arene 8 (3 g, 4.2 mmol) and thioacetic acid (1.3 g, 17 mmol) in tol-
uene (100 mL). The resulting homogeneous mixture was heated at reflux
for 5 h, then the solvent was evaporated to dryness under reduced pres-
sure. The solid residue was dissolved in CH2Cl2 and the organic phase
was washed twice with H2O and with a saturated solution of NaHCO3.
After the removal of the solvent under reduced pressure, the solid resi-
due was purified by column chromatography (silica gel, n-hexane/ethyl
acetate 8:2) to afford 9 as a yellowish sticky solid (70%). M.p. 84.5–
85.58C; 1H NMR (300 MHz, CDCl3): d=8.23 (s, 2H), 7.04 (d, J=7 Hz,
4H), 6.91 (d, J=7 Hz, 4H), 6.73 (t, J=7 Hz, 2H), 6.63 (t, J=7 Hz, 2H),
4.30 (d, J=14 Hz, 4H), 3.99 (t, J=6 Hz, 4H), 3.37 (d, J=14 Hz, 4H),
2.85 (J=6 Hz, 4H), 2.32 (s, 6H), 2.1–2.0 (m, 4H), 1.8–1.7 (m, 4H), 1.6–
1.4 ppm (2m, 28H); 13C NMR (75 MHz, CDCl3): d=153.3, 152.0, 133.4,
128.8, 128.3, 128.1, 125.1, 118.9, 76.6, 31.4, 30.5, 29.9, 29.6, 29.5, 29.4, 29.1,
28.8, 25.9 ppm; MS (ESI): m/z: 904 [M+Na]; elemental analysis calcd
(%) for C54H62O6S2: C 73.51, H 8.22, S 7.27; found: C 73.55, H 8.14, S
6.98.
Calix[4]arene 2: The oily residue obtained from the hydrolysis of 9 was
purified by column chromatography (silica gel, eluent: n-hexane/CH2Cl2
7:3) to afford 2 as a white solid (80%). M.p. 300–3028C; 1H NMR
(300 MHz, CDCl3): d=8.29 (s, 2H), 7.13 (d, J=7 Hz, 4H), 6.96 (d, J=
7 Hz, 4H), 6.8–6.7 (m, 4H), 4.40 (d, J=14 Hz, 4H), 4.07(t, J=6 Hz, 4H),
3.45 (d, J=14 Hz, 4H), 2.7–2.5 (m, 4H), 2.2–2.1 (m, 4H), 1.9–1.8 (m,
4H), 1.8–1.7 (m, 4H), 1.6–1.3 ppm (m, 24H); 13C NMR (75 MHz,
CDCl3): d=153.3, 133.4, 128.8, 128.3, 128.1, 125.2, 118.9, 76.7, 34.0, 31.4,
30.0, 29.6, 29.5, 29.4, 29.1, 28.4, 25.9, 24.6 ppm; ESI (MS): m/z: 818
[M+Na]; elemental analysis calcd (%) for C50H68O4S2: C 75.33, H 8.60, S
8.05; found: C 75.40, H 8.32, S 8.26.
Calix[6]arene 11: Tosylate 4 (0.96 g, 3 mmol) was added to a stirred solu-
tion of calix[6]arene 10 (1 g, 0.98 mmol) and K2CO3 (0.4, 3 mmol) in ace-
tonitrile (200 mL). The resulting heterogeneous mixture was heated at
reflux for 4 d. After this period, the solvent was evaporated to dryness
under reduced pressure. The solid residue was dissolved in a 10% (w/v)
aqueous solution of HCl in water (100 mL) and ethyl acetate (200 mL).
Calix[6]arene 3: The oily residue obtained from the hydrolysis of 12 was
purified by column chromatography (silica gel, eluent: n-hexane/ethyl
acetate 95:5) to afford 3 as a white solid (85%). M.p. 69.5–70.58C;
1H NMR (300 MHz, CDCl3, cone conformer): d=7.28 (s, 6H), 6.64 (s,
6H), 4.58 (d, J=15.0 Hz, 6H), 3.88 (t, J=6.4 Hz, 6H), 3.40 (d, J=
15.0 Hz, 6H), 2.51 (q, J=7.2 Hz, 6H), 2.20 (s, 9H), 2.0–1.8 (m, 6H), 1.7–
11096
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Chem. Eur. J. 2010, 16, 11089 – 11099

Calix[n]arene-Stabilized Au Nanoparticles
FULL PAPER
1.2 (m, 81H), 0.79 ppm (s, 27H); 13C NMR (75 MHz, CDCl3, cone con-
former): d=154.4, 152.0, 145.6, 145.2, 133.6, 133.5, 133.2, 127.9, 123.3,
73.0, 60.1, 34.2, 34.0, 33.9, 31.6, 31.4, 31.3, 31.2, 31.1, 30.6, 30.4, 29.6,
29.53, 29.50, 29.4, 29.0, 28.3, 26.2, 24.6 ppm; ESI (MS): m/z: 1597
[M+Na]; elemental analysis calcd (%) for C102H156O6S3: C 77.76, H 9.91,
S 6.10; found: C 77.61, H 10.10, S 6.10.
77.1, 31.6, 31.4, 30.0, 29.7, 29.0, 27.2, 27.0, 26.4, 22.6, 22.2 ppm; elemental
analysis (%): C 22.65, H 2.59, S 2.41, N 0.13.
Nanoparticles stabilized with the tridentate calix[6]arene ligand 3
Cx6S3/Au (3s): Complex 3 (1.57 g; 1 mmol corresponding to S/Au 3:1)
was used as the gold passivating agent. The solid residue obtained after
the centrifugation cycle required a further purification step to eliminate
the large excess amount of free 3 that was contaminating the nanoparti-
cles. A chromatographic separation was accomplished using a 9:1 mixture
of CH2Cl2/CH3OH as eluent. Organic fraction: 77%; 13C NMR (75 MHz,
CDCl3): d=154.4, 152.0, 145.6, 145.2, 133.6, 133.2, 127.8, 123.3, 77.1, 72.9,
34.2, 31.6, 31.3, 31.1, 29.9, 29.6, 29.2, 26.4, 22.5, 22.3 ppm; elemental anal-
ysis (%): C 59.11, H 7.40, S 4.08, N 0.05.
Synthesis of the calixarene-stabilized gold nanoparticles: The calixarene-
stabilized nanoparticles were synthesized according to the general proce-
dure published by Murray for n-dodecanthiol-stabilized nanoparticles
using S/Au ratios of 3:1, 1:3, and 1:6, respectively.[12c] All the preparations
were carried out using HAuCl4·xH2O (1 mmol) as the source of gold, tet-
raoctylammonium bromide (TOABr; 2.5 mmol) as the transfer catalyst,
and NaBH4 (10 mmol) as the reductant. In each preparation, the amount
of the calix[n]arene ligand used (1–3) was changed according to the
ligand “denticity” to match the requested S/Au ratios. The purification
procedure has been changed as follows: after the gold reduction, toluene
was evaporated to dryness under reduced pressure and the temperature
maintained below 508C. The black sticky residue obtained was suspended
in ethanol (25 mL) and sonicated. The black powder formed was separat-
ed by suction filtration and dissolved in a 5:1 ethanol/dichlorometane
mixture, sonicated, and centrifuged at 14000 rpm for 20 min. The super-
natant solution was eliminated and the solid residue dried under vacuum.
The purification cycle was repeated until the free calixarene ligand was
no longer detectable by TLC in the supernatant solution. The isolated
nanoparticles were characterized by elemental analysis, NMR spectrosco-
py, TEM, and XPS measurements.
Cx6S3/Au (3m): Complex 3 (0.17 g; 0.1 mmol corresponding to S/Au 1:3)
was used as the gold passivating agent. Organic fraction: 50%; 13C NMR
(75 MHz, CDCl3): d=154.4, 152.0, 145.4, 145.2, 133.6, 133.2, 127.8, 123.3,
77.1, 72.9, 34.2, 31.5, 31.6, 31.0, 29.9, 29.5, 29.2, 26.5, 22.5, 22.3 ppm; ele-
mental analysis (%): C 36.78, H 4.57, S 3.08, N 0.08.
Cx6S3/Au (3l): Complex 3 (0.08 g; 0.055 mmol corresponding to S/Au 1:3)
was used as the gold passivating agent. Organic fraction: 32%; 13C NMR
(75 MHz, CDCl3), d=154.4, 152.0, 145.4, 145.2, 133.6, 133.2, 127.8, 123.3,
77.2, 72.9, 34.3, 31.6, 31.4, 31.1, 29.8, 29.5, 29.3, 26.4, 22.5, 22.3 ppm; ele-
mental analysis (%): C 24.73, H 3.28, S 1.95, N 0.14.
Synthesis of calix[4]arene-stabilized Au11 nanoclusters (2ex): NaBH4
(0.033 g, 0.87 mmol) was added in small portions to a heterogeneous mix-
ture of AuPPh3Cl (0.43 g, 0.87 mmol) in absolute ethanol (30 mL) kept
under an inert atmosphere. The resulting reaction mixture turned homo-
geneous within a few minutes and was stirred at room temperature for a
further 2 h. After this period, the solvent was removed to dryness under
reduced pressure and the solid residue take up with dichloromethane
Nanoparticles stabilized with the monodentate calix[4]arene ligand 1
Cx4S1/Au (1s): Complex 1 (1.8 g; 3 mmol corresponding to S/Au 3:1) was
used as the gold passivating agent. The solid residue obtained after the
centrifugation cycle required a further purification step to eliminate the
large excess amount of free 1 that was contaminating the nanoparticles.
A chromatographic separation was accomplished using a 9:1 mixture of
CH2Cl2/CH3OH as eluent. Organic fraction: 60%; 13C NMR (75 MHz,
CDCl3): d=151.4, 150.7, 149.1, 134.1, 129.2, 128.7, 128.4, 126.0, 121.9,
120.9, 77.2, 31.8, 31.6, 31.4, 29.9, 28.9, 26.0 ppm; elemental analysis (%):
found: C 44.12, H 4.36, S 3.03, N 0.12.
(25 mL). The resulting solution that contained Au11ACHTUNGTRENUNG(PPh3)7Cl3 nanoclus-
ters was filtered off to remove the black suspension of colloidal gold, and
the calix[4]arene 2 (0.6 g, 0.75 mmol) was added. After 24 h of stirring at
room temperature, the solvent was removed to dryness under reduced
pressure. The crude residue was purified following the procedure report-
ed for the nanoparticles 2s. Organic fraction: 62%; elemental analysis
(%): C 47.04, H 4.79, S 4.49.
Cx4S1/Au (1m): Complex 1 (0.2 g; 0.33 mmol corresponding to S/Au 1:3)
was used as the gold passivating agent. Organic fraction: 37%; 13C NMR
(75 MHz, CDCl3): d=151.4, 150.7, 149.2, 134.1, 129.2, 128.7, 128.4, 126.0,
121.9, 120.8, 77.1, 31.9, 31.6, 31.4, 29.9, 29.5, 29.2, 28.3, 26.0 ppm; elemen-
tal analysis (%): C 27.52, H 2.74, S 1.88, no N found.
TEM and HAADF-STEM measurements: TEM measurements were
partly carried out at CIGS of the University of Modena (Italy) using a
Jeol JEM 2010 microscope. High-angle annular dark-field (HAADF)
measurements were carried out at CNR-IMEM of Parma using a Jeol
2200FS field-emission microscope working at 200 kV in STEM mode.
The probe size was minimized to about 0.2 nm. The size distribution of
the nanoparticles was determined using ImageJ software[38] by statistical
analysis of more than 300 nanoparticles taken from at least three images
for each sample.
Cx4S1/Au (1l): Complex 1 (0.1 g, 0.17 mmol corresponding to S/Au 1:6)
was used. Addition of the reductant solution to the reaction mixture was
accomplished at room temperature. Organic fraction: 25%; 13C NMR
(75 MHz, CDCl3): d=151.3, 150.7, 149.1, 134.1, 129.2, 128.7, 128.4, 126.0,
121.9, 120.9, 77.1, 31.8, 31.4, 29.9, 26.0 ppm; elemental analysis (%): C
18.60, H 1.92, S 1.29, N 0.10.
XPS measurements: The solid compounds were homogeneously spread
over a graphite tip attached to the XPS sample holder. Photoelectron
spectra were acquired using a modified Omicron NanoTechnology MXPS
system equipped with various photon sources and an Omicron EA-127-7
energy analyzer. The experimental conditions adopted were as follows:
excitation by MgKa photons (hn=1253.6 eV), generated by operating the
anode at 14 kV, 16 mA. XPS atomic ratios for the investigated com-
pounds have been estimated from experimentally determined area ratios
of the relevant core lines, corrected for the corresponding theoretical
atomic cross-sections and for a square-root dependence of the kinetic en-
ergies of the photoelectrons.
Nanoparticles stabilized with the bidentate calix[4]arene ligand 2
Cx4S2/Au (2s): Complex 2 (1.2 g; 1.5 mmol corresponding to S/Au 3:1)
was used as the gold passivating agent. The solid residue obtained after
the centrifugation cycle required a further purification step to eliminate
the large excess amount of free 2 that was contaminating the nanoparti-
cles. A chromatographic separation was accomplished using a 9:1 mixture
of CH2Cl2/CH3OH as eluent. During elution, the desired (2s) nanoparti-
cles were visible as a dark brown band. Organic fraction: 64%; 13C NMR
(75 MHz, CDCl3): d=153.2, 151.8, 133.4, 128.8, 128.3, 128.1, 125.2, 118.9,
77.1, 31.6, 31.4, 30.1, 29.7, 29.1, 29.0, 26.4, 22.5, 22.3 ppm; elemental anal-
ysis (%): C 48.87, H 5.19, S 5.13, N 0.05.
Cx4S2/Au (2m): Complex 2 (0.13 g; 0.17 mmol corresponding to S/Au
1:3) was used as the gold passivating agent. Organic fraction: 46%;
13C NMR (75 MHz, CDCl3): d=153.1, 151.7, 133.5, 129.0, 128.8, 128.3,
128.2, 128.0, 125.2, 118.9, 76.9, 31.6, 31.4, 30.2, 29.7, 29.5, 29.1, 29.0, 28.5,
28.0, 26.3, 26.1, 22.5, 22.2 ppm; elemental analysis (%): C 32.76, H 3.69, S
3.32, no N found.
Acknowledgements
This work was supported by the Italian MIUR (Sistemi supramolecolari
per la costruzione di macchine molecolari, conversione dell’energia, sens-
ing e catalisi) and by Universitꢀ La Sapienza. We thank CIM (Centro In-
terdipartimentale di Misure) “G. Casnati” of the University of Parma for
NMR spectroscopic measurements.
Cx4S2/Au (2l): Complex 2 (0.07 g; 0.08 mmol corresponding to S/Au 1:6)
was used as the gold passivating agent. Organic fraction: 28%; 13C NMR
(75 MHz, CDCl3): d=153.3, 151.9, 133.5, 128.8, 128.3, 128.1, 125.2, 118.9,
Chem. Eur. J. 2010, 16, 11089 – 11099
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11097

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2
that have
a
hypothetical empirical formula of
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a
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empirical
formula
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from
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11098
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Chem. Eur. J. 2010, 16, 11089 – 11099

Calix[n]arene-Stabilized Au Nanoparticles
FULL PAPER
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Received: April 20, 2010
Published online: August 16, 2010
Chem. Eur. J. 2010, 16, 11089 – 11099
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11099