Transformation from [Au25(SCH2CH2CH2CH3)18]0 to Au28(SCH2CH(CH3)Ph)21 gold nanoclusters: Gentle conditions is enough

Article abstract of DOI:10.1039/c9cc08872e

Herein we report the transformation of [Au₂₅(SR)₁₈]⁰ into Au₂₈(SR)₂₁ induced by ligand exchange reaction. In contrast to other reported cluster transformations, which proceed at elevated temperature a

Full text of DOI:10.1039/c9cc08872e

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Transformation from [Au25(SCH2CH2CH2CH3)18]0
to Au28(SCH2CH(CH3)Ph)21 gold nanoclusters:
gentle conditions is enough†
Cite this: Chem. Commun., 2019,
55, 14914
Received 13th November 2019,
Accepted 18th November 2019
Yanan Wang,
Belen Nieto-Ortega
and Thomas Burgi
*
´
¨
DOI: 10.1039/c9cc08872e
rsc.li/chemcomm
Herein we report the transformation of [Au25(SR)18
]
0 into Au28(SR)21
LERs induced transformations can be classified into three
induced by ligand exchange reaction. In contrast to other reported groups. (I) Transformation between structural isomers without
cluster transformations, which proceed at elevated temperature size change. For example, Au28(SR)20 nanocluster reversibly changes
and large excess of incoming ligands, the transformation reported its structure upon ligand exchange between R = c-C6H11 and
here occurs under mild conditions (room temperature, very low R = Ph-tBu at elevated temperatures (e.g., 80 1C).11 (II) Trans-
thiol excess) with a chiral ligand. A difference of one methyl group formations from a larger to a smaller nanocluster.12–16 For
between incoming and leaving thiol is sufficient to induce the instance, highly stable Au144(SC2H4Ph)60 reacted with thiophenol,
transformation. To the best of our knowledge the Au28(SR)21 cluster HSPh, to form a different 99 atom cluster species Au99(SPh)42. (III)
has not been isolated before.
Transformations from a smaller to a larger nanocluster.16–19 An
interesting example is the transformation of Au25(PET)18 into
Au28(TBBT)20 (where TBBT = 4-tert-butylbenzenethiolate).17
Au25(PET)18 is probably the most studied thiolate-protected
nanocluster, due to its prototypical character. At 80 1C and in
large excess of TBBT the cluster is transformed into Au28(TBBT)20.
It is worth to mention that the resulting Au28(TBBT)20 cluster is
chiral, presenting a pair of enantiomers which can be separated
by chiral-HPLC.
Cluster transformations induced by LER typically require
large excess of incoming ligand, normally more than 100 times
compared to the endogenous ligand, and elevated temperatures
to proceed.7 This becomes evident from Table S1 (ESI†), which
compiles the conditions of the reported cluster transforma-
tions. In addition, Jin and coworkers argued that the incoming
thiol must be significantly different from the endogenous thiol
for the transformation to happen.7,20 Herein we report a cluster
transformation that takes place at room temperature in less
than a day and mild condition with ratio of incoming to endo-
genous ligand of 2 : 1. Under these conditions [Au25(SR)18]0 trans-
forms into a larger cluster, Au28(SR)21, by LERs. To our best
knowledge, the Au28(SR)21 cluster has not yet been isolated but
has been detected previously as [Au28(TBBT)21]3À by mass spectro-
metry within a mixture of different clusters in a study of seed-
mediated growth from Au25 to Au44.21
In the last decade, thiolate-protected gold nanoclusters have
gained significant attention due to possible applications in
sensing, catalysis and molecular electronics to name a few.1,2
Also, the interest has grown because nowadays it is possible to
obtain these materials by well-established synthetic methods,3
molecularly pure,4–6 with a precise chemical formula (denoted
as Aun(SR)m, in general), which allows one to address funda-
mental questions concerning matter at the nanoscale. Interest-
ingly, it has been discovered that these materials could also be
obtained by the transformation of one structure into another.7,8
In a recent work, Maran et al. reported that Au25(PET)18 (where
PET = 2-phenylethylthiolate) is able to slowly react with itself,
without any co-reagent, generating Au38(PET)24. The authors
propose that the size transformation reaction involves the formation
of a precursor complex in which van der Waals interactions between
the ligands of the two interacting clusters act as initial driving
force.8 However, according to the current literature, the main way to
trigger such cluster transformation is by ligand exchange reactions
(LERs, here after). LERs is a process that takes advantage of the
dynamic nature of the thiol–gold surface9 and in most cases it does
not cause any change in size or structure of the cluster, only
replacing one capping ligand by another one (the latter in excess in
solution).10
In our work, PET or butanethiol (abbreviated as SBut) were
used as initial ligand for [Au25(SR)18]0, which was synthesized
and purified according to reported protocols.22 LERs were
performed with a chiral free ligand: R- or S-2-phenylpropane-
1-thiol (Scheme 1, abbreviated as R- or S-PPT hereafter, which
´
´
`
Departement de Chimie Physique, Universite de Geneve, 30 Quai Ernest-Ansermet,
`
1211 Geneve 4, Switzerland. E-mail: thomas.buergi@unige.ch
† Electronic supplementary information (ESI) available: Synthesis and character-
ization of Au25 and Au28 nanoclusters and details of ligand exchange reactions.
See DOI: 10.1039/c9cc08872e
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0
0
Scheme 1 Transformation of [Au25(SR)18
] cluster to [Au25(PPT)18] and
Au28(PPT)21. The depicted structure of Au28(PPT)21 is an illustration. The
real structure is unknown.
Fig. 1 Left: UV-vis spectra of both fractions obtained after LER of 0.1 mM of
[Au25(SBut)18]0 with S-PPT at ratio 1 : 2 regarding the ligands at room temperature
0
in dichloromethane. Right: SEC column after LER of [Au25(SBut)18
]
with
were synthesized following a published protocol23 with minor
modifications). Neither thermal activation nor large excess of
the incoming ligand was required to promote the transforma-
S-PPT.
tion. Interestingly, this transformation was observed when the elutes first and which therefore corresponds to a larger cluster,
only difference between the endogenous and the incoming shows a different spectrum. Compared to [Au25(SBut)18]0 the
thiol is one additional methyl group, which leads to a chiral peaks at 400 and 690 nm are missing and a new peak appears
centre and which changes the bulkiness and sterics of the at 557 nm. The purity of fraction 1 was also investigated by NMR
0
thiolate ligand.19 The resulting Au28(SR)21 and [Au25(SR)18
]
(Fig. S15, ESI†). The missing signal B1.40 ppm originating from
clusters were characterized by UV-vis absorption, circular dichroism the –SH group in the cluster spectrum demonstrates the absence
(CD), NMR, matrix-assisted laser desorption ionization time-of- of free ligand.
flight (MALDI-TOF) mass spectrometry and electrospray ionization
(ESI) mass spectroscopy (see ESI† for synthesis and characterization positive mode. As expected, fraction 2 shows a distribution between
details). 6500–7100 Da (Fig. S16, ESI†) and the most abundance cluster with
LERs were similarly performed to that described in previous mass of 6776.58 Da assigned to [Au25(SBut)14(S-PPT)4]0 (theoretical
Both fractions were analysed by ESI mass spectroscopy in
0
0
work:24 10 mg of [Au25(SBut)18
]
or [Au25(PET)18
]
cluster value is 6777.69 Da). Fig. S17 (ESI†) shows the mass spectrum
(the synthesis and characterizations of these two clusters can of fraction 1, which has a distribution of clusters reminiscent of
be found in Section II, ESI†) were dissolved in 10–15 mL of the ligand exchange. The most abundant cluster with a mass of
dichloromethane or toluene solvent together with free chiral m/z = 8255.19 Da is assigned to Au28(SBut)7(S-PPT)14 (theo-
ligand (R- or S-PPT). A molar ratio of 1 : 2 regarding the ligand retical mass 8257.34 Da). Simulated isotopic patterns of the
was used. The reaction mixture was slowly stirred for 18 hours at suspected cluster are in excellent agreement with the experi-
room temperature. All the reaction conditions and UV-vis spectra mental high resolution ESI spectra, providing further evidence
of the different LERs experiments are given in Section III (ESI†). of the new cluster (Fig. S18, ESI†). By running the ligand
In the following, we will focus on the reaction carried out with exchange a second time, after removal of free ligands and Au25,
[Au25(SBut)18]0 and S-PPT, although consistent results were (under the same conditions) the yield of the new species could be
obtained with [Au25(SBut)18]0 or [Au25(PET)18]0 and R-PPT. After increased. The ESI spectrum of fraction 1 of such an experiment
the LERs, the product was purified by size exclusion chromato- is shown in Fig. 2. Again a distribution of Au28(SBut)x(S-PPT)21Àx
graphy (SEC), and the different fractions analysed by UV-vis species is observed with a peak at m/z = 8691.0 Da (theoretical
measurements. According to the obtained UV-vis curves value 8692.04 Da) assigned to the completely exchanged species
(Fig. S5, ESI†), gradual change of the spectra was observed. Apart Au28(S-PPT)21. It is worth to mention that a prominent fragment
from the spectrum of [Au25(SBut)18]0, which eluted later, a of Au24(S-PPT)17 is also detected at m/z = 7297.4, i.e., after the
different spectrum was observed for the first fraction, indicating loss of Au4(S-PPT)4 unit, which is very common in mass spectro-
a new species. Note that the new species and the Au25 cluster are metry of thiolate-protected clusters. Due to the overlap of the two
not clearly separated in the column (inset Fig. 1). Two represen- cluster fractions on the SEC column, clear peaks of [Au25(SBut)x-
tative curves after normalization are depicted in Fig. 1 which (S-PPT)18Àx]0 are also observed (Fig. S19, ESI†). In order to clarify if
illustrate the distinctly different spectra. The absorption spectra in this transformation is unique to the neutral [Au25(SR)18]0 cluster,
Fig. 1 (fraction 1 and fraction 2) were obtained from an experiment (which is an open shell system) we repeat the LER reaction with
À
where only the beginning of fraction 1 and the end of fraction 2 the closed shell anionic [Au25(SR)18
]
cluster and R-PPT. From
were collected (fraction no. 1 and fraction no. 4 from Fig. S5, the UV-vis (Fig S20, ESI†) and mass spectra (Fig S21, ESI†), no
ESI†), in order to avoid mixing of the two species. The spectrum of Au28SR21 cluster could be identified, meaning that the title
fraction 2 is characteristic of [Au25(SBut)18]0. Fraction 1, which transformation does not occur with the anionic species cluster.
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Fig. 2 Electrospray ionization (ESI) mass spectroscopy of the fraction 1
from Fig. 1.
The Au28(S-PPT)21 cluster has similar composition as
Au28(TBBT)20 (they differ in one ligand), which has been
obtained by LERs induced transformation from [Au25(PET)18]0.17
Comparing the UV-vis spectra of both clusters, it is evident that
the UV-vis of Au28(TBBT)20 presents three bands at 365, 480 and
580 nm,17 while the spectrum of Au28(S-PPT)21 shows only two
bands at 448 and 557 nm (Fig. 1). Interestingly, the Au28(TBBT)20
0
Fig. 3 CD spectra of R/S-PPT-substituted Au28(SR)21 (A) and [Au25(SR)18
]
(B) nanoclusters. The average formulations of clusters are calculated by
intensity of related ESI spectra.
cluster undergoes structural change (isomerism) upon LERs with the [Au25(R-PPT)18]0 cluster was prepared by direct synthesis.23
cyclohexanethiol (S-c-C6H11). Note that in the former thiol an Compared with CD spectrum of [Au25(SBut)2(PPT)16]0 the spec-
aromatic ring is directly connected to the sulphur, which is not trum of Au28(SBut)2(PPT)19 is significantly different, showing
the case in the latter.11 The spectrum of Au28(S-c-C6H11)20 presents three intense bands at 325 nm, 380 nm, 460 nm and a weak one
bands at 460 and 550 nm similar to the spectrum in Fig. 1, at 530 nm. As expected, the two spectra of Au28(SBut)2(PPT)19
however the band at 355 nm of Au28(S-c-C6H11)20 spectrum is with two enantiomers of the ligand display good mirror image
missing in Au28(R-PPT)21 as well. Zeta potential measurements relationship.
of the cluster in toluene revealed a slightly negative value (À7.6 Æ
1.2 mV), which may indicate one negative charge anionic cluster performed a LERs in the opposite direction, i.e., the Au28(S-PPT)21
with free electron count of 8 (magic numbers). cluster was reacted with an achiral thiol (specifically SBut thiol,
In order to find out if the core of Au28(PPT)21 is chiral, we
The experiments described above indicate the coexistence 1:2 S-PPT : SBut). A CD spectrum was recorded immediately after
of Au28(S-PPT)21 and [Au25(S-PPT)18]0 clusters after the LER in purification, but no clear CD bands were observed (Fig. S22, ESI†).
mild conditions. In addition, the LERs at different ratio of That means, either the Au28(SR)21 cluster is achiral, or we have a
incoming/endogenous thiol as shown in Section III (ESI†), racemic mixture even after LERs. It is worth to mention that
implied that an increase in the chiral ligand gives rise to an after LERs with SBut, the Au28(SR)21 remains stable and no
increase in the proportion of Au28(R-PPT)21 cluster. Moreover, formation of Au25 was observed (Fig. S11, ESI†). Therefore, the
large amount of the incoming ligand (above 1 : 5 molar ratio) new Au28(SR)21 cluster is stable to LERs.
make both clusters unstable and no clear UV-vis features were
observed for the resulting solution (Fig. S9, ESI†).
This new product of LERs, Au28(SR)21, is indeed very unexpected,
because Au25 has been extensively used for LERs with different kind
CD spectroscopy is a powerful tool, very sensitive to the of ligands in mild-conditions. However, this transformation has not
structure of clusters. Fig. 3 shows the CD spectra of R/S-PPT- been reported until now. In our opinion, the main reason is that
substituted Au28(SBut)21 and [Au25(SBut)18]0 clusters. From the this new cluster is easily fragmenting in MALDI, much more than
distribution of ligands as determined by ESI analysis in Fig. 2 [Au25(SR)18]0, making the cluster difficult to be detected. In addition,
and Fig. S16 (ESI†), an average formula of these two clusters of the cluster is a side-product in LERs with SBut or PET ligands.
0
Au28(SBut)2(PPT)19 and [Au25(SBut)2(PPT)16
]
can be obtained. Further when we performed ligand exchange of [Au25(PET)18]0 with
Intense bands at 305 (positive), 352 (negative), 415 nm (positive) SBut or of [Au25(SBut)18]0 with PET, the amount of Au28 that has
and 480 nm (negative) observed in the CD spectra of [Au25(SBut)2- been obtained was a few % only (Fig. S12 and S13, ESI†). This
(R-PPT)16]0, as expected [Au25(SBut)2(S-PPT)16]0 showed the opposite makes the detection of the cluster difficult.
0
signs. This result of [Au25(SBut)2(R-PPT)16
]
is in good agree-
Clearly, the type of thiol ligand plays a critical role in the
ment with the previous published results by Zhu et al., where transformation of clusters by LERs. Jin et al. proposed that the
14916 | Chem. Commun., 2019, 55, 14914--14917
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difference between incoming/endogenous ligand, mainly the Postdoctoral fellowship (790715 ChiralComm) and Yanan Wang
difference in bulkiness, and the thermal conditions are the thanks the China Scholarship Council fellowship (201706450070).
driving forces for cluster transformations by LERs.25 In our
case the transformation of [Au25(PET)18]0 into Au28(R-PPT)21 has
been observed when the only difference between the two
ligands is a methyl group. Moreover, the electronic conjugation
Conflicts of interest
and even the thiol structure (primary thiol) is the same and the
transformation is still happening in mild conditions. Our work
indicates that very small changes (methyl group) of the ligand
can be sufficient to drive a cluster transformation at mild
conditions. A possible explanation is that the van der Waals
interactions between the ligands are significantly altered by the
methyl group and that the changed interactions within the
ligand shell drive the transformation.
Finally, it is well-known that racemic mixtures often present
a larger degree of packing and less steric hindrance than the
corresponding pure enantiomers.26 In order to explore if the
transformation depends on the enantiopurity of the ligand, we
repeated the LERs in the same experimental conditions but
with a racemic mixture of PPT, which is commercially available.
As shown in Fig. S14 (ESI†), the UV-vis spectrum indicated
the existence of Au28(PPT)21 after LERs without any purifica-
tion. So we can draw the conclusion that for the formation of
Au28(PPT)21 an enantiopure ligand is not necessary.
There are no conflicts to declare.
Notes and references
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TB thanks the Swiss National Science Foundation (grant number
200020_172511) and the University of Geneva for financial support.
´
26 R. Hong, J. M. Fernandez, H. Nakade, R. Arvizo, T. Emrick and
´
Belen Nieto-Ortega thanks the Marie Skłodowska-Curie Action
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This journal is ©The Royal Society of Chemistry 2019
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