Ligand dependence of the synthetic approach and chiroptical properties of a magic cluster protected with a bicyclic chiral thiolate

Article abstract of DOI:10.1039/c2cc00056c

Chiral gold clusters stabilised by enantiopure thiolates were prepared, size-selected and characterised by circular dichroism and mass spectrometry. The product distribution is found to be ligand dependent. Au₂₅ clusters protected with camphorthiol show clear resemblance of their chiroptical properties with their glutathionate analogue. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc00056c

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COMMUNICATION
Ligand dependence of the synthetic approach and chiroptical properties
of a magic cluster protected with a bicyclic chiral thiolatewz
Stefan Knoppe,a Nuwan Kothalawala,b Vijay Reddy Jupally,b Amala Dassb and
Thomas Bu
¨
rgi*a
Received 4th January 2012, Accepted 16th March 2012
DOI: 10.1039/c2cc00056c
Chiral gold clusters stabilised by enantiopure thiolates were
prepared, size-selected and characterised by circular dichroism
and mass spectrometry. The product distribution is found to be
ligand dependent. Au25 clusters protected with camphorthiol
show clear resemblance of their chiroptical properties with their
glutathionate analogue.
A reasonable explanation for the observed optical activity in
the Au25(SR)18 system is the mixing of sulfur orbitals into the
states involved in the respective transitions.2 Surprisingly, the
only two available CD spectra of Au25(SR)18, where SR is
glutathionate5 and 2-methyl-2-phenylethylthiolate,2 respectively,
are quite different, whereas their absorption spectrum is very
similar. In order to understand the subtle effect of the ligand on
the optical activity we considered a different ligand system.
We hereby present the synthesis of a novel camphor-10-thiolate-
protected gold cluster system (in the following Aun(CamS)m).
A one-phase size-focusing approach was used which yields Au25-
(CamS)18 as a major product (along with others, see below).17,18
The crude reaction mixture was size-separated using gel permea-
tion chromatography (GPC).8,19 The isolated fractions were
characterised by MALDI mass spectrometry, UV-Vis spectroscopy
and Electronic Circular Dichroism (CD). In a similar approach, we
used 1-phenylethylthiol (1-PET, Scheme 1). This ligand yields a
polydisperse mixture of clusters, showing that the success of the
one-phase approach towards Au25 is a strongly ligand-dependent
process.
Thiolate-protected gold clusters (o200 Au atoms) have recently
become an intensively studied field. Their optical, magnetic and
electrochemical properties differ widely from bigger nanoparticles
(42 nm) that exhibit surface plasmon resonances.1 Whereas
the general properties (optical spectrum, electronic structure) of
selected clusters (e.g. Au25(SR)18, Au38(SR)24; SR: thiolate) are
well-understood, their chiroptical properties are barely studied.
Several examples of chiral gold nanoclusters have been presented
in the past2–8 and conformational analysis of the adsorbed
ligand was performed using Vibrational Circular Dichroism.9,10
Moreover, the crystal structures of Au38(SR)24 and Au102(SR)44
bear intrinsic chirality.11,12
Since the discovery of optical activity in glutathionate-
protected clusters,5,6 a series of ligands was examined; mostly
cysteine-derivatives.4,9,13,14 Due to lack of methodology,
synthesis and characterisation of monodisperse chiral gold
clusters (and their correct assignment) is rare. Recently,
major breakthroughs were size-conserving ligand exchange
experiments7,15 and successful synthesis of chiral Au25(SR)18.2
This cluster bears a Au13 core covered by six long staples
(SR-Au-SR-Au-SR).16 The core and the arrangement of the
staples on the latter are not chiral, in contrast for example to
Au38(SR)24. Therefore, the observed optical activity has to
be linked to the chirality of the adsorbed thiolates (SR).
The ligands were prepared according to published protocols
(details in the ESIz).20,21 The bicyclic structure of camphorthiol
contains two fixed stereogenic centres. 1-PET is a structural isomer
of the widely used 2-phenylethylthiol. The applied one-phase
approach avoids the use of phase transfer reagents such as TOAB,
which are difficult to remove from the product. It was shown in
earlier studies that the approach yields pure Au25(SR)18 clusters
using simple alkanethiol ligands.17,18
In the camphorthiol case we gained a mixture of clusters of
different sizes. GPC separation yielded three fractions labelled
as 1, 2 and 3 according to increasing elution times (decreasing
hydrodynamic volume). The UV–Vis spectrum of fraction 2 is
in accordance to those reported for Au25 clusters. The spectra
of fractions 1 and 3 are featureless and do not allow direct
assignment of the cluster sizes (ESIz).
a Department of Physical Chemistry, University of Geneva,
`
30 Quai Ernest-Ansermet, 1211 Geneve 4, Switzerland.
E-mail: Thomas.Buergi@unige.ch; Fax: +41 (0)22-379 6103;
Tel: +41 (0)22-379 6552
b Department of Chemistry and Biochemistry,
University of Mississippi, 352 Coulter Hall, Oxford, 38677 MS,
USA. E-mail: amal@olemiss.edu; Fax: +1-662-915 7300;
Tel: +1-662-915 7301
w This article is an invited contribution to the ChemComm ‘‘Chirality’’
web theme.
z Electronic supplementary information (ESI) available: Details on
synthesis of the ligands and clusters and their size-separation; addi-
tional UV-Vis, CD and mass spectra. See DOI: 10.1039/c2cc00056c
Scheme 1 Structures of 1S,4R-camphorthiol (left) and S-1-phenyl-
ethylthiol (right).
c
4630 Chem. Commun., 2012, 48, 4630–4632
This journal is The Royal Society of Chemistry 2012

Fig. 2 MALDI mass spectra of gold nanoclusters using 1-phenyl-
ethylthiolate as a protecting ligand. The size distribution between the
different reaction batches is widely reproducible. No traces of Au25 clusters
are found. A detailed insight into the spectra is given in the ESI.z
activity of Au25 towards reduction of carbonyls.23 The mass
spectrum of fraction 3 shows a series of weak signals in the
mass range below Au25, the signal of strongest intensity (5886 Da)
is in good agreement with Au18(CamS)13 (calcd. 5928 Da).
We conclude that Au18(CamS)13 is the major component of
this fraction.
Fig. 1 Top: MALDI mass spectrum of Au25(1S,4R-CamS)18. Au25-
(2-PET)18, Au38(2-PET)24 and Au40(2-PET)24 were added for internal
calibration. Bottom: the ESI spectrum of Au25(CamS)18 (black) and
simulated isotope distribution patterns for reduced (red) and non-reduced
(blue) keto groups.
In the 1-PET case, a mixture of polydisperse clusters was
gained; notably, no traces of Au25 are found (Fig. 2 and ESIz).
The spectra are highly reproducible between different reaction
batches, regardless of enantiomeric purity of the ligand (the
racemic ligand gives a similar size distribution). Slightly
different synthesis conditions may explain the slight shift
towards higher average mass in the S-1-PET batch (Fig. 2, top).
We attempted to size-select the clusters with GPC but no
significant separation was achieved probably due to the quasi-
continuous size-distribution of the clusters. The fact that no
Au25 was found in the mass spectra (expected position marked
in the ESIz) is highly surprising and reveals the ligand-
dependency of the applied one-phase approach (or stability
of Au25(1-PET)18). Interestingly, the steric demand of 1-PET
compared to CamSH (bicyclic) or glutathione (tripeptide)
seems to be less and is therefore not expected to be the
reason for the non-formation of Au25(1-PET)18. The question
concerning the stability of Au25 that is protected by different
ligands remains open at this stage.
MALDI mass spectra of the isolated fractions were measured
using DCTB as matrix (spectrum of fraction 2 in Fig. 1, top).22
Fraction 1 is—in agreement with the UV-Vis spectra—a poly-
disperse mixture of particles heavier than Au25. The mass range
(11 000–20 000 Da) indicates clusters composed of 38 to 100 Au
atoms. A detailed assignment of the signals is hard to achieve as
fragment peaks overlap with signals of unfragmented clusters. The
mass spectrum of fraction 2 shows only one major signal, assigned
to Au25 (calcd. 8223 Da), as expected from the UV–Vis spectra.
However, the mass found is slightly higher than expected
(8260 Da, internal calibration done with addition of Au25-
(2-PET)18, Au38(2-PET)24 and Au40(2-PET)24, 2-PET: phenyl-
elthylthiolate). Upon reduction of gold with sodium borohydride,
the keto-group in the camphorthiol ligand might have been
reduced, leading to an increase in mass (calcd. 8260 Da). This
conclusion is substantiated by the electrospray ionisation mass
spectrum that shows good agreement with reduced keto groups
(Fig. 1, bottom). The reduction of the carbonyl group is confirmed
by IR spectroscopy which reveals a drastic decrease of the
carbonyl band in the cluster spectrum compared to the free ligand
(ESIz). The remaining weak carbonyl signal might be due to some
(unreduced) free ligands in the sample. We noticed a much
stronger carbonyl signal in less purified samples. The complete
reduction of the carbonyl groups in the Au25(CamS)18 cluster, as
indicated by the mass spectrum, and the presence of unreduced
carbonyls in free CamSH are in accordance with the catalytic
CD spectra of the isolated fractions of Aun(CamS)m and
Aun(1-PET)m were measured for both enantiomers (Fig. 3, top
(Au25) and ESIz). In the camphorthiol-case, the spectra exhibit
good mirror-image relationships; the same is observed for the
1-PET-protected clusters (to a lesser extend which may be due
to the varying size distributions between synthesis batches,
ESIz). As the strength of the CD spectrum is a concentration-
dependent property, anisotropy factors g = DA/A were calculated.
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Chem. Commun., 2012, 48, 4630–4632 4631

camphorthiol case and no Au25 in the 1-PET case. During the
synthesis of Au25 clusters the carbonyl group of adsorbed
camphorthiolate was reduced probably due to the catalytic
activity of the gold cluster. The CD spectra of Au25(CamS)18
show good agreement with the one of Au25(SG)18, in contrast
to Au25(pet*)18. We conclude that it is the relative orientation
of the ligands with respect to the staples, caused by steric
demand, rather than the actual structure of the ligand, which
dictates the optical activity of Au25. This may help to factorize
the contributions of different levels of chirality to the overall
chiroptical activity of gold clusters.
Notes and references
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Fig. 3 Top: CD spectra of 1R,4S- (black) and 1S,4R-camphorthiolate-
protected Au25 clusters. Bottom: CD spectrum of Au25(SG)18 (with
permission from the American Chemical Society).6 The spectrum of
Au25(1S, 4R-CamS)18 is very similar in sign and transition energies.
4 C. Gautier and T. Burgi, J. Am. Chem. Soc., 2008, 130, 7078–7084.
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5 T. G. Schaaff and R. L. Whetten, J. Phys. Chem. B, 2000, 104,
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The observed maximum anisotropy factors drop from 1 ꢀ 10ꢁ3
(fraction 3) over 3 ꢀ 10ꢁ4 (fraction 2) to 1 ꢀ 10ꢁ4 (fraction 1).
A similar trend has been observed earlier in binaphthyldithiol-
protected clusters, but no mass assignment was performed for
these.8 Comparison of the CD spectra of Au25(CamS)18 with
the ones reported by Jin and Whetten reveals some interesting
points.2,6 When comparing to the spectra of Au25(pet*)18 (pet*:
2-methyl-2-phenylethylthiolate), the difference in the spectral
shape is striking.2 In contrast, there is a remarkable resemblance
with the spectrum of Au25(SG)18 (SG: L-glutathionate) (Fig. 3).5,6
Glutathionate and camphorthiolate are both rather bulky
ligands in contrast to pet*. Within the staples (SR-Au-SR-
Au-SR) mentioned above the ligands can adopt different relative
orientations, corresponding to different absolute configurations at
the sulfur atom.16,24 We propose that this configuration at the
sulfur atoms, through mixing of sulfur orbitals into the relevant
electronic states,24 dominates the optical activity rather than the
actual structure of the ligand. The bulkiness of SG and CamS may
result in a similar relative arrangement (steric demand), in contrast
to the small pet* ligand (that also may be stabilised by aromatic
interactions with each other).
The presented camphorthiolate system exhibits the first
example in which an enantiopure, bicyclic ligand is used to
protect gold clusters; 1-PET is the first ligand in which the
thiolate function is directly attached to the stereogenic centre.
The one-phase approach for Au25 synthesis was shown to be
ligand dependent, leading to a polydisperse mixture in the
21 R. P. Volante, Tetrahedron Lett., 1981, 22, 3119–3122.
22 A. Dass, A. Stevenson, G. R. Dubay, J. B. Tracy and
R. W. Murray, J. Am. Chem. Soc., 2008, 130, 5940–5946.
23 Y. Zhu, H. Qian, B. A. Drake and R. Jin, Angew. Chem., Int. Ed.,
2010, 49, 1295–1298.
24 M. Zhu, C. M. Aikens, F. J. Hollander, G. C. Schatz and R. Jin,
J. Am. Chem. Soc., 2008, 130, 5883–5885.
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4632 Chem. Commun., 2012, 48, 4630–4632
This journal is The Royal Society of Chemistry 2012