A chiral gold nanocluster Au20 protected by tetradentate phosphine ligands

Article abstract of DOI:10.1002/anie.201308599

The chirality of a gold nanocluster can be generated from either an intrinsically chiral inorganic core or an achiral inorganic core in a chiral environment. The first structural determination of a gold nanocluster containing an intrinsic chiral inorganic core is reported. The chiral gold nanocluster [Au₂₀(PP₃)₄]Cl₄ (PP ₃=tris(2-(diphenylphosphino)ethyl)phosphine) has been prepared by the reduction of a gold(I)-tetraphosphine precursor in dichloromethane solution. Single-crystal structural determination reveals that the cluster molecular structure has C₃ symmetry. It consists of a Au₂₀ core consolidated by four peripheral tetraphosphines. The Au₂₀ core can be viewed as the combination of an icosahedral Au₁₃ and a helical Y-shaped Au₇ motif. The identity of this Au₂₀ cluster is confirmed by ESI-MS. The chelation of multidentate phosphines enhances the stability of this Au₂₀ cluster.

Full text of DOI:10.1002/anie.201308599

Angewandte
Chemie
DOI: 10.1002/anie.201308599
Gold Nanoclusters
A Chiral Gold Nanocluster Au20 Protected by Tetradentate Phosphine
Ligands**
Xian-Kai Wan, Shang-Fu Yuan, Zhi-Wei Lin, and Quan-Ming Wang*
Dedicated to Professor Xin-Tao Wu on the occasion of his 75th birthday
Abstract: The chirality of a gold nanocluster can be generated
from either an intrinsically chiral inorganic core or an achiral
inorganic core in a chiral environment. The first structural
determination of a gold nanocluster containing an intrinsic
chiral inorganic core is reported. The chiral gold nanocluster
[Au20(PP3)4]Cl4 (PP3 = tris(2-(diphenylphosphino)ethyl)phos-
phine) has been prepared by the reduction of a gold(I)–
tetraphosphine precursor in dichloromethane solution. Single-
crystal structural determination reveals that the cluster molec-
ular structure has C3 symmetry. It consists of a Au20 core
consolidated by four peripheral tetraphosphines. The Au20 core
can be viewed as the combination of an icosahedral Au13 and
a helical Y-shaped Au7 motif. The identity of this Au20 cluster is
confirmed by ESI-MS. The chelation of multidentate phos-
phines enhances the stability of this Au20 cluster.
Both phosphines and thiolates have been used in the
preparation of ligand-protected gold nanoclusters.[14] Phos-
phine-protected gold nanoclusters have a stability issue,
À
because of the weaker Au P bond in comparison with gold–
thiolate bonding. Our strategy to solve this problem is the use
of multidentate phosphines, and the attempt with tetraden-
tate phospine tris(2-(diphenylphosphino)ethyl)phosphine
(PP3) led to the isolation of a chiral gold nanocluster
[Au20(PP3)4]Cl4 (1) with a chiral 20-gold-atom core. Herein,
we present this unprecedented structurally determined Au20
nanocluster with an intrinsic chiral inorganic core. The C3
Au20 core can be viewed as the combination of a centered
icosahedral Au13 and a helical Y-shaped Au7 motif.
The preparation of 1 started with the reduction of
[PP3Au4Cl4] by NaBH4 in CH2Cl2 solution. Solvents were
removed to give a black solid, which was recrystallized in
CH2Cl2/CH3OH/pentane to afford black crystals of 1 in
a typical yield of about 20%.
L
igand-protected gold nanoclusters of atomic precision have
attracted increasing attention owing to their potential appli-
cations in catalysis, sensing, or in biology.[1–5] Some gold
nanoclusters can bear intrinsically chiral features. The chir-
ality of a gold nanocluster can be generated from two origins:
an intrinsically chiral inorganic core or an achiral inorganic
core in a chiral environment.[6] The environment can be chiral
organic ligands or asymmetric arrangements, such as RS-Au-
SR-Au-SR staples. A large number of ligand-protected gold
nanoclusters have been prepared, but only a few have been
structurally determined.[7] Au38(SR)24 is chiral owing to the
asymmetric arrangement of staples, although it has a high
Single-crystal structural analysis[15] revealed that 1 com-
prises a tetracationic cluster [Au20(PP3)4]4+ and chloride
counterions. As shown in Figure 1a, the structure has a Au20
core wrapped by four PP3 ligands. It has a C3 symmetry with
a threefold axis passing through atoms P1, Au1 and Au8.
Geometrically, the Au20 core can be described as a centered
icosahedron Au13 combined with a Au7 motif. The Au20 core
has 16 surface and four interstitial gold atoms. Each surface
gold atom is coordinated by a phosphorus donor of PP3. The
PP3 ligands are of two types: the one on the top bridges four
Au atoms symmetrically; the other three are of the same type,
each connects one Au3 triangle and a neighboring Au atom
(Figure 1b).
For clarity, the Au20 core is illustrated in Figure 1c with Au
atoms highlighted in different colors. The Y-shaped Au7 motif
is in green and the Au13 icosahedron is in orange. A side view
of the core is illustrated in Figure 1d to show the asymmetric
arrangement of gold atoms. As shown in Figure 1e, the 16
surface gold atoms can be classified into six types (from type
A to type E). Accordingly, the 31P NMR should display six
peaks at a ratio of 1:3:3:3:3:3 based on the number of atoms
of each type. Experimentally, five peaks at d = 39.78, 41.32,
48.56, 57.05 and 66.26 ppm in a ratio of 1:3:3:6:3 were
observed (Supporting Information, Figure S1). The 31P NMR
data match the expected, because the peak at d = 57.05 ppm
actually contains the contribution from phosphorous donors
linked to two types of Au atoms (types D and E) with similar
symmetric gold kernel.[8] Au28(SR)20 and Au102(SR)44 are
also chiral for the same reason. However, Au25(SR)18, having
a structure with a Au13 core surrounded by RS-Au-SR-Au-SR
staples, is not chiral.[11] Au36(SR)24 is not chiral either.[12] To
date, there is no structural evidence showing a ligand-
protected gold nanocluster with a chiral inorganic core,
although a theoretical study confirmed a bare gold cluster
Au34À to be intrinsic chiral.[13]
[9]
[10]
[*] X.-K. Wan, S.-F. Yuan, Z.-W. Lin, Prof. Dr. Q.-M. Wang
State Key Lab of Physical Chemistry of Solid Surfaces
Department of Chemistry, College of Chemistry and
Chemical Engineering, Xiamen University
Xiamen, 361005 (P.R. China)
E-mail: qmwang@xmu.edu.cn
[**] This work was supported by the Natural Science Foundation of
China (21125102) and the 973 program (2014CB845603).
À
coordination environments. The Au Au distances are in the
range of 2.572(2)–3.216(2) ꢀ, and the 2.572(2) ꢀ bond length
is among the shortest ones observed for gold clusters. A very
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308599.
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of 0.36(6). The sample is silent in CD
spectroscopy because it is a racemic mixture.
The sample was further characterized by
an ESI-TOF mass spectrometer with an
electrospray ionization source in positive
mode (Figure 2a). The spectrum is quite
clean, with the molecular ion peak [Au20-
(PP3)4]4+ at m/z = 1655.30 being dominant. A
minor peak at m/z = 2219.06 was observed,
which corresponds to [Au20(PP3)4Cl]3+
derived from the binding of a chloride to
[Au20(PP3)4]4+. The observed isotopic pat-
terns of [Au20(PP3)4]4+ and [Au20(PP3)4Cl]3+
are perfectly in agreement with the simulated
(Figure 2b,c). To fulfill the requirement of
charge balance, 16 of the 20 gold atoms
should have a valency of zero. The bonding
energy of Au4f7/2 was determined to be
84.3 eV by XPS (Supporting Information,
Figure S2), which are characteristic of Au0.
Because the Au atoms are largely Au0 in the
nanocluster, the band at 84.9 eV for AuI
could hardly be observed.
Nanoclusters with 20 gold atoms fall in
a special category. Wang et al. reported that
the most stable form of a bare Au20 is
a tetrahedral (Td) structure.[17] They also
synthesized ligand-protected Au20 using
a two-phase system, and the product was
2+
identified as Au20(PPh3)8 by mass spectro-
metry, which was computed to have a tetra-
hedral structure.[18] Jin et al. reported a thio-
late-protected Au20 cluster, and its composi-
tion Au20(SCH2CH2Ph)16 was determined by
mass spectrometry.[19] Zen et al. theoretically
predicted that this thiolate-protected Au20
cluster has a prolate Au8 core with four -RS-
Au-RS-Au-RS-Au-RS- extended staples.[20]
Recently, we reported the first structural
Figure 1. a) The structure of the tetracationic [Au20(PP3)4]4+ cluster in 1. b) Two types of
bridging modes of PP3 ligands. c) Top view of the Au20 core structure in 1; green atoms
denote the Au7 unit, and the Au13 icosahedron is shown in orange; d) side view. e) The
bridging mode of PP3 in 1; the 16 surface gold atoms are of six types (from A to F), the
four interstitial Au atoms are shown in green, and phenyl groups have been omitted for
clarity.
determination of
a phosphine-protected
Au20 cluster [Au20(PhPpy2)10Cl2]Cl4, which
short Au-Au distance of 2.5817(7) ꢀ was found recently in
[Au14(PPh3)8(NO3)4].[16]
has a bivertex-shared double incomplete icosahedral struc-
ture.[21] The present work represents an unprecedented chiral
arrangement of Au20.
The UV/Vis absorption spectrum of 1 in CH2Cl2 shows
three prominent absorption bands at 370, 495, 550 (shoulder),
and a broad band around 833 nm tailing to 1000 nm
(Figure 3). The optical energy gap was determined to be
1.33 eV (Figure 3 inset), which is much lower than 1.77 eV
found in bare Au20, 2.15 eV in Au20(SCH2CH2Ph)16, and
The Au20 core is chiral. The C3 symmetry is clearly shown
in Figure 1c, where the cluster is viewed down the threefold
axis. The helical arrangement of the Au7 motif lowers the
symmetry of the Au20 cluster. This is the first structural
evidence on a gold nanocluster bearing an intrinsic chiral
inorganic core. Previous structurally determined chiral gold
nanoclusters all contain achiral inorganic cores in chiral
environments, and the origin of the chirality is either the
chiral organic ligand or the asymmetric arrangement of
staples.
The absolute configuration of 1 is related to the con-
formation of PP3. Because the flexible PP3 can adopt two
types of conformations, both enantiomers of 1 are present in
the crystal (Supporting Information, Figure S6). Cluster
1 crystallized in a non-centrosymmetric space group, but
racemic twinning occurred as suggested by Flack parameter
2.24 eV in [Au20(PhPpy2)10Cl2]Cl4. Cluster 1 and [Au20-
[21]
(PhPpy2)10Cl2]Cl4
each contains a phosphine-protected
Au20 core, but they have quite different HOMO–LUMO
gaps. This fact shows that the core structures affect the optical
properties of gold nanoclusters containing the same number
of gold atoms.
Although the optical energy gap of 1 is not high, this Au20
cluster is quite stable. It is confirmed by UV/Vis spectra that
no decomposition was observed after its solution had been
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chelating ability of multidentate phosphines enhances the
stability of the Au20. The chiral arrangement of the Au20 core
structure indicates that the protecting ligands can impose
intense influence on the structure of the resulted gold clusters.
We expect that homochiral gold nanoclusters with a chiral
inorganic gold core could be prepared with the use of chiral
multidentate protecting ligands. Further work on modulating
the structures of gold nanoclusters with various bridging
ligands is in progress.
Experimental Section
Synthesis of 1: A freshly prepared solution of NaBH4 (22.1 mg,
0.585 mmol) in ethanol (3.0 mL) was added dropwise under vigorous
stirring to a solution of PP3Au4Cl4 (93.4 mg) in CH2Cl2 (80 mL). The
solution color changed from colorless to brown and finally to black.
The reaction mixture was stirred for 10 h at 308C in air in the absence
of light. Solvent was removed under reduced pressure to give a black
solid. This solid was then dissolved in CH2Cl2 (3.0 mL) and CH3OH
(0.1 mL), and the solution was centrifuged for 4 min at 8000 rminÀ1
.
The brown supernatant was collected and subjected to the diffusion of
pentane, black crystals of 1 deposited after about 5 days (17.8 mg,
yield 23.2% based on Au).
Figure 2. a) Mass spectrum of the Au20 cluster. b),c) The measured
(solid trace) and simulated (dotted trace) isotopic patterns of
b) [Au20(PP3)4]4+ and c) [Au20(PP3)4Cl]3+
.
Received: October 2, 2013
Revised: December 12, 2013
Published online: February 7, 2014
Keywords: chiral structures · cluster compounds · gold ·
.
nanostructures · phosphines
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