Stibine-protected Au13 nanoclusters: Syntheses, properties and facile conversion to GSH-protected Au25 nanocluster

Article abstract of DOI:10.1039/c8sc03132k

Monostibine-protected ionic Au₁₃ nanoclusters, namely, [Au₁₃(L)₈(Cl)₄][Cl] (L= SbPh₃, 2a·Cl; Sb(p-tolyl)₃, 2b·Cl) were prepared by the direct reduction of Au(L)Cl with NaBH₄ in d

Full text of DOI:10.1039/c8sc03132k

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Stibine-Protected Au13 Nanoclusters: Syntheses, Properties and
Facile Conversion to GSH-Protected Au25 Nanocluster†
Received 00th January 20xx,
Accepted 00th January 20xx
Ying-Zhou Li, Rakesh Ganguly, Kar Yiu Hong, Yongxin Li, Malcolm Eugene Tessensohn, Richard
Webster and Weng Kee Leong*
DOI: 10.1039/x0xx00000x
Monostibine-protected ionic Au13 nanoclusters, namely, [Au13(L)8(Cl)4][Cl] (L = SbPh3, 2a.Cl; Sb(p-tolyl)3, 2b.Cl) were
prepared by direct reduction of Au(L)Cl with NaBH4 in dichloromethane. Anion exchange with 2a.Cl afforded
[Au13(SbPh3)8(Cl)4][X] (X = PF6, 2a.PF6; BPh4, 2a.BPh4). All these have been characterized by multinuclear NMR, ESI-MS and
UV-Vis spectroscopy. A crystallographic analysis on 2a.BPh4 reveals that the cation possesses C2v symmetry and the
tridecagold core adopts a closed icosahedron configuration. The weaker coordinating ability of the stibine ligands leads to
the ready reaction of 2b.Cl with PPh3 or glutathione (GSH) to form the smaller phosphine-protected cluster
[Au11(PPh3)8Cl2][Cl] or larger thiolate-protected cluster Au25(SG)18, respectively. In the latter reaction, the addition of a
small amount (0.5 to 3.5 equivalents) of a suitable oxidant such as K3(Fe(CN)6 accelerates the conversion rate significantly.
www.rsc.org/
formulation [Au13(dppm)6][NO3]4, was prepared by NaBH4
reduction of the Au(I) complex [Au2(dppm)][NO3]2 (dppm =
1,1-bis(diphenylphosphino)methane) in ethanol.11 Two other
Introduction
This last decade has witnessed an explosion in the research
output on thiolate-,1 and alkynyl-protected,2 gold nanoclusters.
The earliest work on what is now generally referred to as Au
nanoclusters can, however, be traced back to the pioneering
work of Malatesta on phosphine-protected species.3 A large
number of these, with a variety of metal core configurations,
have already been structurally characterized. They include
some cationic4 and neutral species5 with 8-39 gold atoms, as
well as some even larger ones for which their sizes have only
structurally-related examples, viz., [Au13(dppm)6][BPh4]3 and
[Au13(dppm)6][Cl]5, which contain open icosahedral metal
cores, have also been reported recently; the former was
obtained from a dppm-protected Au18 cluster by thiol-induced
size focusing, while the latter resulted from the direct
reduction of [Au2(dppm)(Cl)2] with NaBH4 in a solution of
methanol and dichloromethane (DCM).12,13 Interestingly, these
three cationic Au13 clusters have the same composition but
carry different charges.
A facile two-step approach to
been
estimated,
such
as,
[Au55(PPh3)12Cl6],6 and
diphosphine-protected Au13 clusters has also been reported;
the addition of aqueous HCl to a polydispersed mixture of Aun
[Au101(PPh3)21(Cl)5].7 Among these, the icosahedral Au13
clusters are of special interest as their cluster core represents
an important sub-unit in some of the larger clusters, such as,
the homometallic Au25, Au37, Au38, Au39 and Au60 clusters,1b-
d,4i,8 and the heterometallic Au13Cun (n = 2, 4, 8, 12) clusters.9
Although the Au13 cluster core is expected to be
thermodynamically stable due to its closed-shell geometry,10
the synthesis of monodispersed Au13 clusters remains a
challenge. Most of the studies have been on those protected
by diphosphines as they exhibit higher thermodynamic
stability arising from the chelating effect of the diphosphine.4d
The first diphosphine-protected cluster, with the probable
clusters (n
= 9-15), obtained from reduction of the
corresponding diphosphine-Au(I) complex, was found to
induce nuclearity convergence to Au13 clusters. This method
yielded
[Au13(L2)5(Cl)2][PF6]3 and [Au13(L)4(Cl)4][Cl] (L = L3, L4, L5),
where denotes an alkyl-bridged diphosphine
Lm
the
diphosphine-protected
Au13
clusters
(Ph2P(CH2)mPPh2.14 The method failed, however, for
monophosphines, and for diphosphines with a longer (L6) or
shorter (L1) alkyl bridge.
In
comparison,
Au13
clusters
stabilized
by
monophosphines are much less well-characterised. Reduction
of [Au(L)X] (L = monophosphine, X = halides, SCN or NO3) with
NaBH4 gave mainly Au11 or other smaller clusters,4a-d, 5a-b,15
although minor amounts of Au13 clusters may also be formed
in particular cases (vide infra). To our knowledge, only three
monophosphine-protected icosahedral Au13 clusters have
been reported. The first was [Au13(PPhMe2)10Cl2][PF6]3,
obtained from reaction of the initially formed
[Au11(PPhMe2)10][PF6]3 with [Et4N]Cl in alcohol.4f This was
Division of Chemistry & Biological Chemistry, Nanyang Technological University, 21
Nanyang Link, Singapore, 637371. Email: chmlwk@ntu.edu.sg
† Electronic Supplementary Information (ESI) available: Experimental,
crystallographic and computational details; table of crystal and refinement data
for 1a
of 1a
,
1b
,
, 2a.BPh4, 3a and 3b ( CCDC 1852139-43); ORTEP and stacking diagrams
3a and 3b; NMR, ESI-MS and UV-Vis spectra of products and some
,
1b
monitoring reactions; TD-DFT excitation transition data of 2a; CV spectra of
2a.PF6; percent buried volumes of stibine ligands; optimized coordinates of 1a
and 2a. See DOI: 10.1039/x0xx00000x.
-
c
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followed 15 years later by [Au13(PPh2Me)8Cl4][C2B9H12], which
The reduction of 1a and 1b with 0.25 equivalents of NaBH4
was obtained from the reaction of [Au11(PPh2Me)10][C2B9H12]3 in the presence of the free stibine gave the corresponding
with [Au(PPh2Me)Cl].16 The third example, [Au13(TOP)8Cl4][Cl] ionic nanoclusters [Au13(L)8(Cl)4][Cl] (L = SbPh3, 2a.Cl; Sb(p-
(where TOP = trioctylphosphine), was obtained as a minor tolyl)3, 2b.Cl) (Scheme 1). Unlike the phosphine-protected Au11
product from the NaBH4 reduction of [Au(TOP)Cl] in aqueous nanoclusters [Au11(PPh3)8Cl2][Cl], these stibine-protected
THF.14b Of these three examples, only the first (with a P:Cl ratio nanoclusters could not be purified by column chromatography
of 10:2) has been fully characterized, while the other two (with but they could be obtained in good purity by precipitation with
P:Cl ratios of 8:4) have only been characterized hexane followed by washing with suitable solvents. Although
spectroscopically and the exact ligand arrangements have not the isolated yields were relatively low (15% and 30% for 2a.Cl
been unambiguously determined.
and 2b.Cl, respectively), the MS and UV-vis spectra of the
In comparison, no stibine-protected Au nanoclusters are crude products did not show any detectable amounts of
known. We present here the first study on monostibine- nanoclusters of other sizes such as Au11 (Figs. S30 and S37).
protected Au13 nanoclusters with Sb:Cl ratios of 8:4, viz., Indeed, fractional crystallization of the crude from
[Au13(L)8Cl4][X] (where L = Ph3Sb, (p-tolyl)3Sb; X = Cl, PF6, BPh4), DCM/hexane (8/4, v/v) in the dark gave mainly
2.Cl and a
and their conversion to the larger glutathione-stabilised colorless crystalline side product identified as the
mononuclear Au(I) complexes [Au(SbPh3)4][SbPh2Cl2], 3a 19
[(p-tolyl)2Cl2Sb]Au[Sb(p-tolyl)3]3, 3b, respectively. Both 3a and
nanocluster Au25(SG)18.
, and
3b
were
characterized
spectroscopically
and
Results and discussion
crystallographically (Figs. S4-S5, S26-S28, S34-S35).
Syntheses and characterizations
Scheme 1. Reduction of 1 with NaBH4.
The Au(I) stibine complexes [Au(L)Cl] (L = SbPh3, 1a; Sb(p-
tolyl)3, 1b) could be prepared by the slow addition of an
equimolar amount of the stibine into a DCM solution of
Me2SAuCl at room temperature (ca. 21 oC) and in the dark. The
products were obtained as white solids after removal of
solvent but quickly decomposed on exposure to light, and
especially in solution. Crystallographic analyses show that
while 1a is a monomer, the p-tolyl analogue 1b is a dimer with
L
L
Au
Au
Cl
Cl
L
Au
Au
Au
Au
NaBH4 (0.25 equiv)/EtOH
L (0.25 equiv)
Cl
Au
Au
Au
Au
L
Au
Au
Cl
3
Au(L)Cl
+
L
DCM, r.t., 4-6h
1
a strong Au
…Au interaction (Figs. S1-S3). This difference in
L
L
structure is probably the result of polymorphism, as both
stacking modes have been observed previously in the
phosphine analogue [(p-tolyl)3PAuCl].17 An ORTEP plot
depicting one of the two crystallographically distinct dimeric
Au
L
Cl
2
.Cl
L = Ph3Sb, a; (p-tolyl)3Sb, b
units observed for 1b is shown in Fig. 1. The Au
…
(2.9441(14) Å and 2.9784(16) Å) is typical for the Au
Au distance
Au
…
The reduction of
1 exhibited markedly different behavior
interaction but is much shorter than that found in its
phosphine analogue (3.375 Å). This aurophilic interaction is
believed to bear significant influence on the reduction process
for the preparation of Au nanoclusters.18
from that of the phosphine analogue [Au(PPh3)Cl]. The latter
was reported to give the ionic nanocluster [Au11(PPh3)8Cl2][Cl]
with 0.25 equivalent of NaBH4 in DCM but the relatively less
stable, neutral nanocluster [Au11(PPh3)7Cl3] with 5 equivalents
of NaBH4 in THF.15 For
1, reduction with more than 0.25
equivalent of NaBH4 led to a decrease in yield without any new
nanocluster isolated or observed (Fig. S38). The use of 5
equivalents of NaBH4 (with either DCM or THF as solvent) led
to the immediate formation of an insoluble black precipitate
and a colorless supernatant, indicating full decomposition into
bulk Au particles. This indicates that while NaBH4 can reduce
1
to form 2.Cl, it also leads to the destruction of 2.Cl;
presumably, the weakly coordinating stibines allows for more
ready decomposition and/or aggregation of the gold cores.
Addition of a small amount (0.25 equiv) of the stibine prior to
reduction was found to aid the formation of 2.Cl, but larger
amounts (1.0 to 3.0 equiv) led to lower yields. This is probably
related to favoring the formation and/or enhancing the
stability of intermediate species, and is consistent with a
recent report that the reduction of [Au(PPh3)Cl] and
[Au(PPh3)2Cl] gave different products.20 We have similarly
observed that reduction of the tetra-stibine coordinated 3a
Fig. 1. ORTEP plot depicting one of the two crystallographically distinct dimers of 1b.
Thermal ellipsoids are drawn at the 50% probability level. Organic hydrogen atoms are
omitted for clarity. Selected bond lengths (Å): Sb(1A)‒Au(1A) 2.460 (2),
=
Sb(2A)‒Au(2A) 2.507(2), Au(1A)‒Cl(1A) 2.327(7), Au(2A)‒Cl(2A) = 2.259(7),
=
=
Au(2A)‒Au(2A) = 2.9441(14).
2 | J. Name., 2012, 00, 1-3
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under the same reaction conditions led to a more complicated superatom rule.24 The electronic structure of 2a, calculated at
mixture, as revealed by the UV-Vis spectrum of the crude the MPW1PW91/LANL2DZ level of theory, clearly shows
mixture, and much less 2a.Cl was obtained (Fig. S39). It is also frontier orbitals corresponding to this superatom complex (Fig.
known that steric bulk of the ligand can have a significant S6). In fact, the three “HOMO” orbitals may be regarded as
influence on the course of the reaction;21 we tested this with comprising a single HOMO (-0.258 eV) and a pair of near-
SbMes3 (Mes = mesityl), which has a larger percent buried degenerate HOMO-1 (-0.265 eV), with p orbital characteristics.
volume (40% for SbMes3 vs 28% for SbPh3 and Sb(p-tolyl)3).22 This is consistent with a breakdown of the icosahedrdal
The reduction of [Au(SbMes3)Cl] (1c) under similar reaction symmetry of the gold core by the ligand sphere; the
conditions, however, afforded a black precipitate in a colorless nanocluster exhibits non-crystallographic mm2 point
1
supernatant; the H NMR spectrum of the latter showed only symmetry. The four Cl ligands lie in two mutually
SbMes3 (Fig. S11). Presumably, the larger steric bulk of SbMes3 perpendicular planes, dividing the eight SbPh3 ligands into
1
prevented nucleation of the intermediate to form three groups in a 1:2:1 ratio, in agreement with the H NMR
nanoclusters.23
spectrum. The peripheral Au-Au bond lengths (2.8487(10) -
The chloride anion in 2a.Cl could be exchanged with [PF6]- 2.9248(11) Å) are clearly longer than those from the central Au
or [BPh4]- to give 2a.PF6 or 2a.BPh4, respectively, as verified by atom (2.7151(10) - 2.7629(10) Å), and comparable to those in
their multinuclear NMR spectra (Figs. S12-S21). Irrespective of [Au13(PPhMe2)10Cl2][PF6]3, the only other crystallographically
the anion, the 1H NMR spectrum is invariant and exhibits three characterized monophosphine-protected icosahedral Au13
doublets, in an integration ratio of 1:2:1, in the 7.15-7.50 ppm cluster (see also Table S3).4f
region (Fig. 2). These resonances are assignable to the ortho-
H’s of the SbPh3 ligand, and suggest the presence of three
chemically non-equivalent stibine ligand environments. The
spectrum of 2b.Cl similarly exhibits three singlets in a 1:2:1
integration ratio, in the 2.04-2.11 ppm region, which are
ascribable to the methyl protons in three chemically non-
equivalent Sb(p-tolyl)3 ligand environments (Fig. S20). This is in
contrast
to
the
phosphine-protected
nanoclusters
[Au11(PPh3)7Cl3] and [Au11(PPh3)8Cl2]Cl, which were reported to
show only one set of resonances in their 1H and 31P NMR
spectra even down to -80 oC. Presumably, therefore, the metal
cores of 2 are structurally more rigid than the undecagold core
in those phosphine-protected nanoclusters.5a,15
Fig. 3. Views of the crystal structure of the cationic fragment of 2a.BPh4 (left: full
structure with hydrogen atoms omitted; right: structure of the core showing the mm2
point symmetry). Au: yellow, Sb: blue, Cl: green, C: grey.
Consistent with the formulation, the ESI-MS(+) spectra of
2a.Cl and 2a.PF6 both shows a main peak at m/z ~2746 with an
isotopic interval of 0.5 amu, indicating a +2 charge (Fig. 4 and
Fig. S31). This is thus attributable to the fragment ion [M-Cl]2+,
i.e., [Au13(SbPh3)8Cl3]2+; the experimental isotopic pattern
matches well with the calculated pattern. Similarly, the mass
spectrum for 2b.Cl shows a major ion fragment at m/z ~2914
assignable to the [M-Cl]2+ ion (Fig. S32).
While a solid sample of 2a.Cl kept at 4 oC without
exclusion of air remained intact after ten months, as
manifested by the 1H NMR spectrum (Fig. S22), a solution in
DCM or acetone showed the formation of a small amount of
an unidentified product after several days under ambient
conditions. Decomposition in acetone was almost complete
after four months; the mass spectrum of the resulting brown
mixture suggests the presence of Ph3SbClX (X = Cl or OH),
indicating dissociation and oxidation of the ligands (Fig. S33).
The decomposition process also appears to be solvent
dependent although the nature of this is unclear (Figs. S23-
S24). The p-tolyl analogue 2b.Cl displayed similar stability in
solution (Fig. S25). These stibine-protected Au13 clusters are
stable to natural light but somewhat sensitive to UV light (254
Fig. 2. 1H NMR spectrum of 2a.PF6 in CD2Cl2 (aromatic region).
Crystallographic analysis on a dark red, diffraction-quality
crystal of 2a.BPh4 confirmed its chemical formulation,
although the crystal exhibited disorder; seven of the peripheral
Au atoms were modelled with ~90:10 disorder. The thirteen
gold atoms of the cluster cation are in an icosahedral
arrangement, with eight SbPh3 and four chlorides forming the
ligand sphere (Fig. 3). The presence of one BPh4 counterion
confirmed that the nanocluster is monocationic and consistent
with a valence electron count of 8 in accordance with the
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nm) in solution. The lower thermal stability of
2
.Cl compared stronger
π
-acidity of the stibines compared to phosphines.25
clusters such as Such a “heavier ligand displacement” effect has previously
to
phosphine-protected
Au11
[Au11(PPh3)8Cl2][Cl] may be ascribable to the lower been observed in thiolate-protected clusters.26
coordinating ability of the stibine ligands. Consistent with this
is the observation that treatment of 2b.Cl with excess PPh3 led
readily to the formation of [Au11(PPh3)8Cl2][Cl] (Figs. S29 and
S40). TGA and DTG measurements on 2a.PF6 and
[Au11(PPh3)8Cl2][Cl] also clearly demonstrate that the former
possesses
lower
thermal
stability;
decomposition
corresponding to loss of all all ligands occurred at ~155 oC and
~237 oC, respectively (Fig. S7).
Fig. 4. ESI-MS(+) spectrum of 2a.PF6. The asterisks indicate fragments corresponding to
loss of SbPh3 ligands. Inset: Calculated and experimental patterns of the main ion
fragment
Optical and electrochemical properties
The electronic spectra of 2a.X (X = Cl, PF6, BPh4) and 2b.Cl, are
essentially identical (Fig. 5), pointing to a common (Au13) metal
core. There are show two main absorption peaks at 352 and
454 nm, together with several weaker absorptions around 490,
530, 580 and 620 nm. The two main peaks are red-shifted
compared to the 340 and 430 nm reported for the phosphine-
protected clusters [Au13(L)4Cl4][Cl] (L = diphosphine) and
[Au13(L)8Cl4][Cl] (L = monophosphine), all of which contain a
closed icosahedral Au13 core.14b,16 The UV-Vis spectra also
appear to be solvent-independent; the main absorption peak
of 2b.Cl in acetonitrile is slightly blue-shifted compared to that
in DCM or ethanol (Fig. S41). A time-dependant DFT (TDDFT)
Fig. 5. Top: Electronic spectra of 2a.X (X = Cl, PF6, BPh4) and 2b.Cl in dichloromethane.
Inset: the enlarged spectra for the lower-energy adsorption peaks. Bottom:
Comparison of the calculated (TDDFT) and experimental UV-vis electronic spectra of 2a.
Thiolate-protected gold nanoclusters display weak
photoluminescence in the visible to near-infrared (NIR) region
with a low quantum yield (< 0.1%),27 although some with
specific core-shell structure,28 or specific ligands such as
peptides,29 have been reported to show stronger emissions,
demonstrating the importance of the protecting ligands in
tuning the luminescence.30 An example is that of
[Au25(SC2H4Ph)18][TOA] and [Au25(SePh)18][TOA], with the latter
displaying a 25 nm red-shifted luminescence compared to the
former.26b Similarly, the diphosphine-protected clusters
[Au13(dppe)5(X)2]3+ (X= Cl; CCPh) have been reported to show
an emission maximum at ca. 800 nm, while that in
[Au13(dppp)4(Cl)4][Cl] and [Au13(TOP)8Cl4][Cl] are blue-shifted
to ca. 775 nm.14b In comparison, the stibine-protected Au13
clusters 2a.X (X = Cl, PF6, BPh4) and 2b.Cl show an NIR emission
calculation
performed
on
the
cationic
fragment
[Au13(SbPh3)8Cl4]+, 2a, at the MPW1PW91/LANL2DZ level of
theory shows two strong peaks in the calculated spectrum at
ca. 338 nm and 436 nm, which may correspond to the
observed peaks at 352 nm and 454 nm (Fig. 5). Both peaks are
due to several transitions; the transitions for the former are
primarily LMCT in nature, and those for the latter are mainly
metal-to-metal transitions (Table S2 and Fig. S8). The red shift
with respect to the phosphine-protected clusters can thus be
understood in terms of the difference in the Au-L interactions
and is believed to be related to the weaker σ-basicity and
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at ca. 740 nm, along with a shoulder at ca. 825 nm, i.e., a
further blue shift of ca. 35 nm (Fig. 6). Thus a more electron-
donating ligand leads to lower energy transitions and
emissions. In addition, it is noted that 2a.Cl and 2b.Cl show
weaker luminescence intensities than those of 2a.PF6 and
2a.BPh4, indicating that the counterion Cl- may have a
quenching effect.
Fig. 6. Photoluminescence spectra of 2a.X (X = Cl, PF6, BPh4) and 2b.Cl (λex = 470 nm) in
DCM solution at room temperature (concentration: ca. 1.0 mg/ml).
The cyclic voltammogram (CV) of 2a.PF6 in DCM with
nBu4NPF6 as the supporting electrolyte shows that the cluster
undergoes irreversible oxidation and reduction (Fig. 8). While
electron transfers occurring between -2.5 and +1.0 V are
reproducible, i.e., the redox peaks in both the positive and
negative potential directions overlap, those occurring beyond
+1.0 V are not, presumably arising from breakdown of the
cluster compound (Fig. S36). The redox behavior of 2a.PF6
between -2.5 and 1.0 V is therefore reliable for analysis of its
electrochemical property. In this region, three oxidation peaks
were found at 0.34, 0.58 and 0.78 V in the square-wave
voltammogram (SWV), probably corresponding to oxidation to
the +2, +3 and +4 states, respectively; seven reduction peaks (-
1.51, -1.61, -1.68, -1.77, -1.97, -2.11, -2.22 V) were observed in
the same region. The electrochemical HOMO-LUMO energy
gap was determined to be about 1.85 eV (without subtracting
the charge energy) from the first oxidation and reduction
potentials (O1 and R1), which is in excellent agreement with
that derived from the UV-Vis spectrum data (620 nm
absorption peak) by extrapolation to zero absorbance. In
Fig. 7. CV (top) and DSV (bottom) spectra of 2a.PF6 in DCM (1.0 mM) with 0.10 M
nBu4NPF6 as the supporting electrolyte. Working electrode: 1 mm diameter planar
glassy carbon; auxiliary electrode: Pt wire; pseudo reference electrode: Ag wire (in 0.50
M
nBu4NPF6 in CH3CN); scan rate: 0.10 V/S; ferrocene (Fc) was added as an internal
reference at the end of the experiment. For CV, the scans were initiated in the positive
(red) and negative (black) potential directions, respectively. For SWV, the red and blue
traces were scanned in the positive and negative potential directions, respectively.
Pulse period (τ) = 25 Hz; potential step = 5.0 mV; pulse amplitude = 20 mV.
Conversion to Au25 cluster
Water-soluble, peptide-protected Au25 clusters possessing
relatively strong fluorescence, such as [Au25(SG)18], 4, have
wide applications in sensing, imaging and biomedicine.32
Although they can be prepared by the modified Brust method,
viz., reduction of the putative [Au(I)-SR]n oligomer with
suitable reducing agents,33 it has been found recently that
treatment of PPh3-protected Au11 nanoclusters or larger
nanoparticles,15,34 or thiolate-protected colloidal clusters
[Aun(SG)m],35 with excessive glutathione (GSH) are more
comparison,
the
other
two
Au13
clusters
[Au13(PPh3)4(SC12H25)2Cl2] and [Au13(dppm)6][Cl]5, with closed
and open icosahedral metal core geometries, have lower
HOMO-LUMO gaps of 1.76 and 1.66 eV, respectively.13,31
efficient synthetic routes to 4. Although the exact mechanism
for the ligand exchange is unclear,36 it is strongly associated
with the stability of the reactant clusters, as indicated by the
observation that while [Au11(PPh3)7Cl3] undergoes ligand
exchange with GSH to give 4, the more structurally stable ionic
clusters [Au11(PPh3)8Cl2][Cl] and [Au11(dppf)4Cl2][Cl] do not
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react with GSH under the same conditions.15,4d Given the Consistent with this was that treatment of 2b.Cl with
weaker coordinating ability of stibines compared to K3[Fe(CN)6] (3.5 equiv) indeed led to decomposition of 2b.Cl
phosphines, the ligand exchange reaction between
2 and and the formation of Prussian blue (Fig. S58).
thiolates may be expected to occur more readily. Indeed,
ligand exchange in 2b.Cl with GSH does occur, and the
formation of
4 can be monitored by its characteristic peak at
~670 nm (Scheme 2). In comparison, unlike the case with PPh3,
the reaction of [Au25(SC2H4Ph)18][TOA] with Sb(p-tolyl)3 (20 and
60 equiv) did not appear to give any of an biicosahedral rod
[Au25{Sb(tolyl)3}10(SR)5Cl2]2+.37
Scheme 2. Ligand exchange reaction of 2b.Cl with GSH.
GSH (20 equiv)
Au25(SG)18
[Au13{Sb(p-tolyl)3}8Cl4]Cl
CHCl3/H2O
45 oC/N2
4
2b
.Cl
The formation of
comparison, [Au11(PPh3)7Cl3] was reported to show the
formation of
after more than 6 h at 50 oC, while the cationic
4 was detectable after 4 h (Fig. S42). In
4
[Au11(PPh3)8Cl2][Cl] showed even lower reactivity although air
was found to aid the reaction.34a The important role of air was
also apparent for 2b.Cl; under aerobic conditions, the
Fig. 8. Electronic spectra of the aqueous solution for the conversion of 2b.Cl to 4 with
different oxidants, at 240 mins reaction time. Redox potential (V) vs SHE for oxidants
given in parenthesis.39
formation of
4 was apparent from about 90 mins (Fig. S43),
with a higher reaction rate than under anaerobic conditions
(Figs. S44-S45). Since the fraction of Au(I) to Au atoms changed
from 0.38 to 0.72 in the conversion of 2b to 4, we hypothesize
that the oxygen in air acts as an oxidant to facilitate the
conversion; such an oxidation-induced transformation of
smaller clusters to larger clusters has received much attention
recently.38 To test the idea that oxidation is involved, the
conversion was carried out with a variety of oxidants (0.50
equiv) with differing oxidation potential and solubility. The
addition of an oxidant clearly affected the rate of conversion,
although we could not find any clear correlation between the
conversion rate and the oxidation potential or solubility of the
oxidant (Fig. 8 and Figs. S46-S53). Besides air, the oxidants
K3[Fe(CN)6] and I2 showed significant acceleration of the
conversion while others showed only moderate to poor effects.
The addition of more K3[Fe(CN)6] from 0.5 equiv to 3.5 equiv
(the theoretical amount required to change the fraction of Au(I)
to Au(0) atoms in 2b.Cl from 0.38 to 0.72) increased the
conversion rate further but further addition to 7.0 equiv led to
a decrease in the conversion rate (Fig. 9 and Figs. S54-S57).
Although antigalvanic reduction is a possibility,40 the
mechanism for the conversion is not entirely clear. In the
conversion mediated by K3[Fe(CN)6], a small amount of
Prussian blue was formed, identified by comparison of its IR
spectral characteristics with that of an authentic sample (Fig.
S58). This is presumably formed from the reduced [Fe(CN)6]4-
and hydrolysed forms of the more labile [Fe(CN)6]3-. Although
the stibine and GSH were found to be able to reduce
K3[Fe(CN)6] to K4[Fe(CN)6] under the same reaction conditions,
no Prussian blue formation was observed (Fig. S58), indirectly
pointing to involvement of the Au core rather than the ligand
sphere of 2b.Cl. An inner sphere electron transfer, presumably
involving bridging CN- ligands, may therefore be operative.
Fig. 9. Electronic spectra of the aqueous solution for the conversion of 2b.Cl to 4 with
varying amount of K3[Fe(CN)6], at 240 mins reaction time (left), and the characteristic
peak integration vs amount of K3[Fe(CN)6] (right).
Conclusion
We have reported here, for the first time, Au nanoclusters
with stibines as protecting ligands. In contrast to phosphines,
the use of stibines as the protecting ligand favors the
formation of icosahedral Au13 clusters rather than Au11 or a
polydispersed mixture of smaller clusters and they made for
interesting comparison with, and a better understanding of,
the structures of the phosphine analogues. The weaker
coordinating ability of the stibine ligands also made them
promising precursors to other Au nanoclusters, for example, to
the thiolate protected Au25 cluster. More importantly, we have
found that the addition of a suitable oxidant can significantly
increase the conversion rate by aiding the size-focusing
process.
Conflicts of interest
There are no conflicts to declare.
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6
G. Schmid, R. Pfeil, R. Boese, F. Bandermann, S. Meyer, G. H.
M. Calis, J. W. A. van der Velden, Chem. Ber. 1981, 114,
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