Crystal structures of Au2 complex and Au25 nanocluster and mechanistic insight into the conversion of polydisperse nanoparticles into monodisperse Au25 nanoclusters

Article abstract of DOI:10.1021/ic2012292

We previously reported a size-focusing conversion of polydisperse gold nanoparticles capped by phosphine into monodisperse [Au₂₅(PPh ₃)₁₀(SC₂H₄Ph)₅Cl ₂]²⁺ nanoclusters in the presence of phenylethylthiol. Herein, we have determined the crystal structure of [Au₂₅(PPh ₃)₁₀(SC₂H₄Ph)₅Cl ₂]²⁺ nanoclusters and also identified an important side-product-a Au(I) complex formed in the size focusing process. The [Au ₂₅(PPh₃)₁₀(SC₂H₄Ph) ₅Cl₂]²⁺ cluster features a vertex-sharing bi-icosahedral core, resembling a rod. The formula of the Au(I) complex is determined to be [Au₂(PPh₃)₂(SC ₂H₄Ph)]⁺ by electrospray ionization (ESI) mass spectrometry, and its crystal structure (with SbF₆^- counterion) reveals Au-Au bridged by-SC₂H₄Ph and with terminal bonds to two PPh₃ ligands. Unlike previously reported [Au₂(PR₃)₂(SC₂H₄Ph)] ⁺ complexes in the solid state, which exist as tetranuclear complexes (i.e., dimers of [Au₂(PR₃)₂(SC ₂H₄Ph)]⁺ units) through a Au??? Au aurophilic interaction, in our case we found that the [Au ₂(PPh₃)₂(SC₂H₄Ph)] ⁺ complex exists as a single entity, rather than being dimerized to form a tetranuclear complex. The observation of this Au(I) complex allows us to gain insight into the intriguing conversion process from polydisperse Au nanoparticles to monodisperse Au₂₅ nanoclusters.

Full text of DOI:10.1021/ic2012292

ARTICLE
pubs.acs.org/IC
Crystal Structures of Au2 Complex and Au25 Nanocluster and
Mechanistic Insight into the Conversion of Polydisperse Nanoparticles
into Monodisperse Au25 Nanoclusters
Huifeng Qian,†,§ William T. Eckenhoff,‡,§ Mark E. Bier,† Tomislav Pintauer,*,‡ and Rongchao Jin*,†
†Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
‡Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, Pennsylvania 15282, United States
S Supporting Information
b
ABSTRACT: We previously reported a size-focusing conversion of poly-
disperse gold nanoparticles capped by phosphine into monodisperse
[Au25(PPh3)10(SC2H4Ph)5Cl2]2+ nanoclusters in the presence of phenyl-
ethylthiol. Herein, we have determined the crystal structure of
[Au25(PPh3)10(SC2H4Ph)5Cl2]2+ nanoclusters and also identified an im-
portant side-product—a Au(I) complex formed in the size focusing process.
The [Au25(PPh3)10(SC2H4Ph)5Cl2]2+ cluster features a vertex-sharing bi-
icosahedral core, resembling a rod. The formula of the Au(I) complex is
determined to be [Au2(PPh3)2(SC2H4Ph)]+ by electrospray ionization
(ESI) mass spectrometry, and its crystal structure (with SbF6À counterion)
reveals AuÀAu bridged by ÀSC2H4Ph and with terminal bonds to two
PPh3 ligands. Unlike previously reported [Au2(PR3)2(SC2H4Ph)]+ com-
plexes in the solid state, which exist as tetranuclear complexes (i.e., dimers of [Au2(PR3)2(SC2H4Ph)]+ units) through a Au Au
3 3 3
aurophilic interaction, in our case we found that the [Au2(PPh3)2(SC2H4Ph)]+ complex exists as a single entity, rather than being
dimerized to form a tetranuclear complex. The observation of this Au(I) complex allows us to gain insight into the intriguing
conversion process from polydisperse Au nanoparticles to monodisperse Au25 nanoclusters.
’ INTRODUCTION
Gold nanoparticles with a size larger than ∼2 nm possess a face-
centered cubic (fcc) structure, whereas ultrasmall Au nanopar-
ticles smaller than ∼2 nm often exhibit non-fcc structures.28,29
A particularly interesting aspect in the conversion of 1À3.5 nm
nanoparticles to [Au25(PPh3)10(SR)5Cl2]2+ nanoclusters is that
the starting polydisperse Au nanoparticles are all converted to a
specific-size cluster in the presence of thiol and triphenyl-
phosphine.20 The conversion process was not fully understood
then, but one important factor should be due to the high stability
of the final cluster, indicating that this is a thermodynamically
driven process. Although the conversion of the nanoparticles in
the presence of thiol ligands led to a single-sized nanocluster, it
was not clear if other smaller side-products were formed simul-
taneously. The identification and structural characterization of
the side-product(s) would offer important insights into the
reaction mechanism.
Recent advances in the synthetic chemistry of ultrasmall gold
nanoparticles (<2 nm) have rendered it possible to prepare
atomically monodisperse nanoclusters protected by ligands.1À10
These ultrasmall nanoparticles (or nanoclusters) exhibit inter-
esting molecular-like optical properties (e.g., multiple absorption
peaks and enhanced fluorescence),11À16 charge transport
properties,17,18 intrinsic magnetism,19 and so forth. A ubiquitous
“size focusing” process has been observed in the synthetic
processes, which permits the attainment of atomically mono-
disperse Aun(SR)m nanoclusters via conversion from initially
polydisperse nanoclusters.2,20 Size conversion from well-defined
starting clusters also allows one to produce new clusters.5,10,14,21
In the goldÀthiolate system, the gold(I)Àthiolate complex
constitutes the predominant side-product of the size evolution
process; the Au(I)ÀSR complex can be easily separated from Au
clusters due to its poor solubility in common solvents such as
toluene and THF.7b Although some experimental and theoretical
investigations have been performed,22À27 the inability to effec-
tively solubilize gold(I)Àthiolate complexes makes it difficult to
unravel their exact size (e.g., the number of AuÀSR units in the
complex) and atomic structures (e.g., ring or linear geometry).
In previous work, we reported the conversion of polydis-
perse Au nanoparticles (1À3.5 nm) to a monodisperse [Au25-
(PPh3)10(SR)5Cl2]2+ nanocluster via one-phase thiol etching.20
In this work, we have identified an important byproduct in
the conversion process from 1 to 3.5 nm nanoparticles to
[Au25(PPh3)10(SR)5Cl2]2+ nanoclusters and successfully deter-
mined the crystal structures of both the gold(I) side-product and
the [Au25(PPh3)10(SR)5Cl2]2+ nanocluster. These results have
Received: June 8, 2011
Published: October 11, 2011
r
2011 American Chemical Society
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ARTICLE
helped our understanding of the size-focusing conversion
process.
’ EXPERIMENTAL SECTION
Chemicals. Tetrachloroauric(III) acid (HAuCl4 3H2O, 99.99%),
3
2-phenylethanethiol (99%), sodium borohydride (NaBH4, 99.99%),
toluene (HPLC grade, 99.9%), ethanol (HPLC grade), pentane
(HPLC grade), chloroform (HPLC grade, 99.9%), and methanol
(HPLC grade, 99%) were purchased from Sigma-Aldrich. Tetraoctyl-
ammonium bromide (TOAB, >98%) was obtained from Fluka. Triphe-
nylphosphine (99%) was obtained from Acros Organics. Sodium
hexafluoroantimonate(V) (NaSbF6, 99.5%) was obtained from Alfa
Aesar. All chemicals are used as received.
Figure 1. Structure of the [Au25(PPh3)10(SC2H4Ph)5Cl2]2+ cluster
(color labels: Au, magenta; P, orange; S, yellow; Cl, green).
Windows and Crystal Maker 7.2 were used to generate molecular
graphics. Crystal data and experimental conditions are given in Support-
ing Information Tables S1 and S2.
Preparation of Triphenylphosphine-Protected Au Nano-
particles. HAuCl4 3H2O (0.118 g, 0.3 mmol, dissolved in 5 mL H2O)
’ RESULTS AND DISCUSSION
3
and TOAB (0.190 g, 0.348 mmol, dissolved in 10 mL toluene) were
combined in a trineck round-bottom flask. Afterwards, the solution was
vigorously stirred for ∼15 min to effect phase transfer of Au(III) into
toluene. The aqueous phase was then removed, and PPh3 (0.9 mmol)
was added to the toluene solution under vigorous stirring. The toluene
solution became whitish cloudy. Then, NaBH4 (0.034 g, 0.9 mmol,
dissolved in 5 mL ethanol) was injected all at once. The solution
immediately turned dark. The reaction was allowed to proceed for 2
hours under an air environment at room temperature. The crude black
product (containing PPh3-protected Au nanoparticles) was obtained
after rotary evaporation of the solvent (toluene). The black product was
washed several times with water and hexane to remove excess PPh3 and
TOAB. The product was further purified through precipitation by
adding a mixed pentane/chloroform (6:1 vol) solvent to a solution of
the nanoparticle product. The precipitated product was collected by
centrifugation and then redissolved in methanol, followed by centrifuga-
tion again to remove insoluble components. The left solution of Au
nanoparticles was dried by rotary evaporation, and the solids were
collected for the subsequent experiments.
Conversion of Polydisperse Au Nanoparticles into Mono-
disperse Bi-icosahedral Au25 Nanoclusters. The bi-icosahedral
[Au25(PPh3)10(SC2H4Ph)5Cl2]2+ clusters were synthesized by adding
∼0.25 mL PhC2H4SH to a CH2Cl2 (20 mL) solution of phos-
phine-protected Au nanoparticles obtained in the above step (the ratio
of PhC2H4SH/Au = 6:1). The solution was vigorously stirred for 12 h
at room temperature. The products were obtained after drying the solution
by rotary evaporation and washing with hexane and finally extraction with
ethanol. The side-product [Au2(PPh3)2(SC2H4Ph)]+ coexists with
[Au25(PPh3)10(SC2H4Ph)5Cl2]2+ clusters. The brownish yellow product
was mixed with excess NaSbF6 (∼80 mg) for 15 min in CH2Cl2. Excess
NaSbF6 (white powder) was removed by centrifugation. The product was
dried and redissolved in CH2Cl2/ethanol (1:1) for crystallization. After
1À2 days, crystals of two colors (orange and black) were obtained. The
black crystals were determined to be [Au25(PPh3)10(SC2Ph)5Cl2]2+, and
the orange ones were [Au2(PPh3)2(SC2H4Ph)]+.
We previously reported the size-focusing synthesis of gold
clusters using 1À3.5 nm Au nanoparticles as the starting
material.20 The formula of the as-synthesized gold cluster was
determined to be [Au25(PPh3)10(SC2H4Ph)5Cl2]2+ by ESI-mass
spectrometry, thermogravimetric analysis (TGA), and nuclear
magnetic resonance spectroscopy (NMR), but the crystal quality
was too low for the structure to be solved.20 Herein, we have
been able to improve the crystal quality of [Au25(PPh3)10-
(SC2H4Ph)5Cl2]2+ clusters through crystallization in ethanol
and CH2Cl2 (1:1, vol). The X-ray structure of the [Au25(PPh3)10-
(SC2H4Ph)5Cl2]2+ cluster is now solved (Figure 1), although the
coordinating ligands are not fully resolved due to disordering. The
Au25 core is composed of two icosahedral Au13 units by sharing one
common vertex (i.e., 13 Â 2 À 1 = 25 Au atoms), resembling a rod.
Five thiolate ligands (only S atoms are shown) bridge the two
icosahedrons, and 10 phosphines are terminally coordinated to the
two Au5 pentagonal rings on the two ends of the rod, and two
chlorides bind to two apical Au atoms of the rod. The organic
ligands (PPh3 and PhC2H4S) could not be resolved through
repeated attempts. It is noteworthy that the core framework of
[Au25(PPh3)10(SC2H4Ph)5Cl2]2+ is identical to that of the
ethylthiolate counterpart [Au25(PPh3)10(SC2H4)5Cl2]2+ reported
by Tsukuda and co-workers previously.5
With respect to the conversion of polydisperse Au nanopar-
ticles to [Au25(PPh3)10(SC2H4Ph)5Cl2]2+ clusters, some Au(I)
complexes could be formed as byproducts in the course of
the conversion process. Since two types of ligands are present
in the solution (PPh3 liberated from the starting nanoparticles
and PhC2H4SH added in the size-focusing step), we speculate
gold(I) complexes of several forms, such as [Au(I)PPh3]+,
neutral Au(I)SC2H4Ph, and Au(I)(PPh3)(SC2H4Ph). We
initially thought that certain [Au(I)SC2H4Ph]n complexes
would most likely form due to stronger AuÀS bonds than AuÀP
bonds, but herein we have indeed identified an interesting
[Au2(PPh3)2(SC2H4Ph)]+ complex as a major byproduct
formed in the size-focusing synthesis of [Au25(PPh3)10-
(SC2H4Ph)5Cl2]2+. Crystals of the [Au2(PPh3)2(SC2H4Ph)]+
complex was formed in the same solution of [Au25(PPh3)10-
(SC2H4Ph)5Cl2]2+ clusters in the crystallization experiment.
Crystals of two colors (orange and black) were found. The black
crystals are [Au25(PPh3)10(SC2H4Ph)5Cl2]2+. To determine the
composition of the orange crystal, a single crystal was picked up
and redissolved in methanol for an electrospray ionization mass
spectrometry (ESI-MS) analysis. As shown in Figure 2, an intense peak
at m/z 1055.14 was found, assigned as [Au2(PPh3)2(SC2H4Ph)]+
(calculated FW: 1055.16). Additionally, the isotope pattern
Characterization. UVÀvis spectra of [Au2(PPh3)2(SC2H4Ph)]+
and [Au25(PPh3)10(SC2H4Ph)5Cl2]2+ (single crystals dissolved in
CH2Cl2) were acquired on a Hewlett-Packard (HP) Agilent 8453 diode
array spectrophotometer at room temperature. ESI-MS analysis was per-
formed on a Waters Q-TOF II mass spectrometer and a Thermo-Fisher
LCQ ESI/APCI ion trap mass spectrometer. The X-ray crystallographic
data were collected at 150 K for [Au2(PPh3)2(SC2H4Ph)]+SbF6À or at
room temperature for [Au25(PPh3)10(SC2H4Ph)5Cl2]2+(SbF6À)2 using
graphite-monochromated Mo Kα radiation (0.71073 Å) with a Bruker
Smart Apex II CCD diffractometer. Data reduction included absorption
corrections with the multiscan method using SADABS. Structures were
solved by Patterson methods and refined by full matrix least-squares
using the SHELXTL 6.1 bundled software package. ORTEP-3 for
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ARTICLE
Scheme 1. Proposed Mechanism for the Conversion of
PPh3-Protected Au Nanoparticles to Gold(I) Complex and
[Au25(PPh3)10(SC2H4Ph)5Cl2]2+ Clusters
Figure 3. Structure of [Au2(PPh3)2(SC2H4Ph)]+[SbF6À] shown with
30% probability ellipsoids. Selected distances (Å) and angles (deg) are
Au(1)ÀAu(2), 3.046; Au(1)ÀP(1), 2.257; Au(1)ÀS(1), 2.318;
Au(2)P(2), 2.262; Au(2)ÀS(1), 2.329; P(1)ÀAu(1)ÀS(1), 175.72;
P(2)ÀA(2)ÀS(1), 173.40; Au(1)ÀS(1)ÀAu(2), 81.92°. Counterion:
SbF6. The X-ray data was collected at 150 K.
We further measured the UVÀvis spectrum of the [Au2(PPh3)2-
(SC2H4R)]+[SbF6À] complex (the orange crystals redissolved in
CH2Cl2; Figure 4A). The absorption spectrum of this complex
exhibits two bands at 268 and 275 nm as well as a weak, broad
absorption tail extending from 280 nm to longer wavelengths, but
no absorption is observed above 400 nm. This absorption spectrum
is quite similar to that observed for diphosphine-bridged dinuclear
Au(I) complex.34À36 The absorption band could be assigned to the
d
σ* f pσ transition, which is an indication of metalÀmetal inter-
action. The optical spectrum of the [Au2(PPh3)2(SC2H4Ph)]+
complex is completely different from that of the [Au25(PPh3)10-
(SC2H4Ph)5Cl2]2+ cluster (Figure 4B). The latter shows a distinct
band at 670 nm and several additional bands at shorter wavelengths.
The absorption at ∼670 nm is assigned to a HOMOÀLUMO
electronic transition due to the dimeric structure, and those
absorption peaks below 600 nm are assigned to the electronic
transitions within individual icosahedral Au13 units.37,38
Figure 2. ESI mass spectrum of the gold(I) complex (positive mode).
shows a peak spacing of unity, indicating a singly charged species.
It is interesting that this Au(I) complex unexpectedly contains
both ligands of PPh3 and -SC2H4Ph.
The structure of the orange crystals, [Au2(PPh3)2(SC2H4-
Ph)]+[SbF6À], was further determined by X-ray crystallographic
analysis (Figure 3). This complex contains two gold(I) centers with a
roughly linear PÀAuÀS motif (angles: 174.5°). The two AuÀP
distances are 2.257 Å and 2.262 Å, which are typical of AuÀP bonds.
The two Au atoms are bridged by the thiolate ligand with AuÀS
distances of 2.318 and 2.329 Å and a AuÀSÀAu angle of 81.92°. The
AuÀAu distance of 3.046 Å is considerably shorter than the AuÀAu
distance in previously reported [Au2(PR3)2SR]+ dimers (3.10À
3.20 Å),30À33 indicating a stronger aurophilic attraction in our case.
It is noteworthy that many gold(I) complexes have been
reported in the literature.30À32 The previously reported gold
complexes typically adopt a polynuclear structure through
AuÀAu aurophilic interactions originating from the relativistic
London forces, with typical energies on the order of hydrogen
bonds (5À15 kcal/mol).31,32 Chen et al. reported a cationic
[Au2(PR3)2SR]+ complex prepared by the oxidation of neutral
mononuclear and dinuclear phosphineÀgold(I)Àthiolate com-
plexes.33 The cationic [Au2(PR3)2(SR)]+ complex existed as a
dimer, i.e., a tetranuclear Au(I) structure with the four Au atoms
arranged in a square planar geometry with pairs of gold atoms
bridged by thiolate ligands. This structure could not be cleaved to
give monomers of dinuclear species in solution.33 In contrast, our
work found that the [Au2(PPh3)2(SC2H4Ph)]+ complex exists
only as a dinuclear Au(I) complex (as opposed to a dimer of the
dinuclear complex) in both solution and solid states; no tetra-
nuclear complexes were found in our analysis.
The identification and structural determination of the new
gold(I) phosphineÀthiolate complex offers important insight
into the conversion of polydisperse Au nanoparticles into
mondisperse [Au25(PPh3)10(SC2H4Ph)5Cl2]2+ clusters. Herein,
we propose a mechanism for this process. The formation of
[Au25(PPh3)10(SC2H4Ph)5Cl2]2+ and [Au2(PPh3)2(SC2H4Ph)]+
may be due to their high stability. The [Au25(SC2H4Ph)5-
(PPh3)10Cl2]2+ has 16 formal valence electrons, i.e., 25(Au 6s) À
5(thiolates) À 2(chloro) À 2(charges) = 16 eÀ); note that PPh3 is
considered not to localize Au(6s) valence electrons and is hence not
counted in. The 16 eÀ count could be considered as a dimer of two
8 eÀ units,39 i.e., electron shell closing.39,40 The bi-icosahedral 25-
atom cluster is a quite ubiquitous structure, which has been observed
in many phosphine-protected bimetal clusters (e.g., Au13Ag12,
Au12Ag13) and trimetal clusters (e.g., Au12Ag12Pt, Au11Ag12Pt2,
Au12Ag12Ni).41 Scheme 1 shows a plausible process for the size
conversion from 1À3.5 nm nanoparticles to Au25 clusters. The
surfaces of Au nanoparticles are initially stabilized by PPh3 ligands;
note that some ClÀ may also bind to nanoparticle surfaces. Excess
PhC2H4SH reacts with Au nanoparticles due to strong interaction
between thiol and gold. PhC2H4SH is expected to etch surface Au
atoms and liberates them by releasing [Au2(PPh3)2(SC2H4Ph)]+.
The partially bare nanoparticle surface would be further etched by
thiols. With continuous etching, the gold particle size steadily
decreases until the entire structure reorganizes to form the thermo-
dynamically favored bi-icosahedral Au25 clusters.
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ARTICLE
Figure 4. UVÀvis spectra of [Au2(PPh3)2(SC2H4Ph)]+ (A) and Au25(PPh3)10(SC2H4Ph)5Cl2]2+ (B).
Figure 5. UVÀvis (A) and ESI-MS (B) monitoring of the reaction process of thiol etching to PPh3-protected Au nanoparticles. Crude samples were
used in the optical and mass spectrometry analyses.
To further support the proposed mechanism, we have ac-
quired time-dependent UVÀvis spectra and ESI mass spectra
during the thiol etching process (Figure 5). The evolution of
the UVÀvis spectra (Figure 5A) indicates the conversion
from polydisperse PPh3-protected Au nanoparticles to mono-
disperse [Au25(PPh3)10(SR)5Cl2]2+ nanoclusters. ESI mass
spectra (Figure 5B) indicate that, prior to thiol etching, a
[Au(PPh3)2]+ complex (m/z 721.2) was observed, which is an
impurity in the starting Au nanoparticles. After 1 h of thiol
etching, we observed the dominant [Au2(PPh3)2(SC2H4Ph)]+
complex, together with minor [Au(PPh3)2]+ and [Au3(PPh3)2-
(SC2H4Ph)2]+ (m/z 1389.2) complexes (Figure 5B); of note,
the accurate m/z values were determined in the 50À2,000 m/z
detection mode, as the 50À4,000 m/z range in the turbo mode is
less accurate for determining the m/z. Throughout the reaction
(from 1 to 12 h), the [Au2(PPh3)2(SC2H4Ph)]+ complex remains a
dominant byproduct; note that the [Au25(PPh3)10(SC2H4Ph)5Cl2]2+
cluster signal (m/z 4152)20 is beyond the mass range in
Figure 5B. Taken together, the proposed mechanism in which
the thiol etching of Au nanoparticles occurs via the release of the
[Au2(PPh3)2(SC2H4Ph)]+ complex should be at work.
Au25 nanocluster have been determined. The [Au25(PPh3)10-
(SC2H4Ph)5Cl2]2+ cluster contains two icosahedral Au13 units
by sharing one common vertex. In the [Au2(PPh3)2SC2H4Ph]+
complex, the two Au atoms form terminal bonds with two
PPh3’s and are bridged by the thiolate ligand. Unlike other
[Au2(PR3)2SR]+ complexes in solid states which exist as tetra-
nuclear complexes (i.e., dimers of Au2(I) complexes) through
Au Au aurophilic interactions, the single unit of [Au2-
3 3 3
(PPh3)2SC2H4Ph]+ was observed in our work. The revealing
of this gold(I) complex provides some important insight into the
intriguing “size focusing” process and facilitates mechanistic
understanding of the conversion process.
’ ASSOCIATED CONTENT
S
Supporting Information. CIF information for [Au25-
b
(PPh3)10(SC2H4Ph)5Cl2]2+ and [Au2(PPh3)2(SC2H4Ph)]+
À
(counterion: SbF6 in both cases). Crystal data and structure
refinement for [Au25(PPh3)10(SR)5Cl2]2+ and [Au2(SC2H4Ph)
(PPh3)2][SbF6]. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ CONCLUSION
’ AUTHOR INFORMATION
In summary, we have identified a new gold(I) complex from
the conversion of polydisperse Au nanoparticles to monodis-
perse [Au25(PPh3)10(SC2H4Ph)5Cl2]2+ clusters. The formula of
the Au(I) complex has been determined to be [Au2(PPh3)2-
SC2H4Ph] + by electrospray ionization (ESI) mass spectrometry.
The crystal structures of both the Au(I) complex and the
Corresponding Author
*E-mail: pintauert@duq.edu, rongchao@andrew.cmu.edu.
Author Contributions
§H.Q. and W. T. E. contributed equally to this work.
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ARTICLE
’ ACKNOWLEDGMENT
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This material is partly based upon work supported by the Air
Force Office of Scientific Research under AFOSR Award No.
FA9550-11-1-9999 (FA9550-11-1-0147).
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