Combining the Single-Atom Engineering and Ligand-Exchange Strategies: Obtaining the Single-Heteroatom-Doped Au16Ag1(S-Adm)13 Nanocluster with Atomically Precise Structure

Article abstract of DOI:10.1021/acs.inorgchem.7b02568

Obtaining cognate single-heteroatom doping is highly desirable but least feasible in the research of nanoclusters (NCs). In this work, we reported a new Au₁₆Ag₁(S-Adm)₁₃ NC, which is synthesized by the combination of single-atom engineering and ligand-exchange strategies. This new NC is so far the smallest crystallographically characterized Au-based NC protected by thiolate. The Au₁₆Ag₁(S-Adm)₁₃ exhibited a tristratified Au₃-Au₂Ag₁-Au₁ kernel capped by staple-like motifs including one dimer and two tetramers. In stark contrast to the size-growth from Au₁₈(S-C₆H₁₁)₁₄ to Au₂₁(S-Adm)₁₅ via just the ligand-exchange method, combining single Ag doping on Au₁₈(S-C₆H₁₁)₁₄ resulted in the size-decrease from Au₁₇Ag₁(S-C₆H₁₁)₁₄ to Au₁₆Ag₁(S-Adm)₁₃. DFT calculations were performed to both homogold Au₁₈ and single-heteroatom-doped Au₁₇Ag₁ to explain the opposite results under the same ligand-exchange reaction. Our work is expected to inspire the synthesis of new cognate single-atom-doped NCs by combining single-atom engineering and ligand-exchange strategies and also shed light on extensive understanding of the metal synergism effect in the NC range.

Full text of DOI:10.1021/acs.inorgchem.7b02568

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Combining the Single-Atom Engineering and Ligand-Exchange
Strategies: Obtaining the Single-Heteroatom-Doped Au16Ag1(S-
Adm)13 Nanocluster with Atomically Precise Structure
Xi Kang,⊥,†,‡ Lin Xiong,⊥,§ Shuxin Wang,*,†,‡ Yong Pei,*,§ and Manzhou Zhu*,†,‡
‡
†Department of Chemistry and Center for Atomic Engineering of Advanced Materials and Anhui Province Key Laboratory of
Chemistry for Inorganic/Organic Hybrid Functionalized Materials, Anhui University, Hefei, Anhui 230601, China
§Department of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education,
Xiangtan University, Xiangtan, Hunan 411105, China
S
* Supporting Information
ABSTRACT: Obtaining cognate single-heteroatom doping is
highly desirable but least feasible in the research of
nanoclusters (NCs). In this work, we reported a new
Au16Ag1(S-Adm)13 NC, which is synthesized by the combina-
tion of single-atom engineering and ligand-exchange strategies.
This new NC is so far the smallest crystallographically
characterized Au-based NC protected by thiolate. The
Au16Ag1(S-Adm)13 exhibited a tristratified Au3−Au2Ag1−Au1
kernel capped by staple-like motifs including one dimer and
two tetramers. In stark contrast to the size-growth from
Au18(S−C6H11)14 to Au21(S-Adm)15 via just the ligand-
exchange method, combining single Ag doping on Au18(S−
C6H11)14 resulted in the size-decrease from Au17Ag1(S−
C6H11)14 to Au16Ag1(S-Adm)13. DFT calculations were performed to both homogold Au18 and single-heteroatom-doped
Au17Ag1 to explain the opposite results under the same ligand-exchange reaction. Our work is expected to inspire the synthesis of
new cognate single-atom-doped NCs by combining single-atom engineering and ligand-exchange strategies and also shed light on
extensive understanding of the metal synergism effect in the NC range.
potential for thoroughly understanding the structure−property
relations.19,23,24,26,27
INTRODUCTION
■
Atomically precise nanoclusters (NCs) have attracted increas-
ing attention from both fundamental and practical aspects
owing to their unique properties and wide applications in
biology, catalysis, and electronics.1−8 During the past decades, a
large number of thiolate-protected metal NCs have been
synthesized, and many of them have been successfully
crystallized.9−17 In the wake of rapid development of
monometallic NCs, bi- or multimetallic NCs have quickly
emerged as new and prominent members due to their
enhanced optical/catalytic properties.1,5,18−25 Different strat-
egies have been developed to synthesize new alloy
NCs,19−24,26,27 and among these alloying strategies, the
“single-atom engineering” strategy has been attracting increas-
ing attention.18,19,23,24,26,27 For instance, a significant boost in
fluorescence has been observed by doping a single Au atom
into the center of Ag25(SR)18 and Ag29(SSR)12(PPh3)4.23,24 In
addition, the Pd1Au24(SR)18 and Pt1Au24(SR)18 show greatly
enhanced catalytic activities compared with that of undoped
Au25(SR)18.18−20 Moreover, compared with proportionally
doped alloy NCs (e.g., AuxAg25−x(SR)18), NCs with only one
heteroatom doping render a much simpler model for density
functional theory (DFT) calculations, which have more
Recently, the metal-exchange method has become a versatile
approach for synthesizing alloy NCs, which has been
demonstrated with the ability to dope both high- and low-
activity metals into parent NCs.21−23,27 Nonetheless, in
previous work, doping single heteroatom into homogold NCs
is usually restricted to the different group metal (that is, doping
Au (group 11) NCs with heteroatom from group 10 or
12).19,20,27 To date, it has remained challenging to dope a single
Ag/Cu atom into homogold NCs. This is probably due to the
similar configuration of the outtermost electrons in cognate
elements (i.e., d10s1 of Cu, Ag, and Au), which is easy to pose a
polydisperse formation in the metal-exchange products (e.g.,
AgxAu25−x(SR)18 or CuxAu25−x(SR)18).27,28
Previous works have demonstrated that doping cognate Ag/
Cu into homogold NCs would significantly affect the electronic
structure of the parent NCs.1,5,27−29 Generally, the change of
electronic structure will alter the binding ability of the metallic
kernel with capped ligands.30,31 For example, increased
Received: October 7, 2017
© XXXX American Chemical Society
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Synthesis of Ag(I)-SC6H11 Complexes. For the Ag(I)SR
complexes synthesis, AgCl (0.17 g, 1 mmol) was dissolved in 5 mL
of CH3OH, and cyclohexanethiol (135 μL, 1.1 mmol) was dissolved in
5 mL of CH3OH and added dropwise to the solution under vigorous
stirring (∼1200 rpm). After reacting for 15 min, the solution mixture
was washed several times with CH3OH. Then, the final product was
used directly.
Synthesis of the Single-Ag-Doped Au17Ag1(S-C6H11)14 NC.
The Au17Ag1(S−C6H11)14 was synthesized by doping Au18(S−
C6H11)14 (prepared based on our previous report35) with Ag(I)SR.
Specifically, Au18(S−C6H11)14 (515 mg, 0.1 mmol) was dissolved in
CH2Cl2 solution (5 mL), and Ag(I)SR (0.6 mg, 0.003 mmol) was
added in under vigorous stirring at 273 K (ice-bath). After 5 min, the
organic layer was separated and dried in a vacuum (note that the
temperature was 273 K when dried) and then washed several times
with CH3OH.
Synthesis of the Single-Ag-Doped Au16Ag1(S-Adm)13 NC.
The as-obtained doping NCs were then used as the precursor to react
with adamantanethiolate (HS-Adm). Typically, both 30 mg of the NC
mixture and 0.5 g of HS-Adm were dissolved in 10 mL of CH2Cl2. The
solution was heated to 40 °C and maintained at this temperature with
vigorous stirring for 24 h. Then, the solvent was removed by rotary
evaporation, and CH3OH (20 mL) was added to the solution. Then,
the mixture was centrifuged at 10000 rpm for 5 min; the supernatant
was discarded, and the precipitate was washed with CH3OH 3 times. A
mixture of Au16Ag1(S-Adm)13 and Au21(S-Adm)15 was obtained. The
mixture of Au16Ag1(S-Adm)13 and Au21(S-Adm)15 dissolved in 1 mL of
CH2Cl2 was pipetted on the PTLC plate (10 cm × 20 cm), and the
separation was conducted in the developing tank (developing solvent:
CH2Cl2/CH3OH = 10/1 v/v) for 60 min. A knife was used to cut the
bands in the PTLC plate, and the products were extracted by pure
CH2Cl2. Then, the collected solution was centrifugated at 10000 rpm
to ensure that there were no nanoparticles in the solution. The organic
layer was separated and dried in vacuum.
Crystallization of Au16Ag1(S-Adm)13 NC. Single crystal of
Au16Ag1(S-Adm)13 NCs grown at room temperature for 2 days in
CH2Cl2/CH3CH2OH. This step was repeated 3 times to obtain high-
quality single crystals. Dark crystals were collected, and the structure of
Au16Ag1(S-Adm)13 was determined. The yield is 4.3% based on the Au
element (calculated from HAuCl4 to synthesize Au18(S−C6H11)14 and
the Au:Ag = 20:1 of the single-heteroatom doping process). The
CCDC number of Au16Ag1(S-Adm)13 NC is 1554060.
flexibility of the Au38(SR)24 NC was obtained by doping more
silver atoms.30 Moreover, when the central Au atom of Au25
was replaced by Pd, the resultant Pd1Au24 exhibited remarkably
enhanced activity in the ligand-exchange process, and this
revealed that the heteroatom doping would have a profound
effect on the ligand-exchange process.31 Recently, the ligand-
exchange method has frequently been used to synthesize new
homo-Au NCs (e.g., Au18−Au21, Au25−Au28, Au38−Au36, and
Au144−Au133).14,32−34 However, it has rarely been tried on
doped NCs, which impeded the full understanding of the
metallic interaction effect as well as adequate analysis of the NC
evolution. Inspired by the remarkable influence of single-atom
engineering on the electronic structure of the homometal NCs,
we rationalize that the products of the same ligand-exchange
reaction on monometallic NCs and single-heteroatom-doped
Au NCs could be different owing to the heteroatom effects.
Furthermore, if the single Ag/Cu heteroatom-doped NC
adopts another evolutional pattern compared with that of the
homogold NC under the same ligand-exchange process, the
mixture (the ligand-exchange resultants of homogold NC and
doped NC) could be easily separated owing to the distinct size
and surface structure. Thus, this strategy (combined the single-
atom engineering and ligand-exchange methods) will not only
facilitate the synthesis of a new class of cognate single-atom-
doped NCs but also shed light on extensively understanding
the metal synergism effect in the NC range.
Herein, different from etching Au18(S−C6H11)14 with HS-
Adm, which produces Au21(S-Adm)15, the same etching
method on the single-heteroatom doped Au17Ag1(S−C6H11)14
(Au17Ag1 for short) produces a different NC: Au16Ag1(S-
Adm)13 (Au16Ag1 for short) with a decreased size. The
preparative thin-layer chromatography (PTLC) was performed
to separate the Au16Ag1 from the ligand-exchange raw products
(i.e., the mixture of Au21(S-Adm)15 and Au16Ag1), and the
cognate single-atom engineering (in group 11) in Au-based NC
was successfully achieved. The crystal structure of as-prepared
Au16Ag1 exhibits an unprecedented Au6Ag1 kernel protected by
staple-like motifs including one dimer and two tetramers. To
date, it is the smallest thiolated Au-based NC with identified X-
ray crystal structure. Combining with DFT calculations, the
HOMO−LUMO in Au16Ag1 exhibits the sp ← d interband
transition, same as the Au18(S−C6H11)14. More importantly, the
Vienna ab initio simulation package (VASP) was employed to
perform the Bader charge analysis on Au18(S-CH3)14 and
Au17Ag1(S-CH3)14. The heterogeneous distribution of charge
density around the Ag atom in Au17Ag1 compared with the
homogold one (i.e., Au18(S−C6H11)14) is proposed as the
fundamental reason for the difference in the ligand-exchange
process. In addition, compared with the symmetrical Au18(S−
C6H11)14, no symmetry element was found in Au16Ag1, which
illustrates that a significant portion of the Au16Ag1 exhibited
enantiomeric chirality.
Theoretical Methods. All geometric optimizations of all NC
structures were performed with the Perdew−Burke−Ernzerhof36
functional, and the d-polarization included basis set (DND) was
used for C, H, S elements. The DFT semicore pseudopototential
(DSPP) approximation with some degree of relativistic corrections
into the core was used for the Au and Ag elements implemented in the
Dmol3 package.37,38 Time-dependent DFT calculations of the UV−vis
optical spectrum and KS orbit implemented in the Amsterdam density
functional (ADF) software packages with the PBE functional with the
triple-ζ-polarized (TZP) basis sets. The TD-DFT calculations
evaluated the lowest 1000 singlet-to-singlet excitation energies. The
calculations on electron density maps were implemented by the
Vienna ab initio simulation package (VASP) software.39,40
Characterization. All UV−vis absorption spectra of the NCs
dissolved in CH2Cl2 were recorded using an Agilent 8453 diode array
spectrometer, whose background correction was made using a CH2Cl2
blank.
Thermogravimetric analysis (TGA) was carried out on a
thermogravimetric analyzer (DTG-60H, Shimadzu Instruments, Inc.)
with 10 mg of Au16Ag1 NC in a SiO2 pan at a heating rate of 10 K
min−1 from room temperature to 1073 K.
X-ray photoelectron spectroscopy (XPS) measurements were
performed on a Thermo ESCALAB 250 configured with a
monochromated Al Kα (1486.8 eV) 150 W X-ray source, 0.5 mm
circular spot size, flood gun to counter charging effects, and analysis
chamber base pressure lower than 1 × 10−9 mbar.
EXPERIMENTAL SECTION
■
Chemicals. All reagents were purchased from Acros Organics or
Sigma-Aldrich and used without further purification: tetrachloroauric
(III) acid (HAuCl4·3H2O, 99.99% metals basis), silver nitrate (AgNO3,
99% metals basis), GSH (γ-Glu-Cys-Gly, Mw = 307.13 Da),
cyclohexanethiol (C6H11SH, 95%), adamantane-1-thiol (C10H15SH,
95%), sodium cyanoborohydride (NaBH3CN, 98%), methylene
chloride (CH2Cl2, HPLC-grade, Aldrich), methanol (CH3OH,
HPLC, Aldrich), ethanol (CH3CH2OH, HPLC, Aldrich), hexane
(C6H14, HPLC, Aldrich).
Inductively coupled plasma-atomic emission spectrometry (ICP-
AES) measurements were performed on an Atomscan advantage
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Figure 1. (A) UV−vis spectrum of the as-prepared mixture of Au16Ag1 and Au21(S-Adm)15 NCs; inset: the PTLC separation result of this mixture.
(B, C) UV−vis and ESI-MS spectra of separated Au21(S-Adm)15 NC; insets: (B) crystal structure and (C) experimental and simulated isotope
patterns of Au21(S-Adm)15 NC. (D, E) UV−vis and ESI-MS spectra of separated Au16Ag1 NC; insets: (D) crystal structure and (E) experimental and
simulated isotope patterns of Au16Ag1 NC. Color legend: orange sphere, Au; cerulean sphere, Ag; red sphere, S. For clarity, the carbon and hydrogen
atoms are not shown.
instrument made by Thormo Jarrell Ash Corporation (USA). The
Au16Ag1 NC was digested by concentrated nitric acid, and the
concentration of the Au16Ag1 NC was set to ∼0.5 mg L−1.
Electrospray ionization time-of-flight mass spectrometry (ESI-TOF-
MS) measurements were performed by MicrOTOF-QIII high-
resolution mass spectrometer. The sample was directly infused into
the chamber at 5 μL/min. For preparing the ESI sample, the NCs were
dissolved in toluene (1 mg/mL) and diluted (v/v = 1:2) by dry
methanol containing 5 mM CsOAc to ionize the NCs by forming the
resultant Cs+-NC.
The data collection for single-crystal X-ray diffraction was carried
out on a Bruker Smart APEX II CCD diffractometer under liquid
nitrogen flow at 130 K using graphite-monochromatized Cu Kα
radiation (λ = 1.54178 Å). Data reductions and absorption corrections
were performed using the SAINT and SADABS programs,
respectively. The structure was solved by direct methods and refined
with full-matrix least-squares on F2 using the SHELXTL software
package. All non-hydrogen atoms were refined anisotropically, and all
the hydrogen atoms were set in geometrically calculated positions and
refined isotropically using a riding model. The crystal structure of
Au16Ag1 was treated with PLATON SQUEEZE, and the diffuse
electron densities from these residual solvent molecules were removed
(with total void volume of 4913 Å3 and squeezed electrons of 559 e).41
remains challenging. Accordingly, the following steps are
necessary and significant to obtain the pure single-heter-
oatom-doped NC. In the second step, the ligand-exchange
method (using HS-Adm ligand) was performed on this mixture,
which converted the precursors into the mixed Au16Ag1 and
Au21(S-Adm)15 NCs. In the last step, we successfully separated
the Au16Ag1 from the mixture by the use of PTLC (shown in
Figure 1).
As shown in Figure 1, Au16Ag1 exhibits a different UV−vis
spectrum from those of Au18(S−C6H11)14 (shown in Figure S2)
and Au21(S-Adm)15. The UV−vis of Au16Ag1 shows two broad
multiband optical absorptions centered around 465 and 590 nm
and two shoulder bands at 320 and 380 nm. In addition, the
TGA was performed to validate the purity of Au16Ag1 NC. As
shown in Figure S3, the weight loss (40.02%) is consistent with
the theoretical value (40.63%), which confirms the purity of the
product. Furthermore, the atomic ratio of Au/Ag (16:1) was
verified by XPS and ICP measurements (Figures S4 and S5 and
Table 1).
Table 1. Atomic Ratio of Au/Ag in the Au16Ag1 NC
Calculated by ICP and XPS Measurements
RESULTS AND DISCUSSION
■
Au atom
Ag atom
Synthetic Strategy. Typically, the synthesis of Au16Ag1
involves three steps (as shown in Figure 1). In the first step,
controlled doping of Au18(S−C6H11)14 with a small amount of
AgI(S−C6H11) generates a mixture of Au17Ag1 and unreacted
Au18(S−C6H11)14. It should be noted that (i) single-atom
engineering has been achieved with the generation of Au17Ag1,
(ii) an excessive dose of AgI(S−C6H11) would produce
Au18−xAgx(S−C6H11)14 (x = 0−3) NCs instead of the
Au17Ag1 and unreacted Au18(S−C6H11)14 NCs, and (iii)
despite huge efforts, the separation of single-Ag-doped
Au17Ag1 from the mixture (i.e., Au18−xAgx(S−C6H11)14)
ICP experimental ratio
XPS experimental ratio
theoretical ratio
93.98%
94.03%
6.02%
5.97%
16/17 (94.12%)
1/17 (5.88%)
In the previous work, we used the HS-Adm ligand to convert
Au18(S−C6H11)14 into Au21(S-Adm)15.32 Therefore, in this
work it is safe to say that Au16Ag1 was prepared from the
Au17Ag1 precursor (Figure 1). Interestingly, the ratio of
Au18(S−C6H11)14 and Au17Ag1 in the mixed precursor (as
well as the ratio of Au21(S-Adm)15 and Au16Ag1 in the mixed
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resultant) could be controlled by the initial single-heteroatom
doping process. ESI-MS was performed to monitor the
processes and identify the ratios. As shown in Figures S6 and
S7, the intensities of the Au17Ag1 and Au18(S−C6H11)14 peaks
in the ESI-MS spectrum positively correlates with the amount
of the added AgI(S−C6H11). In addition, as the ligand-exchange
resultant from these proportional precursors, the percentage of
Au16Ag1 in the mixed resultant is also dependent on the added
AgI(S−C6H11).
from Au(I) complex precursors to small NCs, Xie and co-
workers proposed a theory that the value of free valence
electrons increases accompanied by the growth of the NC size
(i.e., from 0 e− to 2, 4, 6, and 8 e−).44 Unfortunately, the
structural transformation remains uncertain at the atomic level
because the accurate structures of these small-sized NCs are
still not determined.44 For the sake of fundamentally grasping
the structure growth and variation (that is, understanding the
essence of transformation from small molecular complexes to
larger NCs), the atomically precise structures of the relatively
smaller NCs are still highly desirable. The Au16Ag1 in our work
is so far the smallest crystallographically characterized Au-based
NC protected by thiolate. Because of the zero-valence state, the
free electron count of Au16Ag1 is calculated as 4e (that is, 16 +
1 − 13 = 4), which renews the previous “isoelectronic” 4e
small-sized NC family (i.e., Au18(S−C6H11)14, Au20(TBBT)16,
and Au24(SCH2Ph-tBu)20)35,42,45,46 and validates the increment
of free valence electrons from Au complexes to large-sized
NCs.44
Atomic Structure. The total structure of Au16Ag1 was
determined by the single-crystal X-ray diffraction (SC-XRD).
As shown in Figure 2A, the kernel of Au16Ag1 shows a
A previous report has already calculated the most probable
structure of Au17Ag1 in which the single Ag atom was arranged
in the kernel and bonded with the Au2(SR)3 dimer.47
Comparing the composition of the metallic kernel between
Au16Ag1 and Au17Ag1, we found that the Au8Ag1 kernel
transformed into the Au6Ag1 kernel (Figure 3A and C).
Figure 2. (A) Au6Ag1 kernel of Au16Ag1 NC. (B−D) Au6Ag1 kernel
with Au4(SR)5 staple motifs and Au2(SR)3 staple motif. (E) Total
structure of the Au16Ag1 NC. Color legend: orange sphere, Au on the
shell; violet sphere, Au in the kernel; cerulean sphere, Ag; red sphere,
S; pink and green highlighted lines, tetrameric Au4(S-Adm)5 staple
motifs; blue highlighted line, bimeric Au2(S-Adm)3 staple motif.
tristratified arrangement, Au3−Au2Ag1−Au1, instead of the Au3-
M3-Au3 (M = Au/Ag) in (AuAg)18(S−C6H11)14 NCs.35,42,43
The interlayer distances of this tristratified structure are 2.64
and 2.36 Å, which are much more different compared with
those of Au18(S−C6H11)14 NC (2.49 and 2.52 Å, respectively).
In particular, the interlayer distance of Au1-M3 (2.36 Å) is
significantly shorter than the Au3−Au3 distance (2.52 Å) in
Au18(S−C6H11)14,35,42 which is probably caused by the
replacement of top Au3 with only 1 Au. The Au6Ag1 kernel is
further protected by two types of motifs: two tetrameric
Au4(SR)5 staple motifs (Figure 2B and 2C) and one dimeric
Au2(SR)3 staple motif (Figure 2D). The two Au4(SR)5 staples
first constitute almost perpendicular planes and then fully wrap
the Au6Ag1 kernel along with the other Au2(SR)3 staple. As a
result of the irregular interaction between these three staple
motifs, none of symmetry element was found in the structure of
Au16Ag1 NC (Figure 2E). Furthermore, no counterion was
found in the unit cell of Au16Ag1, which indicates the zero-
valence state of Au16Ag1 and is consistent with the ESI-MS and
TGA results.
Figure 3. Structural comparison of Au17Ag1 and Au16Ag1 NCs.
Comparing the top and bottom sections of (A) Au17Ag1 and (C)
Au16Ag1 NCs; two enantiomers of (B) Au17Ag1 and (D) Au16Ag1
NCs. Color legend: orange sphere, Au on the shell; violet sphere, Au
in the kernel; cerulean sphere, Ag; red sphere, S; gray sphere, C.
Remarkably, the position of the single Ag atom (or the
configuration of the bottom two layers: Au3−Au2Ag1) was
retained. In other words, the tristratified Au3−Au2Ag1−Au3
kernel of Au17Ag1 was etched into the Au3−Au2Ag1−Au1
configuration in Au16Ag1 with the two bottom layers
maintained. This configuration of M7 core is observed for the
first time, which is different with another Au7 kernel in
Au20(TBBT)16,45 and both represent the smallest kernel in the
thiolated NCs. Considering the alternation and reduction of the
In the NC field, understanding the grand evolution from
discrete atoms to atomically precise NCs is crucial. In the
previous work, through monitoring the entire transformation
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Figure 4. (A) Kohn−Sham electron states of Au16Ag1 NC; (B) illustration of transition evolution from Au complexes to large Au NCs.
Figure 5. 2D charge density of metallic kernels of (A) Au18(S-CH3)14 and (B) Au17Ag1(S-CH3)14 NCs.
kernel, we speculated that the metallic M7 kernel might
transform into M3 layer-M3 layer configuration (removing the
top Au atom) or hexahedral M5 configuration (removing two
atoms from the bottom M3 layer, like the M9-M7 process)
followed by the further reduction of the NC size. In addition,
the two enantiomers of Au16Ag1 were compared with Au17Ag1
(Figure 3B and D), and it can be observed that the
configuration of Au16Ag1 is more distorted than that of
Au17Ag1. Considering that the single-doped Ag heteroatom in
Au18(S−C6H11)14 would unlikely alter the NC configuration
substantially,43,47 the dramatic distortion probably occurred in
the ligand-exchange process (i.e., from Au17Ag1 to Au16Ag1).
The Kohn−Sham electron states of Au16Ag1 were probed via
DFT computations. As shown in Figure 4A, the HOMO−
LUMO energy gap was calculated as 1.54 eV. In addition, the
HOMO−LUMO in Au16Ag1 exhibits the sp ← d interband
transition. Reviewing the previously reported NCs (Figure 4B
and Table S2), the smaller Au NCs (with 2e or 4e free valence
electrons) as well as the AuI complexes typically exhibit the sp
← d interband transition.35,42−45 By contrast, in larger NCs
(free valence electrons equal to or greater than 6e), the
HOMO−LUMO transition of the electronic excitation mode
mainly involves sp ← sp intraband transition.9,10,32−34 In our
previous work, we have suggested that the emergence of sp ←
sp intraband transition from sp ← d interband transition in the
thiolate-stabilized NC is most likely caused by the increase of
the NC free valence electron.32 As to the Au16Ag1 with 4e free
valence electrons, the sp ← d interband transition verified our
previous suggestion. Additionally, we further suggest that the
HOMOs of small-sized Au-based NCs (smaller than Au16Ag1)
with 4e or 2e free valence electrons are more inclined to
constitute by the d atomic orbitals.
product under the same ligand-exchange reaction on single-
Ag-doped Au17Ag1 is Au16Ag1 with a decreased size. The only
difference in the original reactant is the single heteroatom
doped Ag in the M9 kernel. In this context, the charge densities
of metallic M9 kernels of Au18(S−C6H11)14 and Au17Ag1 were
compared. As shown in Figure 5, we chose the corresponding
kernel cross section of the two NCs to analyze their charge
densities. It can be seen from the 2D charge density map of the
two NCs that the charge distribution of the core is obviously
different. The three kernel Au atoms in Au18(S-CH3)14 have
overlapping charge density, which indicates that the Au atoms
in the core of Au18(S-CH3)14 are averagely bonded by metal
bonds. However, in the Au17Ag1(S-CH3)14 NC, the electron
distribution around the doped Ag atom is obviously higher than
that of the other two Au atoms, which shows strong
polarization characteristics. The charge density distribution
map of Au17Ag1(S-CH3)14 exhibits mainly ionic bonding
between Au and Ag atoms due to the free electron motions,
which is perhaps because of the difference of the charge density
distribution around the Ag atom that results in the opposite
direction of the ligand-exchange process. The 3D charge
density maps are presented in Figure S8.
For analyzing whether there is charge transfer between Au
and Ag atoms in the doping process from Au18(S−C6H11)14 to
Au17Ag1, the charge density differences between Au18(S-CH3)14
and Au17Ag(S-CH3)14 were calculated, namely, Δρ = ρ(Au17Ag-
(S-CH3)14)-ρ(Au18(S-CH3)14). As shown in Figure S9, after
doping the single Ag, the charge at the doping site decreases
significantly (the blue region indicates the decrease in charge of
Au/Ag), and the charge of S (connecting to the doping site)
increases (the yellow region indicates the increase in charge of
S). Moreover, the charges at other positions were almost
maintained. Consequently, the charge transfer between the Au
and Ag atoms in the metallic kernel is small in the doping
Previous work has reported the growth result of Au21(S-
Adm)15 from Au18(S−C6H11)14.32 Interestingly, the only
E
DOI: 10.1021/acs.inorgchem.7b02568
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
ORCID
process, whereas there is a significant charge transfer between
the doping site and the S atoms because of the different
electronegativities of Au and Ag atoms. On the basis of these
results, no obvious charge transfer happens between Au and Ag
atoms in the generation of alloyed Au17Ag1. At the same time,
Hirshfeld charge analysis (Figure S10) shows that the doping
site charge increases from 0.037 to 0.083e and the S charge
decreases from −0.029 to −0.073e, which is consistent with the
differential charge density analyses. In addition, the atomic
charges of the rest kernel atoms have no obvious change after
the doping process.
Shuxin Wang: 0000-0003-0403-3953
Yong Pei: 0000-0003-0585-2045
Manzhou Zhu: 0000-0002-3068-7160
Author Contributions
⊥X.K. and L.X. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
M.Z. acknowledges financial support by NSFC (21372006,
U1532141, and 21631001), the Ministry of Education, the
Education Department of Anhui Province, 211 Project of Anhui
University. Y.P. is supported by NSFC (21373176, 21422305),
Hunan Provincial Natural Science Foundation of China
(12JJ1003), and the Scientific Research Fund of Hunan
Provincial Education Department (13A100).
CONCLUSIONS
■
In summary, Au17Ag1(S−C6H11)14 was first prepared by single-
heteroatom doping strategy and further converted into
Au16Ag1(S-Adm)13 with ligand-exchange strategy. To date, the
as-prepared Au16Ag1(S-Adm)13 is the smallest crystallographi-
cally characterized Au-based nanocluster protected by thiolate.
X-ray crystal structure of Au16Ag1(S-Adm)13 reveals a
tristratified Au3−Au2Ag1−Au1 kernel protected by staple-like
motifs including one dimer and two tetramers. Contrary to the
previous ligand-exchange on the Au18(S−C6H11)14 (Au21(S-
Adm)15 as the resultant), the product of the same ligand-
exchange reaction on Au17Ag1(S−C6H11)14 is Au16Ag1(S-
Adm)13 with decreased size (instead of the size-increased
species). The heterogeneous distribution of charge density
around the Ag atom in the Au17Ag1(S−C6H11)14 nanocluster
compared with the homogold Au18(S−C6H11)14 is proposed as
the fundamental reason for the difference in the ligand-
exchange process. Morever, the Au16Ag1(S-Adm)13 with
HOMO−LUMO sp ← d interband transition exhibits “4e”
free valence electrons, which renews the previous “isoelec-
tronic” 4e small-sized nanocluster family. Furthermore, induced
by the more distorted configuration (having no symmetry
element) of Au16Ag1(S-Adm)13, the enantiomeric chirality is
dramatically enhanced relative to its precursors (Au18(S−
C6H11)14 or Au17Ag1(S−C6H11)14). Overall, the present study
has filled the small-sized nanoclusters gap and, more
significantly, provided a new strategy (combining the single-
heteroatom engineering and ligand-exchange methods) to
synthesize a new class of cognate single-atom-doped nano-
clusters.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02568.
Details of characterizations and DFT calculations (PDF)
Accession Codes
0
Structure of the PdAu24(SR)18 Superatom. Inorg. Chem. 2016, 55,
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data for this paper. These data can be obtained free of charge
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Crystallographic Data Centre, 12 Union Road, Cambridge CB2
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*E-mail: ypnku78@gmail.com.
*E-mail: zmz@ahu.edu.cn.
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