Reactivity and Lability Modulated by a Valence Electron Moving in and out of 25-Atom Gold Nanoclusters

Article abstract of DOI:10.1002/anie.202009278

The emergence of atomically precise metal nanoclusters with unique electronic structures provides access to currently inaccessible catalytic challenges at the single-electron level. We investigate the catalytic behavior of gold Au₂₅(SR)₁₈ nanoclusters by monitoring an incoming and outgoing free valence electron of Au 6s¹. Distinct performances are revealed: Au₂₅(SR)₁₈^? is generated upon donation of an electron to neutral Au₂₅(SR)₁₈⁰ and this is associated with a loss in reactivity, whereas Au₂₅(SR)₁₈⁺ is generated from dislodgment of an electron from neutral Au₂₅(SR)₁₈⁰ with a loss in stability. The reactivity diversity of the three Au₂₅(SR)₁₈ clusters stems from different affinities with reactants and the extent of intramolecular charge migration during the reactions, which are closely associated with the valence occupancies of the clusters varied by one electron. The stability difference in the three clusters is attributed to their different equilibria, which are established between the AuSR dissociation and polymerization influenced by one electron.

Full text of DOI:10.1002/anie.202009278

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Reactivity and Lability Modulated by a Valence Electron Weaving
in and out of the 25-Atom Gold Nanoclusters
Authors: Guangjun Li, Weigang Hu, Yongnan Sun, Jiayu Xu, Xiao
Cai, Xinglian Cheng, Yuying Zhang, Ancheng Tang, Xu Liu,
Mingyang Chen, Weiping Ding, and Yan Zhu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202009278
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10.1002/anie.202009278
Angewandte Chemie International Edition
RESEARCH ARTICLE
Reactivity and Lability Modulated by a Valence Electron Weaving
in and out of the 25-Atom Gold Nanoclusters
Guangjun Li,[a] Weigang Hu,[a] Yongnan Sun,[a] Jiayu Xu,[a] Xiao Cai,[a] Xinglian Cheng,[a] Yuying
Zhang,[a] Ancheng Tang,[a] Xu Liu,[a] Mingyang Chen,*[b] Weiping Ding,[a] and Yan Zhu*[a]
[a]
G. Li, W. Hu, Y. Sun, J. Xu, X. Cai, X. Cheng, Y. Zhang, A. Tang, X. Liu, Prof. W. Ding, Prof. Y. Zhu
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
E-mail: zhuyan@nju.edu.cn
[b]
Prof. M. Chen
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
E-mail: mychen@ustb.edu.cn
Supporting information for this article is given via a link at the end of the document.
Abstract: The ability to pursue heterogeneous catalysis or
1S21P6, 1S21P5 and 1S21P4 superatom-like electron configuration,
–
nanocatalysis at a single-electron level has been largely unsuccessful. respectively.[23-27] Therefore, the three Au25 clusters, that is, Au25
,
0
The emergence of atomically precise metal nanoclusters with unique
electronic structures provides access to currently inaccessible issues.
Here we investigate the catalytic behaviours by performing the
incoming and outgoing of a free valence electron of Au 6s1 on the
Au25(SR)18 nanoclusters. Our studies reveal the distinct
Au25 and Au25+, offer us a platform to assess the nature of the
valence electrons, i.e., play what role in heterogeneous catalysis
or nanocatalysis. Previously, the comparisons of Au25–, Au250, and
Au25+ have been made to understand the three structurally similar
clusters with distinguishable properties.[17,18,28-30] Their different
properties have been mostly attributed to their different valence
occupancies, which is most likely true, but the real connections
between the minorly different electronic structures and exhibited
distinguishable properties have not been clarified explicitly in
most cases.
This work is focused on understanding the nature of how one
electron affects catalytic properties of the Au25 nanoclusters, with
combined experimental and computational efforts. We
demonstrate that the valence electron originating from the Au 6s
orbital can regulate the reactivity and stability of the Au25
nanoclusters by weaving in and out of the nanoclusters, and the
previously inaccessible mechanistic pathways can be reached at
one-electron precision.
–
performances: Au25(SR)18 from donating an electron to neutral
0
+
Au25(SR)18 loses the reactivity and Au25(SR)18 from dislodging an
0
electron from neutral Au25(SR)18 loses the stability. The reactivity
diversity of the three Au25(SR)18 clusters stems from the different
affinities with reactants and different extents of intramolecular charge
migration during the reactions, which are closely associated with the
valence occupancies of the clusters varied by one electron. The
stability difference in the three clusters is attributed to their different
equilibria established between the AuSR dissociation and
polymerization influenced by one electron.
Introduction
Significant research efforts have been witnessed on the single-
electron oxidation or reduction catalysis by organometallic
complexes.[1-3] However, it is extremely challenging to exploit one-
electron-transfer catalysts in heterogeneous catalysis or
nanocatalysis, as the catalysts with identical structure but only
one-electron difference have never been available. The
emergence of atomically precise metal nanoclusters (e.g.,
Aun(SR)m, where SR is the thiol ligand; n = number of gold atoms;
m = number of ligands) brings opportunities to address the above
issues, since crystal structures of such nanoclusters have been
solved and electronic structures can be determined.[4-9] Coupled
with operando spectroscopy and theoritical studies, very
fundamental understanding of catalysis is expected at the single-
atom or single-electron level.[10-12] Atomically precise Aun(SR)m
(abbreviated as Aun hereafter) nanoclusters possess free valence
electrons, which typically differ from the organometallic
complexes, and have been used as model catalysts for
establishing the relationship between catalytic properties and
atomic-level structures, as well as identifing the active sites of
catalysts based on nanoclusters.[13-21]
Results and Discussion
The three Au25(SR)18 (SR: 2-phenylethanethiol, denoted PET
for short) clusters adopt a similar structural framework with a
quasi-D2h symmetry, which is made up of an icosahedral Au13
kernel wrapped by six dimeric staples of Au2(SR)3 (Figure 1).[23]
The average Aucore-Auinnershell bond lengths are respectively 2.775,
2.783, and 2.807 Å for Au25–, Au250, and Au25+, implying the distinct
distortion of Au13 kernels of the three clusters. The continuous
symmetry measure (CSM) is quantified to describe the structural
distortion from the ideal symmetry.[31] The CSM values of Au13
kernels are 0.07, 0.20 and 0.52 for Au25–, Au25 and Au25
,
0
+
respectively, indicating the structural distortion increase with the
Au25 charge increase (Figure 1). The electronic structures of the
three clusters are notably different, as are shown in their UV-vis
spectra (Figure S1a). For Au25–, two prominent bands at 678 and
445 nm, respectively, with a shoulder at 400 nm are observed.
For Au250, a prominent band is at 400 nm with a broad band at
687 nm. For Au25+, the broad band is centered at 660 nm and the
prominent band is at 390 nm. The corresponding HOMO-LUMO
(highest occupied molecular orbital-lowest unoccupied molecular
Within this field, Au25 is a pioneer in opening up new horizons
in catalysis science.[15-18,22] Especially, the Au25 nanoclusters have
three charge states (-1, 0, +1), which can be recognized as the
orbital) gaps are 1.33, 1.47 and 1.54 eV for Au25–, Au250 and Au25
+
1
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10.1002/anie.202009278
Angewandte Chemie International Edition
RESEARCH ARTICLE
+
(Figure S1b), respectively.
that Au25 facilely decomposed. Figure S3 further showed that
UV-vis spectra of the three Au25 clusters all comparatively
+
changed with increased temperature, supporting that Au25 was
impressively labile.
Figure 1. Structure comparison of the anionic, neutral and cationic Au25
nanoclusters. Color code: orange = Au; yellow = S. C and H atoms are omitted
for clarity. The middle panel shows the deviation of Au13 kernels quantified by
CSM. The lowest panel shows an energy level diagram of the P orbitals.
0
Figure 3. Time-resolved in situ UV-vis spectra of (a) Au25–, (b) Au25 and (c)
+
Au25 in DMF at 60 oC and the corresponding absorbance derivatives. The
derivatives at 0 h are set to zero (dotted line), and the derivatives with time refer
to the corresponding differences relative to 0 h.
Intramolecular hydroamination of alkyne toward indole was
selected as a probe reaction to evaluate the catalytic behaviours
of the anionic, neutral and cationic Au25 clusters. As shown in
Figure 2a, regardless of the solvent being N, N-
dimethylformamide (DMF), toluene (TOL) or paraxylene (PX), the
+
0
The missing electron of Au25 compared to Au25 is
considered as a minor perturbation to the total electronic structure;
it, however, completely changes solution stability of Au25. This
implies that the solution stability is likely governed by certain
variable with a critical value. Deciphering such phenomena may
help understand fundamental thermodynamics of gold
nanocluster solutions, and potentially benefit the design and
0
same activity evolution trend was observed: Au25 showed the
–
highest activity, followed by Au25+, and Au25 had little activity,
+
which can be presented in a descending sequence: Au250 > Au25
0
> Au25–. The recyclability of the three catalysts indicated that Au25
+
synthesis of gold nanoclusters. Previously, the lability of Au25
was mainly ascribed to the Jahn-Teller distortion effect due to its
S2P4 valence state,[31] or ascribed to Au25 being susceptible to
–
+
afforded catalytic stability, while both Au25 and Au25 lost
activities after the first run reactions (Figure 2b). From UV-vis
spectra of the spent Au25 catalysts, the Au25 and Au25 clusters
appeared to undergo no significant decomposition during the
reactions, but Au25+ completely degraded (Figure S2). The results
+
–
0
− [30]
redox reactions which form more stable states such as Au25
.
However, the geometry difference of the three Au25 clusters
caused by Jahn-Teller distortion is only significant for the single-
crystal structure, much less for the gas-phase clusters, according
to the density function theory (DFT) calculations, and likely not
considerable for the solvated clusters (the detailed discussion
about geometry distortion of the Au25–, Au250 and Au25+ clusters in
single-crystal, gas and solution phases is shown in Supporting
Information). Apparently, the degradation of Au25+ occurs readily
under mild conditions, which means no external electric field or
redox agents are necessary. Furthermore, the self-redox
reactions of Au25+, such as disproportionation, are improbable
under mild conditions, as removing valence electrons from the
S2P4 Au25 is thermodynamically difficult. UV-vis spectroscopy
showed no sign of conversion of Au25+ into Au250 or Au25− (Figure
3 and S3). Thus, the previous hypotheses are not in accord with
the fact that Au25 facilely degrades in solution without forming
reduced Au25 species. To further understand the lability of Au25
we performed DFT calculations.
0
justifiably revealed that neutral Au25 had the best catalytic
performance, whereas anionic Au25 had the lowest activity and
–
cationic Au25+ had the worst stability.
+
Figure 2. (a) Catalytic performances of the Au25–, Au250, and Au25+ catalysts for
intramolecular hydroamination of 2-ethynylaniline in different solvents. Reaction
conditions: 1 mg Au, 1 mmol substrate, 5 mL solvent, 60 oC. (b) Recyclability of
the spent catalysts in DMF. TOF = turnover frequence. The catalytic data are
averaged over three independent measurements.
+
+
,
First, the dissociation reactions of the three Au25 clusters are
investigated at the DFT level. Au25+, Au250, and Au25− are modelled
with [Au25(SR)18][PF6], [Au25(SR)18]0, and [Au25(SR)18][NR’4],
respectively, where the computationally affordable CH3 group is
used for R and R’ to simplify the actual thiol and octyl groups. In
the solution, the dissociation will most likely yield neutral leaving
fragments, as removal of otherwise ionic fragments has to
overcome strong electrostatic attraction and is hence unfavorable,
so the neutral dissociation reactions with AuSR as the minimal-
Furthermore, time-resolved in situ UV-vis spectroscopy was
employed to illustrate the distinct stability of the three clusters
–
under reaction conditions. Au25 was essentially stable with the
smallest absorbance derivative shown in Figure 3a. Au250 slightly
changed within 1 h and no further changes occurred with time
+
expand (Figure 3b). Of note, the characteristic peaks of Au25
obviously weakened within 5 h and then fully disappeared with the
largest absorbance derivative observed (Figure 3c), indicating
2
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10.1002/anie.202009278
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RESEARCH ARTICLE
sized removal unit are considered. As shown in Figure 4a, for all
three Au25 clusters, the dissociation of first AuSR from the staple
is most endergonic (∆G298 = ~45 kcal mol-1), indicating that the
initial dissociation of the intact staple may be rate-limiting. The
removal of the second AuSR from the staple is endergonic by ~35
kcal mol-1, resulting in Au23(SR)16 with a terminal AuSR containing
an innershell Au and a staple SR. The dissociation of Au23(SR)16
with the removal of the terminal AuSR is predicted to be
endergonic by ~35 kcal mol-1 for Au250 and Au25−, and by ~30 kcal
coordinate ∆. During vertical dissociation the major parts of the
electronic structures of the fragments are close to the electronic
structure of the intact Au25 cluster. The energetics for the vertical
dissociation of AuSR from the three Au25 clusters are shown in
Figure 4b. No dissociation barrier is found for the vertical
dissociation for all three clusters, implying that the activation
energy for the AuSR dissociation can be well approximated by the
reaction endergonicity. The vertical dissociation energies of the
three clusters only differ slightly. At an inter-fragment distance of
+
+
mol-1 for Au25+. Overall, the dissociation of Au25 is more
5 Å, the vertical dissociation energy for Au25 is 68.7 kcal mol-1,
0
−
0
thermodynamically favorable than those of the Au25 and Au25
~1.5 kcal mol-1 lower than those of Au25 and Au25−. This means
counterparts.
that the vertical dissociation only accounts in part for the different
dissociation thermodynamics of the three Au25 clusters and the
relaxation of electronic structures due to the formation of the Au24
clusters might also play sufficient roles. The electronic structures
of the fully relaxed Au25, vertically dissociated Au25 with ∆ = 5 Å,
and fully relaxed Au24 as the dissociation product are compared,
with the transition of molecular orbital levels analyzed (Figure S4-
S6). The analysis shows that upon the dissociation, the S2P4, S2P5
and S2P6 valence states of Au25+, Au250 and Au25− are significantly
altered.[28] For all three clusters, the S orbital of Au25 shifts toward
negative energy and becomes a core orbital of Au24, the three P
orbitals of Au25 remain in the valence, and a core Au-thiolate
bonding orbital (denoted as ‘t’) in Au25 becomes localized around
the terminal thiolate of Au24 which rises in energy to be the new
HOMO (Figure 4c and 4d). For Au25+, one of the P orbitals turns
into a localized orbital, essentially an Au s polaron associated to
the exposed innershell Au of Au24+, due to symmetry breaking; the
localization of P is less apparent for the dissociation of Au250 and
0
−
Au25−. As a result, Au24+, Au24 and Au24 exhibit the t2P2s2, t1P6
+
and t2P6 valence states respectively. The stabilization of the Au24
dissociation product, which contributes to the lability of Au24
+
,
Figure 4. (a) Predicted AuSR dissociation reactions for Au25+, Au250, and Au25
−
could be due to the fact that the energy increase of its HOMO t
orbital is the least and the energy decrease of its S orbital is the
most for the AuSR dissociation of the three clusters (Figure 4c).
at the PBE/LANL2DZ level. Gibbs free energies at 298 K are given in kcal mol-
1. (b) Relative electronic energy of Au25+, Au250, and Au25 with respect to the
−
fully-relaxed intact clusters during the rigid reaction coordinate scan for the
vertical dissociation of the first staple AuSR. The reaction coordinate is chosen
as the inter-fragment distance between leaving AuSR and the remaining cluster
(∆). (c) The molecular orbital level diagram for fully-relaxed gas-phase
Au25(SR)18 (∆ = 0) and Au24(SR)17 (∆ = ∞). (d) The molecular orbital level
diagram for fully-relaxed solution-phase Au25(SR)18 and Au24(SR)17 with the
DMF solvent using the SMD-IEFPCM computational solvation model.
0
We hypothesize that the higher formation energies of Au24 and
−
Au24 are likely due to the conflicting electronic structure
preferences by the molecular structure and by the valence
electron counts: the molecular structure favors forming polaron
over the exposed innershell Au site whereas the valence electron
0
−
counts of Au24 and Au24 favor the full-shell P6 state over the
polaron state. The transition of molecular orbital levels with the
SMD-IEFPCM solvation model suggests that DMF stabilizes the
HOMO t orbital of Au24+ substantially while it does not give notable
changes to the electronic structures of Au240 and Au24− (Figure 4d),
which explains the further divergence of AuSR dissociation
thermodynamics for the three Au25 clusters in DMF.
Figure 4 shows that, in gas phase, the rate-limiting
dissociation of the first AuSR from Au25 is ~3 kcal mol-1 more
favorable than the counterpart reactions for Au25 and Au25
+
0
−
.
Further, the solution-phase dissociation of the first staple AuSR
from the Au25 clusters in DMF is predicted using the SMD-
IEFPCM (the SMD variation of integral-equation-formalism
polarizable continuum) solvation model at the DFT level.[32] In
DMF, the ∆G298 for the removal of AuSR from Au25+, Au250, and
Au25− is reduced to 34.2, 37.4, and 40.3 kcal mol-1, respectively.
+
The AuSR dissociation for Au25 is 6 kcal mol-1 less endergonic
than that for Au25−, suggesting that the difference in lability of the
two clusters becomes more significant with the DMF solvent.
Next, the vertical dissociation of AuSR from the three Au25
clusters is investigated computationally; here, “vertical” indicates
that the geometries of the AuSR and Au24(SR)17 fragments remain
frozen as in the fresh catalyst during the dissociation. This is done
by using a rigid reaction coordinate scan calculation where the
leaving AuSR is pulled away from the equilibrium position until a
distance of 5 Å is reached between AuSR and the remaining
cluster. The inter-fragment distance is defined as the reaction
Figure 5. (a) Optimized geometries for selected (AuSR)n. (b) Predicted
normalized binding energy of AuSR to form (AuSR)n, n ≤ 13. Electronic reaction
energy at 0 K and Gibbs reaction free energy at 298 K are predicted at
PBE/LANL2DZ level in kcal mol-1. (c) Predicted HOMO-LUMO gap for (AuSR)n.
3
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The next question is whether there exists a driven force for
the endergonic AuSR dissociation reaction for Au25+. The key is
to find the bound states for the leaving AuSR fragments. One
straightforward possibility is that AuSR polymerizes into (AuSR)n
clusters. Previously, global minima for the (AuSR)n (R = CH3 and
n ≤ 13) clusters were predicted using a genetic algorithm.[33]
Based on the reported structures, we predicted the polymerization
thermodynamics for the AuSR fragments (Figure 5a and S7). The
normalized binding energy for AuSR to form (AuSR)n, <BE>(n), is
calculated using equation (1):
(AuSR)5 are 3.95 and 3.66 eV (Figure 5c), which forbid absorption
with wavelength longer than 314 and 339 nm respectively (Figure
S11b). Therefore, all of the resultant fragments and consequent
clusters, based on our hypothesis, exhibit absorption properties
that are compatible with the experimental UV-vis spectra, and our
hypothesis holds well against the experimental examination.
Overall, our computation and experiment reveal that the lability of
Au25+ is due to the combination of its facile AuSR removal and the
consequent AuSR aggregation. The relatively high stabilities of
0
−
Au25 and Au25 are owing to the comparatively high AuSR
dissociation energies and pseudo-one-dimensional (AuSR)n
ceasing to grow at small n.
ꢃ ꢅ
ꢃ
ꢅ ⁄
ꢀꢁ ꢂ
ꢄ
ꢁꢃ ꢆꢇꢈꢉ ꢊꢅ ꢄ ꢁꢃꢆꢇꢈꢉꢅ
(1)
The AuSR polymerizations are mildly exergonic, from which the
consumption of AuSR may be conducive to the dissociation of
Au25+. It is found that <BE>(n) Gibbs free energy at 298 K
increases drastically for n ≤ 4 and decreases slightly for n ≥ 4 as
n increases. The early convergence of <BE>(n) is due to (AuSR)n
exclusively being pseudo-one-dimensional torus or linked tori, for
which increasing n does not increase the number of bonds per n
(which cluster stability scales with). This implies (AuSR)4 might be
the dominant (AuSR)n species formed from the AuSR monomers.
<BE>(4) is predicted to be 30.7 kcal mol-1 at 298 K without any
solvent. The existence of (AuSR)5 might also be possible with
<BE>(5) predicted to be 30.4 kcal mol-1. The early convergence
of <BE>(n) and small dominant size for (AuSR)n are key for the
stable existence of many non-stoichiometric Aun(SR)m (n ≠ m)
clusters, because otherwise the spontaneous growth of (AuSR)n
into large clusters will drive the dissociation of most Aun(SR)m
clusters. To confirm our hypothesis, matrix-assisted laser
desorption ionization mass (MALDI-MS) spectrometry for the
+
degraded Au25(PET)18 clusters was measured. Notable peaks
were found for Au3(PET)2, Au4(PET)3, and Au4(PET)4 based
fragments in the low m/z range, which might result from the
fragmenting of Au4(PET)4 (Figure S8). A number of major peaks
for Au25-x(PET)18-x were found in the high m/z range, and frequent
Au(PET) intervals were observed between the major peaks
(Figure S9). It is noted that the Au(PET) intervals were not found
in the high m/z range of MALDI-MS spectra of the fresh
Figure 6. (a) FT-IR spectra of the adsorbed species onto the Au25–, Au250, and
Au25+ clusters in the presence of 2-ethynylaniline. (b) FT-IR spectra of pyridine
adsorbed onto the Au25 clusters. (c) Au 4f XPS profiles of the Au25 clusters. (d)
XANES profiles of the Au25 clusters. Inset: energy in 11916-11922 eV window.
Moreover, to unveil the mystery of the superior reactivity of
+
Au25(PET)18 clusters (Figure S10). Hence, the hypothesis of
0
+
Au25 to both Au25 and Au25–, the chemisorbed modes of 2-
ethynylaniline over the three cluster catalysts were studied by
Fourier-transformed infrared (FT-IR) spectroscopy. As presented
in Figure 6a, two stretching bands at 3280 and 2095 cm-1,
respectively assigned to νs(≡C–H) and νs(C≡C),[34] were observed
on the three catalysts, suggesting the internal binding mode of
ethynylaniline on the three catalysts. Notably, two N–H stretching
bands at 3470 and 3370 cm-1, assigned to free NH2 species, were
found on Au25–, which implied no chemical interaction between N–
H bonds and Au25–. For Au250 and Au25+, one of the N–H stretching
+
Au25 undergoing neutral dissociation reaction with AuSR as
minimal removal unit is supported by the mass experiments.
−
Since the dissociation rate-limiting step of Au25+, Au250 and Au25
is 44.4, 47.9 and 47.4 kcal mol-1 without solvent, the reaction
energy for the net degradation reaction, given by equation (2),
ꢏ ꢃ
ꢆꢇꢋꢌ → ꢆꢇꢋꢍ ꢎ ꢍ ꢆꢇꢈꢉ
ꢅ
ꢍ
(2)
is 13.7, 17.2 and 16.7 kcal mol-1 for Au25+, Au250 and Au25− in the
gas phase, respectively. Using the SMD-IEFPCM solvation model,
<BE>(4) is found to be 22.6 kcal mol-1, and the reaction energy
for the net degradation reaction is evaluated to be 11.6, 14.8 and
17.7 kcal mol-1. Our prediction is in excellent agreement with the
bands at 3470 cm-1 underwent a red shift and the other at 3370
0
cm-1 substantially weakened, especially nearly vanished for Au25
It is proposed that Au-amine species formed onto Au250 and Au25
might effectively boost the hydroamination reaction.
.
+
+
experimental observation from UV-vis spectroscopy that Au25
0
−
readily degrades, Au25 slightly degenerates and Au25 persists
under mild conditions (Figure 3). The low endergonicity for the net
degradation reaction of Au25 implies that this reaction is
The Lewis acidity property of the catalysts was probed by the
IR spectra of adsorbed pyridine. The bands at ~1444 cm-1
assigned to pyridine adsorbed on the Lewis acid sites were
observed on the three Au25 clusters (Figure 6b), suggesting the
electron-deficient sites available on the three catalysts.[35,36]
Additionally, two IR bands in the ranges of 1590~1596 and
1479~1497 cm-1 on Au250 shifted higher wavenumbers than those
on Au25+, the corresponding bands of which in turn shifted higher
wavenumbers than those on Au25–, meaning that the interaction
+
temperature-sensitive, which is confirmed from the evolution of
UV-vis spectra with increased temperature (Figure S3). The UV-
+
vis spectra also show that as Au25 degrades, major absorption
peaks in the visible range essentially disappear. The predicted
evolution of UV-vis spectra for the step-by-step AuSR dissociation
from Au25+ to Au22+ exhibits a qualitatively similar pattern (Figure
S11a). The predicted HOMO-LUMO gaps for (AuSR)4 and
4
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RESEARCH ARTICLE
of the reactant and Au250 may differ from the cases of Au25− and
constants for the three solvents also follow the same order with
DMF (38.25) > TOL (2.38) > PX (2.27).[38] This validates our
proposed mechanism, to some extent. The intramolecular proton
transfer and cyclization lead to CAT[C8H6N(ID)]H. The total
formation reaction of CAT[C8H6N(ID)]H (CAT + ethynylaniline →
CAT[C8H6N(ID)]H) is exergonic for all three catalysts. Formation
of [C8H6N(ID)]H over Au250 is ~10 kcal mol-1 more exergonic than
the corresponding reactions over Au25− and Au25+. The [C8H6N(ID)]
in CAT[C8H6N(ID)]H can recombine with H (on S), which
exergonically forms the indole. Consequently, the cleaved staple
recombines into an intact staple and the catalyst returns to the
initial state.
+
Au25
.
The gold valence states in the three Au25 clusters were
monitored by X-ray photoelectron spectroscopy (XPS). The Au
4f7/2 binding energies on the three clusters were higher than that
of metallic Au (Figure 6c), which was correlated with the electron
deficiency on the gold by the electron withdrawing nature of the
thiol ligands.[37] Notably, the Au 4f7/2 binding energies sequenced
slightly positive shifts from Au25–, Au250, to Au25+, which was
consistent with X-ray absorption near edge structure (XANES)
studies (Figure 6d). Extended X-ray absorption fine structure
(EXAFS) experiments indicated that local coordination
environments in the Au25 clusters were not obviously different
(Figure S12 and Table S1). Taken together, it is suggested that
distinct catalysis of the three Au25 clusters was mainly determined
by the electronic structures, rather than the geometrical structures,
which can influence on their affinities with the reactants.
For the molecular-level understanding of the superior activity
of Au250, the hydroamination reaction pathways on the Au25
catalysts are proposed based on the DFT results. First, 2-
ethynylaniline dissociatively adsorbs at an outer-Au of the catalyst
to form CAT[C8H6N(EA)]H (Figure 7a and S13; EA indicates the
adsorbate being structurally related to ethynylaniline and CAT
stands for Au25+, Au250, or Au25−), during which the Au-S bond of
the staple is cleaved to adsorb the amino-H on S and the C8H6N
on Au. The adsorption Gibbs free energy at 298 K is in the order
0
−
+
of Au25 (25.1 kcal mol-1) < Au25 (29.6 kcal mol-1) < Au25 (31.6
kcal mol-1). The superior activity of Au250 is probably in part related
to its facile adsorption of ethynylaniline. Then, the intramolecular
H+ transfer reaction (followed by cyclization), C8H6N (EA) →
C8H6N (ID), proceeds by transferring a H from N to ethynyl C
(where ID indicates that the structure is related to indole). For this
reaction, the in-situ barrier height is impractical to determine due
to the complex role solvent might play in mediating the proton
transfer and the ex-situ barrier without solvent is also difficult to
predict, as the proton transfer seamlessly leads to cyclization with
high exergonicity which prevents computationally locating the
transition state. The in-situ barrier, though, should scale with the
binding strength of the N-H (or the deprotonation energy) while
also being affected by the choices of the solvent. Therefore, we
provide predictions for the fictitious fully-deprotonated
CAT[C8H5N(EA)]−H and CAT[C8H5N(ID)]−H products, in order to
provide the upper bounds for the intramolecular proton transfer
barriers. Note that for the actual solution-phase reaction, partial
deprotonation likely occurs because of the short proton migration
path and the proton mediation effect of the solvent molecules.
More supplemental discussion about intramolecular proton
transfer of ethynylaniline catalyzed by Au250 with and without DMF
can be found in Supporting Information.
The predicted Gibbs free energy at 298 K for the full
deprotonation of an amino-H of CAT[C8H6N(EA)]H is in the
following ordering: Au25+ (312.9 kcal mol-1) ≈ Au250 (318.4 kcal mol-
1) < Au25− (352.1 kcal mol-1), which explains the lack of reactivity
for Au25−. Based on our proposed proton transfer mechanism,
solvents with high proton affinity and high dielectric constant can
facilitate the intramolecular proton transfer step. The proton
affinity for DMF, TOL, and PX are calculated to be: DMF (202.5
kcal mol-1) > TOL (185.7 kcal mol-1) > PX (182.4 kcal mol-1), which
is consistent with the order of the hydroamination reactivity in
these solvents (Figure 2a). It is found that the CHO moiety of DMF
mainly accounts for its high proton affinity. The dielectric
Figure 7. (a) Predicted reaction Gibbs free energies and enthalpies (kcal mol-1)
0
at 298 K for the proposed hydroamination reaction pathway over Au25
.
Enthalpies are given in brackets. ‘EA’ and ‘ID’ in parentheses indicate
ethynylaniline-like and indole-like respectively. Color code: orange = Au, yellow
= S, black = C, green = H, pink = N. PET is simplified by SCH3. (b) Adsorbate
formal charges based on the predicted natural atomic charges for the adsorbate
fragments during the hydroamination of ethynylaniline over Au250, Au25−, and
Au25+. * denotes CAT. For CAT[C8H6N(EA)]H and CAT[C8H6N(ID)]H, the
adsorbate formal charge is equal to the net atomic charge of the adsorbate
fragments; for CAT[C8H6N(EA)]−H, the adsorbate formal charge is equal to the
net atomic charge of the adsorbates plus 1 as the adsorbate is expected to be
monoanionic after deprotonation.
The electronic structures of the reaction intermediates over
Au25 catalysts are examined to understand their different
performances in catalyzing hydroamination of ethynylaniline. The
amount of charge transferred from the adsorbates to the Au25
catalysts during the reactions can be indicated by the adsorbate
formal charges (Figure 7b) based on the predicted natural atomic
charges at the DFT level. It is shown that upon dissociative
adsorption of ethynylaniline over the catalyst to form
CAT[C8H6N(EA)]H, electron is transferred from the catalyst to its
−
+
adsorbates. The electron transfer is notable for Au25 and Au25
,
but less significant for Au250. We hypothesize that the loss of
valence electron will impair the S2P6 and S2P4 valence states of
−
+
fresh Au25 and Au25 and hence destabilize the corresponding
CAT[C8H6N(EA)]H intermediates. The weaker electron depletion
5
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10.1002/anie.202009278
Angewandte Chemie International Edition
RESEARCH ARTICLE
upon ethynylaniline adsorption over Au250 is probably because in
the ground-state Au250[C8H6N(EA)]H, the spin-polarized C8H6N
adsorbate back-donates electron to the catalyst. Therefore, the
S2P5 valence occupancy of the fresh Au250 catalyst is essentially
retained despite the dissociative adsorption of ethynylaniline,
which might avoid severe destabilization. Note that the ground
state of Au250[C8H6N(EA)]H is 2.5 kcal mol-1 lower in energy than
a low-lying excited state with non-spin-polarized adsorbates
where the electron transfer results in fractional valence
We acknowledge financial support from National Natural Science
Foundation of China (21773109, 91845104, U1930402).
Keywords: nanocluster • catalysis • valence electron • activity •
lability
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7
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10.1002/anie.202009278
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
RESEARCH ARTICLE
Guangjun Li, Weigang Hu, Yongnan
Sun, Jiayu Xu, Xiao Cai, Xinglian
Cheng, Yuying Zhang, Ancheng Tang,
Xu Liu, Mingyang Chen*, Weiping Ding,
and Yan Zhu*
((Insert TOC Graphic here))
Page No.1 – Page No.7
In this work, we investigate the catalytic reactivity and lability by performing the
incoming and outgoing of a free valence electron of Au 6s orbital on the neutral
Au25 nanocluster. We demonstrate that the Au25– nanocluster from donating an
Reactivity and Lability Modulated by
a Valence Electron Weaving in and
out of the 25-Atom Gold Nanoclusters
+
electron to the neutral Au250 nanocluster loses the reactivity and the Au25
nanocluster from dislodging an electron from the neutral Au250 nanocluster loses the
stability. The correlation of the free valence electrons and catalytic properties of the
Au25 nanoclusters can be achieved at a single-electron level.
8
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