A Molecular Hybrid of an Atomically Precise Silver Nanocluster and Polyoxometalates for H2 Cleavage into Protons and Electrons

Article abstract of DOI:10.1002/anie.202106786

Atomically precise silver (Ag) nanoclusters are promising materials as catalysts, photocatalysts, and sensors because of their unique structures and mixed-valence states (Ag⁺/Ag⁰). However, their low stability hinders the in-depth st

Full text of DOI:10.1002/anie.202106786

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Accepted Article
Title: A Molecular Hybrid of an Atomically Precise Silver Nanocluster
and Polyoxometalates for H2 Cleavage into Protons and
Electrons
Authors: Kentaro Yonesato, Seiji Yamazoe, Daisuke Yokogawa,
Kazuya Yamaguchi, and Kosuke Suzuki
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Angewandte Chemie International Edition
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COMMUNICATION
A Molecular Hybrid of an Atomically Precise Silver Nanocluster
and Polyoxometalates for H Cleavage into Protons and Electrons
2
[a]
[b,d]
[c]
[a]
[a,d]
Kentaro Yonesato, Seiji Yamazoe, Daisuke Yokogawa, Kazuya Yamaguchi,* and Kosuke Suzuki*
[
a]
Dr. K. Yonesato, Prof. Dr. K. Yamaguchi, Dr. K. Suzuki
Department of Applied Chemistry, School of Engineering
The University of Tokyo
7
-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: ksuzuki@appchem.t.u-tokyo.ac.jp, kyama@appchem.t.u-tokyo.ac.jp
Prof. Dr. S. Yamazoe
[
[
[
b]
c]
d]
Department of Chemistry, Graduate School of Science
Tokyo Metropolitan University
1
-1 Minami Osawa, Hachioji, Tokyo 192-0397 (Japan)
Dr. D. Yokogawa
Graduate School of Arts and Science
The University of Tokyo
3
-8-1 Komaba, Meguro-ku, Tokyo 153-8902 (Japan)
Prof. Dr. S. Yamazoe, Dr. K. Suzuki
Precursory Research for Embryonic Science and Technology (PRESTO)
Japan Science and Technology Agency (JST)
4
-1-8 Honcho, Kawaguchi, Saitama 332-0012 (Japan)
Supporting information for this article is given via a link at the end of the document.
Abstract: Atomically precise silver (Ag) nanoclusters are promising
photochemical properties.[3] Using lacunary POMs as inorganic
multidentate ligands, various inorganic nanoclusters have been
synthesized.[4] Our previously reported POM-stabilized Ag
nanoclusters, which comprised an {Ag27}17+ nanocluster
materials as catalysts, photocatalysts, and sensors because of their
+
0
unique structures and mixed-valence states (Ag /Ag ). However, their
low stability hinders the in-depth study of their intrinsic reactivity and
catalytic property accompanying their redox processes. Herein, we
surrounded by three C-shaped {Si
2 18 66
W O } units (Ag27;
17+
198)]; TBA = nBu
+
demonstrate that a molecular hybrid of an atomically precise {Ag27
nanocluster and polyoxometalates (POMs) can efficiently cleave H
into protons and electrons. The Ag nanocluster accommodates
}
TBA16(Me NH Ag [Ag27(Si N ; Figure
2
2
)
8
H
5
2
6
W
54
O
4
2
1), showed unprecedented ultrastability in both solid and solution
states; the structure and electronic state of the nanocluster did not
change for at least one week in organic solvents.[5] Considering
that nanosized Ag catalysts on metal oxide supports exhibit
catalytic activity in a variety of reactions such as oxidation,
reduction, hydration, carboxylation, and alkylation,[7] the unique
combination of components in atomically precise mixed-valent Ag
nanoclusters stabilized by POMs could provide the hybrids with
synergistic or cooperative reactivity and/or catalytic ability.
17+
13+
electrons through the redox reaction from {Ag27
}
to {Ag27
}
, and
the POM ligands play the following important roles: (i) a significant
stabilization of the typically unstable Ag nanocluster to preserve its
structure during the redox reaction with H
2
, (ii) formation of a unique
interface between the Ag nanocluster and metal oxides for efficient H
2
cleavage, and (iii) storage of the generated protons on the negatively
charged basic surface.
Silver (Ag) nanoclusters with well-defined molecular structures
and electronic states have recently attracted great interest in
diverse fields such as structural chemistry, photochemistry,
catalysis, electrochemistry, and biochemistry.[1] Recently,
stimulated by the first report on the crystal structure in 2011,[2a]
several atomically precise Ag nanoclusters have been
synthesized by employing organic protecting ligands including
thiols, phosphines, and alkynes, and the correlation between their
unique physicochemical properties and their structures and
electronic states has been extensively explored.[2] Although the
redox property and reactivity of Ag nanoclusters having mixed-
+
0
valence states, i.e., Ag /Ag , and atomically precise structures
generate considerable interest, their low stability, which typically
induces undesired decomposition and agglomeration, hinders the
in-depth study of these properties involved in the redox processes.
Very recently, we reported the development of a new
synthetic method for atomically precise stable Ag nanoclusters
supported by lacunary polyoxometalates (POMs).[5,6] POMs are a
large family of anionic metal oxide clusters that exhibit unique
structures and properties including acidity/basicity and redox and
2
Figure 1. Schematic of the cleavage of H over Ag27 nanoclusters.
1
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In this work, we focused on the cleavage reaction of
To confirm the structure of the Ag nanocluster/POM hybrid
dihydrogen (H
2
) on a molecular hybrid of an Ag nanocluster and
after the reaction with H
2
(Ag27’), the electron spray ionization
was
POMs. Cleavage of the H–H bond is a fundamentally important
reaction that can proceed via a heterolytic or a homolytic
pathway,[8–10] and it is
hydrogenation of organic functional groups,[7h,11–13]
(ESI) mass spectrum of the reaction solution of Ag27 with H
2
measured. As described in our previous report, the ESI mass
spectrum of Ag27 showed a series of complicated sets of signals,
all of which were assignable to the hybrid structure (Figure S3).[5]
a
crucial process in the catalytic
oxidation
H
2
by hydrogenases,[14] or its model catalysts.[15] For example, metal
2
In contrast, after the reaction of Ag27 with H , the corresponding
nanoparticles on suitable metal oxide supports can efficiently
ESI mass spectrum (negative mode) exhibited a simple prominent
set of signals at around m/z 3592, which were assignable to the
sum of two sets of signals, i.e., [I+4H]5− (calcd m/z 3591.892; I =
cleave H
atomically precise nanoclusters, the unique reactivity of gold
through a heterolytic pathway.[8,11] With regard to
2
nanoclusters toward H [
16]
or hydride (H )
– [17]
has been recently
[TBA
16Ag
198)]) and
[I+4H+H
2
O]5−
2
6
(Me
2
NH
2
)
2
H
2
(Ag27Si
6
W
54
O
studied. However, although Ag nanoclusters possessing H–
(calcd m/z 3595.494) (Figure 2c). Importantly, these results
ligands have been synthesized by the reaction of Ag ions and
suggest that the anion structure of Ag27 (Ag27(Si
6
W
54
O
198)) and
–,[18] to the best of our knowledge, no report has dealt with the
two Ag counter cations were preserved even after the reaction
+
BH
4
–
reaction of atomically precise Ag nanoclusters with H
2
or H , most
with H
cleaved and hydrogen species generated, we conducted the
reaction using deuterium (D ) instead of H . The ESI mass
spectrum of the reaction solution of Ag27 with D exhibited a
2 2
. Furthermore, to quantify the amount of H molecules
likely due to their low stability. Herein, we report for the first time
the unique reactivity of an atomically precise Ag nanocluster
2
2
supported by POMs (Ag27) with H
molecular structure of {Ag27}17+ and POMs in Ag27 enabled the
efficient cleavage of H molecules (1 atm) into protons and
2
(Figure 1). The hybrid
2
similar prominent set of signals, which were shifted to higher m/z
regions (at around m/z 3593) compared with those of the reaction
2
electrons, which were stored on the POM frameworks and the Ag
nanocluster, respectively. The process involves most likely a
solution using H
attributable to the sum of [I+4D] (calcd m/z 3592.697) and
[I+4D+D O] (calcd m/z 3596.701; Figure S4), thus showing that
2
(Figure 2d, S4). This set of signals was
heterolytic H
2
cleavage pathway at the interface between the Ag
2
nanocluster and the POMs. Notably, the intrinsic structure of the
Ag nanocluster was preserved even after the redox reaction of the
nanocluster from {Ag27}17+ to {Ag27}13+ through the reaction with H
2
.
This unique reactivity of the Ag nanocluster was facilitated by the
surrounding POM frameworks, which act not only as stabilizing
agents for the Ag nanocluster but also as basic sites to promote
2
the cooperative cleavage of H and store the generated protons.
The UV–vis spectrum of a green solution of Ag27 in N,N-
dimethylformamide (DMF) showed prominent absorption bands at
4
20, 500 (shoulder), and 600 nm (Figures 2a and 2b). By bubbling
into a solution of Ag27 in DMF at 50°C, the color of the reaction
H
2
solution changed from green to brown after 1 h (Figure 2a), and
the corresponding UV–vis spectra changed significantly,
exhibiting two isosbestic points at 410 and 456 nm and new
absorption bands in the visible light region (400, 480, and 580 nm),
whereas the absorption band at 420 nm decreased (Figure 2b).
In contrast, the UV–vis spectrum of Ag27 in DMF did not change
2
under air at 50°C in the absence of H (Figure S1). Considering
that an absorption band due to surface plasmon resonance of Ag
nanoparticles (~400 nm) was not observed, it can be assumed
that undesirable agglomeration of {Ag27} nanoclusters into Ag
nanoparticles did not occur. Additionally, the absence of an
absorption band assignable to the intervalence charge transfer of
W /W in the POMs (typically, 650–1200 nm)[19] indicates that
5+
6+
the W6+ species in the POM frameworks were not reduced to W5+
species during the reaction with H
DFT) calculations revealed that LUMO and LUMO+1 of Ag27
were delocalized over {Ag27} nanocluster, which suggested the
Ag27}17+ could accommodate additional electrons (Figure S2).[20]
2
. Density functional theory
(
{
The absorption bands at 420, 500 (shoulder), and 600 nm of Ag27
can be assigned to the charge transfer from the {Ag27}-based
HOMO to the delocalized W5d orbitals of the POMs, the charge
transfer from the {Ag27}-based HOMO−1 to W5d orbitals of the
POMs, and the intra-{Ag27} charge transfer from HOMO to LUMO,
respectively (Figure S2). These results indicate that the changes
Figure 2. Change in the color (a) and the UV–vis spectra (b) of the reaction
solution of Ag27 and H in DMF (10 µM, 1 cm cell) at 50ºC for 1 h. (c) ESI mass
spectrum (negative mode) of the reaction solution of Ag27 and H in DMF. The
observed set of signals was assignable to the sum of [I+4H]5– (I
TBA (Me NH (Ag27Si 198); calcd m/z 3591.892, red pattern) and
I+4H+H
O]5– (calcd m/z 3595.494, blue pattern). (d) Enlarged ESI mass
spectrum (negative mode) of the solution of Ag27 after the reaction with H (red
line) or D
(blue line) in DMF. (e) 2H NMR spectrum of the reaction solution of
Ag27 and D in DMF.
2
2
=
6
2
2
)
2
H
16Ag
2
6 54
W O
2
in the UV–vis spectrum of Ag27 after the reaction with H are
[
2
likely due to the change in the electronic state of the {Ag27
}
2
nanocluster.
2
2
2
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four additional H or D atoms were attached on Ag27 by the
2
reaction with H
2
or D
2
, respectively. The H NMR spectrum of the
reaction solution of Ag27 with D
2
showed a signal at 3.54 ppm,
which was derived from D+ produced in the cleavage reaction
+
(
Figures 2e and S5). The formation of four D with respect to Ag27
was confirmed by comparing the peak area of D atoms and that
of [D ]dimethyl sulfoxide ([D ]DMSO, internal standard).
Meanwhile, no signal attributable to deuteride (D ) was observed.
These results support the dissociation of two H (or D ) molecules
→ 4H + 4e or
→ 4D+ + 4e ) over Ag27, and the generated protons (or
6
6
−
2
2
+
+
+
−
into four H (or D ) and four electrons (i.e., 2H
2
−
2D
2
deuterons) were probably stored on the oxygen-rich basic surface
of the negatively charged POM frameworks.
Next, to elucidate where the generated electrons were
stored, the electronic state and structure of Ag27’ were
investigated by Ag K-edge X-ray absorption fine structure (XAFS)
studies (Figures 3a–3d). According to reported XAFS studies on
metal nanoclusters,[21] all solid-state XAFS measurements were
conducted at 10 K to reduce the Debye–Waller factors for a
reliable analysis. The X-ray absorption near edge structure
(
XANES) spectrum of Ag27’ was observed between those of
Ag27 and Ag foil, which indicates that the reduction of the {Ag27
nanocluster occurred upon the reaction with H (Figure 3a).
Additionally, monitoring of the reaction solution of Ag27 with H in
DMF by XANES spectroscopy revealed that the absorption edge
energy (E
) changed from 25517.7 to 25517.11 eV,[22] and the
}
2
2
0
absorption in the white line (~25528 eV) decreased during the
reaction (Figures 3b and S6). Notably, the XANES spectrum of
Ag27’ can be fitted by linear combination of 75% of Ag27 and
2
5% of Ag foil (Figure S7). Additionally, X-ray photoelectron
spectroscopy (XPS) spectrum of Ag27’ in Ag 3d region showed
two peaks at 374.63 eV (3d3/2) and 368.63 eV (3d5/2), both of
0
+
which could be fitted as a mixture of Ag and Ag , assuming the
charge of {Ag27} to be +13 (Figure S8). Based on these results
and considering that the W6+ atoms of the POM frameworks were
not reduced, it seems reasonable to conclude that the electrons
Figure 3. (a) Solid-state Ag K-edge XANES spectra of Ag27, Ag27’, Ag foil,
and AgNO
3 2
. (b) Monitoring of the reaction solution of Ag27 and H in DMF at
5
0°C by Ag K-edge XANES spectra. (c) Solid-state k-space and (d) Ag K-edge
Fourier transformed EXAFS spectra of Ag27, Ag27’, and Ag foil (×1/7 scale).
(e) Structure of the {Ag27} core in Ag27. The color codes of the sticks between
Ag atoms indicate the range of the Ag∙∙∙Ag distances.
2
generated by H cleavage were stored in the {Ag27} nanocluster.
Direct current magnetic susceptibility measurement revealed that
Ag27’ was diamagnetic and that its ground state was a singlet.
To gain more insight into the structure of the {Ag27
}
analyzing the Fourier transformed EXAFS spectra of Ag27 and
Ag27’ (see Supporting Information for details). A peak assignable
to the Ag∙∙∙Ag interactions of Ag27’ was observed at 2.79±0.02 Å,
which was slightly longer than that of Ag27 (2.74±0.02 Å; Figure
nanocluster, we conducted a solid-state Ag K-edge extended X-
ray absorption fine structure (EXAFS) analysis. The k-space
EXAFS spectrum of Ag27’ showed oscillation patterns similar to
those of Ag27, albeit clearly different from those of Ag foil (Figure
3
d). Notably, the coordination number (CN) in Ag27 was
3
c), thus indicating that the overall intrinsic structure of the {Ag27}
considerably smaller (CN, 2.7±0.2; Table S1 and Figure S9) than
the expected value according to the X-ray crystallographic
analysis (suggested CN, 4.84; average R, 2.87 Å). Longer Ag∙∙∙Ag
distances were probably not observed in the EXAFS spectrum
because of their broad distribution (Figure 3e). In contrast, the
observed CN in Ag27’ (CN, 4.9±0.2; Table S1 and Figure S10)
was similar to the expected value based on the X-ray structure.
These results indicate that several long Ag∙∙∙Ag distances
nanocluster was preserved during the reaction. However, the
oscillation patterns of Ag27’ were slightly shifted to smaller k
values compared with those of Ag27, indicating that the Ag∙∙∙Ag
distances within the {Ag27} nanoclusters were slightly modified by
the reaction with H
According to our previous single-crystal X-ray analysis of
Ag27, the {Ag27} nanocluster was composed of three {Ag
octahedrons (Ag1–Ag6, Ag7–Ag12, Ag13–Ag18), a face-sharing
dioctahedral {Ag } core (Ag6, Ag12, Ag18, Ag19–Ag24), and
three Ag atoms (Ag25–Ag27) bridging the {Ag } octahedrons
Figure 3e).[5] Although the {Ag27} nanocluster was surrounded by
2
.
6
}
became shorter by the reaction with H
number of Ag atoms that of Ag∙∙∙Ag distances around 2.79 Å likely
increased. In Ag27, the Ag∙∙∙Ag distances on the edge of the {Ag
octahedron and the {Ag } cores were in the range 2.7–3.0 Å
Figure 3e). In contrast, the Ag∙∙∙Ag distances between the
dioctahedral {Ag } core and surrounding {Ag } octahedrons (e.g.,
2
, and as a result, the
9
6
6
}
(
9
the rigid and bulky C-shaped POM frameworks, partial structural
changes in the Ag∙∙∙Ag distances and partial rotations and/or
(
9
6
distortions of the {Ag
6
} octahedrons were allowed. Hence, we
Ag2∙∙∙Ag24, Ag3∙∙∙Ag22) were in the range 3.0–3.3 Å, indicating
that the distances between these Ag atoms were shortened upon
further investigated the structure of the {Ag27} nanocluster by
2
the reaction with H .
3
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Angewandte Chemie International Edition
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COMMUNICATION
These NMR, ESI mass, UV–vis, XAFS, and XPS studies
Research Institute (Proposal number: 2020A1410). We thank Dr.
T. Yabe (Univ. of Tokyo) and Dr. J. Hirayama (TMU) for help with
XAFS measurements.
2
revealed that two H molecules were cleaved by Ag27, and the
four protons and four electrons generated were stored on the
POM frameworks and the internal {Ag27} nanocluster, respectively.
Thus, the {Ag27}17+ nanocluster (Ag27) was reduced to {Ag27}13+
(
Ag27’) by the reaction with H
2
. Overall, the reaction of Ag27 with
Conflict of interest
H
2
can be expressed by the following equation (Eq. 1):
The authors declare no conflict of interest.
[
{Ag27}17+(Si
6
W
54
O
198)]31− + 2H
{Ag27}13+(H+ 198)]31− (Eq. 1)
Si W O
4 6 54
2
→
[
Keywords: cluster compounds • hydrogen • polyoxometalates •
+
–
2
Typically, H is heterolytically cleaved into H and H at the
silver nanoclusters
interface between metal nanoparticles and metal oxide
supports.[8] The H species on the metal nanoparticles supported
–
[
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2
on reducible metal oxides such as TiO can be further dissociated
+
into H and two electrons, both of which migrate to the metal oxide
surface (hydrogen spillover).[8,11] Since POMs are also reducible
molecular metal oxides, POMs with polydisperse Pt(0)
into two H+ and
nanoparticles have been reported to cleave H
two electrons, both of which are stored in the POM molecule to
2
5
415; f) Y. Liu, X. Chai, X. Cai, M. Chen, R. Jin, W. Ding, Y. Zyu, Angew.
Chem. Int. Ed. 2018, 57, 9775–9779; Angew. Chem. 2018, 130, 9923–
927.
5
+
[11d]
form a reduced POM (W ) species.
The present Ag27 system
9
+
exhibited a unique reactivity; H was cleaved into two H and two
2
[
2]
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electrons most likely through a heterolytic pathway at the interface
between the {Ag27} nanocluster and the POMs, and the protons
and electrons generated were stored on the POM surface and the
mixed-valent {Ag27}17+ nanoclusters, respectively, in the hybrid.
2
015, 127, 3773–3777; d) L. Ren, P. Yuan, H. Su, S. Malola, S. Lin, Z.
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The density functional theory calculations showed that the natural
charges of Ag atoms decreased while those of W atoms in POMs
1
remained unchanged by the reaction of Ag27 with two H
molecules (Figure S11), also supporting these experimental
results. After removing the H from the reaction solution, the UV–
2
M. Monahan, K. Kirschbaum, W. P. Griffith, R. L. Whetten, U. Landman,
T. P. Bigioni, Nature 2013, 501, 399–402; f) H. Yang, Y. Wang, H. Huang,
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2
vis spectra of Ag27’ did not change, and the electronic state of
the {Ag27} nanocluster remained unaltered under Ar atmosphere
(
Figure S12). In contrast, when the solution was treated with O
2
at 50°C, the UV–vis spectra gradually changed and became
similar to that of Ag27, although it did not return back to its original
state completely.
1
42, 16905–16909.
In conclusion, we demonstrated that a molecular hybrid of
an atomically precise Ag nanocluster and POMs can efficiently
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2
cleave H into protons and electrons, which were stored on the
POM frameworks and the mixed-valent {Ag27}17+ nanocluster,
respectively. In particular, the POM ligands played important roles
in this system by (i) significantly stabilizing the typically unstable
Ag nanocluster to preserve its structure during the redox reaction
2
4
4
2
717; f) H. N. Miras, J. Yan, D.-L. Long, L. Cronin, Chem. Soc. Rev. 2012,
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with H
2
, (ii) forming a unique interface between the Ag nanocluster
and metal oxides for an efficient H
2
cleavage, and (iii) storing the
generated protons on the negatively charged basic surface. We
believe that these are important results that will open new
[4]
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2
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Acknowledgements
We gratefully acknowledge the financial support from JST
PRESTO Grant Number JPMJPR18T7 and JPMJPR19T9, JSPS
KAKENHI Grant Numbers 20H02749, 20H04659, 17H03037, and
the JSPS Core-to-Core program. A part of the computations was
performed using Research Center for Computational Science,
Okazaki, Japan. The XAFS measurements were conducted at
SPring-8 with the approval of the Japan synchrotron Radiation
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2
2
p-like symmetry and dz -like symmetry, respectively. The disk-like
2
[
[
8]
9]
distorted shape of {Ag27} nanocluster likely caused the lower dz -like
7
orbital energy and higher p-like orbital energy, which resulted in the
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respectively.
2
[23] In the presence of a catalytic amount of Ag27 (0.7 mol%) under H (1
0 3
values for Ag foil and AgNO were 25516.0 and 25518.5 eV,
1
[
[
2
atm), 4-nitrobenzonitrile was efficiently reduced by irradiation of visible
light (l > 400 nm), selectively affording the corresponding azobenzene
(Figure S13). In this case, byproducts such as the corresponding
azoxybenzene and aniline were hardly detected. Since the reaction
hardly proceeded in the absence of photo-irradiation or Ag27, it is clear
that Ag27 functions as a visible-light-responsive photocatalyst. Although
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the reaction proceeded to some extent in the absence of H
the reaction proceeded more efficiently in the presence of
noteworthy. Hence, the nitroarene reduction is considered to be
proceeded with H as the electron and proton sources. Based on the DFT
2
, the fact that
H
2
is
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7
654–7657.
studies and the above-mentioned experimental results, we consider that
the key step for this reaction is the visible-light-induced electron transfer
from {Ag27} nanocluster to the POM frameworks. The electrons stored in
[
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{Ag27} by the cleavage of H (or from other electron sources) were
4
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possibly transferred to the POM framework by photo-irradiation, which
successfully reduced the nitroarene to form the corresponding
azobenzene.
1
5
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Entry for the Table of Contents
A molecular hybrid of an atomically precise mixed-valent {Ag27}17+ nanocluster and polyoxometalate (POM) ligands exhibited unique
reactivity, efficiently cleaving dihydrogen (H ) into protons and electrons, and the protons and electrons generated are stored on the
2
POM surface and the mixed-valent {Ag27}17+ nanoclusters, respectively.
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