A discrete octahedrally shaped [Ag6]4+ cluster encapsulated within silicotungstate ligands

Article abstract of DOI:10.1039/c2cc37591e

By the reaction of TBA₄H₄[γ-SiW ₁₀O₃₆] (TBA = tetra-n-butylammonium) with AgOAc (OAc = acetate) using dimethylphenylsilane as a reductant in acetone, a unique polyoxometalate containing a discrete octahedrally shaped [Ag₆] ⁴⁺ cluster, TBA₈[Ag₆(γ-H ₂SiW₁₀O₃₆)₂]·5H₂O, could be synthesized, and the molecular structure was determined.

Full text of DOI:10.1039/c2cc37591e

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A discrete octahedrally shaped [Ag6]4+ cluster
encapsulated within silicotungstate ligands†
Cite this: Chem. Commun.,
2013, 49, 376
Yuji Kikukawa, Yoshiyuki Kuroda, Kosuke Suzuki, Mitsuhiro Hibino,
Kazuya Yamaguchi and Noritaka Mizuno*
Received 18th October 2012,
Accepted 15th November 2012
DOI: 10.1039/c2cc37591e
www.rsc.org/chemcomm
By the reaction of TBA4H4[c-SiW10O36] (TBA = tetra-n-butylammonium) other hand, there are only a few reports on POMs containing
with AgOAc (OAc = acetate) using dimethylphenylsilane as a ‘‘multimetallic clusters’’.7–9 With regard to multisilver POMs,
reductant in acetone, a unique polyoxometalate containing a discrete POM assemblies with dimeric7a,b or infinite7c,d structures, POMs
octahedrally shaped [Ag6]4+ cluster, TBA8[Ag6(c-H2SiW10O36)2]ꢀ5H2O, with cationic multisilver complexes as counter cations,7e–h and
could be synthesized, and the molecular structure was determined. silver–alkynyl clusters encapsulating POMs (POMs@forty- or
sixty-vertex AgI-alkynyl shells)7i,j that have been reported until
Metal clusters have attracted much attention in various fields of now. To the best of our knowledge, metal clusters including silver
science, such as catalysis, photocatalysis, electrochemistry, substituted or encapsulated by POMs are still very rare compounds.
magnetism, and material science, due to their unique proper- Thus, their rational design and synthesis are challenging sub-
ties, which differ markedly from those of both mononuclear jects to open up a new avenue for POM chemistry.
metal complexes and bulk metals.1 Because their properties are
We have found that a novel POM containing a subvalent
strongly dependent on the numbers of constituent atoms hexasilver cluster, TBA8[Ag6(g-H2SiW10O36)2]ꢀ5H2O (Ag6, TBA =
(cluster sizes) and electrons (valences of clusters) and the tetra-n-butylammonium), could be synthesized by the reaction
shapes, the design and synthesis of well-defined metal of TBA4H4[g-SiW10O36] with AgOAc (OAc = acetate) using
nanoclusters are very important.1 Up to date, template-directed dimethylphenylsilane (1, two-electron reductant) in acetone.
procedures have generally been applied to the synthesis of In the absence of TBA4H4[g-SiW10O36], AgI species were reduced
nanoclusters with well-defined sizes as well as shapes,2 and by 1 and immediately aggregated to form precipitates of
organic ligands, surfactants, and metal organic frameworks metallic silver.‡ Therefore, the lacunary POM ligands could
(MOFs) have frequently been utilized.3
efficiently capture an in situ generated partially reduced [Ag6]4+
We have paid much attention to the unique properties of cluster, resulting in the formation of Ag6. In this communica-
polyoxometalates (POMs).4–6 POMs consist of the group V and tion, we report the synthesis and structural characterization of
VI metals in their highest oxidation states.4 They are thermally Ag6 in detail.
and oxidatively stable in comparison with commonly utilized
Initially, we attempted to synthesize a silver-containing POM
10
organic ligands.4 In addition to their high durabilities, their by the reaction of TBA4H4[g-SiW10O36
]
with 3 equivalents of
chemical and physical properties can precisely be tuned by AgOAc using 1 in acetone (ESI†). In the absence of 1, the
selecting constituent elements and counter cations, and a rich reaction of TBA4H4[g-SiW10O36] with AgOAc gave white precipi-
diversity of POM structures have been reported.4 In particular, tates. The 29Si NMR spectrum of the product showed a single
lacunary POMs have been utilized as rigid multidentate oxygen- signal at ꢁ82.0 ppm assignable to our previously reported POM
donor ligands to synthesize discrete mono-, di-, tri-, and tetra- containing four AgI species, TBA8[Ag4L2(g-H2SiW10O36)2] (Ag4,
metal–oxygen cores by substitution into the lacunary pockets5 L = dimethylsulfoxide or water, Fig. S1 and S2(a), ESI†).9
or encapsulation within two or more lacunary POMs.6 On the When the reaction was carried out with 0.25 equivalents of 1
with respect to TBA4H4[g-SiW10O36], a mixture of Ag6 and Ag4
Department of Applied Chemistry, School of Engineering, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
was obtained (1 : 1, 62% total yield based on TBA4H4-
[g-SiW10O36], Fig. S2(b), ESI†). In the presence of 0.5 equivalents
of 1, Ag6 was obtained as a single product (80% yield, Fig. S2(c),
ESI†).‡ The yield of Ag6 decreased to 53% with 1 equivalent
of 1 due to the undesirable formation of precipitates of
metallic silver. Thus, we hereafter utilized Ag6 obtained by
E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp; Fax: +81-3-5841-7220; Tel: +81-3-5841-7272
† Electronic supplementary information (ESI) available: Experimental details,
Tables S1–S5, Fig. S1–S9, and additional references. CCDC 903979. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c2cc37591e
c
376 Chem. Commun., 2013, 49, 376--378
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the reaction with 0.5 equivalents of 1 (for characterization and in Ag6 are +6 and +4. In each dilacunary silicotungstate unit,
recrystallization). two oxygen atoms with relatively lower BVS values (1.21 and
The elemental analysis of Ag6 showed that the TBA : Si : W : Ag 1.18 for O22 and O23, respectively; the other oxygen atoms were
ratio in Ag6 was 8 : 2 : 20 : 6. The positive-ion cold-spray 1.51–2.09) were found, suggesting that protons are likely
ionization mass (CSI-MS) spectrum of Ag6 exhibited two sets located on these oxygen atoms. We have synthesized several
of signals assignable to {TBA10[Ag6(H2SiW10O36)2]}2+ (signal set metal-substituted POMs using [g-SiW10O36]8ꢁ. Similarly, in
A, centered at m/z 3780) and {TBA9[Ag6(H2SiW10O36)2]}+ (signal these POMs including Ag4, O22 and O23 oxygen atoms were
set B, m/z 7718) (Fig. S3, ESI†). As mentioned above, the also protonated.5b,6a,9 In addition, it was confirmed by the
29Si NMR spectrum of Ag6 showed a single signal (Fig. S4(a), potentiometric titration of Ag6 with TBAOH that Ag6 possessed
ESI†). The 109Ag NMR spectrum showed a signal at 442 ppm four titratable protons (Fig. S6, ESI†) in accord with the BVS
(Fig. S4(c), ESI†). These 29Si and 109Ag NMR results suggest that calculation. Thus, the hexasilver cluster was encapsulated
Ag6 is a single species. The 183W NMR spectrum showed three within two [g-H2SiW10O36]6ꢁ subunits. The BVS values of the
signals at ꢁ115.3, ꢁ129.8, and ꢁ172.8 ppm with an intensity silver atoms were in the range of 0.50–0.56, suggesting that the
ratio of 2 : 2 : 1 (Fig. S4(b), ESI†), respectively, suggesting that hexasilver cluster consists of subvalent silver atoms and that
Ag6 possesses a g-Keggin POM framework.
electrons are delocalized on the cluster. Taking the numbers of
6ꢁ
We successfully obtained high quality single crystals of Ag6 TBA cations and [g-H2SiW10O36
]
subunits, the net valence of
suitable for X-ray analysis by recrystallization from a mixed the hexasilver cluster is +4, i.e., [Ag6]4+, giving an average formal
solvent of nitromethane and diethyl ether (ESI†), and the valence of 2/3 for each silver atom. The magnetic susceptibility
molecular structure could be determined (Fig. 1, Fig. S5, (wM) of Ag6 was ꢁ1.94 ꢂ 10ꢁ3 cm3 molꢁ1 at 300 K in reasonable
Tables S1 and S2, ESI†). The anion had approximately D2h agreement with the sum of diamagnetic susceptibilities of two
inherent symmetry. Eight TBA cations per one Ag6 anion could TBA4H4[g-SiW10O36] (ꢁ0.92 ꢂ 10ꢁ3 cm3 molꢁ1) and six AgI
crystallographically be assigned in accord with the elemental species (ꢁ0.028 ꢂ 10ꢁ3 cm3 molꢁ1),11 showing that Ag6 is
analysis data. The bond valence sum (BVS) values of tungsten diamagnetic. Therefore, an additional two electrons in the
(5.89–6.28) and silicon (3.99) indicate that the respective valences hexasilver cluster are paired and occupy the lowest bonding
molecular orbital (a1g), resulting in the formation of the 2-electron-
6-centered bonding cluster.12
The above-mentioned results (elemental analysis, X-ray
analysis, and titration) and the thermogravimetry data indicate
that the formula of Ag6 is TBA8[Ag6(g-H2SiW10O36)2]ꢀ5H2O.
It was confirmed by GC-MS analysis that acetic acid and the
corresponding silylacetate were detected during the reaction of
TBA4H4[g-SiW10O36] with AgOAc using 1.§ Thus, compound Ag6
can form according to eqn (1).
2TBA4H4[g-SiW10O36] + 6AgOAc + Me2PhSiH -
TBA8[(g-H2SiW10O36)2Ag6] + 5AcOH + Me2PhSiOAc
(1)
Compound Ag6 possessed an octahedrally shaped hexasilver
6ꢁ
cluster encapsulated within two [g-H2SiW10O36
]
subunits.
These types of POMs encupsulating multimetallic clusters have
never been reported so far, to the best of our knowledge. To date,
several compounds containing subvalent [Ag6]4+ units, such as
12a
Ag6Ge10P12
,
Ag6O2,12b and Ag5(XO4) (X = Si or Ge),12c,d have
been reported. The [Ag6]4+ units in these reported compounds
are ‘‘indiscrete’’ and connected with bridging atoms (e.g.,
phosphorus, silver, and/or oxygen) to form infinite crystal
structures.12 In sharp contrast, the [Ag6]4+ clusters in Ag6 were
surrounded by two [g-H2SiW10O36]6ꢁ subunits and were com-
pletely ‘‘discrete’’ from each other; each [Ag6]4+ cluster in Ag6
was separated at least by 18.80 Å (Table S1, Fig. S7, ESI†).
The Ag1 atom in Ag6 was coordinated to the two oxygen
atoms of the central {SiO4} tetrahedron with an average Ag–OSi
distance of 2.43 Å.¶ Thus, the Ag1 atom was deeply located and
stabilized in the building pocket of the [g–H2SiW10O36]6ꢁ sub-
unit. Each silver atom of the square based of octahedron
(Ag2, Ag3, Ag20, and Ag30) was coordinated to two terminal
Fig. 1 (a) Polyhedral and ball-and-stick representation of the anion part of Ag6
(see Fig. S5 for ORTEP, ESI†) and (b) schematic illustration of the hexasilver [Ag6]4+
cluster. The {WO6} and central {SiO4} units are shown as green octahedrons and
gray tetrahedrons, respectively. Black, green, blue, and pink spheres indicate
silver atoms, oxygen atoms of the {WO6} units (OW), oxygen atoms of the {SiO4}
units (OSi), and monoprotonated oxygen atoms, respectively.
c
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oxygen atoms (OW) of two [g-H2SiW10O36]6ꢁ subunits with an
average Ag–OW distance of 2.28 Å. The Ag1–Ag2, Ag1–Ag3,
Ag2–Ag3, and Ag3–Ag2* distances were 2.75 Å, 2.71 Å, 2.76 Å,
and 2.86 Å, respectively. The Ag3–Ag2* distance was slightly
longer than the Ag2–Ag3 distance because of the rectangle
geometry of oxygen donors of [g-H2SiW10O36]6ꢁ subunits
(3.18 Å and 5.21 Å for O1ꢀ ꢀ ꢀO2 and O1ꢀ ꢀ ꢀO3 separations,
respectively). These Ag–Ag distances were shorter than those
of metallic silver (2.88 Å) and the sum of the van der Waals radii
of silver atoms (3.44 Å), suggesting the existence of closed-shell
interactions. It is well known that d10 metals (herein AgI
species) tend to show closed-shell interactions.13 In com-
parison with multimetallic complexes composed of only AgI
species, the hexasilver cluster in Ag6 was subvalent, and the
additional two electrons occupied the bonding molecular
orbital,12 resulting in stronger argentophilic interactions.13
Indeed, the Ag–Ag distances in Ag6 (2.71–2.86 Å) were signifi-
cantly shorter than those of previously reported organometallic
complexes with [Ag6]6+ clusters (2.81–3.28 Å, Table S3, ESI†)14
and Ag4 (2.96–3.07 Å, Fig. S1, ESI†),9 and comparable to those
of previously reported compounds with subvalent [Ag6]4+ units
(2.70–2.86 Å, Table S4, ESI†).12
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a hexasilver cluster. The detailed formation
mechanism and its catalytic applications8 are now under
investigation.
We thank M. Shinoe and T. Tsukahara (The University of
Tokyo) for their help with preliminary experiments. This work
was supported in part by the Japan Society for the Promotion
of Science (JSPS) and Grants-in-Aid for Scientific Research
from the Ministry of Education, Culture, Sports, Science and
Technology.
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‡ When the reaction was carried out without TBA4H4[g-SiW10O36],
a mixture of metallic silver and AgOAc (starting material) was obtained.
§ The corresponding disiloxane was also detected possibly formed from
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