Angewandte
Chemie
formation of 2a and gaseous hydrogen was observed with
almost the same rate as for the first run (86% yield of 2a over
4 min based on newly added 1a; cf. Table 1, entry 4). There-
fore, it is likely that SiW10 can act as an efficient oxygen-
donor ligand to stabilize (active) silver species,[15] resulting in
promotion of the hydrolytic oxidation and prevention of the
aggregation of silver species as is observed for the reaction
without the SiW10 ligand. These results stimulated us to
synthesize a silver-containing POM by employing SiW10 as an
inorganic ligand, which would be expected to be active for
hydrolytic oxidation of silanes and other functional-group
transformations.[13]
Usually, transition metal containing POMs have been
synthesized by reaction of alkali metal salts of lacunary POMs
and the corresponding transition metal salts in aqueous
media, but isomerization and decomposition of lacunary
POMs often proceed in aqueous (acidic) media. Synthesis in
organic media from appropriate lacunary precursors (e.g.,
TBA salts) can avoid isomerization and decomposition of the
original lacunary frameworks. For example, we synthesized
zinc-containing sandwich-type silicotungstate TBA8[{Zn-
(OH2)(m3-OH)}2{Zn(OH2)2}2(g-HSiW10O36)2]·9H2O, which is
not possible to synthesize in aqueous media, by the reaction of
SiW10 with Zn(acac)2 (acac = acetylacetonato) in acetone.[5a]
Likewise, we attempted to synthesize a silver-containing
POM in organic media, while taking the results in Table 1 into
account. Silver-containing POM Ag4 could be synthesized by
reaction of SiW10 and AgOAc in acetone (70% yield based
on SiW10), and single crystals suitable for X-ray crystallo-
graphic analysis were obtained by recrystallization from
acetone/dimethyl sulfoxide (DMSO; see Supporting Infor-
mation).[16] The molecular structure of the anion part of Ag4 is
shown in Figure 1.
values (1.12–1.25 for O20, O21, O56, and O57) found in Ag4
suggest that protons are likely located on these oxygen atoms.
The X-ray, elemental analysis, and thermogravimetric data
show that the formula of Ag4 is TBA8[Ag4(DMSO)2(g-
H2SiW10O36)2]·2DMSO·2H2O. Compound Ag4 can form
according to Equation (1).
2 TBA4H4½g-SiW10O36ꢀ þ 4 AgOAc !
ð1Þ
TBA8½Ag4ðg-H2SiW10O36Þ2ꢀ þ 4 AcOH
As shown in Figure 1, Ag4 has a diamond-shaped
tetrasilver [Ag4]4+ cluster cation encapsulated by two SiW10
subunits. Two DMSO molecules are coordinated to Ag2 and
Ag4. The Ag1 and Ag3 atoms are bridged by two SiW10
subunits in a slightly distorted square-planer environment,
ꢁ
and the average Ag OW distance is 2.38 ꢀ. The Ag2 and Ag4
atoms are coordinated to two oxygen atoms of the central
ꢁ
{SiO4} tetrahedrons with an average Ag OSi distance of
2.46 ꢀ. Thus, the [Ag4]4+ cluster is stabilized in the lacunary
ꢁ
ꢁ
ꢁ
pocket of SiW10. The Ag1 Ag2, Ag2 Ag3, Ag3 Ag4, and
ꢁ
Ag4 Ag1 distances are 2.962(2), 3.070(2), 3.016(2), and
3.074(2) ꢀ, respectively. Since they are shorter than twice
the van der Waals radius of the silver atom (3.44 ꢀ), they
suggest the existence of argentophilic interaction between
silver atoms in Ag4. It is well known that d10 metals tend to
show closed-shell interactions,[17] and such argentophilic
interactions were often observed in previously reported
organometallic complexes with similar [Ag4]4+ cores.[18] The
Ag1···Ag3 and Ag2···Ag4 separations are 4.868(2) and
3.683(2) ꢀ, respectively. The Ag1-Ag2-Ag3 and Ag2-Ag3-
Ag4 angles are 105.66(6)8 and 74.47(5)8, respectively. The
ꢁ
above-mentioned Ag Ag distances, Ag···Ag separations, and
angles in Ag4 are within the ranges of reported complexes
with [Ag4]4+ cores.[18]
Although various kinds of POMs substituting or encap-
sulating metal–oxygen clusters have been synthesized until
now,[2,4,5] we note that POMs containing multimetallic cores
with metal–metal interactions are very rare.[19–22] Polyoxo-
metalate assemblies with dimeric[19] or infinite[20] structures
[19a,b,20]
ꢁ
ꢁ
connected by metal–metal interactions (Ag Ag
or Rh
Rh[19c]) have been reported. The dirhodium-containing POMs
were synthesized by attaching dicarboxylatodirhodium com-
plexes to monolacunary POMs.[21] The g-Keggin silicotung-
states [g-SiW10M2S2O38]6ꢁ (M = W or Mo) with [M(O)(m-
S)2M(O)]2+ oxothio core show M M interactions.
All
[22]
Figure 1. Polyhedral and ball-and-stick representation of the anion part
of Ag4 (see Figure S2 of the Supporting Information for ORTEP). The
{WO6} units and a {SiO4} unit are shown as green octahedra and
a gray tetrahedron, respectively. Black, red, green, blue, and pink
spheres indicate silver atoms, oxygen atoms of DMSO (ODMSO), oxygen
atoms of the {WO6} units (OW), oxygen atoms of the {SiO4} units
(OSi), and monoprotonated oxygen atoms, respectively. DMSO mole-
cules, except for the oxygen atoms, are omitted for clarity.
ꢁ
reported POMs with metal–metal interaction have dimetallic
cores, and multimetallic cores consisting of more than two
metals, as observed in Ag4, have not been reported so far, to
the best of our knowledge.[23]
Finally, we investigated catalysis of hydrolytic oxidation of
silanes by Ag4. As we expected, various structurally diverse
silanes including aromatic, double bond containing, and
aliphatic ones, could selectively be converted to the corre-
sponding silanols in high yields in the presence of Ag4
(Table 2). Selectivities to the desired silanols were greater
than 99% (except for 1d). The hydrolytic oxidation of
commonly examined 1a efficiently proceeded with Ag4
(0.05 mol%), giving 2a in 96% yield under the conditions
described in Table 2.[14,15] In this case, the stoichiometric
Eight TBA cations per Ag4 anion could be crystallo-
graphically assigned, in accord with the result of the elemental
analysis. The bond valence sum (BVS) values of silicon (3.98
and 4.02), tungsten (5.83–6.26), and silver atoms (0.91–1.07) in
Ag4 indicate that the respective valences are + 4, + 6, and
+ 1. In addition, the four oxygen atoms with lower BVS
Angew. Chem. Int. Ed. 2012, 51, 2434 –2437
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2435