Sandwich-type zinc-containing polyoxometalates with a hexaprismane core [{Zn2W(O)O3}2]4+ synthesized by thermally induced isomerization of a metastable polyoxometalate

Article abstract of DOI:10.1021/ic101419a

Two novel sandwich-type silicotungstates, TBA₈[{Zn ₂W(O)O₃}₂H₄{α-SiW ₉O₃₃}₂]·5H₂O (α-Zn4; TBA = tetra-n-butylammonium) and TBA₈[{Zn₂W(O)O ₃}₂H₄{β-SiW₉O ₃₃}₂]·7H₂O (β-Zn4), were synthesized by the solid-state thermally induced isomerization of metastable TBA₈[{Zn(OH₂)(μ₃-OH)}₂{Zn(OH ₂)₂}₂{γ-HSiW₁₀O ₃₆}₂]·9H₂O (γ-Zn4). Compounds α-Zn4 and β-Zn4 consisted of two [SiW₉O₃₃] ^8- subunits sandwiching the unprecedented distorted hexaprismane core [{Zn₂W(O)O₃}₂]⁴⁺.

Full text of DOI:10.1021/ic101419a

8194 Inorg. Chem. 2010, 49, 8194–8196
DOI: 10.1021/ic101419a
Sandwich-Type Zinc-Containing Polyoxometalates with a Hexaprismane Core
[{Zn2W(O)O3}2]4þ Synthesized by Thermally Induced Isomerization of a Metastable
Polyoxometalate
Yuji Kikukawa, Kazuya Yamaguchi, and Noritaka Mizuno*
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan
Received July 16, 2010
Two novel sandwich-type silicotungstates, TBA8[{Zn2W(O)O3}2-
H4{R-SiW9O33}2] 5H2O (r-Zn4; TBA = tetra-n-butylammonium)
proceed in (acidic) aqueous media,2 and the desired metal-
substituted POMs (with retention of the original framework
structures of lacunary precursors) cannot be synthesized in
some cases.3 Very recently, we have successfully synthesized a
γ-Keggin zinc-containing sandwich-type POM, TBA8[{Zn-
3
and TBA8[{Zn2W(O)O3}2H4{β-SiW9O33}2] 7H2O (β-Zn4), were
synthesized by the solid-state thermally induced isomerization of
3
metastable TBA8[{Zn(OH2)(μ3-OH)}2{Zn(OH2)2}2{γ-HSiW10O36}2]
3
9H2O (γ-Zn4). Compounds r-Zn4 and β-Zn4 consisted of two
(OH2)(μ3-OH)}2{Zn(OH2)2}2{γ-HSiW10O36}2] 9H2O (γ-
3
[SiW9O33]8- subunits sandwiching the unprecedented distorted
Zn4), by the reaction of TBA4H4[γ-SiW10O36] with Zn(acac)2
(acac = acetylacetonato) in an organic medium (acetone).3,4
In this case, no isomerization and decomposition of the
lacunary precursor proceeded. To date, there are only a few
reports on the synthesis of POMs in organic media.5
hexaprismane core [{Zn2W(O)O3}2]4þ
.
Polyoxometalates (POMs) are attractive compounds and
are utilized in many fields, e.g., analytical chemistry, medi-
cine, electrochemistry, photochemistry, and catalysis.1 Until
now, various kinds of metal-substituted POMs have been
synthesized by the substitution of metal cations into the
vacant site(s) of lacunary POMs and used as catalysts for
various functional group transformations because of their
unique reactivities depending on their compositions and
structures of active sites.1
Usually, metal-substituted POMs have been synthesized
by the reaction of alkali-metal salts of lacunary POMs and
metal salts in (acidic) aqueous media.1 However, isomeriza-
tion and/or decomposition of lacunary precursors easily
It has been reported that water of crystallization and/or
aquo ligands play an important role in the stabilization of
metal-substituted POMs and that the removal of water by
thermal treatment causes isomerization and/or decomposi-
tion of their structures.6 Compound γ-Zn4 possessed the
tetranuclear zinc-oxygen cluster core [{Zn(OH2)(μ3-OH)}2-
{Zn(OH2)2}2]6þ sandwiched between two [γ-SiW10O36]8-
subunits (Figure 1).4 The zinc atoms in γ-Zn4 possessed
one or two aquo ligand(s). The distances between oxygen
atoms on zinc atoms (aquo ligands) and nearest-neighbor
oxygen atoms from [γ-SiW10O36]8- subunits were in the range
˚
of 2.91-2.93 A (Figure 1), showing the existence of hydrogen-
bonding interaction.7 Thus, as for γ-Zn4, the zinc atoms located
out-of-pocket of the lacunary site of [γ-SiW10O36]8- are likely
*To whom correspondence should be addressed. E-mail: tmizuno@
mail.ecc.u-tokyo.ac.jp. Tel: þ81-3-5841-7272. Fax: þ81-3-5841-7220.
(1) (a) Hill, C. L. In Comprehensive Coordination Chemistry II; McCleverty,
J. A., Meyer, T. J., Eds.; Elsevier: Amsterdam, The Netherlands, 2003; Vol. 4,
pp 679-759. (b) Kozhevnikov, I. V. Catalysis by Polyoxometalates; John Wiley
& Sons, Ltd.: Chichester, U.K., 2002. (c) Hill, C. L., Ed. Thematic issue on
Polyoxometalates. Chem. Rev. 1998, 98, 1-390. (d) Neumann, R. Prog. Inorg.
Chem. 1998, 47, 317. (e) Okuhara, T.; Mizuno, N.; Misono, M. Adv. Catal. 1996,
41, 113. (f) Hill, C. L.; Christina, C.; Prosser-McCartha, C. M. Coord. Chem. Rev.
1995, 143, 407.
(2) For example, see:(a) Kortz, U.; Jeannin, Y. P.; Teze, A.; Herve, G.;
Isber, S. Inorg. Chem. 1999, 38, 3670. (b) Kortz, U.; Isber, S.; Dickman, M. H.;
Ravot, D. Inorg. Chem. 2000, 39, 2915. (c) Kortz, U.; Matta, S. Inorg. Chem.
2001, 40, 815. (d) Hussain, F.; Bassil, B. S.; Bi, L.-H.; Reicke, M.; Kortz, U.
Angew. Chem., Int. Ed. 2004, 43, 3485. (e) Zhang, Z.; Li, Y.; Wang, E.; Wang,
X.; Qin, C.; An, H. Inorg. Chem. 2006, 45, 4313. (f) Lisnard, L.; Mialane, P.;
Dolbecq, A.; Marrot, J.; Clemente-Juan, J. M.; Coronado, E.; Keita, B.; de
(3) Until now, various zinc-containing POMs have been reported:
(a) Kozik, M.; Baker, L. C. W. J. Am. Chem. Soc. 1987, 109, 3159.
ꢀ
ꢀ
(b) Tourne, C. M.; Tourne, G. F.; Zonnevijlle, F. J. J. Chem. Soc., Dalton Trans.
1991, 143. (c) Neumann, R.; Khenkin, A. M. Inorg. Chem. 1995, 34, 5753.
(d) Kortz, U.; Al-Kassem, N. K.; Savelieff, M. G.; Al Kadi, N. A.; Sadakane, M.
Inorg. Chem. 2001, 40, 4742. (e) Wang, J.; Yan, L.; Li, G.; Wang, X.; Ding, Y.;
Suo, J. Tetrahedron Lett. 2005, 46, 7023. (f) Drewes, D.; Limanski, E. M.; Krebs,
B. Eur. J. Inorg. Chem. 2005, 1542.
(4) Kikukawa, Y.; Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed.
2010, 49, 6096.
(5) (a) Khenkin, A. M.; Shimon, L. J. W.; Neumann, R. Eur. J. Inorg.
ꢀ ꢀ
ꢀ
Chem. 2001, 789. (b) Yamaguchi, K.; Mizuno, N. New J. Chem. 2002, 26, 972.
ꢁ
ꢀ
(c) Mialane, P.; Duboc, C.; Marrot, J.; Riviere, E.; Dolbecq, A.; Secheresse, F.
Chem.—Eur. J. 2006, 12, 1950.
(6) (a) Rocchiccioli-Deltcheff, C.; Fournier, M. J. Chem. Soc., Faraday
ꢀ ꢀ
ꢀ
Trans. 1991, 87, 3913. (b) Bayer, R.; Liu, M. F. X.; Teze, A.; Herve, G. J. Mol.
Catal. A 1996, 110, 65.
ꢀ
Oliveira, P.; Nadjo, L.; Secheresse, F. Chem.—Eur. J. 2007, 13, 3525. (g) Luo, Z.;
€
Kogerler, P.; Cao, R.; Hakim, I.; Hill, C. L. Dalton Trans. 2008, 54. (h) Mitchell,
(7) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; W. H. Freeman
and Co.: San Francisco, 1960.
S. G.; Ritchie, C.; Long, D.-L.; Cronin, L. Dalton Trans. 2008, 1415.
r
pubs.acs.org/IC
Published on Web 08/17/2010
2010 American Chemical Society

Communication
Inorganic Chemistry, Vol. 49, No. 18, 2010 8195
Figure 1. Polyhedral and ball-and-stick representation of the anion part
of γ-Zn4. The {WO6} moieties occupy the gray octahedra, and the {SiO4}
groups are shown as internal blue tetrahedra. Green, red, and purple
spheres indicate zinc atoms, oxygen atoms coordinating to zinc, and
oxygen atoms from {WO6} moieties in [γ-SiW10O36]
8-, respectively.
Figure 3. Polyhedral and ball-and-stick representations of the anion
parts of (a) r-Zn4 and (d) β-Zn4. (c) Ball-and-stick representation and (d)
schematic illustration of the hexaprismane core [{Zn2W(O)O3}2]4þ. The
{WO6} moieties occupy the gray octahedra, and {SiO4} groups are shown
by internal blue tetrahedra. Green, gray, red, and purple spheres indicate
zinc atoms, tungsten atoms, oxygen atoms from hexaprismane cores, and
oxygen atoms from {WO6} moieties in [SiW9O33]
8-, respectively.
Figure 2. Solid-state MAS29Si NMR spectra of (a) γ-Zn4, (b) thermally
treated γ-Zn4 (at 150 °C), (c) r-Zn4, and (d) β-Zn4.
(Figure S2 in the Supporting Information). From this solu-
tion, the single crystals of r-Zn4 were obtained upon stand-
ing at room temperature (32% yield based on γ-Zn4). After
r-Zn4 was completely removed by filtration, the white
residue (powder) was obtained by evaporation to dryness.
The analytically pure single crystals of β-Zn4 were successfully
obtained by recrystallization from a mixed solvent of acetone
and acetonitrile (20:1, v/v; 16% yield based on γ-Zn4). The
solid-state MAS 29Si NMR spectra of isolated r-Zn4 and
β-Zn4 showed signals at -78.7 and -79.0 ppm (Figure 2c,d),
respectively. The IR spectrum of the thermally treated γ-Zn4
could be well reproduced by using those of r-Zn4 and β-Zn4
(Figure S3 inthe Supporting Information). All of these results
show that γ-Zn4 was converted into (at least) two new POMs,
r-Zn4 and β-Zn4, by solid-state thermal treatment.8
The molecular structures of r-Zn4 and β-Zn4 could be
determined by X-ray crystallographic analyses. The struc-
tures of the anion parts are shown in Figure 3. The crystallo-
graphic data and the selected bond lengths and angles in
r-Zn4 and β-Zn4 are summarized in Tables S1 and S2 in the
Supporting Information, respectively. The ORTEP represen-
tations are shown in Figure S4 in the Supporting Information.
As shown in Figure 3, compounds r-Zn4 and β-Zn4 possessed
the distorted hexaprismane core[{Zn2W(O)O3}2]4þ sandwiched
between two [SiW9O33]8- subunits ([R-SiW9O33]8- subunits
for r-Zn4; [β-SiW9O33]8- subunits for β-Zn4). Eight TBA
cations per each anion (r-Zn4 and β-Zn4) could be crystal-
lographically assigned in accordance with the results of the
elemental analyses, although they were highly disordered.
The bond balance sum (BVS)9 values of silicon (3.80 for r-
Zn4 and 4.03 for β-Zn4), tungsten (5.96-6.24 for r-Zn4 and
stabilized by hydrogen-bonding interaction. If these aquo
ligands could be removed, isomerization of γ-Zn4 would take
place, resulting in the formation of new zinc-containing
POMs. As expected, we found in this study that two novel
sandwich-type POMs, TBA8[{Zn2W(O)O3}2H4{R-SiW9O33}2]
5H2O (r-Zn4) and TBA8[{Zn2W(O)O3}2H4{β-SiW9O33}2]
3
3
7H2O (β-Zn4), could be synthesized by the solid-state ther-
mally induced isomerization of γ-Zn4.8
The IR and thermogravimetric analyses showed that the
structure of γ-Zn4 was inherently stable upon solid-state
thermal treatment under a vacuum up to 80 °C and that all
aquo ligands as well as water of crystallization in γ-Zn4 were
removed upon thermal treatment under a vacuum at 150 °C.
The IR band positions and intensities of the POM frame-
works changed upon treatment at 150 °C (Figure S1 in the
Supporting Information). The solid-state magic-angle-spin-
ning (MAS) 29Si NMR spectrum of the thermally treated
γ-Zn4 (at 150 °C) showed a broad (overlapped) signal centered
at -79.0 ppm (Figure 2b), and the signal position was diffe-
rent from those of the original γ-Zn4 (-84.2 ppm; Figure 2a).
The thermally treated γ-Zn4 was dissolved in a mixed solvent
of acetone and water (10:1, v/v), and the 29Si NMR spectrum
of the solution showed two signals at -78.6 and -79.0 ppm
(8) Compounds r-Zn4 and β-Zn4 could be obtained only by solid-state
thermally induced isomerization and could not be synthesized by the
reactions of lacunary POMs with zinc precursors in the solution state, e.g.,
the reactions of (i) TBA4H4[γ-SiW10O36] and Zn(acac)2 in organic media, (ii)
K8[γ-SiW10O36], Zn(NO3)2, and TBABr in aqueous media, and (iii) Na10[R-
SiW9O34], Zn(NO3)2, Na2WO4, and TBABr in aqueous media. In addition,
no isomerization of γ-Zn4 to r-Zn4 and β-Zn4 took place when γ-Zn4 was
heated in acetone (reflux) and N,N-dimethylformamide (at 150 °C).
(9) (a) Brese, N. E.; O’Keeffe, M. Acta Crystallogr. 1991, B47, 192. (b)
Brown, I. D.; Altermatt, D. Acta Crystallogr. 1985, B41, 244.

8196 Inorganic Chemistry, Vol. 49, No. 18, 2010
Kikukawa et al.
5.94-6.26 for β-Zn4), and zinc (2.15 and 2.09 for r-Zn4 and
2.17 and 2.10 for β-Zn4) atoms indicate that the respective
valences in these compounds are 4þ, 6þ, and 2þ. The data of
X-ray analyses, elemental analyses, and thermogravimetric
analyses show that the formulas of r-Zn4 and β-Zn4 are
TBA8[{Zn2W(O)O3}2H4{R-SiW9O33}2] 5H2O and TBA8-
3
[{Zn2W(O)O3}2H4{β-SiW9O33}2] 7H2O, respectively. The
3
formations of r-Zn4 and β-Zn4 anions from γ-Zn4 can be
expressed by the following equation (eq 1).
Figure 4. Oxidation of various alcohols. Reaction conditions: r-Zn4 or
β-Zn4 (0.4 mol % with respect to alcohols), alcohol (0.5 mmol), 30%
aqueous H2O2 (0.25 mmol), acetone (1.5 mL), 56 °C. Yields were
determined by gas chromatography analyses and based on H2O2 used.
8 -
½fZnðH2OÞðμ3-OHÞg2fZnðH2OÞ2g2fγ-HSiW10O36g2ꢀ
8 -
f ½fZn2WðOÞO3g2H4fSiW9O33g2ꢀ þ 6H2O ð1Þ
Compounds r-Zn4 and β-Zn4 showed high catalytic
activities for the oxidation of alcohols with H2O2, and the
catalytic activities of r-Zn4 and β-Zn4 were intrinsically the
same. The scope of alcohol oxidation with r-Zn4 and β-Zn4
is summarized in Figure 4. The oxidation of various kinds of
structurally diverse alcohols including aliphatic, benzylic,
allylic, and propargylic ones efficiently proceeded to afford
the corresponding carbonyl compounds in high yields. The
catalytic activities of r-Zn4 and β-Zn4 for the oxidation of
nonactivated aliphatic alcohols such as cyclohexanol, 2-ada-
mantanol, and 2-pentanol were higher than that of γ-Zn4
(Table S3 in the Supporting Information). For activated
alcohols including benzylic and allylic ones, the catalytic
activities of three POMs were almost the same (Table S3 in
the Supporting Information).
After oxidation was completed, the catalyst could be re-
trieved by evaporation of acetone, followed by the addition of
an excess amount of diethyl ether. The IR spectrum of the used
catalyst was intrinsically the same as that of the fresh catalyst
(Figure S5 in the Supporting Information), suggesting that the
framework structure of the catalyst was preserved even after
oxidation. The retrieved catalyst could be reused for alcohol
oxidation; for example, oxidation of cyclohexanol with the
retrieved β-Zn4 under the same conditions gave cyclohexa-
none in 91% yield (88% yield with the fresh catalyst).
Each zinc atom in r-Zn4 and β-Zn4 was five-coordinated
and pyramidally coordinated by three oxygen atoms from the
hexaprismane cores and two oxygen atoms from the {WO6}
moieties in [SiW9O33]8- subunits. The hexaprismane cores were
composed of two six-membered puckered rings with the chair
conformation ({Zn1-O7-Zn2-O1*-W1*-O2*-} and
{O1-W1-O2-Zn1*-O7*-Zn2*-}) (Figure 3). Two puck-
ered rings were essentially located in parallel. The bond lengths
˚
Zn-O in the puckered rings (1.937-2.105 A) were shorter
than those of the Zn-O bonds connecting two puckered
˚
rings (2.122-2.285 A) (Table S2 in the Supporting Infor-
mation). Although complexes with hexaprismane cores com-
posed of monometals and oxygen atoms have been reported,10
the hexaprismane cores (as in r-Zn4 and β-Zn4) composed of
multimetals and oxygen atoms have never been reported, as far
as we know.
As for the hexaprismane core in r-Zn4, the respective BVS
values of oxygen atoms O1, O2, and O7* were 1.98, 1.95, and
1.93, showing that these oxygen atoms are μ3-O ligands
bridging two zinc atoms (Zn1 and Zn2* for O1; Zn1* and
Zn2 for O2; Zn1* and Zn2* for O7*) and one tungsten atom
(W1 for O1 and O2; W1* for O7*). The O8 atom was an oxo
ligand (BVS: 1.71). The BVS values of O4 and O6 were 1.17
and 1.25, respectively, suggesting monoprotonation of these
oxygen atoms. As for the hexaprismane core in β-Zn4, the
respective BVS values of oxygen atoms O2 and O7* were 1.96
and 2.04, showing that O2 and O7* are μ3-O ligands bridging
two zinc atoms (Zn1* and Zn2 for O2; Zn1* and Zn2* for
O7*) and one tungsten atom (W1 for O2; W1* for O7*). In
contrast, the O1 atom is a μ3-OH ligand (BVS: 1.18) bridging
two zinc atoms (Zn1 and Zn2*) and one tungsten atom (W1).
The O8 atom was an oxo ligand (BVS: 1.78). The O4 (BVS:
1.30) and O6 (BVS: 1.90) atoms were μ2-OH and μ2-O
ligands, respectively. Therefore, the bond lengths and angles
in the hexaprismane core in β-Zn4 were somewhat different
from those in r-Zn4 (Table S2 in the Supporting In-
formation).
In summary, the novel POMs r-Zn4 and β-Zn4 could be
obtained by the thermally induced isomerization of γ-Zn4
and showed high catalytic activities for alcohol oxidation.
Acknowledgment. We thank Dr. K. Uehara (The Uni-
versity of Tokyo) for his help with X-ray analyses. This
work was supported, in part, by the Global COE Program
(Chemistry Innovation through Cooperation of Science
and Engineering) and Grants-in-Aid for Scientific Re-
searches from Ministry of Education, Culture, Sports,
Science and Technology. Y.K. is grateful for a JSPS
Research Fellowship for Young Scientists.
Supporting Information Available: X-ray crystallographic
data in CIF format for r-Zn4 and β-Zn4, experimental proce-
dures, Tables S1-S3, and Figures S1-S5. This material is
available free of charge via the Internet at http://pubs.acs.org.
(10) (a) Nekola, H.; Olbrich, F.; Behrens, U. Z. Anorg. Allg. Chem. 2002,
628, 2067. (b) Geier, J.; Gr€utzmacher, H. Chem. Commun. 2003, 2942. (c) Geier,
J.; R€uegger, H.; Gr€utzmacher, H. Dalton Trans. 2006, 129.