Inorganic Chemistry
Communication
different from anisotropic covalent bonds and coordination bonds, and
can easily transform their crystal structures to adapt to polar guest
molecules.2 The structural flexibility of pinacol molecules and ionic
crystals probably contributes to catalysis within the crystal lattice.
(8) Berson, J. A. Angew. Chem., Int. Ed. 2002, 41, 4655.
(9) Compounds Ib and IIb are composed of macroions and possess
electrostatic fields at the internal surfaces suitable for stabilizing the
carbocation intermediate. Such a stabilization of organic cations by
polyoxometalates has been reported.10
(10) (a) Knoth, W. H.; Harlow, R. L. J. Am. Chem. Soc. 1981, 103,
4265. (b) Day, V. W.; Klemperer, G.; Schwartz, C. J. Am. Chem. Soc.
1987, 109, 6030.
(11) Shriver & Atkins Inorganic Chemistry, 4th ed.; Oxford University
Press: Oxford, U.K., 2006.
AUTHOR INFORMATION
Corresponding Author
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ACKNOWLEDGMENTS
■
This work was supported by the Core Research for Evolutional
Science and Technology (CREST) program of the Japan
Science and Technology Agency (JST), Global COE Program
(Chemistry Innovation through Cooperation of Science and
Engineering), the Development in a New Interdisciplinary
Field Based on Nanotechnology and Materials Science
Programs, and Grant-in-Aids for Scientific Research from the
Ministry of Education, Culture, Science, Sports, and Technol-
ogy of Japan.
(12) Hsien, M.; Sheu, H. T.; Lee, T.; Cheng, S.; Lee, J. F. J. Mol.
Catal. A 2002, 181, 189.
́
(13) Palinko, I.; Molnar, A.; Nagy, J. B.; Bertrand, J. C.; Lazar, K.;
́ ́ ́ ́ ́
Valyon, J.; Kiricsi, I. J. Chem. Soc., Faraday Trans. 1997, 93, 1591.
(14) About 8 and 11 out of 12 aqua ligands per formula were
maintained after the treatment of Ia and IIa, respectively. Because the
treatment and reaction were performed at 373 K, the aqua ligands of
Cr3+ and Fe3+ were probably mostly maintained under the reaction
conditions. Moreover, the pinacol rearrangement is a dehydration
reaction, and water molecules that formed in the reaction would
coordinate to the coordinatively unsaturated Cr3+ and Fe3+ sites.
Therefore, protons that dissociated from the aqua ligands (i.e.,
Brønsted acids) are possible active sites.
(15) Compounds Ia and IIa also catalyzed the pinacol rearrangement
to pinacolone with high yields of 76% and 79%, respectively, and these
values were similar to those of Ib and IIb. However, longer times of ca.
12 h (Ia) and 24 h (IIa) were needed to complete the reaction (Figure
S5 in the Supporting Information). This is probably because exchange
between pinacol molecules and crystallization solvents (acetone and
water) would take place within the crystal lattice before the reaction.
(16) Gilson, R.; Durrant, M. C. Dalton Trans. 2009, 10223.
REFERENCES
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(4) K4[α-SiW12O40]·10H2O (0.035 mmol) was dissolved into 10 mL
of water, and NaClO4 (0.15 mmol) was added. The solution was
f i l t e r e d a f t e r s t i r r i n g f o r
5 m i n ( s o l u t i o n A ) .
[Cr3O(OOCC6H5)6(H2O)3](NO3) (0.05 mmol) was dissolved in
10 mL of acetone (solution B). Solution B was added to solution A,
the synthetic solution was kept at room temperature for 1 day, and
green crystals of Ia were obtained (yield 30%). Compound IIa was
synthesized by using [Fe3O(OOCC6H5)6(H2O)3](NO3) in a similar
manner. Compounds Ia and IIa could also be synthesized by using
H4[α-SiW12O40]·nH2O as a starting material. In order to avoid
contamination of Ia and IIa by the acidic protons of H4[α-
SiW12O40]·nH2O, K4[α-SiW12O40]·10H2O was used. The catalytic
activities of Ib and IIb were unchanged by the use of H4[α-
SiW12O40]·nH2O as a starting material.
(5) Crystallographic data for Ia: monoclinic, P21/a, a = 22.3110(2)
Å, b = 33.0752(5) Å, c = 23.0088(3) Å, β = 117.0630(8)°, Z = 2, V =
15120.0(3) Å3, crystal size 0.10 × 0.10 × 0.05 mm3, T = 183.1 K, 41
842 reflections collected, 827 parameters, R1 [I > 2σ(I)] = 0.1043,
wR2 = 0.3298, GOF = 1.174. Crystallographic data for IIa:
monoclinic, P21/a, a = 22.4142(2) Å, b = 33.2833(4) Å, c =
22.9409(3) Å, β = 116.7664(4)°, Z = 2, V = 15280.5(3) Å3, crystal size
0.10 × 0.10 × 0.05 mm3, T = 183.1 K, 41 533 reflections collected, 667
parameters, R1 [I > 2σ(I)] = 0.1191, wR2 = 0.3733, GOF = 1.194.
(6) Calculated lattice parameters of Ib: monoclinic, P21/a, a = 22.3
Å, b = 30.4 Å, c = 20.6 Å, β = 119°, V = 12 200 Å3.
(7) About 5.0 mol mol−1 (ca. 30%) of pinacol was detected in the
solid phase at 20% conversion (after 2 h). Upon completion of the
reaction, 3.5 mol mol−1 (ca. 20%) of pinacolone formed was detected
in the solid phase. These results suggest that the reaction proceeds in
the solid bulk. The molecular diameters of pinacol and pinacolone are
both ca. 7 Å, and geometrical optimization of pinacol sorbed in Ia
(Figure S2 in the Supporting Information) showed that pinacol can be
accommodated into the crystal lattice of Ia. The pinacol molecule is
composed of single bonds and possesses conformational flexibility.
Ionic crystals are composed of isotropic ionic bonds, which are
777
dx.doi.org/10.1021/ic2025186 | Inorg. Chem. 2012, 51, 775−777