β , β-Isomer of Open-Wells – Dawson Polyoxometalate Containing a Tetra-Iron ( III ) Hydroxide Cluster : [ { Fe 4 ( H 2 O ) ( OH ) 5 } ( β , β-Si 2 W 18 O 66 ) ] 9 ́

The β,β-isomer of open-Wells–Dawson polyoxometalate (POM) containing a tetra-iron(III) cluster, K9[{Fe4(H2O)(OH)5}(β,β-Si2W18O66)] ̈ 17H2O (potassium salt of β,β-Fe4-open), was synthesized by reacting Na9H[A-β-SiW9O34] ̈ 23H2O with FeCl3 ̈ 6H2O at pH 3, and characterized by X-ray crystallography, FTIR, elemental analysis, TG/DTA, UV–Vis, and cyclic voltammetry. X-ray crystallography revealed that the {Fe4(H2O)(OH)5} cluster was included in the open pocket of the β,β-type open-Wells–Dawson polyanion [β,β-Si2W18O66]16 ́ formed by the fusion of two trilacunary β-Keggin POMs, [A-β-SiW9O34]10 ́, via two W–O–W bonds. The β,β-open-Wells–Dawson polyanion corresponds to an open structure of the standard γ-Wells–Dawson POM. β,β-Fe4-open is the first example of the compound containing a geometrical isomer of α,α-open-Wells–Dawson structural POM.


Absorption Spectrum
The absorption spectrum of β,β-Fe 4 -open in H 2 O is shown in Figure 4.The shoulder band due to the O to Fe 3+ charge transfer [39] was observed around 450 nm (ε = 98 M ´1¨cm ´1).This spectrum was similar to that of α,α-Fe 4 -open (Figure S3).
On the other hand, the redox waves observed at ´0.212 and 0.380 V can be attributed to the redox processes of Fe 3+ centers, since the Zn-containing open-Wells-Dawson POM displayed no redox waves in this region [23].The area ratio between the second reduction wave of W 6+ (two-electron process, E = ´0.794V) and the reduction of Fe 3+ (E = ´0.212V) is ca.1:2, i.e., 2:4 electrons.Based on the comparison of the area ratio, the wave is likely to be the simultaneous one-electron redox of the four-Fe 3+ center.
On the other hand, the redox waves observed at −0.212 and 0.380 V can be attributed to the redox processes of Fe 3+ centers, since the Zn-containing open-Wells-Dawson POM displayed no redox waves in this region [23].The area ratio between the second reduction wave of W 6+ (two-electron process, E = −0.794V) and the reduction of Fe 3+ (E = −0.212V) is ca.1:2, i.e., 2:4 electrons.Based on the comparison of the area ratio, the wave is likely to be the simultaneous one-electron redox of the four-Fe 3+ center.

Instrumentation and Analytical Procedures
Elemental analyses were carried out by Mikroanalytisches Labor Pascher (Remagen, Germany).The sample was dried overnight at room temperature under 10 ´3-10 ´4 Torr before analysis.Infrared spectra were recorded on a Jasco 4100 FTIR spectrometer (Jasco, Hachioji, Japan) using KBr disks at room temperature.Thermogravimetric (TG) and differential thermal analyses (DTA) were acquired using a Rigaku Thermo Plus 2 series TG8120 instrument (Rigaku, Akishima, Japan).TG/DTA measurements were run under air with a temperature ramp of 4.0 ˝C/min between 20 and 500 ˝C.Absorption spectra in H 2 O were obtained on a JASCO V-630 spectrophotometer (Jasco).Cyclic voltammetry was performed with ALS/CH Instruments (BAS, Sumida-ku, Japan), a Model 610E electrochemical analyzer with a three electrode cell in 0.5 M KOAc/HOAc buffer (pH 4.8) under N 2 atmosphere.A glassy carbon working electrode, a Pt auxiliary electrode and a Ag/AgCl reference electrode were employed.The scan rate was 25 mV¨s ´1.

X-ray Crystallography
For the potassium salt of β,β-Fe 4 -open, a single crystal with dimensions of 0.11 ˆ0.08 ˆ0.07 mm 3 was surrounded by liquid paraffin (Paratone-N) and analyzed at 100(2) K. Measurement was performed using a Bruker SMART APEX CCD diffractometer.The structure was solved by direct methods (SHELXS-97), followed by difference Fourier calculations and refinement by a full-matrix least-squares procedure on F 2 (program SHELXL-97) [41].
Crystal data: triclinic, space group P´1, a = 12.560( 2 R 1 (I > 2.00σ(I)) = 0.0605, R (all data) = 0.0751, wR 2 (all data) = 0.1642, GOF = 1.032.Most atoms in the main part of the structure were refined anisotropically, while the rest (as crystallization solvents) were refined isotropically, because of the presence of disorder.The composition and formula of the POM, containing countercations and crystalline water molecules, were determined by the complete elemental and TG analyses.Similar to other structural investigations of crystals of highly hydrated large polyoxometalate complexes, it was not possible to locate every countercation and hydrated water molecule, due to the extensive disorder of the cations and crystalline water molecules.Further details of the crystal structure investigations may be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: +49-7247-808-666; E-Mail: crysdata@fiz-karlsruhe.de,

Figure 1 .
Figure 1.The isomers of the usual Wells-Dawson structural POMs and the open-Wells-Dawson POMs.Color code: XO4, blue; WO6, gray.

Figure 1 .
Figure 1.The isomers of the usual Wells-Dawson structural POMs and the open-Wells-Dawson POMs.Color code: XO 4 , blue; WO 6 , gray.
These interactions play an important role in the formation of the β,β-type open-Wells-Dawson structural POMs and have been previously reported for the α,α-open-Wells-Dawson POMs [28].
These interactions play an important role in the formation of the β,β-type open-Wells-Dawson structural POMs and have been previously reported for the α,α-open-Wells-Dawson POMs [28].

Figure 3 .
Figure 3. Interactions with K + ions of the terminal oxygen atoms of the WO6 polyhedra serving as a hinge between the two trilacunary β-Keggin units of β,β-Fe4-open.

Figure 3 .
Figure 3. Interactions with K + ions of the terminal oxygen atoms of the WO 6 polyhedra serving as a hinge between the two trilacunary β-Keggin units of β,β-Fe 4 -open.

Figure 3 .
Figure 3. Interactions with K + ions of the terminal oxygen atoms of the WO6 polyhedra serving as a hinge between the two trilacunary β-Keggin units of β,β-Fe4-open.
Hill et al. have reported similar redox processes based on Zn-containing open-Wells-Dawson POMs [23], and noted that the second reduction wave associated with W 6+ is a two-electron process.The cyclic voltammogram of 0.5 mM α,α-Fe 4 -open in 0.5 M KOAc/HOAc buffer (pH 4.8) solution at a scan rate of 25 mV/s was similar to that of β,β-Fe 4 -open (Figure