Multi-Iron Silicotungstates:  Synthesis, Characterization, and Stability Studies of Polyoxometalate Dimers

The reaction of Fe(III) with Na<sup>+</sup> and K<sup>+</sup> salts of the trivacant [α-SiW<sub>9</sub>O<sub>34</sub>]<sup>10-</sup> ligand have been investigated at pH 6 and pH 1. A new dimer, [(α-SiFe<sub>3</sub>W<sub>9</sub>(OH)<sub>3</sub>O<sub>34</sub>)<sub>2</sub>(OH)<sub>3</sub>]<sup>11-</sup> (<b>1</b>), is synthesized by reacting Na<sub>7</sub>H<sub>3</sub>[α-SiW<sub>9</sub>O<sub>34</sub>] or K<sub>10</sub>[α-SiW<sub>9</sub>O<sub>34</sub>] with exactly 3 equiv of Fe(III) in a 0.5 M sodium acetate solution (pH 6). The structure of <b>1</b>, determined by single-crystal X-ray diffraction (<i>a</i> = 22.454(2) Å, <i>b</i> = 12.387(2) Å, <i>c</i> = 37.421(2), β = 100.107(8)°, monoclinic, <i>C</i>2/<i>c</i>, <i>Z</i> = 4, R<sub>1</sub> = 5.11% based on 12739 independent reflections), consists of two [α-SiFe<sub>3</sub>W<sub>9</sub>(OH)<sub>3</sub>O<sub>34</sub>]<sup>4-</sup> units linked by three Fe−μ-OH−Fe bonds. Reaction of K<sub>10</sub>[α-SiW<sub>9</sub>O<sub>34</sub>] with 3 equiv of Fe(III) in water (pH 1) yields [(α-Si(FeOH<sub>2</sub>)<sub>2</sub>FeW<sub>9</sub>(OH)<sub>3</sub>O<sub>34</sub>)<sub>2</sub>]<sup>8-</sup> (<b>2</b>). The structure of <b>2</b> was also determined by single-crystal X-ray diffraction (<i>a</i> = 36.903(2) Å, <i>b</i> = 13.9868(9) Å, <i>c</i> = 21.7839(13) Å, β = 122.709(1)°, monoclinic, <i>C</i>2/<i>c</i>, <i>Z</i> = 4, R<sub>1</sub> = 4.57% based on 11787 independent reflections). It consists of two [α-Si(FeOH<sub>2</sub>)<sub>2</sub>FeW<sub>9</sub>(OH)<sub>3</sub>O<sub>34</sub>]<sup>4-</sup> Keggin units linked by a single edge. The terminal ligand on Fe1 in each trisubstituted Keggin unit becomes a μ<sub>2</sub> oxo ligand bridging to a [WO<sub>6</sub>]<sup>2-</sup> moiety. The UV−vis spectra of both complexes show the characteristic oxygen-to-metal-charge-transfer bands of polyoxometalates as well as an Fe(III)-centered band at 436 nm (ε = 146 M<sup>-1</sup> cm<sup>-1</sup>) and 456 nm (ε = 104 M<sup>-1</sup> cm<sup>-1</sup>) for complexes <b>1</b> and <b>2</b>, respectively. Differential scanning calorimetry data show that complex <b>1</b> decomposes between 575 and 600 °C whereas no decomposition is observed for complex <b>2</b> up to temperatures of 600 °C.