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

The reaction of Fe(III) with Na+ and K+ salts of the trivacant [α-SiW9O34]10- ligand have been investigated at pH 6 and pH 1. A new dimer, [(α-SiFe3W9(OH)3O34)2(OH)3]11- (1), is synthesized by reacting Na7H3[α-SiW9O34] or K10[α-SiW9O34] with exactly 3 equiv of Fe(III) in a 0.5 M sodium acetate solution (pH 6). The structure of 1, determined by single-crystal X-ray diffraction (a = 22.454(2) Å, b = 12.387(2) Å, c = 37.421(2), β = 100.107(8)°, monoclinic, C2/c, Z = 4, R1 = 5.11% based on 12739 independent reflections), consists of two [α-SiFe3W9(OH)3O34]4- units linked by three Fe−μ-OH−Fe bonds. Reaction of K10[α-SiW9O34] with 3 equiv of Fe(III) in water (pH 1) yields [(α-Si(FeOH2)2FeW9(OH)3O34)2]8- (2). The structure of 2 was also determined by single-crystal X-ray diffraction (a = 36.903(2) Å, b = 13.9868(9) Å, c = 21.7839(13) Å, β = 122.709(1)°, monoclinic, C2/c, Z = 4, R1 = 4.57% based on 11787 independent reflections). It consists of two [α-Si(FeOH2)2FeW9(OH)3O34]4- Keggin units linked by a single edge. The terminal ligand on Fe1 in each trisubstituted Keggin unit becomes a μ2 oxo ligand bridging to a [WO6]2- 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-1 cm-1) and 456 nm (ε = 104 M-1 cm-1) for complexes 1 and 2, respectively. Differential scanning calorimetry data show that complex 1 decomposes between 575 and 600 °C whereas no decomposition is observed for complex 2 up to temperatures of 600 °C.