Combination of Lacunary Polyoxometalates and High-Nuclear Transition-Metal Clusters under Hydrothermal Conditions. 5. A Novel Tetrameric Cluster of [{Fe^IIFe^III₁₂(μ₃-OH)₁₂(μ₄-PO₄)₄}(B-α-PW₉O₃₄)₄]^22-

A novel tetrameric cluster, (enH₂)_3.5H₁₅[{Fe^IIFe^III₁₂(μ₃-OH)₁₂(μ₄-PO₄)₄}(B-α-PW₉O₃₄)₄]·83H₂O (1; en = ethylenediamine), has been hydrothermally synthesized and characterized by IR, optical diffuse reflectance, thermogravimetric analysis, and single-crystal X-ray diffraction. The skeleton of 1 is composed of four tri-Fe^III-substituted [Fe^III₃(μ₃-OH)₃(B-α-PW₉O₃₄)]^3- Keggin moieties fused together through a Fe^II₂O₂ unit and four μ₄-PO₄ bridges, resulting in a unique tetramer with C₂ symmetry. Magnetic measurements indicate that 1 reveals the antiferromagnetic interactions between Fe^II and Fe^III centers.