Combination of lacunary polyoxometalates and high-nuclear transition-metal clusters under hydrothermal conditions. 5. A novel tetrameric cluster of [{Fe(II)Fe(III)(12)(mu(3)-OH)(12)(mu(4)-PO(4))(4)}(B-alpha-PW(9)O(34))(4)](22)(-).

A novel tetrameric cluster, (enH2)3.5H15[{FeIIFeIII12(mu3-OH)12(mu4-PO4)4}(B-alpha-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-FeIII-substituted [FeIII3(mu3-OH)3(B-alpha-PW9O34)]3- Keggin moieties fused together through a FeII2O2 unit and four mu4-PO4 bridges, resulting in a unique tetramer with C2 symmetry. Magnetic measurements indicate that 1 reveals the antiferromagnetic interactions between FeII and FeIII centers.