Combination of lacunary polyoxometalates and high-nuclear transition-metal clusters under hydrothermal conditions.: 5.: A novel tetrameric cluster of [{FeIIFeIII12(μ3-OH)12(μ4-PO4)4}(B-α-PW9O34)4]22-

A novel tetrameric cluster, (en H-2)(3.5)H-15[{(FeFeIII)-Fe-II](12)(mu(3)-OH)(12)(mu(4)-PO4)(4)}(B-alpha-PW9O34)(4)]center dot 83H(2)O (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-3(III)(mu(3)-OH)(3)(B-alpha-PW9O34)](3-) Keggin moieties fused together through a (Fe2O2)-O-II unit and four mu(4)-PO4 bridges, resulting in a unique tetramer with C-2 symmetry. Magnetic measurements indicate that 1 reveals the antiferromagnetic interactions between Fe-II and Fe-III centers.