Tetra-hafnium(IV) cluster cation sandwiched between 2 di-lacunary Dawson polyoxotungstate anions: Synthesis and structure of [Hf4(μ3-O)2(μ-OH)2(H2O)4(P2W16O59)2]14−

The title polyoxometalate (POM) 1 was obtained by a 1:2 molar-ratio reaction of tri-lacunary Dawson POM [B-α-P2W15O56]12− with Hf(SO4)2 or HfCl4 in an aqueous solution, and isolated as dimethylammonium salt (Me2NH2-1) and, also, as sodium salt (Na-1). These compounds were unambiguously characterized by complete elemental analysis, including sodium and oxygen analyses, TG/DTA, FTIR, single-crystal X-ray structure analysis, (solid-state and solution) 31P NMR and solution 183W NMR spectroscopy. X-ray crystallography revealed that 1 with Ci symmetry was composed of a tetra-HfIV cluster cation species [Hf4(μ3-O)2(μ-OH)2(H2O)4]10+ sandwiched between 2 di-lacunary Dawson polyoxometalates (POMs), [P2W16O59]12−, and it was isostructural with the previously reported Zr4 analogue. Thus, a pair of Hf- and Zr-containing POMs belonging to the same structure-family was built up.