Relation among the 2:2-, 1:1- and 1:2-type complexes of hafnium(IV)/zirconium(IV) with mono-lacunary α2-Dawson polyoxometalate ligands: Synthesis and structure of the 2:2-type complexes [{α2-P2W17O61M(μ-OH)(H2O)}2]14− (M = Hf, Zr)

The synthesis and characterization of di-nuclear HfIV and ZrIV complexes (the Dawson 2:2-type complexes) sandwiched between 2 mono-lacunary α2-Dawson polyoxometalate (POM) ligands, i.e., (Me2NH2)14[{α2-P2W17O61Hf(μ-OH)(H2O)}2]·17H2O (Me2NH2-1) and (Me2NH2)14[{α2-P2W17O61Zr(μ-OH)(H2O)}2]·16H2O (Me2NH2-2) are described. [Note: the moieties of their polyoxoanions are abbreviated simply as 1 and 2, respectively.] A pair of HfIV- and ZrIV-containing POMs belonging to the same family were herein isolated as dimethylammonium salts and were unambiguously characterized by complete elemental analysis, in addition to potassium analysis, TG/DTA, FTIR, single-crystal X-ray structure analysis, and solid-state (31P CPMAS) and solution (31P and 183W) NMR spectroscopy. Polyoxoanions 1 and 2 were isostructural with each other. The central [M2(μ-OH)2(H2O)2]6+ (M = Hf, Zr) cation unit was composed of 2 polyhedral M units, which were linked through 2 μ-OH groups and contained 1 water molecule coordinated to each metal center. Since the mono-lacunary Dawson POM acts as an oxygen-donor quadridentate ligand, the Hf and Zr centers are 7-coordinate. The Dawson 2:2-type complexes were converted to the Dawson 1:2-type complexes [M(α2-P2W17O61)2]16− (M = Hf, Zr), or vice versa, in solution under appropriate conditions. Also, the Dawson 2:2-type complex can be reversibly converted to the 1:1-type complex under the pH-dependent conditions. The Dawson 2:2-type POMs 1 and 2 can be compared with the recently reported, Keggin 2:2-type POMs, i.e., [M2(μ-OH)2(H2O)2]6+ (M = Hf, Zr) complexes sandwiched between 2 mono-lacunary α-Keggin POM ligands.