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

摘要

The synthesis and characterization of di-nuclear Hf-IV and Zr-IV complexes (the Dawson 2:2-type complexes) sandwiched between 2 mono-lacunary alpha(2)-Dawson polyoxometalate (POM) ligands, i.e., (Me2NH2)(14)[{alpha(2)-P2W17O61Hf(mu-OH)(H2O)}(2)]center dot 17H(2)O (Me2NH2-1) and (Me2NH2) 14[{alpha(2)-P2W17O61Zr(mu-OH)(H2O)}(2)]center dot 16H(2)O (Me2NH2-2) are described. [Note: the moieties of their polyoxoanions are abbreviated simply as 1 and 2, respectively.] A pair of Hf-IV- and Zr-IV-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 (P-31 CPMAS) and solution (P-31 and W-183) NMR spectroscopy. Polyoxoanions 1 and 2 were isostructural with each other. The central [M-2(mu-OH)(2)(H2O)(2)](6+) (M = Hf, Zr) cation unit was composed of 2 polyhedral M units, which were linked through 2 mu-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(alpha(2)-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., [M-2(mu-OH)(2)(H2O)(2)](6+) (M = Hf, Zr) complexes sandwiched between 2 mono-lacunary alpha-Keggin POM ligands.