Synthesis and structure of dinuclear hafnium(IV) and zirconium(IV) complexes sandwiched between 2 mono-lacunary a-Keggin polyoxometalates

摘要

The synthesis and characterization of two dinuclear HfIV and ZrIV complexes sandwiched between 2 mono-lacunary a-Keggin polyoxometalates (POMs), i.e., (Et2NH2)8[{a-PW11O39Hf(m-OH)- (H2O)}2]·7H2O (Et2NH2-1) and (Et2NH2)8[{a-PW11O39Zr(m-OH)(H2O)}2]·7H2O (Et2NH2-2), 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 diethylammonium salts and were unambiguously characterized by complete elemental analysis, including sodium and oxygen analyses, TG/DTA, FT-IR, single-crystal X-ray structure analysis and solution (31P and 183W) NMR spectroscopy. Polyoxoanions 1 and 2 were isostructural with each other. The central [M2(m-OH)2(H2O)2]6+ (M = Hf, Zr) cation unit was composed of 2 edge-sharing polyhedral M units, which were linked through 2 m-OH groups and contained 1 water molecule coordinated to each metal center. Since the mono-lacunary Keggin POM acts as an oxygen-donor quadridentate ligand, the Hf and Zr centers are 7-coordinate. It should be noted that the present 2 Keggin 2 : 2-type compounds, Et2NH2-1 and Et2NH2-2, undergo a reversible conversion to Keggin 1 : 2-type complexes [M(a-PW11O39)2]10-, respectively, in solution under appropriate conditions. The synthesis of Et2NH2-1 and Et2NH2-2 is based on such an interconversion. The Zr compound Et2NH2-2 was rigorously compared with the 3 Zr POMs (OK-1–OK-3), recently reported by Kholdeeva’s group: their POMs in a different protonation-state did not contain any coordinating water molecules.