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

Abstract

The synthesis and characterization of two dinuclear Hf^IV and Zr^IV complexes sandwiched between 2 mono-lacunary α-Keggin polyoxometalates (POMs), i.e., (Et₂NH₂)₈[{α-PW₁₁O₃₉Hf(μ-OH)(H₂O)}₂]·7H₂O (Et₂NH₂-1) and (Et₂NH₂)₈[{α-PW₁₁O₃₉Zr(μ-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 Hf^IV- and Zr^IV-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(μ-OH)₂(H₂O)₂]⁶⁺ (M = Hf, Zr) cation unit was composed of 2 edge-sharing 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 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(α-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.