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.

The syntheses and structures of hafnium(IV) and zirconium(IV) complexes with Keggin tri-lacunary polyoxometalates (POM) [A-α-PW₉O₃₄]⁹⁻, i.e., (Et₂NH₂)₇H₂[Hf₃(μ-OH)₃(A-α-PW₉O₃₄)₂]·11H₂O (Et₂NH₂-1) and (Et₂NH₂)₇H₂[Zr₃(μ-OH)₃(A-α-PW₉O₃₄)₂]·12H₂O (Et₂NH₂-2) are described. The two POMs, Et₂NH₂-1 and Et₂NH₂-2, were obtained as analytically pure, colorless crystals in 30.0 and 40.9% yields, respectively, by 1:3-molar ratio reactions of [A-PW₉O₃₄]⁹⁻ with Hf(SO₄)₂ and Zr(SO₄)₂, respectively, in aqueous solutions at 80–90 °C, followed by refluxing. The two POMs were characterized by elemental analysis, TG/DTA, FTIR, solution (³¹P and ¹⁸³W) NMR and X-ray crystallography. X-Ray structure analysis of Et₂NH₂-1 revealed that in polyoxoanion 1, the three 6-coordinate prismatic Hf^IV ions linked with the three bridging OH groups, i.e., the [Hf₃(μ-OH)₃]⁹⁺ cluster cation was sandwiched between two α-Keggin tri-lacunary POMs. The molecular structure of 1 was isostructural with that of polyoxoanion 2 in Et₂NH₂-2. X-Ray crystallography also showed that the crude crystals of Et₂NH₂-1 and Et₂NH₂-2, which were first formed without refluxing, contained the two isomeric crystalline species in a 7:3 ratio with the α,α- and α,β-junctions of the two Keggin tri-lacunary POMs. These facts were also confirmed by ³¹P NMR in D₂O. By refluxing the solutions containing the crude crystals, only the α,α-isomers of Et₂NH₂-1 and Et₂NH₂-2 were finally isolated.