Sandwich-type HfIV and ZrIVcomplexes composed of tri-lacunary Keggin polyoxometalates: structure of [M3(μ-OH)3(A-α-PW9O34)2]9− (M = Hf and Zr)

Abstract

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.