Construction of polyoxometalates from dynamic lacunary polyoxotungstate building blocks and lanthanide linkers

Abstract

A series of polynuclear metal-oxo clusters have been constructed from the dynamic polyoxometalate (POM) building block {B-α-Sb^IIIW₉O₃₃} and lanthanide (Ln) linkers via a stepwise synthetic strategy with the molecular formulas of [Ln₂(H₂O)₄{WO₂(pic)}₂(SbW₈O₃₀)₂]¹⁰⁻ (Na₄Li₆[La-1]·28H₂O, Na₃Li₇[Pr-2]·30H₂O) and [{Ln(H₂O)}{Ln(pic)}(Sb₃O₄)(SbW₈O₃₁)(SbW₁₀O₃₅)]₂²⁴⁻ (K₂Na₆Li₁₆[Tb-3]·63H₂O, Na₉Li₁₅[Dy-4]·61H₂O, Na₇Li₁₇[Ho-5]·53H₂O) (Hpic = picolinic acid). The five compounds have been characterized by FT-IR, elemental analysis, TG, powder X-ray diffraction (PXRD) and single crystal X-ray diffraction. In compounds 1–5, various POM moieties, such as {B-β-SbW₈O₃₀}, {B-α-SbW₈O₃₁} and {B-α-SbW₁₀O₃₅}, were formed through a series of disassembling and re-assembling processes of the dynamic {B-α-SbW₉O₃₃} precursor with specific pH, reaction temperature and time. Furthermore, the use of oxytropic Ln³⁺ ions as linkers, together with auxiliary organic pic ligands and/or inorganic Sb³⁺ ions, led to diverse connection modes between the POM building blocks and Ln linkers and the assembly of new polynuclear metal-oxo clusters. The polyoxoanions of La-1 and Pr-2 possess the same structural feature, which can be viewed as a sandwich-type cluster composed of two {B-β-SbW₈O₃₀} units connected by two {WO₂(pic)} fragments and two hydrated Ln ions. These sandwich-type polyoxoanions are further linked by the hydrated Ln ions to form a 1-D helical chain. The polyoxoanions of Tb-3, Dy-4 and Ho-5 display the same structural features, although they contain different counter-cations and lattice water molecules. In the polyoxoanions of 3–5, one {B-α-SbW₈O₃₁} POM moiety and one {B-α-SbW₁₀O₃₅} POM unit are connected by one {Sb₃O₄} fragment and one {Ln(pic)} linker, forming an asymmetric sandwich-type metal-oxo cluster. Two of these sandwich-type clusters are further fused together by extra two hydrated Ln ions, leading to an isolated polynuclear metal-oxo cluster with a size of 16.4 × 28.5 Å. The photoluminescence properties of Tb-3 and Dy-4 were investigated. Both compounds exhibit characteristic Tb³⁺ and Dy³⁺ luminescence. The relationship between the luminescence properties and the crystal structure of the polyoxoanion was discussed.