Double-malate bridging tri-lanthanoid cluster encapsulated arsenotungstates: syntheses, structures, luminescence and magnetic properties

Abstract

Five members of a new family of polyoxometalate (POM)-ligated trinuclear lanthanoid (Ln) clusters with the general formula K₂₀Li₂[Ln₃(μ₃-OH)(H₂O)₈(AsW₉O₃₃)(AsW₁₀O₃₅(mal))]₂·17H₂O [Ln = Dy (1Dy), Tb (2Tb), Gd (3Gd), Eu (4Eu), and Sm (5Sm), mal = malate] have been synthesized, all of which consist of the dimeric {[Ln₃(μ₃-OH)(H₂O)₈(AsW₉O₃₃)[AsW₁₀O₃₅(mal)]}¹¹⁻ polyanion constructed from a {AsW₉O₃₃} and a {AsW₁₀O₃₅(mal)} building block linked by a tri-Ln cluster [Ln₃(μ₃-OH)(H₂O)₈]⁸⁺, where the two malate ligands play a key bridging role. The {AsW₁₀O₃₅(mal)} subunit can be viewed as the {AsW₉O₃₃} building block anchoring an additional tungsten center [WO₂(mal)]. The electrospray ionization mass (ESI-MS) spectra indicate that the dimeric fragments of 1Dy and 2Tb are unstable, which are apt to produce the subunit [KH₅Ln₃(OH)(H₂O)(AsW₉O₃₃)₂]⁴⁻ in solution. The solid-state photoluminescence measurements display the yellowish green emission for 1Dy and 5Sm, green emission for 2Tb and reddish orange emission for 4Eu, which are attributed to the Ln^III f–f electron transitions. Magnetic property studies indicate that 1Dy displays probable SMM behaviour with slow magnetization relaxation, whereas the weak antiferromagnetic interactions exist in two {Ln₃} clusters for 2Tb–5Sm.