Synthesis of a 6-nm-Long Transition-Metal–Rare-Earth-Containing Polyoxometalate

An extremely long ribbon-shaped polyoxometalate (POM) octamer, {Eu₁₆Co₇Se₁₆W₁₂₈O₄₄₈(CIT)₁₀(HCIT)₂(NO₃)₄(OH)₄(H₂O)₅₂} (H₄CIT = citric acid), with a maximum length of ca. 6 nm has been obtained, which is the longest molecular POM reported to date and the first Dawson-based 3d-4f-containing selenotungstate(IV). The nanoscale octamer is built from 8 new Dawson-type Eu₂W₂-substituted selenotungstates {(Eu₂W₂O₄)Se₂W₁₄O₅₂} bridged by 12 H₄CIT ligands and 3 Co²⁺ ions, giving rise to a rare giant inorganic-organic hybrid molecular POM with more than 150 metal centers (7 Co^II, 16 Eu^III, 16 Se^IV, and 128 W^VI). Especially, with 128 W centers and more than 800 non-H atoms, the POM octamer also represents the largest known 3d-4f-containing polyoxotungstate.