Trigonal Prismatic Coordination of Discrete Rare Earth Ions, Enforced by the Polyoxotungstate [P₄ W₂₇ O₉₉ (H₂ O)]¹⁶

A family of solution-stable polyanions [Na⊂{Ln^III (H₂ O)}{W^VI O(H₂ O)}P^V ₄ W^VI ₂₆ O₉₈ ]^12- (Ln=Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) represent the first examples of polyoxometalates comprising a single lanthanide(III) or yttrium(III) ion in a rare trigonal prismatic O₆ environment. Their synthesis exploits the reactivity of the organophosphonate-functionalized precursor [P₄ W₂₄ O₉₂ (C₆ H₅ P^V O)₂ ]^16- with heterometal ions and yields hydrated potassium or mixed lithium/potassium salts of composition K_x Ln_y H_12-x-y [Na⊂{Ln(H₂ O)}{WO(H₂ O)}P₄ W₂₆ O₉₈ ]⋅nH₂ O⋅mLiCl (x=8.5-11; y=0-2; n=24-34; m=0-1.5). The Dy, Ho, Er and Yb derivatives are characterized by slow magnetization relaxation.