Heterodinuclear Lanthanoid-Containing Polyoxometalates: Stepwise Synthesis and Single-Molecule Magnet Behavior

Polyoxometalates (POMs) with heterodinuclear lanthanoid cores, TBA(8)H(4)[{Ln((2)-OH)(2)Ln}(-SiW10O36)(2)] (LnLn; Ln=Gd, Dy; Ln=Eu, Yb, Lu; TBA=tetra-n-butylammonium), were successfully synthesized through the stepwise incorporation of two types of lanthanoid cations into the vacant sites of lacunary [-SiW10O36](8-) units without the use of templating cations. The incorporation of a Ln(3+) ion into the vacant site between two [-SiW10O36](8-) units afforded mononuclear Ln(3+)-containing sandwich-type POMs with vacant sites (Ln1; TBA(8)H(5)[{Ln(H2O)(4)}(-SiW10O36)(2)]; Ln=Dy, Gd, La). The vacant sites in Ln1 were surrounded by coordinating WO and LnO oxygen atoms. On the addition of one equivalent of [Ln(acac)(3)] to solutions of Dy1 or Gd1 in 1,2-dichloroethane (DCE), heterodinuclear lanthanoid cores with bis((2)-OH) bridging ligands, [Dy((2)-OH)(2)Ln](4+), were selectively synthesized (LnLn; Ln=Dy, Gd; Ln=Eu, Yb, Lu). On the other hand, La1, which contained the largest lanthanoid cation, could not accommodate a second Ln(3+) ion. DyLn showed single-molecule magnet behavior and their energy barriers for magnetization reversal (E/k(B)) could be manipulated by adjusting the coordination geometry and anisotropy of the Dy3+ ion by tuning the adjacent Ln(3+) ion in the heterodinuclear [Dy((2)-OH)(2)Ln](4+) cores. The energy barriers increased in the order: DyLu (E/k(B)=48K)