Heterodinuclear lanthanoid-containing polyoxometalates: stepwise synthesis and single-molecule magnet behavior

Polyoxometalates (POMs) with heterodinuclear lanthanoid cores, TBA8H4[{Ln(μ2-OH)2Ln'}(γ-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 [γ-SiW10O36](8-) units without the use of templating cations. The incorporation of a Ln(3+) ion into the vacant site between two [γ-SiW10O36](8-) units afforded mononuclear Ln(3+)-containing sandwich-type POMs with vacant sites (Ln1; TBA8H5[{Ln(H2O)4}(γ-SiW10O36)2]; Ln = Dy, Gd, La). The vacant sites in Ln1 were surrounded by coordinating W-O and Ln-O 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(μ2-OH) bridging ligands, [Dy(μ2-OH)2Ln'](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 (ΔE/kB) could be manipulated by adjusting the coordination geometry and anisotropy of the Dy(3+) ion by tuning the adjacent Ln'(3+) ion in the heterodinuclear [Dy(μ2-OH)2Ln'](4+) cores. The energy barriers increased in the order: DyLu (ΔE/kB = 48 K) < DyYb (53 K) < DyDy (66 K) < DyEu (73 K), with an increase in the ionic radii of Ln'(3+); DyEu showed the highest energy barrier.