Crystal Structure and Magnetic Properties of Peacock–Weakley Type Polyoxometalates Na9[Ln(W5O18)2] (Ln = Tm, Yb): Rare Example of Tm(III) SMM

We report Peacock–Weakley complexes, Na9[Ln(W5O18)2]∙35H2O, formed with Tm(III), 1, and Yb(III), 2. Their syntheses, physico-chemical characterizations, crystal structures, and magnetic properties are described. Ab initio calculations are also reported. These polyoxometalate (POM) complexes were obtained using original synthetic conditions where acidification was performed with a stoichiometric amount of nitric acid to an acidity of Z = ν(H+)/ν(WO42–) = 8/10 = 0.80. Both the Tm(III) and Yb(III) derivatives were found to exhibit field-induced slow relaxation of their magnetization likely controlled by Raman and Orbach relaxation processes. 1 is a rare example of a Tm(III)-based single-molecule magnet (SMM) and is a consequence of the oblate tetragonal anti-prismatic symmetry of the coordination sphere.