The Monolanthanide-Containing Silicotungstates [Ln(β₂-SiW₁₁O₃₉)₂]^13- (Ln = La, Ce, Sm, Eu, Gd, Tb, Yb, Lu): A Synthetic and Structural Investigation

We have synthesized and structurally characterized the monolanthanide-containing polyanions [Ln(β2-SiW11O39)2]13- (Ln = La (1), Ce (2), Sm (3), Eu (4), Gd (5), Tb (6), Yb (7), Lu (8)). Synthesis was accomplished by reaction of the respective lanthanide ion with the monolacunary Keggin-type precursor [β2-SiW11O39]8- in a 1:2 molar ratio in 1 M KCl medium at pH 5. Polyanions 1−8 were isolated as potassium salts and then characterized by IR, single-crystal X-ray diffraction, and elemental as well as thermogravimetric analysis. The structures of 1−8 are composed of an eight-coordinated Ln3+ center sandwiched by two chiral (β2-SiW11O39) units. Large Ln3+ ions appear to favor an (R,R) (or (S,S)) configuration (point group C2) of the Keggin units, with an increasing amount of (R,S) (or (S,R)) configuration (point group C1) found in the solid state as the Ln3+ ion decreases in size. This trend is also supported by solution 183W NMR results for the diamagnetic La3+ and Lu3+ derivatives.