Large Cluster Formation through Multiple Substitution with Lanthanide Cations (La, Ce, Nd, Sm, Eu, and Gd) of the Polyoxoanion [(B-α-AsO₃W₉O₃₀)₄(WO₂)₄]^28-. Synthesis and Structural Characterization^†

Several new large polyoxotungstates have been synthesized by reaction of lanthanide cations with the well-known “As4W40” anion, [(B-α-AsO3W9O30)4(WO2)4]28- (1). The heteropolyanions [(H2O)11LnIII(LnIII2OH)(B-α-AsO3W9O30)4(WO2)4]20- (Ln = Ce, Nd, Sm, Gd) (2−4) (Ln3As4W40) and [Mm(H2O)10(LnIII2OH)2(B-α-AsO3W9O30)4(WO2)4](18-m)- (Ln = La, Ce, Gd and M = Ba, K, none) (5−7) (Ln4As4W40) have been isolated as alkali metal and ammonium salts, respectively, and characterized by single-crystal X-ray analysis, elemental analysis, and IR and 183W-NMR spectroscopy. The X-ray analyses revealed interanionic W−O−Ln bonds between adjacent LnxAs4W40 units forming a “dimer” for x = 3 and chains for x = 4. Upon dissolving in water these bonds hydrolyze and the monomeric species form. The straightforward syntheses which require the use of concentrated NaCl solutions (1−4 M) and the addition of stoichiometric amounts of Ba2+ or K+ reemphasize the importance of the presence of appropriate countercations for the assembly of large polyoxometalate structures.