Cation-Directed Dimeric versus Tetrameric Assemblies of Lanthanide-Stabilized Dilacunary Keggin Tungstogermanates

Reaction of mid- to late lanthanide ions with GeO₂ and Na₂WO₄ in NaOAc buffer results in a library of [Ln₂(GeW₁₀O₃₈)]⁶⁻ clusters (Ln₂), which consist of dilacunary Keggin fragments stabilized by the insertion of 4f atoms in the vacant sites and show the ability to undergo cation-directed self-assembly processes. In the presence of Na⁺, two β-Ln₂ subunits assemble by means of Ln-O(WO₅)-Ln bridges to form the chiral [Ln₄(H₂O)₆(β-GeW₁₀O₃₈)₂]¹²⁻ dimeric anions (ββ-Ln₄, Ln=Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). When Cs⁺ is present, two Ln₄-like dimers further assemble into the [{Ln₄(H₂O)₅(GeW₁₀O₃₈)₂}₂]²⁴⁻ species (Ln₈, Ln=Ho, Er, Tm, Yb, Lu). Two types of tetramers coexist in the solid state: One shows a full ββ-Ln₈ architecture, whereas the other one is a mixed αβ-Ln₈ assembly in which each β-subunit is linked to its corresponding α-Ln₂ derivative. Regardless of differences in isomeric forms and the relative arrangement of Ln₂ subunits, all anions display virtually identical {Ln₄} cores as a common structural feature. A combination of ESI mass spectrometry and ¹⁸³W NMR spectroscopy experiments indicates that Ln₈ tetramers fragment into Ln₄ dimers upon dissolution, which undergo partial dissociation into Ln₂ monomers and slow dimer/monomer equilibration. This is most likely followed by β-to-α isomerization of Ln₂ clusters with consequent reassembly, as indicated by isolation of three additional αα-Ln₄ derivatives. Magnetic and photoluminescence properties in the Na-ββ-Ln₄ series are also discussed.