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

摘要

Reaction of mid- to late lanthanide ions with GeO_2 and Na_2WO_4 in NaOAc buffer results in a library of [Ln_2-(GeW_(10)O_(38))]~(6-) clusters (Ln_2), 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-Ln_2 subunits assemble by means of Ln-O(WO_5)-Ln bridges to form the chiral [Ln_4(H_2O)_6(β-GeW_(10)O_(38))_2]~(12-) dimeric anions (ββ-Ln_4, Ln=Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). When Cs~+ is present, two Ln_4-like dimers further assemble into the [{Ln_4-(H_2O)_5(GeW_(10)O_(38))_2}_2]~(24-) species (Ln_8, Ln=Ho, Er, Tm, Yb, Lu). Two types of tetramers coexist in the solid state: One shows α full ββ-Ln_8 architecture, whereas the other one is a mixed αβ-Ln_8 assembly in which each β-subunit is linked to its corresponding α-Ln_2 derivative. Regardless of differences in isomeric forms and the relative arrangement of Ln_2 subunits, all anions display virtually identical {Ln_4} cores as a common structural feature. A combination of ESI mass spectrometry and ~(183)W NMR spectroscopy experiments indicates that Ln_8 tetramers fragment into Ln_4 dimers upon dissolution, which undergo partial dissociation into Ln_2 monomers and slow dimer/monomer equilibration. This is most likely followed by β-to-α isomerization of Ln_2 clusters with consequent reassembly, as indicated by isolation of three additional aa-Ln_4 derivatives. Magnetic and photoluminescence properties in the Na-ββ-Ln_4 series are also discussed.