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 [Ln(2)(GeW10O38)](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 beta-Ln(2) subunits assemble by means of Ln-O(WO5)-Ln bridges to form the chiral [Ln(4)(H2O)(6)(beta-GeW10O38)(2)](12-) dimeric anions (beta beta-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 (Ln(8), Ln= Ho, Er, Tm, Yb, Lu). Two types of tetramers coexist in the solid state: One shows a full bb-Ln(8) architecture, whereas the other one is a mixed ab-Ln(8) assembly in which each b-subunit is linked to its corresponding a-Ln2 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 183W NMR spectroscopy experiments indicates that Ln8 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 b-to-a isomerization of Ln(2) clusters with consequent reassembly, as indicated by isolation of three additional aa-Ln4 derivatives. Magnetic and photoluminescence properties in the Na-bb-Ln(4) series are also discussed.