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 (183)W 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.