Ion-Selective Assembly of Supertetrahedral Selenido Germanate Clusters for Alkali Metal Ion Capture and Separation

Supertetrahedral chalcogenido (semi)metalate cluster-based
frameworks
possess high selectivity for alkali metal cations, matching the specific
charge density of their inner surfaces, which enables their use as
ion-exchange materials. Aggregates of the supertetrahedral chalcogenido
metalate cluster offer even new perspectives for metal ion capture
and separation. Herein, we report on ionothermal preparation of two
corresponding model compounds, (C2C1Im)7[Cs@GeII4(GeIV4Se10)4] (1) and (C2C1Im)10[Na5(CN)6@Cu6(Ge4Se10)4(Cu)] (2). Their formation is reliant on one specific cation type each, Cs+ for 1 and Na+ for 2,
thus providing promising separation potential during crystallization.
Compound 1 is based on the largest discrete binary selenido
germanate cluster reported to date and the first mixed-valent chalcogenido
germanate(II/IV) supertetrahedron. Moreover, it adds to the few examples
of chalcogenides capable of capturing Cs+ ions. Its high
selectivity for Cs+ compared to that of Li+,
Na+, K+, and Rb+ was confirmed by
single-crystal X-ray diffraction, energy-dispersive X-ray spectroscopy,
and electrospray ionization mass spectrometry. Quantum chemical studies
indicate that smaller ions, K+ and Rb+, could
also be embedded in an isolated cluster assembly, but as the cluster
aggregate slightly distorts for crystallization, the selectivity for
Cs+ becomes exclusive in the salt. The anionic substructure
of compound 2 is based on a two-dimensional network of
supramolecular assemblies and exhibits an exclusive preference for
Na+. This work thus provides the first comprehensive insight
into the selective incorporation of specific alkali metal ions into
supramolecular aggregates of supertetrahedral chalcogenide clusters,
as a promising basis for new ion trapping techniquesespecially
for heavy alkali metal ions that pose environmental challenges.