Porous Molecular Crystals by Macrocyclic Coordination Supramolecules

In
this study, we show how the combination of metal ions, counter-anions
and opportunely functionalized and preorganized ligands gives rise
to two distinct supramolecular isomers, coordination polymeric chains
and hexameric macrocycles. The hexamers then aggregate to form a cubic
structure exhibiting permanent microporosity. The supramolecular assemblies
are formed with Ag+, thioether functionalized bis­(pirazolyl)­methane
ligands and CF3SO3–/PF6– as the counter-anions. Five different
ligands were prepared by modifying the peripheral thioether moiety
with naphthyl, methoxy, m-Me, p-Me
and F groups (LSNf, LSPhOMe, LSPhm‑Me, LSPhp‑Me, and LSPhF). Helicoidal coordination
polymeric chains are formed with CF3SO3– (general formula [Ag­(L)]n(CF3SO3)n), whereas
macrocyclic hexamers are formed with PF6– (general formula [Ag­(L)]6(PF6)6). The macrocycles self-assemble into ordered capsules with the shape
of a tetrahedron, and the overall framework is sustained by Ag+···(PF6–)···Ag+ contacts. The capsules generate a highly symmetric structural
arrangement, which is characterized by permanent microporosity arising
from two distinct types of microporous chambers in the structure.
The gas absorption isotherms show that the materials can selectively
adsorb CO2 and N2O over CH4 and N2. The modulation of the microporosity of the materials is
achieved by the different thioether functionalization of the ligands
LSNf, LSPhOMe, LSPhm‑Me, and
LSPhF. The diffusion and localization of the gas molecules
within the cavities were investigated by 2D 1H–13C solid state NMR on samples loaded with enriched 13CO2, showing that both types of cavities are accessible
to guest molecules from the gas phase.