Concerted Functions of Anions and Cations in a Molecular Ionic Crystal with Stable Three-Dimensional Micropores

The
molecular ionic crystal [Cr3O­(OOCCHCH2)6(H2O)3]3[α-PW12O40]·15H2O [Ia] with
stable three-dimensional micropores and a minimum aperture of 3.3
Å was synthesized with a phosphododecatungstate [α-PW12O40]3– (polyoxometalate, POM)
and a macrocation with acrylate ligands [Cr3O­(OOCCHCH2)6­(H2O)3]+. The porous structure of Ia was basically constructed
by an arrangement of macrocations forming a six-membered ring: vinyl
groups (CHCH2) of adjacent macrocations were aligned
parallel to each other, suggesting a weak dispersion force between
them. A guest-free phase [Cr3O­(OOCCHCH2)6­(H2O)3]3­[α-PW12O40] [Ib] was
formed by the treatment of Ia in vacuo at room temperature
without any structure change. Compound Ib showed shape-selective
sorption of CO2 and C2H2 (molecular
size = 3.3 Å) over N2 (3.6 Å) and methane (3.7
Å), and the sorption enthalpy of C2H2 was
larger than that of CO2. The high affinity toward C2H2 was further confirmed as follows: the Monte
Carlo simulations of the optimized geometries of C2H2 in Ib showed that both hydrogen atoms were in
the vicinity of the surface oxygen atoms of POMs. The gas sorption
profiles showed a much faster diffusion for C2H2. All these results suggest that the anion and cation mainly play
the guest-binding and structure-directing roles, respectively, (i.e.,
concerted functions) in an ionic crystal with stable three-dimensional
micropores.