Stabilization of cubic and rhombohedral phases of zinc hexacyanocobaltate (III)

Abstract

Porous solids with the option of hydrophilic or hydrophobic surface within a same composition are not common materials. Zinc hexacyanocobaltate (III) is one of these materials. From room temperature synthesis a porous cubic phase with cavities of ca. 8.5 Å diameter communicated by windows of ca. 4.2 Å is obtained. At the cavity surface six Zn atoms with open coordination sites are available. These sites are responsible for the material affinity for polar species, among them water molecules. In the as-synthesized material these coordination sites are occupied by water molecules. On heating above 333 K the crystal water evolves and the material undergoes a structural transformation to form a rhombohedral structure (R-3c), where the Zn atoms are tetrahedrally coordinated to framework ligands and without available coordination sites. This is the hydrophobic phase. The porous network of this phase is formed by ellipsoidal cavities of ca. 5.1 × 12.7 × 8.3 Å that are communicated by elliptical windows of ca. 3.9 × 5.2 Å. The inverse transformation has a low kinetic and takes place in the presence of a high-chemical potential of water molecules. The structural transformation between the cubic and rhombohedral phases was inhibited by doping the material with a metal that stabilizes one of these structures. The minimum metal concentration that stabilizes a given phase was established.