Structures and Sorption Properties of Ionic Crystals of Macrocation-Dawson-Type Polyoxometalates with Different Charges

The complexation of Dawson-type polyoxometalates of [α-P2W18O62]6-, [α2-P2W17V1O62]7-, and [α-P2W15V3O62]9- with the macrocation ([Cr3O(OOCH)6(H2O)3]+) forms ionic crystals of (NH4)4[Cr3O(OOCH)6(H2O)3]2[α-P2W18O62]·15H2O (1a), (NH4)5[Cr3O(OOCH)6(H2O)3]2[α2-P2W17V1O62]·15H2O (2a), and (NH4)7[Cr3O(OOCH)6(H2O)3]2[α-P2W15V3O62]·15H2O (3a), respectively. The compounds 1a−3a show the honeycomb packing and the symmetry of the constituent ions reflect on the crystal structures. The lengths of the a axes of 1a − 3a are almost the same, while those of the c axes decrease in the order of 1a ≥ 2a > 3a, with the increase in the anion charges of the polyoxometalates. The water of crystallization in 1a − 3a is desorbed by the evacuation at 373 K to form the respective guest-free phases 1b − 3b. The compounds 1b − 3b are crystalline, and the lengths of the a axes of 1b − 3b are almost the same, while those of the c axes decrease in the order of 1b ≥ 2b > 3b. The compounds 1b − 3b possess voids running perpendicular to the c axis. The sizes of the voids decrease in the order of 1b > 2b > 3b and are comparable to those of water and methanol. The water sorption profile of 1b as a function of time is reproduced by the linear driving force mass transfer model with a single rate constant k 1, while that of 3b is reproduced by the two components k 1 and k 2. The component k 1 is attributed to the diffusion of water molecules in the voids, and the component k 2 is attributed to the large increase in the length of the c axis with the water sorption. The compounds 1b and 2b sorb ethanol, while the amount of sorption for 2b is smaller than that for 1b, and 3b completely excludes ethanol. Thus, the crystal structures, water sorption kinetics, and alcohol sorption properties of 1b − 3b systematically change with the anion charges.