Recognition of Small Polar Molecules with an Ionic Crystal of α-Keggin-Type Polyoxometalate with a Macrocation

The complexation of Keggin-type polyoxometalates [α-XW12O40]n- (X = P, Si, B, Co), macrocation [Cr3O(OOCH)6(H2O)3]+, and alkali-metal ions forms ionic crystals of Na2[Cr3O(OOCH)6(H2O)3][α-PW12O40]·16H2O (1a), K3[Cr3O(OOCH)6(H2O)3][α-SiW12O40]·16H2O (2a), Rb4[Cr3O(OOCH)6(H2O)3][α-BW12O40]·16H2O (3a), and Cs5[Cr3O(OOCH)6(H2O)3][α-CoW12O40]·7.5H2O (4a). The space volumes of the ionic crystals decrease in the order of 1a > 2a > 3a > 4a. The water of crystallization in 1a−3a is completely desorbed by evacuation at room temperature, while about 50% of the water of crystallization in 4a is desorbed. The respective 1a−4a after evacuation at room temperature are denoted by 1b−4b, which show the close packing of the constituent ions. The calculated cell volumes per formula decreased in the order of 1b > 2b > 3b > 4b, which would be related to the increase in n. Compound 1b sorbs various ≤C5 polar organic molecules such as 1-butanol, valeronitrile, and methyl propionate. Compound 2b sorbs ethanol, acetonitrile, and methyl formate. Compound 3b sorbs water and methanol, and 4b sorbs only water. Thus, the ionic crystals can discriminate ≤C5 polar organic molecules such as alcohols, nitriles, and esters by one methylene chain, and the decrease in n of [α-XW12O40]n- enables the sorption of molecules with the longer methylene chain. The nature of the sorption properties of 1b−4b can be explained by the lattice energy needed for the expansion of 1b−4b. The selective sorption properties of 1b−4b are successfully applied to the separation of mixtures of alcohols, nitriles, esters, and water.