Unique Guest-Inclusion Properties of a Breathing Ionic Crystal of K 3[Cr3O(OOCH)6(H2O) 3][α-SiW12O40]·16H2O

Abstract

A microstructured ionic crystal, K3[Cr3O(OOCH) 6(H2O)3]["-SiW12O 40]·16H2O (1) was synthesized by the complexation of the Keggin-type polyoxometalate of [α-SiW12O 40]4- with a macrocation of [Cr3O(OOCH) 6(H2O)3]+. Compound 1 possessed a straight channel, with an opening of approximately 0.5 × 0.8 nm, which contained the water of crystallization. The use of the macrocation with large size (0.7 nm) and small charge (+1) reduced the anion-cation interaction and was essential for the channel formation. The molecular structures of the polyoxometalate and the macrocation in 1 were retained under vacuum at 473 K. Analogues of 1 were synthesized with [α-PVW11O 40]4- or [Fe3O(OOCH)6(H 2O)3]+. The water of crystallization in 1 was removed under vacuum at room temperature to form the closely packed guest-free phase 2. Compound 2 reversibly and repeatedly included water and polar organic molecules with two carbon atoms or less. Guest inclusion was highly selective and a difference of even one methylene group in the organic guest molecule was discriminated by the host. Polar organic molecules with longer methylene chains and nonpolar molecules such as dinitrogen and methane were completely excluded. The guest-inclusion properties could be explained by the ion-dipole interaction between the host and the guest, which is proportional to the dipole moment of the guest molecule and inversely proportional to the ion-dipole (host-guest) distance. Thus, small polar molecules were selectively absorbed. These distinctive guest-inclusion properties were successfully applied to the oxidation of methanol from a mixture of C1 and C2 alcohols. These results show unique guest inclusion and catalysis by rationally designed ionic crystals.