Flexibility of Inorganic Tennis Ball Structures Inducing Anion Selectivity

Inorganic tennis balls (ITBs), [[{Pt(betmp)(dach)}2Cu]2(X)][X]3 (in which X=ClO4− (3), NO3− (4), Cl− (5) and Br− (6); dach=trans-1,2-diaminocyclohexane and betmp=bisethylthiomethylidenepropanedioate) and [[{Pt(dteym)(dach)}2Cu]2(PF6)][PF6]3 (7; dteym=1,3-dithiepane-2-ylidenemalonate), were prepared as crystals. Investigation of their X-ray crystal structures revealed that shapes of the cavities in ITBs show significant distortions that depend on the properties of the encapsulated anions. The Cu⋅⋅⋅Cu* distance was observed to be longest in 7 and shortest in 5, the difference between them being 2.05 A. The flexibility of cavity structures of ITBs makes it possible to encapsulate various anions inside the cavity, while their distortions may be a reason for the difference in the encapsulating ability for anions, that is, anion selectivity. Especially, the distortions observed in 7 are so severe that the encapsulating ability of the cavity for PF6− is very low compared to other anions. The shapes of ITBs with ClO4− and BF4− ions inside their cavities are very similar; however, ClO4− is encapsulated by the cavity better than BF4−, which is explicable by the difference of metal–anion interactions. This structural study on ITBs gives a clue to the origin of the anion selectivity of the cavity in ITBs previously investigated by 19F NMR spectroscopy of the ITBs in methanol.