Macrocyclic Polyoxometalates: Selective Polyanion Binding and Ultrahigh Proton Conduction.

The rational development of an anion templation strategy for the construction of macrocycles has been historically limited to small anions, but large polyoxoanions can offer unmatched structural diversity and ample binding sites. Here we report the formation of a {Mo ₂₂ Fe ₈ } macrocycle by using the Preyssler anion, [NaP ₅ W ₃₀ O ₁₁₀ ] ^14- ({P ₅ W ₃₀ }), as a supramolecular template. The {Mo ₂₂ Fe ₈ } macrocycle displays selective anion binding behavior in solution. In the solid state, the 1 : 2 host-guest complex, {P ₅ W ₃₀ } ₂ ⊂{Mo ₂₂ Fe ₈ }, transports protons more effectively, through an extended hydrogen-bonding network, than a related 1 : 1 complex where the guest is completely encapsulated. The results highlight the great potential this anion templation approach has in producing macrocyclic systems for selective anion recognition and proton conduction purposes.
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