Basicity of isostructural porous ionic crystals composed of Nb/Ta-substituted Keggin-type polyoxotungstates.

Three isostructural porous ionic crystals (PICs) based on Keggin-type POMs with different compositions but equal negative charge ([BW ₁₂ O ₄₀ ] ^5- (BW12), [SiW ₁₁ NbO ₄₀ ] ^5- (SiW11Nb), and [SiW ₁₁ TaO ₄₀ ] ^5- (SiW11Ta)) are synthesized. Experimental and theoretical characterizations of the three PICs (1&#95;BW12, 1&#95;SiW11Nb, and 1&#95;SiW11Ta) show that the substitution of Nb/Ta for W in the POMs enhances the basicity of PICs, which increases in the order of 1&#95;BW12 < 1&#95;SiW11Nb < 1&#95;SiW11Ta. These findings clearly show that the increase in basicity is due not only to the increase in negative charge of the POM molecule as is often explained, but also to the character of the substituted element itself.