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

Abstract

Three isostructural porous ionic crystals (PICs) based on Keggin-type POMs with different compositions but equal negative charge ([BW₁₂O₄₀]⁵⁻ (BW₁₂), [SiW₁₁NbO₄₀]⁵⁻ (SiW₁₁Nb), and [SiW₁₁TaO₄₀]⁵⁻ (SiW₁₁Ta)) are synthesized. Experimental and theoretical characterizations of the three PICs (1_BW₁₂, 1_SiW₁₁Nb, and 1_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_BW₁₂ < 1_SiW₁₁Nb < 1_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.