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− ( BW 12 ), [SiW 11 NbO 40 ] 5− ( SiW 11 Nb ), and [SiW 11 TaO 40 ] 5− ( SiW 11 Ta )) are synthesized. Experimental and theoretical characterizations of the three PICs ( 1_BW 12 , 1_SiW 11 Nb , and 1_SiW 11 Ta ) show that the substitution of Nb/Ta for W in the POMs enhances the basicity of PICs, which increases in the order of 1_BW 12 < 1_SiW 11 Nb < 1_SiW 11 Ta . 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. Three isostructural porous ionic crystals (PICs) based on Keggin-type POMs with different compositions but equal negative charge ([BW 12 O 40 ] 5− , [SiW 11 NbO 40 ] 5− , and [SiW 11 TaO 40 ] 5− ) are synthesized.