Structural studies on encapsulation of tetrahedral and octahedral anions by a protonated octaaminocryptand cage

Abstract

Structural aspects of the binding of inorganic anions such as perchlorate, hydrogen sulfate, and hexafluorosilicate with the proton cage of octaaminocryptand L¹, N(CH₂CH₂NHCH₂-p-xylyl-CH₂NHCH₂CH₂)₃N), are examined thoroughly. Crystallographic results for a hexaprotonated perchlorate complex of L¹, [(H₆L¹)⁶⁺(ClO₄^-)]5(ClO₄^-)·11H₂O·CH₃CN (1), an octaprotonated hydrogen sulfate complex of L¹, [(H₈L¹)⁸⁺(HSO₄^-)]7(HSO₄^-)·3H₂O·CH₃OH (2) and an octaprotonated fluorosilicate complex of L¹,[(H₈L¹)⁸⁺(HSiF₆^-)]3(SiF₆^2-)·(HSiF₆^-)·15H₂O (3), show encapsulation of one perchlorate, hydrogen sulfate and hexafluorosilicate, respectively inside the cage of L¹ in their protonated states. Further, detailed structural analysis on complex 1 reveals that the hexaprotonated L¹ encapsulates a perchlorate via two N–H···O and five O–H···O hydrogen bonds from protonated secondary nitrogen atoms of L¹ and lattice water molecules, respectively. Encapsulated hydrogen sulfate in complex 2 is “glued” inside the octaprotonated cage of L¹ via four N–H···O and six C–H···O hydrogen bonds whereas encapsulated HSiF₆⁻ in complex 3 has short contacts via six N–H···F and three C–H···F hydrogen bonds with [H₈L¹]⁸⁺. In the cases of complexes 2 and 3, the cryptand L¹ in octaprotonated state shows monotopic encapsulation of the guest and the final conformation of these receptors is spherical in nature compared to the elongated shape of hexaprotonated state of L¹ in complex 1.