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

Structural aspects of the binding of inorganic anions such as perchlorate, hydrogen sulfate, and hexafluorosilicate with the proton cage of octaaminocryptand L(1), N(CH(2)CH(2)NHCH(2)-p-xylyl-CH(2)NHCH(2)CH(2))(3)N), are examined thoroughly. Crystallographic results for a hexaprotonated perchlorate complex of L(1), [(H(6)L(1))(6+)(ClO(4)(−))]5(ClO(4)(−))·11H(2)O·CH(3)CN (1), an octaprotonated hydrogen sulfate complex of L(1), [(H(8)L(1))(8+)(HSO(4)(−))]7(HSO(4)(−))·3H(2)O·CH(3)OH (2) and an octaprotonated fluorosilicate complex of L(1), [(H(8)L(1))(8+)(HSiF(6)(−))]3(SiF(6)(2−))·(HSiF(6)(−))·15H(2)O (3), show encapsulation of one perchlorate, hydrogen sulfate and hexafluorosilicate, respectively inside the cage of L(1) in their protonated states. Further, detailed structural analysis on complex 1 reveals that the hexaprotonated L(1) encapsulates a perchlorate via two N–H···O and five O–H···O hydrogen bonds from protonated secondary nitrogen atoms of L(1) and lattice water molecules, respectively. Encapsulated hydrogen sulfate in complex 2 is “glued” inside the octaprotonated cage of L(1) via four N–H···O and six C–H···O hydrogen bonds whereas encapsulated HSiF(6)(−) in complex 3 has short contacts via six N–H···F and three C–H···F hydrogen bonds with [H(8)L(1)](8+). In the cases of complexes 2 and 3, the cryptand L(1) 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 L(1) in complex 1.