Spherical versus linear anion encapsulation in the cavity of a protonated azacryptand

Abstract

Grams scale synthesis of an octaaminocryptand L² with high yield is obtained in one-pot by low-temperature [2 + 3] condensation of tris(2-aminoethyl)amine with isophthalaldehyde, followed by sodium borohydride reduction. Structural aspects of octaaminocryptand L²·MeOH, binding of iodide (spherical) and bichloride (linear) in L², (1,4,11,14,17,24,29,36-octa-azapentacyclo-[12.12.12..2^6,9.2^19,22.2^31,34]-tetratetraconta 6(43),7,9(44),19(41),20,22(42),31(39),32,34(40)-nonane, N(CH₂CH₂NHCH₂-m-xylyl-CH₂NHCH₂CH₂)₃N), in the hexaprotonated and tetraprotonated states, respectively, are examined. Crystallographic results show binding of single iodide [H₆L²I](I)₅·4H₂O, (2), in a hexaprotonated cryptand L². Monotopic recognition of iodide is observed via (N−H)⁺···iodide interactions. The tetraprotonation of L² by hydrochloric acid showed the formation and encapsulation of a bichloride inside the cavity, which is examined from the single-crystal X-ray study. Encapsulation and binding of a proton-bridged linear bichloride inside the cavity of tetraprotonated L², [H₄L²(ClHCl)](Cl)₃·nH₂O (3), via (N−H)⁺···chloride interactions is observed in the structural investigation. This study shows that degree of protonation and its distribution in the receptor architecture play an important role in guest encapsulation. Further, it represents the first example of an encapsulated bichloride inside the cavity of an organic host.