Molecular recognition studies of an octaaminocryptand upon different degree of protonation

Abstract

Multigram synthesis of an octaaminocryptand L (1,4,11,14,17,27,29,36-octaazapentacyclo[12.12.12.2.^6,92.^19,222^31,34]tetratetraconta-6(43),7,9(44),19(41),20,22(42),31(39),32,34(40)-nonane, N(CH₂CH₂NHCH₂-p-xylyl-CH₂NHCH₂CH₂)₃N) with high yield was obtained in one pot by low temperature [2 + 3] condensation of tris(2-aminoethyl)amine with terephthalaldehyde followed by sodium borohydride reduction in methanol. L was treated with hydrochloric, hydroiodic, and picric acids in different experimental conditions to obtain crystals of different salts of L having different degrees of protonation. Structural aspects of binding of halides, picrate, and water in protonated L were examined thoroughly. Crystallographic results show that a diprotonated chloride complex of L, [H₂L](Cl)₂·3H₂O (1) and (triprotonated and monoprotonated) species of L in an iodide complex [{H₃L}{HL}](I)₄ (2) do not show any guest encapsulation inside the cavity, whereas one molecule of water resides inside the cavity in the case of its triprotonated state where one of the bridgehead nitrogen atoms is protonated in [2{LH₃(H₂O)}](I)₆·2H₂O (3). Importantly, in a tertraprotonated picrate complex of L, [H₄L(H₂O)_2.50](picrate)₄·2CH₃CN, 5, two water molecules reside inside the two N₄ cavities of L, bridged by a third water molecule showing tritopic binding, whereas in the octaprotonated state a monotopic encapsulation of iodide via N−H···iodide and C−H···iodide interactions was observed inside the receptor cavity of [H₈L(I)](I)₇·9H₂O, (8). This study systematically represents a competition between anion and solvent recognition upon different degrees of protonation of L. Further detailed structural analysis on complex 1 shows the formation of a one-dimensional polymeric water chloride hybrid network of linear rectangular chains sandwiched between the cryptand layers.