Counteranion-controlled water cluster recognition in a protonated octaamino cryptand

Abstract

Structural aspects of binding of water cluster and halides in the octaamino cryptand L (1,4,11,14,17,24,29,36-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)-nonaene, N(CH₂CH₂NHCH₂-p-xylyl-CH₂NHCH₂CH₂)₃N) in a protonated state were examined. Crystallographic results show binding of the acyclic quasiplanar water tetramer [H₄L(H₂O)₄](I)₄·2.57H₂O (1) in a tetraprotonated cryptand L having an iodide counteranion, where two water molecules reside inside the two tren-based cavity, bridged by a third water molecule, and a fourth external water molecule is hydrogen bonded to the bridged water molecule. In the case of complexes [H₆L(Br)][(Br)₆H]·4H₂O·2HBr (2) and [H₆L(Cl)][(Cl)₆H]·10.86H₂O (3), a single bromide and chloride occupied, respectively, the inside of the cryptand cavity, where L is in a hexaprotonated state. Monotopic recognition of bromide/chloride was observed at the center of the cryptand cavity where halides show C−H···halide interactions instead of the N−H···halide interactions reported in the ditopic complexes of halides with the same cryptand, 5 and 6. Thermal analyses on 1−3 were carried out, and the data obtained distinctly differentiate water cluster complex 1 from the anion-encapsulated cryptates 2 and 3. This study represents the first example of anion-controlled cluster formation inside the cavity of a cryptand.