Calixcrown Binding to Alkali Metal Cations—Comparison of Two 1,3-Alternate Calix[4]arenebiscrown-5 Ligands

Abstract

While determination of the crystal structure of 1,3;2,4-biscrown-5-calix[4] arene (chloroform solvate, triclinic. space group Pī, a 11.151(4), b 16.832(6), c 21.929(8) Ǻ, α 98.78(3), β 92.02(3), γ 92.12(3)°, Z = 4 f.u ., conventional R on |F| 0.085 for 4699 'observed', I > 2σ(I), reflections) shows that this ligand has a closely similar solid state conformation to that of its previously characterized, substituted analogue, 1,3;2,4-biscrown-5-p-t-butylcalix[4] arene, the two ligands do differ significantly in their interactions with alkali metal picrates in chloroform solution. Most obviously, the t-butyl substituents of the latter ligand appear to inhibit caesium binding. changing the nature of the interactions to a form similar to that of both ligands with Li⁺ and Na⁺, for which ¹H n.m.r. spectroscopy indicates weak exocyclic binding to some of the polyether oxygen atoms. ¹H n.m.r. spectroscopy also provides evidence of picrate binding to the metals in isomeric forms.