Calixcrown binding to alkali metal cations - Comparison of two 1,3-alternate calix[4]arenebiscrown-5 ligands

While determination of the crystal structure of 1,3;2,4-biscrown-5-calix[4]arene (chloroform solvate, triclinic, space group P <(1)over bar >, a 11.151(4), b 16.832(6), c 21.929(8) Angstrom, alpha 98.78(3), beta 92.02(3), gamma 92.12(3)degrees, Z = 4 f.u., conventional R on \F\ 0.085 for 4699 'observed', I > 2 sigma(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 H-1 n.m.r. spectroscopy indicates weak exocyclic binding to some of the polyether oxygen atoms. H-1 n.m.r. spectroscopy also provides evidence of picrate binding to the metals in isomeric forms.