Divergent and convergent forms of a new Schiff-base cryptand; X-ray crystallographic and molecular mechanics investigations

Abstract

A hexaimino cryptand L¹ has been obtained by condensation of tris(2-aminoethyl)amine with terephthalaldehyde without template. An X-ray crystal structure determination has been carried out on L¹·6H₂O and shows a divergent conformation for the macrocycle in which all six imine nitrogen atoms form hydrogen bonds to water molecules positioned outside the macrobicycle. In the presence of Cu^I and Ag^I a convergent form of the macrocycle which allows encapsulation of a pair of cations is adopted. Tetra- and di-thiocyanato derivatives [M₂L¹(NCS)_x(ClO₄)_y](x= 2 or 4, y= 2 or 0) have also been prepared with M = Co, Ni or Cu. In these molecules the thiocyanate ligand adopts a terminal rather than bridging mode. Molecular mechanics and molecular dynamics calculations indicate that at least four different conformations of L¹ are likely to be found. Conformational preferences of L¹ have been established in the gas phase, in water and in the presence of metal ions.