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

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 CuI and AgI 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.