Macrocyclic polyether tetralactams. Part 3. Spectroscopic study and molecular modelling of their complexes with alkaline-earth cations

Abstract

The formation of Ca²⁺, Sr²⁺ and Ba²⁺ complexes with several 18-, 21 - and 24-membered tetralactams has been investigated by UV, IR and NMR spectroscopy and molecular modelling techniques. These macrocycles form complexes with a 1:1 stoichiometry and the coordination of the metal ion is achieved by the four amide carbonyl groups, and the ether oxygen atoms of the ligand, and also by the anionic counterpart of the metal ion. On the other hand, unlike linear polyamides, it is found that a profound reorganization of these tetralactams occurs upon complexation as evidenced by ¹³C and ¹⁹F NMR spectroscopies. The existence of a single form having a mirror symmetry in the complex, with the carbonyl oxygen atoms oriented towards the cation, is postulated in solution. Molecular modelling of this complex supports this suggestion.