Synthesis and structural characterization of 18-, 19-, 20- and 22-membered Schiff base macrocycles

Six new dioxadiaza macrocyclic Schiff bases with 18-, 19-, 20- and 22-membered rings have been synthesized from diamine and dialdehyde building blocks. An X-ray diffraction study and molecular modelling (at the HF/6-31G(d,p) level of theory) have been performed to determine their molecular geometries and structural configurations. According to the theoretical studies the most stable conformers of the macrocycles are those, in which the lone pair electrons localized on the nitrogen atoms are exo-oriented, and those on the oxygen atoms are endo-oriented with respect to the cavity. In order to obtain more proper metal ion receptors with all lone pair electrons directed into the cavity, two Schiff base macrocycles have been reduced to the corresponding diamines, of which one could be isolated only as a very stable oxaazacyclophane borane adduct. Six dioxadiaza macrocyclic Schiff bases with 18-, 19-, 20- and 22-membered rings were synthesized and their molecular geometries and structural configurations were determined by X-ray diffraction and molecular modelling. In addition, two of these macrocycles were reduced to the corresponding diamines, of which one could be isolated only as a very stable oxaazacyclophane borane adduct.