Synthesis of new macrocyclic ligands for hetero-multinuclear transition-metal complexes

The [3 + 3] macrocyclic compounds 2a–2c were easily prepared by cyclization of the dialdehyde 8 with the appropiate diamines in refluxing CH₃CN/MeOH (1/1), in the presence of Ba2⁺ as a template ion. These macrocycles contain three salen units and are therefore, in combination with the crown ether interior, potentially tetranucleating. The length of the diamine bridge and the temperature at which the reaction is carried out, appear to influence the size of the macrocycle formed. In this way, the [4 + 4] macrocycle 2d could be formed selectively over the [3 + 3] macrocycles by carrying out the reaction at room temperature.