Effect of nitrogen number on the metal binding selectivity of 12-membered macrocycles

The complexes of 12-crown-4, aza-12-crown-4, diaza-12-crown-4, triaza-12-crown-4, and tetraza-12-crown-4 with Be2+, Li+ or Na+ ions were investigated by the density functional theory. Their stable structures were identified by geometry optimization without striction of their geometries. The binding energy of coordination reaction was used to evaluate the metal binding selectivity of these macrocycles. It is found that the number of nitrogens in the macrocycles results in different relative energies, and this effect is discussed through the explicit natural bond orbital analysis. For the systems involved, the calculated results show that diaza-12-crown-4 offers the best binding selectivity to metal ions that could not fit within the cavity, and tetraza-12-crown-4 exhibits the largest metal binding capability to Be2+ ion that is entirely enclosed in the corresponding cavity. Clearly, the number of nitrogens plays an important role in the metal binding selectivity of these systems. This work will make contribution to the design and synthesis of materials of ionic recognition, molecular probes, and other related fields.