Oxatriaza macrocyclic ligands: studies of protonation and metal complexation

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Abstract
Some 12- to 14-membered oxatriaza macrocyclic ligands and their methylated derivatives have been synthesised and their protonation studied by potentiometric and 1H NMR techniques. The stability constants of the complexes formed by these ligands with several first-series transition-metal ions, ZnII, CdII and PbII, were determined by automated potentiometry or by an ‘out-of-cell’ technique in the cases of slow equilibration of the reactions. All the ligands have two very basic nitrogen atoms. The third nitrogen atom is protonated at lower pH, the lower the value the shorter the distance between two protonated nitrogen atoms. Stability constants for complexes of the oxatriaza ligands are lower than those for tetraaza ligands of the same size, the differences being more pronounced for the 14-membered ligands. Larger ions, such as CdII and PbII, seem to prefer smaller rings, contrary to what might be expected if the metals were inserted in the ring. Curiously, the behaviour of the copper(II) and especially nickel(II) complexes is opposite in the two series. Stability constants of complexes of the methylated amines are lower than those of the complexes with the parent amines and the same metal ions, but the differences are smaller for the oxatriaza than for the tetraaza series. This suggests that in the former series both the amines and their methylated derivatives adopt similar conformations in their complexes.