Protonation scheme for some triaza macrocycles studied by potentiometry and NMR spectroscopy

A series of triaza macrocyclic tricarboxylate ligands with ring sizes of nine to twelve (NOTA, DETA, UNTA and DOTRA) have been synthesized. Their acid-base properties and protonation sequence have been established and compared with those of the corresponding cyclic triamines and trimethyl cyclic triamines, using potentiometric measurements and ¹H NMR pH titrations. The first protonation constant of the carboxylate ligands is very high, and larger than the value for the parent cyclic amines. It is also very sensitive to the presence of Na⁺ ions in solution. The first two protonation constants correspond to protonation of the ring nitrogens, and their increase with ring size is non-monotonic, reflecting pH-dependent conformational effects. These effects, due to electrostatic interactions as well as to hydrogen bond formation, caused difficulties in the application of methods (previously used for non-cyclic polyaminocarboxylates) to obtain the protonation sequence from the ¹H NMR pH titrations, similar to a previous occurrence for tetraaza macrocycles. By studying the pH dependence of the shielding parameters, C_N and C_N′, the protonation sequences were then obtained. For each of the four compounds studied, two nitrogen atoms are protonated before the carboxylate groups. Formation of hydrogen bonds between protonated nitrogens and non-protonated carboxylates affects the basicity of the latter, reducing the flexibility of these ligands in certain pH intervals.