Dissociation kinetics study of copper(II) complexes of DO3A, DOTA and its monosubstituted derivatives

摘要

The kinetic inertness of copper(II) complexes with cyclen-based ligands having three (H_3do3a) or four (H_4dota) acetic acid arms and those with three acetic acid and one phosphonic (H_5do3ap), phosphinic (H_5do3ap~(PrA), H_4do3ap~(ABn)) acid or methylpyridine-N-oxide (H_3do3a-pyNox) pendant arms has been studied. The proton-assisted decomplexation reaction was studied under a wide range of conditions to fully assess the parameters influencing the reaction (temperature range 25-60 °C, proton concentration range 0.05-5 M and presence of perchlorate or nitrate anions). The empirical rate law kd, obs = k_0 + k_H × [H~+], involving solvolytic and proton-assisted dissociation pathways, shows that the complexes can be divided into two groups according to their kinetic behavior. Complexes of ligands having only carboxylate pendant arms (H_3do3a, H_4dota) show higher kinetic inertness than those of ligands with mixed pendant arms (H _5do3ap, H_5do3ap~(PrA), H_4do3ap ~(ABn), H_3do3a-pyNox). Linear Free Energy Relationship (LFER) analysis of the activation parameters (ΔH~#, ΔS#) proves that the reaction mechanism is generally the same for all copper(II) complexes and is independent of the inert electrolyte. Rate constants representing both solvolytic and acid-assisted pathways roughly correlate with the basicity of the ring nitrogen atoms. The nitrate anion from the supporting electrolyte was shown to accelerate (by about 3-5 times) the decomplexation reaction when compared to the reaction in perchlorate-containing media; the nitrate anion seems to interact with the reaction intermediates, changing the activation parameters of both reaction pathways. This effect is somewhat suppressed in the complex of the ligand with a hydrophobic p-aminobenzyl substituent on the phosphorus atom, H5do3ap~(ABn). In perchlorate media, complexes of the monophosphorus acid H_4dota analogs are less kinetically inert (τ _(1/2) 2.4-4.8 h at pH 0, 25 °C) than the Cu(II)-H_4dota complex (τ _(1/2) 32 h at pH 0, 25 °C).