Dissociation kinetics study of copper(II) complexes of DO3A, DOTA and its monosubstituted derivatives
Abstract
The kinetic inertness of copper(II) complexes with cyclen-based ligands having three (H3do3a) or four (H4dota) acetic acid arms and those with 3 acetic acid and one phosphonic (H5do3ap), phosphinic (H5do3ap(PrA) , H4do3ap(ABn)) acid or methylpyridine-N-oxide (H3do3a-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 k(d,obs) = k(0) + k(H) x [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 (H3do3a, H4dota) show higher kinetic inertness than those of ligands with mixed pendant arms (H5do3ap, H5do3ap(PrA), H4do3ap(ABn), H3do3a-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 H4dota analogs are less kinetically inert (τ(1/2) 2.4-4.8 h at pH 0, 25 °C) than the Cu(II)-H4dota complex (τ(1/2) 32 h at pH 0, 25 °C).