Formation and decomplexation kinetics of copper(ii) complexes with cyclen derivatives having mixed carboxylate and phosphonate pendant arms

Abstract

The kinetic properties of Cu(II) complexes of H₄dota and its analogues with one (H₅do3ap), two in the 1,7-position (trans-H₆do2a2p), three (H₇doa3p) and four (H₈dotp) phosphonic acid pendant arms were investigated. The formation of a Cu(II) complex with H₄dota, trans-H₆do2a2p and H₈dotp at a slightly acidic pH is faster for the phosphonic acid derivatives than for H₄dota, but with no simple dependence on the number of –CH₂PO₃H₂ substituents (trans-H₆do2a2p > H₈dotp > H₄dota; pH 4–6). Relative differences in the reactivity among the differently protonated species (H_nL^x−) of the same ligand are successively decreased with the more phosphonic acid groups in the ligand. The faster complexation is probably caused by the higher ability of phosphonates to bind the metal ion and/or to assist in the transfer of protons from the ring amine groups to the bulk water. The acid-assisted decomplexation kinetics of the complexes was followed in highly acidic solutions ([H⁺] = 0.01–5 M) and at different temperatures (15–70 °C) to determine the activation parameters of the reaction. The kinetic inertness of the Cu(II) complexes follows the order: H₄dota > H₅do3ap > trans-H₆do2a2p > H₇doa3p > H₈dotp. To obtain information on the influence of additional pendant arms, analogous data were obtained for trans-H₂do2a. The ligand is less reactive than H₄dota, but the kinetic inertness of its Cu(II) complex is similar to that of the H₄dota complex. As it was considered that the published thermodynamics data on the Cu(II)–H₈dotp system are probably incorrect, the system was re-investigated. It showed a very high stability for the [Cu(dotp)]⁶⁻ species and the easy formation of several Cu₂L species in the presence of an excess of the metal ion. Also, the structure of the (H₆doa3p)⁻ anion in the solid state was determined. These experimental data demonstrate that the substitution of acetic acid pendant arms by methylphosphonic acid ones in H₄dota-like ligands increases the rate of complexation but significantly decreases the kinetic inertness of the Cu(II) complexes.