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 H(4)dota and its analogues with one (H(5)do3ap), two (trans-H(6)do2a2p), three (H(7)doa3p) and four (H(8)dotp) phosphonic acid pendant arms were investigated. The formation of a Cu(II) complex with H(4)dota, trans-H(6)do2a2p and H(8)dotp at a slightly acidic pH is faster for the phosphonic acid derivatives than for H(4)dota, but with no simple dependence on the number of -CH2PO3H2 substituents (trans-H(6)do2a2p > H(8)dotp > H(4)dota; pH 4-6).
Relative differences in the reactivity among the differently protonated species (HnLx-) 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 of the complexes was followed in highly acidic solutions ([H+] = 0.01-5 M) and at different temperatures (15-70 degrees C) to determine the activation parameters of the reaction. The kinetic inertness of the of Cu(II) complexes follows the order: H(4)dota > H(5)do3ap > trans-H(6)do2a2p > H(7)doa3p > H(8)dotp.
Analogous data were obtained for trans-H(2)do2a and its Cu(II) complex kinetic inertness is similar to that of the H(4)dota complex. As it was considered that the published thermodynamics data on the Cu(II)-H(8)dotp 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 (H(6)doa3p)(-) anion in the solid state was determined.
These experimental data demonstrate that the substitution of acetic acid pendant arms by methylphosphonic acid ones in H(4)dota-like ligands increases the rate of complexation but significantly decreases the kinetic inertness of the Cu(II) complexes.