Kinetics of the formation and hydrolysis of (desferrioxamine B) aluminium (III) and gallium(lII) complexes in acidic aqueous perchlorate solutions

The kinetics of complexation of desferrioxamine B with M (M = Al(III) and Ga(III) in acidic aqueous perchlorate solutions at 25 °C and 2 M ionic strength are reported. The pseudo-first order complex formation and hydrolysis reactions were followed spectrophotometrically in the UV region. For both metals, a parallel-path mechanism, in which the M(H2Ü)|+ and M(H20)5(OH)2+ ions react with the fully protonated ligand (H4dfb+) to form M(H20)4(H3dfb)3+ complexes, is operative. The calculated formation rate constants for unhydrolyzed and hydrolyzed Al(III) ions are 0.0211(17) s_1 M*1 and 189(3) s-1 M_1, whereas for Ga(III) are 5.45(77) s-1 M-1 and 2.15(3) • 104 s-1 M-1, respectively. For the reaction of H4dfb+ with the unhydrolyzed Al(III) and Ga(III) ions, enthalpies and entropies of activation are: 97.6 ± 8.2 kJ mol-1, 52.5 ± 2.9 J K-1 mol-1, and 87.2 ± 7.2 kJ mol'1, 71.3 ± 2.9 J K_1 mol-1, respectively. The results are discussed in terms of a model in which the loss of the first water molecule coordinated to the metal ions is the rate determining step. The obtained ca. 104 times higher reactivities of the hydrolyzed than the unhydrolyzed metal ions are consistent with the dissociative (Eigen) mechanism