Kinetics and Mechanism of the Complexation of La3+ and Cu2+ Ions with N-Methylacetohydroxamic Acid and Desferrioxamine B1

The kinetics and mechanism of the complexation of La3+ and Cu2+ ions with desferrioxamine B (H4dfb+) and N-methylacetohydroxamic acid (NMHA) in aqueous medium were studied by stopped-flow and 1H NMR methods. The equilibrium constants for reactions (M n+ + HA ⇄ MA(n-1)+ + H+) of NMHA with Cu2+ and La3+ were determined by the combined pH−spectral titration method at 25 ± 0.1 °C as (1.58 ± 0.02) × 10-1 and (3.5 ± 0.2) × 10-4, respectively. In aqueous solution of 2 M ionic strength, the kinetic parameters for complexation of La3+ and Cu2+ with NMHA were determined as k(25 °C) = 3.0 ± 0.3 s-1, ΔH ⧧ = 76 ± 3 kJ mol-1, ΔS ⧧ = 19 ± 7 J K-1 mol-1, ΔV ⧧ = +5.3 ± 0.5 cm3/mol and k(25 °C) = 3.4 ± 0.2 s-1, ΔH ⧧ = 69 ± 1 kJ mol-1, ΔS ⧧ = −3 ± 3 J K-1 mol-1, ΔV ⧧ = +5.0 ± 0.5 cm3/mol, whereas with H4dfb+ they were determined as k(25 °C) = 2.9 ± 0.3 s-1, ΔH ⧧ = 76 ± 1 kJ mol-1, ΔS ⧧ = 34 ± 4 J K-1 mol-1, ΔV ⧧ = +5.2 ± 0.5 cm3/mol; and k(25 °C) = 3.0 ± 0.4 s-1, ΔH ⧧ = 72 ± 1 kJ mol-1, ΔS ⧧ = 5 ± 2 J K-1 mol-1, ΔV ⧧ = +3.4 ± 0.2 cm3/mol, respectively. The rotation about the C−N hydroxamate bonds in NMHA and H4dfb+ is characterized by k(25 °C) = 11 ± 2 s-1, ΔH ⧧ = 76 ± 5 kJ mol-1, ΔS ⧧ = 31 ± 16 J K-1 mol-1, Δr V (cis⇄trans) = +1.5 ± 0.8 cm3/mol, k trans→cis(25 °C) = 2.8 ± 0.5 s-1, ΔV ⧧ trans→cis = +12 ± 4 cm3/mol and by k(25 °C) = 9 ± 1 s-1, ΔH ⧧ = 69 ± 6 kJ mol-1, ΔS ⧧ = 6 ± 18 J K-1 mol-1, Δr V (cis⇄trans) = +0.6 ± 0.3 cm3/mol, k trans→cis(25 °C) = 2.6 ± 0.3 s-1, ΔV ⧧ trans→cis = +5 ± 2 cm3/mol, respectively. The results suggest that the slow rotation around the hydroxamate C−N bond is the rate-determining step for the complexation reactions.