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

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 spectrophotometric, stopped-flow, high-pressure stopped-flow, 1H-NMR and high-pressure 1H-NMR methods. The stability constants of mono(N-methylacetohydroxamato)-lanthanum(III) and -copper(II) complexes were determined by the combined pH-spectral titration method at 25 ą 0.1 oC. The stability constants for Cu2+ and La3+ complexes were determined as (1.58 ą 0.02).10-1 and (3.47 ą.0.23).10-4, respectively. The formation rate constants of the above complexes were found to be independent of the metal and hydronium ion concentration. The kinetic parameters for complexation of La3+ and Cu2+ with NMHA were determined in aqueous solution of 2 M ionic strength (perchlorate) as: k(25 oC) = 3.0 ą 0.3 s-1, DH‡ = 76 ą 3 kJ mol-1, DS‡ = 19 ą 7 J K-1 mol-1, DV‡ = +5.3 ą 0.5 cm3/mol ; and k(25 oC) = 3.4 ą 0.2 s-1, DH‡ = 69 ą 1 kJ mol-1, DS‡ = -3 ą 3 J K-1 mol-1, DV‡ = +5.0 ą 0.5 cm3/mol, respectively. The kinetic parameters for complexation of La3+ and Cu2+ with H4dfb+ were determined in aqueous solution of 2 M ionic strength (perchlorate) as ; k(25 oC) = 2.9 ą 0.3 s-1, DH‡ = 76 ą 1 kJ mol-1, DS‡ = 34 ą 4 J K-1 mol-1, DV‡ = +5.2 ą 0.5 cm3/mol ; and k(25 oC) = 3.0 ą 0.4 s-1, DH‡ = 72 ą 1 kJ mol-1, DS‡ = 5 ą 2 J K-1 mol-1, DV‡ = +3.4 ą 0.2 cm3/mol, respectively. Kinetics of rotation about the C-N hydroxamate bonds in NMHA and H4dfb+ was studied by the 1H-NMR line-broadening method, and is characterized by k(25 oC) = 11 ą 2 s-1 , DH‡ = 76 ą 5 kJ mol-1, DS‡ = 31 ą 16 J K-1 mol-1, DrV (cisVtrans) = +1.5 ą 0.8 cm3/mol, ktransŽcis(25 oC) = 2.8 ą 0.5 s-1, DV‡transŽcis = +12 ą 4 cm3/mol, and by k(25 oC) = 9 ą 1 s-1, DH‡ = 69 ą 6 kJ mol-1, DS‡ = 6 ą 18 J K-1 mol-1, DrV (cisVtrans) = +0.6 ą 0.3 cm3/mol, ktransŽcis(25 oC) = 2.6 ą 0.3 s-1, DV‡transŽcis = +5 ą 2 cm3/mol, respectively. These results suggest that when the ligands assume a trans conformation, the hydroxyl oxygen is not properly positioned for coordination, and the slow rotation around the hydroxamate C-N bond is the rate determining step.

Kinetics; mechanism; complexation; Lanthanum(III); Cuper(II); N-methylacetohydroxamic acid; desferrioxamine B

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