A macrocyclic ligand able to bind gallium(iii) by preorganized pendant arms; coordination and kinetic studies

Abstract

The equilibria and kinetics of the binding of gallium(III) to 4-(N),10-(N)-bis[2-(3-hydroxo-2-oxo-2-H-pyridine-1-y1)acetamido]-1,7-dimethyl-1,4,7,10-tetraazacyclododecane (L) were investigated in acidic medium at ionic strength 1 M (NaClO₄). Spectrophotometric titrations in the UV region revealed that L is able to bind Ga³⁺ also at high H⁺ concentration. The kinetic (stopped-flow) experiments are interpreted on the basis of three parallel reaction paths (i) M³⁺ + H₂L²⁺ = M(H₂L)⁵⁺ where M(H₂L)⁵⁺ is in a steady state, (ii) M(OH)²⁺ + H₂L²⁺ = M(HL)⁴⁺ + H₂O and (iii) M(OH)²⁺ + HL⁺ = ML³⁺ + H₂O. The first-order rate constants for conversion of the outer-sphere into the inner-sphere complexes are similar to those of the Ga(III)/tropolone system which is known to react according to the dissociative I_d mechanism and to the relevant rate constants for water exchange at the metal ion. The effects of pH on the UV-Vis absorption, fluorescence emission properties and NMR spectral features on the Ga(III)/L system were also investigated. Spectrophotometric titrations in the UV region reveal that, in acid medium the prevailing species is M(HL)⁴⁺ whereas the chelate ML³⁺ prevails for [H⁺] < 0.01 M. The results indicate metal coordination at the oxygen atoms of the 3-hydroxo-2-oxopyridine residues.