Complexation of a tripodal amine-catechol ligand tris((2,3-dihydroxybenzylamino) ethyl) amine towards Al(III), Ga(III), and In(III)

The complexation of a tripodal amine-catechol ligand tris((2,3-dihydroxybenzylamino) ethyl) amine (TRENCAT, L) with group-13 metal ions, viz., Al(III), Ga(III), and In(III), were investigated by means of potentiometric titrations and spectrophotometric measurements in an aqueous medium of 0.1 M KCl at 25 +/- 1 degrees C. The ligand shows the potential to form various monomeric complexes of the types MLH3, MLH2, MLH, and ML. At low pH, the ligand is coordinated through three more acidic ortho-catecholic O-atoms to give MLH3 species. With the rise in pH, the species MLH3 releases three protons in steps from the meta-catecholic O-atoms to form MLH2, MLH, and ML. The order of stability Ga(III) > Al(III) > In(III) for the species MLH3 and MLH2 is changed into Al(III) > Ga(III) > In(III) for the species MLH and ML. The coordination modes, binding ability, selectivity, and the change in stability order were explained with the help of experimental evidence, molecular modeling calculations, and available literature.