Tripodal amine catechol ligands: a fascinating class of chelators for aluminium(III)

Complexation constants based on potentiometric titrations and spectrophotometric measurements in an aqueous medium of 0.1M KCl at 25+/-1 degrees C for the complexes of Al(III) with multidentate tripodal polycatechol-amine ligands, cis,cis-1,3,5-tris[(2,3-dihydroxybenzylamino)aminomethyl]cyclohexane (TMACHCAT, L(1)) and N(1),N(3),N(5)-tris(2-(2,3-dihydroxybenzylamino)ethyl)cyclohexane-1,3,5-tricarboxamide (CYCOENCAT, L(2)) have been summarized in this paper. Both the ligands released six protons to form various monomeric complexes of the types AlLH(3), AlLH(2), AlLH and AlL (L=L(1) and L(2)). The first species AlLH(3) depicted at low pH for which a monocapped type geometry was suggested, where the ligands were coordinated through three catecholic oxygens at ortho. Other species are formed subsequently from the species AlLH(3) in steps upon deprotonation and coordination of the catecholic oxygens at meta to give encapsulated tris(catechol) type complexes. The probable structures of the metal complexes formed in solution were proposed through molecular modeling calculations. The pAl values calculated for AlL(1) and AlL(2) are appreciably higher than transferrin. The ligand L(2) showed higher affinity towards Al(III) than L(1) and desferrioxamine (DFO), the only approved drug for the treatment of aluminium intoxication.