Iron(III) complexation behavior of benzene-centered tripodal mono-, di- and tritopic ligands carrying a repeating –Ahe–(HO)Apr– sequence [Ahe = 6-aminohexanoyl; (HO)Apr = 3-(N-hydroxy) aminopropanoyl]

Abstract

To investigate how three-directional molecules behave upon complexation with ferric iron, tripodal 1,3,5-benzene-centered mono-, di- and tritopic iron-chelating ligands {C₆H₃[CO–(Ahe–(HO)Apr)_n–OH]₃: 1, n   =  1; 2, n   =  2; 3, n   =  3}, composed of strands containing an –Ahe–(HO)Apr– sequence [Ahe  =  6-aminohexanoyl; (HO)Apr = 3-(N-hydroxy)aminopropanoyl], were synthesized. These hydroxamate ligands form intramolecular six-coordinate octahedral complexes with Fe(III): Fe₁-1, Fe₁-2, Fe₂-2, Fe₁-3, Fe₂-3 and Fe₃-3. The complexes formed were investigated from a topological viewpoint and examined in terms of stability against attack by H⁺ and HO⁻, monoprotonation equilibrium and iron removal kinetics using a 20-molar excess of EDTA. Fe₁-1, Fe₁-2 and Fe₂-2 have tripodal interstrand structures. In particular, the iron removal reaction of Fe₂-2 shows a consecutive first-order reaction pattern. From kinetic data of the first-order reactions, Fe₁-3, Fe₂-3 and Fe₃-3 are concluded to possess one, two and three discrete ferrioxamine-type (intrastrand) structures, respectively.