Synthesis and spectroscopic characterization of a new tripodal hexadentate iron chelator incorporating catechol units

We report the synthesis and physicochemical properties of a new tripodal hexadentate chelator (catTHC) synthesized by reaction of a flexible tripodal backbone with three bidentate catechol units. To improve the efficiency of the amide coupling reaction, classical conditions using two pairs of coupling reactants were tested, and a significant reduction in reaction time was achieved by using microwave irradiation with the reactants DCC/HOBt. Subsequent removal of the benzyl protecting groups using BCl₃ in dichloromethane provided the final chelator in good yield. The acid-base properties of catTHC in aqueous solution and the affinity of the ligand towards iron(III) were investigated at variable pH and in the presence of iron(III) using spectroscopic methods. The hexadentate ligand forms a 1:1 complex with iron(III) whose stability constant was determined by competition with EDTA. The values obtained for the stability constant and pFe³⁺ are log β₁₁₀ = 36.70 and pFe³⁺ = 26.7.