A Macrocyclic Iron Complex with Pyridinium Amidate, Amidate, and Sulfonamidate <i>N</i> -Donor Atoms as Oxidation Catalysts

The neutral macrocyclic iron(III) complex Fe(H₂O)(L_PYA) (<b>3</b>) is a homogeneous catalyst for the oxidation of organic micropollutants with hydrogen peroxide. The macrocyclic ligand incorporates two hybrid pyridinium amidate/amidate (hybrid PYA/A) <i>N-</i>donors and two sulfonamidate <i>N</i>-donors with an -N(CH₃)- group bridging the two sulfonamidate functions. Anionic analogues of <b>3</b> in which the pyridinium group is replaced by <i>o</i>-phenylene (K[Fe(H₂O)(L_<i>o-</i>P)] (<b>1</b>)) or by nitro-substituted <i>o</i>-phenylene (K[Fe(H₂O)(L_<i>o-</i>P*)] (<b>2</b>)) were also synthesized. Complexes <b>1</b>-<b>3</b> were investigated as potential catalysts for the oxidative destruction by hydrogen peroxide of persistent micropollutants that remain in wastewater after conventional treatment regimens. Complexes <b>1</b>-<b>3</b> catalyzed the oxidation of the surrogate micropollutants Orange II and Safranine O, and kinetic parameters for these reactions were obtained. <b>3</b> also catalyzed the oxidation of carbamazepine (CBZ), which is an actual persistent micropollutant under conditions that are relevant for practical applications ([<b>3</b>] 50 nM, [CBZ] 50 μM, [H₂O₂] 250 μM, i.e., 8.5 ppm, 25 °C, pH 7.0). Complex <b>3</b> catalyzed the oxidation of each of the above substrates at rates three to four times greater than those of the benchmark homogeneous catalyst for reactions of this type.