The Mechanism of Formation of Active Fe-TAMLs Using HClO Enlightens Design for Maximizing Catalytic Activity at Environmentally Optimal, Circumneutral pH.

Fe-TAML/peroxide catalysis provides simple, powerful, ultradilute approaches for removing micropollutants from water. The typically rate-determining interactions of H 2 O 2 with Fe-TAMLs (rate constant k I ) are sharply pH-sensitive with rate maxima in the pH 9-10 window. Fe-TAML design or process design that shifts the maximum rates to the pH 6-8 window of most wastewaters would make micropollutant eliminations even more powerful. Here, we show how the different pH dependencies of the interactions of Fe-TAMLs with peroxide or hypochlorite to form active Fe-TAMLs ( k I step) illuminate why moving from H 2 O 2 (p K a , ca. 11.6) to hypochlorite (p K a , 7.5) shifts the pH of the fastest catalysis to as low as 8.2. At pH 7, hypochlorite catalysis is 100-1000 times faster than H 2 O 2 catalysis. The pH of maximum catalytic activity is also moderated by the p K a 's of the Fe-TAML axial water ligands, 8.8, 9.3, and 10.3, respectively, for [Fe{4-NO 2 C 6 H 3 -1,2-(NCOCMe 2 NSO 2 ) 2 CHMe}(H 2 O) n ] - ( 2 ) [ n = 1-2], [Fe{4-NO 2 C 6 H 3 -1,2-(NCOCMe 2 NCO) 2 CF 2 }(H 2 O) n ] - ( 1b ), and [Fe{C 6 H 4 -1,2-(NCOCMe 2 NCO) 2 CMe 2 }(H 2 O) n ] - ( 1a ). The new bis(sulfonamido)-bis(carbonamido)-ligated 2 exhibits the lowest p K a and delivers the largest hypochlorite over peroxide catalytic rate advantage. The fast Fe-TAML/hypochlorite catalysis is accompanied by slow noncatalytic oxidations of Orange II.