Abstract
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) (pK(a), ca. 11.6) to hypochlorite (pK(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 pK(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 pK(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.