Reactivity and Operational Stability of N‐Tailed TAMLs through Kinetic Studies of the Catalyzed Oxidation of Orange II by H₂O₂: Synthesis and X‐ray Structure of an N‐Phenyl TAML

Abstract

The catalytic activity of the N-tailed ("biuret") TAML (tetraamido macrocyclic ligand) activators [Fe{4-XC6 H3 -1,2-(NCOCMe2 NCO)2 NR}Cl](2-) (3; N atoms in boldface are coordinated to the central iron atom; the same nomenclature is used in for compounds 1 and 2 below), [X, R=H, Me (a); NO2 , Me (b); H, Ph (c)] in the oxidative bleaching of Orange II dye by H2 O2 in aqueous solution is mechanistically compared with the previously investigated activator [Fe{4-XC6 H3 -1,2-(NCOCMe2 NCO)2 CMe2 }OH2 ](-) (1) and the more aggressive analogue [Fe(Me2 C{CON(1,2-C6 H3 -4-X)NCO}2 )OH2 ](-) (2). Catalysis by 3 of the reaction between H2 O2 and Orange II (S) occurs according to the rate law found generally for TAML activators (v=kI kII [Fe(III) ][S][H2 O2 ]/(kI [H2 O2 ]+kII [S]) and the rate constants kI and kII at pH 7 both decrease within the series 3 b>3 a>3 c. The pH dependency of kI and kII was investigated for 3 a. As with all TAML activators studied to-date, bell-shaped profiles were found for both rate constants. For kI , the maximal activity was found at pH 10.7 marking it as having similar reactivity to 1 a. For kII , the broad bell pH profile exhibits a maximum at pH about 10.5. The condition kI ≪kII holds across the entire pH range studied. Activator 3 b exhibits pronounced activity in neutral to slightly basic aqueous solutions making it worthy of consideration on a technical performance basis for water treatment. The rate constants ki for suicidal inactivation of the active forms of complexes 3 a-c were calculated using the general formula ln([S0 ]/[S∞ ])=(kII /ki )[Fe(III) ]; here [Fe(III) ], [S0 ], and [S∞ ] are the total catalyst concentration and substrate concentration at time zero and infinity, respectively. The synthesis and X-ray characterization of 3 c are also described.

A “Beheaded” TAML Activator: A Compromised Catalyst that Emphasizes the Linearity between Catalytic Activity and pKa

“…The p K a value closest to 11.38, of 10.32, has been recently measured for N-tailed “biuret” TAML activator 3a . 3 Such distinct variance prompted us to measure p K a for 1b . A similar approach as described above for 4 (see Figure S1) resulted in the value of 10.35 ± 0.02, which is not significantly higher than that for 3a and 1a (p K a values of both equal 10.3).…”

“…We 3,17,18,26 and others 27–29 employ Orange II dye for catalyzed oxidations by H 2 O 2 and other primary oxidants. Measurements were initiated at pH 7 because of our long-term goal of using TAML activators for purification of water in the environment.…”

“…The data in Figure 6 were fitted to eq 6 assuming K a = 1 × 10 −11.5 M. The fitting procedure was similar to such used by us previously. 3,10 The value of 0.63 ± 0.03 M −1 s −1 , which was measured at pH 7, can be assumed to be the upper limit for the intrinsic rate constant k 1 , as fitting the data gives greater than 100% error for this parameter. The low value of this intrinsic rate constant means that the reaction between [FeL(OH 2 ) 2 ] − and HOOH is by far the slowest of the four possible reaction pathways.…”

“…Designed over the last three decades, 1,2 tetra-amido macrocyclic ligand (TAML) activators 1 , 2 , and 3 (Chart 1), which are catalysts of the first, fourth, and fifth generations, 3 respectively, are particularly effective for the catalyzed degradation of micropollutants by hydrogen peroxide in aqueous media and therefore are extremely promising for the purification of environmental and drinking waters. These catalysts function at the parts per billion (ppb) to parts per trillion (ppt) concentration range and direct the oxidizing power of H 2 O 2 at a wide spectrum of environmental threats, 4 among which endocrine disruptors such as steroid hormone derivatives are particularly hazardous.…”

“…The mechanistic knowledge acquired 14 has been successfully applied for the synthesis of “ideal” TAML activator 1d . 16 Even though TAML activators of later generations, specifically, of fourth ( 2 ) 17,18 and fifth ( 3 ), 3,19 have been designed, the first generation activator 1d remains one of the most efficient catalysts for environmental 5 and other 20 applications.…”