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.
Invited for the cover of this issue are Terrence J. Collins and co-workers at Carnegie Mellon University (USA) and the National Chemical Laboratory (India). The image depicts five generations of tetraamido macrocyclic ligand (TAML) activators, which are small molecule, full-functional mimics of oxidizing enzymes that arguably outperform the peroxidase enzymes they mimic. Read the full text of the article at 10.1002/chem.201406061.
“TAML activators” are full‐functional, small molecule peroxidase mimics that arguably outperform their enzyme progenitors (TAML=tetraamido macrocyclic ligand). In their Full Paper on , S. S. Gupta, A. D. Ryabov, T. J. Collins et al. show that the Generation 5 (Gen‐5) TAML/H2O2 catalytic cycle follows the same path as Gen‐1 through Gen‐4. TAML reactivity in peroxide catalysis is subject to significant variation induced by small structural changes. The intergenerational comparative reactivity decreases in the series Gen‐4>Gen‐5>Gen‐1.
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