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

The catalytic activity of the N-tailed ("biuret") TAML (tetraamido macrocyclic ligand) activators [Fe{4-XC₆H₃-1,2-(NCOCMe₂NCO)₂NR}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 H₂O₂ in aqueous solution is mechanistically compared with the previously investigated activator [Fe{4-XC₆H₃-1,2-(NCOCMe₂NCO)₂CMe₂}OH₂]^- (1) and the more aggressive analogue [Fe(Me₂C{CON(1,2-C₆H₃-4-X)NCO}₂)OH₂]^- (2). Catalysis by 3 of the reaction between H₂O₂ and Orange II (S) occurs according to the rate law found generally for TAML activators (v=k_Ik_II[Fe^III][S][H₂O₂]/(k_I[H₂O₂]+k_II[S]) and the rate constants k_I and k_II at pH 7 both decrease within the series 3 b > 3 a > 3 c. The pH dependency of k_I and k_II was investigated for 3 a. As with all TAML activators studied to-date, bell-shaped profiles were found for both rate constants. For k_I, the maximal activity was found at pH 10.7 marking it as having similar reactivity to 1 a. For k_II, the broad bell pH profile exhibits a maximum at pH about 10.5. The condition k_I 蠐 k_II 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 k_i for suicidal inactivation of the active forms of complexes 3 a-c were calculated using the general formula ln([S₀]/[S_∞])=(k_II/k_i)[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.