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

Abstract

Studies of the new tetra-amido macrocyclic ligand (TAML) activator [Fe^III{(Me₂CNCOCMe₂NCO)₂CMe₂}OH₂]^- (4) in water in the pH range of 2-13 suggest its pseudo-octahedral geometry with two nonequivalent axial H₂O ligands and revealed (i) the anticipated basic drift of the first pK_a of water to 11.38 due to four electron-donating methyl groups alongside (ii) its counterintuitive enhanced resistance to acid-induced iron(III) ejection from the macrocycle. The catalytic activity of 4 in the oxidation of Orange II (S) by H₂O₂ in the pH range of 7-12 is significantly lower than that of previously reported TAML activators, though it follows the common rate law (v/[Fe^III] = k_Ik_II[H₂O₂][S]/(k_I[H₂O₂] + k_II[S]) and typical pH profiles for k_I and k_II. At pH 7 and 25 °C the rate constants k_I and k_II equal 0.63 ± 0.02 and 1.19 ± 0.03 M^-1 s^-1, respectively. With these new values for pK_a, k_I and k_II establishing new high and low limits, respectively, the rate constants k_I and k_II were correlated with pK_a values of all TAML activators. The relations log k = log k⁰ + α × pK_a were established with log k⁰ = 13 ± 2 and 20 ± 4 and α = -1.1 ± 0.2 and -1.8 ± 0.4 for k_I and k_II, respectively. Thus, the reactivity of TAML activators across four generations of catalysts is predictable through their pK_a values.