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

Studies of the new tetra-amido macrocyclic ligand (TAML) activator [Fe{(MeCNCOCMeNCO)CMe}OH] (4) in water in the pH range of 2-13 suggest its pseudo-octahedral geometry with two nonequivalent axial HO ligands and revealed (i) the anticipated basic drift of the first pK 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 HO 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] = kk[HO][S]/(k[HO] + k[S]) and typical pH profiles for k and k. At pH 7 and 25 °C the rate constants k and k equal 0.63 ± 0.02 and 1.19 ± 0.03 M s, respectively. With these new values for pK, k and k establishing new high and low limits, respectively, the rate constants k and k were correlated with pK values of all TAML activators. The relations log k = log k + α × pK were established with log k = 13 ± 2 and 20 ± 4 and α = -1.1 ± 0.2 and -1.8 ± 0.4 for k and k, respectively. Thus, the reactivity of TAML activators across four generations of catalysts is predictable through their pK values.