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

Studies of the new tetra-amido macrocyclic ligand (TAML) activator [FeIII{(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 pKa 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/[FeIII] = kIkII[H2O2][S]/(kI[H2O2] + kII[S]) and typical pH profiles for kI and kII. At pH 7 and 25 °C the rate constants kI and kII equal 0.63 ± 0.02 and 1.19 ± 0.03 M-1 s-1, respectively. With these new values for pKa, kI and kII establishing new high and low limits, respectively, the rate constants kI and kII were correlated with pKa values of all TAML activators. The relations log k = log k0 + α × pKa were established with log k0 = 13 ± 2 and 20 ± 4 and α = -1.1 ± 0.2 and -1.8 ± 0.4 for kI and kII, respectively. Thus, the reactivity of TAML activators across four generations of catalysts is predictable through their pKa values.