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

Studies of the new tetra-amido macrocyclic ligand (TAML) activator [FeIII {(Me2 CNCOCMe2 NCO)2 CMe2 }OH2 ]- (4) in water in the pH range of 2-13 suggest its pseudo-octahedral geometry with two nonequivalent axial H2 O 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 H2 O2 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 ] = kI kII [H2 O2 ][S]/(kI [H2 O2 ] + 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.