title: 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 [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 pKaof 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 H2O2in 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 kIand kII. At pH 7 and 25 °C the rate constants kIand kIIequal 0.63 ± 0.02 and 1.19 ± 0.03 M-1s-1, respectively. With these new values for pKa, kIand kIIestablishing new high and low limits, respectively, the rate constants kIand kIIwere correlated with pKavalues of all TAML activators. The relations log k = log k0+ α × pKawere established with log k0= 13 ± 2 and 20 ± 4 and α = -1.1 ± 0.2 and -1.8 ± 0.4 for kIand kII, respectively. Thus, the reactivity of TAML activators across four generations of catalysts is predictable through their pKavalues.
authors.affiliation: Department of Chemistry, Carnegie Mellon University , 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States.
authors.forename: Matthew R
authors.initials: MR
authors.lastname: Mills
authors.affiliation: Department of Chemistry, Carnegie Mellon University , 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States.
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Weitz
authors.affiliation: Department of Chemistry, Carnegie Mellon University , 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States.
authors.forename: David Z
authors.initials: DZ
authors.lastname: Zhang
authors.affiliation: Department of Chemistry, Carnegie Mellon University , 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States.
authors.forename: Michael P
authors.initials: MP
authors.lastname: Hendrich
authors.affiliation: Department of Chemistry, Carnegie Mellon University , 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States.
authors.forename: Alexander D
authors.initials: AD
authors.lastname: Ryabov
authors.affiliation: Department of Chemistry, Carnegie Mellon University , 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States.
authors.forename: Terrence J
authors.initials: TJ
authors.lastname: Collins