Catalase-Peroxidase Activity of Iron(III)-TAML Activators of Hydrogen Peroxide

Exceptionally high peroxidase-like and catalase-like activities of iron(III)-TAML activators of H2O2 (1: Tetra-Amidato-Macrocyclic-Ligand Fe-III complexes [Fe{1,2-X2C6H2-4,5-(NCOCMe2NCO)(2)CR2}(OH2)](-)) are reported from pH 6-12.4 and 25-45 degrees C. Oxidation of the cyclometalated 2-phenylpyridine organometallic complex, [Ru-II(o-C(6)H(4)py)(phen)(2)]PF6 (2) or "ruthenium dye", occurs via the equation [Ru-II] + 1/2 H2O2 + H+ -> (FeIII-TAML) [Ru-III] + H2O, following a simple rate law rate = k(obs)(per)[1][H2O2], that is, the rate is independent of the concentration of 2 at all pHs and temperatures studied. The kinetics of the catalase-like activity (H2O2 -> (FeIII-TAML) H2O + 1/2 O-2) obeys a similar rate law: rate = kob[1][H2O2]). The rate constants, k(obs)(per) and k(obs)(cat), are strongly and similarly pH dependent, with a maximum around pH 10. Both bell-shaped pH profiles are quantitatively accounted for in terms of a common mechanism based on the known speciation of 1 and H2O2 in this pH range. Complexes 1 exist as axial diaqua species [FeL(H2O)(2)](-) (1(aqua)) which are deprotonated to afford [FeL(OH)(H2O)](2-) (1(OH)) at pH 9-10. The pathways 1(aqua) + H2O2 (k(1)), 1(OH) + H2O2 (k(2)), and 1(OH) + HO2- (k(4)) afford one or more oxidized Fe-TAML species that further rapidly oxidize the dye (peroxidaselike activity) or a second H2O2 molecule (catalase-like activity). This mechanism is supported by the observations that (i) the catalase-like activity of 1 is controllably retarded by addition of reducing agents into solution and (ii) second order kinetics in H2O2 has been observed when the rate of O-2 evolution was monitored in the presence of added reducing agents. The performances of the 1 complexes in catalyzing H2O2 oxidations are shown to compare favorably with the peroxidases further establishing Fe-III-TAML activators as miniaturized enzyme replicas with the potential to greatly expand the technological utility of hydrogen peroxide.