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

2008; American Chemical Society; Volume: 130; Issue: 45 Linguagem: Inglês

10.1021/ja8043689

1943-2984

Anindya Ghosh, Douglas A. Mitchell, Arani Chanda, Alexander D. Ryabov, Delia Laura Popescu, Erin C. Upham, Gregory J. Collins, Terrence J. Collins,

Lanthanide and Transition Metal Complexes

Exceptionally high peroxidase-like and catalase-like activities of iron(III)−TAML activators of H2O2 (1: Tetra-Amidato-Macrocyclic-Ligand FeIII complexes [Fe{1,2−X2C6H2−4,5−(NCOCMe2NCO)2CR2}(OH2)]–) are reported from pH 6−12.4 and 25−45 °C. Oxidation of the cyclometalated 2-phenylpyridine organometallic complex, [RuII(o-C6H4py)(phen)2]PF6 (2) or "ruthenium dye", occurs via the equation [RuII]+1/2 H2O2+H + →□FeIII−TAML [RuIII]+H2O, following a simple rate law rate = kobsper[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 O2) obeys a similar rate law: rate = kobscat[1][H2O2]). The rate constants, kobsper and kobscat, 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]− (1aqua) which are deprotonated to afford [FeL(OH)(H2O)]2− (1OH) at pH 9−10. The pathways 1aqua + H2O2 (k1), 1OH + H2O2 (k2), and 1OH + HO2− (k4) afford one or more oxidized Fe−TAML species that further rapidly oxidize the dye (peroxidase-like 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 O2 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 FeIII−TAML activators as miniaturized enzyme replicas with the potential to greatly expand the technological utility of hydrogen peroxide.