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Role of Carboxylate Bridges in Modulating Nonheme Diiron(II)/O Reactivity

Inorganic Chemistry, ISSN: 0020-1669, Vol: 42, Issue: 23, Page: 7519-7530
2003
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Article Description

A series of diiron(II) complexes of the dinucleating ligand HPTP (N, N, N′,N′-tetrakis(2-pyridylmethyl)-2-hydroxy-1,3-diaminopropane) with one or two supporting carboxylate bridges has been synthesized and characterized. The crystal structure of one member of each subset has been obtained to reveal for subset A a (μ-alkoxo)(μ-carboxylato)-diiron(II) center with one five- and one six-coordinate metal ion and for subset B a coordinatively saturated (μ-alkoxo)bis(μ-carboxylato)diiron(II) center. These complexes react with O in second-order processes to form adducts characterized as (μ-1,2-peroxo)diiron(III) complexes. Stopped-flow kinetic studies show that the oxygenation step is sensitive to the availability of an O binding site on the diiron(II) center, as subset B reacts more slowly by an order of magnitude. The lifetimes of the O adducts are also distinct and can be modulated by the addition of oxygen donor ligands. The O adduct of a monocarboxylate complex decays by a fast second-order process that must be monitored by stopped-flow methods, but becomes stabilized in CHCl/DMSO (9:1 v/v) and decomposes by a much slower first-order process. The O adduct of a dicarboxylate complex is even more stable in pure CHCl and decays by a first-order process. These differences in adduct stability are reflected in the observation that only the O adducts of monocarboxylate complexes can oxidize substrates, and only those substrates that can bind to the diiron center. Thus, the much greater stability of the O adducts of dicarboxylate complexes can be rationalized by the formation of a (μ-alkoxo)(μ-1,2-peroxo)diiron(III) complex wherein the carboxylate bridges in the diiron(II) complex become terminal ligands in the O adduct, occupy the remaining coordination sites on the diiron center, and prevent binding of potential substrates. Implications for the oxidation mechanisms of nonheme diiron enzymes are discussed.

Bibliographic Details

Costas, Miquel; Cady, Clyde W; Kryatov, Sergey V; Ray, Manabendra; Ryan, Meghan J; Rybak-Akimova, Elena V; Que, Lawrence

American Chemical Society (ACS)

Chemistry

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