633291-46-0Relevant academic research and scientific papers
Role of Carboxylate Bridges in Modulating Nonheme Diiron(II)/O2 Reactivity
Costas, Miquel,Cady, Clyde W.,Kryatov, Sergey V.,Ray, Manabendra,Ryan, Meghan J.,Rybak-Akimova, Elena V.,Que Jr., Lawrence
, p. 7519 - 7530 (2003)
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 O2 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 O2 binding site on the diiron(II) center, as subset B reacts more slowly by an order of magnitude. The lifetimes of the O2 adducts are also distinct and can be modulated by the addition of oxygen donor ligands. The O2 adduct of a monocarboxylate complex decays by a fast second-order process that must be monitored by stopped-flow methods, but becomes stabilized in CH2Cl2/DMSO (9:1 v/v) and decomposes by a much slower first-order process. The O2 adduct of a dicarboxylate complex is even more stable in pure CH2Cl2 and decays by a first-order process. These differences in adduct stability are reflected in the observation that only the O2 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 O2 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 O2 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.
