Factors affecting the carboxylate shift upon formation of nonheme diiron-O₂ adducts

Several [Fe^II₂(N-EtHPTB)(μ-O₂X)] ²⁺ complexes (1·O₂X) have been synthesized, where N-EtHPTB is the anion of N,N,N′N′-tetrakis(2-benzimidazolylmethyl)- 2-hydroxy-1,3-diaminopropane and O₂X is an oxyanion bridge. Crystal structures reveal five-coordinate (μ-alkoxo)diiron(II) cores. These diiron(II) complexes react with O₂ at low temperatures in CH ₂Cl₂ (-90 C) to form blue-green O₂ adducts that are best described as triply bridged (μ-η¹:η¹- peroxo)diiron(III) species (2·O₂X). With one exception, all 2·O₂X intermediates convert irreversibly to doubly bridged, blue (μ-η¹:η¹-peroxo)diiron(III) species (3·O₂X). Where possible, 2·O₂X and 3·O₂X intermediates were characterized using resonance Raman spectroscopy, showing respective ν_O-O values of ∼850 and ∼900 cm^-1. How the steric and electronic properties of O ₂X affect conversion of 2·O₂X to 3·O ₂X was examined. Stopped-flow analysis reveals that oxygenation kinetics of 1·O₂X is unaffected by the nature of O ₂X, and for the first time, the benzoate analog of 2·O ₂X (2·O₂CPh) is observed.