Intermediates in the oxygenation of a nonheme diiron(II) complex, including the first evidence for a bound superoxo species

Abstract

The reaction of [Fe(2)(mu-OH)(2)(6-Me(3)-TPA)(2)](2+) (1) [6-Me(3)-TPA, Tris(6-methyl-2-pyridylmethyl)amine] with O(2) in CH(2)Cl(2) at -80 degrees C gives rise to two new intermediates, 2 and 3, before the formation of previously characterized [Fe(2)(O)(O(2))(6-Me(3)-TPA)(2)](2+) (4) that allow the oxygenation reaction to be monitored one electron-transfer step at a time. Raman evidence assigns 2 and 3 as a diiron-superoxo species and a diiron-peroxo species, respectively. Intermediate 2 exhibits its nu(O-O) at 1,310 cm(-1) with a -71-cm(-1) (18)O isotope shift. A doublet peak pattern for the (16)O(18)O isotopomer of 2 in mixed-isotope Raman experiments strongly suggests that the superoxide ligand of 2 is bound end-on. This first example of a nonheme iron-superoxo intermediate exhibits the highest frequency nu(O-O) yet observed for a biomimetic metal-dioxygen adduct. The bound superoxide of 2, unlike the bound peroxide of 4, is readily reduced by 2,4-di-tert-butylphenol via a proton-coupled electron-transfer mechanism, emphasizing that metal-superoxo species may serve as oxidants in oxygen activation mechanisms of metalloenzymes. The discovery of intermediates 2 and 3 allows us to dissect the initial steps of dioxygen binding at a diiron center leading to its activation for substrate oxidation.

Fig. 1.

Formation of 2 (solid line) at –80°C by bubbling O₂ through a solution of 1 (dashed line) (0.2 mM) in CH₂Cl₂. (Inset) Plot of pseudo-first-order rate constants for the formation of 2 (0.1 mM) vs. concentration of O₂ (2.30–5.76 mM) at –80°C.

Fig. 2.

Conversion of 2 (solid line) to 4 (dashed line) initiated by raising the temperature from –80°C to –60°C. (Inset) Plot of the absorbance change at 640 nm against time, showing a sharp decrease followed by a slow increase.

Fig. 3.

Eyring plots for the formation of 2 at lower temperature (▴), the formation of 4 at higher temperature (▵), and two stages of the conversion from 2 to 4 (• and ▪).

Scheme 1.

Proposed mechanism for oxygenation of 1.

Fig. 4.

Resonance Raman spectra of the dark olive green solution generated from the reaction of 1 with ¹⁶O₂ (Upper) or ¹⁸O₂ (Lower) in CH₂Cl₂ at –80°C. All of the spectra were obtained at 77 K with an excitation wavelength of 647.1 nm. Color-coded features associated with 2 (red), 3 (green), and 4 (blue) were fit by using the program grams/ai (Thermo Galactic); solvent bands are labeled with “S.”

Fig. 5.

The ν(O–O) region in the resonance Raman spectra of 2 generated from the reaction of 1 with mixture of ¹⁶O₂ (25%), ¹⁶O¹⁸O (50%), and ¹⁸O₂ (25%) (a); ¹⁶O₂ (b); or ¹⁸O₂ (c) in CD₂Cl₂ at –80°C. All of the spectra were obtained at 77 K with an excitation wavelength of 647.1 nm. (d) Difference spectrum obtained by subtraction of spectra b and c from a. Curves associated with ν(Fe¹⁶O-¹⁶O) (▪), ν(Fe¹⁶O-¹⁸O) (▴), ν(Fe¹⁸O-¹⁶O) (♦), and ν(Fe¹⁸O-¹⁸O) (•) of comparable linewidth (full width at half maximum ∼ 23 cm^–1) were fit by using the program grams/ai (Thermo Galactic).