Functional mimic of dioxygen-activating centers in non-heme diiron enzymes: Mechanistic implications of paramagnetic intermediates in the reactions between diiron(II) complexes and dioxygen

Two tetracarboxylate diiron(II) complexes, [Fe₂(μ-O₂CAr^Tol)₂ (O₂CAr^Tol)₂ (C₅H₅N)₂] (1a) and [Fe₂μ-O₂CAr^Tol)₄ (4-^tBuC₅H₄N)₂] (2a), where Ar^TolCO₂- = 2,6-di(p-tolyl)benzoate, react with 0₂ in CH₂Cl₂ at -78°C to afford dark green intermediates 1b (λ_max ≅ 660 nm; ε = 1600 M^-1 cm^-1) and 2b λmax ≅ 670 nm; ε = 1700 M^-1 cm^-1), respectively. Upon warming to room temperature, the solutions turn yellow, ultimately converting to isolable diiron(III) compounds [Fe₂(μ-OH)₂(μ-0₂CAr^Tol) ₂(O₂CAr^Tol)₂L₂] (L = C₅H₅N (1c), 4-^tBuC₅H₄N (2c)). EPR and Mössbauer spectroscopic studies revealed the presence of equimolar amounts of valence-delocalized Fe^IIFe^III and valence-trapped Fe^IIIFe^IV species as major components of solution 2b. The spectroscopic and reactivity properties of the Fe^IIIFe^IV species are similar to those of the intermediate X in the RNR-R2 catalytic cycle. EPR kinetic studies revealed that the processes leading to the formation of these two distinctive paramagnetic components are coupled to one another. A mechanism for this reaction is proposed and compared with those of other synthetic and biological systems, in which electron transfer occurs from a low-valent starting material to putative high-valent dioxygen adduct(s).