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-O2CArTol)2(O2CArTol)2(C5H5N)2] (1a) and [Fe2(μ-O2CArTol)4(4-tBuC5H4N)2] (2a), where ArTolCO2- = 2,6-di(p-tolyl)benzoate, react with O2 in CH2Cl2 at
−78 °C to afford dark green intermediates 1b (λmax ≅ 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-O2CArTol)2(O2CArTol)2L2] (L = C5H5N (1c),
4-tBuC5H4N (2c)). EPR and Mössbauer spectroscopic studies revealed the presence of equimolar amounts
of valence-delocalized FeIIFeIII and valence-trapped FeIIIFeIV species as major components of solution 2b.
The spectroscopic and reactivity properties of the FeIIIFeIV 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).