Dioxygen Binding to the Singly Alkoxo-Bridged Diferrous Complex: Properties of [<TEX>$Fe^{Ⅱ}_2$</TEX>(N-Et-HPTB)<TEX>$Cl_2$</TEX>]<TEX>$BPh_4$</TEX>

[FeⅡ2(N-Et-HPTB)Cl2]BPh4(1), where N-Et-HPTB is the anion of N,N,N',N'-tetrakis(N-ethyl-2-benzimidazolylmethyl)-2-hydroxy-l,3-diaminopropane, has been synthesized to model dioxygen binding to the diferrous centers of proteins. 1 has a singly bridged structure with a μ-alkoxo of N-Et-HPTB and contains two five-coordinate iron(Ⅱ) centers with two chloride ligands as exogenous ligands. 1 exhibits an electronic spectrum with a λmax at 336 nm in acetone. 1 in acetone exhibits no EPR signal at 4 K, indicating diiron(Ⅱ) centers are antiferromagnetically coupled. Exposure of acetone solution of 1 to O2 at -90 ℃ affords an intense blue color intermediate showing a broad band at 586 nm. This absorption maximum of the dioxygen adduct(1/O2) was found in the same region of μ-l,2-peroxo diiron(Ⅲ) intermediates in the related complexes with pendant pyridine or benzimidazole ligand systems. However, this blue intermediate exhibits EPR signals at g = 1.93, 1.76, and 1.59 at 4 K. These g values are characteristic of S = 1/2 system derived from an antiferromagnetically coupled high-spin Fe(Ⅱ)Fe(Ⅲ) units. 1 is the unique example of a (μ-alkoxo)diferrous complex which can bind dioxygen and form a metastable mixed-valence intermediate. At ambient temperature, most of 1/O2 intermediate decays to form a diamagnetic species. It suggests that the dacay reaction of the intermediate might be bimolecular, implying the formation of mixed-valence tetranuclear species in transition state.