Dioxygen binding to the singly alkoxo-bridged diferrous complex: Properties of [Fe-2(II)(N-Et-HPTB)Cl-2]BPh(4)

Abstract

[Fe-2(II)(N-Et-HPTB)Cl-2] BPh(4) (1), where N-Et-HPTB is the anion of N'N,N',N'-tetrakis(N-ethyl-2-benzimidazolyl methyl)2-hydroxy-1,3-diaminopropane, has been synthesized to model dioxygen binding to the diferrous centers of proteins. 1 has a singly bridged structure with a mu-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 lambda(max) 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 O-2 at -90 degrees C affords an intense blue color intermediate showing a broad band at 586 nm. This absorption maximum of the dioxygen adduct(1/O-2) was found in the same region of mu-1,2-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 (mu-alkoxo)diferrous complex which can bind dioxygen and form a metastable mixed-valence intermediate. At ambient temperature, most of 1/O-2 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.