Characterization of Two Distinct Adducts in the Reaction of a Nonheme Diiron(II) Complex with O₂

Two [FeII2(N-EtHPTB)(μ-O2X)]2+ complexes, where N-EtHPTB is the anion of N,N,N′N′-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane and O2X is O2PPh2 (1·O2PPh2) or O2AsMe2 (1·O2AsMe2), have been synthesized. Their crystal structures both show interiron distances of 3.54 Å that arise from a (μ-alkoxo)diiron(II) core supported by an O2X bridge. These diiron(II) complexes react with O2 at low temperatures in MeCN (−40 °C) and CH2Cl2 (−60 °C) to form long-lived O2 adducts that are best described as (μ-η1:η1-peroxo)diiron(III) species (2·O2X) with νO−O ∼ 850 cm−1. Upon warming to −30 °C, 2·O2PPh2 converts irreversibly to a second (μ-η1:η1-peroxo)diiron(III) intermediate (3·O2PPh2) with νO−O ∼ 900 cm−1, a value which matches that reported for [Fe2(N-EtHPTB)(O2)(O2CPh)]2+ (3·O2CPh) (Dong et al. J. Am. Chem. Soc. 1993, 115, 1851−1859). Mössbauer spectra of 2·O2PPh2 and 3·O2PPh2 indicate that the iron centers within each species are antiferromagnetically coupled with J ∼ 60 cm−1, while extended X-ray absorption fine structure analysis reveals interiron distances of 3.25 and 3.47 Å for 2·O2PPh2 and 3·O2PPh2, respectively. A similarly short interiron distance (3.27 Å) is found for 2·O2AsMe2. The shorter interiron distance associated with 2·O2PPh2 and 2·O2AsMe2 is proposed to derive from a triply bridged diiron(III) species with alkoxo (from N-EtHPTB), 1,2-peroxo, and 1,3-O2X bridges, while the longer distance associated with 3·O2PPh2 results from the shift of the O2PPh2 bridge to a terminal position on one iron. The differences in νO−O are also consistent with the different interiron distances. It is suggested that the O···O bite distance of the O2X moiety affects the thermal stability of 2·O2X, with the O2X having the largest bite distance (O2AsMe2) favoring the 2·O2X adduct and the O2X having the smallest bite distance (O2CPh) favoring the 3·O2X adduct. Interestingly, neither 3·O2AsMe2 nor the benzoate analog of 2·O2X (2·O2Bz) are observed.