Characterization of two distinct adducts in the reaction of a nonheme diiron(II) complex with O₂

Two [Fe^II₂(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 O ₂X is O₂PPh2 (1.O₂PPh₂) or O ₂AsMe₂ (1-O₂AsMe₂), have been synthesized. Their crystal structures both show interiron distances of 3.54 À that arise from a (μ-alkoxo)diiron(II) core supported by an O ₂X 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: ηpexoxo)diiron(III) species (2-O₂X) with ν_0-0 ~ 850 cm^-1. Upon warming to -30 °C, 2-O ₂PPh₂ converts irreversibly to a second (μ- η¹:η¹ -peroxo)diiron(III) intermediate (3-O ₂PPh₂) with v_0-o ~ 900 cm^-1 , a value which matches that reported for [Fe₂(NEtHPTB)(O ₂)(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 Á for 2-O ₂PPh₂ and 3-O₂PPh₂, respectively. A similarly short interiron distance (3.27 Á) is found for 2-O ₂AsMe₂. 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 v_0-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 O ₂X 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-O ₂X (2-O₂Bz) are observed.