Catalytic Nitrene Homocoupling by an Iron(II) Bis(alkoxide) Complex: Bulking Up the Alkoxide Enables a Wider Range of Substrates and Provides Insight into the Reaction Mechanism

Abstract

The reaction of HOR′ (OR′ = di-t-butyl-(3,5-diphenylphenyl)methoxide) with an iron(II) amide precursor forms the iron(II) bis(alkoxide) complex Fe(OR′)2(THF)2 (2). 2 (5-10 mol %) serves as a catalyst for the conversion of aryl azides into the corresponding azoarenes. The highest yields are observed for aryl azides featuring two ortho substituents; other substitution patterns in the aryl azide precursor lead to moderate or low yields. The reaction of 2 with stoichiometric amounts (2 equiv) of the corresponding aryl azide shows the formation of azoarenes as the only organic products for the bulkier aryl azides (Ar = mesityl, 2,6-diethylphenyl). In contrast, formation of tetrazene complexes Fe(OR′)2(ArNNNNAr) (3-6) is observed for the less bulky aryl azides (Ar = phenyl, 4-methylphenyl, 4-methoxyphenyl, 3,5-dimethylphenyl). The electronic structure of selected tetrazene complexes was probed by spectroscopy (field-dependent 57Fe Mössbauer and high-frequency EPR) and density functional theory calculations. These studies revealed that Fe(OR′)2(ArNNNNAr) complexes contain high-spin (S = 5/2) iron(III) centers exchange-coupled to tetrazene radical anions. Tetrazene complexes Fe(OR′)2(ArNNNNAr) produce the corresponding azoarenes (ArNNAr) upon heating. Treatment of a tetrazene complex Fe(OR′)2(ArNNNNAr) with a different azide (N3Ar′) produces all three possible products ArNNAr, ArNNAr′, and Ar′NNAr′. These experiments and quantum mechanics/molecular mechanics calculations exploring the reaction mechanism suggest that the tetrazene functionality serves as a masked form of the reactive iron mono(imido) species.