The role of molecular oxygen in the iron(iii)-promoted oxidative dehydrogenation of amines

Abstract

A mechanistic study is presented of the oxidative dehydrogenation of the iron(III) complex [Fe^IIIL³]³⁺, 1, (L³ = 1,9-bis(2′-pyridyl)-5-[(ethoxy-2′′-pyridyl)methyl]-2,5,8-triazanonane) in ethanol in the presence of molecular oxygen. The product of the reaction was identified by NMR spectroscopy and X-ray crystallography as the identical monoimine complex [Fe^IIL⁴]²⁺, 2, (L⁴ = 1,9-bis(2′-pyridyl)-5-[(ethoxy-2′′-pyridyl)methyl]-2,5,8-triazanon-1-ene) also formed under an inert nitrogen atmosphere. Molecular oxygen is an active player in the oxidative dehydrogenation of iron(III) complex 1. Reduced oxygen species, e.g., superoxide, (O₂˙⁻) and peroxide (O₂²⁻), are formed and undergo single electron transfer reactions with ligand-based radical intermediates. The experimental rate law can be described by the third order rate equation, −d[(Fe^IIIL³)³⁺]/dt = k_OD[(Fe^IIIL³)³⁺][EtO⁻][O₂], with k_OD = 3.80 ± 0.09 × 10⁷ M⁻² s⁻¹ (60 °C, μ = 0.01 M). The reduction O₂ → O₂˙⁻ represents the rate determining step, with superoxide becoming further reduced to peroxide as shown by a coupled heme catalase assay. In an independent study, with H₂O₂, replacing O₂ as the oxidant, the experimental rate law depended on [H₂O₂]: −d[(Fe^IIIL³)³⁺]/dt = k_H2O2[(Fe^IIIL³)³⁺][H₂O₂]), with k_H2O2 = 6.25 ± 0.02 × 10⁻³ M⁻¹ s⁻¹. In contrast to the reaction performed under N₂, no kinetic isotope effect (KIE) or general base catalysis was found for the reaction of iron(III) complex 1 with O₂. Under N₂, two consecutive one-electron oxidation steps of the ligand coupled to proton removal produced the iron(II)-monoimine complex [Fe^IIL⁴]²⁺ and the iron(II)-amine complex [Fe^IIL³]²⁺ in a 1 : 1 ratio (disproportionation), with the amine deprotonation being the rate determining step. Notably, the reaction is almost one order of magnitude faster in the presence of O₂, with k_EtO⁻ = 3.02 ± 0.09 × 10⁵ M⁻¹ s⁻¹ (O₂) compared to k_EtO⁻ = 4.92 ± 0.01 × 10⁴ M⁻¹ s⁻¹ (N₂), documenting the role of molecular oxygen in the dehydrogenation reaction.