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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 [FeIIIL3]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 [FeIIL4]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[(FeIIIL3)3+]/dt = kOD[(FeIIIL3)3+][EtO-][O2], with kOD = 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[(FeIIIL3)3+]/dt = kH2O2[(FeIIIL3)3+][H2O2]), with kH2O2 = 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 [FeIIL4]2+ and the iron(ii)-amine complex [FeIIL3]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 kEtO- = 3.02 ± 0.09 × 105 M-1 s-1 (O2) compared to kEtO- = 4.92 ± 0.01 × 104 M-1 s-1 (N2), documenting the role of molecular oxygen in the dehydrogenation reaction.