Fe(III)-promoted oxidative dehydrogenation of amines by O2 – Mediated cleavage of C–H bond proceeds via hydrogen atom transfer (HAT)

Abstract

The Fe(III) complex, [Fe^IIIL³]³⁺ (1) (L³, 1,9-bis(2′-pyridyl)-5-[(ethoxy-2″-pyridyl)methyl]-2,5,8-triazanonane), is novel because it undergoes an oxidative dehydrogenation reaction in both anoxic (N₂) and oxic (O₂) conditions to form the stable mono-imine Fe(II) complex, [Fe^IIL⁴]²⁺ (2) (L⁴, 1,9-bis(2′-pyridyl)-5-[(ethoxy-2″-pyridyl)methyl]-2,5,8-triazanon-1-ene). This process involves the aminyl radical [Fe^II(L³_N•)]²⁺ (2a) and is shown to proceed with a rate-determining step of cleaving the CH bond through the interaction of O₂, with a kinetic isotope effect CH k^CH/k^CD = 2.30. ²H NMR of the deuterated ligand L²-D and product [Fe^IIL⁴-D]²⁺, indicates a regioselective CH cleavage promoted by O₂ through the hydrogen atom transfer (HAT) mechanism, generating HO₂^•. It is also observed that O₂ does not coordinate to the Fe center to form a high-valent Fe oxo species observed in enzymes and their biomimetic complexes. The values for the activation enthalpy ΔH^≠ = 20.38 kcal mol^-1 and the activation entropy ΔS^≠ = −0.018 kcal mol^-1 K⁻¹ fall within the range typically reported for HAT reactions.