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

• CH/CD kinetic isotope effect supports the O 2 HAT of a Fe-polyamine ODH. • Aminyl radical favors CH cleavage of α-methylene and HAT towards O 2 . • 2 H NMR shows a selective C H/C-D breaking of the 2-pyridylmethyl-amine in the ODH. • O 2 is reduced via HAT activating a CH of a 2-pyridylmethyl-amine iron compound. • A radical HAT mechanism pathway for Fe(III)-2-pyridylmethyl-amine ODH is presented. The Fe(III) complex, [Fe III L 3 ] 3+ ( 1 ) (L 3 , 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 (N 2 ) and oxic (O 2 ) conditions to form the stable mono-imine Fe(II) complex, [Fe II L 4 ] 2+ ( 2 ) (L 4 , 1,9-bis(2′-pyridyl)-5-[(ethoxy-2″-pyridyl)methyl]-2,5,8-triazanon-1-ene). This process involves the aminyl radical [Fe II (L 3 N• )] 2+ ( 2a ) and is shown to proceed with a rate-determining step of cleaving the C H bond through the interaction of O 2 , with a kinetic isotope effect C H k CH /k CD = 2.30. 2 H NMR of the deuterated ligand L 2 -D and product [Fe II L 4 -D] 2+ , indicates a regioselective C H cleavage promoted by O 2 through the hydrogen atom transfer (HAT) mechanism, generating HO 2 • . It is also observed that O 2 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.