On the Mechanism of Iron(III)-Dependent Oxidative Dehydrogenation of Amines

Kinetic and structural data are presented for the iron-promoted dehydrogenation of the amine, [Fe(III)L3]3+ (1), L3 = 1,9-bis(2′-pyridyl)-5-[(ethoxy-2′′-pyridyl)methyl]-2,5,8-triazanonane. Spectroscopic and electrochemical experiments under the exclusion of dioxygen helped to identify reaction intermediates and the final product, the Fe(II)-monoimine complex [Fe(II)L4]2+ (2), L4 = 1,9-bis(2′-pyridyl)-5-[(ethoxy-2′′-pyridyl)methyl]-2,5,8-triazanon-1-ene. 2 is formed by disproportionation of the starting complex 1 by a three-step reaction mechanism, most likely via ligand-centered radical intermediates. The rate law can be described by the second-order rate equation, −d[(Fe(III)L3)3+]/dt = kEtO−[(Fe(III)L3)3+][EtO−], with kEtO− = 4.92 ± 0.01 × 104 M−1 s−1 (60 °C, μ = 0.01 M). The detection of general base catalysis and a primary kinetic isotope effect (kEtO−H/kEtO−D = 1.73) represents the first kinetic demonstration that the deprotonation becomes rate determining followed by electron transfer in the oxidative dehydrogenation mechanism. We also isolated the Fe(II)-monoimine complex 2 and determined its structure in solution (NMR) and in the solid state (X-ray).