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)L-3](3+) (1), L-3 = 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)L-4](2+) (2), L-4 = 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)L-3)(3+)]/dt = k(EtO)(-)[(Fe(III)L-3)(3+)][EtO-], with k(EtO)(-) = 4.92 +/- 0.01 x 10(4) M-1 s(-1) (60 degrees C, mu = 0.01 M). The detection of general base catalysis and a primary kinetic isotope effect (k(EtO)-H/k(EtO)-(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).