Catalytic Four-Electron Reduction of Dioxygen by Ferrocene Derivatives with a Nonheme Iron(III) TAML Complex

A mononuclear nonheme iron(III) complex with a tetraamido macrocyclic ligand (TAML), [(TAML)Fe-III](-) (1), is a selective precatalyst for four-electron reduction of dioxygen by ferrocene derivatives in the presence of acetic acid (CH3COOH) in acetone. This is the first work to show that a nonheme iron(III) complex catalyzes the four-electron reduction of O-2 by one-electron reductants. An iron(V)-oxo complex, [(TAML)Fe-v(O)](-) (2), was produced by oxygenation of 1 with O-2 via the formation of triacetone triperoxide (TATP), acting as an autocatalyst that shortened the induction time for the generation of 2. Decamethylferrocene (Me(10)Fc) and octamethylferrocene (Me(8)Fc) reduced 2 to 1 by two electrons in the presence of CH3COOH to produce decamethylferrocenium cation (Me(10)Fc(+)) and octamethylferrocenium cation (Me(8)Fc(+)), respectively. Then, 1 was oxygenated by O-2 to regenerate 2 via the formation of TATP. In the cases of ferrocene (Fc), bromoferrocene (BrFc) and 1,1'-dibromoferrocene (Br(2)Fc), initial electron transfer from ferrocene derivatives to 2 occurred; however, neither a second proton-coupled electron transfer from ferrocene derivatives to 2 nor a catalytic four-electron reduction of O-2 occurred.