Spectroscopic Characterization of an FeIV Intermediate Generated by Reaction of XO⁻ (X = Cl, Br) with an FeII Complex Bearing a Pentadentate Non‐Porphyrinic Ligand − Hydroxylation and Epoxidation Activity

Mononuclear Fe IV intermediates have been generated in MeOH upon reaction of sodium hypochlorite or hypobromite with a ferrous complex bearing the pentadentate ligands N ‐methyl‐ N , N′ , N′ ‐tris(2‐pyridylmethyl)ethane‐1,2‐diamine (L 5 2 ) or N ‐methyl‐ N , N′ , N′ ‐tris(2‐pyridylmethyl)propane‐1,3‐diamine (L 5 3 ). These highly unstable green complexes are characterized by an absorption band at ca. 750 nm. Mössbauer data indicate that the iron center is low spin ( S = 1) with an axial electronic structure, allowing identification of the mononuclear Fe IV complexes as [L 5 Fe IV O] 2+ or [L 5 Fe IV OCH 3 ] 3+ . In acetonitrile/dichloromethane solutions, the L 5 3 Fe IV system exhibits very selective activities toward hydroxylation of cyclohexane or epoxidation of cyclooctene and cis ‐stilbene. Computational studies performed on [L 5 Fe IV O] 2+ and on the model compound [(NH 3 ) 5 Fe IV O] 2+ reveal that the ground state possesses two unpaired electrons in the two π* orbitals, as known for O 2 . Electronic spectra computed by time‐dependent DFT exhibit only one band in the visible region that is essentially due to d‐d transitions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)