Mononuclear iron complexes relevant to nonheme iron oxygenases. Synthesis, characterizations and reactivity of Fe-Oxo and Fe-Peroxo intermediates

The new ligand L₆²4E (N,N,N′,N′-tetrakis(5-ethyl-2-pyridylmethyl)ethane-1,2-diamine) was designed as a more robust analog of TPEN (N,N,N′,N′-tetrakis(2-pyridylmethyl)ethane-1,2-diamine) for which the ability at stabilizing high valent Fe-Oxo and Fe-(hydro)peroxo has been reported. With respect to the latter, the pyridyl β-substituents in L₆²4E do not modify the Fe coordination chemistry. From the Fe^II precursor, [FeO]²⁺ and Fe^III-(hydro)peroxo intermediates are prepared using the same synthetic methods as those reported for the TPEN analogs. The spectroscopic characteristics of all L₆²4E-Fe complexes are very similar to their TPEN analog. However, [(L₆²4E)FeO]²⁺ has a greater lifetime than that of [(TPEN)FeO]²⁺. This can be explained by a restricted bimolecular autodegradation due to the bulkiness provided by the ethyl substituents. Regarding small organic molecule oxidation, [(L₆²4E)FeO]²⁺ and [(L₆²4E)FeOOH]²⁺ exhibit behaviours that seem to be general for the complexes built with ligands of the TPEN family: [FeO]²⁺ appears to be efficient to epoxidize olefins, whereas [FeOOH]²⁺ hydroxylates the aromatic ring of anisole with efficacy.