Chemical Basis for High Activity in Oxygenation of Cyclohexane Catalyzed by Dinuclear Iron(III) Complexes with Ethereal Oxygen Containing Ligand and Hydrogen Peroxide System
The crystal structures of two binuclear iron(III) complexes with linear μ-oxo bridge, Fe2OCl2 (tfpy)2 (ClO4)2 ·2CH3CN and Fe2OCl2(epy)2(ClO4)2 were determined, where (tfpy) and (epy) represent N,N-bis(2-pyridylmethyl)-tetrahydrofurfurylamine and N,N-bis(2-pyridylmethyl)-2-ethoxyethylamine, respectively. Their structural features are essentially the same as that of the corresponding linear binuclear complex with (tpa)-complex, Fe2OCl2(tpa)2(ClO4)2, where (tpa) is tris(2-pyridylmethyl)amine; the ligands (tfpy) and (epy) act as a tetradentate tripod-like ligand, and Fe-O (ethereal oxygen atom; these are located at the trans-position of bridging oxo-oxygen atom) distances are 2.209(4) and 2.264(2) Å for (tfpy) and (epy) compounds, respectively. These two (tfpy) and (epy) complexes exhibited much higher activity for the oxygenation of cyclohexane in the presence of hydrogen peroxide than that of the (tpa) complex. In contrast to this, the former two complexes exhibit negligible activity for the decomposition of hydrogen peroxide, whereas the catalase-like function of the (tpa) compound is remarkable. These are indicating that an active species for oxygenation of cyclohexane, which is assumed to be an iron(III)-hydroperoxide adduct with η1-coordination mode, should be different from that is operating for decomposition of hydrogen peroxide; for the latter case formation of a (μ-η1:η1-peroxo)diiron(III) species being stressed. The EHMO calculation showed that electronic interaction between the monodentate hydroperoxide adduct of the binuclear iron(IIl)-(tfpy) compound and the tetrahydrofuran ring of the ligand system may lead to facile peroxide-tetrahydrofuran linkage formation, and the interaction described above should promote the O-O cleavage of the peroxide ion heterolytically. Based on these discussions, it was concluded that heterolytic O-O bond cleavage of the iron(III)-hydroperoxide adduct caused by electronic interaction with organic moiety containing an ethereal-oxygen and by approach of the substrate which donates electron to the peroxide adduct should play an important role in producing a high-valent iron-oxo species in these systems. In the case of (tpa) complex, formation of a hydroperoxide adduct linking with the ligand system seems to be unfavorable because of both the steric and electronic reasons.