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 linea r mu-oxo bridge, Fe2OCl2(tfpy)(2)(ClO4)(2) . 2CH(3)CN 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 essen tially the same as that of the corresponding linear binuclear complex with (tpa)-complex, Fe2OCl2(tpa)(2)(ClO4)(2), where (tpa) is tris(2-py ridylmethyl)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 ore-oxygen atom) distances are 2.209 (4) and 2.264(2)A 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 t hat of the (tpa) complex. In contrast to this, the former two complexe s exhibit negligible activity for the decomposition of hydrogen peroxi de, whereas the catalase-like function of the (tpa) compound is remark able. These are indicating that an active species for oxygenation of c yclohexane, which is assumed to be an iron(III)-hydroperoxide adduct w ith eta(1)-coordination mode, should be different from that is operati ng for decomposition of hydrogen peroxide; for the latter case formati on of a (mu-eta(1):eta(1)-peroxo)diiron(III) species being stressed. T he EHMO calculation showed that electronic interaction between the mon odentate hydroperoxide adduct of the binuclear iron(III)-(tfpy) compou nd and the tetrahydrofuran ring of the ligand system may lead to facil e peroxide-tetrahydrofuran linkage formation, and the interaction desc ribed above should promote the O-O cleavage of the peroxide ion hetero lytically. Based on these discussions, it was concluded that heterolyt ic O-O bond cleavage of the iron(III)-hydroperoxide adduct caused by e lectronic interaction with organic moiety containing an ethereal-oxyge n and by approach of the substrate which donates electron to the perox ide adduct should play an important role in producing a high-valent ir on-ore species in these systems. In the case of (tpa) complex, formati on of a hydroperoxide adduct linking with the ligand system seems to b e unfavorable because of both the steric and electronic reasons.