Chemical origin of high activity in oxygenation of cyclohexane by H2O2 catalysed by dinuclear iron(III) complexes with amide-containing ligands

Abstract

The crystal structures of two dinuclear iron(III) complexes containing an oxo bridge, [Fe₂OCl₂L₂][ClO₄]₂·2H₂O 1[L =N,N-bis(2-pyridylmethyl)glycinamide] and [Fe₂OCl₂L₂][ClO₄]₂2(L =N-{2-[bis(2-pyridylmethyl)amino]ethyl}morpholine) were determined. Their structural features are quite similar to those of the corresponding linear dinuclear complex [Fe₂OCl₂(tpa)₂][ClO₄]₂, where tpa is tris(2-pyridylmethyl)amine; the ligands act as tetradentate tripods, and the Fe–O (amide) and average Fe–N (morpholine) distances are 2.165(6) and 2.45(1)Å, respectively. Complex 1 exhibited much higher activity for the hydroxylation of cyclohexane in the presence of H₂O₂, while the activities of the other two complexes are negligible. In contrast, all three complexes exhibited high activity for the decomposition of H₂O₂. These results indicate that the active species for oxygenation of cyclohexane, which may be an iron(III)–peroxide adduct (I), should be different from that for decomposition of H₂O₂, adduct II, and that these two species may exist in the solution of complex 1. It is postulated that adduct I may be a dinuclear iron(III)–η¹-hydroperoxide species stabilized through hydrogen bonding between the hydroperoxide ion and the oxygen atom of the amide group. Extended-Hückel molecular orbital calculations showed that the hydrogen bonding may lead to induction of high ‘oxo-like’ activity in the peroxide adduct. In the cases of the tpa and morpholine complexes the formation of a η¹-hydroperoxide adduct seems unfavourable because of both steric and electronic reasons; instead a (µ-η¹:η¹-peroxo)diiron(III) species, adduct II, is formed which induces high catalase-like activity.