CHEMICAL ORIGIN OF HIGH-ACTIVITY IN OXYGENATION OF CYCLOHEXANE BY H2O2 CATALYZED BY DINUCLEAR IRON(III) COMPLEXES WITH AMIDE-CONTAINING LIGANDS

The crystal structures of two dinuclear iron(In) complexes containing an oxo bridge, [Fe(2)OCl(2)L(2)][ClO4](2) . 2H(2)O 1 [L = N,N-bis(2-py ridylmethyl)glycinamide] and [Fe(2)OCl(2)L(2)][ClO4](2) 2 (L = N-{2-[b is(2-pyridylmethyl)amino]ethyl}morpholine) were determined. Their stru ctural features are quite similar to those of the corresponding linear dinuclear complex [Fe2OCl2(tpa)(2)][ClO4](2), where tpa is tris(2-pyr idylmethyl)amine; the ligands act as tetradentate tripods, and the Fe- O (amide) and average Fe-N (morpholine) distances are 2.165(6) and 2.4 5(1) Angstrom, respectively. Complex 1 exhibited much higher activity for the hydroxylation of cyclohexane in the presence of H2O2, while th e activities of the other two complexes are negligible. In contrast, a ll three complexes exhibited high activity for the decomposition of H2 O2. These results indicate that the active species for oxygenation of cyclohexane, which may be an iron(nr)-peroxide adduct (I), should be d ifferent from that for decomposition of H2O2, adduct II, and that thes e two species may exist in the solution of complex 1. It is postulated that adduct I may be a dinuclear iron(III)eta(1)-hydroperoxide specie s stabilized through hydrogen bonding between the hydroperoxide ion an d the oxygen atom of the amide group. Extended-Huckel molecular orbita l calculations showed that the hydrogen bonding may lead to induction of high 'oxo-like' activity in the peroxide adduct. In the cases of th e tpa and morpholine complexes the formation of a eta(1)-hydroperoxide adduct seems unfavourable because of both steric and electronic reaso ns; instead a (mu-eta(1):eta(1)-peroxo)diiron(III) species, adduct II, is formed which induces high catalase-like activity.