Synthesis of Mononuclear Non-Heme Iron(III)-Hydroperoxo Complex as an Oxidative Catalyst

Abstract

Mononuclear FeII complex [FeII(ppba)]2+ (ppba = (6-pivalamido-2-pyridylmethyl) (2-pyridylmethyl)tert-butylamine) (1) has been synthesized and structurally characterized. The addition of H2O2 (10eq) to an acetone solution of 1 at -78°C generated FeIIIPPBA-OOH species, accompanied by a color change from pale yellow to reddish purple, which showed an intense absorption band corresponding to LMCT band (HOO- to Fe) at 528 nm (ε = 1120 M-1cm-1). The ESR spectrum for FeIIIPPBA-OOH species (g// = 4.22, g⊥ = 7.70 in acetone at 77 K) was typical of a high-spin FeIII species, suggesting that the FeIIIPPBA-OOH species is maintained as a mononuclear complex in the solution state. The resonance Raman spectra, when H2 16O2 and H2 18O2 were used, exhibited intense bands characteristic to the O-O stretching vibrations at 880 cm-1 ν(16O-16O) and 824 cm-1 ν(18O-18O), respectively. These findings suggest that FeIIIPPBA-OOH species is [FeIII(ppba)(OOH)]2+ (2). In order to understand the catalytic reactivity of complex 2, oxidative reactions for some organic substrates by complex 2 were carried out in the presence of H2O2; thioanisole was oxidized to the corresponding sulfoxide and sulfone, respectively, and furthermore, cyclohexene was converted to cyclohexene oxide, cyclohexenol and cyclohexenone.