μ-oxo bridged diiron(III) complexes and hydrogen peroxide: Oxygenation and catalase-like activities

The crystal structures of two dinuclear iron(III) compounds with a bent μ-oxo bridge, [Fe₂O(CH₃CO₂)(tpa)₂][ClO ₄]₃· 2H₂O and [Fe₂O(CH₃CO₂)(mep)₂][ClO ₄]₃, were determined, where tpa and mep represent tris(2-pyridylmethyl)-amine and N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)ethane-1,2-diamine, respectively. Their structural features which include a (μ-oxo)(μ-acetato)diiron(III) core are very similar. In spite of this, these two complexes exhibit very different reactivity towards hydrogen peroxide in solution; the tpa complex showed high catalase-like activity, whereas that of the mep complex was negligible. In the oxygenation of cyclohexane the mep complex-H₂O₂ system exhibited high activity giving cyclohexanol and cyclohexanone, but formation of oxygenated products was negligible with the tpa complex-H₂O₂. The absorption spectrum of the tpa complex shows no change in the presence of H₂O₂; however a drastic change was observed in the spectrum of the mep complex on addition of H₂O₂. Based on the structural and spectroscopic data, the structures of the active species for catalase- and oxygenase-like functions and the origin of the differences in reactivity observed for these two compounds are discussed.