Spectroscopic Study of [Fe2(O2)(OBz)2{HB(pz‘)3}2]: Nature of the μ-1,2 Peroxide−Fe(III) Bond and Its Possible Relevance to O2Activation by Non-Heme Iron Enzymes

Abstract

Spectroscopic and theoretical studies of a binuclear cis μ-1,2 peroxide-bridged Fe(III) complex, [Fe2(O2)(OBz)2{HB(pz‘)3}2], where HB(pz‘)3 represents hydrotris(3,5-diisopropyl-1-pyrazolyl)borate and OBz is benzoate, are presented and discussed. It is shown that the high O−O stretching frequency of 876 cm-1 does not reflect a strong O−O bond (kO-O = 3.1 mdyn/Å) but rather results from substantial mechanical coupling between the Fe−O and O−O stretch motions. The coupling strength in μ-1,2 peroxide-bridged complexes is mainly determined by the metal−O−O bond angle, and this angle is larger in the three structurally characterized Fe−peroxide dimers than in related Cu and Co dimers due to a strong π-bonding interaction in the former. Using absorption, magnetic circular dichroism, and resonance Raman spectroscopies we can identify four peroxide-to-Fe charge transfer (CT) transitions contributing to the optical absorption spectrum below 35 000 cm-1. These transitions give rise to two similarly intense bands centered at ∼14 600 and 26 200 cm-1 (ε ∼ 3800 M-1 cm-1) and to weaker features at 12 500 cm-1 (∼400 M-1 cm-1) and 24 650 cm-1 (∼1600 M-1 cm-1). A complete band assignment based on the experimental data and assisted by density functional calculations is presented and provides significant insight into Fe−peroxide bonding. From a quantitative analysis of the optical absorption and resonance Raman data within the framework of time-dependent theory of electronic spectroscopy it follows that the π contribution to the peroxide → Fe charge donation amounts to 50−65%. The σ contribution, however, is smaller than in the well-characterized trans μ-1,2 peroxide-bridged Cu(II) dimer, and the charge on peroxide is thus similar in the two complexes, suggesting that the bridging peroxide in [Fe2(O2)(OBz)2{HB(pz‘)3}2] has a rather basic/nucleophilic character. The mechanistic implications of these results for the conversion of the putative cis μ-1,2 peroxide-bridged diiron(III) intermediate (P or Hperoxo) of methane monooxygenase to a high-valent diiron(IV)−oxo species are discussed.