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 O2 activation by non-heme iron enzymes

Spectroscopic and theoretical studies of a binuclear cis mu-1,2 peroxide-bridged Fe(III) complex, [Fe(2)(O(2))(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 (k(O-O) = 3.1 mdyn/Angstrom) but rather results from substantial mechanical coupling between the Fe-O and O-O stretch motions. The coupling strength in mu-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 pi-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 similar to 14 600 and 26 200 cm(-1) (E similar to 3800 M(-1) cm(-1)) and to weaker features at 12 500 cm(-1) (similar to 400 M(-1) cm(-1)) and 24 650 cm(-1) (similar to 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 pi contribution to the peroxide --> Fe charge donation amounts to 50-65%. The sigma contribution, however, is smaller than in the well-characterized trans mu-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(O(2))(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 mu-1,2 peroxide-bridged diiron(III) intermediate (P or H(peroxo)) of methane monooxygenase to a high-valent diiron(IV)-oxo species are discussed.