A low-spin alkylperoxo-iron(III) complex with weak Fe-O and O-O bonds: Implications for the mechanism of superoxide reductase

The synthesis of a mononuclear, five-coordinate ferrous complex [([15]aneN₄)Fe^II(SPh)](BF₄) (1) is reported. This complex is a new model of the reduced active site of the enzyme superoxide reductase (SOR), which is comprised of a [(N_His)₄(S_cys)Fe^II] center. Complex 1 reacts with alkylhydroperoxides (tBuOOH, cumenylOOH) at low temperature to give a metastable, dark red intermediate (2a: R = tBu; 2b: R = cumenyl) that has been characterized by UV-vis, EPR, and resonance Raman spectroscopy. The UV-vis spectrum (-80 °C) reveals a 526 nm absorbance (ε = 2150 M^-1 cm^-1) for 2a and a 527 nm absorbance (ε = 1650 M^-1 cm^-1) for 2b, indicative of alkylperoxo-to-iron(III) LMCT transitions, and the EPR data (77 K) show that both intermediates are low-spin iron(III) complexes (g = 2.20 and 1.97). Definitive identification of the Fe(III)-OOR species comes from RR spectra, which give ν(Fe-O) = 612 (2a) and 615 (2b) cm^-1, and ν(O-O) = 803 (2a) and 795 (2b) cm^-1. The assignments for 2a were confirmed by ¹⁸O substitution (tBu¹⁸O¹⁸OH), resulting in a 28 cm^-1 downshift for ν(Fe-¹⁸O), and a 46 cm^-1 downshift for ν(¹⁸O-¹⁸O). These data show that 2a and 2b are low-spin Fe^III-OOR species with weak Fe-O bonds and suggest that a low-spin intermediate may occur in SOR, as opposed to previous proposals invoking high-spin intermediates.