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)FeII(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 [(NHis)4(Scys)FeII] 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 FeIII−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.