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.本研究报道了单核五配位亚铁配合物[([15]aneN4)FeII(SPh)](BF4)（化合物1）的合成。该配合物是超氧化物还原酶（superoxide reductase, SOR）还原活性位点的新型模型，其活性中心结构为[(NHis)4(Scys)FeII]。配合物1可在低温下与烷基氢过氧化物（叔丁基氢过氧化物tBuOOH、枯基氢过氧化物cumenylOOH）反应，生成亚稳态深红色中间体（2a：R = tBu；2b：R = cumenyl），并通过紫外-可见（UV-vis）光谱、电子顺磁共振（EPR）光谱及共振拉曼（resonance Raman）光谱对其进行了表征。-80 ℃下的紫外-可见光谱显示，中间体2a在526 nm处具有特征吸收峰（摩尔消光系数ε = 2150 M⁻¹·cm⁻¹），中间体2b在527 nm处具有特征吸收峰（ε = 1650 M⁻¹·cm⁻¹），该结果印证了烷基过氧根到三价铁的配体到金属电荷转移（LMCT）跃迁的存在；77 K下的电子顺磁共振数据表明，两种中间体均为低自旋三价铁配合物，其g值分别为2.20与1.97。共振拉曼光谱对Fe(III)−OOR物种的精准鉴定结果显示，2a和2b的ν(Fe−O)分别为612 cm⁻¹和615 cm⁻¹，ν(O−O)分别为803 cm⁻¹和795 cm⁻¹。通过对叔丁基氢过氧化物进行氧18同位素取代（tBu18O18OH）验证了2a的谱峰归属：ν(Fe−18O)的波数降低了28 cm⁻¹，ν(18O−18O)的波数降低了46 cm⁻¹。上述实验数据表明，2a和2b均为Fe−O键较弱的低自旋FeIII−OOR物种，这提示超氧化物还原酶的催化过程中可能存在低自旋中间体，而非此前提出的高自旋中间体假说。