Photoinitiated Reactivity of a Thiolate-Ligated, Spin-Crossover Nonheme {FeNO}7 Complex with Dioxygen

The nonheme iron complex, [Fe­(NO)­(N3PyS)]­BF4, is a rare example of an {FeNO}7 species that exhibits spin-crossover behavior. The comparison of X-ray crystallographic studies at low and high temperatures and variable-temperature magnetic susceptibility measurements show that a low-spin S = 1/2 ground state is populated at 0–150 K, while both low-spin S = 1/2 and high-spin S = 3/2 states are populated at T > 150 K. These results explain the observation of two N–O vibrational modes at 1737 and 1649 cm–1 in CD3CN for [Fe­(NO)­(N3PyS)]­BF4 at room temperature. This {FeNO}7 complex reacts with dioxygen upon photoirradiation with visible light in acetonitrile to generate a thiolate-ligated, nonheme iron­(III)-nitro complex, [FeIII(NO2)­(N3PyS)]+, which was characterized by EPR, FTIR, UV–vis, and CSI-MS. Isotope labeling studies, coupled with FTIR and CSI-MS, show that one O atom from O2 is incorporated in the FeIII–NO2 product. The O2 reactivity of [Fe­(NO)­(N3PyS)]­BF4 in methanol is dramatically different from CH3CN, leading exclusively to sulfur-based oxidation, as opposed to NO· oxidation. A mechanism is proposed for the NO· oxidation reaction that involves formation of both FeIII-superoxo and FeIII-peroxynitrite intermediates and takes into account the experimental observations. The stability of the FeIII-nitrite complex is limited, and decay of [FeIII(NO2)­(N3PyS)]+ leads to {FeNO}7 species and sulfur oxygenated products. This work demonstrates that a single mononuclear, thiolate-ligated nonheme {FeNO}7 complex can exhibit reactivity related to both nitric oxide dioxygenase (NOD) and nitrite reductase (NiR) activity. The presence of the thiolate donor is critical to both pathways, and mechanistic insights into these biologically relevant processes are presented.非血红素铁配合物（nonheme iron complex）[Fe(NO)(N3PyS)]BF4是一类罕见的{FeNO}7类物种，展现出自旋交叉行为（spin-crossover behavior）。对其开展的低温与高温X射线晶体学研究，以及变温磁化率测试结果表明：在0~150 K区间内，体系仅以低自旋S=1/2基态为主要占据态；而当温度高于150 K时，体系同时存在低自旋S=1/2与高自旋S=3/2两种占据态。上述结果解释了室温下在氘代乙腈（CD3CN）中检测到[Fe(NO)(N3PyS)]BF4的两处N-O振动模式（波数分别为1737 cm–1与1649 cm–1）的实验现象。该{FeNO}7配合物在乙腈溶剂中经可见光光辐照后可与分子氧发生反应，生成硫醇盐配位的非血红素铁(III)-硝基配合物[FeIII(NO2)(N3PyS)]+，该产物已通过电子顺磁共振波谱（EPR）、傅里叶变换红外光谱（FTIR）、紫外-可见吸收光谱（UV-vis）以及碰撞诱导解离质谱（CSI-MS）完成表征。结合傅里叶变换红外光谱与碰撞诱导解离质谱开展的同位素标记实验表明，氧气中的一个氧原子被掺入至该FeIII-NO2产物中。[Fe(NO)(N3PyS)]BF4在甲醇中的氧气反应活性与在乙腈中存在显著差异，仅发生硫基氧化反应，而非NO·自由基氧化反应。本文针对NO·氧化反应提出了一条反应机理，该机理涉及FeIII-超氧与FeIII-过氧亚硝基两类中间体的生成，且契合所有实验观测结果。该FeIII-亚硝酸盐配合物稳定性有限，[FeIII(NO2)(N3PyS)]+降解后会生成{FeNO}7物种与硫氧化产物。本研究证实，单核硫醇盐配位的非血红素{FeNO}7配合物可同时展现出与一氧化氮双加氧酶（NOD）及亚硝酸盐还原酶（NiR）相关的催化活性。硫醇盐给体在两条反应路径中均发挥关键作用，本文同时阐明了上述与生物过程相关的反应机理。