Multielectron Redox Chemistry of Iron Porphyrinogens

Iron octamethylporphyrinogens were prepared and structurally characterized in three different
oxidation states in the absence of axial ligands and with sodium or tetrafluoroborate as the only counterions.
Under these conditions, the iron- and ligand-based redox chemistry of iron porphyrinogens can be defined.
The iron center is easily oxidized by a single electron (E1/2 = −0.57 V vs NHE in CH3CN) when confined
within the fully reduced macrocycle. The porphyrinogen ligand also undergoes oxidation but in a single
four-electron step (Ep = +0.77 V vs NHE in CH3CN); one of the ligand-based electrons is intercepted for
the reduction of Fe(III) to Fe(II) to result in an overall three-electron oxidation process. The oxidation
equivalents in the macrocycle are stored in Cα−Cα bonds of spirocyclopropane rings, formed between
adjacent pyrroles. EPR, magnetic and Mössbauer measurements, and DFT computations of the redox
states of the iron porphyrinogens reveal that the reduced ligand gives rise to iron in intermediate spin
states, whereas the fully oxidized ligand possesses a weaker σ-donor framework, giving rise to high-spin
iron. Taken together, the results reported herein establish a metal−macrocycle cooperativity that engenders
a multielectron chemistry for iron porphyrinogens that is unavailable to heme cofactors.

我们制备了八甲基卟啉铁配合物（Iron octamethylporphyrinogens），并在无轴向配体、仅以钠离子或四氟硼酸盐作为唯一抗衡离子的条件下，对其三种不同氧化态的结构进行了表征。在此实验条件下，八甲基卟啉铁配合物中铁中心与配体的氧化还原化学行为可被明确界定。当铁中心处于完全还原的大环骨架中时，可通过单电子氧化过程被轻易氧化（在乙腈（CH3CN）溶剂中，相对于标准氢电极（Normal Hydrogen Electrode, NHE）的半波电位E1/2 = -0.57 V）。卟啉原配体自身也会发生氧化，但该过程为一步四电子过程（在乙腈（CH3CN）溶剂中，氧化峰电位Ep = +0.77 V vs NHE）；其中一个配体基电子会被用于将Fe(III)还原为Fe(II)，因此整体表现为三电子氧化过程。大环内的氧化当量会储存于相邻吡咯之间形成的螺环丙烷环（spirocyclopropane rings）的Cα−Cα键中。通过电子顺磁共振（Electron Paramagnetic Resonance, EPR）、磁学测试、穆斯堡尔谱（Mössbauer）测量以及密度泛函理论（Density Functional Theory, DFT）计算对八甲基卟啉铁的氧化态进行表征后发现：还原态配体体系会使铁中心呈现中间自旋态，而完全氧化态配体的σ给电子骨架（σ-donor framework）更弱，因此铁中心为高自旋态。综合上述研究结果，本工作证实了金属-大环协同效应可赋予八甲基卟啉铁多电子氧化还原化学特性，而这一特性是血红素辅因子（heme cofactors）所不具备的。