Bachmann Julien, Nocera Daniel G
Department of Chemistry, 6-335, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4207, USA.
J Am Chem Soc. 2005 Apr 6;127(13):4730-43. doi: 10.1021/ja043132r.
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 (E(1/2) = -0.57 V vs NHE in CH(3)CN) when confined within the fully reduced macrocycle. The porphyrinogen ligand also undergoes oxidation but in a single four-electron step (E(p) = +0.77 V vs NHE in CH(3)CN); 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 C(alpha)-C(alpha) bonds of spirocyclopropane rings, formed between adjacent pyrroles. EPR, magnetic and Mossbauer 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 sigma-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.
制备了八甲基卟吩铁，并在不存在轴向配体且仅以钠或四氟硼酸根作为抗衡离子的情况下，对其三种不同氧化态进行了结构表征。在这些条件下，可以确定卟吩铁基于铁和配体的氧化还原化学性质。当铁中心被限制在完全还原的大环内时，它很容易被单电子氧化（在乙腈中相对于标准氢电极，E(1/2) = -0.57 V）。卟吩配体也会发生氧化，但这是一个单步四电子过程（在乙腈中相对于标准氢电极，E(p) = +0.77 V）；基于配体的一个电子被截获用于将Fe(III)还原为Fe(II)，从而导致一个整体的三电子氧化过程。大环中的氧化当量存储在相邻吡咯之间形成的螺环丙烷环的C(α)-C(α)键中。对卟吩铁氧化还原态的电子顺磁共振、磁性和穆斯堡尔测量以及密度泛函理论计算表明，还原的配体使铁处于中间自旋态，而完全氧化的配体具有较弱的σ-供体框架，从而产生高自旋铁。综上所述，本文报道的结果建立了一种金属-大环协同作用，这种协同作用为卟吩铁带来了血红素辅因子所没有的多电子化学性质。
