Switching orientation of two axial imidazole ligands between parallel and perpendicular in low-spin Fe(III) and Fe(II) nonplanar porphyrinates.
Switching orientation of two axial imidazole ligands between parallel and perpendicular in low-spin Fe(III) and Fe(II) nonplanar porphyrinates.
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DOI：
10.1021/ic300229u
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发表时间：
2012-10
影响因子：
4.6
通讯作者：
R. Patra;Dipankar Sahoo;Soumyajit Dey;Debangsu Sil;S. P. Rath
中科院分区：
文献类型：
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作者：
R. Patra;Dipankar Sahoo;Soumyajit Dey;Debangsu Sil;S. P. Rath
我们在此报道了5,10,15,20-四(五氟苯基)-2,3,7,8,12,13,17,18-八氯卟啉、[Fe(III)(TFPPCl(8))(L)(2)]ClO(4)和低自旋双咪唑配位Fe(III)和Fe(II)配合物的合成、结构和性质Fe(II)(TFPPCl(8))(L)(2)（L = 1-甲基咪唑、4-甲基咪唑、咪唑）。本文报道了 Fe(II)(TFPPCl(8))(1-MeIm)(2) 的 X 射线结构，证明了 Fe(II) 卟啉在高度鞍形扭曲的大环环境中具有近乎垂直的轴向配体取向（两个 1-甲基咪唑之间的二面角为 80.7°）。使用高氯酸铊氧化 Fe(II)(TFPPCl(8))(L)(2) 生成 [Fe(III)(TFPPCl(8))(L)(2)]ClO(4)，同样以固态分离并通过光谱表征。该配合物在 77 K 下在固相和溶液相中均给出了菱形 EPR 光谱，因此代表了鞍形畸变卟啉大环中无阻碍咪唑的近平行轴向配体取向的罕见例子。当使用 1-甲基咪唑作为轴向配体时（两个轴向配体之间的二面角为 8.6°），使用 DFT 进行的几何优化也收敛于平行轴向排列。势能面 (PES) 扫描结果还表明，相对平行的轴向取向对于 Fe(III) 来说在能量上是首选，而垂直取向对于本文报道的 Fe(II) 配合物来说是首选。 Fe(II)(TFPPCl(8))(L)(2) 在二氯甲烷中，在氮气下，在恒定电势下进行本体氧化，将其转化为 [Fe(III)(TFPPCl(8))(L)(2)]ClO(4)，在 77 K 下给出相同的 EPR 谱图，还原后再次生成 Fe(II)(TFPPCl(8))(L)(2)。因此，我们在这里展示了非常罕见的铁卟啉例子，其中在非平面卟啉环境中，仅当铁的氧化态从+3变为+2时，相对轴向咪唑取向就在平行和垂直之间切换。这些观察结果对于理解轴向组氨酸方向对各种血红素蛋白中观察到的类似大环变形的可能影响非常重要。
We have reported here the synthesis, structure, and properties of low-spin bis-imidazole-coordinated Fe(III) and Fe(II) complexes of 5,10,15,20-tetrakis(pentafluorophenyl)-2,3,7,8,12,13,17,18-octachloroporphyrin, [Fe(III)(TFPPCl(8))(L)(2)]ClO(4) and Fe(II)(TFPPCl(8))(L)(2) (L = 1-methylimidazole, 4-methylimidazole, imidazole). The X-ray structure of Fe(II)(TFPPCl(8))(1-MeIm)(2) is reported here, which demonstrated the near-perpendicular axial ligand orientation (dihedral angle between two 1-methylimidazoles is 80.7°) for Fe(II) porphyrins in a highly saddle-distorted macrocyclic environment. Oxidation of Fe(II)(TFPPCl(8))(L)(2) using thianthrenium perchlorate produces [Fe(III)(TFPPCl(8))(L)(2)]ClO(4), which was also isolated in the solid state and characterized spectroscopically. The complex gives rhombic EPR spectra in both solid and solution phases at 77 K and thus represents a rare example of nearly parallel axial ligand orientations for the unhindered imidazoles in a saddle-distorted porphyrin macrocycle. Geometry optimization using DFT also converged to the parallel axial alignment when 1-methylimidazole was used as the axial ligand (the dihedral angle between two axial ligands is 8.6°). The potential energy surface (PES) scan results also show that the relatively parallel axial orientations are energetically preferred for Fe(III), while perpendicular orientations are preferred for the Fe(II) complexes reported here. Bulk oxidation of Fe(II)(TFPPCl(8))(L)(2) in dichloromethane at a constant potential under nitrogen converts it to [Fe(III)(TFPPCl(8))(L)(2)]ClO(4), which gives identical EPR spectra at 77 K and which upon reduction regenerates Fe(II)(TFPPCl(8))(L)(2) again. Thus, we have demonstrated here very rare examples of Fe porphyrins in which the relative axial imidazole orientations switch between parallel and perpendicular just upon changing the oxidation states of iron from +3 to +2, respectively, in a nonplanar porphyrinic environment. These observations could be immensely important for understanding the possible effects of axial histidine orientations on similar macrocyclic deformations observed in various heme proteins.