Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India.
Amity Institute of Click Chemistry Research and Studies, Amity University, Noida, Uttar Pradesh, India.
Dalton Trans. 2023 Jul 4;52(26):8904-8917. doi: 10.1039/d3dt00846k.
A family of five- and six-coordinated Fe-porphyrins which enable us to scrutinize the effects of non-covalent interactions on the out-of-plane displacement of iron and its spin-states and axial ligand orientation in a single distorted macrocyclic environment has been reported. Combined analysis using single-crystal X-ray structure determination and EPR spectral investigation revealed the stabilization of the high-spin state of iron in the five-coordinate complex Fe(TPPBr)(OCHMe), while six-coordinate complexes [Fe(TPPBr)(MeOH)]ClO, [Fe(TPPBr)(HO)]ClO and [Fe(TPPBr)(1-MeIm)]ClO stabilize admixed-high, admixed-intermediate and low-spin states, respectively. The H-bonding interactions between the weak axial HO/MeOH and perchlorate anion resulted in an elongation of the Fe-O bond which eventually shortened the Fe-N(por) distances leading to the stabilization of the admixed spin state of iron which, otherwise, stabilizes the high-spin ( = 5/2) state only. In addition, the iron atom in [Fe(TPPBr)(HO)]ClO is displaced by 0.02 Å towards one of the water molecules engaged in the H-bonding interactions leading to two different Fe-O (HO) distances of 2.098(8) and 2.122(9) Å. In contrast, iron in [Fe(TPPBr)(MeOH)]ClO sits on the plane of the porphyrin since both the axial methanol units are engaged in similar H-bonding interactions with the ClO ion. Moreover, the X-ray structure of low-spin Fe(TPPBr)(1-MeIm) revealed a dihedral angle of 63.0° between two imidazoles which deviates largely from the expected angle of 90° (perpendicular orientations) since the axial imidazole protons are engaged in strong intermolecular C-H⋯π interactions which thereby restrict the axial ligand movement. The complex also displays the shortest Fe-N(1-MeIm) bond along with smallest dihedral angles of 7.8° and 22.4° between the axial imidazole ring and the closest Fe-N axis due to strong π-interactions between iron and the axial imidazole ligand. Our work highlights the influence of non-covalent interactions on the out-of-plane displacement and spin state of iron and axial ligand orientations which are indeed important steps in the functioning of various hemoproteins.
已经报道了一系列五配位和六配位的 Fe-卟啉配合物，这些配合物使我们能够在单个扭曲的大环环境中研究非共价相互作用对铁的面外位移及其自旋态和轴向配体取向的影响。单晶 X 射线结构测定和 EPR 光谱研究的综合分析表明，在五配位配合物 Fe(TPPBr)(OCHMe) 中，铁的高自旋态得到稳定，而六配位配合物 [Fe(TPPBr)(MeOH)]ClO、[Fe(TPPBr)(HO)]ClO 和 [Fe(TPPBr)(1-MeIm)]ClO 分别稳定了混杂高自旋态、混杂中间自旋态和低自旋态。弱轴向 HO/MeOH 与高氯酸根阴离子之间的氢键相互作用导致 Fe-O 键的伸长，最终缩短了 Fe-N(卟啉)距离，从而稳定了铁的混杂自旋态，否则，仅稳定高自旋（=5/2）态。此外，[Fe(TPPBr)(HO)]ClO 中的铁原子向参与氢键相互作用的一个水分子发生 0.02 Å 的位移，导致两个不同的 Fe-O(HO)距离为 2.098(8)和 2.122(9) Å。相比之下，铁在 [Fe(TPPBr)(MeOH)]ClO 中位于卟啉平面上，因为两个轴向甲醇单元都与 ClO 离子发生类似的氢键相互作用。此外，低自旋 Fe(TPPBr)(1-MeIm)的 X 射线结构显示两个咪唑之间的二面角为 63.0°，这与预期的 90°（垂直取向）相差很大，因为轴向咪唑质子参与强分子间 C-H⋯π相互作用，从而限制了轴向配体的运动。该配合物还显示出最短的 Fe-N(1-MeIm)键以及轴向咪唑环与最近的 Fe-N 轴之间的最小二面角 7.8°和 22.4°，这是由于铁和轴向咪唑配体之间的强 π 相互作用所致。我们的工作强调了非共价相互作用对铁的面外位移和自旋态以及轴向配体取向的影响，这些确实是各种血红素蛋白功能的重要步骤。
