Modulation of iron spin states in highly distorted iron(iii) porphyrins: H-bonding interactions and implications in hemoproteins

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-III(TPPBr8)(OCHMe2), while six-coordinate complexes [Fe-III(TPPBr8)(MeOH)(2)]ClO4, [Fe-III(TPPBr8)(H2O)(2)]ClO4 and [Fe-III(TPPBr8)(1-MeIm)(2)]ClO4 stabilize admixed-high, admixed-intermediate and low-spin states, respectively. The H-bonding interactions between the weak axial H2O/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 (S = 5/2) state only. In addition, the iron atom in [Fe-III(TPPBr8)(H2O)(2)]ClO4 is displaced by 0.02 angstrom towards one of the water molecules engaged in the H-bonding interactions leading to two different Fe-O (H2O) distances of 2.098(8) and 2.122(9) angstrom. In contrast, iron in [Fe-III(TPPBr8)(MeOH)(2)]ClO4 sits on the plane of the porphyrin since both the axial methanol units are engaged in similar H-bonding interactions with the ClO4- ion. Moreover, the X-ray structure of low-spin Fe-II(TPPBr8)(1-MeIm)(2) revealed a dihedral angle of 63.0 degrees between two imidazoles which deviates largely from the expected angle of 90 degrees (perpendicular orientations) since the axial imidazole protons are engaged in strong intermolecular C-HMIDLINE HORIZONTAL ELLIPSIS pi 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 degrees and 22.4 degrees between the axial imidazole ring and the closest Fe-N-p axis due to strong pi-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.

Automated summary

This study reports a series of five- and six-coordinated Fe-porphyrins and investigates the effects of non-covalent interactions on the out-of-plane displacement and spin states of iron, as well as the orientation of axial ligands. The results show that weak hydrogen bonding interactions result in elongation of Fe-O bonds and stabilization of the mixed spin state of iron. Additionally, the movement of axial ligands is restricted by strong pi-interactions. This study provides important insights into the functioning of hemoproteins such as hemoglobin.