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(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.