Modulation of Metal Displacements in a Saddle Distorted Macrocycle: Synthesis, Structure, and Properties of High-Spin Fe(III) Porphyrins and Implications for the Hemoproteins

Abstract

A rare family of five and six-coordinated high-spin Fe(III) porphyrins incorporating weak axial ligands are synthesized and structurally characterized which demonstrate, for the first time, stepwise metal displacements in a single distorted macrocyclic environment that has generally been seen in many biological systems. The introduction of four nitro groups into the meso-positions of octaethyl porphyrin severely distorts the porphyrin geometry and provides an interesting modulation of the macrocycle properties which enables the facile isolation of “pure” high-spin FeIII(tn-OEP)Cl, FeIII(tn-OEP)(MeOH)Cl, and FeIII(tn-OEP)(H2O)2+ in excellent yields in a saddle distorted macrocyclic environment that are known to stabilize intermediate spin states. The stepwise out-of-plane displacements of iron are as follows: 0.47 Å for FeIII(tn-OEP)Cl; 0.09 Å for FeIII(tn-OEP)(MeOH)Cl, and 0.01 Å for FeIII(tn-OEP)(H2O)2+ from the mean plane of the porphyrins. However, in both five and six-coordinated Fe(III) porphyrins, the Fe−Np distances are quite comparable while the porphyrin cores have expanded significantly, virtually to the same extent for the six-coordinate complexes reported here. The large size of the high-spin iron(III) atom in FeIII(tn-OEP)(H2O)2+ is accommodated perfectly with no displacement of the metal. This expansion is accompanied by a significant decrease of the saddle distortion with a clear increase of the ruffling. Furthermore, the Fe atom in FeIII(tn-OEP)(MeOH)Cl is not out of plane because of the larger atom size; however, the displacement of the iron depends on both the relative strength of the axial ligands, as well as the nature and extent of the ring deformation. Our characterization demonstrates that increase in ruffling and/or decrease in macrocycle deformation brings the iron atom more into the plane in a distorted macrocyclic environment. Our observations thus suggest that the displacements of iron in proteins are the consequences of nonequivalent axial coordination, as well as protein induced deformations at the heme. The high-spin nature of the complexes reported here is believed to be due to the larger Fe−Np distances which then reduce substantially the interaction between iron dx2−y2 and porphyrin a2u orbital. The FeIII/FeII reduction potential of FeIII(tn-OEP)Cl shows a reversible peak at large positive value (0.20 V), and no ring-centered oxidation was observed within the solvent limit (∼1.80 V). It is thus easier to reduce FeIII(tn-OEP)Cl by almost 700 mV compared to FeIII(OEP)Cl while oxidations are very difficult. Furthermore, the addition of 3-Cl-pyridine to FeIII(tn-OEP)Cl in air undergoes spontaneous auto reduction to produce the rare air-stable FeII(tn-OEP)(3-Cl-py)2 that shows FeII/FeIII oxidation peaks at high positive potential (0.79 V), which is ∼600 mV more anodic compared to [FeII(tn-OEP)Cl]−. This large anodic shift illustrates the effective removal of metal-centered electron density by the macrocycle when the metal is constrained to reside in the porphyrin plane.