Axial Ligand Orientations in a Distorted Porphyrin Macrocycle: Synthesis, Structure, and Properties of Low-Spin Bis(imidazole)iron(III) and Iron(II) Porphyrinates†Dedicated to Prof. Animesh Chakravorty on the occasion of his 75th birthday.

Abstract

We have reported here, for the first time, the parallel and perpendicular orientation preferences of two planar and unhindered imidazoles as axial ligands (L) while coordinated toward iron(III) and iron(II) porphyrins, respectively, in a nonplanar porphyrinic environment. The synthesis and characterization of low-spin Fe(III)(tn-OEP)(L)(2) x ClO(4) and Fe(II)(tn-OEP)(L)(2) are reported. Fe(III)(tn-OEP)(L)(2) x ClO(4) shows rhombic electron paramagnetic resonance (EPR) spectra (at 77 K) in both solid and solution phases that are very characteristic for low-spin (S = 1/2) iron porphyrins with two axial imidazole ligands aligned parallel to each other. Single-point energy calculation is also performed on Fe(III)(tn-OEP)(1-MeIm)(2)(+) using density functional theory (DFT), which shows that the relative parallel orientations of two 1-MeIm are more stable than the perpendicular orientations. X-ray structures of Fe(II)(tn-OEP)(1-MeIm)(2) and Fe(II)(tn-OEP)(1-MeIm)(2).THF are reported that demonstrate, for the first time, the near-perpendicular axial ligand orientation (80.9 and 89.8 degrees, respectively) for iron(II) porphyrins in a distorted macrocyclic environment. Even starting from parallel axial orientations of 1-MeIm, geometry optimization using DFT converged well to the perpendicular axial alignment with a 82.54 degree dihedral angle, which is in close agreement with experiment. This is in sharp contrast to all earlier reports, in which sterically crowded imidazole (such as 2-MeIm) or a nearly planar porphyrin core with a "picket fence" environment that restricts the rotation of the axial ligands is required for perpendicular orientation. Electrochemical data obtained from a cyclic voltammetric study for Fe(II)(tn-OEP)(L)(2) reveal one-electron oxidation at very high positive potential, which readily explains why the complexes are so stable in air. Bulk oxidation of Fe(II)(tn-OEP)(1-MeIm)(2) at a constant potential of 0.69 V in dichloromethane with 0.1 M tetrabutylammonium perchlorate as the supporting electrolyte generates Fe(III)(tn-OEP)(1-MeIm)(2) x ClO(4), which has the same EPR spectrum and which upon reduction at 0.29 V regenerates Fe(II)(tn-OEP)(1-MeIm)(2) again. Thus, we have demonstrated here, for the first time, that iron(II) and iron(III) porphyrinates with two planar and unhindered axial imidazoles have different orientation preferences in a nonplanar porphyrinic environment.

Investigating the Impact of Conformational Molecular Engineering on the Crystal Packing of Cavity Forming Porphyrins

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Herein we report the synthesis of 5,10,15,20-tetraaryl-(X)-substituted-2, 3,7,8,12,13,17, and a structural investigation of their solid-state properties via small molecule X-ray diffraction. A series of halogen (fluorine to iodine), nitrogenous (azido, cyano), alkyl (TMS-acetylene and acetylene) and chained (benzyloxy) porphyrins were chosen as the initial target molecules.

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A Simple, Iron-Catalyzed, Pyridine-Assisted Hydrogen Peroxide Epoxidation System

“…31,32) However, when we performed our model epoxidation reaction with imidazole 3 instead of pyridine, the yields were unsatisfactory and tended to decrease with increasing amounts of imidazole (entries 1-4). Intriguingly, when an excess amount of pyridine was added to the imidazole-FeCl 3…”

Synthesis and characterization of a modified “picket fence” porphyrin complex – stronger π bonding interactions between Fe(ii) and axial ligands

“…The steric 2-MeHIm leads to longer Fe-N Im bond distances relative to the other imidazole derivatives, as seen in Table 2. 47 Type III porphyrin systems are the three picket fence derivatives of [Fe(TpivPP)(R-Im) 2 ] (R-Im = 1-MeIm, 1-EtIm and 1-VinylIm), which feature relative perpendicular orientations of the two axial ligands as well as near planar porphyrin planes. This is in contrast to the case of Fe(III) where the ground state is (d xz , d yz ) 4 (d xy ) 1 and the porphyrin a 2u (π) → Fe(III) π donation requires ruffling.…”