Axial ligand orientations in a distorted porphyrin macrocycle: synthesis, structure, and properties of low-spin bis(imidazole)iron(III) and iron(II) porphyrinates. 2010

Ranjan Patra, and Arvind Chaudhary, and Sudip Kumar Ghosh, and Sankar Prasad Rath
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India.

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.

UI MeSH Term Description Entries
D007093 Imidazoles Compounds containing 1,3-diazole, a five membered aromatic ring containing two nitrogen atoms separated by one of the carbons. Chemically reduced ones include IMIDAZOLINES and IMIDAZOLIDINES. Distinguish from 1,2-diazole (PYRAZOLES).
D007501 Iron A metallic element with atomic symbol Fe, atomic number 26, and atomic weight 55.85. It is an essential constituent of HEMOGLOBINS; CYTOCHROMES; and IRON-BINDING PROTEINS. It plays a role in cellular redox reactions and in the transport of OXYGEN. Iron-56,Iron 56
D008024 Ligands A molecule that binds to another molecule, used especially to refer to a small molecule that binds specifically to a larger molecule, e.g., an antigen binding to an antibody, a hormone or neurotransmitter binding to a receptor, or a substrate or allosteric effector binding to an enzyme. Ligands are also molecules that donate or accept a pair of electrons to form a coordinate covalent bond with the central metal atom of a coordination complex. (From Dorland, 27th ed) Ligand
D008665 Metalloporphyrins Porphyrins which are combined with a metal ion. The metal is bound equally to all four nitrogen atoms of the pyrrole rings. They possess characteristic absorption spectra which can be utilized for identification or quantitative estimation of porphyrins and porphyrin-bound compounds. Metalloporphyrin
D008958 Models, Molecular Models used experimentally or theoretically to study molecular shape, electronic properties, or interactions; includes analogous molecules, computer-generated graphics, and mechanical structures. Molecular Models,Model, Molecular,Molecular Model
D008968 Molecular Conformation The characteristic three-dimensional shape of a molecule. Molecular Configuration,3D Molecular Structure,Configuration, Molecular,Molecular Structure, Three Dimensional,Three Dimensional Molecular Structure,3D Molecular Structures,Configurations, Molecular,Conformation, Molecular,Conformations, Molecular,Molecular Configurations,Molecular Conformations,Molecular Structure, 3D,Molecular Structures, 3D,Structure, 3D Molecular,Structures, 3D Molecular
D004563 Electrochemistry The study of chemical changes resulting from electrical action and electrical activity resulting from chemical changes. Electrochemistries
D004578 Electron Spin Resonance Spectroscopy A technique applicable to the wide variety of substances which exhibit paramagnetism because of the magnetic moments of unpaired electrons. The spectra are useful for detection and identification, for determination of electron structure, for study of interactions between molecules, and for measurement of nuclear spins and moments. (From McGraw-Hill Encyclopedia of Science and Technology, 7th edition) Electron nuclear double resonance (ENDOR) spectroscopy is a variant of the technique which can give enhanced resolution. Electron spin resonance analysis can now be used in vivo, including imaging applications such as MAGNETIC RESONANCE IMAGING. ENDOR,Electron Nuclear Double Resonance,Electron Paramagnetic Resonance,Paramagnetic Resonance,Electron Spin Resonance,Paramagnetic Resonance, Electron,Resonance, Electron Paramagnetic,Resonance, Electron Spin,Resonance, Paramagnetic
D013057 Spectrum Analysis The measurement of the amplitude of the components of a complex waveform throughout the frequency range of the waveform. (McGraw-Hill Dictionary of Scientific and Technical Terms, 6th ed) Spectroscopy,Analysis, Spectrum,Spectrometry
D047028 Macrocyclic Compounds Cyclic compounds with a ring size of approximately 1-4 dozen atoms. Macrocycle Compounds,Spherands,Torands,Compounds, Macrocycle,Compounds, Macrocyclic

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