Chemistry of Iron(III) in a Hemin-like Macrocyclic [N(4)](2)(-) Environment: Relationship between CT Absorption and EPR Spectra

• Published in: Inorganic chemistry Vol. 36; no. 16; pp. 3512 - 3519
• Main Authors: Jäger, Ernst-G., Keutel, Heike
• Format: Journal Article
• Language: English
• Published: United States 30.07.1997
• ISSN: 1520-510X
• DOI: 10.1021/ic9702633

The visible/near-IR and EPR spectra of the octahedral low-spin diadducts of the ethylene-bridged complex 6,13-diethoxycarbonyl-5,14-dimethyl-1,4,8,11-tetraazacyclotetradeca-4,6,12,14-tetraenato[2(-)]iron(III) iodide (1a-I), the ethylene/phenylene 1b-I, and the phenylene-bridged analogous complex 1c-I were investigated as a function of the axial and equatorial ligands. 1a-I forms octahedral low-spin diadducts in polar solvents (water, methanol) with a large variety of bases (pyridines, imidazoles, ammonia, amines, hydroxide, pseudohalides, sulfite, thiosulfate, nitrite, isonitriles, phosphines, phosphites, etc.). The diadducts show a characteristic sharp equatorial ligand (pi) to ferric (t(2g)) ion charge-transfer (CT) transition in the visible or near-IR. The energy of the maximum (E(CT)) shows a bathochromic shift with increasing pi acceptor strength and/or decreasing basicity of the axial ligands. The EPR spectra of the diadducts in frozen solution (77 K) are of rhombic type. In the case of the phosphorus ligand diadducts, a superhyperfine splitting was found, which indicates the coupling of two phosphorus nuclei with the unpaired electron of the iron(III). Using Taylor's model, the relative energies of the t(2g) orbitals was calculated from the g values to characterize the symmetry of the ligand field. The low symmetry of the equatorial ligands is mainly responsible for tetragonal and rhombic ligand field distortion, which is in contrast to the more symmetric phorphyrin ligands which effect only a tetragonal distortion. The splitting of the t(2g) orbitals is greatly enlarged with increasing pi acceptor ability of the axial ligand. In the order [1a-X(2)](+), [1b-X(2)](+), [1c-X(2)](+), FeTPPX(2)(+), the energy E(CT) and the t(2g) orbital splitting decrease as the pi conjugation of the equatorial ligand increases. In contrast to the investigations of hemoproteins, no positive correlation between E(CT) and E(EPR) (the energy of the Fe(III) hole relative to the baricenter of the t(2g) 3d subshell) was found. Generally, the CT energy increases as the orbital splitting decreases.