Chemistry of Iron(III) in a Hemin-like Macrocyclic [N₄]^2-Environment:  Relationship between CT Absorption and EPR Spectra

Abstract

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.

Geometric and Electronic Structure/Function Correlations in Non-Heme Iron Enzymes

“… a im = imidazole; imH = protonated form of imidazole; Meim = methylimidazole; [N 4 ] 2- , [N 2 O 2 ] 2- = macrocyclic Schiff-base ligands depicted in ref ; P = porphyrin-type ligand; TPP = tetraphenylporphyrin; bpy = bipyridine; phen = phenanthroline; PyPep = N -2-mercaptophenyl-2‘-pyridinecarboxamide; ADIT = 6-amino-2,3-dimethyl-4-azahex-3-ene-2-thiolato. b All g -values have been assumed positive in the calculation of Δ/ζ Fe and Δ/ V , and some of the complexes are not in a “proper” coordinate system in the sense discussed by Taylor in ref .…”

Structural, Magnetic, and Electronic Properties of a Pentacoordinated Intermediate-Spin (S = 3/2) Iron(III) Complex with a Macrocyclic [N4]2- Ligand

“…As part of a program to investigate the electronic structure of 3d-metal complexes with varying macrocyclic ligands which are not porphin derivatives but have a similar equatorial [N 4 ] 2- donor set, we synthesized the iron(III) complex 3 and its ethylene/phenylene and phenylene/phenylene bridged analogs, which belong to the general class of the complexes 1 . Recently, we reported about the spectroscopic properties of the octahedral diadducts, the equilibria and kinetics of the stepwise substitution of axial ligands, the redox potentials, and the catalytic performance of these complexes . Preliminary measurements of the magnetic susceptibility of 3 showed a temperature-independent magnetic moment of approximately 4 μ B at 77 K, indicating a pure intermediate spin state ( S = 3 / 2 ).…”

“…Recently, we reported about the spectroscopic properties of the octahedral diadducts, the equilibria and kinetics of the stepwise substitution of axial ligands, the redox potentials, and the catalytic performance of these complexes . Preliminary measurements of the magnetic susceptibility of 3 showed a temperature-independent magnetic moment of approximately 4 μ B at 77 K, indicating a pure intermediate spin state ( S = 3 / 2 ).…”

Trinuclear Triplesalophen Building Blocks with Terminal Cyanides and Implications for the Spin‐Polarization Mechanism for Low‐Spin FeIII and CrIII Ions

“…Complex 4 shows a comparatively strong band in the NIR region at 11 800 cm –1 ( ε = 5.62 × 10 3 m –1 cm –1 ). This band is typical for octahedrally coordinated Fe III low‐spin complexes with Schiff‐base like ligands or porphyrins and was assigned as a LMCT . This band is only visible if the measurement is conducted in a large excess of cyanide ions even under dioxygen and water free conditions.…”