[Fe(OEP^•)(X)]⁺ π-Cation Radicals:  Characterization and Spin−Spin Interactions

Abstract

The preparation and characterization of the π-cation radical derivatives of [Fe(OEP)(X)] (X = Cl-, Br-) is reported. Three different derivatives have been prepared: [Fe(OEP•)(Cl)]ClO4, [Fe(OEP•)(Cl)][SbCl6], and [Fe(OEP•)(Br)][SbCl6]. All derivatives have been characterized by UV−vis, IR, and Mössbauer spectroscopy. In addition, [Fe(OEP•)(Cl)]ClO4 has been characterized by a single-crystal structure determination, and [Fe(OEP•)(Cl)][SbCl6] and [Fe(OEP)(Br)][SbCl6] have been studied by temperature-dependent magnetic susceptibility measurements and Mössbauer measurements in an applied magnetic field. The X-ray structure of [Fe(OEP•)(Cl)]ClO4 reveals a five-coordinate porphyrinate species that forms tight cofacial π−π dimers in which the two porphyrin rings are almost exactly overlapped with an inter-ring separation of 3.24 Å, a lateral shift of 0.2 Å, and a twist angle between the two rings of 31.2°; the two iron atoms are 4.112 Å apart. Crystal data: C37H46FeCl4O4N4, a = 27.454(7) Å, b = 15.322(3) Å, c = 19.802(3) Å, β = 116.14 (2)°, monoclinic, C2/c, Z = 8. Iron(III) is found to be in the high-spin state in all derivatives. The magnetic data (susceptibility and Mössbauer) have been interpreted in terms of two spin coupling models. Both models give a picture of strong coupling between the various spins in the dimeric species. In the model judged to best fit all data with a physically meaningful zero-field splitting, there are three terms in the total Hamiltonian: an axial zero-field splitting parameter D for the high-spin iron, an intramolecular antiferromagnetic coupling −2J Fe - r(S⃗·s⃗) between the iron spin S = 5/2 and the π-cation radical s = 1/2 spin, and an intermolecular antiferromagnetic coupling −2J R - R(S⃗ + s⃗)·(S⃗‘ + s⃗‘) between the total spins on each half of the dimer.

Structural deformations and bond length alternation in porphyrin π-cation radicals

“…2 displays the geometric features that can be used to describe cofacial dimers. A number of similar cofacial dimers of octaethylporphyrin have subsequently been characterized ( [19,20], and W.R. Scheidt et al, unpublished data); these derivatives also display strongly overlapped cores. The high degree of ring-ring overlap in the p-cation radical derivatives, which is much stronger than that seen in the solid-state structures of neutral species [5], suggested that the degree of ring overlap might be related to the oxidation level of the porphyrin ring.…”

The Electronic Structure of Iron Corroles: A Combined Experimental and Quantum Chemical Study

“…[1] This may be appreciated by the structures depicted in Scheme 1, representing intensely investigated iron porphyrin complexes, in which the overall oxidation state is higher by one (1)(2)(3) and two (4) units relative to the ironA C H T U N G T R E N N U N G (III) resting state of the enzymes. Insight into the metal versus porphyrin oxidation dilemma, arising from similar redox potentials of the metal and the porphyrin ligand, has previously been obtained from the combination of various spectroscopic methods, electrochemistry, X-ray crystallography, and computational methods.…”

“…Insight into the metal versus porphyrin oxidation dilemma, arising from similar redox potentials of the metal and the porphyrin ligand, has previously been obtained from the combination of various spectroscopic methods, electrochemistry, X-ray crystallography, and computational methods. [2] The unpaired electrons in 1 a are all of identical spin, while the spin of the single electron from the porphyrin radAbstract: There is a longstanding debate in the literature on the electronic structure of chloroiron corroles, especially for those containing the highly electron-withdrawing meso-tris(pentafluorophenyl)corrole (TPFC) ligand. Two alternative electronic structures were proposed for this and the related iron corrole complexes containing electron-rich corrole ligands.…”

“…[Fe(py) 2 Figure 7. Two metal-based t 2g orbitals, d xy and d yz , are doubly occupied, while one is singly occupied; the two antibonding e g orbitals are unoccupied.…”

Explorations in metalloporphyrin stereochemistry, physical properties and beyond

“…Further, I would take notice of two bodies of work that have not been discussed in this paper, work on the other oxynitrogen-ligated porphyrins (the ligands NO 2 − and NO 3 − ) [112][113][114][115][116][117][118][119][120][121][122][123][124] and the work on the characterization of π-cation radical complexes [125][126][127][128][129][130][131][132][133][134][135][136][137][138][139]. Diagram Illustrating the effect of axial ligand orientation on the axial bond distances in iron (III) species.…”