Iron(III)-porphyrin .pi.-cation radical complexes. Molecular structures and magnetic properties

Buisson, G.; Deronzier, Alain; Duee, E.; Gans, Pierre; Marchon, Jean‐Claude; Regnard, Jean Rene

doi:10.1021/ja00388a059

Cited by 47 publications

(33 citation statements)

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“…Subsequently, the paramagnetism of the mononuclear derivative of [Cu(TMP á )] + [17] (planar core) and the diamagnetism of [Cu(OETPP á )] + [7] and [Cu(OETPP á )(Py)] + [8] (saddled cores) fully con®rmed the importance of core symmetry in controlling the nature of the spin coupling. Similarly, ferromagnetic coupling is observed between iron and the radical in planar, six-coordinate [Fe(TPP á )(OClO 3 ) 2 ] [14] and [Fe(OEP á )(OClO 3 ) 2 ] [16] even though the spin-state of the central iron is high-spin (S=5/2) in the former and admixed intermediate (S=3/2, 5/2) in the latter. There appear to be no exceptions to this symmetry argument for metal-radical coupling.…”

Section: Core Conformation Changes and Ring-ring Interactionsmentioning

confidence: 84%

“…Non-planar cores lead to antiferromagnetically coupled systems since only when the porphinato core deviates signi®cantly from D 4h symmetry are the metal d and ligand a 1u or a 2u orbitals not orthogonal and antiferromagnetic coupling can occur. The diamagnetism of [Cu(TPP á )] + [3,13] and the reduced moment of [Fe(TTP á )(Cl)] + [14], both consistent with antiferromagnetic coupling, were the ®rst derivatives that led to this correlation. Subsequently, the paramagnetism of the mononuclear derivative of [Cu(TMP á )] + [17] (planar core) and the diamagnetism of [Cu(OETPP á )] + [7] and [Cu(OETPP á )(Py)] + [8] (saddled cores) fully con®rmed the importance of core symmetry in controlling the nature of the spin coupling.…”

Section: Core Conformation Changes and Ring-ring Interactionsmentioning

confidence: 91%

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

Scheidt

2001

J. Biol. Inorg. Chem.

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This review discusses the structural changes that occur when the porphyrin ring of metalloporphyrin complexes is oxidized to form a pi-cation radical species. Although various differences in core conformation between the pi-cation derivative and the unoxidized homologue have been observed, there does not appear to be a general pattern of change. A frequently observed feature in pi-cation derivatives is the appearance of an alternating bond distance pattern in the inner ring of the porphyrin consistent with a localized structure rather the delocalized structure usually seen. The pattern, first seen in cofacial dimeric structures, has now been seen in monomeric systems as well. The nature and frequency of the observation and possible explanation are given.

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Section: Core Conformation Changes and Ring-ring Interactionsmentioning

confidence: 84%

Section: Core Conformation Changes and Ring-ring Interactionsmentioning

confidence: 91%

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

Scheidt

2001

J. Biol. Inorg. Chem.

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“…77 The spin density distribution in the porphyrinate ring of these radicals is shown in Figure 9e. In contrast, in the five-coordinate complex having a very saddled porphyrinate core, it would be symmetry-allowed, as pointed out by the authors, 73 for this porphyrinate ring conformation to make possible antiferromagnetic coupling of the d x 2 -y 2 unpaired electron of the high-spin Fe(III) center with the 3a 2u (π) unpaired electron to yield negative spin density on the porphyrinate ring. The spin density distribution in the porphyrinate ring of these radicals is shown in Figure 9f.…”

Section: The Interesting Cases Of Macrocycle Radicalsmentioning

confidence: 99%

“…ppm, and the meso-phenyl-H shift differences, δ m -δ p , and δ m -δ o , are both quite large and positiVe 73 (Table 1), indicating positiVe spin density on the porphyrinate ring. Consistent with this, the magnetic moment of this complex, µ eff ) 6.5 ( 0.2 µ B , 73 indicates six unpaired electrons, with the metal and macrocycle unpaired electrons either uncoupled or weakly ferromagnetically coupled.…”

Section: The Interesting Cases Of Macrocycle Radicalsmentioning

confidence: 99%

“…It is also found to have a strongly saddled porphyrinate ring. 73 Yet another Fe(III) porphyrinate cation radical complex is that of the low-spin Fe(III) complex [TPPFe(ImH) 2 ] 2+ , which has a pyrrole-H shift of -40.1 ppm at -38°C, and the meso-phenyl-H shift differences, δ m -δ p , and δ m -δ o , are both quite large and positiVe 76 (Table 1), indicating positiVe spin density on the porphyrinate ring. Consistent with this, the magnetic moment of the complex (µ eff ) 2.8 ( 0.2 µ B ) 76 indicates two unpaired electrons (one on the metal and one on the ring).…”

Section: The Interesting Cases Of Macrocycle Radicalsmentioning

confidence: 99%

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Pulsed EPR and NMR Spectroscopy of Paramagnetic Iron Porphyrinates and Related Iron Macrocycles: How To Understand Patterns of Spin Delocalization and Recognize Macrocycle Radicals

2003

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Pulsed EPR spectroscopic techniques, including ESEEM (electron spin echo envelope modulation) and pulsed ENDOR (electron−nuclear double resonance), are extremely useful for determining the magnitudes of the hyperfine couplings of macrocycle and axial ligand nuclei to the unpaired electron(s) on the metal as a function of magnetic field orientation relative to the complex. These data can frequently be used to determine the orientation of the g-tensor and the distribution of spin density over the macrocycle, and to determine the metal orbital(s) containing unpaired electrons and the macrocycle orbital(s) involved in spin delocalization. However, these studies cannot be carried out on metal complexes that do not have resolved EPR signals, as in the case of paramagnetic evenelectron metal complexes. In addition, the signs of the hyperfine couplings, which are not determined directly in either ESEEM or pulsed ENDOR experiments, are often needed in order to translate hyperfine couplings into spin densities. In these cases, NMR isotropic (hyperfine) shifts are extremely useful in determining the amount and sign of the spin density at each nucleus probed. For metal complexes of aromatic macrocycles such as porphyrins, chlorins, or corroles, simple rules allow prediction of whether spin delocalization occurs through σ or π bonds, and whether spin density on the ligands is of the same or opposite sign as that on the metal. In cases where the amount of spin density on the macrocycle and axial ligands is found to be too large for simple metal−ligand spin delocalization, a macrocycle radical may be suspected. Large spin density on the macrocycle that is of the same sign as that on the metal provides clear evidence of either no coupling or weak ferromagnetic coupling of a macrocycle radical to the unpaired electron(s) on the metal, while large spin density on the macrocycle that is of opposite sign to that on the metal provides clear evidence of antiferromagnetic coupling. The latter is found in a few iron porphyrinates and in most iron corrolates that have been reported thus far. It is now clear that iron corrolates are remarkably noninnocent complexes, with both negative and positive spin density on the macrocycle: for all chloroiron corrolates reported thus far, the balance of positive and negative spin density yields −0.65 to −0.79 spin on the macrocycle. On the other hand, for phenyliron corrolates, the balance of spin density on the macrocycle is zero, to within the accuracy of the calculations (Zakharieva, O.; Schünemann, V.; Gerdan, M.; Licoccia, S.; Cai, S.; Walker, F. A.; Trautwein, A. X. J. Am. Chem. Soc. 2002, 124, 6636−6648), although both negative and positive spin densities are found on the individual atoms. DFT calculations are invaluable in providing calculated spin densities at positions that can be probed by 1 H NMR spectroscopy, and the good agreement between calculated spin densities and measured hyperfine shifts at these positions leads to increased confidence in the calculated spin densities at position...

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Iron Porphyrin Chemistry

Walker¹,

Simonis²

2005

Encyclopedia of Inorganic Chemistry

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This article reviews most aspects of the chemistry of iron porphyrins, from Fe(0) to Fe(V), including occurrence and roles of natural iron porphyrins (hemes) and their synthetic analogs, structures and electron configurations of iron porphyrins of all oxidation and spin states, π electron configuration of the porphyrin ring, synthesis of metal‐free porphyrins and other related macrocycles, insertion of iron into free‐base porphyrins and related macrocycles, the properties, reactions, uses and biological relevance of iron(0), ‐(I), ‐(II) porphyrins (the latter with S = 0, 1, and 2 spin state possibilities), of iron(II) porphyrin π‐cation radicals, of iron(III) porphyrins (with S = 1/2, 3/2, and 5/2 spin state possibilities), of iron(III) porphyrin and corrole π‐cation radicals, of iron(IV) porphyrins (including five‐ and six‐coordinate ferryl (FeO) 2+ , iron(IV) phenyl, carbene and hydrazine complexes, and the bis‐methoxide complex) and a comparison of iron(IV) porphyrins to iron(III) porphyrin π‐cation radicals, of iron(IV) porphyrin π‐cation radicals, and of the possible existence of iron(V) porphyrins. Included in the Fe(II) part are sections on addition of ligands to four‐coordinate iron(II) porphyrins, including equilibrium binding constants, photodissociation of ligands from PFeL 2 complexes, binding of small molecules (O 2 , CO, NO, HNO) to 5‐coordinate iron(II) porphyrins and design of porphyrin ligands that will mimic the active sites of heme proteins such as myoglobin and hemoglobin, the cytochromes P450 and nitric oxide synthases, and the nitrophorins and guanylyl cyclases. Included in the iron(III) part are sections on both 5‐ and 6‐coordinate high‐spin complexes and their similarities and differences, bridged or through‐space magnetically coupled complexes of high‐spin iron(III) porphyrins with other metal complexes as possible models for cytochrome oxidase and the assimilatory sulfite reductases, coupled oxidation of hemes by hydrogen peroxide or its equivalent, and the relationship of this reactivity to the reactions of heme oxygenase, iron(III) porphyrins as reduction catalysts, and photochemistry of iron(III) porphyrins, possible electron configurations of low‐spin iron(III) porphyrins, the phenomenon and possible electronic consequences of ruffling of the porphinato core in iron(III) porphyrins, the preferred orientation of planar axial ligands bound to low‐spin iron(III) porphyrins, NO complexes of iron(III) porphyrins, reduction potentials, equilibrium constants and rates of axial‐ligand addition and exchange, kinetics of axial‐ligand rotation and porphyrin ring inversion, kinetics of reduction and autoreduction of iron(III) porphyrins, electron self‐exchange between low‐spin iron(III) and iron(II) porphyrins, synthetic ferriheme proteins, and synthesis of five‐coordinate low‐spin iron(III) porphyrins having σ‐alkyl or σ‐aryl groups as axial ligands. The iron(IV) and iron(IV) cation radical sections discuss the high‐valent states of cytochromes P450 and related enzymes.

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Iron(III)-porphyrin .pi.-cation radical complexes. Molecular structures and magnetic properties

Cited by 47 publications

References 3 publications

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

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

Pulsed EPR and NMR Spectroscopy of Paramagnetic Iron Porphyrinates and Related Iron Macrocycles: How To Understand Patterns of Spin Delocalization and Recognize Macrocycle Radicals

Iron Porphyrin Chemistry

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