NMR and EPR Investigations of Iron Corrolates:  Iron(III) Corrolate π Cation Radicals or Iron(IV) Corrolates?

Cai, Shuqun; Walker, F. Ann; Licoccia, Silvia

doi:10.1021/ic990784l

Cited by 83 publications

(124 citation statements)

References 51 publications

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“…4 This orbital has large π spin density at the meso-carbon positions and negligible spin density at the pyrrole -carbons. 4,79,82 We have recently published a detailed study 10 that fully corroborates the earlier interpretations of the NMR data for the chloroiron octaalkylcorrolates, 79 in which several experimental techniques (magnetic susceptibility measurements, Mössbauer, and NMR spectroscopy), as well as DFT calculations, were used to establish unambiguously the electron configuration and spin density distributions of the chloroiron octaalkyl-and triphenylcorrolates, as well as the phenyliron octaalkylcorrolate. 10 In all of the chloroiron complexes, the NMR spectra were strongly indicative of an electron configuration in which the metal is S ) 3 / 2 Fe(III) and the corrolate macrocycle is a cation radical, Corr 2-• .…”

Section: The Special Case Of Iron Corrolatessupporting

confidence: 61%

“…80 We have also recently reported the investigation of a series of chloroiron triphenylcorrolates by 1 H and 19 F NMR spectroscopy; 81 the 1 H NMR spectra of several chloroiron triphenylcorrolates have also been reported by Ghosh and coworkers. 9 The results of all of these studies have shown unambiguously that these five-and six-coordinate iron corrolates are actually iron(III) corrolate(2-•) π cation radicals, 79,80 and that axial ligands such as cyanide can readily autoreduce the corrolate radical, leaving a low-spin Fe(III) mono(cyanide) complex. 80 The nature of the magnetic coupling between the unpaired electrons on the metal and the corrolate(2-•) π cation radical differs, depending on the axial ligand(s) present: Very strong antiferromagnetic coupling is observed in the case of the chloride complexes (as evidenced by the fact that the meso-H resonances are found at +187(1) and +174(2) ppm at 300 K, which can only occur when there is large negatiVe spin density at the meso-carbons, as shown in Figure 9f, to the point of the C m position).…”

Section: The Special Case Of Iron Corrolatesmentioning

confidence: 97%

“…Hence, there is no basis for claims that the electron configuration of the two chloroiron triphenylcorrolates reported by one laboratory 86,87 is different from those reported by all other laboratories, and thus no basis for claims of an electron configuration of any chloroiron corrolate that is different from S ) 3 / 2 Fe(III) coupled antiferromagnetically to a corrolate π cation radical, as originally concluded and stated on the basis of the 1 H NMR spectroscopic data. 79 The…”

Section: The Special Case Of Iron Corrolatesmentioning

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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Section: The Special Case Of Iron Corrolatessupporting

confidence: 61%

Section: The Special Case Of Iron Corrolatesmentioning

confidence: 97%

Section: The Special Case Of Iron Corrolatesmentioning

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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“…The oxidation and reduction of these complexes (Table 1, entries 6, 7) were shown to be corrole-and metal-centered, respectively, which also led to isolation of an authentic iron(IV) corrole cation radical [37]. Still, the interpretations regarding the iron(IV) oxidation state were recently challenged via NMR analysis of (omc)Fe(Cl) and these authors propose that the alternative formulation of iron(III) corrole cation radical cannot be ruled out [38]. The intriguing demonstration that (tpfc)Fe(Cl) is a good oxidation catalyst [10] initiated the full characterization of this and other related iron complexes [20,39].…”

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confidence: 99%

High-valent corrole metal complexes

Gross

2001

J. Biol. Inorg. Chem.

160

108

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This commentary concentrates on corrole complexes with the three metal ions that are most relevant to oxidation catalysis: chromium, manganese, and iron. Particular emphasis is devoted to the only recently introduced meso-triarylcorroles and a comparison with the traditionally investigated b-pyrrole-substituted corroles. Based on a combination of spectroscopic methods, electrochemistry, and X-ray crystallography, it is concluded that in most high-valent metallocorroles the corrole is not oxidized. Both experimental (for (oxo)chromium(V) corrole) and computational (for (oxo)manganese(V) corrole) evidence indicate that the stabilization of high-valent metal ions by corroles originates from a combination of short metal-nitrogen bonds and large metal out-of-plane displacements in the corrole, which lead to quite unexpected interactions of the oxo-metal p* orbitals with the in-plane orbitals of the corrole.Keywords Corrole á Iron á Manganese á Chromium á Cation radicals Corroles may be considered as the non-natural analogs of the cobalt-coordinating corrin ring in Vitamin B 12 with which they share an identical skeleton, or as one meso-carbon short porphyrins with whom they share aromaticity (Scheme 1) [1]. Corrole chemistry, which started in 1964 [2], also led to the ®rst structural characterization of a free-base corrole by Hodgkin and coworkers in 1971 in course of their Vitamin B 12 project [3]. The main research eorts during the years to come were devoted to the synthesis of corroles [4] and to investigation of the coordination chemistry of their metal complexes [5,6]. Still, compared to porphyrins, the research activity in corrole chemistry remained very low. For example, a computer search of``article titles, keywords, or abstract'' in the ISI database for 1998 produced 1143 hits for porphyrin*, but only 10 hits for corrol*. There was also no reported application of corroles or their metal complexes in any scienti®c journal or in patent databases up to 1999, and it took 28 years to obtain the second ever X-ray crystal structure of a metalfree corrole [7]. This situation is however to experience a major change, as much more ecient and simple methodologies for the preparation of corroles are constantly being introduced (for the most recent developments that are not covered by the review [4], see [7,8,9]).One main dierence between the corrin, porphyrin, and corrole macrocycles (Scheme 1) is the number of ionizable protons in their N 4 coordination core, which is 1, 2, and 3, respectively. Accordingly, in their coordination complexes they act as mono-, di-, and trianionic ligands, respectively. This rather naive distinction is actually of importance, as may be appreciated by the following facts. The catalytic action of Vitamin B 12 relies on stabilization of cobalt(I) by the monoanionic corrinate ligand, the most common oxidation states of porphyrin-supported metals are +2 and +3, and the trianionic corrolate ligand supports unusually high metal oxidation states. Within this commentary we will examine the coordinati...

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“…Subsequently, Gross and co-workers developed the coordination chemistry of meso-tris(pentafluorophenyl)corrole extensively [105] and used some of the complexes as catalysts for oxygen-atom transfer, [106][107][108] cyclopropanation, [107] and other reactions. Our laboratory [109,110] and that of Walker [111][112][113] have studied the description of the electronic structure of high-valent transition-metal corroles, a key finding being that corroles often act as highly noninnocent ligands. [114][115][116] …”

Section: Angewandte Chemiementioning

confidence: 99%

A Perspective of One‐Pot Pyrrole–Aldehyde Condensations as Versatile Self‐Assembly Processes

Ghosh

2004

Angew Chem Int Ed

153

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Regarded as the classic one-pot synthetic route to symmetrical porphyrins for well over half a century, pyrrole-aldehyde cyclocondensations have yielded a cornucopia of nonporphyrin macrocycles, such as N-confused porphyrins, corroles, sapphyrins, and expanded porphyrins, and have thus emerged as versatile self-assembly processes. A highlight in this field is the remarkably general one-pot corrole synthesis. The manifold of intermediates generated in the anaerobic phase of a Lindsey-type synthesis have been viewed as a dynamic covalent self-assembly system. This raises the possibility that the addition of a suitable host may alter the equilibrium concentrations of these intermediates by molecular recognition and related phenomena and thus determine the major product formed after oxidative quenching.

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NMR and EPR Investigations of Iron Corrolates: Iron(III) Corrolate π Cation Radicals or Iron(IV) Corrolates?

Cited by 83 publications

References 51 publications

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

High-valent corrole metal complexes

A Perspective of One‐Pot Pyrrole–Aldehyde Condensations as Versatile Self‐Assembly Processes

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