Proton NMR Relaxation in Six-Coordinate Low-Spin Iron(III) Tetraphenylporphyrinates:  Temperature Dependence of Proton Relaxation Rates and Interpretation of NOESY Experiments

Momot, Konstantin I.; Walker, F. Ann

doi:10.1021/jp972194j

Cited by 18 publications

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References 51 publications

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“…(54) In general, if the TjS are less than 10 ms, the likelihood of observing an NOE for protons that are 2.5 Â or more apart for a compound in a non-viscous solvent is very small to negligible. (44) Hence, although we are still convinced that the peak assignments reported earlier are correct, based upon the molecular orbital pictures that had been derived for each of the complexes (54), we cannot prove the assignments by NOESY (or ROESY or ID NOE difference spectroscopy) experiments, because the T,s of the protons involved are just too short to allow buildup of NOEs. Thus, magnitude-NOESY cross peaks can be extremely misleading, and we recommend that magnitude experiments in general, whether NOESY or COSY, not be used, and that spectra never be symmetrized!…”

Section: Expt ( 1 )mentioning

confidence: 71%

“…This has happened in our own research program, where apparent NOE cross peaks, between the very resonances that we expected should give NOE cross peaks, were reported as real. (54) We have not been able to detect these cross peaks in phase-sensitive NOESY spectra (44), where full (usually 16-step) phase cycling is used, and we have shown by measurements of the NOE buildup curve for a related complex and by calculations (44) that the protons which gave the apparent NOE cross peaks in the earlier study are too far apart (>5A) to give a measureable NOE cross peak with the solvent viscosity used in that study (CD 2 C1 2 at -40 to -50 °C). (54) In general, if the TjS are less than 10 ms, the likelihood of observing an NOE for protons that are 2.5 Â or more apart for a compound in a non-viscous solvent is very small to negligible.…”

Section: Expt ( 1 )mentioning

confidence: 99%

“…(32) For systems having multiple unpaired electrons, Curie relaxation (an additional kind of dipolar relaxation) may also contribute a term proportional to B 0 2 S 2 (S+l) 2 x c /T 2 r 6 to the linewidths (32), which could cause the resonances of such complexes to be broader at higher magnetic fields. However, for S = 1/2 systems, where the small value of S usually means that Curie relaxation is not an important contribution (44), the benefits of higher magnetic field (better S/N) always outweigh any (usually undetectable) contribution from Curie relaxation. Peak overlap is usually not a problem for paramagnetic complexes, so if the S/N is adequate, lowerfield (200-300 MHz) spectrometers will provide just as well resolved spectra as will higher-field (500 or 600 MHz) spectrometers.…”

Section: Introductionmentioning

confidence: 99%

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Advances in Single- and Multidimensional NMR Spectroscopy of Paramagnetic Metal Complexes

Walker

1998

ACS Symposium Series

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Proton (and l3 C) NMR signals of paramagnetic metal complexes (and metalloproteins) can often, but not always, be detected by suitable modification of the standard experiments that are a part of the software of any modern NMR spectrometer. These NMR spectra potentially contain important information concerning the electronic and geometrical structure of the complex. Successfully obtaining 1D spectra is usually not difficult, because rapid recycle times can usually be used, and good S/N can usually be obtained in a relatively short time. To successfully obtain 2D spectra of paramagnetic complexes, however, requires careful optimization of the experimental parameters, in line with the relaxation times T 1 and T 2 of the signals of interest. ROESY spectra are in many cases more informative for small to medium-sized inorganic complexes than are NOESY spectra, because ROEs are non-zero for all rotational correlation times. For heteronuclear 2D experiments such as HMQC and HMBC, experiments that include gradient pulses greatly improve S/N and decrease t 1 noise, making it possible to detect weak correlations.NMR spectroscopic investigations of a wide variety of paramagnetic transition or rare earth metal complexes and metalloproteins have been reported over the years, yet many chemists are still convinced that it is impossible to obtain NMR spectra of paramagnetic compounds. Certainly, there are a number of limitations, which will be discussed below, but there are often simple ways of getting around these limitations, and sometimes they can be used to advantage. Thus, it should never be assumed that just because a complex contains one or more unpaired electrons, it is not worth obtaining an NMR spectrum of it! Why should one study the NMR spectra of paramagnetic complexes? Because they are usually exquisitely sensitive to molecular structure (metal-nuclear distances) and 30 31 to the molecular orbital into which the unpaired electron is delocalized. This not only allows one to confirm that the proper product has been obtained, but also, in many cases, to obtain detailed information about molecular shape and conformation and intimate details about covalency and the nature of the molecular orbital(s) of the ligands that interact with the unpaired electron(s) of the metal. One or more unpaired electrons on a metal center cause the protons (or ,3 C nuclei) that are close to the metal through bonds and/or through space to be "illuminated." This "illumination" comes in the form of 1) a much wider range of chemical shifts than are typically observed for diamagnetic compounds and 2) linewidths that vary strongly with the distance between the metal and the nucleus of interest. Tn some of the most extreme cases reviewed herein, ! H and 2 H chemical shifts of greater than ±600 ppm and linewidths of greater than 2 kHz have been detected, and these are by no means limits! (1,2) To observe these large chemical shifts and very broad lines, certain modifications of the typical NMR experiments used by organic chemists are often necessar...

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Section: Expt ( 1 )mentioning

confidence: 71%

Section: Expt ( 1 )mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Advances in Single- and Multidimensional NMR Spectroscopy of Paramagnetic Metal Complexes

Walker

1998

ACS Symposium Series

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“…In the current study, we have undertaken an NMR measurement of the rate of axial ligand rotation and rotational Δ H ⧧ , Δ S ⧧ , using the T 2 's of the pyrrole protons. It has been found that the temperature dependence of transverse relaxation rates of pyrrole protons in [(TMP)Fe(2-MeImH) 2 ] + is bimodal in that the T 2 's increase with temperature at low temperatures and decrease at high temperatures preceding the collapse of the four-peak pyrrole proton pattern. This behavior is compatible with the hypothesis that at low temperatures the T 2 's of the pyrrole protons are the intrinsic T 2 's, which increase with temperature, whereas at the high temperatures the T 2 values are significantly shortened by exchange …”

Section: Introductionmentioning

confidence: 99%

“…In this study, we will use simulation of the experimentally observed pyrrole proton T 2 's in [(TMP)Fe(2-MeImH) 2 ] + to demonstrate that the presence of cyclic chemical exchange quantitatively explains their bimodal behavior . The rate of rotation of axial ligands can be measured from the temperature dependence of the T 2 's as the rate constant of chemical exchange induced by such rotation.…”

Section: Introductionmentioning

confidence: 99%

Rate Constants and Thermodynamic Parameters of Rotation of Axial Ligands in a Bisligated Ferric Tetramesitylporphyrinate Complex Measured from the Temperature Dependence of ¹H Transverse Relaxation Rates

Momot

Walker

1998

J. Phys. Chem. A

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The rate constant of the four-site cyclic chemical exchange between pyrrole protons in tetramesitylporphyrinatoiron(III)bis(2-methylimidazole) has been measured in the temperature range 236−255 K from 1H transverse relaxation times (T 2) observed at the 500 MHz field strength. The values of the rate constant were obtained through the simulation of the observed T 2's under the assumption that their values are dominated by the rate of the chemical exchange. The measurement of the exchange rate constant was also attempted at a lower temperature (230 K), but the use of extrapolated intrinsic T 2's at that temperature introduces a significant error into the simulated rate constant. The thermodynamic activation parameters of the cyclic exchange were determined as ΔH ⧧ = 48 ± 1 kJ/mol and ΔS ⧧ = −10 ± 6 J/K·mol. The value of k varied from 30 to 270 s-1 in the temperature range 236−255 K. The near-zero exchange activation entropy is consistent with two previous studies but contradicts another study based on saturation-transfer measurements previously carried out by the authors of this work. A comparison of all available studies shows that the saturation-transfer experiments overestimate the value of the exchange rate constant by up to a factor of 2. It is shown that equilibrium with the high-spin form of the porphyrin complex does not affect the observed T 2's in the temperature range used in this study but may do so at temperatures above 270 K. It is also shown that neither the linearity of the Curie plot nor the behavior of the T 1 versus temperature plot should be considered indicators of the absence of the low-spin ⇌ high-spin equilibrium.

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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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Proton NMR Relaxation in Six-Coordinate Low-Spin Iron(III) Tetraphenylporphyrinates: Temperature Dependence of Proton Relaxation Rates and Interpretation of NOESY Experiments

Cited by 18 publications

References 51 publications

Advances in Single- and Multidimensional NMR Spectroscopy of Paramagnetic Metal Complexes

Advances in Single- and Multidimensional NMR Spectroscopy of Paramagnetic Metal Complexes

Rate Constants and Thermodynamic Parameters of Rotation of Axial Ligands in a Bisligated Ferric Tetramesitylporphyrinate Complex Measured from the Temperature Dependence of ¹H Transverse Relaxation Rates

Iron Porphyrin Chemistry

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Proton NMR Relaxation in Six-Coordinate Low-Spin Iron(III) Tetraphenylporphyrinates: Temperature Dependence of Proton Relaxation Rates and Interpretation of NOESY Experiments

Cited by 18 publications

References 51 publications

Advances in Single- and Multidimensional NMR Spectroscopy of Paramagnetic Metal Complexes

Advances in Single- and Multidimensional NMR Spectroscopy of Paramagnetic Metal Complexes

Rate Constants and Thermodynamic Parameters of Rotation of Axial Ligands in a Bisligated Ferric Tetramesitylporphyrinate Complex Measured from the Temperature Dependence of 1H Transverse Relaxation Rates

Iron Porphyrin Chemistry

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Rate Constants and Thermodynamic Parameters of Rotation of Axial Ligands in a Bisligated Ferric Tetramesitylporphyrinate Complex Measured from the Temperature Dependence of ¹H Transverse Relaxation Rates