Nuclear magnetic resonance studies of ferric porphyrins

Remote ion-pair interactions in Fe-porphyrin-based molecular catalysts for the hydrogen evolution reaction

The influence of ion-pair interactions between carboxy-containing iron porphyrins and the proton source in the hydrogen evolution reaction is described.

Gold-Clip-Assisted Self-Assembly and Proton-Coupled Expansion-Contraction of a Cofacial FeIII -Porphyrin Cage

A molecular cage {Au₈ (μ-PAnP)₄ [Fe(H₂ O)₂ (TPyP)]₂ (OTf)₂ }(OTf)₈ (1) composed of two cofacial Fe^III -porphyrin can be self-assembled from the gold clip [Au₂ (PAnP)Cl₂ ] and Fe³⁺ (H₂ O)₂ (TPyP)⁺ (PAnP=9,10-bis(diphenylphosphino)anthracene, TPyP=meso-tetra(4-pyridyl)porphyrinato). The height of the cage is 8.579(3) Å. The addition of a base to a solution of the cage leads to a contracted and twisted cage {[Au₈ (μ-PAnP)₄ [Fe₂ (μ-O)(TPyP)₂ ]}(OTf)₈ (2), which has a height of ≈4.4 Å and porphyrin-porphyrin torsional angle of ≈20°. The contracted cage can be synthesized independently from the gold clip and Fe₂ (μ-O)(TPyP)₂ . The spectroscopy and crystal structure of an unclipped analog of the contracted cage, {[AuPPh₃ )₈ [Fe₂ (μ-O)(TPyP)₂ ]}(OTf)₈ (3), supports the DFT-calculated structure of 2. NMR and UV/Vis titrations show that the expansion-untwisting and contraction-twisting of the cage is reversible.

Paramagnetic properties of the low- and high-spin states of yeast cytochrome c peroxidase

Here we describe paramagnetic NMR analysis of the low- and high-spin forms of yeast cytochrome c peroxidase (CcP), a 34 kDa heme enzyme involved in hydroperoxide reduction in mitochondria. Starting from the assigned NMR spectra of a low-spin CN-bound CcP and using a strategy based on paramagnetic pseudocontact shifts, we have obtained backbone resonance assignments for the diamagnetic, iron-free protein and the high-spin, resting-state enzyme. The derived chemical shifts were further used to determine low- and high-spin magnetic susceptibility tensors and the zero-field splitting constant (D) for the high-spin CcP. The D value indicates that the latter contains a hexacoordinate heme species with a weak field ligand, such as water, in the axial position. Being one of the very few high-spin heme proteins analyzed in this fashion, the resting state CcP expands our knowledge of the heme coordination chemistry in biological systems.

Electronic effects on carbon monoxide dissociation from iron(II) tetraphenylporhyrins

We report the kinetics of CO binding to a series of substituted ferrous tetra(4-X-phenyl)porphyrins ( X = CF 3, Cl , H , CH 3, OCH 3) in DMSO solvent using a mixture of 1,2-DMI/1-MeIm as proximal base. The CO dissociation rate constant increases with electron donation of the substituent X; the aggregate Hammett ρ value is -0.38. Using a Swain-Lupton analysis we determine the electronic effects to be transmitted 67% by through-bond or field effect and 33% by resonance. These results indicate that there is significant conjugation between the phenyl rings and the porphyrin core of iron tetraphenylporphyrin systems.

Electronic structure of five- and six-coordinate iron(III) tetraazaporphyrin complexes: pyrrole-Cαchemical shift as a useful probe

Electronic structure of a series of five-coordinate Fe ( OArTAzP ) X ( OAr = octaaryltetraazaporphyrin , X = Cl-, Br-, I-; Ar = 4-tert-butylphenyl) have been examined on the basis of1H NMR,13C NMR, and EPR spectroscopy as well as SQUID magnetometry. These complexes adopt the intermediate-spin state as in the case of analogous complexes reported by Fitzgerald et al. (Inorg. Chem. 1992; 31: 2006-2013) and Stuzhin et al. (Inorg. Chim. Acta 1995; 236: 131-139). The13C NMR studies using13C -enriched complexes at the pyrrole α positions have revealed that the pyrrole- Cαsignals appear at extraordinary upfield positions, i.e. -130 to -250 ppm at 273 K, due to the dz2-a2 uand dπ-3 eginteractions. The Curie plots of the pyrrole- Cαsignals have further revealed that the iodide complex adopts a much purer intermediate-spin state than the bromide and chloride complexes. In contrast to the case of Fe ( OArTAzP ) X , six-coordinate [ Fe ( OArTAzP )( CN )2]-showed the pyrrole- Cαsignal at 47 ppm at 273 K, which indicates that the complex adopts the low-spin state with the ( dxy)2( dxz, dyz)3electron configuration. Thus, the13C NMR chemical shift of the pyrrole- Cαsignal turns out to be quite a good probe to elucidate the spin state and electron configuration of iron(III) tetraazaporphyrins, where the1H NMR spectroscopy is less useful because of the absence of the hydrogen atoms as well as the alkyl or aryl groups directly attached to the meso positions.

Magnetic Resonance Spectroscopic Investigations of the Electronic Ground and Excited States in Strongly Nonplanar Iron(III) Dodecasubstituted Porphyrins

A series of axially ligated complexes of iron(III) octamethyltetraphenylporphyrin, (OMTPP)Fe(III), octaethyltetraphenylporphyrin, (OETPP)Fe(III), its perfluorinated phenyl analogue, (F(20)OETPP)Fe(III), and tetra-(beta,beta'-tetramethylene)tetraphenylporphyrin, (TC(6)TPP)Fe(III), have been prepared and characterized by (1)H NMR spectroscopy: chloride, perchlorate, bis-4-(dimethylamino)pyridine, bis-1-methylimidazole, and bis-cyanide. Complete spectral assignments have been made using 1D and 2D techniques. The temperature dependences of the proton resonances of the complexes show significant deviations from simple Curie behavior and evidence of ligand exchange, ligand rotation, and porphyrin ring inversion at ambient temperatures. At temperatures below the point where dynamics effects contribute, the temperature dependences of the proton chemical shifts of the complexes could be fit to an expanded version of the Curie law using a temperature-dependent fitting program developed in our laboratory that includes consideration of a thermally accessible excited state. The results show that, although the ground state differs for various axial ligand complexes and is usually fully consistent with that observed by EPR spectroscopy at 4.2 K, the excited state often has S = (3)/(2) (or S = (5)/(2) in the cases where the ground state has S = (3)/(2)). The EPR spectra (4.2 K) of bis-4-(dimethylamino)pyridine and bis-1-methylimidazole complexes show "large-g(max)" signals with g(max) = 3.20 and 3.12, respectively, and the latter also shows a normal rhombic EPR signal, indicating the presence of low-spin (LS) (d(xy))(2)(d(xz),d(yz))(3) ground states for both. The bis-cyanide complex also yields a large-g(max) EPR spectrum with g = 3.49 and other features that could suggest that some molecules have the (d(xz),d(yz))(4)(d(xy))(1) ground state. The EPR spectra of all five-coordinate chloride complexes have characteristic features of predominantly S = (5)/(2) ground-state systems with admixture of 1-10% of S = (3)/(2) character.

Kinetics of Ring Inversion in Strongly Nonplanar Iron(III) Octaalkyltetraphenylporphyrinates

The dynamics of porphyrin ring inversion of a number of Fe(III) complexes of octamethyltetraphenylporphyrin, (OMTPP)Fe(III); octaethyltetraphenylporphyrin, (OETPP)Fe(III); octaethyltetra(perfluorophenyl)porphyrin, (F(20)OETPP)Fe(III); and tetra-beta,beta'-tetramethylenetetraphenyl-porphyrin, (TC(6)TPP)Fe(III), having either one (Cl(-), ClO(4-)) or two [4-(dimethylamino)pyridine, 4-Me(2)NPy; 1-methylimidazole, 1-MeIm; tert-butylisocyanide, t-BuNC; or cyanide, CN(-)] axial ligands have been characterized by 1D dynamic NMR (DNMR) and 2D (1)H NOESY/EXSY spectroscopies as a function of temperature. The activation parameters, Delta H++, Delta S++, and Delta G++(298), and the extrapolated rate constants at 298 K for three chloride, one perchlorate, and three bis-(4-Me(2)NPy) complexes as well as [FeOETPP(1-MeIm)(2)]Cl, [FeOETPP(t-BuNC)(2)]ClO(4), and Na[FeOETPP(CN)(2)] have been determined. The results indicate that there is a wide range of flexibility for the porphyrin core (k(ex)(298) = 10-10(7) s(-1)) that decreases in the order TC(6)TPP > OMTPP > F(20)OETPP > or = OETPP, which correlates with increasing porphyrin nonplanarity. To determine the effect of axial ligands, we calculated the free energy of activation, Delta G++(298) for OETPPFe(III) bis-ligated with 4-Me(2)NPy, 1-MeIm, or 4-CNPy (approximately 59 kJ mol(-1)), and for complexes with small cylindrical ligands (t-BuNC and CN(-)) (approximately 37 kJ mol(-1)). These data suggest that the Delta G++(298) for planar ligand rotation is roughly 20-25 kJ mol(-1).

NMR and EPR Studies of the Bis(pyridine) and Bis(tert-butyl isocyanide) Complexes of Iron(III) Octaethylchlorin

The NMR and EPR spectra of a series of pyridine complexes [(OEC)Fe(L)2]+ (L = 4-Me2NPy, Py, and 4-CNPy) have been investigated. The EPR spectra at 4.2 K suggest that, with a decrease of the donor strength of the axial ligands, the complexes change their ground state from (d(xy))2 (d(xz)d(yz))3 to (d(xz)d(yz))4 (d(xy))1. The NMR data from 303 to 183 K show that at any temperature within this range the chemical shifts of pyrrole-8,17-CH2 protons increase with a decrease in the donor strength of the axial ligands. The full peak assignments of the [(OEC)Fe(L)2]+ complexes of this study have been made from COSY and NOE difference experiments. The pyrrole-8,17-CH2 and pyrroline protons show large chemical shifts (hence indicating large pi spin density on the adjacent carbons which are part of the pi system), while pyrrole-12,13-CH2 and -7,18-CH2 protons show much smaller chemical shifts, as predicted by the spin densities obtained from molecular orbital calculations, both Hückel and DFT; the DFT calculations additionally show close energy spacing of the highest five filled orbitals (of the Fe(II) complex) and strong mixing of metal and chlorin character in these orbitals that is sensitive to the donor strength of the axial substituents. The pattern of chemical shifts of the pyrrole-CH2 protons of [(OEC)Fe(t-BuNC)2]+ looks somewhat like that of [(OEC)Fe(4-Me2NPy)2]+, while the chemical shifts of the meso-protons are qualitatively similar to those of [(OEP)Fe(t-BuNC)2]+. The temperature dependence of the chemical shifts of [(OEC)Fe(t-BuNC)2]+ shows that it has a mixed (d(xz)d(yz))4 (d(xy))1 and (d(xy))2 (d(xz),d(yz))3 electron configuration that cannot be resolved by temperature-dependent fitting of the proton chemical shifts, with a S = 5/2 excited state that lies somewhat more than 2kT at room temperature above the ground state; the observed pattern of chemical shifts is the approximate average of those expected for the two S = 1/2 electronic configurations, which involve the a-symmetry SOMO of a planar chlorin ring with the unpaired electron predominantly in the d(yz) orbital and the b-symmetry SOMO of a ruffled chlorin ring with the unpaired electron predominantly in the d(xy) orbital. A rapid interconversion between the two, with calculated vibrational frequency of 22 cm(-1), explains the observed pattern of chemical shifts, while a favoring of the ruffled conformation explains the negative chemical shift (and thus the negative spin density at the alpha-pyrroline ring carbons), of the pyrroline-H of [TPCFe(t-BuNC)2]CF3SO3 (Simonneaux, G.; Kobeissi, M. J. Chem. Soc., Dalton Trans. 2001, 1587-1592). Peak assignments for high-spin (OEC)FeCl have been made by saturation transfer techniques that depend on chemical exchange between this complex and its bis-4-Me2NPy adduct. The contact shifts of the pyrrole-CH2 and meso protons of the high-spin complex depend on both sigma and pi spin delocalization due to contributions from three of the occupied frontier orbitals of the chlorin ring.

Symmetry and bonding in metalloporphyrins. A modern implementation for the bonding analyses of five- and six-coordinate high-spin iron(III)-porphyrin complexes through density functional calculation and NMR spectroscopy

Bonding interactions between the iron and the porphyrin macrocycle of five- and six-coordinate high-spin iron(III)-porphyrin complexes are analyzed within the framework of approximate density functional theory with the use of the quantitative energy decomposition scheme in combination with removal of the vacant pi orbitals of the porphyrin from the valence space. Although the relative extent of the iron-porphyrin interactions can be evaluated qualitatively through the spin population and orbital contribution analyses, the bond strengths corresponding to different symmetry representations can be only approximated quantitatively by the orbital interaction energies. In contrast to previous suggestions, there are only limited Fe --> P pi back-bonding interactions in high-spin iron(III)-porphyrin complexes. It is the symmetry-allowed bonding interaction between d(z)2 and a(2u) orbitals that is responsible for the positive pi spin densities at the meso-carbons of five-coordinate iron(III)-porphyrin complexes. Both five- and six-coordinate complexes show significant P --> Fe pi donation, which is further enhanced by the movement of the metal toward the in-plane position for six-coordinate complexes. These bonding characteristics correlate very well with the NMR data reported experimentally. The extraordinary bonding interaction between d(z)2 and a(2u) orbitals in five-coordinate iron(III)-porphyrin complexes offers a novel symmetry-controlled mechanism for spin transfer between the axial ligand sigma system and the porphyrin pi system and may be critical to the electron transfer pathways mediated by hemoproteins.

Molecular structures and magnetic resonance spectroscopic investigations of highly distorted six-coordinate low-spin iron(III) porphyrinate complexes

Three bis-axially ligated complexes of iron(III) octaethyltetraphenylporphyrin, (OETPP)Fe(III), have been prepared, which are low-spin complexes, each with two axial nitrogen-donor ligands (N-methylimidazole (N-MeIm), 4-(dimethylamino)pyridine (4-NMe(2)Py), and 2-methylimidazole (2-MeImH)). The crystal and molecular structure of the bis-(2-MeImH) complex shows the macrocycle to be in a saddled conformation, with the ligands in perpendicular planes aligned at 14 degrees to the porphyrin nitrogens so as to relieve the steric interaction between the 2-methyl groups and the porphyrin. The Fe-N(por) bond lengths are typical of nonplanar six-coordinate low-spin Fe(III) complexes, while the axial Fe-N(ax) bond lengths are substantially longer than those of [(TPP)Fe(2-MeImH)(2)](+) (2.09(2) A as compared to 2.015(4) and 2.010(4) A). The crystal and molecular structure of the bis-(4-NMe(2)Py) complex also shows the macrocycle to be in a mainly saddled conformation, but with a significant ruffled component. As a result, the average Fe-N(por) bonds are significantly shorter (1.951 A as compared to 1.974 A) than those of the bis-(2-MeImH) complex. One ligand is aligned at 9 degrees to two trans porphyrin nitrogens, while the other is at 79 degrees to the same porphyrin nitrogens, producing a dihedral angle of 70 degrees between the ligand planes. The EPR spectrum of this complex, like that of the bis-(2-MeImH) complex, is of the "large g(max)" type, with g(max) = 3.29 and 3.26, respectively. However, in frozen CD(2)Cl(2), [(OETPP)Fe(N-MeIm)(2)](+) exhibits both "large g(max)" and normal rhombic signals, suggesting the presence of both "perpendicular" and "parallel" ligand orientations. The 1- and 2D (1)H NMR spectra of each of these complexes, as well as the chloroiron(III) starting material, were investigated as a function of temperature. The COSY and NOESY/EXSY spectra of the chloride complex are consistent with the expected J-coupling and saddle inversion dynamics, respectively. Complete spectral assignments for the bis-(N-MeIm) and -(4-NMe(2)Py) complexes have been made using 2D (1)H NMR techniques. In each case, the number of resonances due to methylene (two) and phenyl protons (one each) is consistent with D(2)(d)() symmetry, and therefore an effective perpendicular orientation of the axial ligands on the time scale of the NMR experiments. The temperature dependences of the (1)H resonances of these complexes show significant deviations from Curie behavior, and also evidence of extensive ligand exchange and rotation. Spectral assignment of the eight methylene resonances of the bis-(2-MeImH) complex to the four ethyl groups was possible through the use of 2D (1)H NMR techniques. The complex is fluxional, even at -90 degrees C, and ROESY data suggest that the predominant process is saddle inversion accompanied by simultaneous rotation of the axial ligands. Saddle inversion becomes slow on the 2D NMR time scale as the temperature is lowered in the ligand order of N-MeIm > 4-NMe(2)Py > 2-MeImH, probably due mainly to progressive destabilization of the ground state rather than progressive stabilization of the transition state of the increasingly "hindered" bis-ligand complexes.

Electronic effects in transition metal porphyrins. 10. Effect of ortho substituents on the temperature dependence of the NMR spectra of a series of spin-admixed perchloratoiron(III) tetrakis(2,6- or 2,4,6-phenyl substituted)porphyrinates

The perchloratoiron(III) complexes of a series of 2,6-disubstituted tetraphenylporphyrin ligands, where the 2,6-phenyl substituents were -H, -F, -Cl, -Br, or -OMe, as well as two 2,4,6-phenyl-substituted complexes, where the substituents were -Me and -OMe, have been investigated as a function of temperature by 1H NMR spectroscopy. Curvature in the 1/T dependence was evident in most cases. Forced linear extrapolation of the temperature dependence observed over the range of the study yielded Curie plots that include negative slopes with very large positive 1/T intercepts (Cl approximately Br > Me > H) to negative slope with near zero intercept (tri-OMe) to positive slope with very large negative intercept (F, di-OMe). The NMR results were combined with EPR spectroscopic data and curve-fitting procedures based on an expanded Curie law to arrive at a consistent overview of the variety of temperature-dependence behaviors observed. This overview relies upon the premise that, in addition to the ground state observed by EPR spectroscopy, one (or more) thermally accessible excited state(s) are populated to varying degrees over the temperature range of the NMR measurements. If only one excited state is considered, the analysis is consistent with the ground state being a largely intermediate-spin state (S = 3/2) for the majority of the complexes but a largely high-spin state (S = 5/2) for ((2,6-F2)4TPP)FeOClO3 and ((2,6-(OMe)2)4TPP)FeOClO3.

Structural and Spectroscopic Characterization of Iron(III) Dioxoporphodimethene Complexes and Their Autoreduction to an Iron(II) Complex in Pyridine

Three iron complexes of the meso-dioxo derivative of octaethylporphryin (trans-H(2)OEPO(2)) were characterized by X-ray diffraction. Green ClFe(III)(trans-OEPO(2)).1.5C(6)H(6) crystallizes in the monoclinic space group P2(1)/c with a = 13.766(3) Å, b = 19.075(3) Å, c = 15.217(3) Å, beta = 99.87(2) degrees at 123 K with Z = 4. Refinement of 2712 reflections with F > 6.0sigma(F) and 223 parameters yielded R = 0.0624, R(w) = 0.0596. The iron complex contains a domed dioxoporphodimethene macrocyclic ligand. The observation of a five-coordinate iron(III) ion with an axial Fe-Cl distance of 2.232(2) Å and in-plane Fe-N distances averaging 2.082 Å is consistent with its high-spin (S = (5)/(2)) character. This monomer is readily converted to the green {Fe(III)(trans-OEPO(2))}(2)O using aqueous hydroxide. {Fe(III)(trans-OEPO(2))}(2)O crystallizes in the monoclinic space group C2/c with a = 23.541(8) Å, b = 15.392(5) Å, c = 18.686(8) Å, and beta = 90.09(3) degrees at 294 K with Z = 8. Refinement of 3472 reflections with F > 6.0sigma(F) and 393 parameters yielded R = 0.0484, R(w) = 0.0527. The complex possesses a crystallographically imposed 2-fold symmetry axis that passes through the oxo ligand. The dioxoporphodimethene ligands within the molecule are roof-shaped and fold away from each other. The axial Fe-O distance is 1.749(1) Å with longer in-plane Fe-N distances (average 2.077 Å). The Fe-O-Fe angle of 165.4(2) degrees deviates significantly from linearity and is more acute than related porphyrin complexes. Pyridine solutions of either the iron(III) monomer or &mgr;-oxo dimer autoreduce over a period of days to give (py)(2)Fe(II)(trans-OEPO(2)). This red compound crystallizes in the space group P2(1) with a = 19.177(4) Å, b = 20.039(4) Å, c = 10.547(2) Å, and beta = 100.36(3) degrees at 130 K with Z = 2. Refinement of 5090 reflections with one restraint and 984 parameters yielded R1 = 0.0684, wR2 = 0.1763. The complex crystallizes with two distinct molecules in the asymmetric unit; each molecule contains a different degree of disorder with respect to the trans meso oxygen atoms (50/50, 71/29). Each independent molecule exhibits severe ruffling of the macrocycle. The six coordinate iron(II) center is diamagnetic. The axial Fe-N(pyridine) distances average 1.98 Å, and the in-plane Fe-N(pyrrole) distances average 1.95 Å. A common trend observed for the dioxoporphodimethene macrocycle in all of these structures is an elongation toward the trans oxidized meso carbons. These complexes were originally prepared as cis and trans isomeric mixtures that can be enriched in the trans isomer by fractional crystallization. This is evident in their distinctive (1)H NMR spectra. In addition, these compounds are characterized by electron impact mass spectrometry and UV-visible, ESR, and infrared spectroscopies.

1H NMR studies of Chromatium vinosum cytochrome c'

The cytochrome c' from Chromatium vinosum has been studied through 1H NMR in the pH range 4-11 in both the oxidized and the reduced forms. The 1H NMR spectra are similar to those of the other cytochrome c' systems. Three pKa values of 5.1, 7.0, and 9.2 have been observed for the oxidized species and tentatively assigned to the two carboxylate propionic residues of the heme moiety and to the iron-coordinated histidine 125, respectively. The spectra are consistent with an essentially S = 5/2 state in all the pH ranges investigated. Some evidence is provided for conformational flexibilities. Among the oxidized cytochromes c' the present one is capable of binding cyanide, giving rise to a low spin state. The reduced species is a typical high spin iron(II) system.

NMR and electron-paramagnetic-resonance studies of a dihaem cytochrome from Pseudomonas stutzeri (ATCC 11607) (cytochrome c peroxidase)

A dihaem cytochrome (Mr 37 400) with cytochrome c peroxidase activity was purified from Pseudomonas stutzeri (ATCC 11 607). The haem redox potentials are far apart: one of the haems is completely ascorbate-reducible and the other is only reduced by dithionite. The coordination, spin states and redox properties of the covalently bound haems were probed by visible, NMR and electron paramagnetic resonance (EPR) spectroscopies in three oxidation states. In the oxidized state, the low-temperature EPR spectrum of the native enzyme is a complex superimposition of three components: (I) a low-spin haem indicating a histidinyl-methionyl coordination; (II) a low-spin haem indicating a histidinyl-histidinyl coordination; and (III) a minor high-spin haem component. At room temperature, NMR and optical studies indicate the presence of high-spin and low-spin haems, suggesting that for one of the haems a high-spin to low-spin transition is observed when temperature is decreased. In the half-reduced state, the component I (high redox potential) of the EPR spectrum disappears and induces a change in the g-values and linewidth of component II; the high-spin component II is no longer detected at low temperature. Visible and NMR studies reveal the presence of a high-spin ferric and a low-spin (methionyl-coordinated) ferrous state. The NMR data fully support the haem-haem interaction probed by EPR. In the reduced state, the NMR spectrum indicates that the low-potential haem is high-spin ferrous.

Proton nmr spectroscopy of high-spin ferrous cytochrome P-450 models

Alkyl mercaptide complexes of both synthetic and natural-derivative iron(II) porphyrins have been characterized in DMSO solution by proton nmr spectroscopy. A single mercaptide ligand binds to form a high-spin iron(II) complex as determined by solution magnetic measurements and the nmr isotropic shift pattern. Ligand exchange is slow on the nmr time scale unlike corresponding 2-methyl-imidazole exchange rates which are very rapid. Further comparison of mercaptide and 2-methyl imidazole adducts reveals a downfield bias in isotropic shift values for the mercaptide species, which may be explained by different signs in the dipolar shift term for the two complexes. This apparent magnetic anisotropy of the mercaptide complex is in the same direction, although smaller, than that observed for bacterial cytochrome P-450. Isotropic shift values of at least 250 ppm for methylene resonances of the coordinated mercaptide support a very efficient unpaired spin delocalization for this axial ligand.

1H-NMR and rosonance Raman spectra of octaethylporphyrinatoiron(III) perchlorate and its mono imidazole adduct

The 1H-NMR spectra and the resonance Raman spectra of intermediate spin complex, octaethylporphyrinatoiron (III) perchlorate (OEP-Fe(III)ClO4) and its mono imidazole adduct have been recorded and analyzed. The perchlorate complex was determined to be an intermediate-spin state (S = 3/2) in dichloromethane. The mono imidazole and 2-methylimidazole adducts of OEP-Fe(III)ClO4 were of the high-spin state in dichloromethane, which is a good model for the ferrihemoproteins such as metmyoglobins. The spin state of OEP-Fe(III)ClO4 varies the polarity of solvent from typical high-spin (S = 5/2) to typical low-spin (S = 1/2) state including intermediate-spin state (S = 3/2). The resonance Raman studies of the intermediate-spin complex in various solvents indicate that the complex is a plausible model to reproduce anomalous physico-chemical properties of the ferricytochrome c' at physiological condition.

Carbon-13 nuclear magnetic resonance spectroscopy of high-spin iron(III) porphyrin compounds

13C nuclear magnetic resonance spectra have been obtained for a variety of high-spin iron(III) porphyrin compounds and corresponding mu-oxo-bridged dimeric species. Large hyperfine shifts and significant line broadening are observed. The monomeric complexes exhibit hyperfine shifts which are downfield with the exception of an upfield shift for the meso-carbon atom. Possible unpaired spin delocalization mechanisms and prospects for observing 13C NMR porphyrin resonances in high-spin ferrihemoproteins are discussed. Spectra reported here provide strategy for incorporation of 13C labels in hemoproteins either by biosynthetic or chemical means. The vinyl-CH2 resonances of iron(III) protoporphyrin IX located 260 parts per million downfield from tetramethylsilane are especially attractive from the standpoint of chemical labeling.

Carbon-13 and proton nuclear magnetic resonance spectroscopy of water-soluble porphyrins and metalloporphyrins

Carbon-13 and proton nuclear magnetic resonance (NMR) spectra have been recorded for porphyrins, zinc porphyrins, and iron(III) porphyrin complexes in aqueous media. Spectra of porphyrin-c and hemin-c confirm the structure with thioether linkages at positions alpha to the porphyrin ring. The pattern of NMR isotropic shifts has implications regarding electron transfer in cytochrome-c. Free-base porphyrin-c and meso-substituted porphyrins have been examined for pyrrole nitrogen-hydrogen tautomerism and possible aggregation in aqueous solution. Zinc porphyrin 13C NMR spectra were recorded in order to provide diamagnetic references for paramagnetic iron(III) derivatives. Low-spin iron(III) porphyrin-biscyano complexes in aqueous solution exhibit NMR isotropic shift patterns similar to those previously observed for related compounds in non-aqueous media. The first 13C NMR spectra are reported for mu-oxo-bridged iron(III) porphyrin dimers. A partially resolved spectrum of a high-spin iron(III) porphyrin has also been obtained. Patterns of 13C and proton isotropic shifts are compared, and unpaired spin delocalization mechanisms for 13C resonances are discussed in a qualitative manner.

Axial pertubations on the electronic and magnetic properties of ferric porphyrins. II. Solvent effects on the proton NMR spectra of low-spin cyano complexes

The proton NMR spectra of a series of low-spin bis-cyano ferric complexes of tetraarylporphyrins and octaethylporphyrin in a variety of solvents have been recorded and analyzed. The hyperfine shifts are shown to be very sensitive to the solvent, experiencing an overall downfield bias as the solvent hydrogen-bonding donor strength increased. The characteristic pattern of the contact and dipolar shifts for the meso-aryl group in tetraarylporphyrin complexes are shown to permit a quantitative separation of the dipolar and contact contributions to the hyperfine shift. The separated components indicate that increased solvent hydrogen bonding strength significantly decreases the magnetic anisotropy of the iron and diminishes porphyrin replaced by iron pi bonding. The changes in anisotropy with solvent are shown to be consistent with the coordinated cyanide acting as a proton acceptor. Although similar solvent effects are found to be absent in bis-imidazole complexes, a downfield bias of half the magnitude of the bis-cyano complexes is observed in mixed cyano/imidazole complexes. Hence, the heme hyperfine shifts in cyano-metmyoglobins and -hemoglobins may serve as probes for the protonation of the distal histidyl imidazole.