Ferric Porphyrin Research Articles

Oxidation of N‐(4‐Chlorophenyl)‐N′‐hydroxyguanidine to N‐(4‐Chlorophenyl)urea and Nitric Oxide by Photoexcited Iron Porphyrins

AbstractPhotochemical excitation (λ > 350 nm) of chloro[meso‐tetrakis(2,6‐dichlorophenyl)porphyrin]iron(III) [FeIII(TDCPP)Cl] and chloro[meso‐tetra(α,α,α,α‐pivalamidophenyl)porphyrin]iron(III) [FeIII(TpivPP)Cl] induces the oxidation of coordinated N‐(4‐chlorophenyl)‐N′‐hydroxyguanidine (1) by molecular oxygen, to give iminoxyl radicals and the FeII(O2) [or FeIII(O2·)] adduct. This complex can be accumulated in significant amounts using [FeIII(TpivPP)Cl]. The primary photoproducts give rise to secondary reactions that lead to the formation of N‐(4‐chlorophenyl)urea (2) as the main end‐product of 1. The conversion of 1 into 2 is accompanied by the formation of NO, as revealed both by an ESR spin‐trapping technique and in the form of its stable end‐products NO2− and NO3−. The presence of 1‐methyl imidazole (1‐MeIm) coordinated in axial position has a significant positive effect on the photoinduced production of 2 and NO from the hydroxyguanidine 1. The observation that radical scavengers inhibit the photooxidation process strongly supports the possibility that the reaction pathway resembles a radical‐type autoxidation mechanism, where the very fast reaction of O2 with the ferrous porphyrin in the presence of the photogenerated iminoxyl radical should yield an iron‐peroxo intermediate as precursor of the urea 2 and NO. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Quantitative vibrational dynamics of iron in nitrosyl porphyrins.

We use quantitative experimental and theoretical approaches to characterize the vibrational dynamics of the Fe atom in porphyrins designed to model heme protein active sites. Nuclear resonance vibrational spectroscopy (NRVS) yields frequencies, amplitudes, and directions for 57Fe vibrations in a series of ferrous nitrosyl porphyrins, which provide a benchmark for evaluation of quantum chemical vibrational calculations. Detailed normal mode predictions result from DFT calculations on ferrous nitrosyl tetraphenylporphyrin Fe(TPP)(NO), its cation [Fe(TPP)(NO)]+, and ferrous nitrosyl porphine Fe(P)(NO). Differing functionals lead to significant variability in the predicted Fe-NO bond length and frequency for Fe(TPP)(NO). Otherwise, quantitative comparison of calculated and measured Fe dynamics on an absolute scale reveals good overall agreement, suggesting that DFT calculations provide a reliable guide to the character of observed Fe vibrational modes. These include a series of modes involving Fe motion in the plane of the porphyrin, which are rarely identified using infrared and Raman spectroscopies. The NO binding geometry breaks the four-fold symmetry of the Fe environment, and the resulting frequency splittings of the in-plane modes predicted for Fe(TPP)(NO) agree with observations. In contrast to expectations of a simple three-body model, mode energy remains localized on the FeNO fragment for only two modes, an N-O stretch and a mode with mixed Fe-NO stretch and FeNO bend character. Bending of the FeNO unit also contributes to several of the in-plane modes, but no primary FeNO bending mode is identified for Fe(TPP)(NO). Vibrations associated with hindered rotation of the NO and heme doming are predicted at low frequencies, where Fe motion perpendicular to the heme is identified experimentally at 73 and 128 cm-1. Identification of the latter two modes is a crucial first step toward quantifying the reactive energetics of Fe porphyrins and heme proteins.

Nitric oxide binding to cationic and anionic ferric porphyrins in aqueous solution: reversible formation of ferrous NO species of the cationic porphyrin

Two water-soluble ferric porphyrins, sodium 5α,10β,15α,20β-tetrakis(2-(sulfonatoacetamido)phenyl)porphyrinatoiron(III) (Fe IIITanP) and 5α,10β,15α,20β-tetrakis(2-(N,N,N-trimethylammoniumacetamido)phenyl)porphyrinatoiron(III) chloride (Fe IIITcatP), were synthesized. The p K a values of the coordinated H 2O of Fe IIITanP and Fe IIITcatP were evaluated to be 8.0 and 4.1, respectively. Reactions of NO with the ferric porphyrins were examined spectrophotometrically in aqueous solution. Porphyrin Fe IIITanP binds NO reversibly to give the corresponding ferric NO species at pH 1.3 and pH 3.0, and Fe IIITcatP reacts similarly with NO at pH 1.3. The thermodynamic data for the NO binding were estimated from van't Hoff plots. At pH 3.0, visible and ESR spectral data indicated that Fe IIITcatP binds NO reversibly to produce ferrous NO species depending on NO partial pressures. These results were discussed based on through-space intramolecular interactions between the coordinated H 2O or NO and the ionic substituents of the porphyrins.

Easy preparation of the tris(2-fluoro-6-pyridylmethyl)amine ligand and instantaneous reaction of the corresponding dichloroferrous complex with molecular dioxygen: new access to dinuclear species.

The tris(2-fluoro-6-pyridylmethyl)amine ligand, F3TPA, can easily be prepared by reaction of 2-fluoro-6-bromomethylpyridine with NH4Cl in the presence of NaOH. Complexation to FeCl2 affords the high-spin F3TPAFe(II)Cl2 complex, the X-ray structure of which is reported. The three fluorine substituents provide enough steric hindrance to force the tripod to coordinate in the tridentate mode, affording a trigonal bipyramidal iron center. This complex is thermally stable, and it reacts instantaneously with molecular dioxygen to afford the unsymmetrical micro-oxo dimer F3TPAFe(III)ClOFe(III)Cl3 as the major product, together with small amounts of the mixed salt [F3TPAFe(II)Cl]2, [Fe(III)2OCl6]. These two complexes have been isolated and characterized by X-ray diffraction analysis. A mechanism by which they are obtained is suggested and seems to parallel the well-known process of autoxidation of ferrous porphyrins.

Fast nitroxyl trapping by ferric porphyrins.

Iron(II) porphyrin nitrosyl complexes are obtained in high yields from the reaction of iron(III) porphyrins with the nitroxyl donors sodium trioxodinitrate and toluensulfohydroxamic acid. The reaction was found to proceed both in organic solvents and in aqueous media from iron(III) (meso-tetraphenyl) porphyrinate ([FeIII(TPP)]+) and iron(III) meso-tetrakis (4-sulfonatophenyl) porphyrinate ([FeIII(TPPS)]3-) or iron(III) protoporphyrin IX, respectively. The kinetic rate constant for the reaction of ([FeIII(TPPS)]3-) with sodium trioxodinitrate (kon) was estimated to be 1.00 +/- 0.04 x 107 M-1 s-1. As well as resulting in a versatile method for obtaining ferrous nitrosyl porphyrins, the reaction points at ferric porphyrins as efficient nitroxyl traps and provides a tool to model nitroxyl reactivity toward hemeproteins.

Chemical shift of meso-carbon: a powerful probe to determine the coordination structure and electron configuration of ferric porphyrin complexes.

The meso-13C chemical shifts have been revealed to serve a powerful probe to determine the coordination structure and electron configuration of ferric porphyrin complexes.

Effects of a nonplanar porphyrin rings on the spin-spin interactions in low-spin ferric porphyrin radical cationst.

Low-spin ferric porphyrin radical cations formed by the oxidation of chloro(meso-tetraalkylporphyrinato)iron(III) followed by the addition of bulky 2-methylimidazole show antiferromagnetic coupling, which is interpreted in terms of the interaction between porphyrin a2u and iron d(xy), orbitals caused by the S4 ruffling of the porphyrin core.

The reactivity of alpha-hydroxyhaem and verdohaem bound to haem oxygenase-1 to dioxygen and sodium dithionite.

Recently we have shown that ferric alpha-hydroxyhaem bound to haem oxygenase-1 can be converted to ferrous verdohaem by approximately an equimolar amount of O2 in the absence of exogenous electrons [Sakamoto, H., Omata, Y., Palmer, G., and Noguchi, M. (1999) J. Biol. Chem.274, 18196-18200]. Contrary to those results, other studies have claimed that the conversion requires both O2 and an electron. More recently, Migita et al. have reported that the major reaction product of ferric alpha-hydroxyhaem with O2 is a ferric porphyrin cation radical that can be converted to ferrous alpha-hydroxyhaem with sodium dithionite [Migita, C. T., Fujii, H., Matera, K. M., Takahashi, S., Zhou, H., and Yoshida, T. (1999) Biochim. Biophys. Acta1432, 203-213]. To clarify the reason(s) for the discrepancy, we compared the reactions; i.e. alpha-hydroxyhaem to verdohaem and verdohaem to biliverdin, under various conditions as well as according to the procedures of Migita. We find that complex formation of alpha-hydroxyhaem with haem oxygenase may be small and a substantial amount of free alpha-hydroxyhaem may remain, depending on the reconstitution conditions; this could lead to a misinterpretation of the experimental results. We also find that ferrous verdohaem appears to be air-sensitive and is therefore easily converted to a further oxidized species with excess O2. Finally, we find that dithionite seems to be inappropriate for investigating the haem oxygenase reaction, because it reduces ferrous verdohaem to a further reduced species that has not been seen in the haem degradation system driven by NADPH-cytochrome P450 reductase.

The co-ordination of ligands by iron porphyrins: a comparison of ligand binding by myoglobin from sperm whale and the haem undecapeptide from cytochrome c

The effect of the protein on the ligand binding properties of a ferric porphyrin was investigated by determining spectrophotometrically the temperature dependence of the binding constants, log Keq, and hence the thermodynamic parameters ΔH and ΔS, for coordination of F−, N3−, SCN−, CN− and imidazole by the haemundecapeptide from cytochrome c, known as N-acetyl microperoxidase-11 (NAcMP11), and ferric myoglobin from sperm whale. The axial binding site of the iron porphyrin in NAcMP11 is relatively open to the solvent, but in metMb it is buried in the protein and surrounded by tightly-packed distal amino acid residues. Whereas the N-donor ligands ethanolamine, glycine, pyridine, ammonia and imidazole are readily coordinated by NAcMP11, only the latter is coordinated by metMb, although the affinity of the haemoprotein for imidazole is nearly two orders of magnitude smaller than for NAcMP11 because of a much less favourable reaction enthalpy. Examination of the crystal structures of metMb and its imidazole complex suggests that steric factors are largely responsible. Of all the halides, only F− is coordinated by metMb. In contrast with imidazole, the anionic ligands N3−, SCN−, F− and CN− are coordinated more strongly by metMb than by NAcMP11. In the case of the first two, the difference in affinity is enthalpically driven, although there is a compensating entropic effect; this is attributed to the difference in the polarity of the environment of the porphyrin, with the relatively apolar environment of the haemoprotein stabilising the complex due to an electrostatic interaction between the anionic ligand and the residual positive charge at the metal centre. The effect is not generally true for all anionic ligands. Thus, although both F− and CN− are coordinated more strongly by metMb than by NAcMP11, the difference in affinity now stems largely from an entropic effect. Hence, whereas very significant differences in affinity for an exogenous ligand exist between the relatively exposed ferric porphyrin of the haempeptide and the buried ferric porphyrin of the haemoprotein, such differences cannot be attributed to a single factor only.

Electrochemistry and spectroscopy of sulfate and thiosulfate complexes of iron porphyrins

The electrochemical and spectroscopic properties of the complex formed by the addition of thiosulfate to ferric porphyrins were examined. The NMR spectrum of the thiosulfate–ferric porphyrin complex was consistent with a high-spin ferric complex, while the EPR spectrum at liquid nitrogen temperatures indicated that the complex under these conditions was low-spin. Such behavior has been previously observed for other ferric porphyrin complexes. The visible spectra were characterized by a shift in the Soret band to higher energies, with smaller changes in the longer wavelength region. The complex was reasonably stable in DMF, but slowly reduced over several hours to FeII(TPP) and S4O62−. The voltammetric behavior of the thiosulfate complex in DMF consists of two waves, the first of which was irreversible. The ferric/ferrous reduction in the presence of thiosulfate was shifted negatively about 400 mV, compared to the Fe(TPP)(Cl) reduction. The visible, NMR and EPR spectra were most consistent with a Fe–S bonded ferric porphyrin–thiosulfate complex, Fe(P)(S-SO3)−. The kinetics of the reduction of ferric porphyrin by thiosulfate in DMSO indicated an autocatalytic mechanism, where the first step is the formation of the catalyst. The identity of the catalyst could not be determined because it must be present at low concentrations, but it is formed from the reaction of the ferric complex with thiosulfate. Coordination of thiosulfate to the porphyrin was not necessary for the reduction to occur, and the reduction of Fe(TPP)(Cl) by thiosulfate was accelerated by the addition of sulfate. Under these conditions, sulfate had replaced thiosulfate as the axial ligand for the ferric porphyrin. In the presence of sulfate, the reduction occurred in a single kinetic pseudo-first order step.

Spectroscopic properties of p/p and o/o type iron(III)-metal-free porphyrin dimers and their catalysis as Cytochrome P450 model

Two series of p/p and o/o type iron(III)-metal-free porphyrin dimers were synthesized and characterized. The dimers consist of a ferric porphyrin and a free-base porphyrin, covalently linked with an alkoxy chain - O ( CH2)nO -(n = 2–10) at the para and ortho position of two phenyl rings. The ferric paramagnetic effect and ring current effect on these dimers were discussed by1H NMR study; vibration modes sensitive to the conformations of the dimers were investigated from IR spectra; the electron density and spin state of ferric ion were examined using XPS and EPR spectroscopy. The catalytic activities of p/p and o/o type dimers on the hydroxylation of cyclohexane were studied under mild conditions. As a chemical mimic model of Cyto. P450, these porphyrins exhibit higher catalytic activities than the corresponding monomer FeTPPCl . With increase of the carbon numbers of alkoxy chain, the catalytic activity order of p/p dimers is C 2 < C 3 < C 4 < C 5 < C 6 > C 8 > C 10, the corresponding order of o/o dimers is C 2 > C 4 > C 6 > C 8 > C 10. Catalytic results of dimers are shown to strongly depend on their special conformation equilibrium, which affect the steric hindrance and electron transfer between two porphyrin rings.

Electrochemistry and spectroelectrochemistry of iron porphyrins in the presence of nitrite

The reaction of nitrite with ferric and ferrous porphyrins was examined using visible, infrared and NMR spectroscopy. Solutions of either ferric or ferrous porphyrin were stable in the presence of nitrite, with only complexation reactions being observed. Under voltammetric conditions, though, a rapid reaction between nitrite and iron porphyrins was observed to form the nitrosyl complex, Fe(P)(NO), where P=porphyrin. The products of the reduction of ferric porphyrins in the presence of nitrite were confirmed by visible spectroelectrochemistry to be Fe(P)(NO) and [Fe(P)] 2O. Visible, NMR and infrared spectroscopy were used to rule out the formation of Fe(P)(NO) by the iron-catalyzed disproportionation of nitrite. A reaction between iron porphyrins and nitrite only occurred by the presence of both oxidation states (ferric/ferrous). The kinetics of the reaction were monitored by visible spectroscopy, and the reaction was found to be first-order with respect to Fe(OEP)(Cl) and Fe(OEP). The products were the same as those observed in the spectroelectrochemical experiment. The rate was not strongly dependent upon the concentration of nitrite, indicating that the coordinated, not the free nitrite, was the reaction species. The kinetics observed were consistent with a mixed oxidation state nitrite-bridged intermediate, which carried out the oxygen transfer reaction from nitrite to the iron porphyrin. The effect of nitrite coordination on the reaction rate was examined.

An integrated approach to the mid-spin state (S = 3/2) in six-coordinate iron(III) chiroporphyrins.

An intermediate-spin state very close to the mid-spin state (S = 3/2) can be stabilized in a ferric porphyrin by an integrated approach which combines the favorable effects of a weak axial field strength and of a small macrocycle hole. Axial ligand exchange by reaction of chloroiron(III)tetramethylchiroporphyrin [(TMCP)FeCl] with silver perchlorate in ethanol-chloroform leads to ethanol-ligated ferric chiroporphyrins. Two distinct crystalline products containing a bisethanol complex [[(TMCP)FeIII(EtOH)2]ClO4] and three variants of a mixed ethanol-water complex [[(TMCP)FeIII(EtOH)(H2O)]ClO4] have been structurally characterized in the solid state. The small hole of the ruffled chiroporphyrin and the weak axial oxygen ligation result in strongly tetragonally distorted complexes. The six-coordinate species exhibit long axial Fe-O bond distances (2.173(5)-2.272(4) A) and the shortest equatorial Fe-N(av) distances (1.950(5)-1.978(7) A) found as yet in a ferric porphyrin, reflecting a singly occupied dz2 orbital and a largely depopulated dx2-y2 orbital. An intriguing case of bond-stretch isomerism is seen for the axial Fe-O bonds in two crystallographically independent mixed ethanol-water species, and it is accounted for by their distinct intra- and intermolecular hydrogen-bond arrays. The Mössbauer spectrum (delta = 0.35(1) mm s-1 and delta EQ = 3.79(1) mm s-1 at 77 K) indicates a strong tetragonal distortion around the ferric ion, in agreement with the structural data. The value of the magnetic moment (mu eff = 3.8 mu B in the range 50-300 K) strongly supports a mid-spin state (S = 3/2). The EPR spectrum at 80 K (g perpendicular approximately 4.0, g parallel approximately 2.00) is consistent with a nearly pure mid-spin state (4A2) with little rhombic distortion. The 1H NMR spectra in CDCl3-EtOH exhibit upfield-shifted resonances for the pyrrole protons (delta approximately -30 ppm) which are consistent with the depopulated iron dx2-y2 orbital. Solution equilibria with water and various alcohols, and the spin state of the corresponding species, are discussed on the basis of the NMR data. The bisethanol and ethanol-water species are potential models of unknown hemoprotein ligation states such as Tyr(OH)/Tyr(OH) or Tyr(OH)/H2O that could be obtained by site-directed mutagenesis.

Nitric Oxide Addition to the Ferrous Nitrosyl Porphyrins Fe(P)(NO) Gives trans-Fe(P)(NO)2 in Low-Temperature Solutions

Nitric Oxide Addition to the Ferrous Nitrosyl Porphyrins Fe(P)(NO) Gives <i>trans</i>-Fe(P)(NO)₂ in Low-Temperature Solutions

The Role of the Distal and Proximal Protein Environments in Controlling the Ferric Spin State and in Stabilizing Thiolate Ligation in Heme Systems: Thiolate Adducts of the Myoglobin H93G Cavity Mutant

Recently, heme protein cavity mutants have been engineered in which the proximal coordinating amino acid has been replaced by a smaller, noncoordinating residue leaving a cavity that can be filled by exogenous axial ligands. This approach was pioneered by Barrick (Biochemistry 1994, 33, 6546−6554) with H93G sperm whale myoglobin where the coordinating histidine is replaced by glycine and the proximal cavity is filled with imidazole. In the present study, models for cysteine thiolate-ligated ferric cytochrome P450 have been prepared using H93G myoglobin containing thiolate ligands in the cavity. Despite the availability of water to serve as a distal ligand as occurs in both ferric wild type myoglobin and ferric H93G myoglobin with imidazole in the cavity, the ferric H93G thiolate complexes spectroscopically resemble five-coordinate high-spin substrate-bound ferric P450, which contains a thiolate proximal ligand and lacks a water distal ligand. Thus, the distal protein environment plays a crucial role in controlling whether a five-coordinate thiolate-ligated ferric heme binds water or not. Two advantages pertain to the present thiolate-ligated heme protein model relative to purely synthetic thiolate-ligated ferric porphyrins: (a) aliphatic thiols can form complexes without reduction of the ferric iron and (b) mixed ligand complexes that are stable at ambient temperatures can be prepared with a neutral ligand such as imidazole trans to thiolate. However, when anionic ligands are added to the ferric thiolate adduct in an attempt to prepare mixed ligand complexes with two anionic ligands, the thiolate ligand is displaced (or lost) without formation of a stable mixed ligand derivative. Further, reduction of the ferric-thiolate complex leads to loss of the thiolate ligand even in the presence of CO. The data presented for the thiolate adducts of ferric H93G myoglobin are analyzed in the context of the spectrally related H93C myoglobin mutants. The inability of the thiolate adducts of H93G myoglobin to accommodate a second anionic ligand in the ferric state or to remain thiolate-ligated in the ferrous state is likely due to the lack of (a) correctly positioned hydrogen bond donating groups and (b) a properly oriented helix dipole to stabilize the thiolate ligand as occurs in the proximal protein environment of P450. The present results illustrate the important role of the distal and proximal heme environments in controlling the ferric spin state and in stabilizing thiolate ligation in heme systems, respectively.

Barriers to Inversion of Nonplanar Ferric Porphyrin Radical Cations: Comparison with the Corresponding Neutral Complexes

Barriers to inversion of the highly deformed porphyrin rings in two ferric porphyrin radical cations, [Fe(OMTPP)Cl]SbCl6 and [Fe(OETPP)Cl]SbCl6 where OMTPP and OETPP are dianions of 2,3,7,8,12,13,17,18-octamethyl- and 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin, respectively, have been determined by the dynamic NMR method. Inversion barrier of [Fe(OMTPP)Cl]SbCl6 is estimated to be 14.8 kcal mol-1 (72 °C) which is higher than that of the corresponding neutral porphyrin complex [Fe(OMTPP)Cl] by more than 4 kcal mol-1. Increase in barrier to inversion has also been observed in [Fe(OETPP)Cl]SbCl6, though the precise determination is hampered in this case due to the occurrence of site exchange of the ferric ion across the porphyrin ring; the latter process is verified because the activation free energies obtained from the line shape analysis of the meta and methyl signals are quite close, 18.1 (113 °C) and 17.9 (113 °C) kcal mol-1, respectively. Thus, the inversion barrier in [Fe(OETPP)Cl]SbC...

Syntheses, NMR and EPR Spectroscopy, Electrochemical Properties, and Structural Studies of [5,10,15,20-Tetrakis(perfluoroalkyl)porphinato]iron(II) and -iron(III) Complexes

Syntheses, structural studies, electrochemistry, and spectroscopy of a number of [5,10,15,20-tetrakis(heptafluoropropyl)porphinato]iron derivatives are presented. The X-ray crystal structure of 5,10,15,20-tetrakis(heptafluoropropyl)porphinato]iron(II)·(pyridine)2 exhibits a substantial S4 distortion of the porphyrin macrocycle, with the meso-carbon atoms displaced more than 0.6 Å above and below the porphyrin mean plane defined by the four central nitrogen atoms; the most notable aspect of this ferrous porphyrin structure is the fact that it exhibits metrical features commonly manifested in crystallographically characterized ferric porphyrin complexes. X-ray data are as follows: P21/n with a = 12.772(1) Å, b = 18.895(2) Å, c = 19.756(2) Å, β = 99.960(6)°, V = 4695.7(8) Å3, Z = 4, and dcalc = 1.689 g/cm3. 19F NMR spectroscopy confirms the sensitivity of the 19F nucleus as a probe of macrocycle aromaticity and electronic structure, while 1H NMR spectroscopic studies show large isotropic shifts for the β-protons of the (porphinato)iron(III) chloride derivative (δ = 101.5 and 86.4 ppm). Electrochemical data obtained from cyclic voltammetric and spectroelectrochemical experiments reveal that the E1/2 value for the FeII/III redox couple for 5,10,15,20-tetrakis(heptafluoropropyl)porphinato]iron·(pyridine)2 is shifted by 550 mV relative to that observed for the corresponding (porphinato)iron(III) chloride complex. The cathodic electrochemistry of [5,10,15,20-tetrakis(heptafuoropropyl)porphinato]iron·(pyridine)2 is also unusual in that the first one-electron reduction of this complex produces a largely macrocycle-localized radical anion. EPR spectroscopic data shows that 5,10,15,20-tetrakis(heptafluoropropyl)porphinato]iron(III)·(pyridine)2 manifests a pure axial spectrum (Δ/λ = −26.4; Σg2 = 12.53) congruent with a (dxz,dyz)4(dxy)1 electronic ground state. The extraordinary structural, potentiometric, and spectroscopic properties of these (porphinato)iron species arise from substantially reduced metal-centered electron density effected by the macrocycle's non-π-conjugating, σ-electron-withdrawing meso-perfluoroalkyl substituents.

Synthesis, Molecular Structures, and Properties of Six-Coordinate [Fe(OEP)(L)(NO)]+ Derivatives: Elusive Nitrosyl Ferric Porphyrins

The preparation and characterization of several {FeNO}6 porphyrinates ([Fe(OEP)(L)(NO)]+), where OEP is octaethylporphyrin and L is one of the neutral nitrogen-donating axial ligands 1-methylimidazole (1-MeIm), pyrazole (Pz), indazole (Iz), or pyrazine (Prz), are described. The synthesis and crystallization of these derivatives requires an NO atmosphere and careful attention to conditions. The characterization of these [Fe(OEP)(L)(NO)]+ complexes by IR and UV−vis spectroscopies provides for general distinctions between five- and six-coordination and the two common oxidation states of iron nitrosyl porphyrins. The UV−vis spectra of the [Fe(OEP)(L)(NO)]+ derivatives are all characterized by very distinct α and β bands. The solid-state NO stretching frequencies are found in the narrow range of 1890 to 1921 cm-1. Six X-ray structure determinations have been completed. The molecular structures of [Fe(OEP)(1-MeIm)(NO)]ClO4, [Fe(OEP)(Pz)(NO)]ClO4(1), [Fe(OEP)(Iz)(NO)]ClO4, and {[Fe(OEP)(NO)]2Prz}(ClO4)2 are desc...

Coordination of N-Donor Ligands by the Monomeric Ferric Porphyrin N-Acetylmicroperoxidase-8

The monomeric ferric hemepeptide from cytochrome c, N-acetylmicroperoxidase-8 (NAcMP8), retains the proximal His ligand and has a coordinated water molecule in the second axial site. It provides an opportunity for quantitatively studying in aqueous solution the coordination chemistry of an iron porphyrin with a single accessible coordination site. An examination of the dependence of the spectrophotometrically determined equilibrium constants for coordination of imidazole and cyanide as a function of pH provides a procedure for correcting conditional equilibrium constants for the ionization of coordinated H2O in NAcMP8 and protonation of the entering ligand, L. An examination of the coordination by NAcMP8 of L = primary amines and imidazoles (25 °C, μ = 0.1 M) and pyridines (as a function of temperature, μ = 0.1 M) has shown that the process of binding L to NAcMP8 may be complex. With some ligands such as pyridines and some primary amines there is evidence for formation of an outer-sphere complex before di...

High-Spin (meso-Tetraalkylporphyrinato)iron(III) Complexes As Studied by X-ray Crystallography, EPR, and Dynamic NMR Spectroscopies.

1H NMR spectra of a series of high-spin (meso-tetraalkylporphyrinato)iron(III) chlorides, [Fe(TRP)Cl] where R = Me, Et, Pr, or (i)Pr, have been measured at various temperatures in CD(2)Cl(2) solution. In the case of the Et, Pr, and (i)Pr complexes, either the methyl or the methylene signal split into two signals with equal integral intensities at low temperature. In contrast, the Me complex did not show any splitting even at -100 degrees C. The results have been ascribed to the hindered rotation of the meso-alkyl groups about C(meso)-C(alpha) bonds. The activation free energies for rotation have been determined as 8.0 (-72 degrees C), 8.5 (-60 degrees C), and 8.9 (-62 degrees C) kcal.mol(-1) for the Et, Pr, and (i)Pr complexes, respectively, at coalescence temperatures given in parentheses. The small activation free energy for rotation of the isopropyl groups observed in the present system is explained in terms of the nonplanarity of the porphyrin ring, which has been verified both by the X-ray crystallographic analysis and by the EPR spectrum taken in a frozen CH(2)Cl(2)-toluene solution. The success in observing the hindered rotation of less bulky primary alkyl groups such as ethyl and propyl groups at an easily accessible temperature range is attributed to the large difference in chemical shifts of the mutually exchanging protons, ca. 3500 Hz in the case of the Et complex, caused by the paramagnetism of the five-coordinated ferric porphyrin complexes.