Meso-carbon Atoms Research Articles

Cover Picture: Angew. Chem. Int. Ed. 8/2002

The cover picture shows the simple, one-step synthesis of cyclo[8]pyrroles, a new class of aromatic heteroannulene. Using a newly developed biphasic oxidative coupling procedure that employs aqueous FeCl3 as the oxidant, these novel porphyrin-like systems may be obtained in yields exceeding 70% starting from well-known bipyrrolic precursors. While formally derived from porphyrin as the result of substituting a pyrrole for each of the original bridging meso-carbon atoms, the cyclo[8]pyrroles differ from these quintessential tetrapyrrolic macroycles by virtue of Q-like absorption bands that are red-shifted into the near IR. Cyclo[8]pyrroles also show a propensity for protonation and anion binding that is underscored by the X-ray structure of the sulfate salt that makes up the background for this illustration. More on these exciting new molecules can be found in the communication by J. L. Sessler et al. on pp. 1422–1425.

Pulsed EPR Characterization of the Low-Spin Iron(III) Porphyrinate Complexes with Phenyl Isocyanide Ligands Having the dxy Orbital Ground State

Pulsed electron−nuclear double resonance and electron spin−echo envelope modulation spectroscopies have been used to characterize the bis(phenyl isocyanide) complexes of iron(III)-tetraphenylporphyrin ([TPPFe(PhNC)2]+) and iron(III)-octaethylporphyrin ([OEPFe(PhNC)2]+) that have the dxy orbital configuration. The spin density distribution in the porphyrin π system was obtained, with major spin density (∼0.06) being delocalized to each meso carbon atom. The spin density on the pyrrole β carbons was found to be negligible. The upper limit for spin density on Fe3+ was estimated to be about 0.63. The hyperfine interaction constants on the pyrrole nitrogens (on average, ∼3.3 MHz) were also evaluated. The overall spin density distribution obtained was found to be in agreement with that estimated theoretically (Ghosh, A.; Gonzalez, E.; Vangberg, T. J. Phys. Chem. B 1999, 103, 1363) using density functional theory.

A Remarkable Skeletal Rearrangement of a Coordinated Tetrapyrrole: Chemical Consequences of Palladium π-Coordination to a Bilindione

Pd4(OEB)2, in which a [Pd2]2+ unit is bound in pi-fashion to olefinic sites that are exocyclic to pyrrole rings of the octaethylbilindione ligand, undergoes an unprecedented sequence of reactions that results in the rearrangement of the framework of the bilindione ligand and the formation of trans-Pd(py)2I2. This process of bilindione rearrangement and oxidation occurs as a direct consequence of the pi-coordination of the palladium. The reaction results in the migration of a nitrogen atom from a pyrrole carbon atom to what was formerly a meso carbon atom to transform a former pyrrole ring into a six-membered ring. This process also involves cleavage of the Pd-Pd and Pd-C bonds, oxidation of palladium, and introduction of an oxygen atom (from water) not necessarily in this particular sequence.

Calixphyrins. Hybrid macrocycles at the structural crossroads between porphyrins and calixpyrroles

Calixphyrins are a class of hybrid molecules that lie at the structural crossroads between porphyrins and calixpyrroles. Porphyrins, long known for their versatile metal cation coordination chemistry, are macrocycles that contain only sp2-hybridized bridging meso carbon atoms within their framework. Calix[n]pyrroles, on the other hand, are porphyrin analogs that contain pyrroles bridged exclusively by sp3 meso carbon centers, and in recent years have been shown to display remarkable anion-binding properties. Calix[n]phyrins bear analogy to both the porphyrins and calixpyrroles and are macrocyclic analogs that contain a mixture of sp2- and sp3-hybridized meso carbon bridges. This leads to partial interruptions in the conjugation pathway of the molecule, introduces novel structural features, and leads to interesting anion and cation recognition properties. It also allows for modular syntheses. In the present paper, the chemistry of calix[n]phyrins, still at an early stage of exploration, is reviewed.

Multinuclear ( 13C, 17O, 57Fe) NMR studies of carbonmonoxy heme proteins and synthetic model compounds

13C, 17O and 57Fe NMR spectra of several carbonmonoxy hemoprotein models with varying polar and steric effects of the distal organic superstructure, constraints of the proximal side, and porphyrin ruffling are reported. Both heme models and heme proteins obey a similar excellent linear δ( 13C) versus ν(C–O) relationship which is primarily due to modulation of π-back-bonding from the Fe d π to CO π * orbital by the distal pocket polar interactions. The lack of correlation between δ( 13C) and δ( 17O) suggests that the two probes do not reflect a similar type of electronic and structural perturbation. δ( 17O) is not primarily influenced by the local distal field interactions and does not correlate with any single structural property of the Fe–C–O unit; however, atropisomerism and deformation of the porphyrin geometry appear to play a significant role. 57Fe shieldings vary by nearly 900 ppm among various hemes and an excellent correlation was found between δ( 57Fe) and the absolute crystallographic average displacement of the meso carbon atoms, | C m|, relative to the porphyrin core mean plane. The excellent correlation between iron-57 shieldings and the average shieldings of the meso carbons of the porphyrin skeleton of TPP derivatives suggests that the two probes reflect a similar type of electronic and structural perturbation which is primarily porphyrin ruffling.

A Conformational Study of 5,10,15,20-Tetraalkyl-22H+,24H+-porphyrindiium Salts (Dication Salts)

Abstract N-protonation of 5,10,15,20-tetraalkylporphyrins with n-butyl, isobutyl, isopropyl, 1-ethyl-propyl or tert-butyl substituents yields the respective 22,24-dihydroporphyrins (22H+,24H+-porphyrindiium salts) that exhibit highly nonplanar conformations. Depending on the steric demand of the meso substituents, the free base porphyrins have planar to moderately ruffled macrocycles (primary or secondary alkyl residues) or are severely ruffled (tertiary alkyl resi­dues). X-ray crystallographic studies show that protonation of porphyrins with n-Bu (2), i-Pr (3) or EtPr (4) substituents leads to steric congestion of the core resulting in symmetric, saddle distorted macrocycles as evidenced by average displacements of the Cb positions from the mean plane ranging from 0.79 to 1.01 Å. The structures observed for these porphyrin diacids are very similar in distortion mode and degree of nonplanarity to 5,10,15,20-tetraaryl-22H+,24H+-porphyrindiium salts and to 2,3,7,8,12,13,17,18-octaalkyl-5,10,15,20-tetraaryl-porphyrins. In contrast, protonation of the highly ruffled 5,10,15,20-tetrakis(teA-r-butyl)porph-yrin results in a switch of the conformational distortion mode. The dication 6 shows both saddle distortion (Cb displacements -1.3 Å) and ruffling of the macrocycle (Cm displacements of approx. 0.48 Å). Thus, the need to accomodate four N-H units in the porphyrin core with its associated out-of-plane tilting of the pyrrole rings and the steric strain induced at the meso carbon atoms results in a highly nonplanar porphyrin with mixed distortion modes.

Redox characteristics of nickel and palladium complexes of the open-chain tetrapyrrole octaethylbilindione: a biliverdin model.

Octaethylbiliverdin, C35H46N4O2, is a linear tetrapyrrole that can exist in coordinated form as the fully reduced trianion (OEB)3-, as the two electron oxidized monoanion, (OEBOx)-, or as the one electron oxidized radical, (OEB.)2-. The three-membered electron-transfer series involving [M(OEB)]n with n = +1, 0, and -1, and M = Ni or Pd has been characterized electrochemically. The most highly oxidized members of these series have been isolated in the form of their diamagnetic triiodide salts, [NiII(OEBOx)]I3 and [PdII(OEBOx)]I3, and characterized spectroscopically and by X-ray diffraction. [NiII(OEBOx)]I3 crystallizes in the orthorhombic space group Pbcn with a = 15.121(3) A, b = 16.777(3) A, and c = 14.628(3) A at 130(2) K with Z = 4. Refinement of 3311 reflections with 209 parameters and no restraints yielded wR2 = 0.136 and R = 0.054 for 2643 reflections with I > 2 sigma(I). The structure involves helical coordination of the linear tetrapyrrole ligand about the nickel with all four nitrogen atoms coordinated to the metal and Ni-N distances of 1.867(5) and 1.879(5) A. The triiodide ion is not coordinated to the nickel but sits over one of the meso carbon atoms of the tetrapyrrole. In the solid state, pairs of [NiII(OEBOx)]+ crystallize about a center of symmetry so that two identical tab/slot hydrogen-bonded arrangements involving the lactam oxygen of one complex and methine and two methylene protons of the adjacent cation. Similar hydrogen-bonded motifs are found in other complexes derived from octaethylbilindione. [PdII(OEBOx)]I3 is isomorphic with the nickel analogue and crystallizes in the orthorhombic space group Pbcn with a = 15.2236(6) A, b = 16.7638(7) A, and c = 14.6289(6) A at 90(2) K with Z = 4. Refinement of 6123 reflections with 209 parameters and no restraints yielded wR2 = 0.094 and R = 0.036 for 4042 reflections with I > 2 sigma(I). The odd electron compound PdII(OEB.) has also been isolated and characterized by single-crystal X-ray diffraction. Black PdII(OEB.) crystallizes in the monoclinic space group I2/a with a = 13.274(3) A, b = 18.655(4) A, c = 14.114(3) A, and beta = 116.00(3) degrees at 140(2) K with Z = 4. Refinement of 2030 reflections with 195 parameters and no restraints yielded wR2 = 0.090 and R = 0.042 for 1715 reflections with I > 2 sigma(I). The structure again involves helical coordination of the linear tetrapyrrole to the palladium through the four pyrrole nitrogen atoms. Reduction of the complex causes a slight elongation of the Pd-N bonds from 1.983(2) and 1.986(2) A in [PdII(OEBOx)]I3 to 2.011(4) and 2.012(4) A in PdII(OEB.).

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.

5,10,15,20-Tetrakis(diphenylmethyl)porphyrin - A Nonplanar Porphyrin with Intermediate Degree of Ruffling

The title compound was prepared by condensation of pyrrole and diphenylacetaldehyde in 16% yield. Based on spectroscopic data, the free base porphyrin exhibits a nonplanar macrocycle in solution with a degree of distortion between that of 5,10,15,20-tetracyclohexylporphyrin and 5,10,15,20-tetra(t-butyl)porphyrin. A crystal structure determination of the cobalt(II) complex reveals a ruffled macrocycle conformation with an average out-of-plane displacement of the meso carbon atoms by 0.55 Å.

Theoretical Study of the Q and B Bands of Free-Base, Magnesium, and Zinc Porphyrins, and Their Derivatives

The multireference Møller−Plesset perturbation (MRMP) theory with complete active space self-consistent field (CASSCF) reference functions is applied to the study of the Q and B bands of free-base, magnesium, and zinc porphyrins and their derivatives. The Q band of free-base porphyrin is so weak because the transition is pseudoparity-forbidden because of the alternancy symmetry. The Q band in the visible region can be intensified if the pairing property is broken. This is achieved by various chemical modifications to the basic structure of free-base porphyrin such as external substituents, changes in conjugation pathway, and change in central substituent. The intensity and color of the Q band as well as the macrocyclic porphyrin skeleton are perturbed to a greater or lesser extent by various chemical modifications. Insertion of aza groups in meso position leads to the shrinking of the size of the central hole, whereas hydrogenation to the exo double bond of pyrrole rings expands the central hole. Substitution of nitrogen atoms for meso carbon atoms significantly intensifies the oscillator strength of the higher energy side transition of the Q band with the peak position unchanged. Ring reduction widens the splitting of two peaks of the Q band, giving rise to the characteristically strong far-red band and the weaker visible band. Changes from metal-free to metalloporphyrins have a less dramatic effect on the spectra.

Iron-57 Nuclear Shieldings as a Quantitative Tool for Estimating Porphyrin Ruffling in Hexacoordinated Carbonmonoxy Heme Model Compounds in Solution

57Fe NMR spectra of several carbonmonoxy hemoprotein models with varying polar and steric effects of the distal organic superstructure, constraints of the proximal side, and porphyrin ruffling are reported. 57Fe shieldings, δ(57Fe), vary by nearly 900 ppm among various hemes, and an excellent correlation was found between δ(57Fe) and the absolute crystallographic average displacement of the meso carbon atoms, |Cm|, relative to the porphyrin core mean plane. The great variation of δ(57Fe) as a function of |Cm| (∼140 ppm/0.1 A) demonstrates that iron-57 shieldings can be used in structure refinement protocols for the extraction of more accurate structures for heme rings in heme model compounds. The excellent correlation between iron-57 shieldings and the average shieldings of the meso carbons of the porphyrin skeleton of TPP derivatives suggests that the two probes reflect similar types of electronic and structural perturbations, which are primarily due to porphyrin ruffling. The present findings also empha...

Hydrolytic breakdown of meso-diphenylamine substituted heptamethine dye IR140 and isomerism of the merocyanine breakdown product

Nucleophilic attack by hydroxyl ions is found to occur preferentially at the C 2 position of the benzothiazolium heterocyclic moiety of the laser dye IR140, and not at the eneamine functionality that consists of the mesocarbon atom and the adjacent diphenylamine moiety, which is the other electrophilic site on the dye. The temperature-dependence of the absorption spectrum of the merocyanine breakdown product suggests that similar ground state isomerism is present in the breakdown product, as is found in the initial dye IR140. It is concluded that the non-fluorescent majority conformer of the merocyanine breakdown product is the one in which the meso-amine substituent is twisted so as to withdraw electron density by induction, and that the fluorescent minority conformer is that in which themeso-amine substituent is conjugated to act as an electron donor by resonance.

Structural evolution of the heme group during the allosteric transition in hemoglobin: Insights from resonance raman spectra of isotopically labeled heme

Visible resonance Raman spectra in the low-frequency region (200–500 cm−1) are reported for hemoglobin (Hb) reconstituted with heme that is deuterated at the meso carbon atoms (meso-d4). Spectra were obtained in the deoxy form and in the immediate photoproduct of the carbonmonoxide adduct, HbCO. The isotope shifts permit assignment of two out-of-plane modes, γ6 and γ7, and the in-plane skeletal mode ν8, as well as the well-known iron-histidine [Fe-His] stretching vibration. Important differences between deoxyHb and the immediate photoproduct include 1) a large upshift in the Fe-His frequency, from 216 to 228 cm−1, 2) an upshift in γ6 (349 to 353 cm−1) together with substantial diminution of the ν8 (341 cm−1) intensity, and 3) collapse of two γ7 bands (305 and 296 cm−1) to a single band at 304 cm−1. This last observation implies subunit heterogeneity in deoxyHb but not in the photoproduct. When these bands are monitored in the time-resolved RR spectra following HbCO photolysis, it is seen that subunit heterogeneity is first detectable in the 0.5-μs transient [intermediate S], which has been associated with the initial rearrangement of the subunits to form the T-state contacts, on the basis of ultraviolet RR spectroscopy.1 However the intensification of ν8 does not occur until the 17-μs transient (intermediate T′), in which the T-state contacts are locked in and the Fe-His bond is strained. Implications for the mechanism of Hb allostery are discussed. © 1996 John Wiley & Sons, Inc.

15N NMR spectroscopy of 5-(2-heteroaryl)formazans and their zinc and cadmium chelates

The15N NMR shifts of 5-heteroarylformazans (Het = 3-chloro-2-quinoxalyl and 2-benzothiazolyl) and their complexes with zinc and cadmium and also some model compounds were measured. Procedures were developed for the use of15NMR in structural investigations of formazans and their complexes, and the formation of pseudooctahedral bisformazanates with metals of the zinc subgroup was confirmed. The increments of complexation in the chemical shifts of the15N nitrogens and13C carbons of the tridentate heteroarylformazans are not independent quantities. There are algebraic combinations of the increments in the chemical shifts that are practically independent of the substitution of the zinc by cadmium, substitution of substituents at the 1-aryl and the meso-carbon atom, and even substitution of the heteroaryl. This property is reminiscent of the analogous relations between the increments of the substituents in such π-conjugated systems as benzene derivatives.

Molecular and crystal structure of dichloro(5,10,15,20-tetraphenylporphinato)phosphorus(V) chloride dichloromethane solvate

The crystal structure of dichloro(5,10,15,20-tetraphenylporphinato)phosphorus(V) chloride dichloromethane solvate has been determined by X-ray diffraction at ambient temperature. A saddle shape of the meso-tetraphenylporphinate disc is attributed to marked S 4 deviation from planarity. The mode of the torsion of four phenyl rings around the CC bonds connecting these rings to the respective meso carbon atoms are also of S 4 character. The phosphorus atom lies just in the mean plane of the porphinate nitrogen atoms. The PN bond distances range from 1.804(3) to 1.821(3) Å, which are shorter than those observed in [P(OH) 2(tpp)] + (1.870–1.916 Å), while the axial PCl bond distances [2.149(1) and 2.153(1) Å] are significantly longer than the distance expected for the normal covalent PCl bond (2.10 Å). This phosphorus porphyrin reveals characteristic features peculiar to the structure of metalloporphyrins with the smallest-sized central cations.

The electrochemistry of a stable Zn isoporphyrin

Recently, Xie and Smith [l] described the synthesis of a stable isoporphyrin with Z&I) as the central metal species (Scheme 1). Previously, similar compounds had been formed in solution by several techniques including electro-oxidation [2], but they had not been isolated. Isoporphyrins are especially interesting from an electrochemical point of view because of their unusual redox properties with respect to those of normal porphyrins. Chemically, the macrocyclic conjugation in the isoporphyrin is interrupted at position 5 owing to the presence of a saturated mesocarbon atom [l]. This has interesting consequences which may be possible with respect to the kinetics of electron transfer if the electron is transferred to the ring rather than the central metal ion. Although much is known about the redox chemistry of porphyrins [31, the kinetics of electron transfer to or from these systems have rarely been studied. An important question is whether electron transfer involves the central metal ion or the surrounding ring [41. If electron transfer takes place on the ring, it is expected to be fast [4,5], the magnitude of the rate constant depending on the extent of electron delocalization. Other factors affecting the rate of electron transfer are the spin state of the central metal ion [6] and structural changes accompanying the reaction [7]. The purpose of the present study was to determine the effect of the interruption of conjugation in the isoporphyrin ring on the kinetics of electron transfer. In order to examine this question the kinetic parameters for oxidation and reduction of the zinc isoporphyrin (ZnIP) are compared with those for zinc octaethylporphyrin (ZnOEP), a system known to undergo fast electron transfer [8]. Since the possible extent of delocalization involves the same area in the two molecules, this

1H NMR spectra and electronic structure of reduced iron porphyrins: Fe(II), Fe(I) and Fe(0) porphyrins

Fe(II), Fe(I) and Fe(0) porphyrins have been generated by stepwise reduction with a sodium mirror in vacuum and their 1H NMR spectra have been recorded and analyzed. Fe(II) porphyrins (Fe(II)P) have been examined in three spin states, an S = 0 state in pyridine- d 8, an S = 1 state in benzene- d 6 and an S = 2 state in tetrahydrofuran- d 8 (THF- d 8). The analysis of isotropic shifts for low-spin Fe(II)P ( S = 0) has indicated that no charge transfer has been observed. The ground state configuration is (d xy) 2 (d xz,d yx) 4. The contact shifts for intermediate-spin Fe(II)P ( S = 1) reflect P → Fe π charge transfer. The proposed electron configuration is (d xy) 2 (d z2) 2 (d xz, d yz) 2, which agrees with Mössbauer data. The pattern of contact shifts for high-spin Fe(II)P ( S = 2) is consistent with σ spin transfer, which suggests that the d x2-y2 orbital possesses an unpaired spin. The electron configuration is (d xy) 2 (d xz, d yz 2 (d z2) 1 (d x2-y2) 1. Our results for Fe(II)P ( S = 1,2) agree with the literature 1H NMR data . In the case of Fe(I)P in THF, the separation of isotropic shifts into the dipolar and contact contributions has shown the dominance of the latter. The observed shifts indicate negative π spin density on pyrrole and meso carbon atoms of the ligand, which seems to be due to a strong π-π spin polarization effect. When this fact is taken into account the pattern of contact shifts is consistent with π spin transmission involving both P → Fe π charge transfer out of the ligand-filled molecular (3e(π)) orbital and Fe → P π * charge transfer into the ligand highest unoccupied (4e(π *)) molecular orbital. The occurrence of the unpaired spin in this molecular orbital is consistent with π-radical anion formulation which was found by X-ray crystallography. An S = 1/2 spin state determined by magnetic moment measurements agrees with the most probable electron configuration (d xy) 2 (d xz, d yz 3 (d z2) 2. In the case of Fe(0)P, the isotropic shifts were found to be small, providing evidence of some spin transfer. The ground state configuration is (d xy) 2 (d xz, d yz) 4 (d z2) 2.

Structure and ligand binding properties of leucine 29(B10) mutants of human myoglobin.

Site-specific mutants of human myoglobin (Mb) have been prepared, in which Leu29 (B10) is replaced by Ala(L29A) or Ile(L29I), in order to examine the influence of this highly conserved residue in the hydrophobic clusters of the heme distal site on the heme environmental structure and ligand binding properties of Mb. Structural characterizations of these recombinant Mbs are studied by electronic absorption, infrared (IR), one- and two-dimensional proton nuclear magnetic resonance spectroscopies, and ligand-binding kinetics by laser photolysis measurements under ambient and high pressures (up to 2000 bar). Multiple split carbon monoxide (CO) stretch bands in the IR spectra of mutant Mbs exhibit a relative decrease of the 1945 cm-1 band (approximately 50%) which is associated with an upright binding geometry of CO, accompanied by an increase of the tilted CO conformer at 1932 cm-1. On the basis of these results, replacement of Leu29(B10) by Ala or Ile appears to allow bound CO to rotate from a conformation pointing toward the beta meso carbon of the heme group to the one pointing toward the alpha meso carbon atom, presumably filling the space left by removal of the delta 2 carbon atom of Leu29(B10). These substitutions cause the rate constants for CO and O2 association to decrease almost 3-5-fold. Present results show that CO and O2 bindings to the heme iron of Mb are controlled by Leu29(B10) by influencing the structure of close vicinity of the heme and the geometry of iron-bound ligand. Further, mutant Mbs (Leu72(E15)----Ala and Leu104 (G5)----Ala) which have altered residues in another hydrophobic clusters around proximal and distal site are also examined.

Models of the cytochromes b. 8. Two-dimensional nuclear Overhauser and exchange spectroscopy studies of paramagnetic "cavity" type (tetraphenylporphinato)iron(III) complexes of planar ligands

The paramagnetic bis-2-methylimidazole complex of (tetramesitylporphinato)iron(III) chloride, with short TI and T, relaxation times, was investigated in CD2C12 by NOESY (nuclear Overhauser and exchange spectroscopy) experiments as a function of temperature (-29, -54, -74 C). The two-dimensional (2-D) maps obtained at -29 and -54 C revealed cross peaks due to both chemical exchange and the nuclear Overhauser effect (NOE), while the map obtained at -74 C showed only NOE cross peaks. The NOE cross correlations are consistent with the 2-MeImH ligands being positioned over or near the meso carbon atoms, rather than over the porphyrin nitrogens, as previously suggested by Nakamura and Groves (Tetrahedron 1988, 44, 3225). The nonbonded interactions between the methyl groups of the 2-methylimidazole (2-MeImH) ligands and the mesityl groups of the porphyrin phenyl rings for this orientation of the axial ligands produce a strongly ruffled porphyrin core having two oblong cavities at right angles to each other, one above and one below the plane of the porphyrin ring, which hold the hindered axial ligands in perpendicular planes over the meso positions. The chemical exchange cross peaks are indicative of interconversion of all resonances of a particular type, due to rotation of the axial ligands rather than axial ligand exchange. This interpretation is confirmed by the fact that there are no cross peaks observed between free and coordinated 2-MelmH resonances at any of the temperatures investigated. Rotation of the 2-MeImH ligands requires that alternate phenyl rings flap up and down as the ligands rotate. The coupling pattern between the pyrrole-H protons is consistent with that expected on the basis of the extrapolated intercepts of the Curie plot.

The photo-oxidation of meso-tetraphenylporphyrins

The photo-oxidation of meso-tetraphenylporphyrin (TPP) dianion in methanol affords a purple benzoyl bilinone (5b) by addition of singlet oxygen, followed by ring-opening and addition of methanol; similar products (5a) and (5c) can be formed by addition of water, or ethanol, respectively. The water adduct (5a) had previously been obtained by other workers from the photo-oxidation of metallo derivatives of TPP, by cerium(IV) or thallium(III) oxidation of TPP, or by treatment of the cation radical of TPP with nitrite, but a different structure had been assigned. Our new structure(5) has been deduced from detailed NMR and mass spectral studies including decoupling and NOE measurements, as well as field desorption mass spectra at low wire currents; the presence of quaternary meso-carbon atoms in structures (5a) and (5b) has been shown by the 13C NMR spectra of the meso13C -labelled pigments. Confirmation of these assignments has been provided by photo-oxidation of the meso-tetra(4-methoxyphenyl)porphyrin and further spectral studies of the resulting bilinones (7a) and (7b), including NOE and 13C -1H shift correlated NMR spectra.