Pyrrole Signals Research Articles

Effects of porphyrin deformation on the 13C and1H NMR chemical shifts in high-spin five- and six-coordinate manganese(III) porphyrin complexes

As an extension of our study to reveal the effect of porphyrin deformation on the [Formula: see text]C and1H NMR chemical shifts, both five- and six-coordinate high-spin (S [Formula: see text] 2) Mn(III) complexes such as Mn(Por)Cl and [Mn(Por)(CD3OD)2]Cl have been prepared, where Por is a porphyrin dianion such as TPP, OMTPP, and T[Formula: see text]PrP. Molecular structures of five-coordinate Mn(OMTPP)Cl and Mn(TiPrP)Cl have been determined by the X-ray crystallographic analysis. As expected, Mn(OMTPP)Cl and Mn(TiPrP)Cl have exhibited a highly saddled and highly ruffled porphyrin core, respectively. The [Formula: see text]C NMR spectra have revealed that these complexes generally exhibit the [Formula: see text]-pyrrole signals at the downfield positions and [Formula: see text]-pyrrole an. meso signals at the upfield positions. The results suggest that the spin polarization of Mn(III)–NPσ bonds, which occurs in all the high-spin Mn(III) complexes, is the major factor to determine the chemical shifts of the porphyrin carbon signals (Cheng, R.-J.; Chang, S.-H.; Hung, K.-C. Inorg. Chem. 2007; 46: 1948–1950). Although th. meso and [Formula: see text]-pyrrole signals are observed at the upfield and downfield positions, respectively, these signals are widely dispersed depending on the deformation mode of the porphyrin ring. The results have been explained in terms of the strong spin polarization of the Mn–NPbond together with the specific metal-porphyrin orbital interactions such as: (i) the a2u-dz2 interaction in five-coordinate complexes, (ii) the a2u-dxyinteraction in ruffled complexes, and (iii) the a2u-dx2-y2 interaction in saddled complexes.

The use of electrospray ionization tandem mass spectrometry on the structural characterization of novel asymmetric metallo-organic supermolecules, based on pentafluorophenylporphyrins and ruthenium complexes

The novel asymmetric metallo-organic triads cis- and trans-[B(4-py)BPFPH 2{Ru 3O(Ac) 6(py) 2}{Ru(bpy) 2Cl}](PF 6) 2 ( 5a,b) for which cis- and trans-B(4-py)BPFPH 2 = 5,10-bis(pentafluorophenyl)-15,20-bis(4-pyridyl)porphyrin and 5,15-bis(pentafluorophenyl)-10,20-bis(4-pyridyl)porphyrin, respectively; Ac = acetate; py = pyridine and bpy = 2,2′-bipyridine, as well as their corresponding monosubstituted dyads cis- and trans-[B(4-py)BPFPH 2{Ru 3O(Ac) 6(py) 2}]PF 6 ( 4a,b) have been structurally characterized via electrospray ionization mass spectrometry (ESI-MS and ESI-MS/MS). The ESI-MS of dyads 4a,b display two characteristic Ru-multicomponent clusters of isotopologue ions corresponding to singly charged ions 4a,b + of m/ z 1629 and doubly charged ions [ 4a,b+H] 2+ of m/ z 815 and the triads 5a,b are detected by ESI-MS as the intact doubly charged cluster of isotopologue ions of m/ z 1039 [ 5a,b] 2+. The ESI-MS/MS of 4a,b +, [ 4a,b+H] 2+ and [ 5a,b] 2+ reveal characteristic dissociation pathways, which confirm the structural assignments providing additional information on the intrinsic binding strengths of the gaseous ions. Although the gas-phase behavior of each pair of isomers was rather similar, the less symmetric dyads 4a,b are distinguished via the 1H NMR spectral profile of the pyrrolic signals. Exploratory photophysical assays have shown that both modifying motifs alter the porphyrinic core emission profile, opening the possibility to use these asymmetric systems as photophysical devices.

Interaction Between Terminal Zinc–Porphyrins in a U‐Shaped Trimeric Porphyrin Array Bearing Chiral Amino Acid Tails

AbstractA U‐shaped trimeric porphyrin array with a zinc/free‐base/zinc arrangement (5ZFZ) and its derivatives were synthesized in order to investigate the interacting porphyrins in organic solvents. The porphyrin array is comprised of an αα‐atropisomeric bis(ortho‐aminophenyl)porphyrin free‐base as the template, a pair of zinc–porphyrins at the termini, achiral glycine spacers linking the porphyrins, and chiral L‐leucine derivatives attached to the template in the opposite direction of the zinc–porphyrins. 1H NMR (high‐field shifts of some pyrrole signals), UV/Vis (split Soret band), and CD spectra (exciton‐coupled Cotton effect) show the close assembly of the terminal zinc–porphyrins of 5ZFZ in CDCl3 and CH2Cl2. In contrast, the characteristic spectra for the interacting porphyrins do not appear for the trimeric array with three free‐base porphyrins (5F3) and the dimeric array with a zinc/free‐base arrangement (9ZF). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

O-Confused OxaporphyrinAn Intermediate in Formation of 3-Substituted 2-Oxa-21-Carbaporphyrins

[Reaction: see text]. A condensation of 2,4-bis(phenylhydroxymethyl)furan with pyrrole and p-toluylaldehyde in the presence of ethanol formed 5,20-diphenyl-10,15-di(p-tolyl)-2-oxa-3-ethoxy-3-hydro-21-carbaporphyrin [(H,EtO)OCPH]H2. The new carbaporphyrinoid has 1H NMR features of an aromatic molecule, including the upfield shift of the inner H(21) atom (-5.48 ppm). Addition of acid removes the ethoxy substituent and converts [(H,EtO)OCPH]H2 into the dication of "true" O-confused oxaporphyrin {[(H)OCPH]H3}2+ via an exocyclic C(3)-O bond cleavage followed by an elimination of the ethoxy group as determined by 1H NMR. Addition of ethanol, water, or pyrrole converts {[(H)OCPH]H3]2+ into [(H,EtO)OCPH]H2, [(H,OH)OCPH]H2, or pyrrole appended O-confused porphyrin [(H,pyrrole)OCPH]H2, respectively. The reaction of [(H,OEt)OCPH]H2 with silver(I) acetate yields a stable Ag(III) complex [(H,OEt)OCP]AgIII substituted at the C(3) position by the ethoxy group and hydrogen. Coordination through the nitrogen donors is reflected by the presence of 107/109Ag scalar splitting seen for the selected -H pyrrolic signals. Addition of TFA to [(H,OEt)OCP]AgIII produces a weakly aromatic O-confused porphyrin complex {[(H)OCP]AgIII}+. In the course of this reversible process the tetrahedral-trigonal rearrangements originate at the C(3) atom but extend its consequences on the whole structure. Insertion of silver into the hydroxy analogue of [(H,OEt)OCPH]H2, i.e., [(H,OH)OCPH]H2, was accompanied by ligand oxidation, yielding carbaporpholactone which contains the lactone functionality instead of the regular furan moiety embedded in the carbaporphyrin ligand of [(O)OCP]AgIII. The structure was determined by X-ray crystallography. The macrocycle is only slightly distorted from planarity, and silver(III) is located in the NNNC plane.

Electron Configuration and Spin Distribution in Low-Spin (meso-Tetraalkylporphyrinato)iron(III) Complexes Carrying One or Two Orientationally Fixed Imidazole Ligands1

To understand the orientation effect of coordinated imidazole ligands, a series of low-spin (tetraalkylporphyrinato)iron(III) complexes, [Fe(TRP)(L)2]+ and [Fe(TRP)(L)(CN)], carrying at least one orientationally fixed imidazole (L) have been prepared. The 1H NMR pyrrole signals of a series of [Fe(TRP)(2-MeIm)2]+ have shown considerable downfield shifts as the meso substituent becomes bulkier, from −30.4 (R = H) to +5.6 ppm (R = iPr) at −71 °C. These complexes have exhibited four pyrrole signals at lower temperature due to the hindered ligand rotation. The spread of the pyrrole signals decreases from 9.4 (Me) to 8.2 (Et) and then to 5.7 (iPr) ppm. The downfield pyrrole signals together with the small spread in [Fe(TiPrP)(2-MeIm)2]+ are in sharp contrast to the other low-spin complexes with orientationally fixed imidazole ligands; the chemical shifts and spreads of the pyrrole signals in [tetrakis(2,4,6-trialkylphenyl)porphyrinato]iron(III) complexes [Fe(R-TPP)(2-MeIm)2]+ (R = Me, Et, iPr) are ca. −20 and c...

Fixation of the 2-Methylimidazole Ligand and Anomalous Pyrrole Chemical Shifts in Bis(2-methylimidazole)(meso-tetraalkylporphyrinato)iron(III) Chloride Caused by the Nonplanar Porphyrin Ring.

Dynamic NMR study of bis(2-methylimidazole)(meso-tetraalkylporphyrinato)iron(III) complexes, [Fe(TRP)(2-MeIm)2]+ (R = Me, Et, iPr), has revealed that the 2-methylimidazole ligands were orientationally fixed even in less bulky methyl complex and that the pyrrole signals of the isopropyl complex appeared in a so-called diamagnetic region in spite of the S = 1/2 spin state. The anomaly was ascribed to the nonplanarity of the porphyrin ring.

Rotation and orientation of axially coordinated imidazoles in low-spin ferric porphyrin complexes

The variable temperature 1H NMR spectra of a series of low-spin bis(imidazole)tetrakis(2, 4, 6-trialkylphenyl)porphinatoiron(III) chlorides, (R-TPP)Fe(L) 2Cl, showed that the rotation of axially coordinated 2-alkyl- and benzimidazoles slowed down on the NMR time scale at low temperature to give four pyrrole signals. The 13C NMR spectra of the 13C enriched (Me-TPP)Fe(2- methylimidazole) 2Cl at the meso positions gave two meso signals with equal intensity at the temperature range below −25 °C. These results indicate that each of the axial ligands is perpendicularly aligned over a diagonal C mesoFeC meso axis. The shift range of the pyrrole protons reached as much as 12 ppm at −56 °C. The apparent unfavorable orientation of the ligands was explained in terms of the S 4 deformed structure of the porphinatoiron core in solution. The relatively small slopes in Curie plots of the pyrrole-H and the meso 13C signals were also ascribed to the deformed structure of the core. In the mixed ligand complexes having a hindered 2-isopropylimidazole and an unhindered 1-methylimidazole ligand, it was suggested from the 1H and 13C NMR splitting patterns that the rotation of only one of the ligands, 2-isopropylimidazole, was frozen. In these complexes the spread of the pyrrole signals increased to nearly 20 ppm. The activation free energies for rotation were determined by the dynamic NMR technique. They changed in the range of 11.3 to 13.6 kcal mol −1 depending upon the bulkiness of the axial ligands and o-alkyl substituents. Close examination of the dynamic process using the saturation transfer technique revealed that the dissociation of the axial ligands occurred concomitantly during the rotation process in the case of the bis(2-isopropylimidazole) complexes. In contrast, pure rotation process was observed in the complexes with 2-methyl-, 2-ethyl-, 1, 2-dimethyl- and benzimidazole. X band ESR spectra of these complexes were taken at 4.2 K in CH 2Cl 2 glass. Although complexes with perpendicularly aligned planar ligands tend to exhibit so called ‘strong g max’ type signals, the complexes studied here showed signals with smaller g z values.

Hindered rotation of axially coordinated 2-methylimidazoles in tetramesitylporphyrinato-iron(III) complexes

Hindered rotation of coordinated imidazoles in the title complex has been observed by the dynamic NMR method. At low temperature where the rotation of the imidazoles was slow on the NMR time scale, only the conformer with C 2 symmetry was observed. The dispersion of the pyrrole signals, 7.3 ppm at -48°C, has been ascribed to the deviations from planarity of the porphyrin ring rather than an orientation effect of the axial ligands, since they must be perpendicular to each other. The activation enthalpy and entropy for ligand rotation were determined to be 12.9 ± 0.4 Kcal mol and 3.7±1.6 e. u., respectively.

Characterization of superoxide-metalloporphyrin reaction products: effective use of deuterium NMR spectroscopy

Nuclear magnetic resonance spectroscopy has been utilized to define the solution electronic structures of complexes generated from combination of superoxide ion with iron(II) and manganese(II) porphyrins. Very large proton NMR line widths for the iron porphyrin product dictated the use of deuterium NMR for selectively deuterated synthetic porphyrins. On the basis of the pyrrole deuterium signal at 60 ppm (Me/sub 2/SO solvent, 28/sup 0/C, Me/sub 4/Si reference) and a solution magnetic moment of greater than or equal to5.6 ..mu../sub B/, a peroxoiron(III) porphyrin structure consistent with an earlier formulation is favored. A corresponding manganese porphyrin signal at 32 ppm and a solution magnetic moment of 5.0 ..mu../sub B/ are indicative of a superoxomanganese(II) porphyrin configuration with spin coupling between the superoxide ligand and the manganese center. This species provides an excellent model for what has been described as superoxide coupling to a low-spin iron(III) center in the oxyhemoglobin molecule. The peroxoiron porphyrin complex is likely coordinated in the second axial position by a solvent molecule. The axial ligand combination leaves the iron center in a high-spin state, but the peroxo ligand serves to reduce the zero-field-splitting value and indirectly induce large NMR line widths. Variable-temperature NMR Curie law behavior ismore » consistent with monomeric structures. Absence of splitting in the pyrrole signals suggests no attack of porphyrin nitrogen atoms by peroxide or superoxide species.« less