Tuning of the structures and electronic effects of nanocrystals is of great relevance to the preparation of specialized materials such as artificial enzymes. In this study, binary porphyrin materials have been prepared by ionic self-assembly of oppositely charged Fe(III) and Mn(III) porphyrins. The materials have been characterized by SEM, XPS and UV-vis spectroscopy and tested as biomimetic models of catalase. The heterometallic material [Mn/Fe], prepared by association of MnTHPyP and FeTSPP, showed the highest activity for H2O2 dismutation reaction, 89-95 U center dot mg(-1), over a pH range of 4-7. An enhancement in activity of one to two orders of magnitude can be observed relatively to the individual Fe(III) or Mn(III) porphyrins and to the mono-metallic materials that were prepared by association of a metalloporphyrin (MP) and a metal free porphyrin (H2P). These results support the occurrence of cooperative mechanisms between two metal ion centers within the structure. The materials with the highest catalase-like activity were also tested as peroxidase mimetics in ABTS oxidation. Best activity was observed for the [Mn/Fe] material at pH 4, while at pH 6 the activity decreases by 97%. EPR studies at pH 6 demonstrate the absence of free hydroxyl or superoxide radicals in the mechanism and thus support the activity of the [Mn/Fe] material as a catalytic antioxidant under these conditions. (c) 2023 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Chapter 13 NMR Spectroscopy of Porphyrins Craig J. Medforth, Craig J. Medforth LAQV-REQUIMTE, Department of Chemistry and Biochemistry, University of Porto, Porto, PortugalSearch for more papers by this author Craig J. Medforth, Craig J. Medforth LAQV-REQUIMTE, Department of Chemistry and Biochemistry, University of Porto, Porto, PortugalSearch for more papers by this author Book Editor(s):Penelope J. Brothers, Penelope J. Brothers The Australian National University, Canberra, AustraliaSearch for more papers by this authorMathias O. Senge, Mathias O. Senge Trinity College Dublin, Dublin, IrelandSearch for more papers by this author First published: 17 June 2022 https://doi.org/10.1002/9781119129301.ch13 AboutPDFPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShareShare a linkShare onFacebookTwitterLinked InRedditWechat Summary Since the first nuclear magnetic resonance (NMR) spectrum of a porphyrin was reported in 1959, NMR spectroscopy has become an important tool for studying the structures and properties of porphyrins and related tetrapyrroles. This chapter provides an overview of the NMR properties of porphyrins, discusses the NMR techniques used to study these molecules, and gives examples of the structures and dynamic processes that have been investigated. The chapter begins with a discussion of proton NMR spectroscopy of diamagnetic porphyrins, with emphasis placed on the effect of the porphyrin ring current. The next section describes heteronuclear NMR spectroscopy of diamagnetic porphyrins, including a discussion of the practicalities of measuring the NMR spectra of heteronuclei. Structural studies of diamagnetic porphyrins are then illustrated using examples based on host–guest chemistry, and some dynamic processes of porphyrins are discussed. Finally, NMR spectroscopy of paramagnetic metalloporphyrins is explored, and some uses of solid-state NMR spectroscopy are presented. Fundamentals of Porphyrin Chemistry: A 21st Century Approach, Volume 1 RelatedInformation

The modelling of metabolic activation of the benzofuran nucleus is important to obtain eco-sustainable degradation methods and to understand the related mechanisms. The present work reports the catalytic oxidation of benzofuran, 2-methylbenzofuran, and 3-methylbenzofuran by hydrogen peroxide, at room temperature, in the presence of different Mn(III) porphyrins as models of cytochrome P450 enzymes. Conversions above 95% were attained for all the substrates. The key step is the formation of epoxides, which undergo different reaction pathways depending on factors, such as the position of the methyl group and the reaction and work-up conditions used.

Supramolecular structures were prepared in water by ionic self-assembly of oppositely charged iron(III) porphyrins and tested as a new class of catalase mimics. Iron(III) porphyrins carrying the positively charged groups, N-methyl-4-pyridinium (FeTMPyP), 4-pyridinium (FeTHPyP) and tetrafluoro-4-imidazoliumphenyl (FeTFHImP) were assembled with iron tetrakis(4-sulfonatophenyl)porphyrin (FeTSPP), at an optimized pH. The material with the best catalase activity (FeTHPyP:FeTSPP) showed 65-77 catalase units (U)/mg in the pH range 3.5-7.0, which was significantly higher than the 3 U/mg observed for the individual porphyrins. Analogous metal-free structures had no catalytic activity. Micro-scale structures with nanoscale features were observed by Scanning Electron Microscopy. Studies of the chemical composition of materials by X-ray Photoelectron Spectroscopy showed the formation of structures with a 1:1 ratio for (+):(-) tectons. Characterization of particles in solution was carried out at different pH and before and after catalysis by UV-vis spectroscopy, Dynamic Light Scattering (measurement of hydrodynamic diameters) and Electrophoretic Light Scattering (measurement of zeta potentials). (C) 2020 Elsevier Ltd. All rights reserved.

Herein, we report the first calorimetric study of the protonation of planar and nonplanar free-base porphyrins: H2OETPP (strongly saddled by its substituents), H2T(tBu)P (strongly ruffled by its substituents), and the nominally planar porphyrins (npPs) H2OEP, H2TPP, H2T(nPe)P, and H2T(iPr)P. The observed enthalpies of protonation in solution (ΔHprotsoln) for formation of the dications in 1,1,2,2-tetrachloroethane with 2% trifluoroacetic acid are -45 ± 1 kcal mol-1 for the npPs, -52.0 kcal mol-1 for H2T(tBu)P, and -70.9 kcal mol-1 for H2OETPP. The corresponding enthalpies of protonation (ΔHDFT) obtained from DFT calculations (-27 ± 5, -42, and -63 kcal mol-1, respectively) reproduce this trend. The much more negative enthalpy of protonation seen for H2OETPP is consistent with this molecule being pre-deformed into the saddle structure favored by porphyrin dications. Except for OETPP, the calculated enthalpies of the first protonations (ΔH1) are significantly more positive than the enthalpies of the second protonations (ΔH2). In addition, the structural strain energies for the first protonations (ΔEst(1)) are also significantly more positive than ΔEst(2). According to the calculations, the monocations thus have higher proton affinities than the corresponding free-base porphyrins due to a structural strain effect, which is consistent with the generally elusive nature of the porphyrin monocation. The recent observations of monocations for free-base porphyrins with a high degree of saddling can be rationalized in terms of ΔH1 and ΔH2 being similar; so, the monocation is no longer an unstable intermediate.

Porphyrins and metalloporphyrins are one of the most widely studied platforms for the construction of supramolecular structures. These compounds have an extended aromatic system that allows [Formula: see text]–[Formula: see text] stacking interactions which, together with hydrogen bonds, electrostatic forces and the formation of inter-metallic complexes arising from peripheral groups, offer a versatile platform to control the self-assembly mechanism. In this work, we present the study of nanostructures formed by self-assembly of the water-soluble porphyrins meso-tetra([Formula: see text]-methyl-4-pyridyl)porphyrin (TMPyP) and meso-tetra(4-sulfonatophenyl)porphyrin (TPPS) in the presence of hard nanotemplates. Different nanoparticles (silica, gold, and polystyrene), concentrations and synthetic procedures were explored. The obtained materials were characterized by SEM and AFM microscopies, UV-vis absorption spectroscopy and dynamic light scattering measurements. A clear modification of the SiO2 NP surface roughness using one-pot synthesis was observed. The results were variable depending on the porphyrin–surface interactions and concentrations used. At lower porphyrin concentrations, a shift of the Soret band was observed, while at higher concentrations, free NS were formed. This reflects a competition between surface and solution directed self-assembly.

Oxygen radicals have been proposed to have a role in the metaloporphyrin based biomimetic mechanisms as they can be formed from metal-oxo or metal-hydroperoxy species. Herein EPR studies of biomimetic oxygenation reactions using BMPO as a spin trap are used to investigate the production of reactive oxygen species during the catalytic activation of H2O2 by Fe-III and Mn-III porphyrins. Firstly, reactions in control conditions were studied and the formation of BMPO-OH center dot adducts was observed. These provided a baseline system for further comparative evaluation of the effect of different components of the catalytic reactions, including metaloporphyrins, solvents, co-catalysts and substrates on ROS production and H2O2 consumption. In the absence of co-catalyst or using ammonium acetate as co-catalyst, the metaloporphyrin centered pathways do not generate additional hydroxyl or superoxide radicals. In contrast, in the presence of sodium hydroxide, the production of additional hydroxyl radicals by a metaloporphyrin-mediated process is observed. The EPR results also differentiate the action of intermediates ascribed to metaloporphyrin-H2O2 adducts and high-valent oxo-species by their relative rates of H2O2 consumption and their effect on the production of BMPO-OH center dot.

One-pot green reactions useful for the synthesis of novel and potentially biologically active compounds have been developed using metaloporphyrin catalyzed oxidations of benzofurans. Minor variations of the reaction parameters allow direction of the reactivity (furan versus benzene ring) and the production of multifunctional 2,3-dihydrobenzofurans, salicyl derivatives or an unusual dimerization product. These studies suggest pathways for the metabolic oxidation and activation of pharmaceuticals and other molecules containing the benzofuran core.

Iron(III) fluorinated porphyrins play a central role in the biomimetics of heme enzymes and enable cleaner routes to the oxidation of organic compounds. The present work reports significant improvements in the eco-compatibility of the synthesis of 5,10,15,20-tetrakis-pentafluorophenylporphyrin (H2TPFPP) and the corresponding iron complex [Fe(TPFPP) Cl], and the use of [Fe(TPFPP) Cl] as an oxidation catalyst in green conditions. The preparations of H2TPFPP and [Fe(TPFPP) Cl] typically use toxic solvents and can be made significantly greener and simpler using microwave heating and optimization of the reaction conditions. In the optimized procedure it was possible to eliminate nitrobenzene from the porphyrin synthesis and replace DMF by acetonitrile in the metalation reaction, concomitant with a significant reduction of reaction time and simplification of the purification procedure. The Fe(III) porphyrin is then tested as catalyst in the selective oxidation of aromatics at room temperature using a green oxidant (hydrogen peroxide) and green solvent (ethanol). Efficient epoxidation of indene and selective oxidation of 3,5-dimethylphenol and naphthalene to the corresponding quinones is observed.

To determine if ionic self-assembly can be used to produce nanomaterials from unusual porphyrin ions we have investigated the reactions of a novel octacationic porphyrin. A range of nanomaterials are obtained depending upon the tetra(4-sulfonatophenyl)porphyrin (TPPS) anion(s) used in the reaction: binary ionic solids (from reactions with metal TPPS complexes), ternary ionic solids (from reactions with two different metal TPPS complexes) and binary solids with variable compositions (from reactions with diprotonated TPPS).

An unusual feature of free base tetraarylporphyrins (H2TArP) is the small amount of monocation (H3TArP[Formula: see text] observed during titrations with acids. This anomalous behavior has been known for over forty years and has been explained in terms of the saddle deformation of the porphyrin that occurs upon protonation. In this paper, 1H NMR spectroscopy is used to investigate the protonation of H2OETPP, an H2TArP that is pre-deformed into a highly nonplanar saddle structure by steric crowding of its peripheral substituents. The goal of our studies is to determine if the saddle structure of H2OETPP stabilizes the monocation H3OETPP[Formula: see text], and, if it does, to conduct detailed NMR studies of H3OETPP[Formula: see text]. NMR spectra clearly show the formation of H3OETPP[Formula: see text] when one equivalent of acid is added to H2OETPP in organic solvents, and for picric acid in toluene-d8 the monocation is the dominant species (~70%). Crystals of H3OETPP[Formula: see text] (picrate) suitable for X-ray crystallography could not be obtained, but the structure of H4OETPP[Formula: see text] (picrate)2 was determined. Variable temperature NMR studies of H3OETPP[Formula: see text] (picrate) reveal the presence of two dynamic processes. The first is picrate anion exchange in CD2Cl2, where the activation energy ([Formula: see text]G[Formula: see text] is calculated to be 53 kJ.mol[Formula: see text]. A second process, ND tautomerism ([Formula: see text]G[Formula: see text] 42 kJ.mol[Formula: see text] is also detected for D3OETPP[Formula: see text] at very low temperatures in toluene-d8. The much lower activation energy for ND tautomerism in D3OETPP[Formula: see text] vs. D2OETPP ([Formula: see text]G[Formula: see text] 63 kJ.mol[Formula: see text] may be due to destabilization of the ground state of the monocation due to the presence of cis interactions between the inner deuterium atoms.

Self-assembly of porphyrins is a powerful method for the production of highly functionalized nanomaterials. Porphyrin-based nanomaterials can be self-assembled using a number of procedures including ionic self-assembly (ISA), re-precipitation and coordination polymerization.1 ISA has proven to be a particularly flexible synthetic method and can be used to couple oppositely-charged porphyrin ions (tectons) in a range of environments such as at liquid-liquid interfaces,2 on templates,3 or spontaneously in water.4,5 The resulting materials have been investigated as catalysts for the reduction of O2 to H2O,2 as photosensitizers for the reduction of H2O to H2,6 as adsorbed pigments for functionalizing carbon nanotubes,7 as H2 storage materials,8 and as photoconductors.5 Despite these considerable advances, little is known about the ISA of structurally modified porphyrin tectons that may be useful for producing nanomaterials with specific properties. It is well established that extension of the π-system or nonplanar deformation of the macrocycle significantly alter many properties of porphyrins, including their optical and electronic behaviour. In this talk, we describe recent investigations of the self-assembly of tecton 1 (M = Ni)9 which contains both of these structural modifications, with the standard tecton TPPS 2(M = Sn, Fe, Mn, Cu or 4H) (see Graphic). Porphyrins 1 and 2 are shown to react to produce nanoscale materials together with porous micron-sized wafers, although the reactions are significantly different than those previously reported for ISA of standard water-soluble porphyrins based on the TPP framework. Of particular note are the reactions of 1 with the diprotonated form of 2 (M = 4H)whichcan yield precipitates with stoichiometries lower than or much higher than those expected for ISA. Reasons for the formation of these unusual products are discussed, together with the potential applications of this chemistry to the formation of ternary systems containing 1, a metal complex of 2, and the diprotonated form of 2. [1] For a recent review see: C. J. Medforth, Z. Wang, K. E. Martin, Y. Song, J. L. Jacobsen, J. A. Shelnutt, Chem. Commun.7261 (2009). [2] A. J. Olaya, D. Schaming, P.-F. Brevet, H. Nagatani, T. Zimmermann, J. Vanicek, H.-J. Xu, C. P. Gros, J.-M. Barbe, H. H. Girault, J. Am. Chem. Soc. 134,498 (2012). [3] R. Lauceri, G. F. Fasciglione, A. D'Urso, S. Marini, R. Purrello, M. Coletta, J. Am. Chem. Soc. 130, 10476 (2008). [4] Z. Wang, C. J. Medforth, J. A. Shelnutt, J. Am. Chem. Soc. 126,15954 (2004). [5] K. E. Martin, Z. Wang, T. Busani, R. M. Garcia, Z. Chen, Y. Jiang, Y. Song, J. L. Jacobsen, T. T. Vu, N. E. Schore, B. S. Swartzentruber, C. J. Medforth, J. A. Shelnutt, J. Am. Chem. Soc. 132, 8194 (2010). [6] Y. Tian, K. E. Martin, J. Y.-T. Shelnutt, L. Evans, T. Busani, J. E. Miller, C. J. Medforth, J. A. Shelnutt, Chem. Commun. 47, 6069 (2011). [7] A. Brewer, M. Lacey, J. R. Owen, I. Nandhakumar, E. Stulz, J. Porphyrins Phthalocyanines 15, 257 (2011). [8] M. T. Oztek, M. D. Hampton, D. K. Slattery, S. Loucks, Int. J. Hydrogen Energy 36, 6705 (2011). [9] L. Jiang, R. A. Zaenglein, J. T. Engle, C. Mittal, C. S. Hartley, C. J. Ziegler, H. Wang, Chem. Commun. 48, 6927 (2012). Figure 1

Fluoroquinolones are antibiotics which act by penetrating into bacterial cells and inhibiting enzymes related to DNA replication, and metal complexes of these drugs have recently been investigated as one approach to counteracting bacterial resistance. In this work, we apply a multi-technique approach to studying the partition coefficient (Kp) for the non-fluorescent third-generation fluoroquinolone sparfloxacin or its copper-complex with lipid membrane models of Gram-negative bacteria. The techniques investigated are UV–vis absorption and 19F NMR spectroscopies together with quenching of a fluorescent probe present in the lipids (using steady-state and time-resolved methods). 19F NMR spectroscopy has previously been used to determine the Kp values of fluorinated drugs but in the case of sparfloxacin did not yield useful data. However, similar Kp values for sparfloxacin or its copper-complex were obtained for the absorption and fluorescence quenching methods confirming the usefulness of a multi-technique approach. The Kp values measured for sparfloxacin were significantly higher than those found for other fluoroquinolones. In addition, similar Kp values were found for sparfloxacin and copper-complex suggesting that in contrast to other fluoroquinolones hydrophobic diffusion occurs readily for both of these molecules.

Electrochemical studies of the oxidation of dodecasubstituted and highly nonplanar nickel porphyrins in a noncoordinating solvent have previously revealed the first nickel(III) porphyrin dication. Herein, we investigate if these nonplanar porphyrins can also be used to detect the so far unobserved copper(III) porphyrin dication. Electrochemical studies of the oxidation of (DPP)Cu and (OETPP)Cu show three processes, the first two of which are macrocycle-centered to give the porphyrin dication followed by a CuII/CuIII process at more positive potential. Support for the assignment of the CuII/CuIII process comes from the linear relationships observed between E1/2 and the third ionization potential of the central metal ions for iron, cobalt, nickel, and copper complexes of (DPP)M and (OETPP)M. In addition, the oxidation behavior of additional nonplanar nickel porphyrins is investigated in a noncoordinating solvent, with nickel meso-tetraalkylporphyrins also being found to form nickel(III) porphyrin dications. Finally, examination of the nickel meso-tetraalkylporphyrins in a coordinating solvent (pyridine) reveals that the first oxidation becomes metal-centered under these conditions, as was previously noted for a range of nominally planar porphyrins.

The pH of the solution from which a cooperative binary ionic material is formed can alter the charges of the organic anions and cations and thus the composition, crystal structure, and properties of the materials obtained by ionic self-assembly. In particular, this pH/charge effect provides a means of synthetic control over the properties and morphologies of ionic solids formed from cationic and anionic porphyrins. In the case of titration of an axial hydroxide ligand of the metal of the porphyrin to water, the charge increases by +1 with little perturbation of the molecular structure of the constituent porphyrin anion or cation. The structure of the material is thus altered without significantly altering the electronic structure of the porphyrin tecton. These novel porphyrin-based ionic materials have potential applications as catalysts, photocatalysts, and optoelectronic and sensing materials.

A new water-soluble porphyrin, 5,10,15,20-tetrakis(4-piperidyl)porphyrin (T(4-Pip)P), has been synthesized. T(4-Pip)P is related to the extensively studied water-soluble porphyrin 5,10,15,20-tetrakis(4-pyridyl)porphyrin (T(4-Py)P) but has substituents with different electronic and hydrogen-bonding properties and is soluble over a much larger pH range due to the higher pKa of its conjugate acid T(4-H-Pip)P4+. Investigations of the ionic self-assembly reactions of T(4-H-Pip)P4+ with anionic water-soluble porphyrins reveal that it forms nanoscale materials.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.