Narrow Results

We report the synthesis, characterization and catalytic behavior of iron(III) porphyrins immobilized in layered double hydroxides (LDH). A combination of EPR, UV-visible spectroscopy and powder X-ray diffraction (PXRD) was used to characterize the products. Three different procedures were used to perform the immobilization of iron porphyrin: a) adsorption into the LDH powder, b) co-precipitation and c) rehydration of calcined LDH. Several different porphyrins (Fe(TSPP), Fe(TCPP), Fe(TPP) and Fe(TDFSPP)) were employed in the study. After characterization, the isolated powders were investigated in the oxidation reaction of cyclohexane by iodosylbenzene and hydrogen peroxide in heterogeneous catalysis in comparison to homogeneous media. Some of these immobilized compounds were very good catalysts for this oxidation reaction, stimulating the future investigation of other substrates.

Condensation of pyrrole and 3,5-di-tert-butyl-2,6-dinitrobenzaldehyde in CH₂Cl₂ in the presence of BF₃·Et₂O affords 5,10,15,20-tetrakis-(2',6'-dinitro-3',5'-di-tert-butylphenyl)porphyrin that can be easily metallated with Zn and Fe.

The efficiency of different series of tris-(2-aminoethyl)amine TREN-capped porphyrins as catalysts for the electroreduction of dioxygen has been studied after adsorption of the molecules at the surface of a graphite electrode. The influence of two structural features have been explored: the relative position of the two metals in the iron-copper bimetallic complexes and the nature of the benzyl groups of the tripod itself. These compounds have also been compared with picket porphyrins bearing aromatic amino functions, e.g. quinoline. The conclusions obtained in the case of TREN-capped catalysts cannot be extended to those bearing aromatic pickets.

Cyclodextrins are versatile building blocks for a variety of macromolecules due to the inclusion complexes that are formed with hydrophobic organic molecules. Cyclodextrin-porphyrin interactions are of particular interest since cyclodextrins can serve as a non-covalent binding pocket while metalloporphyrins could serve as the heme analogs in the construction of heme protein model compounds. Various approaches to the design and assembly of biomimetic porphyrin constructs are compared and contrasted in this minireview with a particular emphasis on self-assembled and porphyrin-cyclodextrin systems. Several recent advances from our laboratories are described in this context. A sensitive fluorescent binding probe, 6^A-N-dansyl-permethylated-β-cyclodextrin (Dan-NH-TMCD), was found to form 2:1 complexes with the meso-tetraphenylporphyrins Mn(III)TCPP, Mn(III)TPPS and Mn(III)TF₄TMAP with high binding constants. A perPEGylated cyclodextrin, heptakis(2,3,6-tri-O-2-(2-(2-methoxyethoxy)ethoxy)ethyl)-β-cyclodextrin (TPCD), has been shown by ¹H NMR spectroscopy to form a 1:1 complex with H₂TCPP with a binding constant above 10⁸M^-1. Such a strong binding constant is the largest found for a 1:1 complex between a monomeric cyclodextrin and a guest. TPCD was also found to bind Mn(III)TCPP with a binding constant of 1.2 × 10⁶M^-1. A novel, self-assembled hemoprotein model, hemodextrin is also described. The molecular design is based on a PEGylated cyclodextrin scaffold that bears both a heme-binding pocket and an axial ligand that binds an iron porphyrin. The binding constant for Fe(III)TPPS (iron(III) meso-tetra(4-sulfonatophenyl)porphyrin) by py-PPCD was determined to be 2 × 10⁶M^-1. The pyridyl nitrogen of py-PPCD was shown to ligate to the iron center by observing signal changes in the Fe(II)-porphyrin ¹H NMR spectrum. This hemodextrin ensemble, a minimalist myoglobin, was shown to bind dioxygen reversibly and to form a stable ferryl species.

The preparation, X-ray crystal structure and electrochemistry of a new poly-substituted air-stable tetrabenzoporphyrin iron(II) bis-pyridine complex (1) is reported.

Oxidation of naphthalene was performed with various manganese and iron porphyrins as catalysts and metachloroperbenzoic acid, tert-butylhydroperoxide, and hydrogenperoxide as oxidants, in the presence of imidazole as cocatalyst. With m-CPBA, in low concentration of catalyst, iron porphyrin shows a higher catalytic activity than manganese porphyrin, while a greater amount of manganese porphyrin is needed for having a good catalyst. In the presence of excess amount of H₂O₂ oxidant in CH₃OH solvent, manganese porphyrin acts better than iron porphyrin; however with the same oxidant and in aprotic solvent such as CH₃CN:CH₂Cl₂ again iron shows a higher activity than manganese. Moreover, the effects of metalloporphyrin concentration on the selectivity, activity and stability of catalyst were studied in H₂O₂-catalytic oxidation.

Oxidation of (PyPH)Fe^IIBr, an iron(II) complex of 6,11,16,21-tetraaryl-3-aza-m-benziporphyrin (N-confused pyriporphyrin, (PyPH)H) has been followed, in the presence of pyridine, by ¹H and ²H NMR spectroscopy. One-electron oxidation with dioxygen, accompanied by deprotonation of a C(22)H fragment and formation of a Fe-C(22) bond, produced a low-spin, six-coordinate iron(III) complex [(PyP)Fe^III(py)₂]⁺ as confirmed by combination of ¹H NMR, EPR and structural data. The characteristic patterns of ¹H NMR pyrrole and meso-aryl resonances resemble features assigned to the less common, low-spin ground electronic state ((d_xzd_yz)⁴(d_xy)¹) of iron(III) regular porphyrins. A conformational rearrangement process has been detected which involves two structures differentiated by macrocyclic ruffling. The structure of {H[(PyP)Fe^III(py)₂]₂}(Fe^IIIBr₄)₃·CH₂Cl₂ has been determined by X-ray crystallography. The cationic complex involves a six-coordinate iron atom bound to the N-confused pyriporphyrin through its three nitrogens (Fe-N(23) = 1.924(7), Fe-N(24) = 1.979(7), Fe-N(25) = 1.9343(7) Å) and the pirydyl trigonal C(22) atom (Fe(1)-C(22) = 1.972(10) Å). The porphyrin is strongly ruffled, defining two deep grooves along C_meso-C_meso axes at right angles to each other. Two axial pyridine ligands are located in the prearranged equatorial ligand grooves. The iron lies in the N₃C plane of the macrocycle defined by coordinating nitrogen and carbon atoms. In the solid, pairs of molecules are positioned along the line defined by Fe(1)-C(22) and Fe(2)-C(91) bonds. The structure demonstrates the head-to-head arrangement of two [(PyP)Fe^III(py)₂]⁺ subunits revealing the adjacency of the two perimeter nitrogen atoms (the N(3)⋯N(72) distance = 2.587(10) Å) linked by the N⋯H⋯N hydrogen bond.

Tetraalkylammonium ions react with Fe(TPP)²⁻ to form Fe(TPP)(R)⁻ and trialkylamine. The tetrabutylammonium cation was verified to be the source of the alkyl group in the product, Fe(TPP)(R)⁻, by using (¹H₅C₂)₃(2H₅C₂)N⁻ as the cation and ²H NMR. The reaction of Fe(TPP)²⁻ with Bu₄N⁻ was monitored by cyclic voltammetry and thin layer spectroelectrochemistry. The activation parameters were measured, and were most consistent with an electron transfer (ET) mechanism. The rate of the reaction of tetramethyl and tetraethylammonium ions with Fe(TPP)²⁻ was also examined. The rate constant decreased significantly as the carbon chain length decreased, which was also consistent with an ET mechanism.

Chloro(tetraphenylporphyrinato)iron, Fe(TPP)Cl, is an active catalyst for the Büchner addition of para-substituted methyl 2-phenyldiazoacetates, 1a–d, to substituted benzenes. Yields greater than 70% have been achieved at temperatures ranging from 60–100°C. Reactions of substituted methyl 2-phenyldiazoacetates with benzene gave rapidly equilibrating mixtures of norcaradienecycloheptatriene valence isomers, 2a–d/2′a–d, in yields over 70%. Treatment of chlorobenzene with methyl 2-phenyldiazoacetate produced a regio-isomeric mixture of 7-carbomethoxy-2-chloro-7-phenylnorcaradiene/7-carbomethoxy-2-chloro-7-phenylcycloheptatriene, 3a/3′a, and 7-carbomethoxy-3-chloro-7-phenylnorcaradiene/7-carbomethoxy-3-chloro-7-phenylcycloheptatriene, 4a/4′a. When p-methylanisole was treated with methyl 2-phenyldiazoacetate at 80°C, a product that largely favored a fused cyclopropane structure, 7-carbomethoxy-2-methoxy-5-methyl-7-phenylnorcaradiene, 12a, was obtained along with the benzylic C–H insertion product methyl 3-(p-methoxyphenyl)-2-phenylpropionate, 13a. Heating the norcaradiene product 12a at 110°C yielded the ring-opened diarylacetate, 14a. The diene forms of the fluxional norcaradiene-cycloheptatriene systems were trapped with benzyne to give one stereoisomer of 3,3-disubstituted benzhomobarralenes, 18a–d. The norcaradiene-cycloheptatriene valence isomers were quantitatively converted into ring-opened diaryl acetate products upon acidification in acetonitrile. Rates for the addition of methyl (p-chlorophenyl)diazoacetate to benzene were first order with respect to the diazo reagent. A concerted mechanism involing an iron carbene complex is proposed for these iron porphyrin-catalyzed Büchner reactions.

Iron(III) tetraphenylporphyrin chloride, Fe(TPP)Cl, efficiently catalyzed the insertion of carbenes derived from methyl 2-phenyldiazoacetates into O-H bonds of aliphatic and aromatic alcohols, with yields generally above 80%. Although the analogous N-H insertions are rapid at room temperature, the O-H insertion reactions are slower and required heating in refluxing methylene chloride for about 8 hours using 1.0 mol.% catalyst. Fe(TPP)Cl was also found to be effective for tandem N-H insertion/cyclization reactions when 1,2-diamines and 1,2-alcoholamines were treated with diazo reagents to give piperazinones and morpholinones and related analogs such as quinoxalinones and benzoxazin-2-ones. This approach provides a new one-pot route for synthesizing these classes of heterocyclic compounds.

Iron porphyrin and cobalt porphyrin were respectively modified on glassy carbon (GC) electrodes together with multiwalled carbon nanotubes (MWCNTs), and the modified electrodes were characterized by X-ray photoelectron spectroscopy (XPS). Their electrochemical performance for the reduction of oxygen (O₂), proton (H⁺) and carbon dioxide (CO₂) were respectively investigated and compared by cyclic voltammetry (CV). The results showed that iron porphyrin-MWCNT modified electrode always had a better performance than cobalt porphyrin-MWCNT modified electrode. It is proposed that iron porphyrin-MWCNT modified electrode will have promising application in electrochemical architectures like microbial fuel cells (MFCs) and microbial electrolysis cells (MECs) for its good biocompatibility, easy availability, as well as excellent electrochemical performance.

μ-nitrido diiron porphyrin complex (OEPFe)₂N (OEP = octaethylporphyrinato dianion) was synthesized by the treatment of (OEPFe)Cl with hydrazoic acid followed by the thermal decomposition of formed (OEPFe)N₃ complex. The (OEPFe)₂N complex was investigated by UV-vis, IR, HR ESI-TOF MS and EPR spectroscopic methods. EXAFS study indicated a very short Fe–Fe distance of 3.30 Å in this dimer compared with 3.48 Å distance between iron sites in the related μ-nitrido diiron complex on the tetraphenylporphyrin platform. This difference can be explained by the steric hindrance exerted by phenyl substituents in the latter. The spectral properties of the (OEPFe)₂N complex are discussed. The catalytic properties of (OEPFe)₂N were evaluated in the oxidation of cyclohexene and cyclohexane.

The molecular structure of the μ-nitrido dimer, μ-nitrido-bis(2,3,7,8,12,13,17,18-octaethylporphyrinato)iron, is reported. The axial Fe–N distances average to 1.657(11)Å and the equatorial distances average to 2.005(5)Å. Although not required by symmetry, the two iron centers appear equivalent and are consistent with an assignment of a low-spin state and a formal oxidation state of +3.5. A comparison of this structure with other nitrido-bridged species is given.

Axial ligands play a dominating role in determining the electronic structure and reactivity of iron porphyrin active sites and synthetic models. Several properties unique to the cysteine bound heme enzyme, cytochrome P450, is attributed to the "push effect" of the thiolate axial ligand. In this mini-review the ground state electronic structure of iron porphyrins with imidazole, phenolate and thiolate complexes, derived using a combination of spectroscopy and DFT calculations, are discussed. The differences in kinetics and selectivity of oxygen reduction reaction (ORR), catalyzed by these iron porphyrin complexes with different axial ligands, help elucidate the varying push effects of the different axial ligands on oxygen activation by ferrous porphyrin. The spectroscopic and kinetic data help to develop a quantitative understanding of the "push effect" and, in particular, the electrostatic and covalent contributions to it.

Four bis-imidazole iron(II/III) 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato (TFPP) complexes, [Fe(TFPP)(1-MeIm)${}_{\mathrm{\text{2}}}$], [Fe(TFPP)(1-VinylIm)${}_{\mathrm{\text{2}}}$], [Fe(TFPP)(4-MeHIm)${}_{\mathrm{\text{2}}}$]Cl and [Fe(TFPP)(1-EtIm)${}_{\mathrm{\text{2}}}$]BF${}_{\mathrm{\text{4}}}$ (1-MeIm $=$ 1-methylimidazole, 1-VinylIm $=$ 1-vinylimidazole, 4-MeHIm $=$ 4-methylimidazole and 1-EtIm $=$ 1-ethylimidazole) were synthesized and characterized by single-crystal X-ray and UV-vis spectroscopy. A negative correlation is found between the absolute imidazole orientation ($\varphi )$ and the Fe–N${}_{\mathrm{\text{Im}}}$ distance for the [Fe(II)(Porph)(Im)${}_{\mathrm{\text{2}}}$] (Im $=$ 1-MeIm or 4-MeHIm) complexes where the smaller $\varphi$ angle corresponds to a longer axial distance. Hydrogen bonding, which might affect the orientations of the axial imidazoles is found for Fe(TFPP)(4-MeHIm)${}_{\mathrm{\text{2}}}$]Cl (A and B). The autoreduction of [Fe(III)(TFPP)]Cl to [Fe(II)(TFPP)(1-MeIm)${}_{\mathrm{\text{2}}}$] with 1-methylimidazole has been monitored by UV-vis spectroscopic titration.

Mono-hydroxychlorins are uncommon macrocycles that have only been synthetically realized by modifying porphyrin rings using the harsh oxidizing agent OsO₄. We show here that a more directed delivery of the mono-hydroxychlorin may be concomitantly obtained from the oxidation of porphyrinogen using the mild conditions of the high dilution Lindsey porphyrin forming reaction where water content is minimized by using dry CHCl₃ within the environment of a glovebox. We now report the direct synthesis of 17,18-dihydro-18-hydroxy-5,10,15,20-tetrakis-(4-fluoro,2,6-dimethylphenyl)-porphyrin (2H-TFChl-${}^{\text{β}}$OH) together with the corresponding freebase porphyrin TFP. The TFP has been metalated with FeBr₂ and MgBr₂•OEt₂ resulting in metalloporphyrins Fe(III)TFP(Cl) and Mg(II)-TFP which have been structurally characterized by single-crystal X-ray crystallography. We find that the excited state properties of the mono-hydroxychlorin are similar to that of its parent TFP and Mg(II)TFP porphyrin congeners. Excited state deactivation by vibronic coupling to the high energy O-H oscillator is circumvented with the hydroxyl group remote to the 18$\pi$-electron framework of the chlorin ring. These results reveal that strong H-bonding groups may be introduced on the periphery of the chlorin ring while maintaining the light-gathering properties that lie at the heart of photosynthesis of the chlorin ring.