Narrow Results

The role played by the distal coordination site’s flexibility on oxygen reduction was investigated by rotating ring-disk electrode experiments for a bipyridine-strapped iron(III) porphyrin mimic of cytochrome $c$ oxidase. Compared to a phenanthroline counterpart, the more flexible bipyridine strap improved by up to 15% the selectivity of the four-electron reduction of oxygen to water over the two-electron reduction in the presence of copper(I) coordinated to the distal bipyridine and $N$-methylimidazole bound at the proximal site. The more modest gain in overpotential observed for the bipyridine-strapped porphyrin than for the phenanthroline derivative was attributed to a difference in stability of the iron(III) porphyrins that affects reduction to iron(II) and oxygen binding. Overall, the rotational flexibility imparted by the bipyridine strap allows the ferric porphyrin to remain relatively flat, thus improving its electrocatalytic performance.

Effective systems for the catalytic oxygenation of hydrocarbons have been developed based on metalloporphyrins containing iron, manganese and ruthenium. These metalloporphyrin catalysts were inspired by studies of the mechanism of the heme oxygenase cytochrome P450 and related enzymes. Oxometalloporphyrins, which have now been spectroscopically characterized, have very high reactivity as oxygen transfer agents. A combination of experimental and computational methods has begun to provide an understanding of the structure and reactivity of these oxometalloporphyrin complexes and the mechanisms of oxygen transfer reactions they catalyze.

The interaction between iron(II) tetrasulfophthalocyanine ([Fe^IITSPc]⁴⁺) and histamine results in the oxidation of the central metal by oxygen in the former, with the formation of a complex denoted as [(His)Fe^IIITSPc]³⁻ (where His = histamine). The rate constant for the formation of the complex is k_f = 2.41 × 10⁻²dm³.mol⁻¹.s⁻¹ and an equilibrium constant of 6.3 dm³.mol^-1 was obtained. The oxidation state of the central metal of [Fe^IITSPc]⁴⁻ before and after the coordination of histamine is confirmed by spectroelectrochemistry. Further electrochemical oxidation of this [(His)Fe^IIITsPc]³⁻ derivative results in a metal-based process proposed to involve an Fe^IV phthalocyanine species.

A new family of pyrrole substituted metallophthalocyanine complexes, namely cobalt(II), iron(II), manganese(III), nickel(II) and zinc(II) tetrakis-4-(pyrrol-1-yl)phenoxy phthalocyanines (noted as M(TPhPyrPc), where M is the metallic cation) have been synthesized and fully characterized. In particular, the UV-visible spectra of the iron and nickel complexes showed extensive aggregation even at low concentrations. The cyclic voltammetry of the cobalt, iron and manganese complexes showed three to four redox couples assigned to metal and ring based processes. Spectroelectrochemistry of the manganese derivative confirmed that the synthesized complex is Mn^III(TPhPyrPc^-2) and that the reduction of Mn^II(TPhPyrPc^-2) to be centred on the ring and rather than on the metal, forming the Mn^II(TPhPyrPc^-4) species. Also, the electrochemical polymerization of these newly synthesized pyrrole-substituted phthalocyanines has been demonstrated in the case of the cobalt complex and the electrocatalytic activity of the obtained film has been tested towards the oxidation of L-cysteine.

Novel chiral systems for the catalytic asymmetric oxidation and cyclopropanation of olefins based on metalloporphyrins containing iron, ruthenium and manganese, have been recently introduced. High catalyst turnover numbers and sometimes high enantiomeric excess were observed. New catalytic reactions with metalloporphyrins have recently been reported; these are the olefination of aldehydes and cyclotrimerization of terminal alkynes. Dendrimers and polymers containing metalloporphyrins, have also been found to be efficient catalysts for oxidation and carbene transfer.

The chloroiron complex of 10-acetyl-2,3,7,8,12,13,17,18-octaethylcorrole was isolated as an unexpected product from an attempted preparation of chloroiron-2,2'-bidipyrrin when acetone was used in the solvent mixture. In the molecular structure derived from X-ray crystallographic analysis the orthogonal arrangement of the acetyl group with the corrole macrocycle is clearly apparent, and the UV-vis spectrum of the compound indicates only a small electronic influence of the acetyl group on the corrole π-system. The paramagnetic ¹H NMR spectrum taken at ambient temperature shows two signal sets in a ratio of 2.2:1, indicating two stable orientations of the acetyl group in solution. Attempts to prepare the species from chloroiron-2,2'-bidipyrrin with iron(III)chloride in acetone were successful but gave inconsistent and generally low yields. A rationale for the formation of the new corrole is proposed.

Ferrochelatase (also known as PPIX ferrochelatase; Enzyme Commission number 4.9.9.1.1) catalyzes the insertion of ferrous iron into PPIX to form heme. This reaction unites the biochemically synchronized pathways of porphyrin synthesis and iron transport in nearly all living organisms. The ferrochelatases are an evolutionarily diverse family of enzymes with no more than six active site residues known to be perfectly conserved. The availability of over thirty different crystal structures, including many with bound metal ions or porphyrins, has added tremendously to our understanding of ferrochelatase structure and function. It is generally believed that ferrous iron is directly channeled to ferrochelatase in vivo, but the identity of the suspected chaperone remains uncertain despite much recent progress in this area. Identification of a conserved metal ion binding site at the base of the active site cleft may be an important clue as to how ferrochelatases acquire iron, and catalyze desolvation during transport to the catalytic site to complete heme synthesis.

The chlorido iron(III) complex of octaethyl-2,2′-bidipyrrin has been transformed to a series of pseudohalide complexes by ligand exchange reactions with azide, cyanate, thiocyanate and selenocyanate anions. All new complexes show the expected N-coordination of the axial ligand to the iron(III) center. In the solid state, all four species display an intermediate spin (S = 3/2) ground state, with a gradual increase of a high spin (S = 5/2) contribution at elevated temperatures for the members with the smallest ligand field strengths, i.e. the cyanato and the azido derivatives. In solution, proton NMR, and in particular IR spectroscopic studies support the interpretation of a high-spin state at ambient temperature throughout the series. The dependency of the spin state on the crystalline or dissolved state thus resembles that found for a similar series of halide derivatives before. In dichloromethane solution, the thiocyanato and selenocyanato complexes are very sensitive to aerial oxidation, forming oxacorrole and thiacorrole complexes as the only isolated products. These complexes show a S = 3/2 spin state in the solid as well as in solution, and their structural analyses prove the expected strong π-binding of the linear pseudohalide ion to the iron(III) central metal.

The iron complexes of 5,10,15-tris(4-nitrophenyl)corrole have been prepared and characterized by various spectroscopic techniques. The (nitrosyl)iron complex is diamagnetic and its X-ray structure reveals an almost perfectly linear Fe–N–O bond. EPR spectroscopy in conjunction with ¹⁵N labelling were used to deduce the redox centre of the one-electron reduction and oxidation products of the (nitrosyl)iron corrole.

Recent experimental studies of Liu and Groves (J. Am. Chem. Soc. 2010; 132: 12847) on dioxomanganese(V) porphyrin complexes implicated substrate halogenation in good yield. Currently, little is known of this unique mechanism, therefore to gain understanding on the halogenation mechanism and the chemical features of this oxidant we decided to do a computational (density functional theory) study. We show that the dioxomanganese(V) complex has considerably different molecular (valence) orbitals as compared to monooxomanganese(V) porphyrin due to mixing of the metal 3d orbitals with 2p orbitals on both oxygen atoms. This results in a set of three pairs of orbitals of which the bonding and nonbonding pairs are doubly occupied and the antibonding orbitals are vacant. As a consequence, the bonding character along the Mn–O bond is less in dioxomanganese(V) as compared to monooxomanganese(V) complexes and therefore this bond can formally be described as a double bond rather than a triple bond. The differences in orbital interactions and orbital energies also affect the intrinsic chemical properties of the oxidants, such as the electron affinity and pK_a values, which result in enhanced catalytic potential for dioxomanganese(V) porphyrin. Our calculations predict a halogenation mechanism in line with that proposed by experiment with an initial hydrogen atom abstraction followed by ligand exchange and halogen transfer.

This review summarizes research on the electrochemical and photochemical reduction of CO₂ using a variety of iron and cobalt porphyrins, phthalocyanines and related complexes. Metalloporphyrins and metallophthalocyanines are visible light absorbers with extremely large extinction coefficients. However, yields of photochemically-generated active catalysts for CO₂ reduction are typically low owing to the requirement of a second photoinduced electron. This requirement is not relevant to the case of electrochemical CO₂ reduction. Recent progress on efficient and stable electrochemical systems includes the use of FeTPP catalysts that have prepositioned phenyl OH groups in their second coordination spheres. This has led to remarkable progress in carrying out coupled proton-electron transfer reactions for CO₂ reduction. Such ground-breaking research has to be continued in order to produce renewable fuels in an economically feasible manner.

Coupled oxidation of iron tetraphenylporphyrins bearing either a OMe, Me, F, Cl, COOMe, CF₃ or CN group in the para-position of the phenyl groups gave tetraarylbiladien-ab-1-one and triarylbilindione. The ratios of the yields of the former to those of the latter were linearly correlated with the Hammett substituent constants σ_p⁺, with a positive slope (Δρ = 0.64). The Hammett plot of the oxidation rate vs. the substituent constant σ_p also showed a positive slope (ρ = 0.30). These substituent effects suggest that a nucleophilic step is included in the formation of bilindione. Coupled oxidation of an A3B type tetraarylporphyrin having an electron withdrawing nitro group in one of the phenyl groups indicated that the oxidation leading to biladienone occurred rather statistically in any of the meso-positions, while the oxidation leading to bilindione occurred preferentially in the meso-position bearing the 4-nitrophenyl group.

Ferrochelatase catalyzes the insertion of ferrous iron into protoporphyrin IX to generate heme. Despite recent research on the reaction mechanism of ferrochelatase, the precise roles and localization of individual active site residues in catalysis, particularly those involved in the insertion of the ferrous iron into the protoporphyrin IX substrate, remain controversial. One outstanding question is from which side of the macrocycle of the bound porphyin substrate is the ferrous iron substrate inserted. Pre-steady state kinetic experiments done under single-turnover conditions conclusively demonstrate that metal ion insertion is pH-dependent, and that the conserved active site His-Glu pair coordinately catalyzes the metal ion insertion reaction. Further, p$K_a$ calculations and molecular dynamic simulations indicate that the active site His is deprotonated and the protonation state of the Glu relates to the conformational state of ferrochelatase. Specifically, the conserved Glu in the open conformation of ferrochelatase is deprotonated, while it remains protonated in the closed conformation. These findings support not only the role of the His-Glu pair in catalyzing metal ion insertion, as these residues need to be deprotonated to bind the incoming metal ion, but also the importance of the relationship between the protonation state of the Glu residue and the conformation of ferrochelatase. Finally, the results of this study are consistent with our previous proposal that the unwinding of the $\pi$-helix, the major structural determinant of the closed to open conformational transition in ferrochelatase, is associated with the Glu residue binding the Fe${}^{2+}$ substrate from a mitochondrial Fe${}^{2+}$ donor.

Oxidative N-dealkylation of NN-dimethylaniline catalyzed by substituted tetraaryl porphyrin complexes of iron and manganese with molecular oxygen, gave a mixture of dealkylated, monooxygenated and dimerised compounds as products. Presence of substituents on the catalyst effects nature and yield of products formed. One electron transfer route [E T) predominated over H-atom abstraction [HAT] in most of the reactions.

The corrole ligand serves as a versatile tri-anionic, macrocyclic platform on which to model biological catalytic systems, as well as to effect mechanistically challenging chemical transformations. Herein we describe the synthesis, structure, and characterization of an isomerically pure corrole ligand, selectively mono-brominated at the $\beta$-carbon position adjacent to the corrole C-C bond (2-C) and produced in relatively high yields, as well as its iron chloride complex. Analysis of the iron metalated complex by cyclic voltammetry shows that the bromine being present on the ligand resulted in anodic shifts of +93 and +63 mV for first oxidation and first reduction of the complex respectively. The Mössbauer spectrum of the iron metalated complex shows negligible change relative to the non-brominated analog, indicating the presence of the halide substituent predominantly effects the orbitals of the ligand rather than the metal.

Two iron phthalocyanines peripherally octasubstituted either with electron-withdrawing isobutylsulfonyl moities or electron-donating isobutoxy moieties were designed to investigate the effect of the substitution pattern on their oxidation catalytic activity, and were then tested in oxidation of cyclohexene as a reaction model. For both catalysts, the main product of oxidation was 2-cyclohexen-1-ol which is an allylic oxidation product. The electron-withdrawing isobutylsulfonyl substituted iron phthalocyanine 1 exhibited better catalytic activities than the electron-donating isobutoxy substituted iron phthalocyanine 2.

We report the synthesis, structural characterization and reactivity of perfluoroalkyl (R${}_f)$ substituted perfluoro phthalocyanines, [F${}_{64}$PcFe(II)L₂], L = axial H₂O, methanol or pyridine, and [F${}_{64}$PcFe(III)L], L = methoxy and cyanide anions. X-ray structures of the Fe(II) complexes reveal that axial H₂O and methanol are nested in the hydrophobic R${}_f$ pocket of the macrocycle. The aqua complexes exhibit columnar stacking with a central linear chain of H-bonded H₂O-Fe-H₂O groups that form a hydrophilic, linear “core“embedded in surrounding R${}_f$ groups. The electron-deficient fluoroalkyl fluoro ($i$-C₃F${}_7{)}_8$F₈PcFe(II)L₂ complexes are all low-spin, diamagnetic, unlike the F${}_{16}$PcFe complex, L = H₂O, which is paramagnetic. The P450-related PcFe(II) complexes, which exhibit no macrocyclic C-H bonds, catalyze cyclohexane (C${}_6)$ oxygenations using both t-butyl hydroperoxide (TBHP) and air, without noticeable catalyst decomposition, to produce C₆-alcohol and C₆-ketone in ~1:1 ratio (KA-oil). Relatively high TOFs, 180 and 85/hr, are observed for TBHP and O₂-based oxygenations, respectively. No catalyst decomposition is observed for at least 8 hours, even up to 100${}^{\circ }$C, when O₂ is the oxidant, in the absence of a co-reductant. Fluoroalkylated phthalocyanine iron catalysts lacking C-H bonds could be suitable for other aerobic oxidations and oxygenations.

The molecular structure of the unsubstituted iron(III) phthalocyanine $\mu$-oxo(1) dimer ((PcFe)₂O) was determined by single crystal X-ray diffraction. In agreement with the earlier speculations, the dimer has a bent (Fe-O-Fe angle is 152.4${}^{\circ }$) structure. The interplay between the $\pi$-$\pi$ interactions and steric hindrances caused by the isoindole units led to the observed staggering angle of $\sim$24${}^{\circ }$ between two phthalocyanine ligands. The high-spin iron(III) centers are located significantly above the phthalocyanine N4 planes (0.57–0.58 Å). Several DFT exchange-correlation functionals were used to calculate the absolute value and sign of the Mössbauer quadrupole splitting and antiferromagnetic coupling constant for X-ray determined geometry of (PcFe)₂O. It was demonstrated that the hybrid functionals provide the correct sign of the electric field gradient and the magnitude of the antiferromagnetic coupling constant compared to the pure functionals.