β-Nitro derivatives of iron corrolates

Development of Peripheral Functionalization Chemistry of meso-Free Corroles

In contrast to the extensive development of meso functionalization of porphyrins, that of corroles has been only rarely explored until the development of practical synthetic methods of meso -free corroles in 2015. Ready availability of meso -free corroles opened up meso -functionalization chemistry of corroles, giving rise to successful synthesis of various meso-substituted corroles such as meso -halogen, meso -nitro, meso -amino, meso -oxo, and meso iminocorroles as well as meso-meso linked corrole dimers and corrole tapes. In some cases, 2NH corroles existed as stable or transient radical species. The impacts of meso -functionalization on the structures, electronic properties, optical characteristics, and aromaticity are highlighted in this Minireview.

The Selective Monobromination of a Highly Sterically Encumbered Corrole: Structural and Spectroscopic Properties of Fe(Cl)(2-Bromo-5,10,15-tris(triphenyl)phenyl corrole)

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. Here in we describe the synthesis, structure, and characterization of an isomerically pure corrole ligand, selectively mono-brominated at the β-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 Mossbauer 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.

Infrared Spectroelectrochemistry of Iron-Nitrosyl Triarylcorroles. Implications for Ligand Noninnocence

Recent DFT calculations have suggested that iron nitrosyl triarylcorrole complexes have substantial {FeNO}⁷–corrole^•2– character. With this formulation, reduction of Fe(C)(NO) complexes, where C = triarylcorrole, should be centered on the corrole macrocycle rather than on the {FeNO}⁷ moiety. To verify this proposition, visible and infrared spectroelectrochemical studies of Fe(C)(NO) were carried out and the results were interpreted using DFT (B3LYP/STO-TZP) calculations. The first reduction of Fe(C)(NO) led to significant changes in the Soret and Q-band regions of the visible spectrum as well as to a significant downshift in the ν_NO and changes in the corrole vibrational frequencies. DFT calculations, which showed that the electron was mostly added to the corrole ligand (85%), were also able to predict the observed shifts in the ν_NO and corrole bands upon reduction. These results underscore the importance of monitoring both the corrole and nitrosyl vibrations in ascertaining the site of reduction. By contrast, the visible spectroelectrochemistry of the second reduction revealed only minor changes in the Soret band upon reduction, consistent with the reduction of the FeNO moiety.

For the reduction of FeNO moiety or corrole, infrared spectroelectrochemistry and DFT calculations were performed and experimental evidence was obtained for the noninnocence of the corrole in Fe(triphenylcorrole)(NO).

β-Arylethynyl substituted silver corrole complexes

Silver corrolates are attractive compounds from both practical and theoretical points of view. Indeed, they play a key role in peripheral functionalization reactions occurring at the macrocycle, enabling high-yield and regioselective group insertions useful to further elaborate the molecular skeleton. In parallel, the Janus innocent or noninnocent behavior of the corrole ligand in these complexes makes their description particularly challenging. Herein, we report properties for a series of silver 3,17-disubstituted triarylcorrole complexes with various functionalities (halogens or different phenylethynyl units) that deeply affect the electron density in the macrocyclic ligand, with obvious repercussions on the observed spectral characteristics. The compounds were obtained in yields of 54-92% by applying the Stille coupling reaction with the appropriate tributylethynyl stannane. Among the complexes prepared was a derivative bearing two terminal acetylenic units which opens the way to "click" reactions for new corrole-based architectures. This corrole was structurally characterized by single crystal X-ray crystallography. The addition of substituted ethynyl groups resulted in red-shifts of the electronic absorption spectra, the largest of which was observed for the compound with two β-NO2-Ph-C[triple bond, length as m-dash]C substituents. The remarkable influence of the NO2 groups on the electron density of this macrocycle was further demonstrated by electrochemical measurements, where an easier reduction of this complex derivative was observed as compared to the others. DFT calculations showed full delocalization over the entire p-nitrophenylethynyl unit of 5, largely affecting orbital distributions and the corresponding electronic absorptions. Although a variation of the β-substituents dramatically modifies the Soret- and Q-band positions towards lower energies for all the examined complexes, the saddling of the macrocycle resulting from functionalization is only moderate. The collected results suggest the description of these compounds as AgIII-corrolate3-, a metallocorrole with an innocent macrocyclic ligand.

Iron, iron everywhere: synthesis and characterization of iron 5,10,15-triferrocenylcorrole complexes

A new series of iron triferrocenylcorroles with three different axial ligands, NO, Cl⁻and σ-Ph, is synthesized and characterized using¹H NMR, electrochemical and spectroelectrochemical techniques in nonaqueous media.

Influence of β-octabromination on free-base triarylcorroles: Electrochemistry and protonation-deprotonation reactions in nonaqueous media

Electrochemical and acid-base properties of four free-base triarylcorroles were examined in nonaqueous media. These compounds are represented here as (tdcc)H[Formula: see text], (tpfc)H[Formula: see text], (Br[Formula: see text]tdcc)H[Formula: see text] and (Br[Formula: see text]tpfc)H[Formula: see text], where tdcc and tpfc are the trianions of tris(2,6-dichlorophenyl)corrole and tris(pentafluorophenyl)corrole, respectively. Different spectroscopic and electrochemical properties were observed for the [Formula: see text]-brominated corroles as compared to the non-brominated derivatives, due in part to the corrole ring distortion and in part to the strong electron-withdrawing properties of the Br groups. The brominated free-base corroles are easier to deprotonate than the non-brominated corroles in solution, which was confirmed by electrochemistry and spectroelectrochemistry as well as protonation/deprotonation reactions of the compounds with acid or base in PhCN. The electrochemistry of the protonated and deprotonated corroles is presented and comparisons made with previously published data for other protonated and deprotonated free-base corroles under the same solution conditions.

Electronic Structure of Corrole Derivatives: Insights from Molecular Structures, Spectroscopy, Electrochemistry, and Quantum Chemical Calculations

Presented herein is a comprehensive account of the electronic structure of corrole derivatives. Our knowledge in this area derives from a broad range of methods, including UV-vis-NIR absorption and MCD spectroscopies, single-crystal X-ray structure determination, vibrational spectroscopy, NMR and EPR spectroscopies, electrochemistry, X-ray absorption spectroscopy, and quantum chemical calculations, the latter including both density functional theory and ab initio multiconfigurational methods. The review is organized according to the Periodic Table, describing free-base and main-group element corrole derivatives, then transition-metal corroles, and finally f-block element corroles. Like porphyrins, corrole derivatives with a redox-inactive coordinated atom follow the Gouterman four-orbital model. A key difference from porphyrins is the much wider prevalence of noninnocent electronic structures as well as full-fledged corrole^•2- radicals among corrole derivatives. The most common orbital pathways mediating ligand noninnocence in transition-metal corroles are the metal(d_z2)-corrole("a_2u") interaction (most commonly observed in Mn and Fe corroles) and the metal(d_x2-y2)-corrole(a_2u) interaction in coinage metal corroles. Less commonly encountered is the metal(d_π)-corrole("a_1u") interaction, a unique feature of formal d⁵ metallocorroles. Corrole derivatives exhibit a rich array of optical properties, including substituent-sensitive Soret maxima indicative of ligand noninnocence, strong fluorescence in the case of lighter main-group element complexes, and room-temperature near-IR phosphorescence in the case of several 5d metal complexes. The review concludes with an attempt at identifying gaps in our current knowledge and potential future directions of electronic-structural research on corrole derivatives.

Strategies for Corrole Functionalization

This review covers the functionalization reactions of meso-arylcorroles, both at the inner core, as well as the peripheral positions of the macrocycle. Experimental details for the synthesis of all known metallocorrole types and for the N-alkylation reactions are presented. Key peripheral functionalization reactions such as halogenation, formylation, carboxylation, nitration, sulfonation, and others are discussed in detail, particularly the nucleophilic aromatic substitution and the participation of corroles in cycloaddition reactions as 2π or 4π components (covering Diels-Alder and 1,3-dipolar cycloadditions). Other functionalizations of corroles include a large diversity of reactions, namely Wittig reactions, reactions with methylene active compounds, formation of amines, amides, and imines, and metal catalyzed reactions. At the final section, the reactions involving oxidation and ring expansion of the corrole macrocycle are described comprehensively.

Effect of NO2 substitution and solvent on UV-visible spectra, redox potentials and electron transfer mechanisms of copper β-nitrotriarylcorroles. Proposed electrogeneration of a Cu(I) oxidation state

Three copper triarylcorroles containing a [Formula: see text]-pyrrole nitro substituent were synthesized and characterized as to their spectral and electrochemical properties in nonaqueous media. The examined compounds are represented as [Formula: see text]-NO2(YPh)3CorCu, where Cor is the trianion of a triphenylcorrole and Y is a Cl, H or CH3 substituent at the para-position of the three meso-phenyl rings of the compound. The data from absorption spectra, electrochemistry and thin-layer spectroelectrochemistry are consistent with an initial assignment of Cu[Formula: see text]-Cor[Formula: see text] in CH2Cl2, DMF and pyridine and electrogeneration of a formal Cu(II) corrole with an unreduced macrocycle, represented as Cu[Formula: see text]-Cor[Formula: see text], after the first one-electron reduction in these solvents. The doubly reduced [Formula: see text]-nitrocorrole has a sharp Soret band at 439 nm and a well-defined Q-band at 611 nm in CH2Cl2. Similar absorption spectra are seen for the three examined doubly reduced nitrocorroles in DMF and pyridine, suggesting formation of a Cu(I) species with an unreduced macrocycle which is represented as Cu[Formula: see text]-Cor[Formula: see text]. Changes in redox potentials and absorption spectra of the nitrocorroles are examined as a function of solvent and substituents on the meso-phenyl rings of the compounds and comparisons are made between spectral and electrochemical data of the newly synthesized corroles and that of structurally related tetraarylcorroles lacking a [Formula: see text]-nitro group.

The impact of solvents on the singlet and triplet states of selected fluorine corroles – absorption, fluorescence, and optoacoustic studies

This paper examines the influence of aprotic solvents on the spectroscopic properties as well as the energy deactivation of two free-base corrole dyes substituted with C₆F₅ and/or 4-NO₂C₆H₄ groups.

Corrole and nucleophilic aromatic substitution are not incompatible: a novel route to 2,3-difunctionalized copper corrolates

The insertion of a -NO2 group onto the corrole framework represents a key step for subsequent synthetic manipulation of the macrocycle based on the chemical versatility of such a functionality. Here we report results of the investigation of a copper 3-NO2-triarylcorrolate in nucleophilic aromatic substitution reactions with "active" methylene carbanions, namely diethyl malonate and diethyl 2-chloromalonate. Although similar reactions on nitroporphyrins afford chlorin derivatives, nucleophilic attack on carbon-2 of corrole produces 2,3-difunctionalized Cu corrolates in acceptable yields (ca. 30%), evidencing once again the erratic chemistry of this contracted porphyrinoid.

Redox properties of nitrophenylporphyrins and electrosynthesis of nitrophenyl-linked Zn porphyrin dimers or arrays

Five nitrophenylporphyrins were investigated as to their electrochemical properties in CH 2 Cl 2 containing 0.1 M TBAP. The investigated compounds are represented as ( NO 2 Ph )x Ph 4-x PorM , where Por represents the dianion of the porphyrin macrocycle, Ph is a phenyl group on meso-position of the macrocycle, NO 2 Ph is a meso-substituted nitrophenyl group, M = 2 H , Pd II or Zn II and x = 1 or 2. Each porphyrin undergoes an initial one electron reduction at E1/2 = -1.07 to -1.12 V where the added negative charge is almost totally localized on the meso-nitrophenyl group of the compound. This reversible reduction is then followed by one or more irreversible reductions of the nitrophenyl anion at more negative potentials which overlap with reduction of the conjugated porphyrin macrocycle. The initial one electron addition was monitored by thin-layer UV-vis spectroelectrochemistry which confirmed formation of a reduced nitrophenyl group in each case but also gave spectral evidence for a linkage of the one-electron reduction products in the case of the Zn derivatives, giving Zn porphyrin dimers or arrays which are characterized by a 14–15 nm red-shifted Soret band and two well-defined Q-bands, consistent with conversion from an unreduced four coordinate Zn II nitrophenylporphyrin to a five-coordinate Zn II complex with an unreduced porphyrin macrocycle.

3-NO2-5,10,15-triarylcorrolato-Cu as a versatile platform for synthesis of novel 3-functionalized corrole derivatives

β-Nitrocorroles are potentially valuable platforms for the preparation of a wide range of more elaborated corrole derivatives possessing unique chemical functionalities and electronic properties. Here we report our results on the chemical manipulation of a copper 3-NO2-triarylcorrolate using different organic reactions, all involving the reduction of -NO2 to -NH2 at an early stage, followed by further transformations. By way of a β-acylated copper corrolate, a novel corrole derivative bearing an alkyl azide group on the peripheral positions was obtained and exploited in the Huisgen 1,3-dipolar cycloaddition.

Phenyl derivative of iron 5,10,15-tritolylcorrole

The phenyl-iron complex of 5,10,15-tritolylcorrole was prepared by reaction of the starting chloro-iron complex with phenylmagnesium bromide in dichloromethane. The organometallic complex was fully characterized by a combination of spectroscopic methods, X-ray crystallography, and density functional theory (DFT) calculations. All of these techniques support the description of the electronic structure of this phenyl-iron derivative as a low-spin iron(IV) coordinated to a closed-shell corrolate trianion and to a phenyl monoanion. Complete assignments of the (1)H and (13)C NMR spectra of the phenyl-iron derivative and the starting chloro-iron complex were performed on the basis of the NMR spectra of the regioselectively β-substituted bromo derivatives and the DFT calculations.

Adventures in corrole features by electrospray ionization mass spectrometry studies

In this short review the importance of electrospray mass spectrometry in corrole chemistry is highlighted.

Synthesis and electrochemistry of β-pyrrole nitro-substituted cobalt(ii) porphyrins. The effect of the NO2 group on redox potentials, the electron transfer mechanism and catalytic reduction of molecular oxygen in acidic media

Four cobalt porphyrins were synthesized and characterized as to their electrochemical, spectroelectrochemical and electrocatalytic properties.

β-Nitro-substituted free-base, iron(III) and manganese(III) tetraarylporphyrins: synthesis, electrochemistry and effect of the NO2 substituent on spectra and redox potentials in non-aqueous media

Two free-base and four metal derivatives of substituted tetraarylporphyrins containing a nitro-substituent on the β-pyrrole position of the macrocycle were synthesized and characterized by UV-vis, FTIR, 1 H NMR and mass spectrometry as well as electrochemistry and spectroelectrochemistry in non-aqueous media. The porphyrins are represented as ( NO 2 TmPP ) M and ( NO 2 TdmPP ) M , where M = 2 H , Fe III Cl or Mn III Cl , m is a CH 3 group on the para-position of the four meso-phenyl rings of the tetraphenylporphyrin (TPP) and dm represents two OCH 3 substituents on the meta-positions of each phenyl ring of the TPP macrocycle. UV-visible spectra of the nitro-substituted porphyrins exhibit absorption bands which are red-shifted by 4–11 nm as compared to bands of the same substituted tetraarylporphyrins lacking a nitro substituent. Three or four reductions are observed for each iron and manganese nitroporphyrin, the first of which is metal-centered, leading to formation of an Fe ( II ) or Mn ( II ) complex. Further reduction at the metal center occurs for the iron porphyrins but this reaction proceeds via an Fe ( II ) π anion radical in the case of the two nitro-substituented derivatives. The β-nitro-substituted porphyrins are easier to reduce and harder to oxidize than the corresponding compounds lacking a nitro group. The effect of NO 2 substituent on reduction/oxidation potentials and the site of electron transfer was also discussed.

β-Nitro-5,10,15-tritolylcorroles

Functionalization of the β-pyrrolic positions of the corrole macrocycle with -NO(2) groups is limited at present to metallocorrolates due to the instability exhibited by corrole free bases under oxidizing conditions. A careful choice of the oxidant can limit the transformation of corroles into decomposition products or isocorrole species, preserving the corrole aromaticity, and thus allowing the insertion of nitro groups onto the corrole framework. Here we report results obtained by reacting 5,10,15-tritolylcorrole (TTCorrH(3)) with the AgNO(2)/NaNO(2) system, to give mono- and dinitrocorrole derivatives when stoichiometry is carefully controlled. Reactions were found to be regioselective, affording the 3-NO(2)TTCorrH(3) and 3,17-(NO(2))(2)TTCorrH(3) isomers as the main products in the case of mono- and disubstitution, in 53 and 20% yields, respectively. In both cases, traces of other mono- and disubstituted isomers were detected, which were structurally characterized by X-ray crystallography. The influence of the β-nitro substituents on the corrole properties is studied in detail by UV-visible, electrochemical, and spectroelectrochemical characterization of these functionalized corroles. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations of the ground and excited state properties of these β-nitrocorrole derivatives also afforded significant information, closely matching the experimental observations. It is found that the β-NO(2) substituents conjugate with the π-aromatic system of the macrocycle, which initiates significant changes in both the spectroscopic and redox properties of the so functionalized corroles. This effect is more pronounced when the nitro group is introduced at the 2-position, because in this case the conjugation is, for steric reasons, more efficient than in the 3-nitro isomer.