Trans‐A2B‐corroles Bearing a Coumarin Moiety ‐ From Synthesis to Photophysics

Four dyads comprised of corrole and coumarin units have been synthesised. Three coumarincarboxaldehydes were synthesized and transformed into the corresponding trans‐A₂B‐corroles by reaction with 5‐(pentafluorophenyl)dipyrromethane. It has been proven that this type of direct condensation can lead to the corresponding corroles in moderate yields. The reaction of hydroxybenzaldehydes with vinylphosphonium salts has been identified as the most general method for the preparation of formyl‐coumarins with various patterns of substituents. The dyad consisting of ketobiscoumarin and corrole was synthesized by Sonogashira coupling. Spectroscopic and photophysical investigations revealed that there is an efficient energy transfer from the coumarin moiety to corrole in all four dyads. Energy transfer can be clearly ascribed to a dipole–dipole mechanism (Förster) for all dyads that contain luminescent coumarins and to an electron exchange mechanism (Dexter) for the dyad with the non‐luminescent one. In the case of the dyad that bears coumarin with a hydroxy group at position 5, an electron‐transfer was detected from corrole to coumarin. The latter process is possible because of the suitably low reduction potential of coumarins of this type.     

meso‐Ester Corroles

The introduction of ester groups on the 5‐ and 15‐meso positions of corroles stabilizes them against oxidation and induces a redshift of their absorption and emission spectra. These effects are studied through the photophysical and electrochemical characterization of up to 16 different 5,15‐diester corroles, in which the third meso position is free or occupied by an aryl group, a long alkyl chain, or an ester moiety. Single‐crystal X‐ray structure analysis of five 5,15‐diestercorroles and DFT and time‐dependent DFT calculations show that the strong electron‐withdrawing character of the 5,15 ester substituents is reinforced by their π overlap with the macrocyclic aromatic system. The crystal packing of corroles 2 , 4 , 6 , 9 , and 15 features short distances between chromophores that are stacked into columns thanks to the low steric hindrance of meso‐ester groups. This close packing is partially due to intermolecular interactions that involve inner hydrogen and nitrogen atoms, and thereby, stabilize a single, identical corrole tautomeric form.     

A Spectroscopic and Theoretical Investigation of a Free‐Base meso‐Trithienylcorrole

The unique optical properties of free‐base meso‐tris(5‐methylthien‐2‐yl)corrole were compared to those of the widely investigated meso‐triphenyl‐substituted analogue. A combination of spectroscopic and computational experiments was undertaken to elucidate the relationship between structural features of the neutral, mono‐anionic and mono‐cationic forms of the corroles and their corresponding optical properties. A general bathochromic shift was measured for the thienyl‐substituted corrole. The experimental spectra are supported by excited state calculations. A systematic series of ground state minimizations were performed to determine energy minima for the flexible and solvent‐sensitive molecules. Trithienylcorrole was found to have a more nonplanar macrocycle in conjunction with a high degree of π‐overlap with the meso‐substituents. Both structural features contribute to their bathochromically shifted optical spectra. The configurational character of the thienyl‐substituted corrole is shown to have a larger degree of molecular orbital mixing and doubly excited character, which suggest a more complex electronic structure that does not fully adhere to the Gouterman four‐orbital model. The reactivity of the thienyl groups, particularly with respect to their ability to be (electro)‐polymerized, combined with the tight coupling of the meso‐thienyl groups with the corrole chromophore elucidated in this work, recommends the meso‐thienylcorroles as building blocks in, for instance, organic semiconductor devices.     

Development of the Peripheral Functionalization Chemistry of meso-Free Corroles

In contrast to the extensive development of the meso-functionalization of porphyrins, that of corroles had rarely been explored until the development of practical synthetic methods for meso-free corroles in 2015. The 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 exist as stable or transient radical species. The impact of meso-functionalization on the structures, electronic properties, optical characteristics, and aromaticity of corroles are highlighted in this Minireview.     

Stable meso–meso‐Linked 2NH‐Corrole Radical Dimers as a Key Intermediate to Corrole Tape

While oxidation of 5,5′,15,15′‐tetramesityl‐10‐10′‐linked 3NH‐corrole dimer with DDQ gave the corresponding triply linked 2NH‐corrole tape, the use of an equimolar amount of p‐chloranil as a milder oxidant resulted in the formation of a 10‐10′‐linked neutral 2NH‐corrole radical dimer as a stable product. The stability of this peculiar product is ascribed largely to strong antiferromagnetic interaction of the two spins. Further oxidation of this diradical produced corrole tape, suggesting its involvement as a reaction intermediate to the corrole tape. Oxidation of 10‐10′‐linked bis‐pyridine‐coordinated Co^III corrole dimer with DDQ produced a cobalt corrole radical dimer and a doubly linked corrole dimer both as stable compounds bearing pyridine and cyanide axial ligands. This type of oxidative transformation involving neutral diradical intermediates is a unique reaction mechanism specific for corrole dimers.     

Bis‐copper(II) Complex of Triply‐linked Corrole Dimer and Its Dication

Copper complexes of corroles have recently been a subject of keen interest due to their ligand non‐innocent character and unique redox properties. Here we investigated bis‐copper complex of a triply‐linked corrole dimer that serves as a pair of divalent metal ligands but can be reduced to a pair of trivalent metal ligands. Reaction of triply‐linked corrole dimer 2 with Cu(acac)₂ (acac=acetylacetonate) gave bis‐copper(II) complex 2Cu as a highly planar molecule with a mean‐plane deviation value of 0.020 Å, where the two copper ions were revealed to be divalent by ESR, SQUID, and XPS methods. Oxidation of 2Cu with two equivalents of AgBF₄ gave complex 3Cu , which was characterized as a bis‐copper(II) complex of a dicationic triply‐linked corrole dimer not as the corresponding bis‐copper(III) complex. In accord with this assignment, the structural parameters around the copper ions were revealed to be quite similar for 2Cu and 3Cu . Importantly, the magnetic spin–spin interaction differs depending on the redox‐state of the ligand, being weak ferromagnetic in 2Cu and antiferromagnetic in 3Cu .     

Iron(III) and iron(IV) corroles: synthesis,spectroscopy, structures,and no indications for corrole radicals

A delicate control of reaction conditions allows the isolation of several distinctively different iron complexes of tris(pentafluorophenyl)- and tris(2,6-dichlorophenyl)corrole. As long as coordinating ligands are present, the iron(III) complexes are stable in solution. Otherwise they are aerobically oxidized to either mononuclear chloroiron(IV) or dinuclear (mu-oxo)iron(IV) complexes, in acidic and basic solutions, respectively (the latter holds only for tris(pentafluorophenyl)corrole). When treated with NaNO(2), the mononuclear chloroiron(IV) corroles are efficiently converted into diamagnetic iron nitrosyl complexes. The low- and intermediate-spin iron(III), iron nitrosyl, and chloroiron(IV) corroles were fully characterized by a combination of spectroscopic methods and X-ray crystallography. There was no indication for an open-shell corrole in any of the complexes.     

Synthesis and characterization of germanium, tin, phosphorus, iron, and rhodium complexes of tris(pentafluorophenyl)corrole, and the utilization of the iron and rhodium corroles as cyclopropanation catalysts

The germanium(IV), tin(IV). and phosphorus(v) complexes of tris(pentafluorophenyl)corrole were prepared and investigated by electrochemistry for elucidation of the electrochemical HOMO-LUMO gap of the corrole and the spectroscopic characteristics of the corrole pi radical cation. This information was found to be highly valuable for assigning the oxidation states in the various iron corroles that were prepared. Two iron corroles and the rhodium(I) complex of an N-substituted corrole were fully characterized by X-ray crystallography and all the transition metal corroles were examined as cyclopropanation catalysts. All iron (except the NO-ligated) and rhodium corroles are excellent catalysts for cyclopropanation of styrene, with the latter displaying superior selectivities. An investigation of the effect of the oxidation state of the metal and its ligands leads to the conclusion that for iron corroles the catalytically active form is iron(III), while all accesible oxidation states of rhodium are active.     

meso‐Free BIII Subporphyrins with Electron‐donating Groups

meso‐Free B^III 5,10‐bis(p‐dimethylaminophenyl)subporphyrins were synthesized. They display red‐shifted absorption and fluorescence spectra, bathochromic behaviors in polar solvents, a high fluorescence quantum yield (Φ_F=0.57), and a small HOMO–LUMO gap mainly due to destabilized HOMO as compared with meso‐free B^III 5,10‐diphenylsubporphyrin. This subporphyrin serves as a nice precursor of various meso‐substituted B^III subporphyrins such as B^III meso‐nitrosubporphyrin, B^III meso‐aminosubporphyrin, and meso‐meso’ linked B^III azosubporphyrin dimer. Reactions of meso‐free B^III subporphyrins with NBS or bis(2,4,6‐trimethylpyridine)bromonium hexafluorophosphate gave meso‐meso′ linked subporphyrin dimers, often as a major product along with meso‐bromosubporphyrins.     

New and efficient arrays for photoinduced charge separation based on perylene bisimide and corroles

The bichromophoric systems C2-PI, C3-PI, and C3-PPI consisting of corrole and perylene bisimide units and representing one of the rare cases of elaborate structures based on corrole, have been synthesized. Corroles C2 and C3 are, respectively, meso-substituted corroles with 2,6-dichlorophenyl and pentafluorophenyl substituents at the 5 and 15 positions. The three dyads were prepared by divergent strategy with the corrole-forming reaction as the last step of the sequence. C2-PI and C3-PI differ in the nature of the corroles, whereas C3-PI differs from C3-PPI in the presence of a further phenyl unit in the linker between photoactive units. The dyads display spectroscopic properties which are the superposition of the component spectra, indicating a very weak electronic coupling. Excitation of the corrole unit leads to charge separation with a rate which decreases from 2.4 x 10(10), to 5.0 x 10(9), and to 4.9 x 10(7) s(-1) for C2-PI, C3-PI, and C3-PPI, respectively, where the reaction is characterized by a delta G degrees >0. Excitation of the perylene bisimide unit is followed by competing reactions of: 1) energy transfer to the corrole unit, which subsequently deactivates to the charge-separated state and; 2) electron transfer to directly form the charge-separated state. The ratio of electron-to-energy-transfer rates is 9:1 and 1:1 for C3-PI and C3-PPI, respectively. The yield of charge separation is essentially 100 % for C2-PI and C3-PI, and approximately 50 % (excitation of peryleneimide) or 15 % (excitation of the corrole) for C3-PPI. The lifetime of the charge-separated state, observed for the first time in corrole-based structures, is 540 ps for C2-PI, 2.5 ns for C3-PI, and 24 ns for C3-PPI, respectively. This is in agreement with an inverted behavior, according to Marcus theory.     

Cationic metal-corrole complexes: design, synthesis, and properties of guanine-quadruplex stabilizers

A series of pyridinium and quaternary ammonium copper corroles has been designed and synthesized. All new compounds have been fully characterized by NMR spectroscopy, high-resolution mass spectrometry, UV/Vis spectrscopy, and elemental analysis. Biochemical studies have indicated that all of these corrole derivatives can stabilize G-quadruplex structures, with corrole 4 being the most effective according to the results of circular dichroism (CD) melting experiments, polymerase chain reaction (PCR) stop assays, and surface plasmon resonance (SPR) experiments. Moreover, both corroles 3 and 4 tend to induce the human telomeric sequence to form hybrid G-quadruplex structures, whereas corroles 8 and 9 are more inclined to induce the human telomeric sequence to form antiparallel G-quadruplex structures.     

A survey of acid catalysis and oxidation conditions in the two-step, one-flask synthesis of meso-substituted corroles via dipyrromethanedicarbinols and pyrrole

The reaction of dipyrromethanedicarbinols with pyrrole leading to meso-substituted corroles was investigated to determine whether mild acid catalysts [Dy(OTf)(3), Yb(OTf)(3), Sc(OTf)(3), and InCl(3)] known to provide porphyrins from dipyrromethanecarbinol species while suppressing undesired reversibility (resulting in scrambling) are applicable to reactions affording corrole, and to explore the requirements of the oxidation step. We examined a model reaction leading to meso-triphenylcorrole (TPC) to survey the effect of acid catalyst, acid concentration, ratio of pyrrole to dipyrromethanedicarbinol, oxidant, oxidant quantity, and reaction time on the yield of TPC (by UV-vis) in reactions performed at room temperature in CH(2)Cl(2). Key to this survey was a modification of the well-known spectrophotometric method for monitoring reactions leading to porphyrin. The survey revealed that TPC could be prepared via a subset of the mild acid catalysts [Dy(OTf)(3) and Yb(OTf)(3)], and a preparative-scale reaction afforded an isolated yield of TPC of 49%, devoid of porphyrin. Suppression of reversible processes was further demonstrated by the synthesis of three corroles bearing different meso substituents in defined locations in isolated yields ranging from 50% to 80%. The reaction conditions were applicable to a dipyrromethanedicarbinol bearing electron-withdrawing pentafluorophenyl substituents-provided that the reaction time of the condensation step was increased. We identified circumstances under which DDQ can cause severe interference with the detection and isolation of some corroles, we found that the yield and purity of the corrole depend on judicious selection of oxidation conditions, and we assessed the sensitivity toward light of dilute solutions of the corroles prepared in this study.     

Iridium(III) Complexes with Sulfonyl and Fluorine Substituents: Synthesis,Stereochemistry and Effect of Functionalisation on their Photophysical Properties

The synthesis and photophysical and electrochemical characterisation of new heteroleptic iridium complexes with electron‐withdrawing sulfonyl groups and fluorine atoms bound to phenylpyridine ligands are reported. The emission energy of these materials strongly depends on the position of the sulfonyl groups and on the number of fluorine substituents. A 90 nm wide tuning range of photoluminescence from the blue‐green (λ_em=468 nm) of iridium(III)bis[2‐(4′‐benzylsulfonyl)phenylpyridinato‐N,C2′][3‐(pentafluorophenyl)‐pyridin‐2‐yl‐1,2,4‐triazolate] to the orange (λ_em=558 nm) of iridium(III)bis[2‐(3′‐benzylsulfonyl)phenylpyridinato‐N,C2′](2,4‐decanedionate) has been achieved. Emission quantum yields ranging from 47 to 71 % have also been found for degassed solutions of the complexes, and a surprisingly high value of 16 % was recorded for iridium(III)bis[2‐(5′‐benzylsulfonyl‐3′,6′‐difluoro)phenylpyridinato‐N,C2′](2,4‐decanedionate) in air‐equilibrated dichloromethane. A unusual stereochemistry of the benzylsulfonyl‐substituted dimer and heteroleptic complexes has been detected by ¹H NMR spectroscopy, and is characterised by the mutual cis disposition of the pyridyl nitrogen atoms of the phenylpyridine ligands, which differs from the most common trans arrangement reported in the literature.     

Configurational assignments of the diastereomers of 3,3′‐(1,2‐ethanediyl)bis[2‐(3‐fluorophenyl)‐5‐methyl‐4‐thiazolidinone] derivative with four stereogenic centers

Diastereomers of antiinflammatory/analgesic and antihistaminic 3,3′[(1,2‐ethanediyl)bis(2‐aryl‐4‐thiazo‐lidinone)] derivatives possessing two stereogenic centers (indicated as BIS 2*C) have been widely investigated in recent years. The 5,5′‐dimethyl analogues (BIS 4*C), now reported, have been synthesized by reaction of (±) α‐mercaptopropionic acid and N,N'‐di(3‐fluorobenzyliden)ethylenediamine. Because the 2 and 2′carbons bear the same groups and similarly the 5 and 5′ carbons, and the latter groups are different from the former, four enantiomeric pairs and two meso forms exist in this situation. These diastereomers were identified by the concerted use of nmr spectroscopy and hplc on chiral stationary phase.     

Wolves in Sheep's Clothing: μ‐Oxo‐Diiron Corroles Revisited

For well over 20 years, μ‐oxo‐diiron corroles, first reported by Vogel and co‐workers in the form of μ‐oxo‐bis[(octaethylcorrolato)iron] (Mössbauer δ 0.02 mm s^?1, ΔE_Q 2.35 mm s^?1), have been thought of as comprising a pair antiferromagnetically coupled low‐spin Fe^IV centers. The remarkable stability of these complexes, which can be handled at room temperature and crystallographically analyzed, present a sharp contrast to the fleeting nature of enzymatic, iron(IV)‐oxo intermediates. An array of experimental and theoretical methods have now shown that the iron centers in these complexes are not Fe^IV but intermediate‐spin Fe^III coupled to a corrole^.2?. The intramolecular spin couplings in {Fe[TPC]}₂(μ‐O) were analyzed via DFT(B3LYP) calculations in terms of the Heisenberg–Dirac–van Vleck spin Hamiltonian H=J_Fe–corrole(S_Fe?S_corrole)+J_Fe–Fe′(S_Fe?S_Fe′)+J_{Fe′–corrole}(S_Fe′?S_corrole′), which yielded J_Fe–corrole=J_{Fe′–corrole′}=0.355 eV (2860 cm^?1) and J_Fe–Fe′=0.068 eV (548 cm^?1). The unexpected stability of μ‐oxo‐diiron corroles thus appears to be attributable to charge delocalization via ligand noninnocence.     

Photophysical properties of a series of free-base corroles

Four free-base corroles with electron-donating or electron-withdrawing groups on the para or 2 through 6-positons of the meso phenyl rings were prepared via either Paolesse or Gross conditions and investigated for their absorption and emission properties. The triaryl corroles 5,10,15-triphenylcorrole, 5,10,15-tris(pentafluorophenyl)corrole, 5,10,15-tris(p-nitrophenyl)corrole, and 5,10,15-tris(p-methoxyphenyl)corrole were examined. Absorption, steady-state, and time-resolved fluorescence measurements were performed on all compounds in both nonpolar (dichloromethane) and polar (dimethylacetamide) solvents. The experimental evidence points to hydrogen bonding with an internal N-H group as the most likely factor in the solvent-dependent photophysical behavior of these corroles, that is also highly dependent upon substitution.     

First Example of a Modular Porphyrinoid Assembly Capable of Stabilizing Different Metal Ions in a Single Molecular Scaffold

We report the synthesis and characterization of porphyrin–corrole–porphyrin (Por‐Cor‐Por) hybrids directly linked at the meso–meso positions for the first time. The stability and solubility of the trimer are carefully balanced by adding electron‐withdrawing substituents to the corrole ring and sterically bulky groups on the porphyrins. The new hybrids are capable of stabilizing more than one metal ion in a single molecular scaffold. The versatility of the triad has been demonstrated by successfully stabilizing homo‐ (Ni) and heterotrinuclear (Ni‐Cu‐Ni) coordination motifs. The solid‐state structure of the NiPor‐CuCor‐PorNi hybrid was revealed by single‐crystal X‐ray diffraction studies. The Ni^II porphyrins are significantly ruffled and tilted by 83° from the plane of corrole. The robustness of the synthesized hybrids was reflected in the electrochemical investigations and the redox behaviour of the hybrids show that the oxidation processes are mostly corrole‐centred. In particular it is worth noting that the Por‐Cor‐Por hybrid can further be manipulated due to the presence of substituent‐free meso‐positions on both the terminals.     

Cross‐Conjugated Hexaphyrins and Their Bis‐Rhodium Complexes

A cross‐conjugated hexaphyrin that carries two meso‐oxacyclohexadienylidenyl (OCH) groups 9 was synthesized from the condensation of 5,10‐bis(pentafluorophenyl)tripyrrane with 3,5‐di‐tert‐butyl‐4‐hydroxybenzaldehyde. The reduction of 9 with NaBH₄ afforded the Möbius aromatic [28]hexaphyrin 10 . Bis‐rhodium complex 11 , prepared from the reaction of 10 with [{RhCl(CO)₂}₂], displays strong Hückel antiaromatic character because of the 28 π electrons that occupy the conjugated circuit on the enforced planar structure. The oxidation of 11 with 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) yielded complexes 12 and 13 depending upon the reaction conditions. Both 12 and 13 are planar owing to bis‐rhodium metalation. Although complex 12 bears two meso‐OCH groups at the long sides and is quinonoidal and nonaromatic in nature, complex 13 bears 3,5‐di‐tert‐butyl‐4‐hydroxyphenyl and OCH groups and exhibits a moderate diatropic ring current despite its cross‐conjugated electronic circuit. The diatropic ring current increases upon increasing the solvent polarity, most likely due to an increased contribution of an aromatic zwitterionic resonance hybrid.