A Stable Radical Species from Facile Oxygenation of meso-Free 5,10,20,25-Tetrakis(pentafluorophenyl)-Substituted [26]Hexaphyrin(1.1.1.1.1.1)

Carbazole-Containing Carbadecaphyrins: Non-aromatic Expanded Porphyrins that Undergo Proton-Triggered Conformational Changes

A pair of meso-unsubstituted expanded carbaporphyrins containing two carbazole moieties were prepared in high isolated yields (82 and 76 %, respectively). The two macrocycles, namely 3 and 4, differ with respect to their substitution at the carbazole N-atoms i. e. by H and i-Bu, respectively. As prepared in their free-base forms, macrocycles 3 and 4 adopt figure-of-eight conformations and are best characterized as 40 π-electron, non-aromatic species possessing a decaphyrin(1.1.0.0.0.1.1.0.0.0) skeleton. Protonation of 3 with either trifluoroacetic acid (TFA) or perchloric acid (HClO₄ ) produces a parallelogram-shaped structure. A similar structure is produced when N-functionalized system 4 is treated with TFA. In contrast, protonation of 4 with HClO₄ leads it to adopt a twisted Möbius strip-like structure in the solid state, thus allowing access to three distinct conformational states as a function of the conditions.

A Robust Porphyrin-Stabilized Triplet Carbon Diradical

The synthesis of robust high-spin carbon radicals is an important topic in organic chemistry. Toward this end, several porphyrin-stabilized radicals have been systematically explored. A singly naphthalene-fused porphyrin radical was synthesized by a reaction sequence consisting of a Suzuki-Miyaura coupling of β-borylated porphyrin with 2-bromobenzaldehyde, addition of mesityl Grignard reagent, intramolecular Friedel-Crafts alkylation, and final oxidation with DDQ or tBuOK/O₂ . This strategy was also used to synthesize doubly naphthalene-fused porphyrins and syn- and anti-fused-anthracene-bridged porphyrin dimers. While singly naphthalene-fused porphyrin radical has been shown to be a stable monoradical, doubly naphthalene-fused porphyrins and anti-fused-anthracene-bridged porphyrin dimers have been shown to be closed-shell molecules. Finally, the syn-dimer was characterized as a surprisingly stable radical (t_1/2 =28 days under ambient air and at 80 °C) that is storable for more than several months, despite its high-spin triplet ground-state carbon diradical.

Nucleophilic Aromatic Substitution (SNAr) and Related Reactions of Porphyrinoids: Mechanistic and Regiochemical Aspects

The nucleophilic substitution of aromatic moieties (SNAr) has been known for over 150 years and found wide use for the functionalization of (hetero)aromatic systems. Currently, several "types" of SNAr reactions have been established and notably the area of porphyrinoid macrocycles has seen many uses thereof. Herein, we detail the SNAr reactions of seven types of porphyrinoids with differing number and type of pyrrole units: subporphyrins, norcorroles, corroles, porphyrins, azuliporphyrins, N-confused porphyrins, and phthalocyanines. For each we analyze the substitution dependent upon: a) the type of nucleophile and b) the site of substitution (α, β, or meso). Along with this we evaluate this route as a synthetic strategy for the generation of unsymmetrical porphyrinoids. Distinct trends can be identified for each type of porphyrinoid discussed, regardless of nucleophile. The use of nucleophilic substitution on porphyrinoids is found to often be a cost-effective procedure with the ability to yield complex substituent patterns, which can be conducted in non-anhydrous solvents with easily accessible simple porphyrinoids.

Dicarba[26]hexaporphyrinoids(1.1.1.1.1.1) with an Embedded Cyclopentene Moiety-Conformational Switching

Incorporation of cyclopentene fragments into a skeleton of parental [26]hexaphyrin(1.1.1.1.1.1) afforded extended carbaporphyrinoids: 31,34-dicarbahexa[26]chlorin and its derivatives: the first externally substituted by ethoxy and 2,4,6-trimethylbenzylidene groups and the second one formed by selective oxidation of one cyclopentene ring. Macrocycles adopt dumbbell-shaped conformations with two meso hydrogen atoms located inside the macrocyclic cavity. Protonation of 31,34-dicarba[26]hexachlorins provided dications existing in dumbbell-shaped and rectangular conformations.

Expanded Carbaporphyrinoids

This Review outlines the progress in the field of synthetic expanded carbaporphyrinoids. The evolution of this topic is demonstrated with expanded porphyrin-inspired systems with a variety of incorporated entities that introduce one or more carbon atoms into the cavity. The discussion starts with platyrins-the macrocycles that were identified as parent molecules of not only the expanded carbaporphyrinoids, but the carbaporphyrinoid class in general. After historic considerations, the plethora of expanded porphyrin-like macrocycles containing N-confused or neo-confused pyrrole motifs and different carbocyclic subunits are presented. Special emphasis is given to applications of expanded carbaporphyrinoids in different areas, including organometallic chemistry, switching systems, or aromaticity, concluding with the demonstration of a covalent cage based on an expanded carbaporphyrinoid.

Bis-palladium(II) complex of doubly N-confused octaphyrin(1.1.1.1.1.1.1.1): Möbius aromaticity and chiroptical properties

A novel bis-palladium(II) complex of doubly N-confused octaphyrin (Pd[Formula: see text]-2) adopting a Möbius-twisted topological structure was synthesized and characterized. Due to the effective 36[Formula: see text]-electronic delocalization over the Möbius-twisted octaphyrin scaffold, characteristic Soret and Q-like absorption features were observed in the near-infrared (NIR) region. However, the complex Pd[Formula: see text]-2 exhibited weak aromatic character attributed to the distinct cross-conjugated resonance contribution as inferred from the 1H NMR chemical shifts as well as theoretical assessments ([Formula: see text] nucleus-independent chemical shift (NICS)). Since the bis-palladium(II) complexation of doubly N-confused octaphyrin 2 imparted significant conformational stability, topologically chiral enantiomers of Pd[Formula: see text]-2 were successfully separated as ([Formula: see text]- and ([Formula: see text]-twisted forms. The resulting isomers revealed relatively large circular dichroism (CD) responses with an absorption anisotropy factor of [Formula: see text] [Formula: see text] 0.009 in the NIR region ([Formula: see text] 823 nm). In addition, the cyclic voltammogram of Pd[Formula: see text]-2 revealed redox-rich properties due to its large [Formula: see text]-conjugated system.

5,20-Diheterohexaphyrins: metal-template-free synthesis and aromaticity switching

As the first example of expanded heteroporphyrins with heteroatoms at meso-positions, 5,20-dithiahexaphyrins and 5,20-diazahexaphyrins were synthesized via nucleophilic substitution reactions of α,α'-dibromotripyrrin as the key step. While 5,20-dithiahexaphyrins are practically nonaromatic, 5,20-[28]diazahexaphyrins show weakly antiaromatic characters and can be cleanly converted into aromatic 5,20-[26]diazahexaphyrins upon oxidation with PbO2, demonstrating distinct aromaticity switching with maintenance of dumbbell-like conformations.

Stable radical versus reversible σ-bond formation of (porphyrinyl)dicyanomethyl radicals

(Porphyrinyl)dicyanomethyl radicals were produced by oxidation of dicyanomethyl-substituted porphyrins with PbO2. These radicals constitute a rare example displaying stable radical versus dynamic covalent chemistry (DCC) depending upon the substitution position of the dicyanomethyl radical. meso-Dicyanomethyl-substituted radicals exist as stable monomeric species and do not undergo any dimerization processes either in the solid state or in solution. In contrast, β-dicyanomethyl-substituted radicals are isolated as σ-dimers that are stable in the solid-state but display reversible σ-dimerization behavior in solution; monomeric radical species exist predominantly at high temperatures, while σ-dimerization is favoured at low temperatures. This dynamic behaviour has been confirmed by variable-temperature 1H NMR, UV-vis and EPR measurements. The structures of the stable radical and σ-dimer have been revealed by single-crystal X-ray diffraction analysis. The observed different reactivities of the two (porphyrinyl)dicyanomethyl radicals have been rationalized in terms of their spin delocalization behaviours.

A Stable Trimethylenemethane Triplet Diradical Based on a Trimeric Porphyrin Fused π-System

Trimethylenemethane (TMM) diradical is the simplest non-Kekulé non-disjoint molecule with the triplet ground state (ΔE_ST =+16.1 kcal mol^-1 ) and is extremely reactive. It is a challenge to design and synthesize a stable TMM diradical with key properties, such as actual aliphatic TMM diradical centers and the triplet ground state with a large positive ΔE_ST value, since such species provide detailed information on the electronic structure of TMM diradical. Herein we report a TMM derivative, in which the TMM segment is fused with three Ni^II meso-triarylporphyrins, that satisfies the above criteria. The diradical shows delocalized spin density on the propeller-like porphyrin π-network and the triplet ground state owing to the strong ferromagnetic interaction. Despite the apparent TMM structure, the diradical can be handled under ambient conditions and can be stored for months in the solid state, thus allowing its X-ray diffraction structural analysis.

A Benzene-1,3,5-Triaminyl Radical Fused with ZnII -Porphyrins: Remarkable Stability and a High-Spin Quartet Ground State

A benzene-1,3,5-triaminyl radical fused with three Zn^II -porphyrins was synthesized through a three-fold oxidative fusion reaction of 1,3,5-tris(Zn^II -porphyrinylamino)benzene followed by oxidation with PbO₂ as key steps. This triaminyl radical has been shown to possess a quartet ground state with a doublet-quartet energy gap of 3.1 kJ mol^-1 by superconducting quantum interference device (SQUID) studies. Despite its high-spin nature, this triradical is remarkably stable, which allows its separation and recrystallization under ambient conditions. Moreover, this triradical can be stored as a solid for more than one year without serious deterioration. The high stability of the triradical is attributed to effective spin delocalization over the porphyrin segments and steric protection at the nitrogen centers and the porphyrin meso positions.

Porphyrinoids as a platform of stable radicals

The non-innocent ligand nature of porphyrins was observed for compound I in enzymatic cycles of cytochrome P450. Such porphyrin radicals were first regarded as reactive intermediates in catabolism, but recent studies have revealed that porphyrinoids, including porphyrins, ring-contracted porphyrins, and ring-expanded porphyrins, display excellent radical-stabilizing abilities to the extent that radicals can be handled like usual closed-shell organic molecules. This review surveys four types of stable porphyrinoid radical and covers their synthetic methods and properties such as excellent redox properties, NIR absorption, and magnetic properties. The radical-stabilizing abilities of porphyrinoids stem from their unique macrocyclic conjugated systems with high electronic and structural flexibilities.

Metalation Control of Open-Shell Character in meso-meso Linked Porphyrin meso-Oxy Radical Dimers

Control of open-shell character of meso-meso linked porphyrin meso-oxy radical dimers has been demonstrated by core metalation. Namely, Ni^II -porphyrin dimer 6Ni exhibits a clear ¹ H NMR spectrum and a distorted but rather coplanar quinonoidal structure consisting of two ruffled porphyrin rings, in accordance with the previous report. Freebase dimer 6H₂ shows a similar quinonoidal structure in the solid state but displays slightly broader and temperature-dependent ¹ H NMR spectra, indicating a partial diradical character in solution that increases at high temperature. In sharp contrast, bis-imidazole-coordinated Zn^II -porphyrin dimer 6ZnIm₂ exhibits a perpendicular structure consisting of two planar Zn^II -porphyrins and has been characterized as a distinct open-shell diradical on the basis of its non-observable ¹ H NMR signals, a clear ESR signal, and a characteristic absorption spectrum reaching about 1700 nm. Despite the distinct diradical character, 6ZnIm₂ is an extremely stable molecule.

Selective synthesis of 5-aryl-10-(nitromethyl) substituted 15-azatripyrrane, 15-oxatripyrrane and 15-thiatripyrrane: access to nitromethyl functionalized A3B-porphyrins

A chemical strategy toward the selective synthesis of unsymmetrical tripyrranes was developed for the building of meso-aryl and -nitromethyl substituted A₃B porphyrins.

Hetero Cu(III)-Pd(II) Complex of a Dibenzo[g,p]chrysene-Fused Bis-dicarbacorrole with Stable Organic Radical Character

Bis-dicarbacorrole (bis-H₃) with two adj-CCNN subunits was synthesized by incorporating a dibenzo[g,p]chrysene moiety into the macrocyclic structure. The two trianionic cores in bis-H₃ can stabilize two Cu(III) ions (bis-Cu) or concurrently a Cu(III) cation and a Pd(II) ion in the form of a hetero bis-metal complex (mix-Cu/Pd). As prepared, mix-Cu/Pd displays organic π radical character, as confirmed by various techniques, including electron paramagnetic resonance spectroscopy, cyclic voltammetry, femtosecond transient absorption measurements, and DFT calculations. Radical formation is ascribed to one-electron transfer from the dicarbacorrole backbone to the Pd center allowing the d⁸ Pd(II) center to be accommodated in a square planner coordination geometry. Nucleus-independent chemical shift and anisotropy of the induced current density calculations provide support for the conclusion that bis-H₃ and bis-Cu both display antiaromatic character and contain two formally 16 π-electron dicarbacorrole subunits. On this basis, we suggest that mix-Cu/Pd is best considered as containing a fused 15 π-electron nonaromatic radical subunit and a 16 π-electron antiaromatic subunit. The spectroscopic observations are consistent with these assignments.

Electrochemistry and spectroelectrochemistry of metallohexaphyrins containing bis-copper or bis-zinc central metal ions

Two meso-substituted bis-metallohexaphyrins were synthesized and characterized as to their electrochemical and spectroelectrochemical properties in nonaqueous media. The examined compounds are represented as HexaPyM2, where HexaPy represents an expanded macrocycle containing six pyrroles and M [Formula: see text] Cu(II) or Zn(II). Each hexaphyrin undergoes four reversible one-electron reductions and two irreversible oxidations in PhCN or CH2Cl2 containing 0.1 M tetrabutylammonium perchlorate. The HexaPyCu2 is much easier to reduce than the corresponding Cu porphyrin with the same meso-substituents and it is also easier to reduce than a structurally related Cu(II) tripyrrinone. All four reductions of HexaPyM2 are assigned as [Formula: see text]-ring centered electron transfers to give [HexaPyM2][Formula: see text], [HexaPyM2][Formula: see text], [HexaPyM2][Formula: see text] and [HexaPyM2][Formula: see text], respectively. The neutral HexaPyCu2 and HexaPyZn2 also exhibit two oxidations, both of which are irreversible at a scan rate of 0.10 V/s. The peak separation between the first one-electron addition and the first electron abstraction of HexaPyM2 ranges from 1.46 to 1.72 V. These non-thermodynamic HOMO–LUMO gaps are similar to the range of HOMO–LUMO gaps for previously characterized sapphyrins which contain five pyrrole ring in the macrocycle.

Flexible Porphyrinoids

This review underscores the conformational flexibility of porphyrinoids, a unique class of functional molecules, starting from the smallest triphyrins(1.1.1) via [18]porphyrins(1.1.1.1) and concluding with a variety of expanded porphyrinoids and heteroporphyrinoids, including the enormous [96]tetracosaphyrin(1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0). The specific flexibility of porphyrinoids has been documented as instrumental in the designing or redesigning of macrocyclic frames, particularly in the search for adjustable platforms for coordination or organometallic chemistry, anion binding, or mechanistic switches in molecular devices. A structural prearrangement to coordinate one or more metal ions has been outlined. The coverage of the topic focuses on representative examples of geometry or conformational rearrangements for each selected class of the numerous porphyrinoids accordingly categorized by the number of built-in carbo- or heterocycles.

Stable neutral radicals of planar N2O2-type dipyrrin platinum complexes: hybrid radicals of the delocalized organic π-orbital and platinum d-orbital

Neutral radicals of N2O2-dipyrrin platinum complexes were synthesized by the reaction of dipyrrin ligands with PtCl2(cod) and successive one-electron oxidation. The radicals are very stable even under aerobic and ambient conditions. X-ray crystallographic analysis revealed the stacking array of the planar dipyrrin complex moieties. The ESR signals were broadened and significantly downfield shifted. The absorption spectra exhibited NIR bands. These results indicated a delocalized radical character with a contribution by the platinum d-orbital.

Highly planar diarylamine-fused porphyrins and their remarkably stable radical cations

Oxidative fusion of a meso-phenoxazino Ni(ii) porphyrin at high temperature gave a doubly phenoxazine-fused porphyrin as a highly planar diarylamine-fused porphyrin. One-electron oxidation of the corresponding β,β-dichlorinated compound gave a remarkably stable radical cation.

Oxidative fusion reactions of meso-phenoxazino Ni(ii) porphyrin were found to be temperature dependent, giving rise to either a doubly phenylene-fused product at room temperature or a singly phenoxazine-fused product at 70 °C. The latter was further oxidized to a doubly phenoxazine-fused Ni(ii) porphyrin, which was subsequently converted to the corresponding free base porphyrin and Zn(ii) porphyrin. Compared to previously reported diphenylamine-fused porphyrins that displayed a molecular twist, doubly phenoxazine-fused porphyrins exhibited distinctly different properties owing to their highly planar structures, such as larger fluorescence quantum yields, formation of an offset face-to-face dimer both in solution and the solid state, and the generation of a mixed-valence π-radical cation dimer upon electrochemical oxidation. One-electron oxidation of the phenoxazine-fused Ni(ii) porphyrin with Magic Blue gave the corresponding radical cation, which was certainly stable and could be isolated by separation over a silica gel column but slowly chlorinated at the reactive β-positions in the solid state. This finding led to us to examine β,β′-dichlorinated phenoxazine-fused and diphenylamine-fused Ni(ii) porphyrins, which, upon treatment with Magic Blue, provided remarkably stable radical cations to an unprecedented level. It is actually possible to purify these radical cations by silica gel chromatography, and they can be stored for over 6 months without any sign of deterioration. Moreover, they exhibited no degradation even after the CH₂Cl₂ solution was washed with water. However, subtle structural differences (planar versus partly twisted) led to different crystal packing structures and solid-state magnetic properties.

Switchable π-electronic network of bis(α-oligothienyl)-substituted hexaphyrins between helical versus rectangular circuit

The switching phenomena of conformation with π-electronic network through deprotonation–protonation processes were investigated by employing a series of 5,20-bis(α-oligothienyl) substituted hexaphyrins(1.1.1.1.1.1).

The switching phenomena of conformation with π-electronic network through deprotonation–protonation processes were investigated by employing a series of 5,20-bis(α-oligothienyl) substituted hexaphyrins(1.1.1.1.1.1). They showed significant changes in the absorption and emission spectra with deprotonation, and returned to the initial state with protonation. Through NMR measurements and single crystal X-ray diffraction analysis, we found that the 5,20-bis(α-oligothienyl) substituted hexaphyrins, which possess acyclic, helical electronic networks involving oligothienyl chains in dumbbell conformations (type-I) in a neutral form, underwent effective deprotonation upon treatment with tetrabutylammonium fluoride (TBAF) to generate the corresponding dianions, which display cyclic electronic networks with enhanced aromaticity in rectangular conformations (type-II). Our quantum calculation results provide an unambiguous description for the switchable conformation and π-conjugation, which revealed that a deprotonation-induced enhanced aromatic conjugation pathway is involved in the switchable π-electronic network.