Meso-Fused Carbatriphyrins(2.1.1) and Its Organo Phosphorus(V) Complex

Pyrrolizinofluorene-embedded dithiacalixcorrole(1.0.1.1)s

An unprecedented synthesis of pyrrolizinofluorene moiety embedded dithiacalixcorrole(1.0.1.1)s via [3 + 2] condensation of fluorene-based tripyrrane and appropriate bithiophene diol under TFA-catalyzed conditions is reported. Single crystal X-ray structural analysis revealed that the fluorene unit was inverted and involved in the intramolecular C-N bond formation, resulting in the generation of an unusual pyrrolizinofluorene moiety as part of the dithiacalixcorrole(1.0.1.1) framework. These macrocycles exhibit strong absorption in the UV-visible region and electrochemical studies revealed their electron-rich nature.

Porphyrinoid framework embedded with polycyclic aromatic hydrocarbons: new synthetic marvels

The highly conjugated tetrapyrrolic porphyrin macrocycle and its contracted and expanded congeners have been extensively used for a wide range of applications across diverse research domains because of their captivating and intriguing features. Over the years, the porphyrin framework and electronic properties of porphyrinoids have been modified and tuned by replacing one or more pyrrole ring(s) with five- and six-membered heterocycles/carbacycles, and their resulting properties have been explored. In recent times, polycyclic aromatic hydrocarbons (PAHs), such as biphenyl, terphenyl, naphthalene, anthracene, phenanthrene, fluorene, pyrene and dibenzo[g,p]chrysene, have been used to replace one or more pyrrole rings of porphyrinoids, and resulting polycyclic-aromatic-embedded porphyrinoids show unique features that differ from those of other modified porphyrinoids. The polycyclic aromatic hydrocarbons in the porphyrinoid macrocyclic framework induce different π-conjugation pathways in macrocycles, exhibit variable degrees of aromaticity from nonaromatic to aromatic and antiaromatic and provide a unique ligand environment to form stable coordination and organometallic complexes in which metals show uncommon oxidation states and unusual reactivity. This review presents an overview of the synthesis, coordination chemistry, structure and properties of various porphyrinoids with an embedded PAH that have been reported to date.

Synthesis of Phenothiazine Embedded Heteroporphyrins

A series of phenothiazine embedded heteroporphyrins containing one phenothiazine unit, two pyrrole rings and one heterocycle such as furan, thiophene, selenophene and tellurophene connected via four meso carbons were synthesized. The macrocycles were synthesized by condensing the phenothiazine based tripyrrane with corresponding 2,5-bis(hydroxymethyl)heterocycle under BF3  ⋅ OEt2 catalyzed conditions and compared the structural, spectral, and electrochemical properties with the reported phenothiazinophyrins. The studies showed that the phenothiazine embedded heteroporphyrins were nonaromatic and electronic properties were significantly altered by replacing the pyrrole ring from phenothiazinophyrin with different heterocycles. The X-ray structure of phenothiazine embedded thiaporphyrin revealed that the macrocycle was distorted with an inverted thiophene ring. Both mono-protonated and diprotonated derivatives of macrocycles were generated by the controlled addition of trifluoroacetic acid to the macrocycles. The macrocyclic protons experienced upfield/downfield shifts in protonated derivatives compared to their corresponding neutral phenothiazine embedded heteroporphyrins. However, the heterocyclic ring in both mono- and diprotonated derivatives retained its inverted conformation. The macrocycles in their neutral and protonated form exhibit nonaromatic absorption features. The studies indicated the electron rich nature of macrocycles and DFT/TD-DFT studies were carried out to justify the experimental observations.

Synthesis and Properties of Nonaromatic meso-fused Heterobenzihomoporphyrin(2.1.1.1)s

Fluorene-based tripyrrane has been used as a fused precursor to synthesize three novel examples of nonaromatic meso-fused thia and selenabenzihomoporphyrin(2.1.1.1)s by condensing it with appropriate 2,5-bis(hydroxymethyl)aryl thiophene or selenophene under acid catalyzed conditions. The meso-fused heterobenzihomoporphyrins contain one fluorene unit, two pyrrole rings and one thiophene/selenophene ring connected via five meso-carbons in the macrocyclic framework. The macrocycles were thoroughly characterized by HR-MS, 1D and 2D NMR, absorption, cyclic voltammetry and DFT/TD-DFT studies. NMR, absorption, and DFT studies indicated the nonaromatic nature of meso-fused heterobenzihomoporphyrins. The macrocycles displayed one intense band at ∼380 nm along with a shoulder band at 450 nm and a broad band in the region of 590-850 nm which were bathochromically shifted in the monoprotonated derivatives and absorbed prominently in the NIR region with the peak maxima at ∼1035 nm. The electrochemical studies revealed that the macrocycles showed three well-defined oxidations and reductions, and TD-DFT studies corroborated experimental observations.

Synthesis of stable nonaromatic phenothiazinophyrins

Stable and nonaromatic phenothiazinophyrins which resulted from the replacement of one of the pyrrole rings of porphyrin with a phenothiazine unit were synthesized by condensing phenothiazine based tripyrrane with aryl aldehyde and pyrrole under acid catalysed conditions. NMR studies revealed that the pyrrole ring that is across the phenothiazine unit is inverted and DFT studies also supported that the pyrrole ring inverted phenothiazinophyrins were more stable. Phenothiazinophyrins and their protonated derivatives showed panchromatic absorption features and absorbed in the visible to NIR region.

Recent Advances in Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds

This review surveys recent progress in the chemistry of polycyclic heteroaromatic molecules with a focus on structural diversity and synthetic methodology. The article covers literature published during the period of 2016-2020, providing an update to our first review of this topic (Chem. Rev. 2017, 117 (4), 3479-3716).

Recent advances in subphthalocyanines and related subporphyrinoids

Subporphyrinoids constitute a class of extremely versatile and attractive compounds. Herein, a comprehensive review of the most recent advances in the fundamentals and applications of these cone-shaped aromatic macrocycles is presented.

Synthesis, Structure, Spectral, and Anion Sensing Studies of an Aromatic meso-Fused Boron(III) Benzitriphyrin(2.1.1) Complex

An aromatic tricoordinated organo B(III) complex of benzitriphyrin(2.1.1) was synthesized by treating the nonaromatic phlorin analogue of meso-fused benzitriphyrin(2.1.1) with BCl₃ in triethylamine/toluene and refluxing for 2 h. The X-ray structure revealed that B(III) was in a trigonal-planar geometry and was coordinated to two nitrogen atoms and one carbon. Insertion of the small B(III) ion into the meso-fused phlorin analogue resulted in transforming the nonaromatic phlorin analogue to an aromatic B(III) benzitriphyrin(2.1.1) complex. Spectral and electrochemical studies supported the aromatic nature of the B(III) complex of benzitriphyrin(2.1.1). Theoretical studies supported the major contribution of the 18π delocalization pathway toward the aromatic nature of the B(III) meso-fused benzitriphyrin(2.1.1) in comparison to the 22π delocalization pathway. The B(III) benzitriphyrin(2.1.1) complex acts as a specific colorimetric and fluorogenic F^-/CN^- ion sensor.

Changes in porphyrin’s conjugation based on synthetic and post-synthetic modifications

Porphyrins or more broadly defined porphyrinoids are the structures where the extended π-cloud can be significantly modified by several factors. The broad range of introduced structural motifs has shown a possibility of modification of conjugation by a controlled synthetic approach, leading to expected optical or magnetic behaviour, and also by post-synthetic modifications (i.e. redox or protonation/deprotonation), Both approaches lead to noticeab changes in observed properties but also open a potential for further utilization. Thus, this already constituted big family of macrocyclic structures with specific highly extended π-delocalization shows a significant contribution in several fields from fundamental studies, leading to understanding behaviour of skeletons like that with a substantial influence on biological studies and material science. The presented material focuses on the most significant examples of modifications of porphyrinoids skeleton leading to drastic changes in optical response and magnetic properties. Through the presentation, the focus will be placed on the changes leading to the most red-shifted transition as the parameter indicating extending the π-delocalization. Significantly different magnetic character will be also discussed based on the switching between aromatic/antiaromatic character assigned to macrocyclic structures that will be included.

PtCl2 mediated peripheral transformation of carbatriphyrin(3.1.1) into a meso-fused β-β' dimer and its monomer analogue

An unprecedented formation of a meso-fused β-β' carbaporphyrin dimer and its monomer with a keto group was described. These analogues were synthesized from carbatriphyrin(3.1.1.) by a metal assisted strategy using PtCl2 salt in a single step without any prefunctionalized precursors. Upon dimerization, the monomeric ligand with a dianionic core is transformed into a dimeric structure with unique trianionic cores.

Synthesis of Azulitriphyrins(1.2.1) and Related Benzocarbatriphyrins

Bis(pyrrolylmethyl)azulene dialdehydes underwent intramolecular McMurry coupling, and following oxidation with DDQ and addition of trifluoroacetic acid, gave aromatic azulitriphyrin cations. The proton NMR spectra for these contracted carbaporphyrinoids showed the internal CH upfield at ca. 2 ppm, while the bridging methine units appeared downfield between 8 and 10 ppm, demonstrating that the macrocycles possess significant 14π electron diamagnetic ring currents. Although protonated azulitriphyrins(1.2.1) proved to be very stable, the free base forms were unstable and could not be isolated. Treatment of an azulitriphyrin with KOt-Bu-t-BuOOH gave rise to oxidative ring contractions that afforded the first examples of benzocarbatriphyrins. These contracted porphyrinoids also exhibit aromatic ring currents, and the internal CH resonances shifted upfield to 0.5-0.7 ppm. Protonation afforded cations with slightly enhanced diatropic properties. Nucleus independent chemical shift and anisotropy of induced current density calculations confirmed that benzocarbatriphyrins are 14π electron delocalized aromatic systems that bypass the fused arene units.

Antiaromatic Carbaporphyrinoids: Fluorene as a Fused Motif toward the Synthesis of meso-Fused Heterobenziporphyrins

meso-Tetraphenyl meta-benziporphyrins are nonaromatic macrocycles which can be changed to antiaromatic by fusing the core benzene ring with one of the adjacent meso-phenyl groups, as demonstrated here by adopting a premodification method. We used a fluorene moiety in place of the m-phenylene ring as a premodified fused aromatic motif to synthesize fused heterobenziporphyrins. Spectral and X-ray data indicated that the macrocycles are antiaromatic, which was supported by DFT, ACID, and NICS calculations. These macrocycles formed organopalladium complexes.

28-Hetero-2,7-Naphthiporphyrins: Horizontal Expansion of the m-Benziporphyrin Macrocycle

Replacement of the m-phenylene moiety of m-benziporphyrins with the 2,7-naphthalenyl subunit yielded 28-hetero-2,7-naphthiporphyrins-macrocycles that can be considered as expanded carbaporphyrinoids. This group retains some features of parent m-benziporphyrins, but due to larger size and different shape of the macrocyclic cavity, their coordination properties are different. Upon reduction and conformational rearrangement the 28-thia- and 28-selena-2,7-naphthiporphyrin form organophosphorus(V) complexes.