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

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.

Organometallic Chemistry within the Structured Environment Provided by the Macrocyclic Cores of Carbaporphyrins and Related Systems

The unique environment within the core of carbaporphyrinoid systems provides a platform to explore unusual organometallic chemistry. The ability of these structures to form stable organometallic derivatives was first demonstrated for N-confused porphyrins but many other carbaporphyrin-type systems were subsequently shown to exhibit similar or complementary properties. Metalation commonly occurs with catalytically active transition metal cations and the resulting derivatives exhibit widely different physical, chemical and spectroscopic properties and range from strongly aromatic to nonaromatic and antiaromatic species. Metalation may trigger unusual, highly selective, oxidation reactions. Alkyl group migration has been observed within the cavity of metalated carbaporphyrins, and in some cases ring contraction of the carbocyclic subunit takes place. Over the past thirty years, studies in this area have led to multiple synthetic routes to carbaporphyrinoid ligands and remarkable organometallic chemistry has been reported. An overview of this important area is presented.