meso-Aryl [20]π Homoporphyrin: The Simplest Expanded Porphyrin with the Smallest Möbius Topology

Synthesis and Properties of Stable 20π Porphyrinoids

The 20π porphyrinoids are immediate higher homologues of 18π porphyrins and differ from porphyrins in aromaticity which in turn affects the structure, properties and chemical reactivities. Research over the years indicated that the 20π porphyrinoids can be stabilized as non-aromatic/anti-aromatic or Mobius aromatic macrocycles using different strategies such as core-modification of porphyrins, non-metal/metal complexation of porphyrins, peripheral modification of porphyrins and expanded porphyrinoids. The structural properties such as aromaticity of the macrocycle can be controlled by choosing the right synthetic strategy. This review will provide an overview of the development in the chemistry of 20π porphyrinoids giving emphasize on the synthesis, structure and electronic properties of these macrocycles which have huge potential for various applications.

Synthesis of N-Fused Dithia and Dibenzi Homoporphyrins

N-fused dithia and dibenzi homoporphyrins have been synthesized using [2+2] approach by condensing one equivalent of appropriate dithienyl/di p-phenylene ethene diol with one equivalent of N-confused dipyrromethane in CH2Cl2 under...

Recent Advances in the Design and Syntheses of Porphyrinoids by Embedding Higher Analogues of Arene and Pyridine Units

Due to enthralling applications in various fields and augmenting fundamental wisdom, π-conjugated macrocycles in general and porphyrin systems in particular are constantly explored. Subtle modifications of porphyrin structure can amend the rudimentary properties. Pursuing innovative properties provides impetus to underpin arene or pyridine moiety embedded porphyrin derivatives. There have been several reviews related to arene incorporated carbaporphyrinoids; however, recent developments of porphyrin analogues by introducing higher analogues of arenes and pyridine units are not adequately inspected. This mini-review mainly focuses on biphenyl, bipyridine, terphenyl, and mixed arene pyridine embedded porphyrin analogues and their coordination chemistry.

Tetrabromo[36]octaphyrin: A Promising Precursor of Directly Fused Porphyrin(2.1.1.1) Dimer and meso-α Fused N-Confused Porphyrin Dimer

3,7,23,27-Tetrabromo[36]octaphyrin 2 was synthesized as a novel octaphyrin bearing two meso-free positions. Surprisingly, its Zn^II and Ni^II complexation reactions produced a directly fused porphyrin(2.1.1.1) dimer 6, and a meso-α fused N-confused porphyrin (NCP) dimer 7, as the first example of NCP tape, respectively, via transannular C-C bond formation. While 6 exhibits a diatropic ring-current effect owing to the global 36π Möbius aromaticity, 7 shows a paratropic ring-current effect due to the global Hückel 36π antiaromaticity. In addition, the oxidation of 7 with PbO₂ allowed for formation of its two-electron oxidized species 9 that exhibited a diatropic ring-current effect due to the global Hückel 34π aromaticity. This work has demonstrated that meso-free large expanded porphyrins can be a promising platform to produce novel fused porphyrinoids.

Electrophilic aromatic substitution reactions of compounds with Craig-Möbius aromaticity

Electrophilic aromatic substitution (EAS) reactions are widely regarded as characteristic reactions of aromatic species, but no comparable reaction has been reported for molecules with Craig-Möbius aromaticity. Here, we demonstrate successful EAS reactions of Craig-Möbius aromatics, osmapentalenes, and fused osmapentalenes. The highly reactive nature of osmapentalene makes it susceptible to electrophilic attack by halogens, thus osmapentalene, osmafuran-fused osmapentalene, and osmabenzene-fused osmapentalene can undergo typical EAS reactions. In addition, the selective formation of a series of halogen substituted metalla-aromatics via EAS reactions has revealed an unprecedented approach to otherwise elusive compounds such as the unsaturated cyclic chlorirenium ions. Density functional theory calculations were conducted to study the electronic effect on the regioselectivity of the EAS reactions.

Synthesis and properties of covalently linked di-p-benzihomoporphyrin-BODIPY conjugates

Two mono meso-functionalized [20]di-[Formula: see text]-benzihomoporphyrins containing [Formula: see text]-formylphenyl and [Formula: see text]-iodophenyl groups at meso-position respectively were synthesized by condensing one equivalent of appropriate tetrapyrrane with one equivalent of [Formula: see text]-formyl benzaldehyde/[Formula: see text]-iodo benzaldehyde in CH2Cl2 under mild acid catalyzed conditions. The meso-formylphenyl and meso-iodophenyl functionalized di-[Formula: see text]-benzihomoporphyrins were used to synthesize two covalently linked di-[Formula: see text]-benzihomoporphyrins-BODIPY conjugates. The meso-formylphenyl-functionalized di-[Formula: see text]-benzihomoporphyrin was converted to corresponding meso-dipyrrolyl substituted di-[Formula: see text]-benzihomoporphyrin by treating with excess pyrrole under acid catalyzed conditions. In the next step, the meso-dipyrrolyl di-[Formula: see text]-benzihomoporphyrin was subjected to oxidation followed by BF2 complexation to afford the directly linked di-[Formula: see text]-benzihomoporphyrin-BODIPY conjugate. The meso-iodophenyl functionalized di-[Formula: see text]-benzihomoporphyrin was coupled with ethynyl-functionalized BODIPY under mild Pd(0) coupling condition to synthesize diphenylethyne-bridged di-[Formula: see text]-benzihomoporphyrin-BODIPY conjugate. The two conjugates were characterized by HR-MS, NMR, absorption, electrochemical, fluorescence and DFT studies. The spectral and electrochemical studies indicated that the two constituents, di-[Formula: see text]-benzihomoporphyrin and BODIPY units in the conjugates interact weakly and retain their individual characteristic features. DFT studies indicated a possibility of charge transfer between di-[Formula: see text]-benzihomoporphyrin and BODIPY units in conjugates.

Synthesis and Structure of meso-Substituted Dibenzihomoporphyrins

Bench-stable meso-substituted di(p/m-benzi)homoporphyrins were synthesized through acid-catalyzed condensation of dipyrrole derivatives with aryl aldehydes. The insertion of a 1,1,2,2-tetraphenylethene (TPE) or but-2-ene-2,3-diyldibenzene unit in the porphyrin framework results in the formation of dibenzihomoporphyrins, merging the features of hydrocarbons and porphyrins. Single crystal X-ray analyses established the non-planar structure of these molecules, with the phenylene rings out of the mean plane, as defined by the dipyrromethene moiety and the two meso-carbon atoms. Spectroscopic and structural investigations show that the macrocycles exhibit characteristics of both TPE or but-2-ene-2,3-diyldibenzene and dipyrromethene units indicating the non-aromatic characteristics of the compounds synthesized. Additionally, the dibenzihomoporphyrins were found to generate singlet oxygen, potentially allowing their use as photosensitizers.

Core-Modified Pentaphyrins(2.1.1.1.1) and Bis(difluoroborane) Complex: Synthesis, Structure, and Spectral and Redox Properties

A series of hetero analogues of pentaphyrins(2.1.1.1.1) such as oxapentaphyrins(2.1.1.1.1) and thiapentaphyrins(2.1.1.1.1) were synthesized by 3 + 2 condensation of dipyrroethenedicarbinol with 16-oxatripyrrane/16-thiatripyrrane under mild acid-catalyzed reaction conditions. The stable macrocycles are freely soluble in organic solvents, and their identities were confirmed by a corresponding molecular-ion peak in high-resolution mass spectrometry, 1D and 2D NMR spectroscopy, and X-ray structure obtained for one of the oxapentaphyrin(2.1.1.1.1) macrocycles. The crystal structure and NMR studies indicated that the heterocyclic ring, such as furan in oxapentaphyrins(2.1.1.1.1) and thiophene in thiapentaphyrins(2.1.1.1.1), was inverted. In absorption spectra, the macrocycles showed one sharp band at ∼516 nm and one broad band at ∼744 nm. The spectral and X-ray studies supported the nonaromatic nature of these macrocycles. This is in contrast to the recently reported aza analogue of pentaphyrins(2.1.1.1.1), which showed antiaromatic behavior. Upon protonation, the core-modified pentaphyrins(2.1.1.1.1) macrocycles exhibited bathochromically shifted absorption bands with a distinct change in the color of the solution. The ¹H NMR, nucleus-independent chemical shift, and anisotropy-induced current density studies indicated the presence of Mobius aromaticity in the protonated macrocycles. The core-modified pentaphyrins(2.1.1.1.1) can act as good coordinating ligands, as shown here by synthesizing a bis(difluoroborane) complex of one of the thiapentaphyrins(2.1.1.1.1).

Structurally Isomerized Bis-Biphenyl Moieties Embedded in Hexaphyrin(3.1.1.3.1.1) and Octaphyrin(1.1.1.0.1.1.1.0)

An o-p-biphenyl moiety-incorporated 32π hexaphyrin(3.1.1.3.1.1) is successfully achieved. Replacing ortho with meta connectivity in the biphenyl unit of hexaphyrin leads to the formation of its structural isomer, octaphyrin(1.1.1.0.1.1.1.0). Spectral and structural analyses reveal the lack of planarity in hexaphyrin and the presence of an m-arene unit in octaphyrin, thus affording nonaromatic characteristics.

Di-( m- m- m)terphenyl-Embedded Decaphyrin and Its Bis-Rh(I) Complex

A new rectangular-shaped carbadecaphyrin was successfully synthesized by introducing a terphenylene unit ( m- m- m) in the macrocyclic core. The terphenylene moiety offers an open framework with multiple binding pockets to stabilize two Rh(I) ions in the core. The photophysical and structural studies reveal the non-aromatic character of the ligand and its bis-Rh(I) complex.

Homocarbaporphyrinoids: The m-o-m and p-o-p Terphenyl Embedded Expanded Porphyrin Analogues and Their RhI Complexes

Stable homocarbaporphyrinoids were successfully synthesized by incorporating the m-o-m and p-o-p terphenyl units into the porphyrin core. The distinct bonding modes of terphenyl in the macrocycle generated two structural isomers with two and four carbon atoms in the macrocyclic environment. The core was utilized to stabilize the Rh^I ion. The spectral and structural analyses revealed that the restricted (m-o-m) and allowed (p-o-p) conjugation in the macrocyclic core provide overall non-aromatic characteristics both to the free bases and their complexes.

24π Core-Modified Nonfused Pentaphyrin with Möbius Aromaticity

Syntheses of core-modified 24π pentaphyrin 7 and 20π homoporphyrin 8 are reported. The freebase form of 7 exhibits optical and ¹ H NMR spectroscopic behavior corresponding to a 4nπ nonaromatic topology at 298 K. However, upon lowering the temperature to 188 K, 7 exhibits weak aromaticity because of a small conformational change. Protonation of the imine pyrrolic nitrogen atoms results in sharpening of the absorption band with more than 100 nm red shift and two-fold increase in ϵ values. The ¹ H NMR spectra show large shielding and deshielding of specific protons accompanied by twisting of pyrrole and thiophene rings from the mean plane of the macrocycle. These observations suggest a major structural change upon protonation. The single-crystal X-ray structure of protonated 7 a reveals a severe twisting of the macrocycle, especially at the ethene bridge, and the two thiophene rings adjacent to the ethene bridge are pointed above and below the plane of the ethene bridge (49.07° and 49.08°). The nucleus-independent chemical shift (NICS) values calculated at the center of the macrocycle for the freebase and protonated derivatives of 7 a and 7 b are -5.6, -8.3 ppm and -5.3, -8.4 ppm, respectively. The anisotropy-induced current density (AICD) plots suggest a clockwise circulation of π electrons. Taken together, the changes occurring in the optical spectra, ¹ H NMR spectra, single-crystal X-ray structure and NICS calculation reflect a Möbius aromatic topology for protonated 7. However, the homoporphyrin 8 remains nonaromatic both in the freebase and protonated form.

[30]Hexaphyrin(2.1.2.1.2.1) as Aromatic Planar Ligand and Its Trinuclear Rhodium(I) Complex

Expanded porphyrins are attractive research targets because of their large and flexible structures, optical and electrochemical properties, and diverse coordination abilities. We are interested in the use of double bonds within expanded porphyrins because double bonds could conduct isomerization, expansion of π-conjugation, and giving different molecular geometry. We, thus, report [30]hexaphyrin(2.1.2.1.2.1) 3H-1, which was synthesized by a simple condensation reaction of 1,2-di(pyrrol-2-yl)ethene and pentafluorobenzaldehyde under an acidic condition. The compound 3H-1 exhibited 30π aromatic property with a highly planar structure, displaying intense Soret- and weak Q-like absorption bands. The compound 3H-1 has a sufficient space and dipyrrin-like coordination sites in its cavity. Trinuclear rhodium(I) complex 3Rh-1 was obtained with [Rh(CO)₂Cl]₂ and exhibited six redox potentials.

Coarctate and Möbius: The Helical Orbitals of Allene and Other Cumulenes

As brought to the attention of the community by Hendon et al. and noted by previous workers, the π orbitals of the equilibrium geometry odd-carbon (even number of double bonds = n) [n]cumulenes may be written in either rectilinear or helical form. We trace the origins and detailed composition of the helical orbitals of cumulenes, which emerge in the simplest Hückel model and are not much modified in advanced computations. For the α,ω-disubstituted even [n]cumulenes, the helical representation is obligatory as the symmetry is reduced from D_2d to C₂. A relationship is apparent between these helical orbitals of the even [n]cumulenes, seen as a Herges coarctate system, and the corresponding Möbius cyclic polyene orbitals. The twist of the orbitals varies in interesting ways along the helix, and so does the contribution of the component atomic orbitals. Though the electronic structures of even [n]cumulenes and Möbius cyclopolyenes are closely related, they differ for higher n in intriguing ways; these are linked to the constrained rotation of the basis orbitals along the helical twist itinerary. Relations are constructed between the level patterns of the π-systems of even [n]cumulenes and ideas of Hückel and Möbius aromaticity.