Synthesis of meso-Tetraaryl Triphyrins(2.1.1)

Synthesis of unsymmetrical 2,3,7,8-tetrabromo meso-5,10,11,16-tetraaryl- triphyrin(2.1.1) and its use in the synthesis of sterically crowded 2,3,5,7,8,10,11,16-octaarylated triphyrin(2.1.1)s

Triphyrin(2.1.1) is a 14π aromatic contracted congener of an 18π aromatic porphyrin(1.1.1.1). An unsymmetrical 2,3,7,8-tetrabromo meso-tetraaryl triphyrin(2.1.1) containing four bromides at the β-pyrrole carbons of two out of three pyrrole rings of the triphyrin core was synthesized for the first time in 90% yield by treating meso-tetraaryl triphyrin(2.1.1) with five equivalents of N-bromosuccinimide in 1,2-dichloroethane (DCE) under reflux for 8 h. The X-ray structure revealed that the triphyrin(2.1.1) macrocycle was significantly distorted in 2,3,7,8-tetrabromo meso-tetraaryl triphyrin compared to planar meso-tetraaryl triphyrin. A series of novel sterically crowded 2,3,5,7,8,10,11,16-octaaryl triphyrin(2.1.1)s were synthesized by coupling 2,3,7,8-tetrabromo meso-tetraaryl triphyrin with six different aryl boronic acids under Suzuki-Miyaura coupling conditions. NMR, absorption, electrochemical and theoretical studies revealed that the structure and electronic properties were drastically altered in the 2,3,5,7,8,10,11,16-octaaryl triphyrin(2.1.1) series due to the presence of four additional aryl groups at the β-pyrrole carbons which caused steric crowding at the periphery of the triphyrin core resulting in a decrease in effective π-conjugation in the triphyrin(2.1.1)s.

Dibenzi Heteroheptaphyrin(2.0.1.1.1.1.0)s: Synthesis, Spectral, Redox and Theoretical Studies

Three examples of dibenzi heteroheptaphyrin(2.0.1.1.1.1.0)s were synthesized by condensing bis(phenylene ethene) based hexapyrrane with appropriate diol, 2,5-bis(α-hydroxy-α-arylmethyl) thiophene or selenophene in CH2 Cl2 under BF3  ⋅ OEt2 catalyzed inert atmosphere conditions followed by DDQ oxidation in open air. HR-MS analyses confirmed the identities of dibenzi heteroheptaphyrins. The DFT optimized structures revealed that dibenzi heteroheptaphyrins were highly distorted nonplanar macrocycles with two thiophene rings preferred to be in an inverted conformation. 1D & 2D NMR helped in deducing the molecular structures of dibenzi heteroheptaphyrins and supported their nonaromatic nature. The theoretical NMR calculations were carried which matched closely with the experimental NMR data. NMR studies also revealed that the π-delocaliztion was significantly altered in dibenzi heteroheptaphyrins compared to previously reported dibenzi hexaphyrins. The dibenzi heptaphyrins showed one sharp absorption band in 400-500 nm region and a broad band in the region of 600-800 nm which were bathochromically shifted in their diprotonated derivatives. The theoretical absorption calculations corroborate the slight hypsochromic shift of the broad absorption band in the lower energy region of dibenzi heptaphyrins compared to dibenzi hexaphyrins. The electrochemical studies revealed that the dibenzi heptaphyrins were easier to reduce but difficult to oxidize compared to dibenzi hexaphyrins.

Pd(II), Ni(II), and Cu(II) complexes of α,α'-ditolylmethanone dipyrroethene

Dipyrromethenes containing two pyrrole rings connected by one meso-carbon are versatile monoanionic bidentate ligands and form coordination complexes with many metals/nonmetals/metalloids. Dipyrroethenes containing one additional meso-carbon compared to dipyrromethenes have more space between coordinating pyrrole nitrogens and provide a good coordination environment but have not been explored as ligands in coordination chemistry. Dipyrroethenes are dianionic bidentate ligands and by suitable modifications, the coordination environment of dipyrroethenes can be changed further. Herein, we successfully synthesized α,α'-ditolylmethanone dipyrroethene which is a bipyrrolic tetradentate ligand with an ONNO ligand core and used it for the synthesis of novel Pd(II), Ni(II), and Cu(II) metal complexes by treating it with respective metal salts in CH₂Cl₂/CH₃OH at room temperature. The X-ray crystallographic structure of the metal complexes showed that the M(II) ion was coordinated to the ONNO atoms of the ligand in a perfect square planar geometry. The NMR studies of Pd(II) and Ni(II) complexes also supported the highly symmetric nature of the metal complexes. The absorption spectra of the metal complexes showed strong bands in the region of 300-550 nm. The electrochemical studies of metal complexes revealed that only ligand-based oxidation and reduction were observed. The DFT and TD-DFT studies were in agreement with the experimental observations. Our preliminary studies indicated that the Pd(II) complex can be used as a catalyst for the Fujiwara-Moritani olefination reaction.

Chain Length Modulated Dimerization and Cyclization of Terminal Thienyl-Blocked Oligopyrranes

Oxidation of thienyl-blocked bilane and pentapyrrane afforded chain length dependent products of the symmetric dimer D1 and the thienyloligopyrrin-appended pentaphyrin analogue P2, respectively, with the latter formed by simultaneous dimerization and cyclization. Coordination of D1 and P2 with Cu(II) afforded di- and monometallic complexes D1-Cu2 and P2-Cu, respectively. These compounds exhibit distinct NIR absorption, with the absorption tail of D1-Cu2 extended to ca. 1900 nm despite its smaller conjugation framework than that of P2-Cu.

Effect of Meso-Thienyl and Meso-Furyl Groups on The Electronic Properties of Aromatic 14π Triphyrin(2.1.1)s

A series of triphyrin(2.1.1)s containing five membered heterocycles such as thienyl and furyl groups in place of six membered aryl groups at two or four meso -positions were synthesized by [2+1] condensation of appropriate dipyrroethene diols with pyrrole under acid catalyzed conditions. The meso -thienyl and meso -furyl triphyrin(2.1.1)s were characterized and studied by HR-MS, 1D & 2D NMR spectroscopy, X-ray crystallography, absorption, cyclic voltammetry and DFT/TD-DFT studies. The X-ray structure obtained for one of the meso -thienyl trirphyrins indicated that the macrocycle was nearly planar and showed strong intramolecular hydrogen bonding. The spectral and electrochemical studies revealed that in meso -thienyl and meso -furyl triphyrin(2.1.1)s, the resonances of triphyrin core protons downfield shifted in 1 H NMR, the absorption bands bathochromically shifted and redox potentials shifted cathodically and anodically compared to meso -tetra( p -tolyl) triphyrin(2.1.1). These observations indicated that the thienyl/furyl groups at meso -positions alter the electronic properties of triphyrin(2.1.1)s significantly and the maximum effects were noted in meso -tetra(2-thienyl) triphyrin(2.1.1) and meso -tetra(2-furyl) triphyrin(2.1.1)s. The DFT and TD-DFT studies corroborated experimental observations.

Doubly Fused Unsymmetrical Calixdicarbahexaphyrins

Three novel doubly fused unsymmetrical calixdicarbahexaphyrins were synthesized by mild acid-catalyzed (4+2) condensation of dicarbatetrapyrrane with dipyrroethene diol followed by oxidation. The condensation formed doubly fused calixdicarbahexaphyrins instead of π-conjugated dicarbahexaphyrins, due to the unusual fusion of the pyrrole N with the α-carbon of the adjacent pyrrole ring to form a tripentacyclic ring and one usual fusion of the pyrrole N with the adjacent phenylene C to form a fused moiety containing two pentacycles and one hexacycle ring. Both fusions occurred on one side of the macrocycle, making the macrocycles unsymmetric. The crystal structure obtained for one of the macrocycles exhibited a saddle-shaped structure with two benzene rings and four pyrrole rings connected via two ethylene and four methene meso-carbon atoms. The crystal structure also revealed unusual fusions in the macrocyclic framework and the presence of one sp³ carbon that disrupts the π-electron delocalization. ¹H, ¹H-¹H COSY, NOESY, ¹³C, and HMBC NMR techniques were used to characterize the macrocycles. The absorption spectra of the macrocycles showed one intense sharp band at ∼485 nm along with a shoulder in the lower-energy region, suggesting its non-aromatic nature. Electrochemical studies indicated their electron rich nature, and DFT/TD-DFT studies corroborated the experimental observations.

Synthesis of the β-dipyrrinyl triphyrin(2.1.1) ligand and its coordination complexes

Functionalized β-formylphenyl triphyrin(2.1.1) was prepared by coupling β-bromo triphyrin(2.1.1) with 4-formylphenyl boronic acid under Pd(0) coupling conditions. β-Formylphenyl triphyrin(2.1.1) was treated with excess pyrrole under acid catalyzed conditions in CH₂Cl₂ to obtain dipyrramethanyl triphyrin(2.1.1), which was subjected to oxidation by treating it with DDQ to obtain the desired ligand, β-dipyrrinyl triphyrin(2.1.1). The dipyrrinyl unit of triphyrin(2.1.1) can act as a bidentate ligand to form interesting coordination complexes. Thus, the dipyrrinyl triphyrin(2.1.1) ligand was treated with BF₃·(OEt)₂ as well as metal salts such as [Ru(p-cymene)Cl₂]₂, Pd(acac)₂ and Zn(CH₃COO)₂ to obtain BODIPY-triphyrin(2.1.1), Pd(II)dipyrrin-triphyrin(2.1.1), Ru(II)-dipyrrin-triphyrin(2.1.1) and bis(Zn dipyrrin)-triphyrin(2.1.1) conjugates in good yields. The ligand and all four conjugates were freely soluble in common organic solvents and thoroughly characterized and studied by HR-MS, 1D & 2D NMR spectroscopy, absorption, cyclic voltammetry and DFT/TD-DFT studies. The optimized structures indicated that the triphyrin(2.1.1) and BODIPY/metal dipyrrin units in the conjugates were oriented w.r.t each other with an angle in the range of 25.18°-77.55°. The spectral studies indicated that the two moieties in the conjugates interact weakly and retain their individual characteristics, whereas electrochemical studies revealed their electron deficient nature. The TD-DFT studies were in agreement with the experimental observations.

Tellurophene-Containing Core-Modified Pentaphyrin(2.1.1.1.1)s

Various core-modified tellurophene-containing pentaphyrin(2.1.1.1.1)s were synthesized via (3 + 2) condensation of 16-telluratripyrrane with different heterodiols under mild acid catalyzed conditions in 5-12% yields. The formation of pentaphyrin (2.1.1.1.1) with a N₂O₂Te core was not successful due to its inherent instability. The new pentaphyrins were characterized and studied by HR-MS, 1D and 2D NMR, X-ray crystallography for one of the pentaphyrins, absorption and DFT/TD-DFT techniques. The NMR studies indicated their nonaromatic nature. The X-ray structure obtained for pentaphyrin(2.1.1.1.1) with N₄Te core revealed that the macrocycle exists in a highly distorted nonplanar structure. The DFT studies showed that the macrocycles are nonaromatic and exists in highly distorted nonplanar geometry. Furthermore, as the core heterocyclic groups at ethene moiety were changed from pyrrole to furan to thiophene to benzene, the macrocycles tended toward more planar structures. The absorption spectra of pentaphyrins showed one strong sharp band in the region of 450-540 nm along with a broad band in the region of 700-800 nm. The pentaphyrin(2.1.1.1.1) with N4Te core upon protonation showed distinct color change in solution and large bathochromic shifts in absorption bands with an absorption in the NIR region.

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.

Regioselective Stepwise Bromination of [14]Triphyrins(2.1.1) and Their Effects on Structural, Spectral, and Redox Properties

Regioselective stepwise bromination of meso-tetraaryl [14]triphyrins(2.1.1) was explored to investigate the effect of bromine substitution at the β-pyrrole carbons of triphyrin(2.1.1) on the structural, spectral, photophysical, and redox properties. A series of β-monobromo to β-hexabromo triphyrins(2.1.1) 2-7 were synthesized by treating triphyrin(2.1.1) 1 with appropriate equivalents of N-bromosuccinimide at ambient temperature in decent yields. The regiochemistry of bromines in β-brominated triphyrins(2.1.1) 3-5 and 7 was confirmed by X-ray crystallography, and the analysis revealed the effect of bromination of triphyrin(2.1.1) on the structural framework was significant in the case of hexabromotriphyrin(2.1.1) 7 compared to other macrocycles. Absorption spectroscopy showed that stepwise substitution of bromines at β-pyrrole carbons of triphyrin(2.1.1) resulting in bathochromic shifts of absorption bands relative to triphyrin(2.1.1) 1 and hexabromotriphyrin(2.1.1) 7 exhibited absorption bands at longer wavelengths. The redox studies revealed that compounds 2-7 were easier to reduce than triphyrin(2.1.1) 1, and the first reduction potential wave shifted anodically with an increase in the number of bromine substituents at β-pyrrole carbons of triphyrin(2.1.1) 1 from one to six. These structural, spectral, and electrochemical properties were also predicted by density functional theory calculations, and the analysis was consistent with the experimental observations.

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.

(Aza)Acenes Share the C2 Bridge with (Anti)Aromatic Macrocycles: Local vs. Global Delocalization Paths

A strong conjugation present in fused systems plays a crucial role in tuning of the properties that would be showing a dependence on the efficiency of π-electrons coupling. The π-cloud available in the final structure can be drastically influenced by a side- or a linear fusion of unsaturated and conjugated hydrocarbons. The linear welding of naphthalene/anthracene or quinoxaline/benzo[g]quinoxaline with triphyrin(2.1.1) gives structures where the competition between local and global delocalization is distinguished. The aromatic character observed in skeletons strongly depends on the oxidation state of the macrocyclic flanking and is either extended over the whole system or kept as a composition of local currents (diatropic and paratropic) of incorporated units. The hybrid systems show the properties derived from the π-conjugations that interlace one another but also show a significant independence of (aza)acene subunits reflected in the observed spectroscopic properties.

Switching of Aromatic Free Base Triphyrin(2.1.1) to Antiaromatic Phosphorus(V) Complexes of Triphyrin(2.1.1)

The aromatic 14π meso-tetraaryl triphyrin(2.1.1)s were switched to stable antiaromatic 16π P(V) complexes of triphyrin(2.1.1) by refluxing free base triphyrin(2.1.1)s with PCl₃ in a mixture of solvents toluene/triethylamine for 4 h. The P(V) triphyrin(2.1.1)s were characterized by high-resolution mass spectrometry (HRMS) and nuclear magnetic resonance (NMR) spectroscopy and their properties were studied in detail by absorption, electrochemical, and density functional theory (DFT) studies. The studies suggested that PCl₃ reduces 14π triphyrins to 16π triphyrins, which were then complexed to form stable antiaromatic P(V) complexes.

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.

Porphyrinoids with Vinylene Bridges

Porphyrinoids containing vinylene bridges, such as triphyrin(2.1.1), porphycene, porphyrin(2.1.2.1), and hexaphyrin(2.1.2.1.2.1), are a relatively new family of porphyrinoids. Vinylene bridges give porphyrinoids a lower symmetry and a flexibility of the framework and they permit cis/trans-isomerization reactions; these factors confer unique properties to these substances, such as coordination to metal ions and aromaticity switching. In this account, the synthesis, crystal structures, and properties of new porphyrinoids containing vinylene bridges are summarized.

1 Introduction

2 Triphyrin(2.1.1)

3 Porphycene

4 Porphyrin(2.1.2.1)

5 Hexaphyrin(2.1.2.1.2.1)

6 Conclusion

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).

Aromatic and Antiaromatic Pathways in Triphyrin(2.1.1) Annelated with Benzo[b]heterocycles

Understanding of the aromatic properties and magnetically induced current densities of highly conjugated chromophores is important when designing molecules with strongly delocalized electronic structure. Linear extension of the triphyrin(2.1.1) skeleton with an annelated benzo[b]heterocycle fragment modifies the aromatic character by extending the electron delocalization pathway. Two-electron reduction leads to an antiaromatic triphyrin(2.1.1) ring and an aromatic benzo[b]heterocycle subunit. Current-density calculations provide detailed information about the observed pathways and their strengths.

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

The first examples of phlorin analogues of meso-fused carbatriphyrins(2.1.1) and organo P(V) complex of one of the meso-fused carbatriphyrins were obtained by adopting a premodification strategy starting with fluorene as a fused polyaromatic precursor over a sequence of steps. The meso-fused carbatriphyrins(2.1.1) were obtained as their nonaromatic phlorin analogues owing to the unstable nature of the fully conjugated meso-fused carbatriphyrins(2.1.1). The organo P(V) complex of one of the meso-fused carbatriphyrins was prepared by treating the phlorin analogue of carbatriphyrin(2.1.1) with PCl₃ in toluene/triethylamine at reflux for 1 h. The P(V) complex of the meso-fused carbatriphyrin(2.1.1) was found to be moderately aromatic and the resulting global conjugation pathways in the P(V) complex significantly alters the aromaticity of the fluorene unit. The nonaromatic nature of the phlorin analogues of the meso-fused carbatriphyrin(2.1.1) and the moderately aromatic nature of its P(V) complex were supported by X-ray crystallography, absorption spectroscopy, NMR studies together with nucleus-independent chemical shifts, anisotropy-induced current density, and harmonic oscillation stabilization energy calculations.

Covalently linked meso-tetraaryl triphyrin(2.1.1)-ferrocene(s) conjugates: synthesis and properties

Functionalized meso-tetraaryl triphyrins(2.1.1) containing two meso-iodophenyl groups or a bromo group at the β-pyrrole carbon were synthesized over a sequence of steps. The covalently linked conjugates such as a triphyrin(2.1.1)-(ferrocene)₂ triad and a triphyrin(2.1.1)-ferrocene dyad were obtained in 47-51% yields by coupling the appropriate functionalized triphyrin with ethynylferrocene under mild Pd(0) coupling reaction conditions. The functionalized triphyrins(2.1.1) and their conjugates were thoroughly characterized by HR-mass spectrometry and 1D and 2D NMR spectroscopy. The absorption, electrochemical and DFT studies suggested weak interactions between the triphyrin(2.1.1) and ferrocenyl moieties in triads but relatively strong interactions in dyads.