η5-Cyclopentadienyliron(II)-[14]Triphyrin(2.1.1) Sandwich Compounds: Synthesis, Characterization, and Stable Redox Interconversion

Contracted porphyrins and calixpyrroles: synthetic challenges and ring-contraction effects

Ring-contracted porphyrin analogues, such as subporphyrins and calix[3]pyrroles, have recently attracted considerable attention not only as challenging synthetic targets but also as functional macrocyclic compounds. Although canonical porphyrins and calix[4]pyrrole are selectively generated via acid-catalyzed condensation reactions of pyrrole monomers, their tripyrrolic analogues are always missing under similar conditions. Recent progress in synthesis has shown that strain-controlled approaches using boron(iii)-templating, core-modification, or ring tightening provide access to various contracted porphyrins. The tripyrrolic macrocycles are a new class of functional macrocycles exhibiting unique ring-contraction effects, including strong boron chelation and strain-induced ring expansion. This Perspective reviews recent advances in synthetic strategies and the novel ring-contraction effects of subporphyrins, triphyrins(2.1.1), calix[3]pyrroles, and their analogous.

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.

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.

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

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.

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.

Benzofuran-/Benzothiophene-Incorporated NIR-Absorbing Triphyrins(2.1.1)

Novel benzofuran-/benzothiophene-incorporated triphyrins(2.1.1) were synthesized using readily available precursors under mild reaction conditions. The X-ray structure revealed that the benzofuran-incorporated triphyrin(2.1.1) macrocycle was slightly ruffled. The triphyrins(2.1.1) exhibit more absorption in the NIR region compared to previously reported dibenzofuran-/dibenzothiophene-incorporated hybrid macrocycles. The benzofuran-incorporated triphyrin shows significant red shift in absorption bands compared to that of benzothiophene-based triphyrin due to its more coplanar arrangement as supported by X-ray and DFT studies.

Stereoretentive Ligand Exchange Reactions of N-Fused Porphyrin Ruthenium(II) Complexes

The ligand exchange reactions of the ruthenium(II) complex of N-fused tetraphenylporphyrin, Ru(NFp)(CO)₂Cl (2), with various anions were investigated. The chloride ligand of the isomers 2a-c was stereoretentively exchanged with bromide (Br^-), iodide (I^-), and acetate (AcO^-) anions in toluene at 100 °C, structures of which were confirmed by ¹H NMR as well as single crystal X-ray diffraction analysis. The silver (AgOAc, AgOTf) and boron (NaBPh₄) reagents also afforded the corresponding stereoretentive products. On the other hand, the reaction with NaBH₄ afforded the hydride complex Ru(NFp)(CO)₂H (7) with low stereospecificity, showing a higher reactivity of 2c than other isomers. The ligand dissociation mechanism was proposed with the help of theoretical calculations on the plausible five-coordinated intermediates.

Hybrid Macrocycles of Subporphyrins and Triphyrins

The dibenzofuran/dibenzothiophene-based nonaromatic hybrid macrocycles, exhibiting the features of both contracted macrocycles, subporphyrins and triphyrins, have been synthesized under simple reaction conditions using readily available precursors. The monoanionic new macrocyclic ligands with three donor atoms, such as two pyrrole nitrogens and one dibenzofuran oxygen or dibenzothiophene sulfur, readily form Re(I) complexes.

Recent Advances in Subporphyrins and Triphyrin Analogues: Contracted Porphyrins Comprising Three Pyrrole Rings

Subporphyrinato boron (subporphyrin) was elusive until the syntheses of tribenzosubporphine in 2006 and meso-aryl-substituted subporphyrin in 2007. These novel contracted analogues possess a 14π-electron conjugated system embedded in a bowl-shaped structure. They exhibit absorption and fluorescence in the UV/vis region and nonlinear optical properties due to their octupolar structures. The unique coordination geometry around the central boron atom in the structure of subporphyrin enabled investigation of rare boron species, such as borenium cations, boron hydrides, and boron peroxides. Along with the burgeoning development of the chemistry of subporphyrins, analogous triphyrin systems have also emerged. Their rich coordination chemistry as a result of their free-base structures, which are different from the boron-coordinating structure of subporphyrins, has been intensively investigated. On the basis of the unique structures and reactivities of subporphyrins and their related triphyrin analogues, supramolecular architectures and covalently linked multicomponent systems have also been actively pursued. This Review provides an overview of the development of subporphyrin and triphyrin chemistry in the past decade and future prospects in this field, which may inspire molecular design toward applications based on their unique properties.

Synthesis, characterization and electrochemistry of rhodium(iii) complexes of meso-substituted [14]tribenzotriphyrin(2.1.1)

A thermal reaction using a series of [14]tribenzotriphyrins(2.1.1) (TriPs,1a–d) with Rh₂(C₈H₁₂)Cl₂provides Rh^III–TriP complexes (2a–d) in 40−52% yields.

Catalytic two-electron reduction of dioxygen catalysed by metal-free [14]triphyrin(2.1.1)

The catalytic two-electron reduction of dioxygen (O₂) by octamethylferrocene (Me₈Fc) occurs with a metal-free triphyrin (HTrip) in the presence of perchloric acid (HClO₄) in benzonitrile (PhCN) at 298 K to yield Me₈Fc⁺ and H₂O₂. Detailed kinetic analysis has revealed that the catalytic two-electron reduction of O₂ by Me₈Fc with HTrip proceeds via proton-coupled electron transfer from Me₈Fc to HTrip to produce H₃Trip˙⁺, followed by a second electron transfer from Me₈Fc to H₃Trip˙⁺ to produce H₃Trip, which is oxidized by O₂via formation of the H₃Trip/O₂ complex to yield H₂O₂. The rate-determining step in the catalytic cycle is hydrogen atom transfer from H₃Trip to O₂ in the H₃Trip/O₂ complex to produce the radical pair (H₃Trip˙⁺ HO₂˙) as an intermediate, which was detected as a triplet EPR signal with fine-structure by the EPR measurements at low temperature. The distance between the two unpaired electrons in the radical pair was determined to be 4.9 Å from the zero-field splitting constant (D).

Time-resolved fluorescence spectroscopy study of excited state dynamics of alkyl- and benzo-substituted triphyrin(2.1.1)

Non-radiative relaxation in triphyrin(2.1.1) is found to be due to non-planarity of the macrocycle.