Bidipyrrin AuIII Complex as a Helical Charged π-Electronic System.

A series of ion pairs based on a bidipyrrin-AuIII complex that acts as a stable helical π-electronic cation have been prepared via ion-pair metathesis. The helical cation, which exhibits NIR absorption and phosphorescence emission, formed solid-state ion-pairing assemblies, whose assembling modes depended on the properties of coexisting counteranions.

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, structure, and properties of palladium(II) complex of α-formyl pyrrolyl dipyrromethene.

A simple α-formyl pyrrolyl dipyrromethene ligand was synthesized by deboronation of a BF2 complex of α-formyl pyrrolyl dipyrrin under Lewis acid catalyzed conditions. The α-formyl pyrrolyl dipyrrin ligand was treated with PdCl2 in CH3CN/Et3N at room temperature under inert atmosphere conditions for 3 h followed by recrystallization to afford a Pd(II) complex of α-formyl pyrrolyl dipyrrin in 61% yield. The X-ray structure revealed that Pd(II) was coordinated to three pyrrole Ns of the α-formyl pyrrolyl dipyrrin ligand and the fourth position was occupied by a water molecule with an overall distorted square planar geometry around the Pd(II) ion. The formyl group present in the appended pyrrole ring was not involved in bonding with the Pd(II) ion but helps in stabilizing the complex via hydrogen bonding interaction with the coordinated water molecule. The Pd(II) complex was further characterized and studied thoroughly by 1D and 2D NMR, absorption, cyclic voltammetry and DFT/TD-DFT techniques. The absorption spectrum of the Pd(II) complex showed one sharp intense band at 630 nm and less-intense bands at 585, 435 and 348 nm and the electrochemical studies indicated the electron-deficient nature of the Pd(II) complex. The DFT/TD-DFT studies were in agreement with the experimental observations. The Pd(II) complex was tested as a catalyst for the Suzuki-Miyaura coupling of aryl bromides with aryl boronic acid and the studies supported the catalyst's efficiency in the coupling reaction to form biaryl compounds.

Inverted and fused expanded heteroporphyrins.

Expanded heteroporphyrins are a class of porphyrin macrocycles containing pyrrole, thiophene, furan, selenophene and other heterocyclic rings that are connected to form an internal ring pathway containing a minimum of 17 atoms and more than 18 delocalized π electrons in their conjugated macrocyclic framework. Considering that expanded heteroporphyrins are large in size, these macrocycles are structurally flexible and prefer to adopt various conformations in which one or more pyrrole(s)/heterocycle(s) tend to be in an inverted conformation and pointed outward from the centre of the macrocyclic core. The inverted expanded heteroporphyrins are divided into two classes as follows: (1) N-inverted expanded heteroporphyrins and (2) hetero-atom inverted expanded heteroporphyrins. Both inverted expanded heteroporphyrins show quite unique features in terms of their structure, aromaticity, and electronic and coordination properties. Sometimes, inverted expanded heteroporphyrins lead to the formation of fused expanded heteroporphyrins because of the intramolecular fusion of the pyrrole "N" with the "C" of the inverted heterocycle ring, which also exhibit unique features compared to inverted expanded heteroporphyrins. In this review, we attempt to describe the synthesis, structure, and aromatic, electronic and coordination properties of inverted and fused expanded heteroporphyrins. This review covers the synthesis, structure and properties of inverted and fused expanded heteroporphyrins containing a combination of pyrrole/heterocycle rings starting with five pyrrole/heterocycle-containing pentaphyrins, and then expanded heteroporphyrins containing six, seven, eight and more pyrrole/heterocyclic rings in their porphyrin macrocyclic framework.

Bis-Palladium Complex of α-Benzimidazole 9-Pyrrolyl Dipyrromethene: Synthesis, Structure, and Spectral and Catalytic Properties.

A new ligand is designed and synthesized in two steps starting from α-formyl 3-pyrrolyl BODIPY. In the first step, the α-formyl 3-pyrrolyl BODIPY was condensed with 1,2-diaminobenzene in toluene at reflux and afforded α-benzimidazole 3-pyrrolyl BODIPY in 16% yield. In the second step, α-benzimidazole 3-pyrrolyl BODIPY was decomplexed upon being treated with Lewis acid AlCl3 and afforded the desired ligand α-benzimidazole 9-pyrrolyl dipyrromethene. However, the ligand was not very stable and reacted further with PdCl2 in CH3CN for 1 h at reflux followed by recrystallization and afforded a novel bis-palladium complex of α-benzimidazole 9-pyrrolyl dipyrromethene in 36% yield. The bis-palladium complex was characterized and studied by high-resolution mass spectrometry, one- and two-dimensional nuclear magnetic resonance, X-ray crystallography, absorption, and density functional theory/time-dependent DFT (DFT/TD-DFT) studies. The X-ray structure revealed that two ligands and two Pd(II) ions were involved in forming a unique complex in which each Pd(II) ion was coordinated to three pyrrole N atoms of the first ligand and the benzimidazole N atom of the second ligand in a distorted square planar geometry. The absorption spectrum of the bis-palladium complex shows ill-defined, broad, and less intense bands in the region of 345-425 nm along with split bands in the higher-wavelength region of 600-630 nm. The bis-palladium complex was nonfluorescent, and the results of DFT/TD-DFT studies were in agreement with the experimental observations. The preliminary studies indicated that the bis-palladium complex can act as an efficient catalyst for coupling different aryl bromides with phenylboronic acid.

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.

Dibenzofuran/Dibenzothiophene-Embedded Dithia-bis(calix)-sapphyrins.

A series of first examples of dibenzofuran (DBF)/dibenzothiophene (DBT)-embedded dithia-bis(calix)-sapphyrins were synthesized by condensing 1 equiv of dibenzofuran/dibenzothiophene-based tripyrrane with 1 equiv of [2,2'-bithiophene]-5,5'-diylbis(aryl)methanol under mild acid-catalyzed conditions in CH2Cl2 followed by oxidation with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) and alumina column chromatographic purification afforded new dithia-bis(calix)-sapphyrins with two meso-sp3 carbons in 5-7% yields. The DBF/DBT-embedded dithia-bis(calix)-sapphyrins were characterized by HRMS (high-resolution mass spectrometry), 1H and 13C NMR, 1H-1H COSY, 1H-1H NOESY, 1H-13C HSQC, and 1H-13C HMBC spectroscopy, absorption spectroscopy, cyclic voltammetry, and density functional theory (DFT) studies. The macrocycles showed one broad absorption band at ∼553 nm with a shoulder peak at the higher energy side along with a sharp intense band at ∼415 nm. However, the protonated dithia-bis(calix)-sapphyrins showed large bathochromic shifts in the absorption bands, indicating that the electronic properties of dithia-bis(calix)-sapphyrins were altered significantly upon protonation of dithia-bis(calix)-sapphyrins. The electrochemical study indicated that dithia-bis(calix)-sapphyrins are relatively easier to reduce but difficult to oxidize. The DFT studies revealed that macrocycles adopt a highly folded half-chair conformation due to the disruption of conjugation of the macrocycle because of the presence of two sp3 meso-carbons. The DFT studies also support the significant bathochromic shifts observed for protonated dithia-bis(calix)-sapphyrins macrocycles.

Effects of Core Modification on Electronic Properties of para-Benziporphyrins.

In contrast to the nonaromatic meta-benziporphyrins, the para-benziporphyrins possess aromatic character depending on the type of five-membered ring present in the macrocyclic core. The effects of changing the para-benziporphyrinic core from C2N3 to C2NSN, C2NSeN, and C2NTeN by replacing the pyrrole with other five-membered heterocycles such as thiophene, selenophene, and tellurophene on aromatic properties of p-benziporphyrins are described here using spectral, electrochemical, X-ray, and density functional theory (DFT) studies. The missing core-modified p-benziporphyrins with C2NSeN and C2NTeN cores were synthesized by condensing 1 equiv of benzitripyrrane and 1,3-benzene-bis((4-phenyl)methanol with an appropriate diol such as 2,5-bis[(p-tolyl)hyroxymethyl]selenophene and 2,5-bis(hydroxymethyl)tellurophene under mild acid-catalyzed conditions at room temperature and characterized in detail by high-resolution mass spectrometry (HR-MS), one- & two-dimensional NMR, and X-ray crystallography of the one of the macrocycles, Selena p-benziporphyrin. The X-ray structure of Selena p-benziporphyrin revealed that the macrocycle was almost planar apart from the p-phenylene ring, which was deviated by 49.71° from the mean plane of the macrocycle defined by four meso carbons, unlike Selena m-benziporphyrin, which is relatively more distorted. NMR studies revealed that, as the core changes from C2N3 to C2NSN, C2NSeN, and C2NTeN, the diatropic ring current decreases, indicating that the aromatic character also decreases in the same order. X-ray structure and DFT studies also revealed that the distortion in the macrocycle increases as the pyrrole ring of p-benziporphyrin was replaced with other heterocycles such as furan, thiophene, selenophene, and tellurophene and that the tellura p-benziporphyrin was the most distorted macrocycle among core-modified p-benziporphyrins. Absorption and electrochemical properties were in agreement with these observations. Our repeated attempts on metalation of these p-benziporphyrins resulted in the successful synthesis of a Pd(II) complex of tellura p-benziporphyrin. The Pd(II) complex was characterized by HR-MS and NMR techniques, and the structure was optimized by DFT. The studies indicated that the Pd(II) ion was bonded to one of the pyrrolic nitrogens, tellurophene, tellurium, and two chloride ions in distorted square-planar geometry.

Synthesis of Nonaromatic and Aromatic Dithia Benzisapphyrins.

A series of [24π] dithia meta-benzisapphyrins and [22π] dithia para-benzisapphyrins were synthesized by 3 + 2 condensation of appropriate benzitripyrrane with bithiophene diol under mild acid catalyzed conditions. The dithia m-benzisapphyrins and dithia p-benzisapphyrins were thoroughly characterized by HR-MS, 1D and 2D NMR, absorption, and electrochemical techniques. Our studies showed that dithia m-benzisapphyrins are nonaromatic, whereas the dithia p-benzisapphyrins are aromatic in nature. Thus, we demonstrated here that the dithiabenzisapphyrins can be made aromatic by replacing the m-phenylene moiety of the benzisapphyrin macrocycle with a p-phenylene unit. Furthermore, the studies also indicated that the aromaticity of the dithia p-benzisapphyrins was relatively more compared to the reported heterosapphyrins. The structural and spectral characteristics including aromaticity of the m-benzisapphyrins and p-benzisapphyrins were also discussed with the help of DFT, NICS, and TD-DFT studies.