Pyrrole ßC-B-N Fused Porphyrins: Molecular Structures and Opto-Electrochemical Studies.

Herein, we report the design, synthesis, structure, and electrochemical study of doubly ßC-B-N fused Ni(II) porphyrins (1-trans, 1-cis, 2-trans, and 2-cis). These compounds have been synthesized from A2B2 type dipyridyl Ni(II) porphyrins (Ar=Ph for 1 a; Ar=C6F5 for 2 a) via Lewis base-directed electrophilic aromatic borylation reactions. The solution state structures of these compounds have been established using 1H NMR, 11B NMR, 1H-1H COSY, 1H-13C HSQC, and 19F-13C HSQC NMR techniques. Single crystal X-ray analysis have revealed that 1-trans, 1-cis, and 2-trans adopt ruffled conformations, with alternate meso-carbons on the opposite sides of the mean porphyrin plane. The Soret bands in the absorption spectra of the B-N fused molecules are ~40 nm redshifted compared to unfused Ni(II) porphyrin precursors. The B-N fusion have diminished the redox potential of fused porphyrins. Although 1-trans and 1-cis, show four oxidation processes, 2-trans and 2-cis show only three oxidation processes. DFT studies have revealed that the tetrahedral geometry of the boron has induced a twist in the π-conjugation, which destabilizes the HOMO and stabilizes the LUMO in 1-trans, 1-cis, 2-trans, and 2-cis.

Isolation of potassium salt of oxadiazole-2-thione and in vitro anticancer activities of its Cu(II) and Zn(II) complexes against MDA-MB-231 human breast carcinoma cells.

A potassium 4-(pyridyl)-1,3,4-oxadiazole-2-thione was isolated in a basic medium, and its complexes [Cu(en)2(pot)2] (1) and [Zn(en)2(pot)2]HBr·CH3OH (2) containing ethylenediamine (en) as secondary ligand were synthesized and fully characterized. Upon changing the reaction conditions, the Cu(II) complex (1) adopts an octahedral geometry around the metal center. The cytotoxic activity of ligand (Kpot·H2O) along with complexes 1 and 2 was tested, and their anticancer activity against MDA-MB-231 human breast cancer cells was demonstrated, with complex 1 exhibiting superior cytotoxicity against these cells as compared to Kpot·H2O and complex 2. According to the DNA nicking assay, the ligand (Kpot·H2O) was found to be more potent to scavenge hydroxyl radicals even at a lower concentration (50 µg mL-1) than that of both complexes. The wound healing assay revealed that ligand Kpot·H2O and its complexes 1 and 2 attenuated the migration of the above-mentioned cell line. The loss of cellular and nuclear integrity and induction in the activity of Caspase-3 suggest the anticancer potential of ligand Kpot·H2O and its complexes 1 and 2 against MDA-MB-231 cells.

A2B- and A3-Type Boron(III)Subchlorins Derived from meso-Diethoxycarbonyltripyrrane: Synthesis and Photophysical Exploration.

The first direct fabrication of A2B- and A3-type B(III)subchlorins from meso-ethoxycarbonyl-substituted tripyrrane has been realized by condensation with appropriate acid chlorides (benzoyl chloride, butyryl chloride, and ethyl chlorooxoacetate). The aliphatic acid chloride-based annulation reaction is new to subporphyrinoid chemistry. The phenyl (6a)- or n-propyl (6b)-substituted derivatives could be oxidized to the corresponding B(III)subporphyrins upon refluxing with DDQ, whereas the triethoxycarbonyl moiety (6c) was found to be resistant to oxidation and exhibits the most red-shifted absorption (587 nm) and emission (604 nm). The study indicates that absorption and emission behaviors of the B(III)subchlorin can be tuned by the introduction of electron-rich or electron-deficient substituents at the meso-position. B(III)subchlorins 6a and 6c generate singlet oxygen efficiently (44 and 40%, respectively) and, thus, may find application as potential photosensitizers in photodynamic therapy (PDT).

1,2-Phenylene-Incorporated Smallest Expanded Calix[4]pyrrole via One-Step Synthesis of Tetrapyrrane: A Fluorescent Host for Fluoride Ion.

Synthesis of tetrapyrrane 8 from acetone and pyrrole via one-step condensation was achieved for the first time along with a much-improved yield of the tripyrrane 9. Diborylation of the tetrapyrrane and subsequent "1 + 1" cyclocoupling with 1,2-diiodobenzene following the Suzuki protocol generated novel o-phenylene incorporated macrocycle belonging to the smallest meso-expanded calix[4]pyrrole family. The latter macrocycle displays exclusive turn-on fluorescence sensing of fluoride ion upon complexation via a unique partial cone conformation supported by DFT analysis in acetonitrile solvent.

Post Synthetic Modification of Planar Antiaromatic Hexaphyrin (1.0.1.0.1.0) by Regio-Selective, Sequential SNAr.

In spite of unique structural, spectroscopic and redox properties, the synthetic variants of the planar, antiaromatic hexaphyrin (1.0.1.0.1.0) derivatives 2, has been limited due to the low yields and difficulty in access to the starting material. A chemical modification of the meso-substituents could be good alternative overcoming the synthetic barrier. Herein, we report a regio-selective nucleophilic aromatic substitution (SNAr) of meso-pentafluorophenyl group in rosarrin 2 with catechol. The reaction afforded benzodioxane fused rosarrin 3 as single product with high yield. The intrinsic antiaromatic character of the starting rosarrin 2 retained throughout the reactions. Clean, two electron reduction was achieved by treatment of 3 with SnCl2•2H2O affording 26π-electron aromatic rosarrin 4. The synthesized compounds exhibited noticeable changes in photophysical and redox properties compared with starting rosarrin 2.

Meso-Free Boron(III)subchlorin and Its µ-Oxo Dimer with Interacting Chromophores.

Meso-free B(III)subchlorin 1 has been realized exclusively for the first time from meso-ethoxycarbonyl-substituted tripyrrane along with the first subchlorin dimer 2 as its µ-oxo analogue via a facile one-pot approach. The subchlorin is highly stable toward oxidation; hence, it was not contaminated with the corresponding subporphyrin analogue 3. The subchlorin (56%) and its dimer (30%) exhibit singlet oxygen generation ability for the first time. The B-O-B dimer displays strong exciton coupling between the two macrocycles.

A robust bis-rhodium(i) complex of π-extended planar, anti-aromatic hexaphyrin[1.0.1.0.1.0].

ß,ß'-Phenylene bridged hexaphyrin[1.0.1.0.1.0] (naphthorosarin), an expanded porphyrin possessing C3v-symmetry, has been shown to possess unique electronic features. We now report a bimetallic Rh(i)-complex of naphthorosarin retaining 24 π-antiaromatic characteristics. The two Rh(i) cations reside on opposite sides of the macrocyclic π-system and are separated at a distance consistent with a possible Rh(i)-Rh(i) metallic bond interaction.

Hexaethylsubporphyrins: ß-alkyl analogues in the subporphyrin family.

Two new subporphyrins were synthesized for the first time from a ß-substituted pyrrole i.e. 3,4-diethylpyrrole via pyridine-tri-N-(3,4-diethylpyrrolyl)borane as building blocks. These ß-hexaethylsubporphyrins are true contracted congeners of ß-octaethylporphyrin (OEP). While the meso-triphenyl derivative of hexaethylsubporphyrin could be synthesized by following the reported method, the meso-free analogue could only be synthesized by condensation with trioxane, in the presence of catalytic methanesulfonic acid. These contracted macrocycles display interesting absorption, and emission behaviour including substituent dependent S2 fluorescence owing to the presence of flexible electron donating ethyl groups at their ß-positions. The optical response and ultrafast S2 state dynamics of these systems suggest that it may be possible to tune the properties of the subporphyrin to develop efficient systems for solar energy capture and conversion processes.

Calix[2]bispyrrolylarenes: new expanded calix[4]pyrroles for fluorometric sensing of anions via extended π-conjugation.

Two new expanded calix[4]pyrroles 3 and 4 were synthesized by '2 + 2' cyclocoupling of easily prepared diboryldipyrromethane 7 (by Ir-catalyzed CH-bond activation) with appropriate diiodoarenes using the Suzuki protocol. Owing to the unique design, both macrocycles exhibited extended π-conjugation and enhanced fluorescence. Upon complexation with anions (fluoride and acetate), receptor 3 displayed turn-on sensing of fluorescence, whereas 4 showed turn-off sensing.