Advent and features of pyriporphyrinoids: an overview of a pyridine-based porphyrin analogue

Pyriporphyrinoids have recently attracted a significant proliferation of attention due to their versatile characters, which stem from structural motifs in which the pyridine moiety is involved. The evolution of pyriporphyrin chemistry revealed the subtle modifications of the macrocyclic core that tweak the electronic structure as compared to the parental macrocycle. The amendment of π-electronic organization inside the core manifests exceptional photophysical and coordination properties that cover a vast range of seemingly contradictory fields. In fundamental chemistry, the pyridine unit acts as a modulator of π-conjugated porphyrinoid systems, resulting in aromaticity swapping. From the applied chemistry perspective, these macrocycles are primarily utilized as (i) sensors, (ii) NIR absorbing photoacoustic dyes, (iii) electrochemical catalysts, (iv) singlet biradicaloid generation and (v) contributors to generate metal complexes with intriguing binding modes. Surprisingly, despite their prominence, pyriporphyrinoids are inadequately investigated, while pyridine unit-embedded calixphyrin, calixpyridinopyrrole and calixpyridine are barely reported. This review article illustrates the controlled formation of specific porphyrinic scaffolds with pyridine unit(s) and diverse functionalized heterocyclic and/or carbocyclic building block(s), and demonstrates a substantial influence on the macrocyclic properties.

Boron calixphyrin complexes: exploring the coordination chemistry of a BODIPY/porphyrin hybrid

Boron complexes of calix[4]phyrins (1.1.1.1) were prepared by reacting the free-base ligands with BF₃·Et₂O. The reaction conditions can be efficiently tailored to produce mono- or di-boron calixphyrins. Mono-BF₂ calixphyrins with boron coordinating to either the dipyrrin, BF₂[H(Calix)], or dipyrromethane, BF₂[H(Calix)] and BF₂[H₂(Calix)]⁺, bonding sites were isolated. The dipyrromethane isomer, BF₂[H(Calix)], isomerises into BF₂[H(Calix)] which kinetic studies and DFT calculations indicate is an intramolecular process. Two isomers of B₂OF₂(Calix) were isolated, one isomer bonding via the dipyrrin sites with the FBOBF moiety in cisoid geometry, and the second isomer bonding via the dipyrromethane sites with the FBOBF moiety in transoid geometry. Although the cisoid/dipyrrin isomer was calculated to be most energetically favourable for B₂OF₂(Calix), the isolation of the transoid/dipyrromethane isomer is postulated to occur via the presumed intermediate (BF₂)₂(Calix), for which DFT indicated a preference for transoid/dipyrromethane geometry.