No abstract

A new method of activating corrole macrocycles via an in situ generated SCN radical has been developed at very mild conditions at room temperature. This photoredox reaction resulted in the generation of tetrathiocyanatocorroles in good yields. The synthesis of tetrathiocyanatocorroles was never reported earlier. Single-crystal XRD analysis reveals that the insertion of four thiocyanate moieties at the four β-pyrrolic positions has imparted significant distortion to the corrole macrocycle. The generated tetrathiocyanatocorroles are different from the parent corroles in many ways. The photophysical properties of the newly synthesized tetrathiocyanatocorroles are dramatically altered from the parent corroles. The absorption feature of these modified corrole derivatives (both position and intensity) bears a nice similarity with the chlorophyll-a macrocycle. Thus, these newly synthesized molecules can be considered as spectroscopic model systems for chlorophyll-a pigments. The observed absorption and emission spectra of these tetrathiocyanatocorroles certainly point out that these newly developed ligand scaffolds and their various metal complexes will have immense potential as pigments in solar cells and also as NIR-emissive dyes. The observed C-H•••Au weak interactions in a representative Au(III)-corrole complex point out that these complexes are capable of activating the unfunctionalized C−H groups and thus will have potential implications in C-H activation reactions.

This article covers the chemistry of corroles, tetrapyrrole macrocycles having relevance with biologically relevant corrin ring of vitamin B12. Corroles are 18π electron aromatic systems like porphyrin. Despite few similarities, corroles have some major differences with porphyrin. Unlike porphyrin, which is dianionic in nature, the inner N4 coordination core of corroles consists of three amino nitrogens and one imino nitrogen, and thus corroles act as trianionic chelating ligands. Corroles are more acidic than porphyrin. As the corrole core is more compressed than porphyrin, the inner π system is more electron rich. Thus, it can stabilize higher oxidation states of metals. Various interesting spectroscopic properties and wide range of applications make corrole one of the most intriguing macrocycles in the porphyrinoid chemistry.