https://doi.org/10.1021/acs.joc.0c01146

https://doi.org/10.1039/C9DT03573G

Johnson and Kay have synthesized a new macrocycle called corrole in the year 1965. It possesses the structural similarity to the central corrin ring of vitamin B₁₂. Both corrin and corrole rings are tetrapyrrole macrocycles with similar numbers of carbon atoms with direct pyrrole-pyrrole bond (C1‒C19), however corroles are 18π electron aromatic system like porphyrin. Thus corrole ring can be considered as an intermediate of porphyrin and corrin. Despite of few similarities, corrole has some major differences with porphyrin.

Unlike porphyrin, which is dianionic in nature, the inner N4 coordination core of corrole consist of three amino nitrogens and one imino nitrogen and thus corrole act as a trianionic chelating ligand. There is a loss of symmetry in the corrole ring due to the absence of C-20 carbon atom. The X-ray structure of corrole shows slight deviation from planarity in order to reduce the steric strain of the inner NH protons. As a result, 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.

https://doi.org/10.1002/9781119951438.eibc2729