porphyrin

A macrocyclic molecule that contains four pyrrole rings linked together by single carbon atom bridges between the alpha positions of the pyrrole rings. Porphyrins usually occur in their dianionic form coordinated to a metal ion (e.g. V, Ni, Fe, Ga, Ti) in thermally stable complexes.

Metalloporphyrins are ubiquitously present in natural energy resources, such as shale oil or coal. Their fate becomes of concern during burning and processing of these materials.