Cobalt corroles: Synthesis and applications

Cobalt corrole chemistry has become its own corpus of corrole literature in recent years, with findings involving synthesis, catalysis, sensing, electrocatalysis, etc reported with increasing frequency. Cobalt is abundant and biologically essential (e.g., cobalamin). Since Co metallocorroles are relatively straightforward to synthesize, this makes Co corroles already a practical and sustainable macrocycle class suitable for further research and "post functionalization." In addition, the metal is redox active and (therefore) vastly catalytically active. The versatility of redox active cobalt as a central atom in corroles helps achieve oxidation states ranging from Co1+ to Co5+; this has allowed researchers to synthesize, and therefore experimentally access, the chemistry of a large number of cobalt metallocorroles that have been substituted with various groups at the β–, meso–, or axial– positions. From these published examples, a wide array of structural properties and characteristics continue to unfold; a review is now due to help focus the researchers concentrating on this area to enable for outstanding future investigations. Their stabilization into different oxidation states has also made Co corroles valuable and heavily pursued in electrocatalysis. These complexes are frequently used for oxygen and reduction reactions, hydrogen evolution reactions (HERs), and oxygen evolution reactions (OERs). Co–corroles have thus widespread usage beyond catalysis per se, including sensing of anions and various gas molecules and, in some cases, small molecule activation. When N2 and O2 are present, Co complexes may preferentially bind and detect CO. This current review helps (i) summarize the accomplishments to date in synthesis and characterization, (ii) consolidate all the key characteristics of cobalt corroles within a common area of access, and (iii) expand our understanding of how these "active" macrocycles might be used across different scientific and technological disciplines (see future work section).