Catalysis of the electrochemical reduction of carbon dioxide by iron(“0”) porphyrins

Transforming wastes or hazardous materials into value-added chemical products is of great concern from economic, environmental, and sustainable prospects. Various approaches were implemented to convert CO2 into valuable/fine chemicals, e.g., chemical fixation, electro-, thermal-, and photo-catalysis. The electrochemical reduction pathway of CO2 stands as the most promising one with its multiple advantages and the growing abundance of renewable sources of electricity. Though heterogeneous catalysts are advantageous for their robustness and recyclability, molecular catalysts are well-established for controlling and designing reaction sites within well-defined structures for a better understanding of catalytic reaction pathways. The straightforward modifications of pincer-type ligands, along with their robust chelation, pre-designed geometries, and redox activities, make pincer-based molecular catalysts of broad worthy interest in molecular catalysis. Thus, the primary aim of this review is to focus on the application of pincer-type complexes in CO2 reduction reaction (CO2RR) that are being widely investigated and have provided stimulating findings about reaction mechanisms, reactive intermediates, deactivation pathways, etc. The insights gained from these studies have the potential to drive the development of catalysts of high selectivity and reasonable stability at low overpotential and high reaction rates. The product selectivity is generally controlled by the central transition metal, where CO, syngas (CO + H2 mixture), and formate are the most frequent products of most reported pincer-based catalysts. However, modulating the pincer ligand structure could also switch product selectivity. A simple modification in the ligand motif(s) can significantly change the reaction mechanism and catalytic rates. We have also discussed the reaction mechanisms by which the reaction products are formed as well as the stability issues of these catalysts. Finally, some examples of immobilized molecular pincer catalysts for CO2RR are highlighted. It is obvious that introducing a variety of metals and manipulating the coordination sites in the pincer ligands still require further detailed investigations, so that this field may lead to fulfilling the demands of global scientific and technological prosperity.