Molecular catalysts design: Intramolecular supporting site assisting to metal center for efficient CO2 photo- and electroreduction

Carbon dioxide (CO2) transformation is a hot topic in both fundamental research and industrial application. Transition metal complexes of tunable and definite structure exhibit unique advantage in the study of CO2 reduction. Catalytic transformation of CO2 at metal complex catalyst is a multi-electron and multi-atom involved process. In this regard, the local chemical environment around the metal center (M) is critical to the catalytic activity. Intramolecularly inducing a supporting site/group into the metal complex catalyst enables to synergistically stabilize M-CO2 or M-COOH adducts with the metal center, which always lowers the energy barrier and/or alters transformation route. Some typical strategies had been applied to design such metal complex catalysts for photo- and/or electrocatalytic CO2 reduction. In this review, we are going to review such representative molecular catalysts (based on earth-abundant metals) that contain an intramolecular supporting site to assist the metal center for efficient photo- and/or electrocatalytic CO2 reduction. According to the structural and functional features of the catalysts, three types of catalyst containing an intramolecular supporting site are summarized. In the first type, one of metal atoms in binuclear complex acts as an intramolecular supporting site to synergistically work with another metal atom for catalytic CO2 reduction. In the second type, hydrogen bond (HD) donor group (such as amide or phenol groups) at secondary coordination sphere acts as an intramolecular supporting site to stabilize M-CO2 adduct via an intramolecular hydrogen bond. In the third type, cis-coordination atoms in several cis-cobalt-complexes function as an intramolecular supporting site in photocatalytic CO2 reduction. The structural characteristics, catalytic performances, and mechanisms of the typical examples in these three types are reviewed.