Transition Metal Complexes with N,N’,O-Donor Ligands as Catalysts for the Polymerisation Reaction and Electrochemical CO2 Reduction

The concept of metal-ligand cooperativity has not only been important for biological systems but also for the application in synthetic systems. Especially transformation where both electron and protons are transferred in a synergistic manner, are essential for a wide range of energy conversion reactions. These conversions are often possible due to proton-coupled electron transfer (PCET) reactions in biological and synthetic systems which include processes such as water splitting, dioxygen and dinitrogen activation, carbon dioxide reduction as well as the combustion of fossil fuels in cars. In transition metal complexes the protons are usually transferred from and to the ligand, whereas the electrons are often transferred from and to the metal centre. The term PCET was first used for transition metals in 1981 to describe the concerted electron and proton transfer process in the comproportionation reaction of RuIV(bpy)2(py)(O)2+ and RuII(bpy)2(py)(OH2)2+. Through the concerted transfer of an electron and proton between two systems high energy intermediates are avoided and coupling leads to favourable effects on the energetic profile. Nowadays the term PCET is more broadly used to describe the thermodynamic coupling of the electron and proton transfer without regard to the mechanism.