Copper, zinc and cobalt complexes witz NH-imidazolyl moieties

A lot of redox reactions encountered in nature and in synthetic chemistry are coupled to the transfer of protons. These reactions include bioenergetic processes such as water splitting, lightdriven reduction of carbon dioxide or the activation of nitrogen as well as the combustion of fossil fuels in cars. The term proton-coupled electron transfer (PCET) was coined in 1981 for the description of chemical reactions in which both, electrons and protons, are transferred. Thus, high energy intermediates caused by high charge separations are avoided and the coupling might have favourable effects on the energetic profile of activation processes. A PCET reaction is not limited to the transfer of a single electron and proton, but also multielectron and -proton reactions are considered as such. All three reactions depicted in Scheme 1.1 are multielectron processes, in which the uncatalysed reactions show rather large activation barriers. A catalyst can lower the activation barrier by forming intermediates, which are ideally neither of high nor of low energy. Such a energetic profile can be realised by a PCET reaction by coupling the transfer of negative and positive charges and thus circumventing the formation of charged species, which would arise by the transfer of solely electrons (ET) or protons (PT).