Redox-active ligand promoted multielectron reactivity at earth-abundant transition metal complexes

Abstract

The introduction of redox-active ligands into transition metal complexes can lead to novel redox behavior due to the ability of these ligands to serve as electron reservoirs. This feature is especially attractive in earth-abundant transition metal systems that typically favor one-electron redox processes and radical reactivity, as opposed to the two-electron redox cycles common with the noble metals. The redox flexibility afforded by redox-active ligands can enable substrate activation and/or bond forming and breaking processes that would otherwise be inaccessible with traditional redox-innocent ligands. This review discusses key examples of stoichiometric substrate activation and organic transformations facilitated by redox-active ligand-promoted multielectron reactivity at earth-abundant metal complexes. We highlight the electrochemical properties of these systems in relation to their substrate reactivity, where, in many cases, the complexes exhibit sequential one-electron redox events. Only a few examples have achieved electrocatalytic reactivity based on two-electron redox features, which underscores the untapped potential for further development of redox-active ligand systems to expand the capabilities of earth-abundant metal complexes in electrocatalysis.