Redox Non-Innocent Ligands: Versatile New Tools to Control Catalytic Reactions

In this (tutorial overview) perspective we highlight the use of “redox non-innocent” ligands in catalysis. Two main types of reactivity in which the redox non-innocent ligand is involved can be specified: (A) The redox active ligand participates in the catalytic cycle only by accepting/donating electrons, and (B) the ligand actively participates in the formation/breaking of substrate covalent bonds. On the basis of these two types of behavior, four main application strategies of redox-active ligands in catalysis can be distinguished: The first strategy (I) involves oxidation/reduction of the ligand to tune the electronic properties (i.e., Lewis acidity/basicity) of the metal. In the second approach (II) the ligand is used as an electron reservoir. This allows multiple-electron transformations for metal complexes that are reluctant to such transformations otherwise (e.g., because the metal would need to accommodate an uncommon, high-energy oxidation state). This includes examples of (first row) transition metals that have a tendency to react via one-electron pathways, and even “oxidative addition” reactions for d0 transition metal complexes become possible with redox active ligands as electron reservoirs. The electron-reservoir function of the ligand tolerates the metal to maintain its most common or most stable oxidation state by delivering or accepting the electron density associated with the multielectron transformation (most typically two-electron transformations such as oxidative addition/reductive elimination). The third strategy (III) involves the generation of reactive ligand-radicals that actively participate in the making and breaking of chemical bonds during catalysis. Cooperative substrate activation by the redox non-innocent ligand and the metal allows reactions that are difficult to achieve otherwise. The last strategy (IV) involves (radical-type) activation of the substrates or modification of the substrate reactivity in cases where the substrate itself acts as a redox non-innocent ligand. These four approaches are illustrated by recent literature data.