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

Abstract

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.

Synthesis and Characterization of (pyNO−)2GaCl: A Redox-Active Gallium Complex

“…However, the N-O bond distance in 1 does fit in the range of similar bonding parameters reported for metal complexes coordinated by the aminoxyl anion form of the pyridyl nitroxide ligand. For example, the cerium complexes {Ce III (µ-R pyNO − )( R pyNO − ) 2 } 2 and Ce IV ( R pyNO − ) 4 reported by the Schelter group have average N-O distances of 1.37 and 1.38 Å), respectively [12,13]. The [Cu 2+ (pyNO − )(pyNO • )] + cation reported by the Ishida group has an N-O distance for the pyNO − ligand in the range of 1.4092(8)-1.423(2) Å [18].…”

“…The synthesis of metal complexes of redox-active and non-innocent ligands is a research area that has been expanding in recent years and has resulted in new classes of complexes for metals from throughout the periodic table [1][2][3][4]. Of specific interest to us is the ability to modulate the reactivity of redox-active ligands through coordination of Lewis acids.…”

EPR Spectroscopy as a Tool in Homogeneous Catalysis Research

“…An example includes C-H activation/arylation, by the group of Fensterbank, applying iron complexes bearing a bisiminopyridine ligand [57]. Several other examples have been described using this concept [23], but in most cases these were not investigated with EPR spectroscopy.…”

“…Instead of changes in the d-electron count of the metal, oxidation or reduction of the redox active ligand can occur, often leading to formation of ligand centered radicals in the coordination sphere of the metal. These are important to understand the reactivity of open-shell organometallic compounds [22,23], and offer interesting opportunities to control radical-type reactions. Being able to control radical reactivity is a difficult challenge in synthetic organic and organometallic chemistry, but in the coordination sphere of (transition) metals reactions of radicals become more controlled.…”

Catalytic Dibenzocyclooctene Synthesis via Cobalt(III)–Carbene Radical and ortho‐Quinodimethane Intermediates

“…[8] Upon formation of a Fischertype carbene at the low-spin cobalt(II) center of these catalysts, the redox-active Fischer-type carbene undergoes one-electron reduction by the cobalt(II) center, forming a "cobalt(III)-carbene radical" complex. The p orbital of the carbene moiety is dominant in the SOMO of these reactive intermediates, effecting their typical radical-type behavior towards, for example, C À H, C = C, and C C bonds ( Figure 2).…”