Radical-Type Reactivity and Catalysis by Single-Electron Transfer to or from Redox-Active Ligands

Abstract

Controlled ligand-based redox-activity and chemical non-innocence are rapidly gaining importance for selective (catalytic) processes. This Concept aims to provide an overview of the progress regarding ligand-to-substrate single-electron transfer as a relatively new mode of operation to exploit ligand-centered reactivity and catalysis based thereon.

Keywords: non-innocent ligands; radical; redox-active ligands; single-electron transfer; substrate.

Figure 1

General depiction of reversible ligand centered redox process. Green=oxidized ligand; red=reduced ligand.

Figure 2

General depiction of three “established” modes (i–iii) of redox‐active ligand reactivity that go beyond two‐electron transfer to a metal center, as well as the new fourth mode (iv), highlighted in this Concept. Red=reduced ligand; green=oxidized ligand; light brown=metal in low oxidation state; dark brown=metal in high oxidation state.

Figure 3

Metalloradical based vs. ligand‐centered single‐electron transfer reactivity. Red=reduced; green=oxidized (either metal or ligand).

Scheme 1

Top: Oxygen atom transfer reaction of A with Ph₃C^. to produce Ph₃COH and two rhenium complexes B and C. Bottom: Homolysis of O₂ on bis(catecholate) derivative of A to give Re^V‐dioxo derivative.

Scheme 2

Ligand‐facilitated single‐electron oxidation on a Zr^IV(NNN) complex.

Scheme 3

Ligand‐to‐substrate single‐electron transfer using o‐phenylenediamine‐derived chromium(III) complexes upon reaction with aryl and sulfonylazides, and catalytic formation of benzosultam product H.

Scheme 4

Ligand‐to‐substrate single‐electron transfer to homolytically cleave disulfides and generate mixed‐valent dinuclear Pd‐ligand species.

Scheme 5

Selected examples of outer‐sphere ligand‐to‐substrate single‐electron transfer to CF₃ ⁺.

Scheme 6

Generation of a nitrene radical in the coordination sphere of Pd^II by ligand‐to‐substrate single‐electron transfer and follow‐up C−H amination reactivity. Bottom: Fe^III‐catalyzed C−H amination.

Scheme 7

Postulated cycle for the C−H amination of a broad range of organoazides (4‐phenylbutyl azide chosen as an example) using crystallographically characterized Fe^III‐precatalyst Q.

Scheme 8

A Rh^III(ONNO) catalyst for intermolecular C−H amination of xanthene using tosyl azide.

Scheme 9

Copper(II) mediated aziridination of vinylic substrates by ligand‐centered two‐state reactivity.

Scheme 10

Stepwise single‐electron transfer from a U^IV(tetrabenzyl) complex to an incoming iminobenzoquinone ligand to afford complex W.

Scheme 11

Formation of a trinuclear ruthenium complex with two bridging nitrides by single‐electron transfer from a nitride ligand to the redox‐active ligand.