Controlling Radical-Type Single-Electron Elementary Steps in Catalysis with Redox-Active Ligands and Substrates

Abstract

Advances in (spectroscopic) characterization of the unusual electronic structures of open-shell cobalt complexes bearing redox-active ligands, combined with detailed mapping of their reactivity, have uncovered several new catalytic radical-type protocols that make efficient use of the synergistic properties of redox-active ligands, redox-active substrates, and the metal to which they coordinate. In this perspective, we discuss the tools available to study, induce, and control catalytic radical-type reactions with redox-active ligands and/or substrates, contemplating recent developments in the field, including some noteworthy tools, methods, and reactions developed in our own group. The main topics covered are ( i ) tools to characterize redox-active ligands; ( ii ) novel synthetic applications of catalytic reactions that make use of redox-active carbene and nitrene substrates at open-shell cobalt–porphyrins; ( iii ) development of catalytic reactions that take advantage of purely ligand- and substrate-based redox processes, coupled to cobalt-centered spin-changing events in a synergistic manner; and ( iv ) utilization of redox-active ligands to influence the spin state of the metal. Redox-active ligands have emerged as useful tools to generate and control reactive metal-coordinated radicals, which give access to new synthetic methodologies and intricate (electronic) structures, some of which are yet to be exposed.

A Copper(I) Complex with Two Unpaired Electrons, Synthesised by Oxidation of a Copper(II) Complex with Two Redox‐Active Ligands

“…Thei ntegration of redox-active ligands in molecular coordination compounds leads to intriguing electronic structures and opens up new possibilities for applications in catalysis and materials science. [1][2][3][4][5] Intramolecular ligandmetal electron transfer (IET) processes in these complexes could be used for the activation of substrates in catalytic…”

A Single Bioinspired Hexameric Nickel Catechol–Alloxazine Catalyst Combines Metal and Radical Mechanisms for Alkene Hydrosilylation

“…Redox‐active ligands, inspired by natural redox cofactors found in metalloenzymes, [60] operate by providing access to metal‐ and ligand‐based redox events, thus enlarging the mechanistic possibilities and catalytic activity of 3d metals. They can promote bis ‐electronic steps at first‐row transition metals [61] but also sustain outer‐sphere SET to generate radicals [62–66] . We show here that a new class of bioinspired catechol‐alloxazine redox‐active ligands [18] can promote a dual mechanism involving outer‐sphere and inner‐sphere reactivities.…”

“…They can promote bis-electronic steps at first-row transition metals [61] but also sustain outer-sphere SET to generate radicals. [62][63][64][65][66] We show here that a new class of bioinspired catechol-alloxazine redox-active ligands [18] can promote a dual mechanism involving outer-sphere and inner-sphere reactivities.…”

Combining metal–metal cooperativity, metal–ligand cooperativity and chemical non-innocence in diiron carbonyl complexes

“… 11 The two main strategies involve proton-responsive ligands that facilitate bond heterolysis over the ligand and metal 12 or redox non-innocent ligands that can participate in electron transfer processes. 13 In recent years researchers have developed ligand systems that can enable both MMC and MLC for the activation of chemical bonds. Most prominently, the group of Uyeda has demonstrated how dinickel complexes of a redox non-innocent 1,8-naphthyridinediimine-based ligand can catalyze chemical transformations with superior activity and selectivity or distinct reactivity from what is possible with mononuclear analogues.…”