Redox-Active Pincer Ligands
This review aims to provide a comprehensive overview of the emerging field of redox-active pincer ligands. As such, following a short general introduction on ligand-centred redox activity, the recent literature is discussed in two separate sections. The f
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Redox-Active Pincer Ligands Jarl Ivar van der Vlugt
Contents 1 General Introduction 2 Pincer Ligands That Undergo Reductive Chemistry 2.1 Bis(imino)pyridine as Redox-Active Pincer Platform 2.2 2,20 ,200 -Terpyridine and Substituted Derivatives Thereof 2.3 Other Bis-Azadiene-Based Pincer Platforms 2.4 Miscellaneous Pincer Systems Amenable to Ligand-Centred Reduction 3 Pincer Ligands That Undergo Oxidative Chemistry 3.1 Trianionic Pincer Ligands 3.2 Dianionic Pincer Systems 3.3 Monoanionic Pincer Ligands 3.4 Neutral Pincer Systems 4 Concluding Remarks References
Abstract This review aims to provide a comprehensive overview of the emerging field of redox-active pincer ligands. As such, following a short general introduction on ligand-centred redox activity, the recent literature is discussed in two separate sections. The first deals with ligand platforms that predominantly display reductive chemistry from the ‘parent’ scaffold; the second focusses more on systems that readily undergo oxidative chemistry. Fundamental stoichiometric reactivity as well as, where explored, catalytic applications will be discussed. This combined survey hopefully inspires the exploration and uncovering of many new avenues within the realm of redox-active pincer chemistry. Keywords Homogeneous catalysis · Pincer ligand · Redox-active ligand · Singleelectron transfer · Transition metals
J. I. van der Vlugt (*) Bioinspired Coordination Chemistry and Catalysis Group, Institute of Chemistry Carl von Ossietzky University Oldenburg, Oldenburg, Germany e-mail: [email protected]
J. I. van der Vlugt
1 General Introduction Traditionally, when considering the design of a metal complex for a specific desired (stoichiometric or catalytic) transformation, the metal centre (with an oxidation state commonly ranging from 0 to VI for 1st and 2nd row mid-to-late transition metals) is the locus for chemical bond activation and follow-up reactivity, with one or more ligands bound to this metal centre acting as spectators that stabilize, tune and/or sterically restrict the coordination sphere around it in order to invoke selective substrate coordination, activation and transformation. Redox-chemistry in these complexes is typically confined to the metal centre, because the energy input required to induce the transfer of an electron to (reduction) or from (oxidation) a spectator, or ‘redox-inert’, ligand is much higher than the cost associated with changing the oxidation state of the metal. Often, the ability of a metal to change its oxidation state whilst mediating a chemical conversion is essential for catalytic processes. Generally speaking, catalytic processes can be distinguished in reductive, oxidative or redox-neutral processes. Many (industrially relevant) reactions, even when formally redox-neutral, involve two-electron redox steps and are thus linked to, e.g. 2nd and 3rd row (noble) transition metals. Base metals generally prefer to undergo one-electron redox events, but controlled (‘metal-mediated’) odd-electron pathways are sti
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