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  • Review Article
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Charge frustration in ligand design and functional group transfer

Abstract

Molecules with different resonance structures of similar importance, such as heterocumulenes and mesoionics, are prominent in many applications of chemistry, including ‘click chemistry’, photochemistry, switching and sensing. In coordination chemistry, similar chameleonic/schizophrenic entities are referred to as ambidentate/ambiphilic or cooperative ligands. Examples of these had remained, for a long time, limited to a handful of archetypal compounds that were mere curiosities. In this Review, we describe ambiphilicity — or, rather, ‘charge frustration’ — as a general guiding principle for ligand design and functional group transfer. We first give a historical account of organic zwitterions and discuss their electronic structures and applications. Our discussion then focuses on zwitterionic ligands and their metal complexes, such as those of ylidic and redox-active ligands. Finally, we present new approaches to single-atom transfer using cumulated small molecules and outline emerging areas, such as bond activation and stable donor–acceptor ligand systems for reversible 1e chemistry or switching.

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Fig. 1: Applications of zwitterionic compounds in coordination chemistry.
Fig. 2: Electronic structure of zwitterionic ligands.
Fig. 3: Organic zwitterions for organocatalysis, photochemistry and sensing.
Fig. 4: Nucleophilic and electrophilic bond activation by ambiphilic compounds.
Fig. 5: Fragmentation of heteroallenes results in single-atom transfer.
Fig. 6: Heterocyclic carbene complexes or zwitterions.
Fig. 7: Zwitterionic ancillary ligand design.
Fig. 8: Geminal dianions are stabilized by cationic substituents.
Fig. 9: Remote zwitterionic ligands.

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Acknowledgements

We thank Sadig Aghazada for insightful discussions. We acknowledge the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) for generous financial support. D.M. acknowledges financial support by the Fonds der Chemischen Industrie im Verband der Chemischen Industrie e.V. (Liebig Fellowship), as well as Research Corporation for Science Advancement (RCSA) and Fulbright Germany for a Fulbright-Cottrell Award.

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Munz, D., Meyer, K. Charge frustration in ligand design and functional group transfer. Nat Rev Chem 5, 422–439 (2021). https://doi.org/10.1038/s41570-021-00276-3

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