Anionic N-heterocyclic carbenes: Synthesis, coordination chemistry and applications in homogeneous catalysis

• Anionic N-heterocyclic carbenes (NHCs) bear an exo- or endo-cyclic anionic substituent.
• Anionic NHCs containing delocalized or non-delocalized anionic charges are described.
• Anionic NHCs react with transition-metal or main-group precursors to afford overall neutral zwitterionic complexes or ate-complexes.
• Comparison of the catalytic performance of these overall neutral zwitterionic and their cationic congeners is described.

N-Heterocyclic carbenes (NHCs) are nowadays ubiquitous and indispensable as ancillary ligands in organotransition metal chemistry, with an enormous variety of NHC types known to date. Among these, NHCs which bear an overall anionic charge represent one of the latest additions to this ever-growing area of research, and the chemistry and application of these species is summarized in this review by gathering all records about anionic NHCs since their first appearance in 1996. The first part describes the synthesis and coordination chemistry of anionic NHCs that contain an anionic charge in π-conjugation with the heterocycle and their potential to undergo post-functionalization reactions by use of electrophilic reagents. The second part deals with anionic NHCs, in which the anionic charge is not π-conjugated with the heterocycle, and their use in transmetallation reactions to predominantly provide zwitterionic and electron-poor metal complexes.

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