N-Heterocyclic carbene-carbodiimide (NHC-CDI) betaine adducts: synthesis, characterization, properties, and applications

Abstract

N-Heterocyclic carbenes (NHCs) are an important class of reactive organic molecules used as ligands, organocatalysts, and σ-donors in a variety of electroneutral ylide or betaine adducts with main-group compounds. An emerging class of betaine adducts made from the reaction of NHCs with carbodiimides (CDIs) form zwitterionic amidinate-like structures with tunable properties based on the highly modular NHC and CDI scaffolds. The adduct stability is controlled by the substituents on the CDI nitrogens, while the NHC substituents greatly affect the configuration of the adduct in the solid state. This Perspective is intended as a primer to these adducts, touching on their history, synthesis, characterization, and general properties. Despite the infancy of the field, NHC-CDI adducts have been applied as amidinate-type ligands for transition metals and nanoparticles, as junctions in zwitterionic polymers, and to stabilize distonic radical cations. These applications and potential future directions are discussed.

Fig. 1. (A) Ground-state electronic structure of imidazol-2-ylidenes with numbering at the heterocycle shown. (B) NHCs of the imidazolidium and imidazolidinium based classes, with unsaturated and saturated backbones, respectively.

Fig. 2. Zwitterionic betaine adducts made from N-heterocyclic carbenes with allenes, ketenes, or heteroallenes.

Fig. 3. Common synthetic routes to (i) free NHC by deprotonation of an imidazolium salt precursor, (ii) free CDI by desulfurization of a thiourea, and (iii) NHC–CDI adduct.

Fig. 4. Previously synthesized imidazol-2-ylidene based NHC–CDI adducts.

Fig. 5. Previously synthesized imidazolidin-2-ylidene based discrete NHC–CDI adducts.

Fig. 6. (A) Priorities for assigning the geometry of amidinate CN bonds and (B) overall geometry for NHC–CDI adducts.

Fig. 7. (A) Comparison of binding equilibria of SIMes with N,N′-diaryl, N-aryl-N′-alkyl, and N,N′-dialkyl CDIs. (B) Competitive binding experiment to show relative stability of NHC–CDIs with N-aryl-N′-alkyl and N,N′-diaryl amidinates. (C) Control experiment testing CDI exchange for two N,N′-diaryl CDIs.

Fig. 8. Coordination modes of amindinate ligands. R² groups omitted from paddlewheel complexes for clarity.

Fig. 9. Carbodiimides such as CDI^CY can be used to probe the non-innocent bonding of NHCs to group 13 atoms.

Fig. 10. Copper complexes of NHC–CDIs. (A) Mono- and bimetallic complexes. (B) Heating complexes to 90 °C causes ejection of CDI^DiPP fragment and formation of a bis-NHC Cu(i) complex.

Fig. 11. NHC–CDI complexes with Mg and Zn reported by Nembenna and co-workers.

Fig. 12. (A) Single crystal structure of methylene-bridged aminal 13. Hydrogen bonds shown in dotted lines. Ellipsoids plotted at 30% probability. Reproduced from ref. with permission from The Royal Society of Chemistry. (B) Reaction of NHC–CDI ICyCDI^pTol with ZnCl₂ in DCM, as reported by Cámpora and Mosquera. (Cy = cyclohexyl, p-Tol = 4-methylphenyl, DCM = dichloromethane).

Fig. 13. Synthesis of NHC–CDI-stabilized nanoparticles. (A) Ru(COD)(COT) to form RuNPs, (B) Ni(COD)₂ to form NiNPs, and (C) Pt(NBE)₃ to form PtNPS.

Fig. 14. Structures of previously synthesized poly(azolium amidinates) (PAzAms). (Mes = 2,4,6-trimethylphenyl).

Fig. 15. NHC–CDI distonic radical cation reported by Johnson and co-workers. (Mes = 2,4,6-trimethylphenyl, [O] = oxidant).

Jessica R. Lamb

Christopher M. Brown

Jeremiah A. Johnson