Coinage metal N-heterocyclic carbene complexes

[1] While the general form can be represented as (R2N)2C:–M (R = various alkyl or aryl groups), the exact nature of the bond between NHC and M has been investigated extensively through computational modeling and experimental probes.

[1] Coinage metal NHC complexes show effective activity as catalysts for various organic transformations functionalizing C-H and C-C bonds, and as antimicrobial and anticancer agents in medicinal chemistry.

[5] Of the three coinage metals discussed, the library of Cu NHC complexes has been historically the least developed, due to comparatively lower stabilities.

[4] The vast majority of these complexes are synthesized through method 2, due to the ease of using Ag2O with air stable imidazolium salts.

[6] Linear, trigonal, and tetrahedral coordination geometries are the most commonly reported, although a wide variety of more complex configurations have also been characterized.

[11] More in-depth computational analyses also highlight the role of M to NHC π back-donation, with respect to a description of an M-NHC bond.

[12] In an initial analysis of coinage metal NHC bonding from 1998, Boehme and Frenking performed quantum mechanical ab initio calculations for the three MICl(imidazol-2-ylidene) complexes, as well as with the corresponding silylene and germylene NHC-type ligands.

[13] The same trend emerges with the other NHC-type ligands, with Au having a rather strong bond to the NHC, and Ag having the relatively weakest.

[13] These computational results are corroborated in Density Functional Theory analyses of other coinage metal NHC complexes.

[1] For example, CuI NHC complexes are effective catalysts for various carboxylation or C-C coupling reactions, oftentimes including utilization of CO2 as a C1 source.

[20] Chiral Cu NHC complexes can also accomplish asymmetric catalysis for reactions such as allylic substitutions or conjugate additions.

[20] CuI NHC complexes have also shown promising pharmacological applications, with notable antitumor activity attributed to low biotoxicity, sufficient stability to reach sites of interest, and the ability to react with intracellular dioxygen to produce reactive oxygen species which attack DNA.

[1] As alkyne and alkene activations are often achieved by these complexes, commonly catalyzed organic transformations are heteroatom additions, cyclizations, and some cross-couplings.

Schematic examples of coinage metal NHC complexes synthesized through the four typical methods highlighted in the text. [ 2 ]
Examples of reported coinage metal NHC complexes displaying various structural types. [ 1 ] [ 5 ]
Contour line diagrams of the Laplacian distribution of three (NHC)MCl complexes. Solid contours show charge accumulation; dashed contours show charge depletion; blue lines show bond paths; blue dots show bond critical points. (See Boehme and Frenking 1998 for a thorough discussion). [ 13 ]