In this frontier article we overview the emergence and scope of NHC-based CCC and CNC pincer systems, i.e. complexes containing mer-tridentate ligands bearing two NHC donor groups, comment on their effectiveness in applications, and highlight areas for future development and exploitation.

The reaction of CO 2 with Et 3 SiH catalyzed by the nickel complex [(dippe)Ni(μ-H)] 2 (1) afforded the reduction products Et 3 SiOCH 2 OSiEt 3 (12%), Et 3 SiOCH 3 (3%), and CO, which were characterized by standard spectroscopic methods. Part of the generated CO was found as the complex [(dippe)Ni(CO)] 2 (2), which was characterized by single-crystal Xray diffraction. When the same reaction was carried out in the presence of a Lewis acid, such as Et 3 B, the hydrosilylation of CO 2 efficiently proceeded to give the silyl formate (Et 3 SiOC(O)H) in high yields (85−89%), at 80 °C for 1 h. Further reactivity of the silyl formate to yield formic acid, formamides, and alkyl formates was also investigated.

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A method using CO2 and PhSiH3 for the methylation of primary and secondary aliphatic amines catalyzed by Ni (0) complexes was developed, selectively producing the monomethylated products in moderate to good yields. For that purpose, two catalysts were used: [(dippe)Ni(μ-H)]2 and the commercially available Ni(COD)2/dcype, both of which were rather efficient in this process. With a slight experimental modification, the reaction allowed the production of monomethylated ureas in good yields by using low amounts of PhSiH3. On the basis of the experimental results, we propose a possible reaction mechanism for the formation of the new C–N bond.