Thermodynamics of N-heterocyclic carbene dimerization: The balance of sterics and electronics

The past decade has seen N-heterocyclic carbenes (NHC) emerging as a credible alternative to ubiquitous tertiary phosphines as ligands in organic and organometallic chemistry. In many cases NHC ligands have proven more effective than phosphines as ancillary ligands in catalysis, while in some cases they also enable unexpected and welcomed chemical routes to new products. These initial discoveries have paved the way to a continued search for new NHCs able to promote more effective and/or unprecedented catalysis. Nevertheless, the search for new and useful NHCs remains an arduous task, because, to date, useful NHCs have to be synthesized as monomeric species, while some monomeric NHCs have the disappointing tendency to easily dimerize as shown in eq 1.

2 NHC: <---> NHC=CHN + –E_dim (1)