Persistent carbene

N- Heterocyclic carbenes or abbreviated NHC (sometimes Arduengo carbenes ) are a group of organic chemical compounds. These are electron-rich, aromatic or unsaturated heterocyclic compounds, which can form stable carbenes.

Structure and Stability

The high stability of NHCs compared to other carbenes is mainly caused by the I M, and effects of the adjacent heteroatoms. The electron density of the empty p orbital on carbon is increased. In addition to nitrogen, sulfur or oxygen can stabilize the carbene. NHCs with imidazole as the backbone are additionally stabilized by its aromaticity.

Education

Free carbenes are usually obtained by the deprotonation of imidazolium salts or the corresponding cationic precursor compounds. Typically, strong bases such as sodium hydride or alcoholates are used.

Dimerization

Some NHCs dimerize to Tetraaminoethylen derivatives. You are in chemical equilibrium, the so-called Wanzlick equilibrium with the corresponding carbenes. Other than initially suspected that balance is irrelevant for most NHCs.

Application

NHCs play an important role as ligands for metal complexes. Due to its properties as a strong sigma donors and their kinetic inertness make it a popular alternative for phosphines especially for transition metal catalysis dar. transition metal -NHC complexes are often temperature stable and less sensitive to oxidation than phosphine.

A well-known example of the use of transition metal -NHC complexes in catalysis is the olefin metathesis catalysts of the second generation of Robert H. Grubbs.

Another important feature is the ability of the NHC to the formation of charge-transfer complexes.