Transition metal carbene complexes are widely used in organic synthesis, polymer production and even in supramolecular chemistry. Nobel prizes have been awarded for pioneering work in this area of chemistry.

 

In particular, in 1991 the significant discovery of isolable N-heterocyclic carbenes (NHC) by Arguengo spurred on a new generation of carbenes - five-membered heterocyclic carbenes with two nitrogens - which are used successfully as ligands for homogeneous catalysis. 

 

Remarkably, only five-membered heterocyclic rings have really been considered as ligands for catalysis; chiefly those containing two nitrogen atoms in the heterocycle. In this Dalton Transactions Perspective, Helgard Raubenheimer and Stephanie Cronje examine the forgotten six-membered carbene family. In particular, they discuss an emerging concept in NHCs: having the nitrogen remote from the carbene carbon rather than right next to it.

 

It is important to compare the catalytic activity with that of the better known five-membered NHC complexes and also to determine whether differences in bonding could be established between the normal NHCs and those with remote nitrogens, says Raubenheimer.