Laboratoire de Chimie de Coordination.
Toulouse,
France

Outline

According to the Green formalism, most monodentate N-heterocyclic carbenes are neutral L type 2e^- donor ligands. We have now designed the archetype of a new family of stable NHCs whose main characteristic is the presence of a remote anionic functional group within their heterocyclic backbone.

Anionic NHCs of type B, named maloNHCs are readily available from pyrimidinium betaïnes of type A by simple deprotonation under ambient conditions. Their further reaction with a variety of Transition Metal complexes provides zwitterionic species of type C.

A valuable characteristic of the resulting coordinatively unsaturated zwitterionic complexes is that the remote anionic moiety pointing toward the outer coordination sphere does not interfere with the metal’s cavity, whose shape is controlled exclusively by the size of the NHC ring and the nature of its substituents. The steric and electronic properties of the ligand can thus be independently adjusted.

 
 
Current research line

While extending the scope of this concept to other transition metals, we are also currently at the point to demonstrate that a remote control of the catalytic activity of such complexes can be achieved by a simple modulation of their outer coordination sphere with suitable protecting groups.

Leading reference

 V. César, N. Lugan, G. Lavigne, J. Am. Chem. Soc. 2008, 130, 11286
"A Stable Anionic N-Heterocyclic Carbene and its first Zwitterionic Complexes."