Complexes with protic (NH,NH and NH,NR) N-heterocyclic carbene ligands

• Methods for the synthesis of complexes bearing protic NHC ligands, i.e. NHCs with an NH,R- or NH,NH-substitution pattern, have been reviewed.
• The reactivity of complexes bearing protic NHCs is discussed.
• Applications of protic NHCs in the synthesis of macrocycles containing NHC donors have been reviewed.
• New trends regarding the application of complexes with protic NHC ligands in catalysis are presented.

The review describes methods for the preparation of complexes bearing protic NHC ligands, i.e. NHCs featuring an NH,NH or NH,NR substitution pattern. The protic NHC ligands are easily functionalized at the ring nitrogen atoms after N–H deprotonation. Thus the introduction of various functional groups at the ring nitrogen atoms is possible giving access to new complexes with NHC ligands and even allowing the linkage of individual protic NHCs to give macrocyclic ligands with NHC donors. Selected applications for complexes bearing protic NHCs will be described.