Complexes with Poly(N-heterocyclic carbene) Ligands: Structural Features and Catalytic Applications

Easy access to NHC precursors has allowed the preparation of a wide set of poly-NHC (N-hetereocyclic carbene) ligands with an unlimited number of architectures. An important class of bis-NHCs is that introducing a donor-functional group between the two carbene edges. The design of non-chelating bis-NHCs is also of great importance since this type of ligand can provide bimetallic structures that might lead to new advances in the development of electronic materials. bis-NHCs with short linkers force the two azole rings to lie on the sterically crowded plane of molecule. The chelating coordination of bis-NHCs can be achieved if the metal has small coligands that produce a low steric hindrance in the xy plane. Developments have been reported in poly-NHC ligands that provide enhanced catalytic properties, and are also found to afford processes not previously known for complexes with other ligands.