Poly-NHCs

Bridging bis-NHCs allow the coordination of two metals and may facilitate the preparation of dimetallic discrete molecules or polymeric systems. While some poly-NHCs have been designed to prevent chelation and therefore favor the formation of discrete polymetallic complexes, some others rely on the synthetic... 

The option to alkylate the coordinated NH,NH-NHC ligand offers the possibility to generate macrocyclic poly-NHC ligands in a template-controlled approach. For example, four NH,NH-NHC ligands coordinated to Pt in a square-planar complex have been linked to a macrocyclic tetra-NHC ligand with crown-ether topology [57]. 

Poly-NHC Complexes of Transition Metals Recent Applications and New Trends... 

Poly-NHC Ligands Containing Abnormal, Remote, or Mesoionic Carbenes 216... 

The success enjoyed in the last 20 years by N-heterocycHc carbenes (NHCs) as hgands toward transition metal centers has fuelled from the very beginning the parallel development of a great number of polytopic ligands featuring more than one NHC moiety (poly-NHCs). Such polydentate... 

Lots of previously unreported poly-NHC ligands have been disclosed in the literature during the last 5 years. However, most of the investigations can be categorized according to a small number of target compound classes, namely (i) chiral poly-NHC ligands (ii) pincer or pseudopincer poly-NHC... 

Research on chiral NHCs has flourished in the course of the last 20 years parallehng the success of NHCs as hgands for catalyticaUy active metal complexes and as organocatalysts in their own right. Several chiral poly-NHCs have been also proposed and prepared, but success in their apphcation as Hgands for enantioselective metal catalysts has been up to now moderate, although in the course of the last few years remarkable degrees of asymmetric induction have been achieved in a few instances. The topic has been reviewed by Shi. ... 

Figure 1 Recently developed chiral poly-NHC ligand architectures.

Figure 2 Recently developed poly-NHC pincer and pseudopincer ligand architectures.

Figure 3 Formation of bimetallic complexes from poly-NHC metal complexes bearing free NH moieties.

An interesting and specific characteristic of poly-NHC ligands is the possibility of combining different carbene donors to form the so-called hetero-poly-NHC hgands. As already reported by Huynh and coworkers, the combination of two different donors did not lead always to Hgands with intermediate properties with respect to the corresponding homoleptic compounds. In fact, enhanced properties (e.g., catalytic properties) may be recorded in the corresponding complexes, which have been interpreted in terms of an electronic asymmetry . The most recent examples on this topic are reported in Fig. 8. 

Figure 8 Recent examples on "hetero-poly-NHC" ligands.

Some additional reports on novel and/or interesting types of poly-NHC hgand architectures have appeared in the recent literature (Fig. 9). 

In the course of the last 5 years, an enormous number of new metal complexes of poly-NHCs has been disclosed in the hterature, which sum... 

---