Poly-NHCs ligands

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 Ligands Containing Abnormal, Remote, or Mesoionic Carbenes 216... 

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... 

Figure 1 Recently developed chiral poly-NHC ligand architectures.

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.

In recent years, there has been an increasing interest toward the synthesis of heteropolymetallic systems supported by poly-NHC ligands.The... 

Finally, it is worth mentioning that nickel or palladium complexes with poly-NHC ligands have been also employed as electrocatalysts for CO2 reduction (Fig. 31). Although the reaction rates are modest, the catalysts, particularly the Pd ones, appear stable against dimerization pathways ultimately leading to catalyst decomposition hence, they hold promise for this reaction provided their activity can be improved. 

Figure 31 Nickel and palladium complexes with poly-NHC ligands employed as electrocatalysts for CO2 reduction.

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