N-heterocyclic carbenes as bridgehead donors in metal pincer complexes

Rigid terdentate 'pincer' ligands containing a bridgehead N-heterocyclic carbene (NHC) donor attract considerable interest as spectators in transition metal complexes with broad scope and potential applications in the fields of catalysis, medicinal chemistry and material science. This review aims at compiling and evaluating the developments occurring in the area of complexes with these ligands. The various donor groups in the side-arms that can be associated with the bridgehead NHC donor provide considerable diversity and tunability of the pincer system. In addition to the distinct structural and reactivity features that are discussed in detail, comparisons with related pincer complexes based on all-classical donor sets or mixed donor sets with NHCs at the side-arm(s) only have been undertaken. The review comprises five major sections: the first attempting to differentiate the NHC bridgehead donor as structural element in the pincer ligand design; the following three describing in detail the synthesis, characterization and reactivity of metal complexes with ligand topologies featuring one, two or three NHC donors, respectively; finally the fifth compiling in a tabular format the catalytic applications of the complexes discussed previously, organized as a function of the catalytic reaction, the metal and the ligand type. The review provides a rationally organized coverage of the literature since 2005 in a diverse and fast expanding field dealing with pincer ligand design.