Synthesis and coordination chemistry of macrocyclic ligands featuring NHC donor groups

Poly-NHC (NHC = N-heterocyclic carbene) ligands emerged almost immediately after the first stable NHCs had been described. Macrocyclic ligands, featuring NHC donor groups and their metal complexes, however, remained rare until recently. This perspective highlights modern developments in the fields of synthesis and coordination chemistry of macrocyclic poly-NHC ligands. These include the synthesis of tetracarbene ligands which were obtained from complexes of beta-functionalized isocyanides followed by cyclization of the coordinated iscocyanide ligands to NH, NH-functionalized NHCs and the subsequent metal template controlled bridging alkylation of the NH, NH-NHCs to yield the macrocycle. The template synthesis of ligands featuring a mixed NHC/phosphine donor set like [11] ane-P2CNHC and [16] ane-P2C2NHC by linkage of NH, NH-NHCs to different phosphines is also presented. Finally, methods for the preparation of cyclic polyazolium salts, their deprotonation and metalation and the different modes of coordination of such macrocyclic poly-NHC ligands are discussed.