Synthesis, Characterization, and Encapsulation Properties of Rigid and Flexible Porphyrin Cages Assembled from <i>N</i>-Heterocyclic Carbene–Metal Bonds

Abstract

Four porphyrins equipped with imidazolium rings on the <i>para</i> positions of their <i>meso</i> aryl groups were prepared and used as tetrakis(<i>N</i>-heterocyclic carbene) (NHC) precursors for the synthesis of porphyrin cages assembled from eight NHC-M bonds (M = Ag⁺ or Au⁺). The conformation of the obtained porphyrin cages in solution and their encapsulation properties strongly depend on the structure of the spacer -(CH₂)_<i>n</i>- (<i>n</i> = 0 or 1) between <i>meso</i> aryl groups and peripheral NHC ligands. In the absence of methylene groups (<i>n</i> = 0), porphyrin cages are rather rigid and the short porphyrin-porphyrin distance prevents the encapsulation of guest molecules like 1,4-diazabicyclo[2.2.2]octane (DABCO). By contrast, the presence of methylene functions (<i>n</i> = 1) between <i>meso</i> aryl groups and peripheral NHCs offers additional flexibility to the system, allowing the inner space between the two porphyrins to expand enough to encapsulate guest molecules like water molecules or DABCO. The peripheral NHC-wingtip groups also play a significant role in the encapsulation properties of the porphyrin cages.