D<inf>3h</inf>-Symmetric Porphyrin-Based Rigid Macrocyclic Ligands for Multicofacial Multinuclear Complexes in a One-Nanometer-Sized Cavity

The one-step synthesis of D<inf>3h</inf>-symmetric cyclic porphyrin trimers 1 composed of three 2,2′-[4,4′-bis(methoxycarbonyl)]bipyridyl moieties and three porphyrinatozinc moieties was achieved from a nickel-mediated reductive coupling of meso-5,15-bis(6-chloro-4-methoxycarbonylpyrid-2-yl)porphyrinatozinc. Although cyclic trimers 1 were obtained as a mixture that included other cyclic and acyclic porphyrin oligomers, an extremely specific separation was observed only for cyclic trimers 1 when using columns of silica gel modified with pyrenylethyl, cyanopropyl, and other groups. Structural analysis of cyclic trimers 1 was carried out by means of NMR spectroscopy and X-ray crystallography. Treatment of an η³-allylpalladium complex with a cyclic trimer gave a tris(palladium) complex containing three η³-allylpalladium groups inside the space, which indicated that the bipyridyl moieties inside the ring could work as bidentate metalloligands. Separation anxiety: Chromatography on a cyanopropyl-modified column supplied a cyclic porphyrin trimer efficiently from a mixture of acyclic and cyclic oligomers (see scheme; cod=1,5-cyclooctadiene, bpy=2,2′-bipyridyl). Treatment of an η³-allylpalladium complex with the cyclic trimer gives a tris(palladium) complex, which indicates that the cyclic trimer is a versatile macrocyclic ligand for multicofacial multimetallic complexes.