The one-step synthesis of D3h -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 η(3) -allylpalladium complex with a cyclic trimer gave a tris(palladium) complex containing three η(3) -allylpalladium groups inside the space, which indicated that the bipyridyl moieties inside the ring could work as bidentate metalloligands.

Double-bridged cofacial Ni porphyrin dimers 2 with 2,2′-bipyridyl pillars were effectively prepared by a one-step reductive homocoupling reaction of bis(chloropyridyl)-substituted Ni porphyrin derivatives followed by a specific separation of a cyanopropyl-modified silica gel column using pyridine eluent systems. The structural analyses of 2 and its Pd complex were carried out in their solid and solution states by means of X-ray single crystal analysis and NMR, respectively. The complexation of η 3 -allylpalladium chloride (Pd) with 2 on the spatially restricted 2,2-bipyridine moieties on 2 gave a 2:1 (Pd:2) complex, in which the 2,2′-bipyridine ligands only provided one of the N atoms on a 2,2′-bipyridine ligand to a Pd. Therefore, the 2,2-bipyridine moieties acted as a monodentate ligand.

Weak affinity chromatography (WAC) is effective for isolating target compounds from analogous compounds with similar functional groups. Previously, a few chromatographic behaviors based on WAC using organic solvents were observed in a series of cyclic multiporphyrin systems on cyanopropyl-modified silica gel (CN-MS). Here, three cyclic porphyrin trimers with various rigidity were examined on CN-MS to understand the mechanism of the specific interactions between porphyrin derivatives and functional groups on modified silica gel. In addition to CN-MS, six modified silica-gel columns were tested to compare their retention abilities for a cyclic nickel porphyrin dimer (C4Ni2MsCP2). We examined the cosolvent effects of the pyridine eluents for C4Ni2MsCP2. Apparent dissociation constants of C4Ni2MsCP2 with functional groups on the MS columns and effective amounts of the functional groups were estimated by frontal affinity chromatography (FAC). 1H NMR titrations of acetonitrile and nitrobenzene to C4Ni2MsCP2 were conducted to compare their association constants with movable guest molecules to the dissociation constants with immobilized functional groups obtained in FAC. We found rigidity of cyclic porphyrin derivatives and immobilization of functional groups on silica gel is necessary for significant retentions using WAC. The affinity interaction does not occur at the center of C4Ni2MsCP2, but probably occurs on the surface composed of a bipyridyl moiety and the adjacent edges of the two porphyrins. Polar solvents, such as nitrobenzene, acetonitrile, and methanol, weakened the interaction. Although C4Ni2MsCP2 dissolves well in chloroform, the interactions between C4Ni2MsCP2 and the MS columns are considerably strengthened in the presence of chloroform. The competitiveness of solvents and cosolvents with the interaction of the porphyrin on WAC is independent of the solubility of the analyte.