Functionalized nanorings are expected to provide unique reaction fields in which specific and selective molecular transformation occurs. Here, we designed new cyclic porphyrin trimers (CP3s) in which three porphyrin moieties were connected through 2,2’-bipyridyl groups at their 3,3’-positions. CP3s were synthesized by nickel-mediated reductive coupling reaction of bis(2-chloro-4-methoxycarbonyl-6-pyridyl)porphyrinatozinc derivatives (1).(ref. 1) In a CP3, three bipyridyl parts are directed toward the center of the ring. Unlike metal-organized porphyrin cages formed from tetrapyridyl substituted porphyrins and metal ions, the three bisnitrogen pairs on the three bipyridyl moieties in CP3 are not used to construct the macrocyclic structures. Therefore, active metal species, such as coordinatively unsaturated complexes can be generated inside the rings, whereas generation of such active species in metal-oraganized nanocages is generally difficult because of destabilization of the skeletons. As well as the three bidentate 2,2’-bipyridyl ligands, three tetradentate porphyrinato parts gives metal ligands. The two kinds of metal ligands connected in a circular manner to provide a macrocyclic ligand for multicofacial and multinuclear complexes within a one-nanometer cavity. In fact, three π-allyl palladium complexes were incorporated into a zincporphyrin nanoring to give a hexametallic complex.

During the synthesis of CP3s, an efficient separation method by using modified silica-gel columns was discovered. Reductive coupling of bis(chloropyridyl)porphyrin 1 gave mixtures of acyclic and cyclic porphyrin oligomers. They were eluted in the order of hydrodynamic volume of the oligomers on gel permeation chromatography (GPC), and repeated GPC processes were necessary to obtain pure material of CP3. To reduce the time- and solvent-consuming processes, several modified silica-gel columns, COSMOSIL series supplied from Nacalai Tesque, PYE (pyrene-modified), π-NAP (naphtalene-modified), PBB (pentabromobenzene-modified), NBE (nitrobenzene-modified), and CN-MS (cyanopropyl), were tested. When a mixture of pyridine and toluene was used as an eluent, extremely specific retention behavior of CP3 was observed on the modified silica-gel columns. Under the conditions, other acyclic oligomers were eluted without retention. Therefore, the target CP3s can be much easily isolated from reaction mixtures compared with GPC separation. The observed specificity is probably related to molecular recognition between CP3s and substituents on the used modified silica-gel columns.

Reference

1) Ohkoda, Y., Asaishi, A., Namiki, T., Hashimoto, T., Yamada, M., Shirai, K., Katagami, Y., Sugaya, T., Tadokoro, M. and Satake, A., Chem. Eur. J., 2015, 21, 11745–11756. doi:10.1002/chem.201501854.