One-step template-directed synthesis of a macrocyclic tetraarylporphyrin hexamer based on supramolecular interactions with a C3-symmetric tetraarylporphyrin trimer

Taking into consideration the model geometry of the macrocyclic hexaporphyrin 1 as a host molecule, the structure of a benzene-centered porphyrin trimer bearing pyridine rings at the apical positions has been designed with the aim to use the latter as a template for the synthesis of its own host. Indeed, in the presence of the porphyrin trimer 5, the yield of the cyclization of a linear porphyrin hexamer, as a precursor of 1, could be improved from 8 to 30% (variable yield) to 50% (reproducible yield). Even the condensation of equimolecular amounts of porphyrin monomers 20b and 21b in the presence of 5 led-probably through a loose preorganized complex between the latter and the Zn(II) chelate 20b-to the formation of I in only five steps from 19, as compared with 13 steps of the synthesis via linear porphyrin hexamer in the absence of template. As evidenced by H-1 NMR spectroscopic analysis of the supramolecular complex between 5 and an analogue of 1b in which all H-atoms at the pyrrole rings have been replaced by deuterium, in the presence of unlabeled 1b, a rapid dissociation and recombination of the host and guest molecules forming the supramolecular complex takes place even at low temperature (-40 degrees C). As at 55 degrees C all six Zn(II) porphyrinate rings of the complex 1b + 5 become magnetically equivalent in the 500 MHz H-1 NMR time scale, approximate kinetic data for the ligand exchange process could be obtained.