The effects of catenation on the acidity constants of porphyrin [2]catenanes

The acidity constants (pKa₃ and pKa₄) of a series of porphyrins and their corresponding [2]catenanes where the porphyrin is over-arched with a hydroquinol or naphthoquinol polyether strap and mechanically linked to a bipyridinium macrocycle cyclobis-paraquat(4,4'-biphenylene), are calculated from acid-base titration data. As the length of the strap decreases from tetraethylene through triethylene to diethylene, the pKa values decrease, and the separation between pKa₃ and pKa₄ increases. In the diethylene glycol strapped derivatives, only the monocation is accessible above pH = 0. The trend in values reflects the ease of protonation which becomes increasingly difficult as the coulombic repulsions between the protonated porphyrin and the tetracationic macrocycle becomes more severe, as a consequence of enforced proximity of the two charged centres. On protonation, any electronic interactions between the interlinked subunits is replaced by strong electrostatic repulsions. This is alleviated by a 90˚ rotation of the tetracation and a movement away from the porphyrin; such a reorientation is possible only in the longer strapped derivatives, and is prevented in the "tight" analogues.