Outstanding Enhancement in the Axial Coordination Ability of the Highly Rigid Cofacial Cyclic Metalloporphyrin Dimer

Copper- and nickel-porphyrin complexes show extremely weak axial coordination ability without any electron-withdrawing groups. Herein, we report axial ligation on Cu^II- and Ni^II-porphyrins in a highly rigid cofacial porphyrin dimer with a bidentate ligand, 1,4-diazabicyclo[2.2.2]octane (DABCO). To the best of our knowledge, this is the first report on the use of Cu^II- and Ni^II-porphyrins for coordination-induced guest binding of porphyrin-based host molecules without the help of other metal ions. The high rigidity of the dimer induces guest binding through the cooperative effect of weak axial ligation. The results showed that Cu^II- and Zn^II-complexes bind one DABCO molecule inside their cavities, whereas the Ni^II-complex binds two additional DABCO molecules outside to form a stable 6-coordinate paramagnetic Ni^II-complexes.