Synthesis of tetra-porphyrin tubes for catalysis and host-guest chemistry

We report a reaction between tetra-3-pyridyl porphyrin and tetra-3-bromomethylphenyl porphyrin leading to the formation of a tetra-porphyrin tube with alkylpyridinium linkers. The 24-hour synthesis resulted in a 26% isolated yield. The free-base tube (FB4-tube) was characterized using 1D NMR, 2D NMR, HR-MS, UV-Vis, and single-crystal X-ray diffraction. This architecture was subsequently metalated with zinc(II) and cobalt(II) resulting in tetra-nuclear cores. Cobalt porphyrins are often used as catalysts for the Oxygen Reduction Reaction (ORR). Therefore, we evaluated the tube as a homogenous ORR catalyst. The Co4-tube showed a Ecat1/2 value of –1.0 V versus FcH+/FcH. Additionally, porphyrin cages with large cavities have been used to sequester pollutants and other organic molecules like fullerenes. With our tube, encapsulation of C60 and C70 occurred with high binding affinity, which was verified via NMR and HR-MS. Due to the large cavity of the tube, the fullerenes sit off-centered, breaking the symmetry to C2v from D2h. This was confirmed by proton NMR shifts associated with through-space interactions.