Synthesis of Tetra-Porphyrin Tubes for Catalysis and Host-Guest Chemistry

We report a reaction between tetra-3-pyridyl and tetra-3-bromomethylphenyl porphyrins forming a tetra-porphyrin tube with alkylpyridinium linkers (24 h, 26% yield). The free-base tube (FB₄-tube) was characterized using NMR, HR-MS, UV-vis, and single-crystal X-ray diffraction. Metalating the tolyl porphyrin prior to synthesis afforded Zn₂FB₂-tube which was further metalated with Co(II) to a heterobimetallic tetranuclear Zn₂Co₂-tube. FB₄-tube was subsequently metalated with zinc(II) and cobalt(II) giving homometallic tetranuclear cores. Cobalt porphyrins are catalysts for the Oxygen Reduction Reaction (ORR); therefore, we evaluated the tubes as homogeneous ORR catalysts. The Co₄-tube showed an Ecat_1/2 of -1.0 V vs FcH⁺/FcH. The tubes were evaluated for ORR selectivity via reduction with ferrocene. The Co₄-tube had a selectivity of 81.7% for H₂O versus H₂O₂ while that of the heterobimetallic tube was 37.8%. The former depleted O₂ at ∼4 times the rate. Porphyrin cages with large cavities have been used to sequester fullerenes. The FB₄-tube encapsulated C₆₀ with a binding constant K_C60⊂tube of 2.33 ± 0.03 × 10⁵ M^-1. Binding was verified via NMR, HR-MS, and electronic absorption spectroscopy. Tetra-porphyrin architectures are rare, and this facile synthesis of homometallic and heterobimetallic variants may populate a wide library of tetranuclear cores for catalysis and supramolecular chemistry.