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 (<b>FB₄-tube</b>) was characterized using NMR, HR-MS, UV-vis, and single-crystal X-ray diffraction. Metalating the tolyl porphyrin prior to synthesis afforded <b>Zn₂FB₂-tube</b> which was further metalated with Co(II) to a heterobimetallic tetranuclear <b>Zn₂Co₂-tube</b>. <b>FB₄-tube</b> 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 <b>Co₄-tube</b> showed an Eca<i>t</i>_1/2 of -1.0 V vs FcH⁺/FcH. The tubes were evaluated for ORR selectivity via reduction with ferrocene. The <b>Co₄-tube</b> 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 <b>FB₄-tube</b> encapsulated C₆₀ with a binding constant <i>K</i>_{C60<b>⊂</b>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.