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 ( FB 4 -tube ) was characterized using NMR, HR-MS, UV–vis, and single-crystal X-ray diffraction. Metalating the tolyl porphyrin prior to synthesis afforded Zn 2 FB 2 -tube which was further metalated with Co(II) to a heterobimetallic tetranuclear Zn 2 Co 2 -tube . FB 4 -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 Co 4 -tube showed an Eca t 1/2 of −1.0 V vs FcH + /FcH. The tubes were evaluated for ORR selectivity via reduction with ferrocene. The Co 4 -tube had a selectivity of 81.7% for H 2 O versus H 2 O 2 while that of the heterobimetallic tube was 37.8%. The former depleted O 2 at ∼4 times the rate. Porphyrin cages with large cavities have been used to sequester fullerenes. The FB 4 -tube encapsulated C 60 with a binding constant K C60 ⊂ tube of 2.33 ± 0.03 × 10 5 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.