Cyclo[2]pyridine[8]pyrrole: An Expanded Porphyrinoid with Three Closed-Shell Oxidation States

We report here an expanded porphyrinoid, cyclo[2]pyridine[8]pyrrole, 1, that can exist at three closed-shell oxidation levels. Macrocycle 1 was synthesized via the oxidative coupling of two open chain precursors and fully characterized by means of NMR and UV-vis spectroscopies, MS, and X-ray crystallography. Reduction of the fully oxidized form (1, blue) with NaBH₄ produced either the half-oxidized (2, teal) or fully reduced forms (3, pale yellow), depending on the amount of reducing agent used and the presence or absence of air. Reduced products 2 or 3 can be oxidized to 1 by various oxidants (quinones, FeCl₃, and AgPF₆). Macrocycle 1 also undergoes proton-coupled reductions with I^-, Br^-, Cl^-, SO₃^2-, or S₂O₃^2- in the presence of an acid. Certain thiol-containing compounds likewise reduce 1 to 2 or 3. This conversion is accompanied by a readily discernible color change, making cyclo[2]pyridine[8]pyrrole 1 able to differentiate biothiols, such as cysteine (Cys), homocysteine (Hcy), and glutathione (GSH).