Electron-transfer and acid-base properties of a two-electron oxidized form of quaterpyrrole that acts as both an electron donor and an acceptor

Electron-transfer interconversion between the four-electron oxidized form of a quaterpyrrole (abbreviated as P₄ for four pyrroles) and the two-electron oxidized form (P₄H₂) as well as between P₄H₂ and its fully reduced form (P₄H ₄) bearing analogous substituents in the α- and β-pyrrolic positions was studied by means of cyclic voltammetry and UV-visible spectroelectrochemistry combined with ESR and laser flash photolysis measurements. The two-electron oxidized form, P₄H₂, acts as both an electron donor and an electron acceptor. The radical cation (P ₄H₂^̇+) and radical anion (P ₄H₂^̇-) are both produced by photoinduced electron transfer from dimeric 1-benzyl-1,4-dihydronicotinamide to P ₄H₂, whereas the cation radical form of the compound is also produced by electron-transfer oxidation of P₄H₂ with [Ru(bpy)₃]³⁺. The ESR spectra of P₄H ₂^̇+ and P₄H₂^̇- were recorded at low temperature and exhibit spin derealization over all four pyrrole units. Thus, the two-electron oxidized form of the quaterpyrrole (P ₄H₂) displays redox and electronic features analogous to those seen in the case of porphyrins and may be considered as a simple, open-chain model of this well-studied tetrapyrrolic macrocycle. The dynamics of deprotonation from P₄H₂^̇+ and disproportionation of P₄H₂ were examined by laser flash photolysis measurements of photoinduced electron-transfer oxidation and reduction of P₄H₂, respectively.