Synthesis, Characterization, Protonation Reactions, and Electrochemistry of Substituted Open-Chain Pentapyrroles and Sapphyrins in Nonaqueous Media

Open-chain pentapyrroles were isolated as side-products from the synthesis of triaryl-corroles and then converted to the corresponding sapphyrins by catalytic oxidation in acidic media. The investigated compounds were characterized by UV–vis and ¹H NMR spectroscopy, mass spectrometry, electrochemistry, and spectroelectrochemistry and are represented as (Ar)₄PPyH₃ and (Ar)₄SH₃, where Ar is a F^– or Cl^– substituted phenyl group, PPy is a trianion of the open-chain pentapyrrole, and S is a trianion of the sapphyrin. Cyclic voltammetry and thin-layer UV–vis spectroelectrochemistry measurements were carried out in PhCN and CH₂Cl₂ containing 0.1 M tetra-<i>n</i>-butylammonium perchlorate. The open-chain pentapyrroles undergo two reversible one-electron reductions and two reversible one-electron oxidations to generate [(Ar)­PPyH₃]⁻, [(Ar)­PPyH₃]^2–, [(Ar)­PPyH₃]⁺, and [(Ar)­PPyH₃]²⁺ which were spectroscopically characterized. The corresponding sapphyrins exhibit two or three reversible one-electron oxidations in PhCN, but the reductions of these compounds are irreversible because of coupled chemical reactions following electron transfer. Comparisons are made between redox potentials and spectral properties of the open-chain pentapyrroles, sapphyrins, and structurally related corroles. Protonation of the open-chain pentapyrroles and sapphyrins was also carried out in CH₂Cl₂, and equilibrium constants were calculated by monitoring the spectral changes during titrations with trifluoroacetic acid. The pentapyrroles undergo a simultaneous two-proton addition to generate [(Ar)₄PPyH₅]²⁺ while the sapphyrins undergo two stepwise single proton additions to give [(Ar)₄SH₄]⁺ and [(Ar)₄SH₅]²⁺, respectively