Electrochemistry and Protonation of Open-Chain Pentapyrroles and Sapphyrins with Highly Electron-Withdrawing Meso-Substituents

Tuesday, 30 May 2017
Grand Ballroom (Hilton New Orleans Riverside)
W. Shan (University of Houston), M. L. Naitana, N. Desbois, V. Blondeau-Patissier, V. Quesneau, C. Gros (Université de Bourgogne Franche-Comté), W. Huang, Z. Ou (Jiangsu University), and K. M. Kadish (University of Houston)
A series of open-chain pentapyrroles and sapphyrins with highly electron-withdrawing meso-tetraaryl groups were synthesized and characterized as to their electrochemistry and protonation reactions in nonaqueous media. The chemical conversion of pentapyrroles to sapphyrins upon oxidation in acid media is known but it was not known if sapphyrins could be electrosynthesized from pentapyrroles under the appropriate solution conditions. This was investigated in the present study which compares the redox behavior and acid-base properties of pentapyrroles to that of sapphyrins having the same meso-substituents. The investigated compounds were each characterized by UV−vis and 1H NMR spectroscopy, mass spectrometry and electrochemistry, in addition to spectroelectrochemistry. Structures of the investigated compounds are shown in Chart 1 and are represented as (Ar)4PpyH3 and (Ar)4SapH3.

Cyclic voltammetry was used to measure redox potentials and spectroelectrochemistry was used to monitor the products of each redox reaction. The protonation processes were monitored by UV-vis absorption spectroscopy as protons were added to the investigated compounds in the form of trifluoroacetic acid (TFA). Equilibrium constants were calculated using both the Hill equation and the Mole Ratio method. The electrochemical data, the protonation constants and previous reported mechanisms for reduction and oxidation of pentapyrroles and sapphyrins with other meso-tetraaryl groups were then analyzed as a function of the solvent properties and para-substituents on the four phenyl rings of compounds in the two series.