Synthesis, Characterization, and Electrochemistry of Open‐Chain Pentapyrroles and Sapphyrins with Highly Electron‐Withdrawing meso‐Tetraaryl Substituents

Abstract

A series of open-chain pentapyrroles and sapphyrins with highly electron-withdrawing substituents (i.e., CN, CF , or CO Me) on the meso-phenyl rings was synthesized and characterized as to the spectral properties, protonation reactions, and electrochemistry in non-aqueous media. The investigated compounds are represented as (Ar) PPyH and (Ar) SapH where PPy and Sap correspond to the tri-anion of the open-chain pentapyrrole and sapphyrin, respectively, and Ar=p-CNPh, p-CF Ph, or p-CO MePh. UV/Vis and H NMR spectroscopy as well as mass spectrometry data are given for the confirmation of the structures for the newly synthesized compounds. An X-ray structure for one of the pentapyrroles, that is, (p-CF Ph) PPyH (2), is also presented. The protonation processes were examined by UV/Vis absorption spectroscopy during the titration of the compounds with trifluoroacetic acid (TFA) in CH Cl . Equilibrium constants for the protonation reactions were calculated by using both the Hill equation and the mole ratio method. The protonation-initiated conversion of pentapyrroles to sapphyrins upon oxidation was also investigated. Cyclic voltammetry was used to measure the redox potentials in CH Cl , PhCN, and/or pyridine (Py). Electrochemical properties, protonation constants, and chemical reactions of the six compounds in the two series were then analyzed as a function of the solvent properties and the type of the electron-withdrawing groups on the meso-phenyl rings.

Synthesis, electrochemistry, protonation and X-ray analysis of meso-aryl substituted open-chain pentapyrroles

“…Open-chain pentapyrroles (or pentapyrrotetramethenes) with meso-tetraaryl substituents were first isolated as a major side product in the synthesis of corroles, being obtained in high yield (11%, the same as the target product) [1], and these molecules have since attracted great attention as synthetic precursors for the related sapphyrin macrocycles [1][2][3][4][5][6][7][8].…”

“…Our own interest in open-chain pentapyrroles has involved a characterization of their electrochemistry, spectroelectrochemistry and acid-base properties in nonaqueous media, as well as a study of their conversion to the corresponding sapphyrins [5][6][7][8]. The open-chain pentapyrroles were shown to undergo two reversible oneelectron reductions in CH 2 Cl 2 , PhCN or pyridine, with the E 1/2 values for these processes varying only slightly with the utilized electrochemical solvent [5,6].…”

“…Cations and dications of the pentapyrroles could be generated and characterized prior to sapphyrin formation in CH 2 Cl 2 or PhCN [5] but this was not the case in pyridine where deprotonation was proposed to occur after Synthesis, electrochemistry, protonation and X-ray analysis of meso-aryl substituted open-chain pentapyrroles the abstraction of one electron in the basic solvent [6]. Additional oxidation processes were also observed in the cyclic voltammograms of pentapyrroles with sterically hindered or highly electron-withdrawing groups, and it was suggested [5,8] that this might be related to the products formed in a proton-induced conversion to the corresponding sapphyrins.…”

“…The acid-base properties of several open-chain pentapyrroles were previously examined in our laboratory and the equilibrium constants for proton addition to these compounds were determined utilizing spectroscopic monitoring of the reaction. Two protons could be easily added to the open-chain pentapyrroles with the measured logb 2 for this reaction ranging from 8.1 to 11.0 [5,8]. Two protons could also be abstracted with logb 2 values ranging from 5.1 to 6.4 [6].…”

Rhodium Complexes of a New‐Generation Sapphyrin: Unique Structures, Axial Chirality, and Catalysis

“…Meanwhile, their coordination chemistry has remained limited; and only rhodium complexes have been reported for meso ‐aryl‐substituted sapphyrins . The two reasons for the restricted progress are: a) the N 5 core of sapphyrins is a much less suitable coordination sphere than the N 4 cores of porphyrins and corroles, and b) rotation of pyrrole ring A (Scheme ) dramatically alters the chelating properties of the sapphyrin core . Neutral meso ‐aryl‐substituted sapphyrins have only recently been fully characterised, and structures both with and without an inverted pyrrole ring A have been disclosed .…”