Deprotonation Reactions and Electrochemistry of Substituted Open-Chain Pentapyrroles and Sapphyrins in Basic Nonaqueous Media

Abstract

Electrochemical and spectroelectrochemical properties of three open-chain pentapyrroles and the corresponding sapphyrins were examined in pyridine containing 0.1 M tetra-n-butylammonium perchlorate and dichloromethane (CH2Cl2) or benzonitrile (PhCN) containing tetra-n-butylammonium hydroxide (TBAOH). The investigated compounds 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. The pentapyrroles, (Ar)4PPyH3, undergo two reversible one-electron reductions in pyridine, while the structurally related sapphyrins exhibit four reductions in this solvent, the first two of which are irreversible due to coupled chemical reactions following the electron transfers. Both series of neutral compounds could be deprotonated in CH2Cl2 or PhCN by addition of TBAOH to solution, and the progress of these reactions was monitored as a function of the base concentration by cyclic voltammetry and UV-vis spectroscopy. The neutral pentapyrroles were spectroscopically shown to undergo a loss of two protons in a single step to generate the [(Ar)4PPyH](2-) dianion while the sapphyrins could only be monodeprotonated, leading to formation of the [(Ar)4SH2](-) monoanion under the same solution conditions. The deprotonation constants were measured for each series of compounds in benzonitrile, and oxidation-reduction mechanisms are examined as a function of the solution 'basicity'.

Electrochemistry and spectroelectrochemistry of metallohexaphyrins containing bis-copper or bis-zinc central metal ions

“…For example, the values of DE 1/2 for meso-substituted sapphyrins range from 1.4 to 1.7 V, depending upon the type of substituents and the solvent [56][57][58]. In the case of the currently investigated bis-metallic hexaphyrins 1b and 2b, the HOMO-LUMO gaps are also smaller than for regular porphyrins but similar to that of copper tripyrrinones [46].…”

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

“…[12] Since then, sapphyrins have remained at opic of high interest, [13][14][15][16] in part because of their potential applicationsi nt he area of photodynamic therapy and their structurals imilarity to the well-known porphyrin and corrole macrocycles, [10,11,[17][18][19][20][21] and in part because of their unique properties as ar eceptor of neutraland anionic substrates. [55][56][57][58][59][60][61] Sapphyrins containing four meso-aryl substituentsc an serve as favorable building blocks fort he further designo fm olecules, which are similar to meso-aryl-substituted porphyrins, whose redox properties and chemicalr eactions can be finely tuned. [55][56][57][58][59][60][61] Sapphyrins containing four meso-aryl substituentsc an serve as favorable building blocks fort he further designo fm olecules, which are similar to meso-aryl-substituted porphyrins, whose redox properties and chemicalr eactions can be finely tuned.…”

“…[55] Although meso-tetraaryl sapphyrins can be isolated as side products of the Rothemund reaction, [55] or synthesized from the self-coupling reaction of dipyrromethane, [56] they can also be easily prepared from the respective open-chain pentapyrroles by acid-catalyzed oxidative coupling. [57][58][59][60][61][62] Our own interest in sapphyrins and open-chain pentapyrroles has involved in large part the characterization of their electrochemistry and acid-basep roperties in non-aqueous sol-vents. [57][58][59][60][61][62] Our own interest in sapphyrins and open-chain pentapyrroles has involved in large part the characterization of their electrochemistry and acid-basep roperties in non-aqueous sol-vents.…”

“…The structurally relateds apphyrins were also shownt oe xhibit two or sometimes three reversible oneelectron oxidations, [60] but this is not the case for the reduction where deprotonation followst he first one-electrona ddition leadingt oa no verall irreversible redox process, similar to what has been reported in the case of corroles. Open-chain pentapyrroles can also gain or lose protons from the pyrrole nitrogen atoms, [59,60] with the extent of theser eactions being related to both the properties of the non-aqueous solventa nd the type of the meso substituents on the compounds. Open-chain pentapyrroles can also gain or lose protons from the pyrrole nitrogen atoms, [59,60] with the extent of theser eactions being related to both the properties of the non-aqueous solventa nd the type of the meso substituents on the compounds.…”

“…[22][23][24][25][26][27][28][29] Al arge number of studies have characterizedd ifferent sapphyrind erivatives, including b-alkylateds apphyrins, [30][31][32][33] coremodified sapphyrins with heteroatoms (O, S, or Se), [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50] Nfused/N-confused sapphyrins, [51][52][53][54] and meso-tetraaryl-substituted sapphyrins. [55][56][57][58][59][60][61] Sapphyrins containing four meso-aryl substituentsc an serve as favorable building blocks fort he further designo fm olecules, which are similar to meso-aryl-substituted porphyrins, whose redox properties and chemicalr eactions can be finely tuned. [55] Although meso-tetraaryl sapphyrins can be isolated as side products of the Rothemund reaction, [55] or synthesized from the self-coupling reaction of dipyrromethane, [56] they can also be easily prepared from the respective open-chain pentapyrroles by acid-catalyzed oxidative coupling.…”