Free-base corroles: determination of deprotonation constants in non-aqueous media

Abstract

A series of free-base corroles with different electron-donating or electron-withdrawing substituents were reacted with piperidine, 4-aminopyridine, 2-methylimidazole, 2-aminopyridine or pyridine in PhCN and the UV-visible spectral changes monitored during conversion of ( Cor ) H 3 to [( Cor ) H 2]- as a function of the concentration and strength of the added organic base. Analysis of the UV-visible spectral changes as a function of the added base concentration enabled calculation of equilibrium constants ( logK ) for deprotonation of each corrole under the given experimental conditions. Relationships are examined between the experimentally measured logK values and previously published spectroscopic and structural properties of the compounds.

Effect of Solvent and Protonation/Deprotonation on Electrochemistry, Spectroelectrochemistry and Electron‐Transfer Mechanisms of N‐Confused Tetraarylporphyrins in Nonaqueous Media

“…Radical anions and radical cations are generated in an initial one-electron addition and one-electron abstraction, respectively, but both of these species are highly unstable on the electrochemical and spectroelectrochemical timescale and only the deprotonated and protonated N-confused porphyrins are obtained as the final products of electroreduction and electrooxidation, respectively. Similar deprotonation and protonation reactions have been reported in the case of free-base corroles [38,45,46] and free-base tetraarylporphyrins after reduction and oxidation, [39,47] suggesting that extreme care must be taken in correctly assigning spectra to a specific product of electron transfer upon oxidation or reduction of a given N-confused free-base porphyrin.…”

Aura of Corroles

“…In more recent work, Kadish and co‐workers measured the equilibrium constants for deprotonation of ten free‐base corroles in the nonaqueous solvent benzonitrile by piperidine titration and analysis by the Hill equation 18. Similar spectral changes were observed for all corroles upon deprotonation, regardless of the number (1–3) of sterically hindered meso ‐substituents.…”

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

“…[59][60][61] The pentapyrroles previously examined in our laboratory,w ere shown to undergot wo reversible one-electron reductions and two reversible one-electron oxidations, [60] with an averageH OMO-LUMO gap of (1.34 AE 0.04) Vi nC H 2 Cl 2 .H owever,a sd escribed in the currentm anuscript, the reversibility is dependentu pont he solvent and the type of substituent on the meso-aryl groups. [63,64] Corroles are easily protonated and deprotonated in nonaqueous media [63][64][65][66] and this is also the case for sapphyrins, which can lose one pyrrole nitrogen proton or gain two pyrrole protons in non-aqueous media under the same solution conditions. [63,64] Corroles are easily protonated and deprotonated in nonaqueous media [63][64][65][66] and this is also the case for sapphyrins, which can lose one pyrrole nitrogen proton or gain two pyrrole protons in non-aqueous media under the same solution conditions.…”

“…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. [63,64] Corroles are easily protonated and deprotonated in nonaqueous media [63][64][65][66] and this is also the case for sapphyrins, which can lose one pyrrole nitrogen proton or gain two pyrrole protons in non-aqueous media under the same solution conditions. 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.…”