Formation and Cleavage of Aromatic Disulfide Radical Anions

Abstract

The electron transfer (ET) to a series of para-substituted diaryl disulfides, having the general formula (X-C(6)H(4)S-)(2), has been studied. The X groups were selected as to have a comprehensive variation of the substituent effect, being X = NH(2), MeO, H, F, Cl, CO(2)Et, CN, and NO(2). The reduction was carried out experimentally, using N,N-dimethylformamide as the solvent, and by molecular orbital (MO) ab initio calculations. The ET was studied heterogeneously, by voltammetric reduction and convolution analysis, and homogeneously, by using electrogenerated radical anions as the solution electron donors. The reduction is dissociative, leading to the cleavage of the S-S bond in a stepwise manner. Both experimental approaches led us to estimate the E degrees and the intrinsic barrier values for the formation of the radical anions. Comparison of the independently obtained results allowed obtaining, for the first time, a quantitative description of the correlation between heterogeneous and homogeneous rate constants of ETs associated with significant inner reorganization energy. The experimental outcome was fully supported by the theoretical calculations, which provided information about the disulfide lowest unoccupied MOs (LUMOs) and singly occupied MO (SOMO), the bond dissociation energies, and the most significant structural modifications associated with radical anion formation. With disulfides bearing electron-donating or mildly electron-withdrawing groups, the inner reorganization is particularly large, which reflects the significant stretching of the S-S bond experienced by the molecule upon ET. The process entails formation of loose radical anion species in which the SOMO is heavily localized, as the LUMO, onto the frangible bond. As a consequence of the formation of these sigma-radical anions, the S-S bond energy of the latter is rather small and the cleavage rate constant is very large. With electron-withdrawing groups, the extent of delocalization of the SOMO onto the aryl system increases, leading to a decrease of the reorganization energy for radical anion formation. Interestingly, while the LUMO now has pi character, the actual reduction intermediate (and thus the SOMO) is still a sigma-type radical anion. With the nitro-substituted disulfide, very limited inner reorganization is required and a pi-radical anion initially forms. A nondissociative type intramolecular ET then ensues, leading to the formation of a new radical anion whose antibonding orbital has similar features as those of the SOMO of the other diaryl disulfides. Therefore, independently of the substituent, the actual S-S bond cleavage occurs in a quite similar way along the series investigated. The S-S bond cleavage rate, however, tends to decrease as the Hammett sigma increases, which would be in keeping with an increase of both the electronic and solvent reorganization energies.

Asymmetry and Non‐Adiabaticity in Fragmentation of Disulfide Bonds upon Electron Capture

“…This situation reflects, at least partially, the difficulties inherent to an adequate description of the intimate internal reorganization of the system required to finally produce the SÀS bond fission. Some interesting theoretical attempts to clarify this question have been performed, such as the quantum chemical model study reported by Uggerud, [16] in which four possible mechanisms for ECD of the disulfide bonds were investigated at different levels of theory, or the analysis of the electron transfer (ET) within the series of parasubstituted diaryl disulfides carried out by Antonello et al, [17] which showed that this process entails the formation of a loose radical anion with a singly occupied molecular orbital (SOMO) localized onto the SÀS bond. Despite these efforts, the actual situation is that the description of the SÀS bond cleavage is still a theoretical challenge even in the simplest disulfide system exhibiting a disulfide bond, S 2 H 2 , and the measured experimental value could only be reproduced when high-level G2 and complete basis set (CBS) methods were used.…”

A Definitive Answer to a Bonding Quandary? The Role of One‐Electron Resonance Structures in the Bonding of a {Cu3S2}3+ Core

“…To the best of my knowledge this species has never before been described, although it shares close similarities with organic disulfide radical anions that have been studied in molecular systems as well as in proteins as reactive intermediates in S À S bond cleavage reactions. [13][14][15][16][17] These radical species contain a three-electron s bond, which derives from population of the SÀS s bonding orbital by two electrons and population of the s* orbital by one electron.…”

Electrochemical Reduction of 4-Iodophenyl Sulfonyl Chloride

“…5). A value close to or higher than 0.5 is expected in the case of a stepwise mechanism, while for a concerted Et mechanism a value much lower than 0.5 is expected (18)(19)(20)(21)(22)(23)(24).…”