2,2′‐Bipyridine Equipped with a Disulfide/Dithiol Switch for Coupled Two‐Electron and Two‐Proton Transfer

Abstract

[1,2]Dithiino[4,3-b:5,6-b']dipyridine (1) and its protonated open form 3,3'-dithiol-2,2'-bipyridine (2) were synthesised and their interconversion investigated. The X-ray structure of 2 revealed an anti orientation of the two pyridine units and a zwitterionic form. In depth electrochemical studies in combination with DFT calculations lead to a comprehensive picture of the redox chemistry of 1 in the absence and presence of protons. Initial one-electron reduction at E =-1.20 V results in the formation of the radical anion 1 with much elongated S-S bond, which readily undergoes further reduction at E =-1.38 V. Water triggers a potential inversion (E≥-1.13 V for the second reduction) as the radical anion 1 is protonated at its basic N atom. DFT studies revealed that S-S bond breaking and twisting of the pyridine units generally occurs after the second reduction step, whereas the potential inversion induced by protonation is a result of charge compensation. The CV data were simulated to derive rate constants for the individual chemical and electrochemical reactions for both scenarios in the absence and presence of protons.

Exploiting Potential Inversion for Photoinduced Multielectron Transfer and Accumulation of Redox Equivalents in a Molecular Heptad

“…Cyclic voltammetry of the PhSSPh reference compound (Scheme 1b) shows the typical features of potential inversion as reported previously for this and closely related compounds (SI page S16). 10,12 From cathodic scans, a peak potential of -1.6 V vs. SCE is measurable, while on return scans the corresponding reoxidation wave is detected at -0.3 V vs. SCE. The shift of 1.3 V between corresponding (two-electron) half-waves is a manifestation of the redox potential inversion.…”

Robust Binding of Disulfide-Substituted Rhenium Bipyridyl Complexes for CO2 Reduction on Gold Electrodes

“…Recently, the molecule 3,3 ′ -disulfide-2,2 ′ -bipyridine (SSbpy), with a functional disulfide bridge, was synthesized for the first time showing interesting redox capabilities (Cattaneo et al, 2018). The redox active dithiol/disulfide switch coupled to proton transfer provides the ligand with promising properties for heterogenization as well as multi-electron storage and redox control.…”

Proton-coupled multi-electron transfer and its relevance for artificial photosynthesis and photoredox catalysis

“…109 In non-proton-coupled redox reactions, every additional redox step is usually thermodynamically less favored than the preceding step, except in the somewhat special cases of redox potential inversion. 71,72,[74][75][76]110 As noted in the introduction, temporary storage of redox equivalents is needed to synchronize the fast primary photoinduced electron transfer events with the slower multi-electron turnovers in catalytic reaction centers, but there is also the important issue of photon flux. Two-electron transfer is difficult to induce upon single excitation.…”

“…5d), for which it was found that water triggers redox potential inversion due to protonation of the radical monoanion at its basic N-atom. 76 It will be interesting to see how that bpy ligand behaves electrochemically and photochemically when coordinated to a metal center.…”

“…5 Compounds exhibiting redox potential inversion. [74][75][76] Fig . 6 Reductive amination by photoredox catalysis: 80 has been further exploited for the enantioselective synthesis of amines by combined photoredox and enzymatic catalysis.…”