2,2'-Bipyridine Equipped with a Disulfide/Dithiol Switch for Coupled Two-Electron and Two-Proton Transfer.

[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 E1 =-1.20 V results in the formation of the radical anion 1red with much elongated S-S bond, which readily undergoes further reduction at E2 =-1.38 V. Water triggers a potential inversion (E≥-1.13 V for the second reduction) as the radical anion 1red 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.