Synthesis and characterization of novel p-terphenoquinone analogues involving a central dihydrothiophenediylidene structure

The title quinones, 2,5-bis(4-oxo-2,5-cyclohexadien-l-ylidene)-2,5-dihydrothiophene and its tetraalkyl derivatives, were synthesized via 2-(4-alkoxyphenyl)thiophene and 2,5-bis(4-alkoxyphenyl)thiophene conveniently prepared by using palladium(0)-catalyzed cross coupling reaction of 4-alkoxyaryl halides and thienylzinc chloride. The ground-state structures and redox properties of these quinones were fully characterized based on spectroscopic data (IR, ¹H NMR, ¹³C NMR, UV-vis, and MS), X-ray analysis, cyclic voltammetry, CNDO/2 calculation, and EPR spectroscopic investigation. These quinones show an intense absorption maximum in the region of 531–558 nm of their electronic spectra. Despite their three-ring system, these quinones exist in a coplanar conformation in the solid state in which the terminal six-membered rings incline 9.1° and 11.1°, respectively, from the central five-membered ring, and the twisting angle between the mean planes of the two terminal rings is 1.8°. Rotation about the intercyclic bonds does not take place at ordinary temperatures in solution. Noticeably, these quinones exhibit a half-wave oxidation potential (E₁^ox value) of 1.26–1.46 V along with half-wave reduction potentials (E₁^red value) of −0.11 to −0.52 V and (E₂^red value) of −0.20 to −0.67 V vs SCE, so that they have been proven to be a new type of quinones exhibiting an amphoteric redox property undergoing two-stage one-electron reduction up to the dianions and one-electron oxidation up to the radical cations. The reduction and oxidation potentials of these quinones are in good correspondence with the calculated energy levels of the HOMO and the LUMO. Decrease in the HOMO-LUMO gap is mainly ascribed to the destabilized HOMO lying at a higher level by 0.97 eV than that of diphenoquinone. Both the cation and anion radicals were generated by the electrochemical reduction and oxidation of these amphoteric quinones and have been definitely characterized by EPR spectroscopy.