INTERACTION OF 9,10-ANTHRAQUINONE WITH TETRACHLOROALUMINATE AND PROTON IN BASIC ALUMINUM-CHLORIDE - 1-ETHYL-3-METHYLIMIDAZOLIUM CHLORIDE ROOM-TEMPERATURE MOLTEN-SALTS

The electrochemical behavior of 9, 10-anthraquinone (AQ) in a basic room-temperature molten salt composed of a mixture of AlCl3 and 1-ethyl-3-methylimidazolium chloride (ImCl) is described. In the absence of a proton source, AQ is reduced via a quasireversible two-electron transfer to AQ(AlCl3)22-. The homogeneous chemical steps coupled to the electron transfers are displacement of Cl- from AlCl4- to form AQ(AlCl3)22-. Addition of a proton source, imidazolium hydrogen dichloride (ImHCl2), causes the coupled chemical reactions to shift from solvent leveling of reduced AQ by AlCl4- to more facile protonation steps. Visible spectroscopy of AQ in 0.8:1 melt and neat ImHCl2 show that AQ is present as the neutral, unprotonated quinone in its oxidized form. The two-electron, two-proton reduction product 9,10-dihydroxyanthracene (AQH-2) is formed under these conditions but is unstable in the AlCl3:IMCl melt. It is converted to AQ(AlCl3)22- which is the reoxidizable form, regardless of whether protons are present in the melt. AQH-2 is stable on the voltammetric time scale in neat ImHCl2. Under these conditions, the hydroquinone is formed exclusively and the electrochemistry tends toward that of the classical 2e-, 2H+ case found in aqueous systems. Experiments in molten ImCl, at 90-degrees-C, show that the quinone anion radical and dianion are stable only under conditions where no Lewis acid is available for adduct formation.