Molybdenum(II) Chloride in the Aluminum Chloride–1-Methyl-3-ethylimidazolium Chloride Molten Salt. Electrochemical and Spectroscopic Characterization of the [(Mo₆Cl₈)Cl₆]²⁻Ion in Neutral and Basic Melts

Absorption spectroscopy and titration experiments suggest that molybdenum(II) chloride is complexed as the hexanuclear [(Mo₆Cl_g)Cl₆]²⁻cluster in the basic and the neutral-basic aluminum chloride-1-methyl-3-ethylimidazolium chloride molten salt. Only a single voltammetric reduction wave at E_p^c= ~ 1.23 V versus aluminum in the 66.7/33.3 mol % melt can be seen for this complex at a glassy carbon electrode in basic melt, but the reduction product is not stable. However, if all of the unbound chloride ion in basic melt containing [(Mo₆Cl₈)Cl₆]²⁻is carefully removed by titration with acidic melt, it is possible to also observe a reversible one-electron oxidation wave with E_1/2= 1.55 V corresponding to the oxidation of this complex to [(Mo₆Cl₈)Cl₆]⁻. This species is stable on the voltammetric time scale, but attempts to produce bulk solutions of this oxidant were unsuccessful because it is slowly reduced by some component of the neutral melt. The [(Mo₆Cl₈)Cl₆]²⁻complex luminesces in basic melt with an emission maximum centered at ca. 700 nm (uncorrected) when irradiated at 400 nm. Molybdenum(II) is very soluble in moderately acidic melt (>55 mol % A1C1₃), but it is not electroactive. However, it luminesces in this melt at 710 nm (uncorrected) when irradiated at 400 nm, suggesting that the (Mo₆Cl₈)⁴⁺core is retained after dissolution.