ELECTROCHEMICAL AND SPECTROSCOPIC CHARACTERIZATION OF (NB6CL12)Z+ CHLORIDE CLUSTERS IN THE ALUMINUM CHLORIDE-1-METHYL-3-ETHYLIMIDAZOLIUM CHLORIDE MOLTEN-SALT

The electrochemistry and electronic absorption spectroscopy of metal-metal-bonded chloride clusters derived from the {Nb6Cl12}z+ (z = 1-4) cores were investigated in the room-temperature molten salt aluminum(III) chloride-1-methyl-3-ethylimidazolium chloride (AlCl3-MeEtimCl). Anionic complexes of this core cluster of the type [(Nb6Cl12)Cl6](6-z)- were identified in basic melt. Reversible electrode reactions with E1/2 values of -0.02 and 0.39 V versus the Al3+/Al couple in the 66.7/33.3 mol % AlCl3-MeEtimCl melt at 100-degrees-C were found for the oxidation of the chloride complexes derived from the {Nb6Cl12}2+ and {Nb6Cl12}3+ cores, respectively. The overall redox scheme for these complexes is [(Nb6Cl12)Cl6]5-? +e- half-arrow-right-over-half-arrow-left -e- [(Nb6Cl12)Cl6]4- +e- half-arrow-right-over-half-arrow-left -e- [(Nb6Cl12)Cl6]3- +e- half-arrow-right-over-half-arrow-left -e- [(Nb6Cl12)Cl6]2-[(Nb6Cl12)Cl6]5- is moderately stable on the voltammetric time scale but decomposes during controlled-potential electrolytic reduction of the parent species. Likewise, [(Nb6Cl12)Cl6]2- is voltammetrically stable, but bulk solutions of this complex are slowly reduced to [(Nb6Cl12)Cl6]3- by chloride ion in basic melt. Reversible electrochemical reactions involving the {Nb6Cl12}2+/3+ and {Nb6Cl12}3+/4+ redox systems were also found in acidic melt at 1.13 and 1.50 V, respectively, and solutes containing the {Nb6Cl12}z+ (z = 2-4) cores exhibited long-term stability. The approximate 1-V positive shift in the E1/2 values for these redox reactions in acidic melt relative to the E1/2 values found in basic melt suggests that as many as six of the labile chloride ions that are associated with the [(Nb6Cl12)Cl6](6-z)- clusters in basic melt may be replaced by [AlCl4]- ions when these clusters are dissolved in acidic melt. The Stokes-Einstein products of the niobium clusters in both acidic and basic melts are essentially independent of the oxidation level of the core. The average values of these products are 8.2 x 10(-11) (40-degrees-C) and 1.2 x 10(-10) (100-degrees-C) g cm s-2 K-1, respectively.