Electrochemical and Spectroscopic Study of Ce(III) Coordination in the 1-Butyl-3-methylpyrrolidinium Bis(trifluoromethylsulfonyl)imide Ionic Liquid Containing Chloride Ion

Cyclic staircase voltammetry, controlled potential coulometry, and electronic absorption spectroscopy were used to probe the coordination and accessible oxidation states of Ce3+ dissolved in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BuMePyroTf(2)N) before and after the addition of chloride ion as BuMePyroCl. Controlled potential coulometry indicated that the oxidation of Ce metal in this ionic liquid produces only Ce3+. Spectroscopic examination of the resulting solutions indicated that Ce3+ was weakly solvated by Tf2N- ions as [Ce(Tf2N)]((x-3)-), x >= 3. This species can be reduced at negative potentials, probably to a related Ce2+ species, but the latter is unstable and quickly disproportionates to Ce3+ and Ce-0; the latter appears to react with the ionic liquid. The addition of Cl- to solutions of [Ce((TfN)-N-2)]((x-3)-) causes the precipitation of CeCl3(s), providing a convenient route to the nondestructive recovery of Ce3+ from the ionic liquid. However, as the cr concentration is further increased, the CeCl3(s) redissolves as the octahedral complex, [CeCl6](3-), and the voltammetric and spectroscopic signature for [Ce(Tf2N)]((x-3)-) disappears. Absorption spectroscopy indicated that the bulk controlled potential oxidation of solutions containing [CeCl6](3-) produces [CeCl6](2-). Although stable on the time scale of voltammetry, this species slowly reacts with the ionic liquid and is converted back to [CeCl6](3-).