Spectroscopy and electrochemistry of uranium complexes in aluminum chloride-1-ethyl-3-methylimidazolium chloride

The redox chemistry of UCl{dollar}sb4{dollar} was investigated in the solvent AlCl{dollar}sb3{dollar}-1-ethyl-3-methylimidazolium chloride (AlCl{dollar}sb3{dollar}-EMIC), a room temperature ionic liquid. In basic solutions ({dollar}<{dollar}50 mol % AlCl{dollar}sb3{dollar}) the reduction of U(IV) to U(III) on glassy carbon electrodes is reversible. Visible/near-IR spectroscopic data and potentiometric measurements for U(IV)/U(III) as a function of the basic composition of AlCl{dollar}sb3{dollar}-EMIC indicate U(IV) and U(III) are both hexachloro anions. Diffusion coefficients were measured for the U(IV) and U(III) complexes by two methods. The values are comparable to those of transition metal hexachloro anionic complexes in the same solvent, supporting the assignment of UCl{dollar}sb6sp{lcub}2-{rcub}{dollar} and UCl{dollar}sb6sp{lcub}3-{rcub}{dollar}.; The redox behavior of UO{dollar}sb2sp{lcub}2+{rcub}{dollar} was examined in acidic AlCl{dollar}sb3{dollar}-EMIC ({dollar}>{dollar}50 mol % AlCl{dollar}sb3{dollar}). UO{dollar}sb2sp{lcub}2+{rcub}{dollar} is reduced in bulk solution by a chloroaluminate species (AlCl{dollar}sb4sp-{dollar} or Al{dollar}sb2{dollar}Cl{dollar}sb7sp-{dollar}) producing UCl{dollar}sb6sp-{dollar}. The kinetics of the reduction were investigated spectroscopically by monitoring the growth of UCl{dollar}sb6sp-{dollar} in the visible and near-IR regions of the spectrum. Two first order pathways were observed, with over 90% reducing by the slower pathway and less than 10% reducing by a faster reaction. The two pathways reflect a competition between the formation of a UO{dollar}sb2sp{lcub}2+{rcub}{dollar} chloroaluminate complex from UO{dollar}sb2{dollar}Cl{dollar}sb{lcub}rm n{rcub}sp{lcub}rm (2-n)+{rcub}{dollar} which undergoes slow reduction to UCl{dollar}sb6sp-{dollar}, and an outer-sphere electron transfer of UO{dollar}sb2{dollar}Cl{dollar}sb{lcub}rm n{rcub}sp{lcub}rm (2-n)+{rcub}{dollar} to UO{dollar}sb2{dollar}Cl{dollar}sb{lcub}rm n{rcub}sp{lcub}rm (1-n)+{rcub}{dollar} which quickly forms UCl{dollar}sb6sp-{dollar}. The faster pathway was also studied spectroscopically following the disappearance of a U(V) oxo-species in the near-IR. Cyclic voltammetric results of UO{dollar}sb2sp{lcub}2+{rcub}{dollar} solutions in acidic AlCl{dollar}sb3{dollar}-EMIC are consistent with the spectroscopic data.