Binary lanthanide(iii)/nitrate and ternary lanthanide(iii)/nitrate/chloride complexes in an ionic liquid containing water: optical absorption and luminescence studies

Abstract

The formation of binary Ln(III)/nitrate and ternary Ln(III)/nitrate/chloride complexes in a water-saturated ionic liquid, 1-butyl-3-methyl imidazolium bis(trifluoromethanesulfonyl)imide (denoted BumimTf₂N), was investigated by absorption spectrophotometry and luminescence spectroscopy. Four successive binary complexes, Nd(NO₃)²⁺, Nd(NO₃)₂⁺, Nd(NO₃)₃, and Nd(NO₃)₄⁻, were identified, and their stability constants in water-saturated BumimTf₂N are several orders of magnitude higher than those in aqueous solutions, but much lower than those observed in dry BumimTf₂N. The complexation of lanthanides with nitrate in wet BumimTf₂N proceeds via the replacement of water molecules by bidentate nitrate anions from the inner solvation spheres of Ln³⁺ cations. In the absence of nitrate, the precipitation of Ln(III)/chloride complex(es) occurs at low ratios of C_Cl/C_Ln (<6) in BumimTf₂N, which precludes the determination of stability constants of binary Ln(III)/chloride complexes by spectrophotometry or luminescence spectroscopy. However, using a competition approach, the formation of two ternary complexes, Ln(NO₃)₃Cl₂²⁻ and Ln(NO₃)₂Cl₄³⁻, has been observed and their stability constants in wet BumimTf₂N were determined. Data indicate that both nitrate and chloride are stronger ligands than water for lanthanides in BumimTf₂N.