(BMI)₃LnCl₆ Crystals as Models for the Coordination Environment of LnCl₃ (Ln = Sm, Eu, Dy, Er, Yb) in 1‑Butyl-3-methylimidazolium Chloride Ionic-Liquid Solution

Abstract

A series of (BMI)₃LnCl₆ (Ln = Sm, Eu, Dy, Er, Yb) crystals was prepared from solutions of LnCl₃ dissolved in the ionic liquid, 1-butyl-3-methylimidazolium chloride (BMICl). Crystals with Ln = 5% Sm + 95% Gd and with Ln = 5% Dy + 95% Gd were also grown to assess the importance of cross-relaxation in the Sm and Dy samples. The crystals are isostructural, with monoclinic space group <i>P</i>2₁/<i>c</i> and four formula units per unit cell. The first coordination sphere of Ln³⁺ consists of six Cl^– anions forming a slightly distorted octahedral LnCl₆^3– center. The second coordination sphere is composed of nine BMI⁺ cations. The emission spectra and luminescence lifetimes of both (BMI)₃LnCl₆ crystals and LnCl₃ in BMICl solution were measured. The spectroscopic similarities suggest that crystalline (BMI)₃LnCl₆ provides a good model of the Ln³⁺ coordination environment in BMICl solution