Hexahalometallate salts of trivalent scandium, yttrium and lanthanum: cation–anion association in the solid state and in solution

Abstract

The hexahalide salts, [NⁿBu₄]₃[LaCl₆], [BMPYRR]₃[LaCl₆] (BMPYRR = 1-butyl-1-methylpyrrolidinium), [EMIM]₃[MX₆] (EMIM = 1-ethyl-3-methylimidazolium; M = La, X = Cl, Br, I; M = Sc, Y, Ce, X = Cl) and [EDMIM]₃[MX₆] (EDMIM = 1-ethyl-2,3-dimethylimidazolium; M = Y, X = Cl; M = La, X = Cl, I) have been prepared and X-ray crystal structures determined for several of them, with a view to probing the effect of varying the trivalent metal ion, the halide and the counter-cation on the structures adopted in the solid state. The crystal structures of the EMIM and EDMIM salts show extensive H-bonding between the halide ligands and organic cations; based upon the H-bonding distances, this appears to be strongest for the [EMIM]₃[MCl₆] salts, becoming progressively weaker for heavier metal ion or halide. In terms of the cations, changing from EMIM to EDMIM also reduces the strength of the H-bonding. The strength of the cation–anion pairing in solution has also been probed in solution via NMR spectroscopy where possible (⁴⁵Sc, ⁸⁹Y and ¹⁸⁹La) and, for the EMIM salts, via the shift of δ(H2) relative to [EMIM]Cl at a standard concentration. The trends observed in solution mirror those determined in the solid state.