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

The hexahalide salts, [(NBu4)-Bu-n](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 (Sc-45, Y-89 and La-189) and, for the EMIM salts, via the shift of delta(H2) relative to [EMIM]Cl at a standard concentration. The trends observed in solution mirror those determined in the solid state.