Synthesis and structures of alkaline earth metal salts of bis[(trifluoromethyl)sulfonyl]imide

A series of alkaline earth metal salts of the bis[(trifluoromethyl)sulfonyl]imide anion have been prepared and structurally characterized. The magnesium cation is fully hydrated with no direct interaction with the anion, although there is extensive hydrogen bonding involving coordinated and lattice water molecules and the anion. The calcium cation is heavily hydrated, but also directly interacts with two anions. As with the magnesium salt, hydrogen bonding plays a major role in determining the crystal packing. The strontium salt is anhydrous, and the eight-coordinate cation interacts directly with several anions to form a two-dimensional layered structure. The barium salt is a monohydrate, with a nine-coordinate cation. As with strontium, it also forms a layered structure. Despite the differences, all of the structures exhibit extensive fluorine segregation which results in the formation of hydrophilic and hydrophobic domains. In all but the magnesium salt, the anion is chelated to the metal and has a cisoid conformation with the trifluoroalkyl groups lying to the same side of the SNS plane. This is in keeping with our previous observations that the cisoid conformation is preferred when the anion is coordinated to a metal ion, while the transoid is preferred for noncoordinating cations.