Highly Hydrated Cations: Deficiency, Mobility, and Coordination of Water in Crystalline Nonahydrated Scandium(III), Yttrium(III), and Lanthanoid(III) Trifluoromethanesulfonates

Trivalent lanthanide‐like metal ions coordinate nine water oxygen atoms, which form a tricapped trigonal prism in a large number of crystalline hydrates. Water deficiency, randomly distributed over the capping positions, was found for the smallest metal ions in the isomorphous nonahydrated trifluoromethanesulfonates, [M(H2O)n](CF3SO3)3, in which M=ScIII, LuIII, YbIII, TmIII or ErIII. The hydration number n increases (n=8.0(1), 8.4(1), 8.7(1), 8.8(1) and 8.96(5), respectively) with increasing ionic size. Deuterium (2H) solid‐state NMR spectroscopy revealed fast positional exchange between the coordinated capping and prism water molecules; this exchange started at temperatures higher than about 280 K for lutetium(III) and below 268 K for scandium(III). Similar positional exchange for the fully nonahydrated yttrium(III) and lanthanum(III) compounds started at higher temperatures, over about 330 and 360 K, respectively. An exchange mechanism is proposed that can exchange equatorial and capping water molecules within the restrictions of the crystal lattice, even for fully hydrated lanthanoid(III) ions. Phase transitions occurred for all the water‐deficient compounds at ≈185 K. The hydrated scandium(III) trifluoromethanesulfonate transforms reversibly (ΔH°=−0.80(1) kJ mol−1 on cooling) to a trigonal unit cell that is almost nine times larger, with the scandium ion surrounded by seven fully occupied and two partly occupied oxygen atom positions in a distorted capped trigonal prism. The hydrogen bonding to the trifluoromethanesulfonate anions stabilises the trigonal prism of water ligands, even for the crowded hydration sphere of the smallest metal ions in the series. Implications for the Lewis acid catalytic activity of the hydrated scandium(III) and lanthanoid(III) trifluoromethanesulfonates for organic syntheses performed in aqueous media are discussed. Water on the move: Coordinated water molecules exchange positions in the series of nonahydrated trifluoromethanesulfonates [M(H2O)n](CF3SO3)3 (M=metal, see structure), and water deficiency occurs when the tricapped trigonal prism becomes too crowded for the smaller trivalent metal ions. The hydration number n increases with increasing ionic size for M=Sc, Lu, Yb, Tm and Er (n=8.0(1), 8.4(1), 8.7(1), 8.8(1) and 8.96(5), respectively).