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

Abstract

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 = Sc(III), Lu(III), Yb(III), Tm(III) or Er(III). 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 approximately 185 K. The hydrated scandium(III) trifluoromethanesulfonate transforms reversibly (DeltaH degrees = -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.

Hydration of Lanthanoid(III) Ions in Aqueous Solution and Crystalline Hydrates Studied by EXAFS Spectroscopy and Crystallography: The Myth of the “Gadolinium Break”

“…A recent detailed crystallographic and 2D solidstate NMR investigation of the hydrated trifluoromethanesulfonate salts showed reduced occupancy of the capping positions for the heavy lanthanoidA C H T U N G T R E N N U N G (III) ions Er-Lu. [23] This water deficiency starts at holmiumA C H T U N G T R E N N U N G (III) with 2.91 water molecules in the three capping positions, reported in this study, and increases for erbiumA C H T U N G T R E N N U N G (III), thuliumA C H T U N G T R E N N U N G (III), ytterbiumA C H T U N G T R E N N U N G (III), and lutetiumA C H T U N G T R E N N U N G (III) with 2.96, 2.8, 2.7, and 2.4 water molecules, respectively. [23] The hydrated lanthanoidA C H T U N G T R E N N U N G (III) ions in the bromate salts also crystallize as tricapped trigonal prisms, but with much smaller differences between the Ln À O bond lengths in the prismatic and capping positions (Table 1).…”

“…[23] salts (Ln = La-Ho) are nine-coordinate in tricapped trigonal-prismatic configurations with a narrow distribution of the Ln À O bond lengths ( 0.04 ). The eight-coordinate hydrated lanthanoidA C H T U N G T R E N N U N G (III) ions in the bromide and iodide salts with square-antiprismatic configuration display fairly wide bond-length distributions of about 0.10 and 0.05 , respectively (Table 1), [34,35] as is also found for the hydrated yttriumA C H T U N G T R E N N U N G (III) ion in its solid hydrates.…”

“…Two almost complete series of isomorphous crystal structures have been reported, the nonaaqualanthanoidA [19][20][21][22][23] and the…”

“…The light and large hydrated lanthanoidA C H T U N G T R E N N U N G (III) ions in the iodide [a] Refs. [19][20][21][22][23].…”

Advanced Extraction Methods for Actinide/Lanthanide Separations

“…The coordination number of trivalent lanthanide hydrated salts in solution vary from 9 to 8 across the series, 47,48 and in the solid state, the lanthanide aqua complexes adopt nearly rigorous tricapped trigonal prismatic (TTP) geometry. 49 increased by factor of three ( Figure 19).…”