Crown ether complexation of scandium(III), yttrium(III) and lanthanum(III) halides. Synthesis and spectroscopic characterisation of anhydrous cationic metal-oxacrown species and crystal structure of a scandium benzocrown

Abstract

Halide abstraction reactions of MCl₃(M = Sc, Y or La) with SbCl₅ in the presence of a crown ether provided a series of anhydrous cationic M^III-crown compounds of the types [MCl₂(crown)][SbCl₆][M = Sc, crown = 1, 4, 7, 10, 13, 16- hexaoxacyclooctadecane (18-crown-6), 2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecine (benzo-15-crown-5), or 1,4,7,10,13-pentaoxacyclo- pentadecane (15-crown-5)], [ScCl₂(12-crown-4)(MeCN)][SbCl₆](12-crown-4 =1,4,7,10-tetraoxacyclododecane),[M(18-crown-6)(MeCN)₃][SbCl₆]₃(M = Y or La) and [M(12-crown-4)(MeCN)₅]-[SbCl₆]₃(M = Y or La). Analytical and spectroscopic (¹H and ¹³C NMR) characterisation of these compounds supports their formulation as hexachloroantimonate(v) salts. The structure of [ScCl₂(benzo-15-crown-5)][SbCl₆] has been determined by X-ray diffraction. Crystals are orthorhombic, space group Pbca with a= 17.015(10), b= 13.296(8), c= 22.236(12)Å and Z= 8. The cation has a threaded structure with an essentially linear ScCl₂⁺ unit [Sc–Cl (mean) 2.395 Å, Cl–Sc–Cl 175.5°]. All the oxygen atoms of the oxacrown ligand are coplanar and are bonded to the central scandium atom, Sc–O 2.166(4)–2.223(4)Å, (mean) 2.188 Å with the benzene ring titled out of this plane at an angle of 24.9°. The resulting geometry of the seven-co-ordinate scandium atom is pentagonal bipyramidal.