Lanthanide(III)chloride-tetrahydrofuran solvates: Structural patterns within the series LnCl₃(THF)_n, where n = 2, 3, 3.5 and 4: Crystal and molecular structures of [PrCl(μ-Cl)₂(THF)₂]_n, [Nd(μ-Cl)₃(H₂O)(THF)]_n and GdCl₃(THF)₄

Praseodymium(III), neodymium(III) and gadolinium(III) chloride adducts with tetrahydrofuran (THF) have been prepared and structurally characterized by X-ray crystallography. Removal of water from the corresponding hexahydrate LnCl₃(H₂O)₆ using thionyl chloride in the presence of excess THF provides light-green cubic crystals of [PrCl₃(THF)₂]_n (1), blue block crystals of [NdCl₃(H₂O)(THF)]_n (2) and colourless needle crystals of GdCl₃(THF)₄ (3). In 1 each praseodymium atom is seven-coordinate and is linked to (two) adjacent metal centres by double (μ-Cl)₂ halogen bridging units, resulting in a polymeric chain structure. The metal geometry approximates to distorted pentagonal bipyramidal with Pr-Cl_bridge 2.852(6) 2.891(5), Pr-Cl_terminal 2.668(6), Pr-O_THF 2.51(1) (axial), 2.53(2) Å (equatorial). In 2 the structure is a two-dimensional cross-linked polymer in which each neodymium atom is connected to (three) others via double (μ-Cl)₂ halogen bridge bonds. The coordination sphere of each metal centre comprises six chlorine atoms [Nd-Cl_bridge 2.816(3)-2.938(3) Å] and two oxygen atoms belonging to a THF molecule [Nd-O, 2.548(6) Å] and a water molecule [Nd-O, 2.490(7) Å], respectively. Hydrogen bonding interactions involving halogen atoms and coordinated water molecules from adjacent metal units (intermolecular) are observed, O-H·Cl 3.147-3.350 Å. In 3 the molecular structure is based on a seven-coordinate pentagonal bipyramidal metal geometry in which two chlorides occupy the axial positions with the other chloride and the four solvate (THF) molecules making up the equatorial plane. Gd-Cl 2.60(2)-2.66(2), Gd-O 2.40(2)-2.52(3) Å. In addition, general comments concerning structural relationships within the series LnCl₃(THF)_n, where n = 2, 3, 3.5 and 4, are discussed.