Structural Variety in Solvated Lanthanoid(III ) Halide Complexes

Abstract

Treatment of lanthanum metal with CH₂Br₂ or CH₂I₂ in tetrahydrofuran (thf) under ultrasound conditions yields the corresponding [LaX₃(thf)₄] (X = Br, I) complexes in good yield. Recrystallization of [LaBr₃(thf)₄] from 1,2-dimethoxyethane (dme) or bis(2-methoxyethyl) ether (diglyme) generates [LaBr₂(µ-Br)(dme)₂]₂ and [LaBr₂(dig-lyme)₂][LaBr₄(diglyme)]. Treatment of lanthanoid metals with hexachloroethane in dme yields [LnCl₃(dme)₂] (Ln = La, Nd, Er or Yb) and in acetonitrile [YbCl₂(MeCN)₅]₂[YbCl₃(MeCN)(-Cl)₂YbCl₃(MeCN)]. The reaction of Yb metal pieces with 1,2-dibromoethane in thf and dme gave single crystals of [YbBr₃(thf)₃] and [YbBr₃(dme)₂], respectively. The X-ray determined structure of [LaBr₃(thf)₄] shows a seven-coordinate monomer with pentagonal-bipyramidal stereochemistry and apical bromide ligands. For [YbBr₃(thf)₃], a monomeric structure with mer-octa-hedral stereochemistry is observed. In [LaBr₂(µ-Br)(dme)₂]₂, two eight-coordinate La centres are linked by two bridging bromides. The dme ligands have a trans relationship to each other, and cis terminal bromides are transoid to the bridging bromides with dodecahedral stereochemistry for La. By contrast, the 1: 1.5 diglyme adduct is found to be ionic [LaBr₂(diglyme)₂][LaBr₄(diglyme)], with an eight-coordinate bicapped trigonal-prismatic lanthanum cation and a seven-coordinate pentagonal-bipyramidal lanthanum anion. In the cation, the bromide ligands are cis to each other, and in the anion, two bromides are equatorial and two are axial. In [YbBr₃(dme)₂], [YbCl₃(dme)₂] and [ErCl₃(dme)₂], a seven-coordinate pentagonal-bipyramidal arrangement exists with apical halogen ligands. Far-infrared data, and in particular the absence of absorptions attributable to Ì(La–Cl_ter), suggest that [LaCl₃(dme)] is polymeric with six bridging chlorides per lanthanum. For [YbCl₂(MeCN)₅]₂[YbCl₃(MeCN)(-Cl)₂YbCl₃-(MeCN)], a remarkable ionic structure, with pentagonal-bipyramidal [YbCl₂(MeCN)₅]⁺ cations and octahedral di-nuclear [YbCl₃(MeCN)(-Cl)₂YbCl₃(MeCN)]^2– counter ions, is observed. In the former, chloride ligands are apical, while the MeCN ligands of the latter are transoid.