Synthesis and molecular structure of pentadienyl complexes of the rare-earth metals

Abstract

The coordination chemistry of the sterically encumbered pentadienyl ligand pdl′ (pdl′ = 2,4-(Me₃C)₂C₅H₅) towards a series of rare-earth metals was systematically explored and the resulting metal complexes were fully characterized by several techniques including X-ray diffraction, elemental analysis, NMR spectroscopy and solid-state magnetic susceptibility studies. Three different reaction products were isolated depending on the redox-potentials and ionic radii of the metal atoms. They can be classified as (a) salt-metathesis, (b) metal reduction–ligand oxidation and (c) ligand deprotonation products. While for the larger and difficult to reduce metal ions, M = La, Ce, Pr and Nd, trivalent compounds [(η⁵-pdl′)₃M] (1-M) were isolated, for the more readily reduced metal ions the corresponding divalent compounds [[(η⁵-pdl′)₂M(thf)_n] (2-M; with M = Sm (n = 2); Eu, Yb (n = 1)) were formed. A more complex structural motif was observed for the smaller and also difficult to reduce metal ions, M = Sc, Y, Gd, Tb, Dy, Ho, Er, Tm and Lu, which yielded the bimetallic complexes of the type [(pdl′)(pdl′^−1H)(pdl′^−2H)M₂(thf)₂] (3-M). In these dimeric complexes the pdl′ ligand acts as a result of deprotonation reactions not only as a monoanionic [pdl′]⁻, but also as a dianionic [pdl′^−1H]²⁻ and a trianionic [pdl′^−2H]³⁻ ligand scaffold, which form unusual structural motifs including a six-membered metallacycle. Solid-state magnetic susceptibility studies revealed the expected free-ion magnetic moments at T = 300 K for all investigated compounds, whereas at lower temperatures crystal-field effects dominate. Furthermore, for 3-Gd, 3-Tb, 3-Dy and 3-Er weak ferromagnetic exchange interactions were observed at low temperature.