Borane and Borohydride Complexes of the Rare-Earth Elements: Synthesis, Structures, and Butadiene Polymerization Catalysis

摘要

The reaction of potassium 2,5-bis[N-(2,6-diisopropylphenyl)iminomethyl]pyrrolyl [(dip₂-pyr)K] with the borohydrides of the larger rare-earth metals, [Ln(BH₄)₃(thf)₃] (Ln=La, Nd), afforded the expected products [Ln(BH₄)₂(dip₂-pyr)(thf)₂]. As usual, the trisborohydrides reacted like pseudohalide compounds forming KBH₄ as a by-product. To compare the reactivity with the analogous halides, the dimeric neodymium complex [NdCl₂(dip₂-pyr)(thf)]₂ was prepared by reaction of [(dip₂-pyr)K] with anhydrous NdCl₃. Reaction of [(dip₂-pyr)K] with the borohydrides of the smaller rare-earth metals, [Sc(BH₄)₃(thf)₂] and [Lu(BH₄)₃(thf)₃], resulted in a redox reaction of the BH₄− group with one of the Schiff base functions of the ligand. In the resulting products, [Ln(BH₄){(dip)(dip-BH₃)-pyr}(thf)₂] (Ln=Sc, Lu), a dinegatively charged ligand with a new amido function, a Schiff base, and the pyrrolyl function is bound to the metal atom. The by-product of the reaction of the BH₄− anion with the Schiff base function (a BH₃ molecule) is trapped in a unique reaction mode in the coordination sphere of the metal complex. The BH₃ molecule coordinates in an η2 fashion to the metal atom. The rare-earth-metal atoms are surrounded by the η2-coordinated BH₃ molecule, the η3-coordinated BH₄− anion, two THF molecules, and the nitrogen atoms from the Schiff base and the pyrrolyl function. All new compounds were characterized by single-crystal X-ray diffraction. Low-temperature X-ray diffraction data at 6 K were collected to locate the hydrogen atoms of [Lu(BH₄){(dip)(dip-BH₃)-pyr}(thf)₂]. The (DIP₂-pyr)− borohydride and chloride complexes of neodymium, [Nd(BH₄)₂(dip₂-pyr)(thf)₂] and [NdCl₂(dip₂-pyr)(thf)]₂, were also used as Ziegler–Natta catalysts for the polymerization of 1,3-butadiene to yield poly(cis-1,4-butadiene). Very high activities and good cis selectivities were observed by using each of these complexes as a catalyst in the presence of various cocatalyst mixtures.