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

• Published in: Chemistry : a European journal 2010-05, Vol.16 (18), p.5472-5480
• Main Authors: Jenter, Jelena, Meyer, Nils, Roesky, Peter W., Thiele, Sven K.-H., Eickerling, Georg, Scherer, Wolfgang
• Format: Article
• Language: English
• Subjects: Anions; Borohydrides; boron; Byproducts; chelates; Chemistry; Diffraction; Earth; Ligands; N ligands; Neodymium; Polymerization; Rare earth metals; rare earths; Schiff bases
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200902923

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. Unusual coordination and reactivity: Depending on the ionic radius of the central metal atom, the BH4− group, which usually reacts in lanthanide chemistry as a pseudohalide, can be involved in redox chemistry. The resulting product contains an NBH3 unit that binds in an unusual η2 fashion onto the metal atom (see graphic). The Nd complexes were used as Ziegler–Natta catalysts for the polymerization of 1,3‐butadiene to poly(cis‐1,4‐butadiene).