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

The reaction of potassium 2,5-bis[N-(2,6-diisopropylphenyl)imino-methyl]pyrrolyl [(dip(2)-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)(dip(2)-pyr)(thf)(2)]. As usual, the trisborohydrides reacted like pseudo-halide compounds forming KBH4 as a by-product. To compare the reactivity with the analogous halides, the dimeric neodymium complex [NdCl2(dip(2)-pyr)(thf)](2) was prepared by reaction of [(dip(2)-pyr)K] with anhydrous NdCl3. Reaction of [(dip(2)-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 resuiting 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 eta(2) fashion to the metal atom. The rare-earth-metal atoms are surrounded by the eta(2)-coordinated BH3 molecule, the eta(3)-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)(dip(2)-pyr)-(thf)(2)] and [NdCl2(dip(2)-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.