Preparation and structural characterization of an enantiomerically pure, C-2-symmetric, single-component Ziegler-Natta alpha-olefin polymerization catalyst

A new linked bis(cyclopentadienyl) ligand, {(C5H3-2-SiMe(3)-4-CMe(3))(2)Si(OC10H6C10H6O)} (BnBpH(2)), has been designed to coordinate to transition metals tp afford a single enantiomeric C-2-symmetric ansa-metallocene. The syntheses of its dipotassium salt and (BnBp)YCl(THF) are described. Steric interactions between the 3- and 3'-methine positions of the 1,1'-binaphth-2,2'-diolate rings of the chiral linker with the alpha-trimethylsilyl substituents on the cyclopentadienyl rings force enantioselective metalation of this ligand, Thus, coordination to yttrium occurs in an entirely diastereoselective manner: the ligand prepared from (R)-(+)-1,1'-bi-2-naphthol directs formation of the (S)-yttrocene, (R,S)-(BnBp)YCl(THF), while that from (S)-(-)-1,1'-bi-2-naphthol directs formation of the (R)-yttrocene, (S,R)-(BnBp)YCl(THF). Removal of coordinated tetrahydrofuran allows the preparation of (BnBp)YCH(SiMe(3))(2). Treatment of rac-(BnBp)YCH(SiMe(3))(2) with H-2 yields a kinetic mixture of both heterochiral and homochiral dimers (e.g. (R,S)-(BnBp)Y(mu(2)-H)(2)-(S,R)-(BnBp) and (R,S)-(BnBp)Y(mu(2)-H)(2)-(R,S)-Y(BnBp), respectively). Over several hours this mixture undergoes conversion to the pure homochiral dimers. As anticipated, hydrogenolysis of enantiopure (BnBp)YCH(SiMe(3))(2) (e.g. R,S-(BnBp)YCH(SiMe(3))(2)) affords directly only enantiopure homochiral dimer (e.g. (R,S)-(BnBp)Y(mu(2)-H)(2)-(R,S)-Y(BnBp)). The hydride, generated in situ from (R,S)-(BnBp)YCH(SiMe(3))(2), polymerizes 1-pentene to highly isotactic poly-1-pentene (M(n) = 119 000, PDI = 1.44, mmmm >95%). The much higher solubility of the enantiopure homochiral dimers was used to grow crystals of the racemate at the interface of layered solutions of the pure optical isomers. rac-(BnBp)Y(mu(2)-H)(2)(BnBp) crystallizes in the centrosymmetric space group P (1) over bar (#2) with a = 11.925(4) Angstrom, b = 19.461(8) Angstrom, c = 23.814(10) Angstrom, and alpha = 82.80(3)degrees, beta = 84.64(4)degrees, gamma = 81.30(3)degrees, with a volume of 5404(4) Angstrom(3) and Z = 2.