Ym. Sun et al., INTRAMOLECULAR ION-ION INTERACTIONS IN ZWITTERIONIC METALLOCENE OLEFIN POLYMERIZATION CATALYSTS DERIVED FROM TUCKED-IN CATALYST PRECURSORS AND THE HIGHLY ELECTROPHILIC BORANES XB(C6F5)(2) (X=H, C6F5), Journal of the American Chemical Society, 119(22), 1997, pp. 5132-5143
The reactions of so called ''tuck-in'' permethyl zirconocene compounds
Cp(eta(5)-eta(1)-C5Me4CH2)ZrX (X = Cl (1a), C6H5 (1b), CH3 (1c)) wit
h the highly electrophilic boranes HB(C6F5)(2) and B(C6F5)(3) are desc
ribed. The products are zwitterionic olefin polymerization catalysts.
Reactions with la and Ib yielded single products cleanly, but reaction
s with tuck-in methyl starting material Ic gave mixtures. Spectroscopi
c and structural studies showed that the electrophilic zirconium cente
r in the product zwitterions was stabilized by a variety of mechanisms
. In the products of reaction between 1a and 1b with HB(C6F5)(2), Cp[
eta(5) eta(1)-C5Me4CH2B(C6F5)(2)(mu-H)]ZrX (X = Cl (2a), 74%), C6H5 (2
b, 62%)), the metal is chelated by a pendant hydridoborate moiety. Chl
oride product 2a was characterized crystallographically. In the reacti
on of B(C6F5)(3) with 1a, the fluxional zwitterionic product Cp[eta(5
)-C5Me4CH2B(C6F5)(3)]ZrCl (3a, 84%) is stabilized by a weak donor inte
raction between one of the ortho fluorine atoms of the -CH2B-(C6F5)(3)
counterion and the zirconium center (Zr-F = 2.267(5) Angstrom). In th
e product of the reaction between 1b and B(C6F5)(3), Cp[eta(5)-C5Me4C
H2B(C6F5)(3)]ZrC6H5 (3b, 82%), a similar ortho-fluorine interaction wa
s found in a yellow kinetic product (y-3b), which converted upon heati
ng gently to a thermodynamic orange polymorph (o-3b) in which the zirc
onium center is compensated via an agostic interaction from an ortho C
-H bond of the phenyl group and an interaction between the methylene g
roup of the -CH2B-(C6F5)(3) counteranion. These compounds were both ch
aracterized by X-ray crystallography. Zwitterion o-3b reacts with H2 t
o form the zwitterionic hydride Cp[eta(5)-C5Me4CH2B(C6F5)(3)]ZrH (4,
77%), characterized by NMR spectroscopy and X-ray crystallography to r
eveal a return to the ortho-fluorine mode of stabilization. Compounds
2a, 3a, o-3b, and 4 were all found to be active ethylene polymerizatio
n catalysts; the chloride derivatives required minimal amounts of meth
ylaluminoxane (MAO) to alkylate the zirconium center. Polymerization d
ata are discussed in light of the structural findings for the catalyst
s employed.