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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 contribution
posted on 1997-06-04, 00:00 authored by Yimin Sun, Rupert E. v. H. Spence, Warren E. Piers, Masood Parvez, Glenn P. A. YapThe reactions of so called “tuck-in” permethyl zirconocene
compounds
Cp*(η5-η1-C5Me4CH2)ZrX
(X =
Cl (1a), C6H5 (1b),
CH3 (1c)) with the highly electrophilic boranes
HB(C6F5)2 and
B(C6F5)3 are described.
The
products are zwitterionic olefin polymerization catalysts.
Reactions with 1a and 1b yielded single
products cleanly,
but reactions with tuck-in methyl starting material 1c gave
mixtures. Spectroscopic and structural studies showed
that the electrophilic zirconium center 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*[η5,η1-C5Me4CH2B(C6F5)2(μ-H)]ZrX
(X = Cl (2a),
74%), C6H5 (2b, 62%)), the metal
is chelated by a pendant hydridoborate moiety. Chloride product
2a was
characterized crystallographically. In the reaction of
B(C6F5)3 with 1a, the
fluxional zwitterionic product Cp*[η5-C5Me4CH2B(C6F5)3]ZrCl
(3a, 84%) is stabilized by a weak donor interaction between
one of the ortho fluorine
atoms of the
−CH2B-(C6F5)3
counterion and the zirconium center (Zr−F = 2.267(5) Å).
In the product of the
reaction between 1b and
B(C6F5)3,
Cp*[η5-C5Me4CH2B(C6F5)3]ZrC6H5
(3b, 82%), a similar ortho-fluorine
interaction
was found in a yellow kinetic product (y-3b), which
converted upon heating gently to a thermodynamic orange
polymorph (o-3b) in which the zirconium center is
compensated via an agostic interaction from an ortho C−H
bond
of the phenyl group and an interaction between the methylene group of
the
−CH2B-(C6F5)3
counteranion. These
compounds were both characterized by X-ray crystallography.
Zwitterion o-3b reacts with H2 to form the
zwitterionic
hydride
Cp*[η5-C5Me4CH2B(C6F5)3]ZrH
(4, 77%), characterized by NMR spectroscopy and X-ray
crystallography
to reveal a return to the ortho-fluorine mode of
stabilization. Compounds 2a, 3a,
o-3b, and 4 were all found to be
active ethlyene polymerization catalysts; the chloride derivatives
required minimal amounts of methylaluminoxane
(MAO) to alkylate the zirconium center. Polymerization data are
discussed in light of the structural findings for the
catalysts employed.
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Keywords
Zwitterionic Metallocene Olefin Polymerization Catalysts Derivedortho fluorine atomsethlyene polymerization catalystsMAO4 CH 2ZrC 6 H 5HBzwitterionic olefin polymerization catalystspendant hydridoborate moietymaterial 1 czirconium centerelectrophilic zirconium centerXBinteractionC 6 F 5NMR1 bCpC 5Chloride product 2
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