Synthesis and Structure of Bifunctional Zirconocene/Borane Complexes and Their Activation for Ethylene Polymerization

The potassium salt of 1 was transmetalated
by reaction
with (Cp-allyl)­ZrCl3(dme), yielding the complex (NC-nacnac)­Cp-allylZrCl2 (2). Addition of 1 molar equiv of B­(C6F5)3 gave [(C6F5)3B-NC-nacnac]­Cp-allylZrCl2 (3), and
the subsequent addition of 1 molar equiv of HB­(C6F5)2 to 3 gave the hydroborated species
[((C6F5)3B-NC-nacnac)­Cp-(CH2)3-B­(C6F5)2]­ZrCl2 (4). The hydroboration of 2 with
1 molar equiv of HB­(C6F5)2 gave an
insoluble product. This product was fully characterized by 11B and 13C solid-state NMR and IR, which showed that the
hydroboration followed by intermolecular adduct formation (R–B­(C6F5)2- - -NC- - -R′)
occurs, resulting in an organometallic polymer (5). In
addition to the CN-free [nacnac]­Cp-allylZrCl2 (6), the hydroborated derivative [(nacnac)­Cp-(CH2)3-B­(C6F5)2] (7) was
also prepared. Complexes 2–7 were
activated with methylaluminoxane (MAO) and gave active catalysts for
ethylene polymerization. The polymers produced were linear and had
high molecular weight, with polydispersities (PDI) of less than 2,
indicating that the catalysts are single site. The presence of the
allyl group on the Cp ring was found to increase the molecular weight
of the materials produced. In addition to discrete complexes, an organometallic
polymer (5) was formed. This compound is air stable and,
while not very soluble, polymerizes ethylene with good activity.