Web Release Date: October 18,
Dynamic NMR Studies of Cationic Bis(2-phenylindenyl)zirconium Pyridyl Complexes: Evidence for syn Conformers in Solution
Department of Chemistry, Stanford University, Stanford, California 94305-5080
Received June 9, 2005
Abstract:
The conformationally dynamic unbridged metallocene (2-PhInd)2ZrMe2 (1) was activated
with trispentafluorophenylborane (B(C6F5)3, B2) or trityl tetrakispentafluorophenylborate
("trityl borate", [Ph3C+][B(C6F5)4-], B4) to generate the ion pair [(2-PhInd)2ZrMe+][MeB(C6F5)3-] (2a) or [(2-PhInd)2ZrMe+][B(C6F5)4-] (2b), respectively. Activation parameters for
ion-pair separation were determined by line-shape analysis (2a: 
H
ips = 20 ± 1 kcal/mol,
Sips = 17 ± 4 eu; 2b: Hips = 15 ± 2 kcal/mol, Sips = 13 ± 7 eu). For 2a, a much slower
B(C6F5)3 dissociation-reassociation process was also observed (G83
C = 18.9 ± 0.1 kcal/mol). Both 2a and 2b were treated with a series of o-substituted pyridines, and the behavior
of the resulting zirconocenium-pyridyl complexes (3a-8b) was studied by 1H, 13C, and 19F
NMR over the temperature range -100 to 100 C. Below room temperature, 1H NMR NOESY
spectra revealed signals characteristic of a Cs-symmetric syn conformation.
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