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Behind the Scenes of Group 4 Metallocene Catalysis: Examination of the Metal–Carbon Bond
journal contribution
posted on 2018-08-15, 21:06 authored by Martin
R. Machat, Andreas Fischer, Dominik Schmitz, Marcel Vöst, Markus Drees, Christian Jandl, Alexander Pöthig, Nicola P. M. Casati, Wolfgang Scherer, Bernhard RiegerThis contribution
provides the first detailed analysis of the nature
of the M–C σ-bond of three alkylated, isostructural group 4 (M = Ti,
Zr, Hf) metallocenes, thereby elucidating individual peculiarities
of each metal center in the catalytic conversion of olefins. Therefore,
the subtle electronic differences of the individual M–C σ-bonds,
which are considered crucial for several subprocesses in the coordinative
polymerization of olefins, were examined by detailed experimental
charge density studies. These studies provided measures of the increasing
ionic character of the M–C bonds along the group 4 elements
(Ti–C < Zr–C < Hf–C). These results are
further supported by high-pressure diffraction studies showing that
the predominantly ionic Hf–C bond is more compressible than
the more covalent Zr–C bond in line with a smaller degree of
electron localization in the valence shell of the hafnium relative
to the zirconium atom along the M–C bond directions. The Ti–C
bond displays the largest degree of electron localization in these
group 4 metallocenes as witnessed by a pronounced bonded charge concentration
in the valence shell of the titanium atom–a rare phenomenon
in transition metal alkyls. All findings were then complemented by
experimental and theoretical studies of the kinetic aspects of M–C
σ-bond cleavage in group 4 metallocenes. These studies show
that the entropy of activation is distinctly more negative for a Zr–C
relative to a Hf–C bond dissociation. The combined results
of the kinetic and electronic analysis herein shed new light on the
different catalytic behavior of group 4 metallocenes with regard to
the applied transition metal atom. In this context, deviations between
zirconium- and hafnium-based catalysts concerning the catalytic activity
and the stereoregularities became clearly explainable, just as the
well-known “hafnium-effect” in the production of extraordinarily
high molecular weight polypropylenes.