Molecular Mechanics Study of Oligomeric Models for Poly(ferrocenylsilanes) Using the Extensible Systematic Forcefield (ESFF)

Molecular mechanics have been employed to study the
conformations of the oligomeric metallocene species
Fc[Fe(C5H4)2XMe2]nH
(Fc =
Fe(C5H5)(C5H4);
X = Si, n = 1 (1); X = C, n =
1 (2); X = Si, n = 2 (3); X =
C,
n = 2 (4); X = Si, n = 4
(5)) and their cations. The calculations utilize the
new generalized ESFF forcefield; they
show that the neutral molecules are conformationally flexible with the
lowest energy configurations having close
contacts between the positively charged iron atoms of the ferrocene
units and the negatively charged cyclopentadienyl
rings of their neighbors. The carbon-bridged species have harder
potential surfaces than their silicon analogues, but
replacement of the methyl groups of the bridge with longer alkyl chains
was found to have very little effect on the
relative orientation of the ferrocene units. The conformations of
the molecules 3, 4, and 5 in their
experimental
crystal structures were found to be significantly different from the
calculated low energy isolated conformers. However,
the solid state conformations allow intermolecular
iron−cyclopentadienyl interactions analogous to the
intramolecular
interactions found for the isolated species. In contrast, the
conformation of (3)3+ in the crystal structure
is very
similar to the calculated low energy isolated conformer, since the
conformations are primarily determined by Fe−Fe
repulsion. Implications of this work for the structures of
high-molecular weight poly(ferrocenylsilanes) are
discussed.