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

Abstract

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.