The solid-state structure of 1,3-diferrocenyl-1,1,3,3-tetramethyldisiloxane (1) exhibits
significant differences between the pure crystal and the cocrystal with ferrocene. The
structure of (1) shows that the two silicon atoms have eclipsed constituents when viewed
along the Si−O−Si linkage. The structure of 1,3-diferrocenyl-1,1,3,3-tetramethyldisiloxane·ferrocene (2) indicates that the siloxane now exhibits a staggered conformation when viewed
along the Si−O−Si linkage. To understand the origin of these differences, a nonempirical
study of the energetics of very large clusters, as models of the solid state, was undertaken.
PRDDO/M calculations of the relative energetics of the two different conformers of 1 show
that methyl−methyl steric interactions dominate the conformational energetics and that
the monomer is ∼5 kcal/mol more stable in the staggered form found in the cocrystal.
However, a detailed analysis of intermolecular interactions in the pure- and cocrystals
demonstrate that intermolecular interactions are more favorable in the pure crystal than
in the cocrystal, resulting in the high-energy eclipsed form of the monomer being stabilized.
The importance of intermolecular interactions are evaluated by calculating insertion energies
of a central monomer into large clusters of molecules derived from the full lattice. The role
of specific intermolecular interactions, such as ferrocene−ferrocene, ferrocene−tetramethyldisiloxane, etc., are also investigated by a similar procedure, in which either the bridge or
the ferrocene components of the lattice are removed. We find that the stabilization of the
eclipsed form of the monomer in the pure crystal is due dominantly to intermolecular
interactions between the tetramethyldisiloxane bridges and the neighboring ferrocenes.