Conformational Preferences of 1,3-Diferrocenyl-1,1,3,3-tetramethyldisiloxane and Its Cocrystal with Ferrocene: A Theoretical and Crystallographic Study of the Role of Intra- and Intermolecular Interactions

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.