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 INTRAMOLECULAR AND INTERMOLECULAR INTERACTIONS

The solid-state structure of 1,3-diferrocenyl-1,1,3,3-tetramethyldisil oxane (1) exhibits significant differences between the pure crystal an d the cocrystal with ferrocene. The structure of (1) shows that the tw o silicon atoms have eclipsed constituents when viewed along the Si-O- Si linkage. The structure of 1,3-diferrocenyl-1,1,3,3-tetramethyldisil oxane . ferrocene (2) indicates that the siloxane now exhibits a stagg ered conformation when viewed along the Si-O-Si linkage. To understand the origin of these differences, a nonempirical study of the energeti cs of very large clusters, as models of the solid state, was undertake n. PRDDO/M calculations of the relative energetics of the two differen t conformers of 1 show that methyl-methyl steric interactions dominate the conformational energetics and that the monomer is similar to 5 kc al/mol more stable in the staggered form found in the cocrystal. Howev er, a detailed analysis of intermolecular interactions in the pure- an d cocrystals demonstrate that intermolecular interactions are more fav orable in the pure crystal than in the cocrystal, resulting in the hig h-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 derive d from the full lattice. The role of specific intermolecular interacti ons, such as ferrocene-ferrocene, ferrocene-tetramethyldisiloxane, etc ., are also investigated by a similar procedure, in which either the b ridge 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.