Coordinating Benzenes Stack Stronger than Noncoordinating Benzenes, even at Large Horizontal Displacements

Abstract

Stacking interactions between two benzene molecules that coordinate transition metal ions within organometallic sandwich and half-sandwich compounds were investigated by performing Cambridge Structural Database (CSD) search and DFT-D calculations. Calculations of interaction energies revealed that stacking interactions between coordinating benzenes of sandwich (-3.69 kcal/mol) and half-sandwich compounds (-3.29 kcal/mol) are significantly stronger than the stacking interaction between noncoordinating benzenes (-2.7 kcal/mol). At large horizontal displacements (offset r = 5.0 angstrom), these sandwich parallel to sandwich interactions are remarkably strong (-3.03 kcal/mol)) while half-sandwich parallel to half-sandwich interactions are significantly weaker (-1.27 kcal/mol). The results of calculations are in good agreement with the data in the crystal structures from the CSD, where 76% of sandwich parallel to sandwich contacts have large horizontal displacements, which is significantly more than 46% of half-sandwich parallel to half-sandwich contacts arranged in this fashion. The study provides valuable information about interactions of aromatic molecules relevant to crystal engineering, materials design, and molecular recognition.