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 (−2.73 kcal/mol). At large horizontal displacements (offset r = 5.0 Å), these sandwich∥sandwich interactions are remarkably strong (−3.03 kcal/mol), while half-sandwich∥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∥sandwich contacts have large horizontal displacements, which is significantly more than 46% of half-sandwich∥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.