Probing Attractive OH-π Interactions and Repulsive n-π Interactions in a Phenol Molecular Balance.

Molecular balances have emerged as invaluable chemical systems for examinations of noncovalent interactions. Here, we report physical-organic chemistry studies on a 2,6-diarylphenol molecular balance that enables the examination of competing attractive OH-π interactions and repulsive O-π interactions between two different flanking aromatic rings. Integrated structural, NMR spectroscopic, and quantum chemical analyses revealed that the phenolic hydroxyl group preferentially interacts with the electron-rich aromatic ring over the electron-poorer counterpart via through-space OH-π interactions. Quantum chemical analyses based on canonical energy decomposition analysis furthermore showed that the avoidance of the repulsive O-π interactions provides an important contribution to the noncovalent interactions between the phenol and aromatic rings that constitute the molecular balances. The work is important because it demonstrates that subtle differences in the electron density of aromatic rings can be recognized by the phenolic OH group, a useful piece of knowledge for the design of drugs and catalysts.