Unveiling Superacidity in Alcohol-BF₃ Complexes Using a Vibrational Probe

Boron trifluoride, a powerful Lewis acid, instantly forms coordination complexes with Lewis n-donor bases such as water and alcohols generating strong conjugate Brønsted acid systems, including those in the superacidity range. These complexes are efficient acid catalysts and reagents that enable a wide range of transformative chemistry. Despite their widespread use, their hydrogen bonding capabilities have not been systematically studied. It is known that some common aliphatic alcohols with low Brønsted acidity turn into strong acids when complexed with BF₃. In this work, we use deuterated acetonitrile as a vibrational probe for evaluating hydrogen bonding in BF₃-alcohol complexes. As previously reported, we observe a linear blue shift in the CN vibrational frequency with increasing acidity of the alcohols. However, the complexation of alcohols with BF₃ induces a substantially larger blue shift on the probe, pushing the CN frequency into the range expected for known superacids. The most acidic alcohol (HFIP)-BF₃ complex induces a frequency change in the probe that is larger than that of triflic acid. Our work is further supported by computations, which confirm the increase in acidity of the alcohol-BF₃ complex and additionally provide insights into the changes in the electronic structure of the alcohols upon complex formation. Our work shows that nitrile frequency can serve as a sensitive probe of hydrogen bonding, and by extension acidity, even in the superacidity range. This work will help in quantifying the chemical environment of superacidic systems and extend their design scope for next-generation reagents.