Two faces of triel bonds in boron trihalide complexes

Area of Science:

• Quantum Chemistry
• Chemical Bonding
• Computational Chemistry

Background:

• Boron trihalides (BX3) are important Lewis acids.
• Understanding triel bonds (N⋅⋅⋅B) is crucial for Lewis acid-base interactions.
• Previous studies have explored BX3 complexes, but the nuanced behavior with different bases requires further investigation.

Purpose of the Study:

• To theoretically analyze the N⋅⋅⋅B triel bonds in BX3 complexes with nitrogen Lewis bases (NH3, N2, HCN).
• To investigate the influence of Lewis acid strength and Lewis base strength on the nature and stability of these bonds.
• To explore the structural characteristics, including tautomerism, of these complexes.

Main Methods:

• Ab initio calculations using MP2/aug-cc-pVTZ.
• Analysis of molecular electrostatic potentials (EPs).
• Examination of electron density distributions using the Quantum Theory of Atoms in Molecules (QTAIM).
• Decomposition of interaction energy.

Main Results:

• Lewis acidity order of boron center varies: BCl3 > BF3 with strong bases (NH3), but BF3 > BCl3 with weak bases (N2).
• BX3⋅⋅⋅HCN complexes (X=Cl, Br, I) exhibit two tautomeric forms with distinct N⋅⋅⋅B distances, one resembling a covalent bond.
• BF3⋅⋅⋅HCN shows a single energetic minimum.
• Differences in EP distribution between BF3 and other BX3 species influence acidity and tautomerism.

Conclusions:

• The strength and nature of N⋅⋅⋅B triel bonds are sensitive to both the boron trihalide and the Lewis base.
• Electrostatic interactions play a significant role in determining the observed acidity trends and structural diversity.
• The study provides detailed insights into the electronic structure and bonding in these important chemical complexes.