Understanding the Structure of Protonated Box Ligands: Insights from Spectroscopy and Computational Studies

Protonation and tautomerization significantly impact the conformational landscape and non-covalent interactions in bis(oxazoline) (BOX) ligands, which are crucial for their applications in catalysis. These interactions influence properties such as binding affinity and catalytic efficiency. While tautomerization has been reported for neutral BOX ligands, its role in protonated forms remains less understood. Here we report the structural and spectroscopic characterization of (S,S−Ph-BOX)H⁺ and its tautomerization-resistant derivative (S,S−Ph-diMeBOX)H⁺. We show through IRMPD spectroscopy and TIMS experiments, combined with DFT calculations that (S,S−Ph-BOX)H⁺ is present in its tautomeric form, while a broader conformational landscape is observed for (S,S−Ph-diMeBOX)H⁺, in which an N−H⋅⋅⋅N proton-shared conformer is identified. These findings provide a deeper understanding of the relationship between tautomerization and non-covalent interactions in protonated BOX ligands, offering insights for the design of catalytic systems.