Tuning the Lewis Basicity of Pyridine: Structural and Theoretical Characterization of the SO ₃ Adducts of Pyridine Derivatives

The synthesis and isolation of seven novel Lewis adducts between substituted pyridine derivatives and sulfur trioxide (SO₃) is reported. The compounds were systematically studied by structural analyses and computational studies. The Lewis basicity correlates with the nature of the substituents of the pyridine ring, as can be seen from the S-N distances and the formation of higher SO₃ adducts, i.e., further uptake of sulfur trioxide. Neat pyridine (C₅H₅N, "py") acts as a base toward SO₃ and increased amounts of SO₃ result in the formation of [py·S₂O₆] moieties or the adduct py·SO₃·SO₃, both represented in the crystal structure of [C₅H₅N·S₂O₆][C₅H₅N·SO₃] (SO₃) (1). The basicity of the N donor atom can be tuned by modifying the pyridine scaffold. Electron-withdrawing or electron-donating substituents lead to significantly enlarged or decreased S-N distances compared to py·SO₃. An increasing basicity (shorter S-N bonds) fosters the formation of S₂O₆ adducts. The influence of the substituents on the pyridine scaffold is strongly corroborated by density functional theory (DFT) investigations, resulting MEP surface plots, and QTAIM and NBO analyses. Moreover, we successfully identified and characterized chalcogen bonding (ChB) interactions in all structures, confirming that the free SO₃ units act as chalcogen bond donors.