Influence of the protonation of pyridine nitrogen on pnicogen bonding: competition and cooperativity

Abstract

Ab initio MP2/aug-cc-pVTZ calculations were performed to investigate the pnicogen-bonded complexes of PyZX₂ (Py = pyridine, Z = P and As, X = H and F) and their protonated analogues. The selected Lewis bases include H₂S, PH₃, H₂O, NH₃, and H₂CO. The relative stability of pnicogen-bonded complexes is related to the nature of PyZX₂ and bases. When the nitrogen atom of the pyridine ring in PyZX₂ is protonated, the protonated complexes are more stabilized than their neutral counterparts, with the interaction energies increased by 8.5–34.6 kJ mol⁻¹ and the binding distances shortened by 0.050–0.574 Å. Protonation strengthens the pnicogen bond, from a weak interaction to one of moderate strength. In the neutral complexes of PyZX₂ and H₂O, the formation of a N⋯H–O hydrogen bond is favorable compared to the pnicogen bond. Such inclination is more prominent in the complexes of protonated PyZX₂ and NH₃. In H₂O⋯PyZX₂⋯H₂O, pnicogen bonding is strengthened by hydrogen bonding due to positive synergistic effects; however, in NH₃⋯H⁺-PyZX₂⋯NH₃, pnicogen bonding is weakened by hydrogen bonding due to negative synergistic effects.