The Schiff base of gossypol with 2-(aminomethyl)-15-crown-5 complexes with monovalent cations studied by MS, ¹H NMR, FT-IR and PM5 semiempirical methods

A new Schiff base of gossypol with 2-(aminomethyl)-15-crown-5 [GSCB] was shown to be capable of complexation of monovalent cations. This process of complex formation was studied by electrospray ionization (ESI) mass spectrometry, ¹H NMR and FT-IR spectroscopy, as well as by PM5 semiempirical method. It was found that the gossypol Schiff base can form 1 ∶ 1 and 1 ∶ 2 complexes with monovalent cations. With H⁺, Li⁺ or Na⁺ cations the Schiff base preferentially forms 1 ∶ 2 complexes. In the complex with protons they are localised on the N atoms of the Schiff base and the molecule exists in the imine–imine tautomeric form. The 1 ∶ 2 complexes of the Schiff base with Li⁺ and Na⁺ cations occur only in the enamine–enamine tautomeric form and the cations are coordinated only by the oxygen atoms of the crowns. The complexes of the Schiff base with K⁺, Rb⁺ or Cs⁺ cations are of 1 ∶ 1 and 1 ∶ 2 type with the oxygen atoms of the crowns, as well as O₁H and O_1′H groups coordinating the cations. The structures of the complexes are calculated by PM5 semiempirical method and disscussed.