Structure of a new Schiff base of gossypol with 1-(3-aminopropyl)-2-pyrrolidinone studied by the X-ray, FT-IR, NMR, ESI-MS and PM5 methods

Crystals of the Schiff base derivative of gossypol with 1-(3-aminopropyl)-2-pyrrolidinone (GSPP) have been obtained and examined by X-ray diffraction, FT-IR, 1H and 13C NMR methods. The Schiff base crystallizes in ethanol solution as an inclusion compound with ethanol and H2O molecules. The crystal space group is P21/n with a = 7.5555(5), b = 18.7583(12), c = 29.415(2) Å, β = 94.270(6)°, and Z = 4. In the inclusion compound the Schiff base exists in the enamine–enamine tautomeric form. This tautomeric form is also conserved in the chloroform solution. Water and ethanol molecules are enclosed in hydrophilic channels where they are hydrogen-bonded to pyrrolidinone moieties and O5H groups of the Schiff base. The FT-IR spectral features of the inclusion compound are in agreement with the X-ray data indicating different bonding and arrangement of pyrrolidinone moieties. The FT-IR spectrum of GSPP in chloroform implies the structural equivalence of these two pyrrolidinone groups. The ESI-MS studies of mixtures of GSPP with formic acid have demonstrated the formation of the mono- and di-protonated species. The PM5 semiempirical calculations allow visualization of the most energetically favourable structures of protonated GSPP species.