Influence of peripheral substituents on the stability of sandwich-type complexes of crown ether-containing anthraquinoneimines
Benzo-15-crown-5 ether derivatives of 1-hydroxy-9,10-anthraquinone-9-imine bearing a substituted benzoylamino group, NHCO(p-C6H4NO2) or NHCOC6F5, in position 2 were synthesized by a photochemical method. The structure of the fluorine-containing compound was determined by single-crystal X-ray diffraction. Both synthesized dyes, as well as their analogue without substituents on the benzoyl ring, are capable of binding Sr2+ and Ba2+ to form sandwich-type 2:1 (ligand:metal) complexes. In MeCN, these complexes exhibit higher stability constants (K2:1) than the corresponding 1:1 complexes (K1:1). The para-substitution of the benzoyl ring with a nitro group leads to a significant increase in both the K2:1 constant and the K2:1/K1:1 ratio. The fluorination of the benzoyl ring insignificantly affects the stability of sandwich complexes. Density functional theory calculations showed that this is related to steric interactions in the benzoylamino group. The calculated structures of the dyes and their sandwich complexes with Ba2+ are consistent with the 2D NOESY data.