Structural characterization for metal-ion complexation and isomerization of crowned bis(spirobenzopyran)s

Metal ion complexes of crown ether derivatives bearing two spirobenzopyran moieties were synthesized and their structural features were studied by X-ray crystallography and NMR spectroscopy. The resulting X-ray crystal structure for the Ca2+ complex of diaza-12-crown-4-bis(spirobenzopyran) showed that both of two spirobenzopyran moieties were isomerized from their spiropyran forms to the corresponding merocyanine forms and their phenolate anions interacted with Ca2+ bound to its crown ether moiety. The crystal structure also revealed its syn conformation whose two spirobenzopyran moieties oriented on the same side of the crown ether moiety. Furthermore, density functional theories (DFT) calculations were carried out in order to predict energetically stable conformation of diaza-18-crown-6-bis(spirobenzopyran).