High stability constants for multivalent metal ion complexes of crown ether derivatives incorporating two spirobenzopyran moieties

Abstract

The metal ion complexing behavior of diazacrown ether derivatives possessing two spirobenzopyran moieties has been studied under dark conditions. Introduction of two spirobenzopyran moieties into diazacrown ethers resulted in interesting metal ion complexation abilities, along with isomerization of the spirobenzopyran moieties to the corresponding merocyanine form. The extremely high affinity for multivalent metal ions observed is due to an ionic interaction between the two phenolate anions and the multivalent metal ion. This finding is supported by absorption spectral measurements and electrospray ionization mass spectrometry. Stability constants for complex formation with various metal ions in methanol under dark conditions have been determined for the first time using mass spectrometry. It was found that the stability constants for crowned bis(spirobenzopyran) complexes with high charge density metal ions are much greater than those for the parent crown ether rings. Specifically, the La³⁺complex of the diaza-18-crown-6 derivative carrying two spirobenzopyran moieties possesses an extraordinarily high stability constant of greater than 10¹¹ M⁻¹ in methanol.