Interaction of [Ln(DO2A)(H2O)2-3]+ and [Ln(DO2P)(H2O)2-3]- with phosphate, acetate and fluoride anions in aqueous solution

Abstract

Mixed complexation of the lanthanide(III) chelates of the 1,7-disubstituted tetraazacyclododecane macrocycles DO2A and DO2P with phosphate, acetate and fluoride anions was studied in aqueous solutions using high resolution 1H and 31P NMR spectroscopy. The [La(DO2A)(H2O)2-3]+ and [Ce(DO2A)(H2O)2-3]+ chelates readily decompose in aqueous solutions containing phosphate anions. [Nd(DO2A)(H2O)2-3]+ and [Eu(DO2A)(H2O)2-3]+ chelates remain stable in an acetate environment. Association constants for the 1:1 adduct were obtained via 1H NMR titrations. In a fluoride environment, [La(DO2A)(H2O)2-3]+ was found to be unstable, in contrast to [Eu(DO2A)(H2O)2-3]+, for which the association constant for the 1:1 adduct was calculated. Chelate formation, as well as mixed complexation with fluoride and phosphate was studied for [Eu(DO2P)(H2O)2-3]-. The 31P chemical shifts of [Eu(DO2P)(H2O)2-3]- indicate that the DO2P macrocycle forms an [`]in-cage' complex with binding through all four nitrogens. The [Eu(DO2P)(H2O)2-3]- chelate gradually decomposes in a phosphate environment, but remains stable in an fluoride environment.