Interaction of [Ln(DO2A)(H 2O) 2−3] + and [Ln(DO2P)(H 2O) 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)(H 2O) 2−3] + and [Ce(DO2A)(H 2O) 2−3] + chelates readily decompose in aqueous solutions containing phosphate anions. [Nd(DO2A)(H 2O) 2−3] + and [Eu(DO2A)(H 2O) 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)(H 2O) 2−3] + was found to be unstable, in contrast to [Eu(DO2A)(H 2O) 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)(H 2O) 2−3] −. The 31P chemical shifts of [Eu(DO2P)(H 2O) 2−3] − indicate that the DO2P macrocycle forms an ‘in-cage’ complex with binding through all four nitrogens. The [Eu(DO2P)(H 2O) 2−3] − chelate gradually decomposes in a phosphate environment, but remains stable in an fluoride environment.