Europium(III) complexes of polyfunctional carboxylic acids: Luminescence probes of possible binding sites in fulvic acid

The ⁷Fe₀ → ⁵D₀ excitation spectra of Eu(III) complexed with polyfunctional dicarboxylic acid containing neutral O, N, and S donors were investigated using Eu(III) luminescence spectroscopy. Three distinct peaks are seen in the excitation spectra in the Eu(III)-diglycolic acid (ODA) and Eu(III)-iminodiacetic acid (IMDA) systems showing the formation of three species, where each tridentate ODA^2- and IMDA^2- displace approximately three coordinated water molecules; however, the central S doner of Eu(III)-thiodiglycolate and 2,5-thiophenedicarboxyrate are shown to be poor chelators for free EU³⁺ ions. The neutral O and N donors in combination with neighboring carboxylates may form strong metal chelation sites an fulvic acid.