Study on Coordination Chemistry of U(Ⅵ) with N,N-dimethyl-6-amide-pyridine-2-carboxylic Acid

The very strong complexing capacity of pyridine-2,6-dicarboxylic acid (DPA) with lanthanides/actinides of different oxidation states in aqueous solutions makes it a selective complexing agent or stripping agent in various separation systems. Besides DPA, its derivatives and their coordination chemistry with lanthanides/actinides have received significant attention for the past a few years as well. Amongst, the mono-amide derivative of DPA, N,N-dimethyl-6-amide-pyridine-2-carboxylic acid (DMAPA, HL) as a multidentate ligand is very appealing because it can bond to metal ions in varying modes through oxygen atoms in both amide and carboxyl groups, and the nitrogen atom of the pyridine-ring. In this work, the coordination chemistry of DMAPA and UO²⁺₂ in aqueous solution was studied with fluorescence spectroscopy, potentiometry, Raman spectroscopy in combination with X-ray single crystal diffraction analysis of the solid complex UO₂L₂. It is found that besides the two previously reported complexes, UO₂L⁺ and UO₂L₂, formed by the deprotonated DMAPA (L-), a protonated complex UO₂(HL)²⁺ complex is formed by the neutral DMAPA (HL) molecule. Due to the combination between UO²⁺₂ and L^- in the UO₂L⁺ and UO₂L₂ complexes are too strong, the stability constants can’t be directly determined with potentiometric titration. Therefore, by using HEDTA as a competing ligand, the stability constants of UO₂L⁺ and UO₂L₂ are determined to be 105.45±0.06 and 107.67±0.10 respectively, which are apparently different from previously reported values of 105.65±0.10 and 108.95±0.15. Because the protonated complex UO₂(HL)²⁺ species exist in relatively acidic solutions in which pH electrode can’t work appropriately, potentiometry method is not suitable for determining the stability constant. Therefore, the stability constant of UO₂(HL)²⁺ is determined to be 106.32±0.09 by fluorescence spectral titration. Interestingly, different from the enhanced fluorescence of U(Ⅵ) in the UO₂(DPA) complex, the fluorescence intensity of U(Ⅵ) in UO₂L⁺ and UO₂(HL)²⁺ is lowered, but these observations can’t be well explained by a solid mechanism. By comparing the Raman spectra of the complexes in aqueous solutions and in the solid complex UO₂L₂, the coordination modes of L- and HL in the complexes of UO₂L⁺ and UO₂(HL)²⁺ are found to be same, L^- and HL bond to U(Ⅵ) as tridentate. For the UO₂(HL)²⁺ species, the protonation happens in the carboxyl group, which has no obvious effect on the Raman vibration of uranyl, hence the Raman shift of uranyl in UO₂(HL)²⁺ is almost the same with that in UO₂L⁺.