A combined spectroscopic and theoretical study to investigate the coordination modes of 2,3- and 3,4-pyridine dicarboxylates with Eu(III)

The complex formation of Eu(III) with 2,3-pyridine dicarboxylic acid (23PDCA) and 3,4-pyridine dicarboxylic acid (34PDCA) has been studied by luminescence spectroscopy in aqueous medium. About three orders of luminescence enhancement of Eu(III) was observed in Eu(III)-PDCA complexes compared to its aqua complex, Eu(H2O)93+. Stability constant values (Log KMLi) were obtained for various metal–ligand (MLi (i = 1–3)) complexes formed in the aqueous medium. Complexation studies along with luminescence lifetime data suggested that both the ligands bind with Eu(III) in a bidentate fashion. Interestingly, the bi-dentate mode of 23PDCA interaction with Eu(III) is dominantly through nitrogen atom of pyridyl and the oxygen atom of nearby carboxylate group, whereas in case of 34PDCA, the bi-dentate coordination is through the two oxygen atoms of both the carboxylate groups. The density functional theory (DFT) calculations provided insights into the complex formation at the molecular level and the theoretical results show excellent agreement with the experimental observations.