The structure and spectroscopy of lanthanide(III) complexes with picolinic acid N-oxide in solution and in the solid state

The spectroscopic characteristics and the crystal structure of Eu(III) complex with picolinic acid N-oxide ligand, picNO, at room and liquid-nitrogen temperatures are discussed. Studies concerning the Eu(III) ion luminescence (intensity, luminescence lifetime measurements, and excitation spectra of the ⁵D₀ → ⁷F₀ transition) are presented. The selective excitation luminescence spectroscopy of Eu(III) in the range of the ⁵D₀ → ⁷F₀ transition is used for the study of Eu/picNO complexes in solution equilibria. In the crystal the complex molecules build the two-dimensional structures with additional Na⁺ cations and water molecules. This structure consists of edge-sharing chains of Na distorted octahedral, interconnected by Eu polyhedra (distorted square antiprisms). The ligand, pyridine-2-carboxylate-1-oxide, coordinates to the Eu(III) ion as an ionic bidentate chelate, forming the NaEu(picNO)₄] complex of six-membered chelate rings with the bite angles of ca. 70.5°. The complex is symmetrical; the Eu(III) ion is eight-coordinated.